<?xml version="1.0" encoding="us-ascii" standalone="yes"?>
<!--Created by IRIS RegTech Solutions_Direct, Powered by IRIS Carbon, Unique Code 6a070bd58fdfe617203479e8, Generated At Fri, 15 May 2026 12:05:57 GMT-->
<link:linkbase xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xl="http://www.xbrl.org/2003/XLink" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrli="http://www.xbrl.org/2003/instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" />
  <link:roleRef roleURI="http://60degreespharma.com/role/PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofExchangeRatesalongwithHistoricalRatesDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofExchangeRatesalongwithHistoricalRatesDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDBALANCESHEETSUnauditedParentheticals" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDBALANCESHEETSUnauditedParentheticals" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DocumentAndEntityInformation" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DocumentAndEntityInformation" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofLiabilitiesMeasuredatFairValueDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofLiabilitiesMeasuredatFairValueDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INTANGIBLEASSETSScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INTANGIBLEASSETSScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DEBTDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DEBTDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONScheduleofShareBasedCompensationExpensesDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SHAREBASEDCOMPENSATIONScheduleofShareBasedCompensationExpensesDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DERIVATIVELIABILITIESScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DERIVATIVELIABILITIESScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/STOCKHOLDERSEQUITYScheduleofEquityClassifiedWarrantsDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#STOCKHOLDERSEQUITYScheduleofEquityClassifiedWarrantsDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DERIVATIVELIABILITIESDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DERIVATIVELIABILITIESDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/NATUREOFOPERATIONSDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#NATUREOFOPERATIONSDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/COMMITMENTSANDCONTINGENCIESDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#COMMITMENTSANDCONTINGENCIESDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUBSEQUENTEVENTSDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUBSEQUENTEVENTSDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SHAREBASEDCOMPENSATIONDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/STOCKHOLDERSEQUITYDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#STOCKHOLDERSEQUITYDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDBALANCESHEETSUnaudited" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDBALANCESHEETSUnaudited" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSSUNAUDITED" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSSUNAUDITED" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSSUNAUDITEDCalc2" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSSUNAUDITEDCalc2" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITEDParentheticals" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITEDParentheticals" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFCASHFLOWSUNAUDITED" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDSTATEMENTSOFCASHFLOWSUNAUDITED" />
  <link:roleRef roleURI="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFCASHFLOWSUNAUDITEDParentheticals" xlink:type="simple" xlink:href="sxtp-20260331.xsd#CONSOLIDATEDCONDENSEDSTATEMENTSOFCASHFLOWSUNAUDITEDParentheticals" />
  <link:roleRef roleURI="http://60degreespharma.com/role/NATUREOFOPERATIONS" xlink:type="simple" xlink:href="sxtp-20260331.xsd#NATUREOFOPERATIONS" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIES" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIES" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INVENTORY" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INVENTORY" />
  <link:roleRef roleURI="http://60degreespharma.com/role/PROPERTYANDEQUIPMENT" xlink:type="simple" xlink:href="sxtp-20260331.xsd#PROPERTYANDEQUIPMENT" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INTANGIBLEASSETS" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INTANGIBLEASSETS" />
  <link:roleRef roleURI="http://60degreespharma.com/role/STOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="sxtp-20260331.xsd#STOCKHOLDERSEQUITY" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DEBT" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DEBT" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DERIVATIVELIABILITIES" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DERIVATIVELIABILITIES" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INCOMETAXES" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INCOMETAXES" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SHAREBASEDCOMPENSATION" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SHAREBASEDCOMPENSATION" />
  <link:roleRef roleURI="http://60degreespharma.com/role/COMMITMENTSANDCONTINGENCIES" xlink:type="simple" xlink:href="sxtp-20260331.xsd#COMMITMENTSANDCONTINGENCIES" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUBSEQUENTEVENTS" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUBSEQUENTEVENTS" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESPolicies" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESPolicies" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INVENTORYTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INVENTORYTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/PROPERTYANDEQUIPMENTTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#PROPERTYANDEQUIPMENTTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INTANGIBLEASSETSTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INTANGIBLEASSETSTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/STOCKHOLDERSEQUITYTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#STOCKHOLDERSEQUITYTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DEBTTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DEBTTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DERIVATIVELIABILITIESTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DERIVATIVELIABILITIESTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONTables" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SHAREBASEDCOMPENSATIONTables" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INVENTORYScheduleofInventoryDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INVENTORYScheduleofInventoryDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INVENTORYDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INVENTORYDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/PROPERTYANDEQUIPMENTDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#PROPERTYANDEQUIPMENTDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INTANGIBLEASSETSDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INTANGIBLEASSETSDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INTANGIBLEASSETSScheduleofIntangibleAssetsDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INTANGIBLEASSETSScheduleofIntangibleAssetsDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/DEBTScheduleoftheCurrentFuturePaymentObligationsDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#DEBTScheduleoftheCurrentFuturePaymentObligationsDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/INCOMETAXESDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#INCOMETAXESDetails" />
  <link:roleRef roleURI="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONScheduleofFairValueofOptionsGrantedDetails" xlink:type="simple" xlink:href="sxtp-20260331.xsd#SHAREBASEDCOMPENSATIONScheduleofFairValueofOptionsGrantedDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/PROPERTYANDEQUIPMENTScheduleofPropertyandEquipmentDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:title="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" xlink:title="us-gaap_PropertyPlantAndEquipmentGross" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentLineItems to us-gaap_PropertyPlantAndEquipmentGross" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:title="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentLineItems to us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" xlink:title="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentLineItems to us-gaap_PropertyPlantAndEquipmentNet" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentLineItems to us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="arc" order="3" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:title="us-gaap_PropertyPlantAndEquipmentByTypeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:title="Definition : us-gaap_ScheduleOfPropertyPlantAndEquipmentTable to us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:title="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentByTypeAxis to us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_LabEquipmentMember" xlink:label="sxtp_LabEquipmentMember" xlink:title="sxtp_LabEquipmentMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="sxtp_LabEquipmentMember" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentTypeDomain to sxtp_LabEquipmentMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MachineryAndEquipmentMember" xlink:label="us-gaap_MachineryAndEquipmentMember" xlink:title="us-gaap_MachineryAndEquipmentMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_MachineryAndEquipmentMember" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentTypeDomain to us-gaap_MachineryAndEquipmentMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" xlink:title="us-gaap_ComputerEquipmentMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentTypeDomain to us-gaap_ComputerEquipmentMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" xlink:title="us-gaap_FurnitureAndFixturesMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentTypeDomain to us-gaap_FurnitureAndFixturesMember" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" xlink:title="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain_1" xlink:title="Definition : us-gaap_PropertyPlantAndEquipmentByTypeAxis to us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofExchangeRatesalongwithHistoricalRatesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntercompanyForeignCurrencyBalanceLineItems" xlink:label="us-gaap_IntercompanyForeignCurrencyBalanceLineItems" xlink:title="us-gaap_IntercompanyForeignCurrencyBalanceLineItems" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_AverageExchangeRate" xlink:label="sxtp_AverageExchangeRate" xlink:title="sxtp_AverageExchangeRate" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IntercompanyForeignCurrencyBalanceLineItems" xlink:to="sxtp_AverageExchangeRate" xlink:title="Definition : us-gaap_IntercompanyForeignCurrencyBalanceLineItems to sxtp_AverageExchangeRate" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyExchangeRateTranslation1" xlink:label="us-gaap_ForeignCurrencyExchangeRateTranslation1" xlink:title="us-gaap_ForeignCurrencyExchangeRateTranslation1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IntercompanyForeignCurrencyBalanceLineItems" xlink:to="us-gaap_ForeignCurrencyExchangeRateTranslation1" xlink:title="Definition : us-gaap_IntercompanyForeignCurrencyBalanceLineItems to us-gaap_ForeignCurrencyExchangeRateTranslation1" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SummaryofSignificantAccountingPoliciesScheduleofExchangeRatesalongwithHistoricalRatesDetailsTable" xlink:label="sxtp_SummaryofSignificantAccountingPoliciesScheduleofExchangeRatesalongwithHistoricalRatesDetailsTable" xlink:title="sxtp_SummaryofSignificantAccountingPoliciesScheduleofExchangeRatesalongwithHistoricalRatesDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_IntercompanyForeignCurrencyBalanceLineItems" xlink:to="sxtp_SummaryofSignificantAccountingPoliciesScheduleofExchangeRatesalongwithHistoricalRatesDetailsTable" xlink:title="Definition : us-gaap_IntercompanyForeignCurrencyBalanceLineItems to sxtp_SummaryofSignificantAccountingPoliciesScheduleofExchangeRatesalongwithHistoricalRatesDetailsTable" xlink:type="arc" order="2" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CurrencyAxis" xlink:label="srt_CurrencyAxis" xlink:title="srt_CurrencyAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_SummaryofSignificantAccountingPoliciesScheduleofExchangeRatesalongwithHistoricalRatesDetailsTable" xlink:to="srt_CurrencyAxis" xlink:title="Definition : sxtp_SummaryofSignificantAccountingPoliciesScheduleofExchangeRatesalongwithHistoricalRatesDetailsTable to srt_CurrencyAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/currency/2025/currency-2025.xsd#currency_AllCurrenciesDomain" xlink:label="currency_AllCurrenciesDomain" xlink:title="currency_AllCurrenciesDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_CurrencyAxis" xlink:to="currency_AllCurrenciesDomain" xlink:title="Definition : srt_CurrencyAxis to currency_AllCurrenciesDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/currency/2025/currency-2025.xsd#currency_USD" xlink:label="currency_USD" xlink:title="currency_USD" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="currency_AllCurrenciesDomain" xlink:to="currency_USD" xlink:title="Definition : currency_AllCurrenciesDomain to currency_USD" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/currency/2025/currency-2025.xsd#currency_AllCurrenciesDomain" xlink:label="currency_AllCurrenciesDomain_1" xlink:title="currency_AllCurrenciesDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_CurrencyAxis" xlink:to="currency_AllCurrenciesDomain_1" xlink:title="Definition : srt_CurrencyAxis to currency_AllCurrenciesDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDBALANCESHEETSUnauditedParentheticals">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:title="us-gaap_StatementLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:title="us-gaap_PreferredStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_PreferredStockParOrStatedValuePerShare" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" xlink:title="us-gaap_PreferredStockSharesAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesAuthorized" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_PreferredStockSharesAuthorized" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" xlink:title="us-gaap_PreferredStockSharesIssued" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesIssued" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_PreferredStockSharesIssued" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" xlink:title="us-gaap_PreferredStockSharesOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_PreferredStockSharesOutstanding" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" xlink:title="us-gaap_CommonStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" xlink:title="us-gaap_CommonStockSharesAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_CommonStockSharesAuthorized" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" xlink:title="us-gaap_CommonStockSharesIssued" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesIssued" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_CommonStockSharesIssued" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" xlink:title="us-gaap_CommonStockSharesOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_CommonStockSharesOutstanding" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:title="us-gaap_StatementTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_StatementTable" xlink:type="arc" order="8" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:title="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:title="Definition : us-gaap_StatementTable to us-gaap_StatementClassOfStockAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="us-gaap_SeriesAPreferredStockMember" xlink:title="us-gaap_SeriesAPreferredStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_SeriesAPreferredStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain_1" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DocumentAndEntityInformation">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationLineItems" xlink:label="dei_DocumentInformationLineItems" xlink:title="dei_DocumentInformationLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName" xlink:label="dei_EntityRegistrantName" xlink:title="dei_EntityRegistrantName" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityRegistrantName" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityRegistrantName" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType" xlink:label="dei_DocumentType" xlink:title="dei_DocumentType" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentType" xlink:title="Definition : dei_DocumentInformationLineItems to dei_DocumentType" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag" xlink:label="dei_AmendmentFlag" xlink:title="dei_AmendmentFlag" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_AmendmentFlag" xlink:title="Definition : dei_DocumentInformationLineItems to dei_AmendmentFlag" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey" xlink:label="dei_EntityCentralIndexKey" xlink:title="dei_EntityCentralIndexKey" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCentralIndexKey" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityCentralIndexKey" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate" xlink:label="dei_DocumentPeriodEndDate" xlink:title="dei_DocumentPeriodEndDate" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentPeriodEndDate" xlink:title="Definition : dei_DocumentInformationLineItems to dei_DocumentPeriodEndDate" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CurrentFiscalYearEndDate" xlink:label="dei_CurrentFiscalYearEndDate" xlink:title="dei_CurrentFiscalYearEndDate" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_CurrentFiscalYearEndDate" xlink:title="Definition : dei_DocumentInformationLineItems to dei_CurrentFiscalYearEndDate" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalYearFocus" xlink:label="dei_DocumentFiscalYearFocus" xlink:title="dei_DocumentFiscalYearFocus" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentFiscalYearFocus" xlink:title="Definition : dei_DocumentInformationLineItems to dei_DocumentFiscalYearFocus" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="dei_DocumentFiscalPeriodFocus" xlink:title="dei_DocumentFiscalPeriodFocus" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentFiscalPeriodFocus" xlink:title="Definition : dei_DocumentInformationLineItems to dei_DocumentFiscalPeriodFocus" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentQuarterlyReport" xlink:label="dei_DocumentQuarterlyReport" xlink:title="dei_DocumentQuarterlyReport" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentQuarterlyReport" xlink:title="Definition : dei_DocumentInformationLineItems to dei_DocumentQuarterlyReport" xlink:type="arc" use="optional" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentTransitionReport" xlink:label="dei_DocumentTransitionReport" xlink:title="dei_DocumentTransitionReport" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_DocumentTransitionReport" xlink:title="Definition : dei_DocumentInformationLineItems to dei_DocumentTransitionReport" xlink:type="arc" use="optional" order="9" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber" xlink:label="dei_EntityFileNumber" xlink:title="dei_EntityFileNumber" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityFileNumber" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityFileNumber" xlink:type="arc" use="optional" order="10" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode" xlink:label="dei_EntityIncorporationStateCountryCode" xlink:title="dei_EntityIncorporationStateCountryCode" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityIncorporationStateCountryCode" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityIncorporationStateCountryCode" xlink:type="arc" use="optional" order="11" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber" xlink:label="dei_EntityTaxIdentificationNumber" xlink:title="dei_EntityTaxIdentificationNumber" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityTaxIdentificationNumber" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityTaxIdentificationNumber" xlink:type="arc" use="optional" order="12" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1" xlink:label="dei_EntityAddressAddressLine1" xlink:title="dei_EntityAddressAddressLine1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressAddressLine1" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityAddressAddressLine1" xlink:type="arc" use="optional" order="13" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine2" xlink:label="dei_EntityAddressAddressLine2" xlink:title="dei_EntityAddressAddressLine2" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressAddressLine2" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityAddressAddressLine2" xlink:type="arc" use="optional" order="14" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown" xlink:label="dei_EntityAddressCityOrTown" xlink:title="dei_EntityAddressCityOrTown" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressCityOrTown" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityAddressCityOrTown" xlink:type="arc" use="optional" order="15" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince" xlink:label="dei_EntityAddressStateOrProvince" xlink:title="dei_EntityAddressStateOrProvince" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressStateOrProvince" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityAddressStateOrProvince" xlink:type="arc" use="optional" order="16" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode" xlink:label="dei_EntityAddressPostalZipCode" xlink:title="dei_EntityAddressPostalZipCode" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityAddressPostalZipCode" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityAddressPostalZipCode" xlink:type="arc" use="optional" order="17" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode" xlink:label="dei_CityAreaCode" xlink:title="dei_CityAreaCode" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_CityAreaCode" xlink:title="Definition : dei_DocumentInformationLineItems to dei_CityAreaCode" xlink:type="arc" use="optional" order="18" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber" xlink:label="dei_LocalPhoneNumber" xlink:title="dei_LocalPhoneNumber" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_LocalPhoneNumber" xlink:title="Definition : dei_DocumentInformationLineItems to dei_LocalPhoneNumber" xlink:type="arc" use="optional" order="19" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle" xlink:label="dei_Security12bTitle" xlink:title="dei_Security12bTitle" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_Security12bTitle" xlink:title="Definition : dei_DocumentInformationLineItems to dei_Security12bTitle" xlink:type="arc" use="optional" order="20" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol" xlink:label="dei_TradingSymbol" xlink:title="dei_TradingSymbol" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_TradingSymbol" xlink:title="Definition : dei_DocumentInformationLineItems to dei_TradingSymbol" xlink:type="arc" use="optional" order="21" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName" xlink:label="dei_SecurityExchangeName" xlink:title="dei_SecurityExchangeName" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_SecurityExchangeName" xlink:title="Definition : dei_DocumentInformationLineItems to dei_SecurityExchangeName" xlink:type="arc" use="optional" order="22" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCurrentReportingStatus" xlink:label="dei_EntityCurrentReportingStatus" xlink:title="dei_EntityCurrentReportingStatus" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCurrentReportingStatus" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityCurrentReportingStatus" xlink:type="arc" use="optional" order="23" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityInteractiveDataCurrent" xlink:label="dei_EntityInteractiveDataCurrent" xlink:title="dei_EntityInteractiveDataCurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityInteractiveDataCurrent" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityInteractiveDataCurrent" xlink:type="arc" use="optional" order="24" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFilerCategory" xlink:label="dei_EntityFilerCategory" xlink:title="dei_EntityFilerCategory" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityFilerCategory" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityFilerCategory" xlink:type="arc" use="optional" order="25" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitySmallBusiness" xlink:label="dei_EntitySmallBusiness" xlink:title="dei_EntitySmallBusiness" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntitySmallBusiness" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntitySmallBusiness" xlink:type="arc" use="optional" order="26" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany" xlink:label="dei_EntityEmergingGrowthCompany" xlink:title="dei_EntityEmergingGrowthCompany" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityEmergingGrowthCompany" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityEmergingGrowthCompany" xlink:type="arc" use="optional" order="27" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityExTransitionPeriod" xlink:label="dei_EntityExTransitionPeriod" xlink:title="dei_EntityExTransitionPeriod" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityExTransitionPeriod" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityExTransitionPeriod" xlink:type="arc" use="optional" order="28" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityShellCompany" xlink:label="dei_EntityShellCompany" xlink:title="dei_EntityShellCompany" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityShellCompany" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityShellCompany" xlink:type="arc" use="optional" order="29" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="dei_EntityCommonStockSharesOutstanding" xlink:title="dei_EntityCommonStockSharesOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntityCommonStockSharesOutstanding" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntityCommonStockSharesOutstanding" xlink:type="arc" use="optional" order="30" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntitiesTable" xlink:label="dei_EntitiesTable" xlink:title="dei_EntitiesTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="dei_DocumentInformationLineItems" xlink:to="dei_EntitiesTable" xlink:title="Definition : dei_DocumentInformationLineItems to dei_EntitiesTable" xlink:type="arc" order="31" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:title="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="dei_EntitiesTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:title="Definition : dei_EntitiesTable to us-gaap_StatementClassOfStockAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:title="us-gaap_CommonStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_CommonStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" xlink:title="us-gaap_WarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_WarrantMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_WarrantMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain_1" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESScheduleofLiabilitiesMeasuredatFairValueDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:title="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitiesCurrent" xlink:label="us-gaap_DerivativeLiabilitiesCurrent" xlink:title="us-gaap_DerivativeLiabilitiesCurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="us-gaap_DerivativeLiabilitiesCurrent" xlink:title="Definition : us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to us-gaap_DerivativeLiabilitiesCurrent" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable" xlink:label="sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable" xlink:title="sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable" xlink:title="Definition : us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems to sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable" xlink:type="arc" use="optional" order="1" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="us-gaap_FinancialInstrumentAxis" xlink:title="us-gaap_FinancialInstrumentAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable" xlink:to="us-gaap_FinancialInstrumentAxis" xlink:title="Definition : sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable to us-gaap_FinancialInstrumentAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:title="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:title="Definition : us-gaap_FinancialInstrumentAxis to us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeMember" xlink:label="us-gaap_DerivativeMember" xlink:title="us-gaap_DerivativeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_DerivativeMember" xlink:title="Definition : us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain to us-gaap_DerivativeMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" xlink:title="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FinancialInstrumentAxis" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_1" xlink:title="Definition : us-gaap_FinancialInstrumentAxis to us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:title="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:title="Definition : sxtp_SummaryofSignificantAccountingPoliciesScheduleofLiabilitiesMeasuredatFairValueDetailsTable to us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:title="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:title="Definition : us-gaap_FairValueByFairValueHierarchyLevelAxis to us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member" xlink:title="us-gaap_FairValueInputsLevel1Member" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel1Member" xlink:title="Definition : us-gaap_FairValueMeasurementsFairValueHierarchyDomain to us-gaap_FairValueInputsLevel1Member" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member" xlink:title="us-gaap_FairValueInputsLevel2Member" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" xlink:title="Definition : us-gaap_FairValueMeasurementsFairValueHierarchyDomain to us-gaap_FairValueInputsLevel2Member" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member" xlink:title="us-gaap_FairValueInputsLevel3Member" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" xlink:title="Definition : us-gaap_FairValueMeasurementsFairValueHierarchyDomain to us-gaap_FairValueInputsLevel3Member" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" xlink:title="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" xlink:title="Definition : us-gaap_FairValueByFairValueHierarchyLevelAxis to us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INTANGIBLEASSETSScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:title="us-gaap_FiniteLivedIntangibleAssetsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear" xlink:title="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseRemainderOfFiscalYear" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:title="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseNextTwelveMonths" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:title="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearTwo" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:title="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearThree" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:title="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseYearFour" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:title="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to us-gaap_FiniteLivedIntangibleAssetsAmortizationExpenseAfterYearFive" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNet" xlink:title="us-gaap_FiniteLivedIntangibleAssetsNet" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsNet" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to us-gaap_FiniteLivedIntangibleAssetsNet" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_IntangibleAssetsScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetailsTable" xlink:label="sxtp_IntangibleAssetsScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetailsTable" xlink:title="sxtp_IntangibleAssetsScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:to="sxtp_IntangibleAssetsScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetailsTable" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsLineItems to sxtp_IntangibleAssetsScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetailsTable" xlink:type="arc" use="optional" order="7" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:title="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_IntangibleAssetsScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetailsTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:title="Definition : sxtp_IntangibleAssetsScheduleofEstimatedFutureAmortizationExpenseforOurPatentsandWebsiteDevelopmentCostsDetailsTable to us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:title="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis to us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PatentsMember" xlink:label="us-gaap_PatentsMember" xlink:title="us-gaap_PatentsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_PatentsMember" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain to us-gaap_PatentsMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" xlink:label="us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" xlink:title="us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain to us-gaap_SoftwareAndSoftwareDevelopmentCostsMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" xlink:title="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_1" xlink:title="Definition : us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis to us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DEBTDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="us-gaap_DebtInstrumentLineItems" xlink:title="us-gaap_DebtInstrumentLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:label="us-gaap_DebtInstrumentFaceAmount" xlink:title="us-gaap_DebtInstrumentFaceAmount" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentFaceAmount" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to us-gaap_DebtInstrumentFaceAmount" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:label="us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:title="us-gaap_DebtInstrumentInterestRateEffectivePercentage" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:label="us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:title="us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to us-gaap_DebtInstrumentPeriodicPaymentTermsBalloonPaymentToBePaid" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:label="us-gaap_DebtInstrumentMaturityDate" xlink:title="us-gaap_DebtInstrumentMaturityDate" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentMaturityDate" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to us-gaap_DebtInstrumentMaturityDate" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCostsAndDiscounts" xlink:label="us-gaap_AmortizationOfFinancingCostsAndDiscounts" xlink:title="us-gaap_AmortizationOfFinancingCostsAndDiscounts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_AmortizationOfFinancingCostsAndDiscounts" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to us-gaap_AmortizationOfFinancingCostsAndDiscounts" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtCurrent" xlink:label="us-gaap_LongTermDebtCurrent" xlink:title="us-gaap_LongTermDebtCurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebtCurrent" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to us-gaap_LongTermDebtCurrent" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesNoncurrent" xlink:label="us-gaap_LiabilitiesNoncurrent" xlink:title="us-gaap_LiabilitiesNoncurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LiabilitiesNoncurrent" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to us-gaap_LiabilitiesNoncurrent" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_DebtDetailsTable" xlink:label="sxtp_DebtDetailsTable" xlink:title="sxtp_DebtDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="sxtp_DebtDetailsTable" xlink:title="Definition : us-gaap_DebtInstrumentLineItems to sxtp_DebtDetailsTable" xlink:type="arc" use="optional" order="7" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" xlink:title="us-gaap_DebtInstrumentAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_DebtDetailsTable" xlink:to="us-gaap_DebtInstrumentAxis" xlink:title="Definition : sxtp_DebtDetailsTable to us-gaap_DebtInstrumentAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" xlink:title="us-gaap_DebtInstrumentNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" xlink:title="Definition : us-gaap_DebtInstrumentAxis to us-gaap_DebtInstrumentNameDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SmallBusinessAdministrationMember" xlink:label="sxtp_SmallBusinessAdministrationMember" xlink:title="sxtp_SmallBusinessAdministrationMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="sxtp_SmallBusinessAdministrationMember" xlink:title="Definition : us-gaap_DebtInstrumentNameDomain to sxtp_SmallBusinessAdministrationMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_BalloonPaymentMember" xlink:label="sxtp_BalloonPaymentMember" xlink:title="sxtp_BalloonPaymentMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="sxtp_BalloonPaymentMember" xlink:title="Definition : us-gaap_DebtInstrumentNameDomain to sxtp_BalloonPaymentMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SBAMember" xlink:label="sxtp_SBAMember" xlink:title="sxtp_SBAMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="sxtp_SBAMember" xlink:title="Definition : us-gaap_DebtInstrumentNameDomain to sxtp_SBAMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain_1" xlink:title="us-gaap_DebtInstrumentNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain_1" xlink:title="Definition : us-gaap_DebtInstrumentAxis to us-gaap_DebtInstrumentNameDomain" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONScheduleofShareBasedCompensationExpensesDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:title="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" xlink:title="us-gaap_AllocatedShareBasedCompensationExpense" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:title="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:type="arc" use="optional" order="1" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis" xlink:title="us-gaap_IncomeStatementLocationAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:to="us-gaap_IncomeStatementLocationAxis" xlink:title="Definition : us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable to us-gaap_IncomeStatementLocationAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain" xlink:title="us-gaap_IncomeStatementLocationDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" xlink:title="Definition : us-gaap_IncomeStatementLocationAxis to us-gaap_IncomeStatementLocationDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember" xlink:title="us-gaap_ResearchAndDevelopmentExpenseMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_ResearchAndDevelopmentExpenseMember" xlink:title="Definition : us-gaap_IncomeStatementLocationDomain to us-gaap_ResearchAndDevelopmentExpenseMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember" xlink:title="us-gaap_GeneralAndAdministrativeExpenseMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_GeneralAndAdministrativeExpenseMember" xlink:title="Definition : us-gaap_IncomeStatementLocationDomain to us-gaap_GeneralAndAdministrativeExpenseMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain_1" xlink:title="us-gaap_IncomeStatementLocationDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain_1" xlink:title="Definition : us-gaap_IncomeStatementLocationAxis to us-gaap_IncomeStatementLocationDomain" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DERIVATIVELIABILITIESScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:title="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeLiabilitiesCurrent" xlink:label="us-gaap_DerivativeLiabilitiesCurrent" xlink:title="us-gaap_DerivativeLiabilitiesCurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:to="us-gaap_DerivativeLiabilitiesCurrent" xlink:title="Definition : us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems to us-gaap_DerivativeLiabilitiesCurrent" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisMarkToMarketAdjustment" xlink:label="sxtp_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisMarkToMarketAdjustment" xlink:title="sxtp_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisMarkToMarketAdjustment" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:to="sxtp_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisMarkToMarketAdjustment" xlink:title="Definition : us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems to sxtp_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisMarkToMarketAdjustment" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" xlink:label="sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" xlink:title="sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems" xlink:to="sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" xlink:title="Definition : us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems to sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" xlink:type="arc" order="3" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:title="us-gaap_FairValueByMeasurementFrequencyAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" xlink:to="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:title="Definition : sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable to us-gaap_FairValueByMeasurementFrequencyAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain" xlink:title="us-gaap_FairValueMeasurementFrequencyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain" xlink:title="Definition : us-gaap_FairValueByMeasurementFrequencyAxis to us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="us-gaap_FairValueMeasurementsRecurringMember" xlink:title="us-gaap_FairValueMeasurementsRecurringMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementFrequencyDomain" xlink:to="us-gaap_FairValueMeasurementsRecurringMember" xlink:title="Definition : us-gaap_FairValueMeasurementFrequencyDomain to us-gaap_FairValueMeasurementsRecurringMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="us-gaap_FairValueMeasurementFrequencyDomain_1" xlink:title="us-gaap_FairValueMeasurementFrequencyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByMeasurementFrequencyAxis" xlink:to="us-gaap_FairValueMeasurementFrequencyDomain_1" xlink:title="Definition : us-gaap_FairValueByMeasurementFrequencyAxis to us-gaap_FairValueMeasurementFrequencyDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:title="us-gaap_DerivativeInstrumentRiskAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:title="Definition : sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable to us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:title="us-gaap_DerivativeContractTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:title="Definition : us-gaap_DerivativeInstrumentRiskAxis to us-gaap_DerivativeContractTypeDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ContingentMilestonePaymentMember" xlink:label="sxtp_ContingentMilestonePaymentMember" xlink:title="sxtp_ContingentMilestonePaymentMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="sxtp_ContingentMilestonePaymentMember" xlink:title="Definition : us-gaap_DerivativeContractTypeDomain to sxtp_ContingentMilestonePaymentMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_BridgeSharesMember" xlink:label="sxtp_BridgeSharesMember" xlink:title="sxtp_BridgeSharesMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="sxtp_BridgeSharesMember" xlink:title="Definition : us-gaap_DerivativeContractTypeDomain to sxtp_BridgeSharesMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" xlink:title="us-gaap_WarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_WarrantMember" xlink:title="Definition : us-gaap_DerivativeContractTypeDomain to us-gaap_WarrantMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain_1" xlink:title="us-gaap_DerivativeContractTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain_1" xlink:title="Definition : us-gaap_DerivativeInstrumentRiskAxis to us-gaap_DerivativeContractTypeDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:title="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:title="Definition : sxtp_DerivativeLiabilitiesScheduleofDerivativeLiabilitiesMeasuredatFairValueonaRecurringBasisDetailsTable to us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:title="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:title="Definition : us-gaap_FairValueByFairValueHierarchyLevelAxis to us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member" xlink:title="us-gaap_FairValueInputsLevel3Member" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel3Member" xlink:title="Definition : us-gaap_FairValueMeasurementsFairValueHierarchyDomain to us-gaap_FairValueInputsLevel3Member" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" xlink:title="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_1" xlink:title="Definition : us-gaap_FairValueByFairValueHierarchyLevelAxis to us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/STOCKHOLDERSEQUITYScheduleofEquityClassifiedWarrantsDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightLineItems" xlink:label="us-gaap_ClassOfWarrantOrRightLineItems" xlink:title="us-gaap_ClassOfWarrantOrRightLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsGranted" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsGranted" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsGranted" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsGranted" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsGranted" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExercised" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExercised" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExercised" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExercised" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExercised" xlink:type="arc" use="optional" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:type="arc" use="optional" order="9" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:type="arc" use="optional" order="10" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsForfeited" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsForfeited" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsForfeited" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsForfeited" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsForfeited" xlink:type="arc" use="optional" order="11" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExpired" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExpired" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExpired" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExpired" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExpired" xlink:type="arc" use="optional" order="12" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExpired" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExpired" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExpired" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExpired" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExpired" xlink:type="arc" use="optional" order="13" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExpired" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExpired" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExpired" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExpired" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageRemainingContractualLifeYearsExpired" xlink:type="arc" use="optional" order="14" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms1" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms1" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms1" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageRemainingContractualTerms1" xlink:type="arc" use="optional" order="17" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:type="arc" use="optional" order="18" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableWeightedAverageGrantDateFairValue" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableWeightedAverageGrantDateFairValue" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableWeightedAverageGrantDateFairValue" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableWeightedAverageGrantDateFairValue" xlink:type="arc" use="optional" order="19" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableOutstandingWeightedAverageRemainingContractualTerms" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableOutstandingWeightedAverageRemainingContractualTerms" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableOutstandingWeightedAverageRemainingContractualTerms" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableOutstandingWeightedAverageRemainingContractualTerms" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisableOutstandingWeightedAverageRemainingContractualTerms" xlink:type="arc" use="optional" order="20" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightTable" xlink:label="us-gaap_ClassOfWarrantOrRightTable" xlink:title="us-gaap_ClassOfWarrantOrRightTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ClassOfWarrantOrRightLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightTable" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightLineItems to us-gaap_ClassOfWarrantOrRightTable" xlink:type="arc" order="21" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" xlink:title="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ClassOfWarrantOrRightTable" xlink:to="us-gaap_ClassOfWarrantOrRightAxis" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightTable to us-gaap_ClassOfWarrantOrRightAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" xlink:title="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightAxis to us-gaap_ClassOfWarrantOrRightDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" xlink:title="us-gaap_WarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="us-gaap_WarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to us-gaap_WarrantMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain_1" xlink:title="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain_1" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightAxis to us-gaap_ClassOfWarrantOrRightDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DERIVATIVELIABILITIESDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:label="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:title="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockSharesConverted1" xlink:label="us-gaap_ConversionOfStockSharesConverted1" xlink:title="us-gaap_ConversionOfStockSharesConverted1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="us-gaap_ConversionOfStockSharesConverted1" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to us-gaap_ConversionOfStockSharesConverted1" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction" xlink:label="us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction" xlink:title="us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ExcessStockSharesAuthorized" xlink:label="us-gaap_ExcessStockSharesAuthorized" xlink:title="us-gaap_ExcessStockSharesAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="us-gaap_ExcessStockSharesAuthorized" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to us-gaap_ExcessStockSharesAuthorized" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_MilestonePayment" xlink:label="sxtp_MilestonePayment" xlink:title="sxtp_MilestonePayment" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="sxtp_MilestonePayment" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to sxtp_MilestonePayment" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockRedemptionTerms" xlink:label="us-gaap_PreferredStockRedemptionTerms" xlink:title="us-gaap_PreferredStockRedemptionTerms" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="us-gaap_PreferredStockRedemptionTerms" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to us-gaap_PreferredStockRedemptionTerms" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_RoyaltyPercentage" xlink:label="sxtp_RoyaltyPercentage" xlink:title="sxtp_RoyaltyPercentage" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="sxtp_RoyaltyPercentage" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to sxtp_RoyaltyPercentage" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DerivativeGainLossOnDerivativeNet" xlink:label="us-gaap_DerivativeGainLossOnDerivativeNet" xlink:title="us-gaap_DerivativeGainLossOnDerivativeNet" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="us-gaap_DerivativeGainLossOnDerivativeNet" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to us-gaap_DerivativeGainLossOnDerivativeNet" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ValuationOfContingentMilestonePaymentDiscountRate" xlink:label="sxtp_ValuationOfContingentMilestonePaymentDiscountRate" xlink:title="sxtp_ValuationOfContingentMilestonePaymentDiscountRate" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="sxtp_ValuationOfContingentMilestonePaymentDiscountRate" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to sxtp_ValuationOfContingentMilestonePaymentDiscountRate" xlink:type="arc" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_DerivativeLiabilitiesDetailsTable" xlink:label="sxtp_DerivativeLiabilitiesDetailsTable" xlink:title="sxtp_DerivativeLiabilitiesDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems" xlink:to="sxtp_DerivativeLiabilitiesDetailsTable" xlink:title="Definition : us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems to sxtp_DerivativeLiabilitiesDetailsTable" xlink:type="arc" order="8" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:title="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_DerivativeLiabilitiesDetailsTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:title="Definition : sxtp_DerivativeLiabilitiesDetailsTable to us-gaap_StatementClassOfStockAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:label="sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:title="sxtp_SeriesANonVotingConvertiblePreferredStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain_1" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/NATUREOFOPERATIONSDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:title="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:title="us-gaap_StockIssuedDuringPeriodSharesNewIssues" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesIssuedPricePerShare" xlink:label="us-gaap_SharesIssuedPricePerShare" xlink:title="us-gaap_SharesIssuedPricePerShare" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_SharesIssuedPricePerShare" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_SharesIssuedPricePerShare" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceInitialPublicOffering" xlink:label="us-gaap_ProceedsFromIssuanceInitialPublicOffering" xlink:title="us-gaap_ProceedsFromIssuanceInitialPublicOffering" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_ProceedsFromIssuanceInitialPublicOffering" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_ProceedsFromIssuanceInitialPublicOffering" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingTerm" xlink:label="us-gaap_WarrantsAndRightsOutstandingTerm" xlink:title="us-gaap_WarrantsAndRightsOutstandingTerm" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstandingTerm" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_WarrantsAndRightsOutstandingTerm" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:title="us-gaap_CashAndCashEquivalentsAtCarryingValue" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:title="us-gaap_NetCashProvidedByUsedInOperatingActivities" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SBAEIDLincludingAccruedInterestCurrent" xlink:label="sxtp_SBAEIDLincludingAccruedInterestCurrent" xlink:title="sxtp_SBAEIDLincludingAccruedInterestCurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="sxtp_SBAEIDLincludingAccruedInterestCurrent" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to sxtp_SBAEIDLincludingAccruedInterestCurrent" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" xlink:label="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" xlink:title="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:label="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:title="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to us-gaap_LesseeOperatingLeaseLiabilityPaymentsDueNextTwelveMonths" xlink:type="arc" use="optional" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NatureofOperationsDetailsTable" xlink:label="sxtp_NatureofOperationsDetailsTable" xlink:title="sxtp_NatureofOperationsDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="sxtp_NatureofOperationsDetailsTable" xlink:title="Definition : us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems to sxtp_NatureofOperationsDetailsTable" xlink:type="arc" order="9" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" xlink:title="us-gaap_SubsidiarySaleOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_NatureofOperationsDetailsTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" xlink:title="Definition : sxtp_NatureofOperationsDetailsTable to us-gaap_SubsidiarySaleOfStockAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:title="Definition : us-gaap_SubsidiarySaleOfStockAxis to us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IPOMember" xlink:label="us-gaap_IPOMember" xlink:title="us-gaap_IPOMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_IPOMember" xlink:title="Definition : us-gaap_SaleOfStockNameOfTransactionDomain to us-gaap_IPOMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_TradeableWarrantsMember" xlink:label="sxtp_TradeableWarrantsMember" xlink:title="sxtp_TradeableWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="sxtp_TradeableWarrantsMember" xlink:title="Definition : us-gaap_SaleOfStockNameOfTransactionDomain to sxtp_TradeableWarrantsMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NontradeableWarrantsMember" xlink:label="sxtp_NontradeableWarrantsMember" xlink:title="sxtp_NontradeableWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="sxtp_NontradeableWarrantsMember" xlink:title="Definition : us-gaap_SaleOfStockNameOfTransactionDomain to sxtp_NontradeableWarrantsMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_TradeableAndNonTradeableWarrantsMember" xlink:label="sxtp_TradeableAndNonTradeableWarrantsMember" xlink:title="sxtp_TradeableAndNonTradeableWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="sxtp_TradeableAndNonTradeableWarrantsMember" xlink:title="Definition : us-gaap_SaleOfStockNameOfTransactionDomain to sxtp_TradeableAndNonTradeableWarrantsMember" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="Definition : us-gaap_SubsidiarySaleOfStockAxis to us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/COMMITMENTSANDCONTINGENCIESDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsLineItems" xlink:label="us-gaap_OtherCommitmentsLineItems" xlink:title="us-gaap_OtherCommitmentsLineItems" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NumberOfDirectors" xlink:label="sxtp_NumberOfDirectors" xlink:title="sxtp_NumberOfDirectors" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="sxtp_NumberOfDirectors" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to sxtp_NumberOfDirectors" xlink:type="arc" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyPendingClaimsNumber" xlink:label="us-gaap_LossContingencyPendingClaimsNumber" xlink:title="us-gaap_LossContingencyPendingClaimsNumber" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_LossContingencyPendingClaimsNumber" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_LossContingencyPendingClaimsNumber" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseCost" xlink:label="us-gaap_OperatingLeaseCost" xlink:title="us-gaap_OperatingLeaseCost" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_OperatingLeaseCost" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_OperatingLeaseCost" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CompensationAndBenefitsTrust" xlink:label="us-gaap_CompensationAndBenefitsTrust" xlink:title="us-gaap_CompensationAndBenefitsTrust" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_CompensationAndBenefitsTrust" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_CompensationAndBenefitsTrust" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NumberOfNonAuditCommitteeChairs" xlink:label="sxtp_NumberOfNonAuditCommitteeChairs" xlink:title="sxtp_NumberOfNonAuditCommitteeChairs" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="sxtp_NumberOfNonAuditCommitteeChairs" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to sxtp_NumberOfNonAuditCommitteeChairs" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm" xlink:label="us-gaap_LesseeOperatingLeaseRenewalTerm" xlink:title="us-gaap_LesseeOperatingLeaseRenewalTerm" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_LesseeOperatingLeaseRenewalTerm" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_LesseeOperatingLeaseRenewalTerm" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:title="us-gaap_BusinessCombinationContingentConsiderationLiability" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_LicensedTechnologyTerm" xlink:label="sxtp_LicensedTechnologyTerm" xlink:title="sxtp_LicensedTechnologyTerm" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="sxtp_LicensedTechnologyTerm" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to sxtp_LicensedTechnologyTerm" xlink:type="arc" use="optional" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_AnnualRoyaltyPaymentPercentage" xlink:label="sxtp_AnnualRoyaltyPaymentPercentage" xlink:title="sxtp_AnnualRoyaltyPaymentPercentage" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="sxtp_AnnualRoyaltyPaymentPercentage" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to sxtp_AnnualRoyaltyPaymentPercentage" xlink:type="arc" use="optional" order="9" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RoyaltyExpense" xlink:label="us-gaap_RoyaltyExpense" xlink:title="us-gaap_RoyaltyExpense" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_RoyaltyExpense" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_RoyaltyExpense" xlink:type="arc" use="optional" order="10" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_PercentageOfRoyaltyPaymentsOfNetSales" xlink:label="sxtp_PercentageOfRoyaltyPaymentsOfNetSales" xlink:title="sxtp_PercentageOfRoyaltyPaymentsOfNetSales" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="sxtp_PercentageOfRoyaltyPaymentsOfNetSales" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to sxtp_PercentageOfRoyaltyPaymentsOfNetSales" xlink:type="arc" use="optional" order="11" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_PercentageOfTreatmentOfBabesiosis" xlink:label="sxtp_PercentageOfTreatmentOfBabesiosis" xlink:title="sxtp_PercentageOfTreatmentOfBabesiosis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="sxtp_PercentageOfTreatmentOfBabesiosis" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to sxtp_PercentageOfTreatmentOfBabesiosis" xlink:type="arc" use="optional" order="12" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationPercentage" xlink:label="us-gaap_RevenueRemainingPerformanceObligationPercentage" xlink:title="us-gaap_RevenueRemainingPerformanceObligationPercentage" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="us-gaap_RevenueRemainingPerformanceObligationPercentage" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to us-gaap_RevenueRemainingPerformanceObligationPercentage" xlink:type="arc" use="optional" order="13" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CommitmentsandContingenciesDetailsTable" xlink:label="sxtp_CommitmentsandContingenciesDetailsTable" xlink:title="sxtp_CommitmentsandContingenciesDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_OtherCommitmentsLineItems" xlink:to="sxtp_CommitmentsandContingenciesDetailsTable" xlink:title="Definition : us-gaap_OtherCommitmentsLineItems to sxtp_CommitmentsandContingenciesDetailsTable" xlink:type="arc" order="14" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" xlink:title="us-gaap_SubsidiarySaleOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_CommitmentsandContingenciesDetailsTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" xlink:title="Definition : sxtp_CommitmentsandContingenciesDetailsTable to us-gaap_SubsidiarySaleOfStockAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:title="Definition : us-gaap_SubsidiarySaleOfStockAxis to us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IPOMember" xlink:label="us-gaap_IPOMember" xlink:title="us-gaap_IPOMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_IPOMember" xlink:title="Definition : us-gaap_SaleOfStockNameOfTransactionDomain to us-gaap_IPOMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="Definition : us-gaap_SubsidiarySaleOfStockAxis to us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:title="srt_RangeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_CommitmentsandContingenciesDetailsTable" xlink:to="srt_RangeAxis" xlink:title="Definition : sxtp_CommitmentsandContingenciesDetailsTable to srt_RangeAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:title="srt_RangeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:title="Definition : srt_RangeAxis to srt_RangeMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember" xlink:label="srt_MinimumMember" xlink:title="srt_MinimumMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" xlink:title="Definition : srt_RangeMember to srt_MinimumMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:title="srt_MaximumMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:title="Definition : srt_RangeMember to srt_MaximumMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember_1" xlink:title="srt_RangeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember_1" xlink:title="Definition : srt_RangeAxis to srt_RangeMember" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis" xlink:label="srt_TitleOfIndividualAxis" xlink:title="srt_TitleOfIndividualAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_CommitmentsandContingenciesDetailsTable" xlink:to="srt_TitleOfIndividualAxis" xlink:title="Definition : sxtp_CommitmentsandContingenciesDetailsTable to srt_TitleOfIndividualAxis" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:title="srt_TitleOfIndividualWithRelationshipToEntityDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:title="Definition : srt_TitleOfIndividualAxis to srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_BoardOfDirectorsChairmanMember" xlink:label="srt_BoardOfDirectorsChairmanMember" xlink:title="srt_BoardOfDirectorsChairmanMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="srt_BoardOfDirectorsChairmanMember" xlink:title="Definition : srt_TitleOfIndividualWithRelationshipToEntityDomain to srt_BoardOfDirectorsChairmanMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_DirectorMember" xlink:label="srt_DirectorMember" xlink:title="srt_DirectorMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="srt_DirectorMember" xlink:title="Definition : srt_TitleOfIndividualWithRelationshipToEntityDomain to srt_DirectorMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain_1" xlink:title="srt_TitleOfIndividualWithRelationshipToEntityDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain_1" xlink:title="Definition : srt_TitleOfIndividualAxis to srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="us-gaap_RelatedPartyTransactionAxis" xlink:title="us-gaap_RelatedPartyTransactionAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_CommitmentsandContingenciesDetailsTable" xlink:to="us-gaap_RelatedPartyTransactionAxis" xlink:title="Definition : sxtp_CommitmentsandContingenciesDetailsTable to us-gaap_RelatedPartyTransactionAxis" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain" xlink:title="us-gaap_RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionAxis" xlink:to="us-gaap_RelatedPartyTransactionDomain" xlink:title="Definition : us-gaap_RelatedPartyTransactionAxis to us-gaap_RelatedPartyTransactionDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CherylXuMember" xlink:label="sxtp_CherylXuMember" xlink:title="sxtp_CherylXuMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_CherylXuMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_CherylXuMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_PaulFieldMember" xlink:label="sxtp_PaulFieldMember" xlink:title="sxtp_PaulFieldMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_PaulFieldMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_PaulFieldMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CharlesAllenMember" xlink:label="sxtp_CharlesAllenMember" xlink:title="sxtp_CharlesAllenMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_CharlesAllenMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_CharlesAllenMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StephenTooveyMember" xlink:label="sxtp_StephenTooveyMember" xlink:title="sxtp_StephenTooveyMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_StephenTooveyMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_StephenTooveyMember" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_USGovernmentSalesMember" xlink:label="sxtp_USGovernmentSalesMember" xlink:title="sxtp_USGovernmentSalesMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_USGovernmentSalesMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_USGovernmentSalesMember" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_USArmyAgreementMember" xlink:label="sxtp_USArmyAgreementMember" xlink:title="sxtp_USArmyAgreementMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_USArmyAgreementMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_USArmyAgreementMember" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_TuftsMedicalCenterAgreementMember" xlink:label="sxtp_TuftsMedicalCenterAgreementMember" xlink:title="sxtp_TuftsMedicalCenterAgreementMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_TuftsMedicalCenterAgreementMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_TuftsMedicalCenterAgreementMember" xlink:type="arc" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain_1" xlink:title="us-gaap_RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RelatedPartyTransactionAxis" xlink:to="us-gaap_RelatedPartyTransactionDomain_1" xlink:title="Definition : us-gaap_RelatedPartyTransactionAxis to us-gaap_RelatedPartyTransactionDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_CommitmentsandContingenciesDetailsTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="Definition : sxtp_CommitmentsandContingenciesDetailsTable to us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" xlink:title="us-gaap_RelatedPartyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" xlink:title="Definition : us-gaap_RelatedPartyTransactionsByRelatedPartyAxis to us-gaap_RelatedPartyDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="sxtp_CharlesAllenMember" xlink:title="Definition : us-gaap_RelatedPartyDomain to sxtp_CharlesAllenMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain_1" xlink:title="us-gaap_RelatedPartyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain_1" xlink:title="Definition : us-gaap_RelatedPartyTransactionsByRelatedPartyAxis to us-gaap_RelatedPartyDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUBSEQUENTEVENTSDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems" xlink:label="us-gaap_SubsequentEventLineItems" xlink:title="us-gaap_SubsequentEventLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" xlink:title="us-gaap_CommonStockSharesIssued" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_CommonStockSharesIssued" xlink:title="Definition : us-gaap_SubsequentEventLineItems to us-gaap_CommonStockSharesIssued" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable" xlink:label="us-gaap_SubsequentEventTable" xlink:title="us-gaap_SubsequentEventTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_SubsequentEventTable" xlink:title="Definition : us-gaap_SubsequentEventLineItems to us-gaap_SubsequentEventTable" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="us-gaap_SubsequentEventTypeAxis" xlink:title="us-gaap_SubsequentEventTypeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_SubsequentEventTypeAxis" xlink:title="Definition : us-gaap_SubsequentEventTable to us-gaap_SubsequentEventTypeAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain" xlink:title="us-gaap_SubsequentEventTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain" xlink:title="Definition : us-gaap_SubsequentEventTypeAxis to us-gaap_SubsequentEventTypeDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember" xlink:label="us-gaap_SubsequentEventMember" xlink:title="us-gaap_SubsequentEventMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventMember" xlink:title="Definition : us-gaap_SubsequentEventTypeDomain to us-gaap_SubsequentEventMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain_1" xlink:title="us-gaap_SubsequentEventTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain_1" xlink:title="Definition : us-gaap_SubsequentEventTypeAxis to us-gaap_SubsequentEventTypeDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:label="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:title="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:title="Definition : us-gaap_SubsequentEventTable to us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:label="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:title="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:to="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:title="Definition : us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis to us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ConsultingAgreementMember" xlink:label="sxtp_ConsultingAgreementMember" xlink:title="sxtp_ConsultingAgreementMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:to="sxtp_ConsultingAgreementMember" xlink:title="Definition : us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain to sxtp_ConsultingAgreementMember" xlink:type="arc" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:label="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_1" xlink:title="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:to="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_1" xlink:title="Definition : us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis to us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="us-gaap_RelatedPartyTransactionAxis" xlink:title="us-gaap_RelatedPartyTransactionAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_RelatedPartyTransactionAxis" xlink:title="Definition : us-gaap_SubsequentEventTable to us-gaap_RelatedPartyTransactionAxis" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain" xlink:title="us-gaap_RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionAxis" xlink:to="us-gaap_RelatedPartyTransactionDomain" xlink:title="Definition : us-gaap_RelatedPartyTransactionAxis to us-gaap_RelatedPartyTransactionDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_HCWainwrightAndCoLLCMember" xlink:label="sxtp_HCWainwrightAndCoLLCMember" xlink:title="sxtp_HCWainwrightAndCoLLCMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_HCWainwrightAndCoLLCMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_HCWainwrightAndCoLLCMember" xlink:type="arc" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain_1" xlink:title="us-gaap_RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RelatedPartyTransactionAxis" xlink:to="us-gaap_RelatedPartyTransactionDomain_1" xlink:title="Definition : us-gaap_RelatedPartyTransactionAxis to us-gaap_RelatedPartyTransactionDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateAxis" xlink:label="us-gaap_AwardDateAxis" xlink:title="us-gaap_AwardDateAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_AwardDateAxis" xlink:title="Definition : us-gaap_SubsequentEventTable to us-gaap_AwardDateAxis" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain" xlink:title="us-gaap_AwardDateDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain" xlink:title="Definition : us-gaap_AwardDateAxis to us-gaap_AwardDateDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_AtTheMarket2025AgreementMember" xlink:label="sxtp_AtTheMarket2025AgreementMember" xlink:title="sxtp_AtTheMarket2025AgreementMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_AtTheMarket2025AgreementMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_AtTheMarket2025AgreementMember" xlink:type="arc" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain_1" xlink:title="us-gaap_AwardDateDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain_1" xlink:title="Definition : us-gaap_AwardDateAxis to us-gaap_AwardDateDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:title="srt_RangeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="srt_RangeAxis" xlink:title="Definition : us-gaap_SubsequentEventTable to srt_RangeAxis" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:title="srt_RangeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:title="Definition : srt_RangeAxis to srt_RangeMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:title="srt_MaximumMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:title="Definition : srt_RangeMember to srt_MaximumMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember_1" xlink:title="srt_RangeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember_1" xlink:title="Definition : srt_RangeAxis to srt_RangeMember" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:title="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_AwardTypeAxis" xlink:title="Definition : us-gaap_SubsequentEventTable to us-gaap_AwardTypeAxis" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="Definition : us-gaap_AwardTypeAxis to us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockOptionsMember" xlink:label="sxtp_StockOptionsMember" xlink:title="sxtp_StockOptionsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="sxtp_StockOptionsMember" xlink:title="Definition : us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain to sxtp_StockOptionsMember" xlink:type="arc" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" xlink:title="us-gaap_RestrictedStockUnitsRSUMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" xlink:title="Definition : us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain to us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" xlink:title="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" xlink:title="Definition : us-gaap_AwardTypeAxis to us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis" xlink:label="srt_TitleOfIndividualAxis" xlink:title="srt_TitleOfIndividualAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="srt_TitleOfIndividualAxis" xlink:title="Definition : us-gaap_SubsequentEventTable to srt_TitleOfIndividualAxis" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:title="srt_TitleOfIndividualWithRelationshipToEntityDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:title="Definition : srt_TitleOfIndividualAxis to srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_DirectorMember" xlink:label="srt_DirectorMember" xlink:title="srt_DirectorMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="srt_DirectorMember" xlink:title="Definition : srt_TitleOfIndividualWithRelationshipToEntityDomain to srt_DirectorMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_DirectorOneMember" xlink:label="sxtp_DirectorOneMember" xlink:title="sxtp_DirectorOneMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="sxtp_DirectorOneMember" xlink:title="Definition : srt_TitleOfIndividualWithRelationshipToEntityDomain to sxtp_DirectorOneMember" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain_1" xlink:title="srt_TitleOfIndividualWithRelationshipToEntityDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_TitleOfIndividualAxis" xlink:to="srt_TitleOfIndividualWithRelationshipToEntityDomain_1" xlink:title="Definition : srt_TitleOfIndividualAxis to srt_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:title="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:title="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:title="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:title="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:title="us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" xlink:title="us-gaap_AllocatedShareBasedCompensationExpense" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Cash" xlink:label="us-gaap_Cash" xlink:title="us-gaap_Cash" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_Cash" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_Cash" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SharesIssuedPayrollTaxesWithholdings" xlink:label="sxtp_SharesIssuedPayrollTaxesWithholdings" xlink:title="sxtp_SharesIssuedPayrollTaxesWithholdings" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_SharesIssuedPayrollTaxesWithholdings" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_SharesIssuedPayrollTaxesWithholdings" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_UnamortizedBalance" xlink:label="sxtp_UnamortizedBalance" xlink:title="sxtp_UnamortizedBalance" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_UnamortizedBalance" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_UnamortizedBalance" xlink:type="arc" use="optional" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseNoncurrent" xlink:label="us-gaap_PrepaidExpenseNoncurrent" xlink:title="us-gaap_PrepaidExpenseNoncurrent" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_PrepaidExpenseNoncurrent" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_PrepaidExpenseNoncurrent" xlink:type="arc" use="optional" order="9" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NumberOfExecutives" xlink:label="sxtp_NumberOfExecutives" xlink:title="sxtp_NumberOfExecutives" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_NumberOfExecutives" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_NumberOfExecutives" xlink:type="arc" order="10" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="arc" use="optional" order="11" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NumberOfTranches" xlink:label="sxtp_NumberOfTranches" xlink:title="sxtp_NumberOfTranches" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_NumberOfTranches" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_NumberOfTranches" xlink:type="arc" order="12" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward" xlink:type="arc" use="optional" order="13" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_UnamortizedBalancePrepaidAssets" xlink:label="sxtp_UnamortizedBalancePrepaidAssets" xlink:title="sxtp_UnamortizedBalancePrepaidAssets" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_UnamortizedBalancePrepaidAssets" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_UnamortizedBalancePrepaidAssets" xlink:type="arc" order="14" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_UnamortizedBalanceOfPrepaidAssetsRecognizedRatablyOverRemainingServicePeriod" xlink:label="sxtp_UnamortizedBalanceOfPrepaidAssetsRecognizedRatablyOverRemainingServicePeriod" xlink:title="sxtp_UnamortizedBalanceOfPrepaidAssetsRecognizedRatablyOverRemainingServicePeriod" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_UnamortizedBalanceOfPrepaidAssetsRecognizedRatablyOverRemainingServicePeriod" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_UnamortizedBalanceOfPrepaidAssetsRecognizedRatablyOverRemainingServicePeriod" xlink:type="arc" order="15" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_UnamortizedBalanceOfPrepaidAssetsRemainingBalanceRecognizedAsResearchAndDevelopmentServices" xlink:label="sxtp_UnamortizedBalanceOfPrepaidAssetsRemainingBalanceRecognizedAsResearchAndDevelopmentServices" xlink:title="sxtp_UnamortizedBalanceOfPrepaidAssetsRemainingBalanceRecognizedAsResearchAndDevelopmentServices" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_UnamortizedBalanceOfPrepaidAssetsRemainingBalanceRecognizedAsResearchAndDevelopmentServices" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_UnamortizedBalanceOfPrepaidAssetsRemainingBalanceRecognizedAsResearchAndDevelopmentServices" xlink:type="arc" order="16" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalGrantsEachMonth" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalGrantsEachMonth" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalGrantsEachMonth" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalGrantsEachMonth" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalGrantsEachMonth" xlink:type="arc" order="17" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_DescriptionConsultingAgreementTerm" xlink:label="sxtp_DescriptionConsultingAgreementTerm" xlink:title="sxtp_DescriptionConsultingAgreementTerm" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_DescriptionConsultingAgreementTerm" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_DescriptionConsultingAgreementTerm" xlink:type="arc" order="18" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc" use="optional" order="19" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="arc" use="optional" order="20" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationDetailsTable" xlink:label="sxtp_ShareBasedCompensationDetailsTable" xlink:title="sxtp_ShareBasedCompensationDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="sxtp_ShareBasedCompensationDetailsTable" xlink:title="Definition : us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems to sxtp_ShareBasedCompensationDetailsTable" xlink:type="arc" order="21" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" xlink:title="us-gaap_PlanNameAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_ShareBasedCompensationDetailsTable" xlink:to="us-gaap_PlanNameAxis" xlink:title="Definition : sxtp_ShareBasedCompensationDetailsTable to us-gaap_PlanNameAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" xlink:title="us-gaap_PlanNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" xlink:title="Definition : us-gaap_PlanNameAxis to us-gaap_PlanNameDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_TwoThousandTwentyTwoEquityIncentivePlanMember" xlink:label="sxtp_TwoThousandTwentyTwoEquityIncentivePlanMember" xlink:title="sxtp_TwoThousandTwentyTwoEquityIncentivePlanMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="sxtp_TwoThousandTwentyTwoEquityIncentivePlanMember" xlink:title="Definition : us-gaap_PlanNameDomain to sxtp_TwoThousandTwentyTwoEquityIncentivePlanMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain_1" xlink:title="us-gaap_PlanNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain_1" xlink:title="Definition : us-gaap_PlanNameAxis to us-gaap_PlanNameDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:label="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:title="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_ShareBasedCompensationDetailsTable" xlink:to="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:title="Definition : sxtp_ShareBasedCompensationDetailsTable to us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:label="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:title="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:to="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:title="Definition : us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis to us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ConsultingAgreementMember" xlink:label="sxtp_ConsultingAgreementMember" xlink:title="sxtp_ConsultingAgreementMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:to="sxtp_ConsultingAgreementMember" xlink:title="Definition : us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain to sxtp_ConsultingAgreementMember" xlink:type="arc" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:label="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_1" xlink:title="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis" xlink:to="us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_1" xlink:title="Definition : us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis to us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:title="us-gaap_StatementEquityComponentsAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_ShareBasedCompensationDetailsTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:title="Definition : sxtp_ShareBasedCompensationDetailsTable to us-gaap_StatementEquityComponentsAxis" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:title="us-gaap_CommonStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_CommonStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain_1" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain_1" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:title="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_ShareBasedCompensationDetailsTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:title="Definition : sxtp_ShareBasedCompensationDetailsTable to us-gaap_StatementClassOfStockAxis" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_CommonStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain_1" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis" xlink:label="us-gaap_VestingAxis" xlink:title="us-gaap_VestingAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_ShareBasedCompensationDetailsTable" xlink:to="us-gaap_VestingAxis" xlink:title="Definition : sxtp_ShareBasedCompensationDetailsTable to us-gaap_VestingAxis" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain" xlink:title="us-gaap_VestingDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain" xlink:title="Definition : us-gaap_VestingAxis to us-gaap_VestingDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:label="us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:title="us-gaap_ShareBasedCompensationAwardTrancheOneMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:title="Definition : us-gaap_VestingDomain to us-gaap_ShareBasedCompensationAwardTrancheOneMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain_1" xlink:title="us-gaap_VestingDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain_1" xlink:title="Definition : us-gaap_VestingAxis to us-gaap_VestingDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:title="us-gaap_AwardTypeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_ShareBasedCompensationDetailsTable" xlink:to="us-gaap_AwardTypeAxis" xlink:title="Definition : sxtp_ShareBasedCompensationDetailsTable to us-gaap_AwardTypeAxis" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:title="Definition : us-gaap_AwardTypeAxis to us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockOptionsMember" xlink:label="sxtp_StockOptionsMember" xlink:title="sxtp_StockOptionsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="sxtp_StockOptionsMember" xlink:title="Definition : us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain to sxtp_StockOptionsMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="us-gaap_RestrictedStockUnitsRSUMember" xlink:title="us-gaap_RestrictedStockUnitsRSUMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_RestrictedStockUnitsRSUMember" xlink:title="Definition : us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain to us-gaap_RestrictedStockUnitsRSUMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" xlink:title="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_1" xlink:title="Definition : us-gaap_AwardTypeAxis to us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChangeInAccountingEstimateLineItems" xlink:label="us-gaap_ChangeInAccountingEstimateLineItems" xlink:title="us-gaap_ChangeInAccountingEstimateLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityReverseStockSplit" xlink:label="us-gaap_StockholdersEquityReverseStockSplit" xlink:title="us-gaap_StockholdersEquityReverseStockSplit" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ChangeInAccountingEstimateLineItems" xlink:to="us-gaap_StockholdersEquityReverseStockSplit" xlink:title="Definition : us-gaap_ChangeInAccountingEstimateLineItems to us-gaap_StockholdersEquityReverseStockSplit" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1" xlink:label="us-gaap_ConcentrationRiskPercentage1" xlink:title="us-gaap_ConcentrationRiskPercentage1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ChangeInAccountingEstimateLineItems" xlink:to="us-gaap_ConcentrationRiskPercentage1" xlink:title="Definition : us-gaap_ChangeInAccountingEstimateLineItems to us-gaap_ConcentrationRiskPercentage1" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsAndShortTermInvestments" xlink:label="us-gaap_CashCashEquivalentsAndShortTermInvestments" xlink:title="us-gaap_CashCashEquivalentsAndShortTermInvestments" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ChangeInAccountingEstimateLineItems" xlink:to="us-gaap_CashCashEquivalentsAndShortTermInvestments" xlink:title="Definition : us-gaap_ChangeInAccountingEstimateLineItems to us-gaap_CashCashEquivalentsAndShortTermInvestments" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" xlink:title="us-gaap_ResearchAndDevelopmentExpense" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ChangeInAccountingEstimateLineItems" xlink:to="us-gaap_ResearchAndDevelopmentExpense" xlink:title="Definition : us-gaap_ChangeInAccountingEstimateLineItems to us-gaap_ResearchAndDevelopmentExpense" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageExpectedDividend" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageExpectedDividend" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageExpectedDividend" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ChangeInAccountingEstimateLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageExpectedDividend" xlink:title="Definition : us-gaap_ChangeInAccountingEstimateLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageExpectedDividend" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:label="sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:title="sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ChangeInAccountingEstimateLineItems" xlink:to="sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:title="Definition : us-gaap_ChangeInAccountingEstimateLineItems to sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:type="arc" order="5" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:title="us-gaap_StatementEquityComponentsAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:title="Definition : sxtp_SummaryofSignificantAccountingPoliciesDetailsTable to us-gaap_StatementEquityComponentsAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ReverseStockSplitsMember" xlink:label="sxtp_ReverseStockSplitsMember" xlink:title="sxtp_ReverseStockSplitsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="sxtp_ReverseStockSplitsMember" xlink:title="Definition : us-gaap_EquityComponentDomain to sxtp_ReverseStockSplitsMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NetLossPerCommonShareMember" xlink:label="sxtp_NetLossPerCommonShareMember" xlink:title="sxtp_NetLossPerCommonShareMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="sxtp_NetLossPerCommonShareMember" xlink:title="Definition : us-gaap_EquityComponentDomain to sxtp_NetLossPerCommonShareMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain_1" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain_1" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="us-gaap_ConcentrationRiskByTypeAxis" xlink:title="us-gaap_ConcentrationRiskByTypeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:to="us-gaap_ConcentrationRiskByTypeAxis" xlink:title="Definition : sxtp_SummaryofSignificantAccountingPoliciesDetailsTable to us-gaap_ConcentrationRiskByTypeAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain" xlink:title="us-gaap_ConcentrationRiskTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain" xlink:title="Definition : us-gaap_ConcentrationRiskByTypeAxis to us-gaap_ConcentrationRiskTypeDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerConcentrationRiskMember" xlink:label="us-gaap_CustomerConcentrationRiskMember" xlink:title="us-gaap_CustomerConcentrationRiskMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskTypeDomain" xlink:to="us-gaap_CustomerConcentrationRiskMember" xlink:title="Definition : us-gaap_ConcentrationRiskTypeDomain to us-gaap_CustomerConcentrationRiskMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain_1" xlink:title="us-gaap_ConcentrationRiskTypeDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain_1" xlink:title="Definition : us-gaap_ConcentrationRiskByTypeAxis to us-gaap_ConcentrationRiskTypeDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MajorCustomersAxis" xlink:label="srt_MajorCustomersAxis" xlink:title="srt_MajorCustomersAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:to="srt_MajorCustomersAxis" xlink:title="Definition : sxtp_SummaryofSignificantAccountingPoliciesDetailsTable to srt_MajorCustomersAxis" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain" xlink:label="srt_NameOfMajorCustomerDomain" xlink:title="srt_NameOfMajorCustomerDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_MajorCustomersAxis" xlink:to="srt_NameOfMajorCustomerDomain" xlink:title="Definition : srt_MajorCustomersAxis to srt_NameOfMajorCustomerDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SignificantCustomersMember" xlink:label="sxtp_SignificantCustomersMember" xlink:title="sxtp_SignificantCustomersMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_NameOfMajorCustomerDomain" xlink:to="sxtp_SignificantCustomersMember" xlink:title="Definition : srt_NameOfMajorCustomerDomain to sxtp_SignificantCustomersMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CustomerTwoMember" xlink:label="sxtp_CustomerTwoMember" xlink:title="sxtp_CustomerTwoMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_NameOfMajorCustomerDomain" xlink:to="sxtp_CustomerTwoMember" xlink:title="Definition : srt_NameOfMajorCustomerDomain to sxtp_CustomerTwoMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CustomerThreeMember" xlink:label="sxtp_CustomerThreeMember" xlink:title="sxtp_CustomerThreeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_NameOfMajorCustomerDomain" xlink:to="sxtp_CustomerThreeMember" xlink:title="Definition : srt_NameOfMajorCustomerDomain to sxtp_CustomerThreeMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SignificantCustomerOneMember" xlink:label="sxtp_SignificantCustomerOneMember" xlink:title="sxtp_SignificantCustomerOneMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_NameOfMajorCustomerDomain" xlink:to="sxtp_SignificantCustomerOneMember" xlink:title="Definition : srt_NameOfMajorCustomerDomain to sxtp_SignificantCustomerOneMember" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SignificantCustomerTwoMember" xlink:label="sxtp_SignificantCustomerTwoMember" xlink:title="sxtp_SignificantCustomerTwoMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_NameOfMajorCustomerDomain" xlink:to="sxtp_SignificantCustomerTwoMember" xlink:title="Definition : srt_NameOfMajorCustomerDomain to sxtp_SignificantCustomerTwoMember" xlink:type="arc" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SignificantCustomerThreeMember" xlink:label="sxtp_SignificantCustomerThreeMember" xlink:title="sxtp_SignificantCustomerThreeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_NameOfMajorCustomerDomain" xlink:to="sxtp_SignificantCustomerThreeMember" xlink:title="Definition : srt_NameOfMajorCustomerDomain to sxtp_SignificantCustomerThreeMember" xlink:type="arc" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain" xlink:label="srt_NameOfMajorCustomerDomain_1" xlink:title="srt_NameOfMajorCustomerDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_MajorCustomersAxis" xlink:to="srt_NameOfMajorCustomerDomain_1" xlink:title="Definition : srt_MajorCustomersAxis to srt_NameOfMajorCustomerDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:title="us-gaap_ConcentrationRiskByBenchmarkAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_SummaryofSignificantAccountingPoliciesDetailsTable" xlink:to="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:title="Definition : sxtp_SummaryofSignificantAccountingPoliciesDetailsTable to us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:title="us-gaap_ConcentrationRiskBenchmarkDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:title="Definition : us-gaap_ConcentrationRiskByBenchmarkAxis to us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableMember" xlink:label="us-gaap_AccountsReceivableMember" xlink:title="us-gaap_AccountsReceivableMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:to="us-gaap_AccountsReceivableMember" xlink:title="Definition : us-gaap_ConcentrationRiskBenchmarkDomain to us-gaap_AccountsReceivableMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SalesRevenueNetMember" xlink:label="us-gaap_SalesRevenueNetMember" xlink:title="us-gaap_SalesRevenueNetMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:to="us-gaap_SalesRevenueNetMember" xlink:title="Definition : us-gaap_ConcentrationRiskBenchmarkDomain to us-gaap_SalesRevenueNetMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_AccountsReceivableOrSalesRevenueNetMember" xlink:label="sxtp_AccountsReceivableOrSalesRevenueNetMember" xlink:title="sxtp_AccountsReceivableOrSalesRevenueNetMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:to="sxtp_AccountsReceivableOrSalesRevenueNetMember" xlink:title="Definition : us-gaap_ConcentrationRiskBenchmarkDomain to sxtp_AccountsReceivableOrSalesRevenueNetMember" xlink:type="arc" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain_1" xlink:title="us-gaap_ConcentrationRiskBenchmarkDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain_1" xlink:title="Definition : us-gaap_ConcentrationRiskByBenchmarkAxis to us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/STOCKHOLDERSEQUITYDetails">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems" xlink:label="us-gaap_ClassOfStockLineItems" xlink:title="us-gaap_ClassOfStockLineItems" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleConversionPrice" xlink:label="us-gaap_PreferredStockConvertibleConversionPrice" xlink:title="us-gaap_PreferredStockConvertibleConversionPrice" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockConvertibleConversionPrice" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_PreferredStockConvertibleConversionPrice" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" xlink:title="us-gaap_CommonStockSharesAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_CommonStockSharesAuthorized" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" xlink:title="us-gaap_CommonStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" xlink:title="us-gaap_PreferredStockSharesAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockSharesAuthorized" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_PreferredStockSharesAuthorized" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:title="us-gaap_PreferredStockParOrStatedValuePerShare" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_PreferredStockParOrStatedValuePerShare" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ExcessStockSharesAuthorized" xlink:label="us-gaap_ExcessStockSharesAuthorized" xlink:title="us-gaap_ExcessStockSharesAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ExcessStockSharesAuthorized" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ExcessStockSharesAuthorized" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" xlink:title="us-gaap_CommonStockSharesIssued" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockSharesIssued" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_CommonStockSharesIssued" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" xlink:title="us-gaap_CommonStockSharesOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_CommonStockSharesOutstanding" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="us-gaap_PreferredStockSharesIssued" xlink:title="us-gaap_PreferredStockSharesIssued" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockSharesIssued" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_PreferredStockSharesIssued" xlink:type="arc" use="optional" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" xlink:title="us-gaap_PreferredStockSharesOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_PreferredStockSharesOutstanding" xlink:type="arc" use="optional" order="9" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityReverseStockSplit" xlink:label="us-gaap_StockholdersEquityReverseStockSplit" xlink:title="us-gaap_StockholdersEquityReverseStockSplit" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_StockholdersEquityReverseStockSplit" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_StockholdersEquityReverseStockSplit" xlink:type="arc" use="optional" order="10" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized" xlink:type="arc" use="optional" order="11" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="us-gaap_SaleOfStockPricePerShare" xlink:title="us-gaap_SaleOfStockPricePerShare" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_SaleOfStockPricePerShare" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_SaleOfStockPricePerShare" xlink:type="arc" use="optional" order="12" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:title="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights" xlink:type="arc" use="optional" order="13" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingTerm" xlink:label="us-gaap_WarrantsAndRightsOutstandingTerm" xlink:title="us-gaap_WarrantsAndRightsOutstandingTerm" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstandingTerm" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_WarrantsAndRightsOutstandingTerm" xlink:type="arc" use="optional" order="14" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstanding" xlink:label="us-gaap_WarrantsAndRightsOutstanding" xlink:title="us-gaap_WarrantsAndRightsOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstanding" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_WarrantsAndRightsOutstanding" xlink:type="arc" use="optional" order="15" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:title="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:type="arc" use="optional" order="16" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:label="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:title="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ProceedsFromIssuanceOfPrivatePlacement" xlink:type="arc" use="optional" order="17" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:label="us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:title="us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction" xlink:type="arc" use="optional" order="18" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_UnitsIssuedDuringPeriodSharesNewIssues" xlink:label="sxtp_UnitsIssuedDuringPeriodSharesNewIssues" xlink:title="sxtp_UnitsIssuedDuringPeriodSharesNewIssues" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_UnitsIssuedDuringPeriodSharesNewIssues" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_UnitsIssuedDuringPeriodSharesNewIssues" xlink:type="arc" use="optional" order="19" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding" xlink:title="us-gaap_ClassOfWarrantOrRightOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightOutstanding" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ClassOfWarrantOrRightOutstanding" xlink:type="arc" use="optional" order="20" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromDebtNetOfIssuanceCosts" xlink:label="us-gaap_ProceedsFromDebtNetOfIssuanceCosts" xlink:title="us-gaap_ProceedsFromDebtNetOfIssuanceCosts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ProceedsFromDebtNetOfIssuanceCosts" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ProceedsFromDebtNetOfIssuanceCosts" xlink:type="arc" use="optional" order="21" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" xlink:label="us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" xlink:title="us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested" xlink:type="arc" use="optional" order="22" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_OtherOfferingExpensesPaid" xlink:label="sxtp_OtherOfferingExpensesPaid" xlink:title="sxtp_OtherOfferingExpensesPaid" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_OtherOfferingExpensesPaid" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_OtherOfferingExpensesPaid" xlink:type="arc" use="optional" order="23" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SharesOfWarrants" xlink:label="sxtp_SharesOfWarrants" xlink:title="sxtp_SharesOfWarrants" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_SharesOfWarrants" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_SharesOfWarrants" xlink:type="arc" use="optional" order="24" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesOther" xlink:label="us-gaap_StockIssuedDuringPeriodSharesOther" xlink:title="us-gaap_StockIssuedDuringPeriodSharesOther" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesOther" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_StockIssuedDuringPeriodSharesOther" xlink:type="arc" use="optional" order="25" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantExercisePriceIncrease" xlink:label="us-gaap_WarrantExercisePriceIncrease" xlink:title="us-gaap_WarrantExercisePriceIncrease" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantExercisePriceIncrease" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_WarrantExercisePriceIncrease" xlink:type="arc" use="optional" order="26" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NumberofWarrantExercisable" xlink:label="sxtp_NumberofWarrantExercisable" xlink:title="sxtp_NumberofWarrantExercisable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_NumberofWarrantExercisable" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_NumberofWarrantExercisable" xlink:type="arc" use="optional" order="27" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NumberOfShares" xlink:label="sxtp_NumberOfShares" xlink:title="sxtp_NumberOfShares" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_NumberOfShares" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_NumberOfShares" xlink:type="arc" use="optional" order="28" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" xlink:title="us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_OtherComprehensiveIncomeLossCashFlowHedgeGainLossAfterReclassificationAndTax" xlink:type="arc" use="optional" order="29" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockDividendRatePercentage" xlink:label="us-gaap_PreferredStockDividendRatePercentage" xlink:title="us-gaap_PreferredStockDividendRatePercentage" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockDividendRatePercentage" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_PreferredStockDividendRatePercentage" xlink:type="arc" use="optional" order="30" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DividendsPreferredStock" xlink:label="us-gaap_DividendsPreferredStock" xlink:title="us-gaap_DividendsPreferredStock" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_DividendsPreferredStock" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_DividendsPreferredStock" xlink:type="arc" use="optional" order="31" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockConvertibleConversionPriceIncrease" xlink:label="us-gaap_CommonStockConvertibleConversionPriceIncrease" xlink:title="us-gaap_CommonStockConvertibleConversionPriceIncrease" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_CommonStockConvertibleConversionPriceIncrease" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_CommonStockConvertibleConversionPriceIncrease" xlink:type="arc" use="optional" order="32" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:type="arc" use="optional" order="33" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_AgreedToReturnShares" xlink:label="sxtp_AgreedToReturnShares" xlink:title="sxtp_AgreedToReturnShares" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_AgreedToReturnShares" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_AgreedToReturnShares" xlink:type="arc" order="34" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromWarrantExercises" xlink:label="us-gaap_ProceedsFromWarrantExercises" xlink:title="us-gaap_ProceedsFromWarrantExercises" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ProceedsFromWarrantExercises" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ProceedsFromWarrantExercises" xlink:type="arc" use="optional" order="35" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CommonStockAggregateOfferingPrice" xlink:label="sxtp_CommonStockAggregateOfferingPrice" xlink:title="sxtp_CommonStockAggregateOfferingPrice" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_CommonStockAggregateOfferingPrice" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_CommonStockAggregateOfferingPrice" xlink:type="arc" order="36" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ATMOfferingShares" xlink:label="sxtp_ATMOfferingShares" xlink:title="sxtp_ATMOfferingShares" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_ATMOfferingShares" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_ATMOfferingShares" xlink:type="arc" order="37" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ProceedsFromCommonStockSalesAgreementNet" xlink:label="sxtp_ProceedsFromCommonStockSalesAgreementNet" xlink:title="sxtp_ProceedsFromCommonStockSalesAgreementNet" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_ProceedsFromCommonStockSalesAgreementNet" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_ProceedsFromCommonStockSalesAgreementNet" xlink:type="arc" order="38" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromOtherEquity" xlink:label="us-gaap_ProceedsFromOtherEquity" xlink:title="us-gaap_ProceedsFromOtherEquity" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ProceedsFromOtherEquity" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ProceedsFromOtherEquity" xlink:type="arc" use="optional" order="39" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:label="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:title="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:type="arc" use="optional" order="40" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockSharesConverted1" xlink:label="us-gaap_ConversionOfStockSharesConverted1" xlink:title="us-gaap_ConversionOfStockSharesConverted1" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ConversionOfStockSharesConverted1" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ConversionOfStockSharesConverted1" xlink:type="arc" use="optional" order="41" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantExercisePriceDecrease" xlink:label="us-gaap_WarrantExercisePriceDecrease" xlink:title="us-gaap_WarrantExercisePriceDecrease" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantExercisePriceDecrease" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_WarrantExercisePriceDecrease" xlink:type="arc" use="optional" order="42" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockConvertibleSharesIssuable" xlink:label="us-gaap_PreferredStockConvertibleSharesIssuable" xlink:title="us-gaap_PreferredStockConvertibleSharesIssuable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_PreferredStockConvertibleSharesIssuable" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_PreferredStockConvertibleSharesIssuable" xlink:type="arc" use="optional" order="43" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable" xlink:label="us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable" xlink:title="us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ClassOfWarrantOrRighstDateFromWhichWarrantsOrRightsExercisable" xlink:type="arc" use="optional" order="44" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingMaturityDate" xlink:label="us-gaap_WarrantsAndRightsOutstandingMaturityDate" xlink:title="us-gaap_WarrantsAndRightsOutstandingMaturityDate" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_WarrantsAndRightsOutstandingMaturityDate" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_WarrantsAndRightsOutstandingMaturityDate" xlink:type="arc" use="optional" order="45" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SalesAgreementTerminationDate" xlink:label="sxtp_SalesAgreementTerminationDate" xlink:title="sxtp_SalesAgreementTerminationDate" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_SalesAgreementTerminationDate" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_SalesAgreementTerminationDate" xlink:type="arc" order="46" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CommonStockAggregateGrossSalesProceedAuthorized" xlink:label="sxtp_CommonStockAggregateGrossSalesProceedAuthorized" xlink:title="sxtp_CommonStockAggregateGrossSalesProceedAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_CommonStockAggregateGrossSalesProceedAuthorized" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_CommonStockAggregateGrossSalesProceedAuthorized" xlink:type="arc" order="47" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_IncreasedCommonStockAggregateGrossSalesProceedAuthorized" xlink:label="sxtp_IncreasedCommonStockAggregateGrossSalesProceedAuthorized" xlink:title="sxtp_IncreasedCommonStockAggregateGrossSalesProceedAuthorized" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_IncreasedCommonStockAggregateGrossSalesProceedAuthorized" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_IncreasedCommonStockAggregateGrossSalesProceedAuthorized" xlink:type="arc" order="48" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SalesAgentCommissionPercentage" xlink:label="sxtp_SalesAgentCommissionPercentage" xlink:title="sxtp_SalesAgentCommissionPercentage" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_SalesAgentCommissionPercentage" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_SalesAgentCommissionPercentage" xlink:type="arc" order="49" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNumberOfWarrantsExercised" xlink:type="arc" use="optional" order="50" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ProceedsFromExerciseOfPreFundedWarrants" xlink:label="sxtp_ProceedsFromExerciseOfPreFundedWarrants" xlink:title="sxtp_ProceedsFromExerciseOfPreFundedWarrants" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_ProceedsFromExerciseOfPreFundedWarrants" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_ProceedsFromExerciseOfPreFundedWarrants" xlink:type="arc" use="optional" order="51" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:title="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:title="Definition : us-gaap_ClassOfStockLineItems to us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="arc" use="optional" order="52" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsExercisable" xlink:type="arc" use="optional" order="53" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:label="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:title="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:type="arc" order="54" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:label="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:title="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageExercisePriceExercised" xlink:type="arc" use="optional" order="55" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ClassOfWarrantOrRightsExpiryTerm" xlink:label="sxtp_ClassOfWarrantOrRightsExpiryTerm" xlink:title="sxtp_ClassOfWarrantOrRightsExpiryTerm" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_ClassOfWarrantOrRightsExpiryTerm" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_ClassOfWarrantOrRightsExpiryTerm" xlink:type="arc" order="56" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:label="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:title="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:type="arc" use="optional" order="57" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_NetProceedsFromFebruary2025Offering" xlink:label="sxtp_NetProceedsFromFebruary2025Offering" xlink:title="sxtp_NetProceedsFromFebruary2025Offering" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_NetProceedsFromFebruary2025Offering" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_NetProceedsFromFebruary2025Offering" xlink:type="arc" order="58" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_CommonStockAggregateGrossSalesProceedAuthorizedIncrease" xlink:label="sxtp_CommonStockAggregateGrossSalesProceedAuthorizedIncrease" xlink:title="sxtp_CommonStockAggregateGrossSalesProceedAuthorizedIncrease" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_CommonStockAggregateGrossSalesProceedAuthorizedIncrease" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_CommonStockAggregateGrossSalesProceedAuthorizedIncrease" xlink:type="arc" order="59" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockholdersEquityDetailsTable" xlink:label="sxtp_StockholdersEquityDetailsTable" xlink:title="sxtp_StockholdersEquityDetailsTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ClassOfStockLineItems" xlink:to="sxtp_StockholdersEquityDetailsTable" xlink:title="Definition : us-gaap_ClassOfStockLineItems to sxtp_StockholdersEquityDetailsTable" xlink:type="arc" order="60" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" xlink:title="us-gaap_SubsidiarySaleOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to us-gaap_SubsidiarySaleOfStockAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:title="Definition : us-gaap_SubsidiarySaleOfStockAxis to us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OverAllotmentOptionMember" xlink:label="us-gaap_OverAllotmentOptionMember" xlink:title="us-gaap_OverAllotmentOptionMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_OverAllotmentOptionMember" xlink:title="Definition : us-gaap_SaleOfStockNameOfTransactionDomain to us-gaap_OverAllotmentOptionMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IPOMember" xlink:label="us-gaap_IPOMember" xlink:title="us-gaap_IPOMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_IPOMember" xlink:title="Definition : us-gaap_SaleOfStockNameOfTransactionDomain to us-gaap_IPOMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_1" xlink:title="Definition : us-gaap_SubsidiarySaleOfStockAxis to us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:title="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to us-gaap_StatementClassOfStockAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="us-gaap_SeriesAPreferredStockMember" xlink:title="us-gaap_SeriesAPreferredStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_SeriesAPreferredStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:label="sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:title="sxtp_SeriesANonVotingConvertiblePreferredStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to sxtp_SeriesANonVotingConvertiblePreferredStockMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:title="us-gaap_CommonStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_CommonStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_CommonStockMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleCommonStockMember" xlink:label="us-gaap_ConvertibleCommonStockMember" xlink:title="us-gaap_ConvertibleCommonStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_ConvertibleCommonStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_ConvertibleCommonStockMember" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain_1" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="us-gaap_RelatedPartyTransactionAxis" xlink:title="us-gaap_RelatedPartyTransactionAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="us-gaap_RelatedPartyTransactionAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to us-gaap_RelatedPartyTransactionAxis" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain" xlink:title="us-gaap_RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionAxis" xlink:to="us-gaap_RelatedPartyTransactionDomain" xlink:title="Definition : us-gaap_RelatedPartyTransactionAxis to us-gaap_RelatedPartyTransactionDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_WallachBethCapitalLLCMember" xlink:label="sxtp_WallachBethCapitalLLCMember" xlink:title="sxtp_WallachBethCapitalLLCMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_WallachBethCapitalLLCMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_WallachBethCapitalLLCMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_TrevallyLLCMember" xlink:label="sxtp_TrevallyLLCMember" xlink:title="sxtp_TrevallyLLCMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_TrevallyLLCMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_TrevallyLLCMember" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_HCWainwrightAndCoLLCMember" xlink:label="sxtp_HCWainwrightAndCoLLCMember" xlink:title="sxtp_HCWainwrightAndCoLLCMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionDomain" xlink:to="sxtp_HCWainwrightAndCoLLCMember" xlink:title="Definition : us-gaap_RelatedPartyTransactionDomain to sxtp_HCWainwrightAndCoLLCMember" xlink:type="arc" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="us-gaap_RelatedPartyTransactionDomain_1" xlink:title="us-gaap_RelatedPartyTransactionDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RelatedPartyTransactionAxis" xlink:to="us-gaap_RelatedPartyTransactionDomain_1" xlink:title="Definition : us-gaap_RelatedPartyTransactionAxis to us-gaap_RelatedPartyTransactionDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain" xlink:title="us-gaap_RelatedPartyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" xlink:title="Definition : us-gaap_RelatedPartyTransactionsByRelatedPartyAxis to us-gaap_RelatedPartyDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="sxtp_WallachBethCapitalLLCMember" xlink:title="Definition : us-gaap_RelatedPartyDomain to sxtp_WallachBethCapitalLLCMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="us-gaap_RelatedPartyDomain_1" xlink:title="us-gaap_RelatedPartyDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain_1" xlink:title="Definition : us-gaap_RelatedPartyTransactionsByRelatedPartyAxis to us-gaap_RelatedPartyDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:title="us-gaap_StatementEquityComponentsAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to us-gaap_StatementEquityComponentsAxis" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_CommonStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain_1" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain_1" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateAxis" xlink:label="us-gaap_AwardDateAxis" xlink:title="us-gaap_AwardDateAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="us-gaap_AwardDateAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to us-gaap_AwardDateAxis" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain" xlink:title="us-gaap_AwardDateDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain" xlink:title="Definition : us-gaap_AwardDateAxis to us-gaap_AwardDateDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_January2024OfferingMember" xlink:label="sxtp_January2024OfferingMember" xlink:title="sxtp_January2024OfferingMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_January2024OfferingMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_January2024OfferingMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_July2025OfferingMember" xlink:label="sxtp_July2025OfferingMember" xlink:title="sxtp_July2025OfferingMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_July2025OfferingMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_July2025OfferingMember" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_AtTheMarket2025AgreementMember" xlink:label="sxtp_AtTheMarket2025AgreementMember" xlink:title="sxtp_AtTheMarket2025AgreementMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_AtTheMarket2025AgreementMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_AtTheMarket2025AgreementMember" xlink:type="arc" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_AtTheMarket2026OfferingMember" xlink:label="sxtp_AtTheMarket2026OfferingMember" xlink:title="sxtp_AtTheMarket2026OfferingMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_AtTheMarket2026OfferingMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_AtTheMarket2026OfferingMember" xlink:type="arc" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_January2025OfferingMember" xlink:label="sxtp_January2025OfferingMember" xlink:title="sxtp_January2025OfferingMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_January2025OfferingMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_January2025OfferingMember" xlink:type="arc" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_February2025OfferingMember" xlink:label="sxtp_February2025OfferingMember" xlink:title="sxtp_February2025OfferingMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_February2025OfferingMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_February2025OfferingMember" xlink:type="arc" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_March2026AtTheMarketAgreementMember" xlink:label="sxtp_March2026AtTheMarketAgreementMember" xlink:title="sxtp_March2026AtTheMarketAgreementMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="sxtp_March2026AtTheMarketAgreementMember" xlink:title="Definition : us-gaap_AwardDateDomain to sxtp_March2026AtTheMarketAgreementMember" xlink:type="arc" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain_1" xlink:title="us-gaap_AwardDateDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain_1" xlink:title="Definition : us-gaap_AwardDateAxis to us-gaap_AwardDateDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightAxis" xlink:label="us-gaap_ClassOfWarrantOrRightAxis" xlink:title="us-gaap_ClassOfWarrantOrRightAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="us-gaap_ClassOfWarrantOrRightAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to us-gaap_ClassOfWarrantOrRightAxis" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain" xlink:title="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightAxis to us-gaap_ClassOfWarrantOrRightDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_PlacementAgentWarrantsMember" xlink:label="sxtp_PlacementAgentWarrantsMember" xlink:title="sxtp_PlacementAgentWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_PlacementAgentWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_PlacementAgentWarrantsMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember" xlink:title="us-gaap_WarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="us-gaap_WarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to us-gaap_WarrantMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_PreFundedWarrantMember" xlink:label="sxtp_PreFundedWarrantMember" xlink:title="sxtp_PreFundedWarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_PreFundedWarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_PreFundedWarrantMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JanuaryTwoThousandTwentyFourPreFundedWarrantsMember" xlink:label="sxtp_JanuaryTwoThousandTwentyFourPreFundedWarrantsMember" xlink:title="sxtp_JanuaryTwoThousandTwentyFourPreFundedWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JanuaryTwoThousandTwentyFourPreFundedWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JanuaryTwoThousandTwentyFourPreFundedWarrantsMember" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JanuaryTwoThousandTwentyFourWarrantMember" xlink:label="sxtp_JanuaryTwoThousandTwentyFourWarrantMember" xlink:title="sxtp_JanuaryTwoThousandTwentyFourWarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JanuaryTwoThousandTwentyFourWarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JanuaryTwoThousandTwentyFourWarrantMember" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JanuaryRepresentativeWarrantsMember" xlink:label="sxtp_JanuaryRepresentativeWarrantsMember" xlink:title="sxtp_JanuaryRepresentativeWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JanuaryRepresentativeWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JanuaryRepresentativeWarrantsMember" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SeriesAWarrantsMember" xlink:label="sxtp_SeriesAWarrantsMember" xlink:title="sxtp_SeriesAWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_SeriesAWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_SeriesAWarrantsMember" xlink:type="arc" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SeriesBWarrantsMember" xlink:label="sxtp_SeriesBWarrantsMember" xlink:title="sxtp_SeriesBWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_SeriesBWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_SeriesBWarrantsMember" xlink:type="arc" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SeptemberTwoThousandTwentyFourAgentWarrantsMember" xlink:label="sxtp_SeptemberTwoThousandTwentyFourAgentWarrantsMember" xlink:title="sxtp_SeptemberTwoThousandTwentyFourAgentWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_SeptemberTwoThousandTwentyFourAgentWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_SeptemberTwoThousandTwentyFourAgentWarrantsMember" xlink:type="arc" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_SeriesAAndSeriesBWarrantsMember" xlink:label="sxtp_SeriesAAndSeriesBWarrantsMember" xlink:title="sxtp_SeriesAAndSeriesBWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_SeriesAAndSeriesBWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_SeriesAAndSeriesBWarrantsMember" xlink:type="arc" order="9" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JulyTwoThousandTwentyFivePreFundedWarrantsMember" xlink:label="sxtp_JulyTwoThousandTwentyFivePreFundedWarrantsMember" xlink:title="sxtp_JulyTwoThousandTwentyFivePreFundedWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JulyTwoThousandTwentyFivePreFundedWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JulyTwoThousandTwentyFivePreFundedWarrantsMember" xlink:type="arc" order="10" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JulyTwoThousandTwentyFiveAOneWarrantsMember" xlink:label="sxtp_JulyTwoThousandTwentyFiveAOneWarrantsMember" xlink:title="sxtp_JulyTwoThousandTwentyFiveAOneWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JulyTwoThousandTwentyFiveAOneWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JulyTwoThousandTwentyFiveAOneWarrantsMember" xlink:type="arc" order="11" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JulyTwoThousandTwentyFiveATwoWarrantsMember" xlink:label="sxtp_JulyTwoThousandTwentyFiveATwoWarrantsMember" xlink:title="sxtp_JulyTwoThousandTwentyFiveATwoWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JulyTwoThousandTwentyFiveATwoWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JulyTwoThousandTwentyFiveATwoWarrantsMember" xlink:type="arc" order="12" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JulyTwoThousandTwentyFivePlacementAgentWarrantsMember" xlink:label="sxtp_JulyTwoThousandTwentyFivePlacementAgentWarrantsMember" xlink:title="sxtp_JulyTwoThousandTwentyFivePlacementAgentWarrantsMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JulyTwoThousandTwentyFivePlacementAgentWarrantsMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JulyTwoThousandTwentyFivePlacementAgentWarrantsMember" xlink:type="arc" order="13" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JanuaryTwoThousandTwentyFiveWarrantMember" xlink:label="sxtp_JanuaryTwoThousandTwentyFiveWarrantMember" xlink:title="sxtp_JanuaryTwoThousandTwentyFiveWarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JanuaryTwoThousandTwentyFiveWarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JanuaryTwoThousandTwentyFiveWarrantMember" xlink:type="arc" order="14" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_JanuaryTwoThousandTwentyFiveAgentWarrantMember" xlink:label="sxtp_JanuaryTwoThousandTwentyFiveAgentWarrantMember" xlink:title="sxtp_JanuaryTwoThousandTwentyFiveAgentWarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_JanuaryTwoThousandTwentyFiveAgentWarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_JanuaryTwoThousandTwentyFiveAgentWarrantMember" xlink:type="arc" order="15" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_FebruaryTwoThousandTwentyFiveWarrantMember" xlink:label="sxtp_FebruaryTwoThousandTwentyFiveWarrantMember" xlink:title="sxtp_FebruaryTwoThousandTwentyFiveWarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_FebruaryTwoThousandTwentyFiveWarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_FebruaryTwoThousandTwentyFiveWarrantMember" xlink:type="arc" order="16" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_FebruaryTwoThousandTwentyFiveAgentWarrantMember" xlink:label="sxtp_FebruaryTwoThousandTwentyFiveAgentWarrantMember" xlink:title="sxtp_FebruaryTwoThousandTwentyFiveAgentWarrantMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfWarrantOrRightDomain" xlink:to="sxtp_FebruaryTwoThousandTwentyFiveAgentWarrantMember" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightDomain to sxtp_FebruaryTwoThousandTwentyFiveAgentWarrantMember" xlink:type="arc" order="17" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightDomain" xlink:label="us-gaap_ClassOfWarrantOrRightDomain_1" xlink:title="us-gaap_ClassOfWarrantOrRightDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ClassOfWarrantOrRightAxis" xlink:to="us-gaap_ClassOfWarrantOrRightDomain_1" xlink:title="Definition : us-gaap_ClassOfWarrantOrRightAxis to us-gaap_ClassOfWarrantOrRightDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis" xlink:label="srt_RangeAxis" xlink:title="srt_RangeAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="sxtp_StockholdersEquityDetailsTable" xlink:to="srt_RangeAxis" xlink:title="Definition : sxtp_StockholdersEquityDetailsTable to srt_RangeAxis" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember" xlink:title="srt_RangeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" xlink:title="Definition : srt_RangeAxis to srt_RangeMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember" xlink:label="srt_MaximumMember" xlink:title="srt_MaximumMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" xlink:title="Definition : srt_RangeMember to srt_MaximumMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember" xlink:label="srt_RangeMember_1" xlink:title="srt_RangeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember_1" xlink:title="Definition : srt_RangeAxis to srt_RangeMember" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITED">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:title="us-gaap_StatementLineItems" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCosts" xlink:label="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCosts" xlink:title="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCosts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCosts" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCosts" xlink:type="arc" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_IssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCostsPaid" xlink:label="sxtp_IssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCostsPaid" xlink:title="sxtp_IssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCostsPaid" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_IssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCostsPaid" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_IssuanceOfCommonStockPursuantToATMOfferingNetOfOfferingCostsPaid" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:label="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:title="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:label="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:title="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToJanuary2025OfferingNetOfOfferingCosts" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:label="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:title="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:label="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:title="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockAndWarrantsPursuantToFebruary2025OfferingNetOfOfferingCosts" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodValueStockIssuanceOfCommonStockUponExerciseOfPreFundedWarrants" xlink:label="sxtp_StockIssuedDuringPeriodValueStockIssuanceOfCommonStockUponExerciseOfPreFundedWarrants" xlink:title="sxtp_StockIssuedDuringPeriodValueStockIssuanceOfCommonStockUponExerciseOfPreFundedWarrants" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodValueStockIssuanceOfCommonStockUponExerciseOfPreFundedWarrants" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodValueStockIssuanceOfCommonStockUponExerciseOfPreFundedWarrants" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:label="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:title="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodIssuanceOfCommonStockUponExerciseOfPreFundedWarrantsinShares" xlink:type="arc" use="optional" order="7" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodValueIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxes" xlink:label="sxtp_StockIssuedDuringPeriodValueIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxes" xlink:title="sxtp_StockIssuedDuringPeriodValueIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxes" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodValueIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxes" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodValueIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxes" xlink:type="arc" use="optional" order="8" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxesinShares" xlink:label="sxtp_StockIssuedDuringPeriodIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxesinShares" xlink:title="sxtp_StockIssuedDuringPeriodIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxesinShares" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxesinShares" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodIssuanceOfSharesForAnnualPerformanceBonusesNetOfSharesWithheldForTaxesinShares" xlink:type="arc" use="optional" order="9" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_ReverseStockSplitFractionalSharesRoundingAdjustment" xlink:label="sxtp_ReverseStockSplitFractionalSharesRoundingAdjustment" xlink:title="sxtp_ReverseStockSplitFractionalSharesRoundingAdjustment" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_ReverseStockSplitFractionalSharesRoundingAdjustment" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_ReverseStockSplitFractionalSharesRoundingAdjustment" xlink:type="arc" order="10" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:title="us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition" xlink:type="arc" use="optional" order="11" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:title="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax" xlink:type="arc" use="optional" order="12" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfitLoss" xlink:label="us-gaap_ProfitLoss" xlink:title="us-gaap_ProfitLoss" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProfitLoss" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_ProfitLoss" xlink:type="arc" use="optional" order="13" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="sxtp-20260331.xsd#sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockForFractionalSharesPursuantToReverseStockSplitRoundingAdjustment" xlink:label="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockForFractionalSharesPursuantToReverseStockSplitRoundingAdjustment" xlink:title="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockForFractionalSharesPursuantToReverseStockSplitRoundingAdjustment" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockForFractionalSharesPursuantToReverseStockSplitRoundingAdjustment" xlink:title="Definition : us-gaap_StatementLineItems to sxtp_StockIssuedDuringPeriodValueIssuanceOfCommonStockForFractionalSharesPursuantToReverseStockSplitRoundingAdjustment" xlink:type="arc" use="optional" order="14" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits" xlink:label="us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits" xlink:title="us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_StockIssuedDuringPeriodSharesReverseStockSplits" xlink:type="arc" use="optional" order="15" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:title="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:type="arc" use="optional" order="16" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesOutstanding" xlink:label="us-gaap_SharesOutstanding" xlink:title="us-gaap_SharesOutstanding" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharesOutstanding" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_SharesOutstanding" xlink:type="arc" use="optional" order="17" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:title="us-gaap_StatementTable" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:title="Definition : us-gaap_StatementLineItems to us-gaap_StatementTable" xlink:type="arc" order="18" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:title="us-gaap_StatementEquityComponentsAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:title="Definition : us-gaap_StatementTable to us-gaap_StatementEquityComponentsAxis" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember" xlink:label="us-gaap_PreferredStockMember" xlink:title="us-gaap_PreferredStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_PreferredStockMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_PreferredStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:title="us-gaap_CommonStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_CommonStockMember" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember" xlink:title="us-gaap_AdditionalPaidInCapitalMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AdditionalPaidInCapitalMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_AdditionalPaidInCapitalMember" xlink:type="arc" use="optional" order="2" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsUnappropriatedMember" xlink:label="us-gaap_RetainedEarningsUnappropriatedMember" xlink:title="us-gaap_RetainedEarningsUnappropriatedMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_RetainedEarningsUnappropriatedMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_RetainedEarningsUnappropriatedMember" xlink:type="arc" use="optional" order="3" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:title="us-gaap_AccumulatedOtherComprehensiveIncomeMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:type="arc" use="optional" order="4" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ParentMember" xlink:label="us-gaap_ParentMember" xlink:title="us-gaap_ParentMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_ParentMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_ParentMember" xlink:type="arc" use="optional" order="5" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncontrollingInterestMember" xlink:label="us-gaap_NoncontrollingInterestMember" xlink:title="us-gaap_NoncontrollingInterestMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_NoncontrollingInterestMember" xlink:title="Definition : us-gaap_EquityComponentDomain to us-gaap_NoncontrollingInterestMember" xlink:type="arc" use="optional" order="6" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain_1" xlink:title="us-gaap_EquityComponentDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain_1" xlink:title="Definition : us-gaap_StatementEquityComponentsAxis to us-gaap_EquityComponentDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis" xlink:title="us-gaap_StatementClassOfStockAxis" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementClassOfStockAxis" xlink:title="Definition : us-gaap_StatementTable to us-gaap_StatementClassOfStockAxis" xlink:type="arc" use="optional" order="1" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesAPreferredStockMember" xlink:label="us-gaap_SeriesAPreferredStockMember" xlink:title="us-gaap_SeriesAPreferredStockMember" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ClassOfStockDomain" xlink:to="us-gaap_SeriesAPreferredStockMember" xlink:title="Definition : us-gaap_ClassOfStockDomain to us-gaap_SeriesAPreferredStockMember" xlink:type="arc" use="optional" order="0" xbrldt:closed="false" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="us-gaap_ClassOfStockDomain_1" xlink:title="us-gaap_ClassOfStockDomain" />
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_ClassOfStockDomain_1" xlink:title="Definition : us-gaap_StatementClassOfStockAxis to us-gaap_ClassOfStockDomain" xlink:type="arc" order="1" xbrldt:closed="false" />
  </link:definitionLink>
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDBALANCESHEETSUnaudited" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSSUNAUDITED" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSSUNAUDITEDCalc2" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFSHAREHOLDERSEQUITYUNAUDITEDParentheticals" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFCASHFLOWSUNAUDITED" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/CONSOLIDATEDCONDENSEDSTATEMENTSOFCASHFLOWSUNAUDITEDParentheticals" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/NATUREOFOPERATIONS" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIES" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INVENTORY" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/PROPERTYANDEQUIPMENT" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INTANGIBLEASSETS" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/STOCKHOLDERSEQUITY" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DEBT" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DERIVATIVELIABILITIES" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INCOMETAXES" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SHAREBASEDCOMPENSATION" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/COMMITMENTSANDCONTINGENCIES" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUBSEQUENTEVENTS" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESPolicies" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INVENTORYTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/PROPERTYANDEQUIPMENTTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INTANGIBLEASSETSTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/STOCKHOLDERSEQUITYTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DEBTTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DERIVATIVELIABILITIESTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONTables" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INVENTORYScheduleofInventoryDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INVENTORYDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/PROPERTYANDEQUIPMENTDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INTANGIBLEASSETSDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INTANGIBLEASSETSScheduleofIntangibleAssetsDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/DEBTScheduleoftheCurrentFuturePaymentObligationsDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/INCOMETAXESDetails" />
  <link:definitionLink xlink:type="extended" xlink:role="http://60degreespharma.com/role/SHAREBASEDCOMPENSATIONScheduleofFairValueofOptionsGrantedDetails" />
</link:linkbase>
