<?xml version="1.0" encoding="US-ASCII" standalone="no"?>
    <!-- Field: Doc-Info; Name: Generator; Value: GoFiler Complete; Version: 6.4b -->
    <!-- Field: Doc-Info; Name: VendorURI; Value: https://www.novaworks.com -->
    <!-- Field: Doc-Info; Name: Status; Value: 0x00000000 -->
<link:linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" 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:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Cover" roleURI="http://xeneticbio.com/role/Cover" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#ConsolidatedBalanceSheets" roleURI="http://xeneticbio.com/role/ConsolidatedBalanceSheets" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#ConsolidatedBalanceSheetsParenthetical" roleURI="http://xeneticbio.com/role/ConsolidatedBalanceSheetsParenthetical" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#ConsolidatedStatementsOfOperations" roleURI="http://xeneticbio.com/role/ConsolidatedStatementsOfOperations" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#ConsolidatedStatementsOfStockholdersEquity" roleURI="http://xeneticbio.com/role/ConsolidatedStatementsOfStockholdersEquity" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#ConsolidatedStatementsOfCashFlows" roleURI="http://xeneticbio.com/role/ConsolidatedStatementsOfCashFlows" />
    <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/cyd/2025/cyd-2025.xsd#CybersecurityRiskManagementAndStrategyDisclosure" roleURI="http://xbrl.sec.gov/cyd/role/CybersecurityRiskManagementAndStrategyDisclosure" />
    <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/cyd/2025/cyd-2025.xsd#MaterialCybersecurityIncidentDisclosure" roleURI="http://xbrl.sec.gov/cyd/role/MaterialCybersecurityIncidentDisclosure" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Company" roleURI="http://xeneticbio.com/role/Company" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#RisksAndUncertainties" roleURI="http://xeneticbio.com/role/RisksAndUncertainties" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SummaryOfSignificantAccountingPolicies" roleURI="http://xeneticbio.com/role/SummaryOfSignificantAccountingPolicies" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SignificantStrategicCollaborations" roleURI="http://xeneticbio.com/role/SignificantStrategicCollaborations" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#AccruedExpensesAndOtherCurrentLiabilities" roleURI="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilities" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#FairValueMeasurements" roleURI="http://xeneticbio.com/role/FairValueMeasurements" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#IncomeTaxes" roleURI="http://xeneticbio.com/role/IncomeTaxes" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#StockholdersEquity" roleURI="http://xeneticbio.com/role/StockholdersEquity" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpense" roleURI="http://xeneticbio.com/role/Share-basedExpense" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#EmployeeBenefitPlans" roleURI="http://xeneticbio.com/role/EmployeeBenefitPlans" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#CommitmentsAndContingencies" roleURI="http://xeneticbio.com/role/CommitmentsAndContingencies" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#RelatedPartyTransactions" roleURI="http://xeneticbio.com/role/RelatedPartyTransactions" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SubsequentEvents" roleURI="http://xeneticbio.com/role/SubsequentEvents" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SummaryOfSignificantAccountingPoliciesPolicies" roleURI="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesPolicies" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SummaryOfSignificantAccountingPoliciesTables" roleURI="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesTables" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#AccruedExpensesAndOtherCurrentLiabilitiesTables" roleURI="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilitiesTables" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#IncomeTaxesTables" roleURI="http://xeneticbio.com/role/IncomeTaxesTables" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpenseTables" roleURI="http://xeneticbio.com/role/Share-basedExpenseTables" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SummaryOfSignificantAccountingPoliciesDetails-ConsolidatedNetLoss" roleURI="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesDetails-ConsolidatedNetLoss" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SummaryOfSignificantAccountingPoliciesDetailsNarrative" roleURI="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#SignificantStrategicCollaborationsDetailsNarrative" roleURI="http://xeneticbio.com/role/SignificantStrategicCollaborationsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#AccruedExpensesAndOtherCurrentLiabilitiesDetails-AccruedExpensesAndOtherCurrentLiabilities" roleURI="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilitiesDetails-AccruedExpensesAndOtherCurrentLiabilities" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#AccruedExpensesAndOtherCurrentLiabilitiesDetailsNarrative" roleURI="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilitiesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#IncomeTaxesDetails-IncomeByGeographicRegions" roleURI="http://xeneticbio.com/role/IncomeTaxesDetails-IncomeByGeographicRegions" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#IncomeTaxesDetails-IncomeTaxBenefit" roleURI="http://xeneticbio.com/role/IncomeTaxesDetails-IncomeTaxBenefit" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#IncomeTaxesDetails-DeferredTaxAssets" roleURI="http://xeneticbio.com/role/IncomeTaxesDetails-DeferredTaxAssets" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#IncomeTaxesDetailsNarrative" roleURI="http://xeneticbio.com/role/IncomeTaxesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#StockholdersEquityDetailsNarrative" roleURI="http://xeneticbio.com/role/StockholdersEquityDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpenseDetails-ShareBasedExpense" roleURI="http://xeneticbio.com/role/Share-basedExpenseDetails-ShareBasedExpense" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpenseDetails-AssumptionsEmployee" roleURI="http://xeneticbio.com/role/Share-basedExpenseDetails-AssumptionsEmployee" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpenseDetails-EmployeeStockOptionActivity" roleURI="http://xeneticbio.com/role/Share-basedExpenseDetails-EmployeeStockOptionActivity" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpenseDetails-Non-vestedEmployeeOptionActivity" roleURI="http://xeneticbio.com/role/Share-basedExpenseDetails-Non-vestedEmployeeOptionActivity" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpenseDetails-Non-employeeOptionActivity" roleURI="http://xeneticbio.com/role/Share-basedExpenseDetails-Non-employeeOptionActivity" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#Share-basedExpenseDetailsNarrative" roleURI="http://xeneticbio.com/role/Share-basedExpenseDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#EmployeeBenefitPlansDetailsNarrative" roleURI="http://xeneticbio.com/role/EmployeeBenefitPlansDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#CommitmentsAndContingenciesDetailsNarrative" roleURI="http://xeneticbio.com/role/CommitmentsAndContingenciesDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="xbio-20251231.xsd#RelatedPartyTransactionsDetailsNarrative" roleURI="http://xeneticbio.com/role/RelatedPartyTransactionsDetailsNarrative" />
    <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#PvpExecutiveCategoriesOnly" roleURI="http://xbrl.sec.gov/ecd/role/PvpExecutiveCategoriesOnly" />
    <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#PvpAdjustmentsOnly" roleURI="http://xbrl.sec.gov/ecd/role/PvpAdjustmentsOnly" />
    <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#IndividualsOnly" roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" />
    <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#AwardTypeOnly" roleURI="http://xbrl.sec.gov/ecd/AwardTypeOnly" />
    <link:roleRef xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#TradingArrangmentsOnly" roleURI="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" arcroleURI="http://xbrl.org/int/dim/arcrole/all" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#notAll" arcroleURI="http://xbrl.org/int/dim/arcrole/notAll" />
    <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Cover" xlink:title="00000001 - Document - Cover" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/ConsolidatedBalanceSheets" xlink:title="00000002 - Statement - CONSOLIDATED BALANCE SHEETS">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaapStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaapStatementTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStatementTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaapStatementClassOfStockAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapStatementTable" xlink:to="loc_us-gaapStatementClassOfStockAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaapClassOfStockDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapStatementClassOfStockAxis" xlink:to="loc_us-gaapClassOfStockDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaapClassOfStockDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapStatementClassOfStockAxis" xlink:to="loc_us-gaapClassOfStockDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="loc_us-gaapSeriesBPreferredStockMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockDomain" xlink:to="loc_us-gaapSeriesBPreferredStockMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract" xlink:label="loc_us-gaapAssetsAbstract_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapAssetsAbstract_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="loc_us-gaapAssetsCurrentAbstract_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapAssetsAbstract_50" xlink:to="loc_us-gaapAssetsCurrentAbstract_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents" xlink:label="loc_us-gaapCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapAssetsCurrentAbstract_50" xlink:to="loc_us-gaapCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_50" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="loc_us-gaapPrepaidExpenseAndOtherAssetsCurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapAssetsCurrentAbstract_50" xlink:to="loc_us-gaapPrepaidExpenseAndOtherAssetsCurrent_50" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent" xlink:label="loc_us-gaapAssetsCurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapAssetsCurrentAbstract_50" xlink:to="loc_us-gaapAssetsCurrent_50" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="loc_us-gaapOtherAssetsNoncurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapAssetsAbstract_50" xlink:to="loc_us-gaapOtherAssetsNoncurrent_50" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets" xlink:label="loc_us-gaapAssets_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapAssetsAbstract_50" xlink:to="loc_us-gaapAssets_50" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="loc_us-gaapLiabilitiesAndStockholdersEquityAbstract_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapLiabilitiesAndStockholdersEquityAbstract_50" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="loc_us-gaapLiabilitiesCurrentAbstract_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapLiabilitiesAndStockholdersEquityAbstract_50" xlink:to="loc_us-gaapLiabilitiesCurrentAbstract_50" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent" xlink:label="loc_us-gaapAccountsPayableCurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapLiabilitiesCurrentAbstract_50" xlink:to="loc_us-gaapAccountsPayableCurrent_50" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndOtherAccruedLiabilitiesCurrent" xlink:label="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapLiabilitiesCurrentAbstract_50" xlink:to="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_50" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent" xlink:label="loc_us-gaapLiabilitiesCurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapLiabilitiesCurrentAbstract_50" xlink:to="loc_us-gaapLiabilitiesCurrent_50" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities" xlink:label="loc_us-gaapLiabilities_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapLiabilitiesAndStockholdersEquityAbstract_50" xlink:to="loc_us-gaapLiabilities_50" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="loc_us-gaapCommitmentsAndContingencies_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapLiabilitiesAndStockholdersEquityAbstract_50" xlink:to="loc_us-gaapCommitmentsAndContingencies_50" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="loc_us-gaapStockholdersEquityAbstract_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapLiabilitiesAndStockholdersEquityAbstract_50" xlink:to="loc_us-gaapStockholdersEquityAbstract_50" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue" xlink:label="loc_us-gaapPreferredStockValue_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapPreferredStockValue_50" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValue" xlink:label="loc_us-gaapCommonStockValue_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapCommonStockValue_50" xlink:type="arc" order="16" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="loc_us-gaapAdditionalPaidInCapital_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapAdditionalPaidInCapital_50" xlink:type="arc" order="17" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="loc_us-gaapRetainedEarningsAccumulatedDeficit_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapRetainedEarningsAccumulatedDeficit_50" xlink:type="arc" order="18" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaapAccumulatedOtherComprehensiveIncomeLossNetOfTax_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapAccumulatedOtherComprehensiveIncomeLossNetOfTax_50" xlink:type="arc" order="19" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonValue" xlink:label="loc_us-gaapTreasuryStockCommonValue_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapTreasuryStockCommonValue_50" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaapStockholdersEquity_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStockholdersEquity_50" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="loc_us-gaapLiabilitiesAndStockholdersEquity_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapLiabilitiesAndStockholdersEquity_50" xlink:type="arc" order="22" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:title="00000003 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaapStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaapStatementTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStatementTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaapStatementClassOfStockAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapStatementTable" xlink:to="loc_us-gaapStatementClassOfStockAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaapClassOfStockDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapStatementClassOfStockAxis" xlink:to="loc_us-gaapClassOfStockDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaapClassOfStockDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapStatementClassOfStockAxis" xlink:to="loc_us-gaapClassOfStockDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="loc_us-gaapSeriesBPreferredStockMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockDomain" xlink:to="loc_us-gaapSeriesBPreferredStockMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaapPreferredStockSharesAuthorized_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapPreferredStockSharesAuthorized_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="loc_us-gaapPreferredStockParOrStatedValuePerShare_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapPreferredStockParOrStatedValuePerShare_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="loc_us-gaapPreferredStockSharesIssued_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapPreferredStockSharesIssued_50" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="loc_us-gaapPreferredStockSharesOutstanding_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapPreferredStockSharesOutstanding_50" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaapCommonStockParOrStatedValuePerShare_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapCommonStockParOrStatedValuePerShare_50" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaapCommonStockSharesAuthorized_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapCommonStockSharesAuthorized_50" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued" xlink:label="loc_us-gaapCommonStockSharesIssued_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapCommonStockSharesIssued_50" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaapCommonStockSharesOutstanding_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapCommonStockSharesOutstanding_50" xlink:type="arc" order="7" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/ConsolidatedStatementsOfOperations" xlink:title="00000004 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/ConsolidatedStatementsOfStockholdersEquity" xlink:title="00000005 - Statement - CONSOLIDATED STATEMENTS OF STOCKHOLDERS&apos; EQUITY">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaapStatementLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaapStatementTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStatementTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaapStatementEquityComponentsAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapStatementTable" xlink:to="loc_us-gaapStatementEquityComponentsAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaapEquityComponentDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapStatementEquityComponentsAxis" xlink:to="loc_us-gaapEquityComponentDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaapEquityComponentDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapStatementEquityComponentsAxis" xlink:to="loc_us-gaapEquityComponentDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockMember" xlink:label="loc_us-gaapPreferredStockMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEquityComponentDomain" xlink:to="loc_us-gaapPreferredStockMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaapCommonStockMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEquityComponentDomain" xlink:to="loc_us-gaapCommonStockMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaapAdditionalPaidInCapitalMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEquityComponentDomain" xlink:to="loc_us-gaapAdditionalPaidInCapitalMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaapRetainedEarningsMember_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEquityComponentDomain" xlink:to="loc_us-gaapRetainedEarningsMember_30" xlink:type="arc" order="31" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaapAccumulatedOtherComprehensiveIncomeMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEquityComponentDomain" xlink:to="loc_us-gaapAccumulatedOtherComprehensiveIncomeMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonMember" xlink:label="loc_us-gaapTreasuryStockCommonMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEquityComponentDomain" xlink:to="loc_us-gaapTreasuryStockCommonMember_50" xlink:type="arc" order="51" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaapStockholdersEquity_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStockholdersEquity_80" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesOutstanding" xlink:label="loc_us-gaapSharesOutstanding_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapSharesOutstanding_80" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="loc_us-gaapStockIssuedDuringPeriodValueNewIssues_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStockIssuedDuringPeriodValueNewIssues_80" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueConversionOfConvertibleSecurities" xlink:label="loc_us-gaapStockIssuedDuringPeriodValueConversionOfConvertibleSecurities_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStockIssuedDuringPeriodValueConversionOfConvertibleSecurities_80" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockSharesIssued1" xlink:label="loc_us-gaapConversionOfStockSharesIssued1_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapConversionOfStockSharesIssued1_80" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockSharesConverted1" xlink:label="loc_us-gaapConversionOfStockSharesConverted1_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapConversionOfStockSharesConverted1_80" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaapStockIssuedDuringPeriodSharesNewIssues_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStockIssuedDuringPeriodSharesNewIssues_80" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:label="loc_us-gaapStockIssuedDuringPeriodValueEmployeeStockPurchasePlan_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStockIssuedDuringPeriodValueEmployeeStockPurchasePlan_80" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="loc_us-gaapStockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapStockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_80" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_ExerciseOfPurchaseWarrantsValue" xlink:label="loc_xbioExerciseOfPurchaseWarrantsValue_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_xbioExerciseOfPurchaseWarrantsValue_80" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_ExerciseOfPurchaseWarrantsShares" xlink:label="loc_xbioExerciseOfPurchaseWarrantsShares_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_xbioExerciseOfPurchaseWarrantsShares_80" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="loc_us-gaapAdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapAdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_80" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaapNetIncomeLoss_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapStatementLineItems" xlink:to="loc_us-gaapNetIncomeLoss_80" xlink:type="arc" order="13" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/ConsolidatedStatementsOfCashFlows" xlink:title="00000006 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cyd/role/CybersecurityRiskManagementAndStrategyDisclosure" xlink:title="00000012 - Disclosure - Cybersecurity Risk Management and Strategy Disclosure" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/cyd/role/MaterialCybersecurityIncidentDisclosure" xlink:title="00000013 - Disclosure - Material Cybersecurity Incident Disclosure" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Company" xlink:title="999014 - Disclosure - The Company" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/RisksAndUncertainties" xlink:title="999015 - Disclosure - Risks and Uncertainties" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SummaryOfSignificantAccountingPolicies" xlink:title="999016 - Disclosure - Summary of Significant Accounting Policies" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SignificantStrategicCollaborations" xlink:title="999017 - Disclosure - Significant Strategic Collaborations" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilities" xlink:title="999018 - Disclosure - Accrued Expenses and other current liabilities" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/FairValueMeasurements" xlink:title="999019 - Disclosure - Fair Value Measurements" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/IncomeTaxes" xlink:title="999020 - Disclosure - Income Taxes" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/StockholdersEquity" xlink:title="999021 - Disclosure - Stockholders&#8217; Equity" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpense" xlink:title="999022 - Disclosure - Share-Based Expense" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/EmployeeBenefitPlans" xlink:title="999023 - Disclosure - Employee Benefit Plans" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/CommitmentsAndContingencies" xlink:title="999024 - Disclosure - Commitments and Contingencies" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/RelatedPartyTransactions" xlink:title="999025 - Disclosure - Related Party Transactions" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SubsequentEvents" xlink:title="999026 - Disclosure - Subsequent Events" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:title="999027 - Disclosure - Summary of Significant Accounting Policies (Policies)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:title="999028 - Disclosure - Summary of Significant Accounting Policies (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilitiesTables" xlink:title="999029 - Disclosure - Accrued Expenses and other current liabilities (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/IncomeTaxesTables" xlink:title="999030 - Disclosure - Income Taxes (Tables)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpenseTables" xlink:title="999031 - Disclosure - Share-Based Expense (Tables)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaapAwardTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="loc_us-gaapAwardTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaapEmployeeStockOptionMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaapEmployeeStockOptionMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_NonEmployeeStockOptionsMember" xlink:label="loc_xbioNonEmployeeStockOptionsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioNonEmployeeStockOptionsMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCompensationCostForShareBasedPaymentArrangementsAllocationOfShareBasedCompensationCostsByPlanTableTextBlock" xlink:label="loc_us-gaapScheduleOfCompensationCostForShareBasedPaymentArrangementsAllocationOfShareBasedCompensationCostsByPlanTableTextBlock_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfCompensationCostForShareBasedPaymentArrangementsAllocationOfShareBasedCompensationCostsByPlanTableTextBlock_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:label="loc_us-gaapScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock_40" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:label="loc_us-gaapScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock_40" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfNonvestedShareActivityTableTextBlock" xlink:label="loc_us-gaapScheduleOfNonvestedShareActivityTableTextBlock_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfNonvestedShareActivityTableTextBlock_40" xlink:type="arc" order="3" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesDetails-ConsolidatedNetLoss" xlink:title="999032 - Disclosure - Summary of Significant Accounting Policies (Details - Consolidated net loss)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SummaryOfSignificantAccountingPoliciesDetailsNarrative" xlink:title="999033 - Disclosure - Summary of Significant Accounting Policies (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureLineElements" xlink:label="loc_xbioDisclosureLineElements" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureTable" xlink:label="loc_xbioDisclosureTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_xbioDisclosureTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaapBalanceSheetLocationAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_xbioDisclosureTable" xlink:to="loc_us-gaapBalanceSheetLocationAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaapBalanceSheetLocationDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapBalanceSheetLocationAxis" xlink:to="loc_us-gaapBalanceSheetLocationDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaapBalanceSheetLocationDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapBalanceSheetLocationAxis" xlink:to="loc_us-gaapBalanceSheetLocationDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_ResearchAndDevelopmentExpensesMember" xlink:label="loc_xbioResearchAndDevelopmentExpensesMember_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapBalanceSheetLocationDomain" xlink:to="loc_xbioResearchAndDevelopmentExpensesMember_10" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetImpairmentCharges" xlink:label="loc_us-gaapAssetImpairmentCharges_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapAssetImpairmentCharges_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssets" xlink:label="loc_us-gaapPrepaidExpenseAndOtherAssets_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapPrepaidExpenseAndOtherAssets_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndOtherAccruedLiabilitiesCurrent" xlink:label="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_50" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="loc_us-gaapAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_50" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfOperatingSegments" xlink:label="loc_us-gaapNumberOfOperatingSegments_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapNumberOfOperatingSegments_50" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments" xlink:label="loc_us-gaapNumberOfReportableSegments_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapNumberOfReportableSegments_50" xlink:type="arc" order="5" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/SignificantStrategicCollaborationsDetailsNarrative" xlink:title="999034 - Disclosure - Significant Strategic Collaborations (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:label="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:label="loc_us-gaapScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis" xlink:label="loc_srtCounterpartyNameAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="loc_srtCounterpartyNameAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_srtRepurchaseAgreementCounterpartyNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srtCounterpartyNameAxis" xlink:to="loc_srtRepurchaseAgreementCounterpartyNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain" xlink:label="loc_srtRepurchaseAgreementCounterpartyNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_srtCounterpartyNameAxis" xlink:to="loc_srtRepurchaseAgreementCounterpartyNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_TakedaMember" xlink:label="loc_xbioTakedaMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtRepurchaseAgreementCounterpartyNameDomain" xlink:to="loc_xbioTakedaMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_CatalentPharmaSolutionsMember" xlink:label="loc_xbioCatalentPharmaSolutionsMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtRepurchaseAgreementCounterpartyNameDomain" xlink:to="loc_xbioCatalentPharmaSolutionsMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_ScrippsResearchMember" xlink:label="loc_xbioScrippsResearchMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtRepurchaseAgreementCounterpartyNameDomain" xlink:to="loc_xbioScrippsResearchMember_50" xlink:type="arc" order="51" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_UniversityOfVirginiaMember" xlink:label="loc_xbioUniversityOfVirginiaMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtRepurchaseAgreementCounterpartyNameDomain" xlink:to="loc_xbioUniversityOfVirginiaMember_120" xlink:type="arc" order="121" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srtProductOrServiceAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="loc_srtProductOrServiceAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srtProductsAndServicesDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srtProductOrServiceAxis" xlink:to="loc_srtProductsAndServicesDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srtProductsAndServicesDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_srtProductOrServiceAxis" xlink:to="loc_srtProductsAndServicesDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_RoyaltyRevenueMember" xlink:label="loc_xbioRoyaltyRevenueMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtProductsAndServicesDomain" xlink:to="loc_xbioRoyaltyRevenueMember_0" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransactionTypeAxis" xlink:label="loc_us-gaapTransactionTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="loc_us-gaapTransactionTypeAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransactionDomain" xlink:label="loc_us-gaapTransactionDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapTransactionTypeAxis" xlink:to="loc_us-gaapTransactionDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransactionDomain" xlink:label="loc_us-gaapTransactionDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapTransactionTypeAxis" xlink:to="loc_us-gaapTransactionDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DNaseTechnologyMember" xlink:label="loc_xbioDNaseTechnologyMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapTransactionDomain" xlink:to="loc_xbioDNaseTechnologyMember_50" xlink:type="arc" order="52" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_ScrippsAgreementMember" xlink:label="loc_xbioScrippsAgreementMember_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapTransactionDomain" xlink:to="loc_xbioScrippsAgreementMember_100" xlink:type="arc" order="102" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_UVAAgreementMember" xlink:label="loc_xbioUVAAgreementMember_120" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapTransactionDomain" xlink:to="loc_xbioUVAAgreementMember_120" xlink:type="arc" order="122" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaapSubsequentEventTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="loc_us-gaapSubsequentEventTypeAxis" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaapSubsequentEventTypeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapSubsequentEventTypeAxis" xlink:to="loc_us-gaapSubsequentEventTypeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaapSubsequentEventTypeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapSubsequentEventTypeAxis" xlink:to="loc_us-gaapSubsequentEventTypeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaapSubsequentEventMember_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapSubsequentEventTypeDomain" xlink:to="loc_us-gaapSubsequentEventMember_90" xlink:type="arc" order="93" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:label="loc_srtScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable" xlink:to="loc_srtScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="loc_srtEquityMethodInvesteeNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srtScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="loc_srtEquityMethodInvesteeNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain" xlink:label="loc_srtEquityMethodInvesteeNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_srtScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="loc_srtEquityMethodInvesteeNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_PharmsynthezMember" xlink:label="loc_xbioPharmsynthezMember_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtEquityMethodInvesteeNameDomain" xlink:to="loc_xbioPharmsynthezMember_150" xlink:type="arc" order="151" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RoyaltyIncomeNonoperating" xlink:label="loc_us-gaapRoyaltyIncomeNonoperating_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapRoyaltyIncomeNonoperating_180" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="loc_us-gaapResearchAndDevelopmentExpense_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapResearchAndDevelopmentExpense_180" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="loc_us-gaapPrepaidExpenseAndOtherAssetsCurrent_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapPrepaidExpenseAndOtherAssetsCurrent_180" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndOtherAccruedLiabilitiesCurrent" xlink:label="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_180" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssets" xlink:label="loc_us-gaapOtherAssets_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapOtherAssets_180" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractualObligation" xlink:label="loc_us-gaapContractualObligation_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapContractualObligation_180" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_ContractualObligationAddition" xlink:label="loc_xbioContractualObligationAddition_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_xbioContractualObligationAddition_180" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage" xlink:label="loc_us-gaapEquityMethodInvestmentOwnershipPercentage_180" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems" xlink:to="loc_us-gaapEquityMethodInvestmentOwnershipPercentage_180" xlink:type="arc" order="7" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilitiesDetails-AccruedExpensesAndOtherCurrentLiabilities" xlink:title="999035 - Disclosure - Accrued Expenses and other current liabilities (Details - Accrued expenses and other current liabilities)" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/AccruedExpensesAndOtherCurrentLiabilitiesDetailsNarrative" xlink:title="999036 - Disclosure - Accrued Expenses and other current liabilities (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OffsettingAssetsLineItems" xlink:label="loc_us-gaapOffsettingAssetsLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OffsettingAssetsTable" xlink:label="loc_us-gaapOffsettingAssetsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapOffsettingAssetsLineItems" xlink:to="loc_us-gaapOffsettingAssetsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransactionTypeAxis" xlink:label="loc_us-gaapTransactionTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapOffsettingAssetsTable" xlink:to="loc_us-gaapTransactionTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransactionDomain" xlink:label="loc_us-gaapTransactionDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapTransactionTypeAxis" xlink:to="loc_us-gaapTransactionDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransactionDomain" xlink:label="loc_us-gaapTransactionDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapTransactionTypeAxis" xlink:to="loc_us-gaapTransactionDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_SeparationAgreementsMember" xlink:label="loc_xbioSeparationAgreementsMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapTransactionDomain" xlink:to="loc_xbioSeparationAgreementsMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LaborAndRelatedExpense" xlink:label="loc_us-gaapLaborAndRelatedExpense_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapOffsettingAssetsLineItems" xlink:to="loc_us-gaapLaborAndRelatedExpense_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation" xlink:label="loc_us-gaapShareBasedCompensation_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapOffsettingAssetsLineItems" xlink:to="loc_us-gaapShareBasedCompensation_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableAndOtherAccruedLiabilitiesCurrent" xlink:label="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapOffsettingAssetsLineItems" xlink:to="loc_us-gaapAccountsPayableAndOtherAccruedLiabilitiesCurrent_50" xlink:type="arc" order="2" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/IncomeTaxesDetails-IncomeByGeographicRegions" xlink:title="999037 - Disclosure - Income Taxes (Details - Income by geographic regions)">
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureLineElements" xlink:label="loc_xbioDisclosureLineElements" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureTable" xlink:label="loc_xbioDisclosureTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_xbioDisclosureTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srtStatementGeographicalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_xbioDisclosureTable" xlink:to="loc_srtStatementGeographicalAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srtSegmentGeographicalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srtStatementGeographicalAxis" xlink:to="loc_srtSegmentGeographicalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srtSegmentGeographicalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_srtStatementGeographicalAxis" xlink:to="loc_srtSegmentGeographicalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US" xlink:label="loc_countryUS_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtSegmentGeographicalDomain" xlink:to="loc_countryUS_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB" xlink:label="loc_countryGB_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtSegmentGeographicalDomain" xlink:to="loc_countryGB_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_DE" xlink:label="loc_countryDE_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtSegmentGeographicalDomain" xlink:to="loc_countryDE_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CH" xlink:label="loc_countryCH_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtSegmentGeographicalDomain" xlink:to="loc_countryCH_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaapNetIncomeLoss_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapNetIncomeLoss_110" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/IncomeTaxesDetails-IncomeTaxBenefit" xlink:title="999038 - Disclosure - Income Taxes (Details - Income tax benefit)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationLineItems" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaapIncomeTaxAuthorityAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationTable" xlink:to="loc_us-gaapIncomeTaxAuthorityAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaapIncomeTaxAuthorityDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapIncomeTaxAuthorityAxis" xlink:to="loc_us-gaapIncomeTaxAuthorityDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaapIncomeTaxAuthorityDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapIncomeTaxAuthorityAxis" xlink:to="loc_us-gaapIncomeTaxAuthorityDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember" xlink:label="loc_us-gaapDomesticCountryMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeTaxAuthorityDomain" xlink:to="loc_us-gaapDomesticCountryMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCountryMember" xlink:label="loc_us-gaapForeignCountryMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeTaxAuthorityDomain" xlink:to="loc_us-gaapForeignCountryMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:label="loc_us-gaapIncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate_70" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_70" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxCredits" xlink:label="loc_us-gaapIncomeTaxReconciliationTaxCredits_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationTaxCredits_70" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxCredits" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationTaxCredits_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationTaxCredits_70" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationOtherReconcilingItems" xlink:label="loc_us-gaapIncomeTaxReconciliationOtherReconcilingItems_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationOtherReconcilingItems_70" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent_70" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="loc_us-gaapIncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_70" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_70" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost" xlink:label="loc_us-gaapIncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost_70" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitPercent" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitPercent_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationShareBasedCompensationExcessTaxBenefitPercent_70" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationOtherAdjustments" xlink:label="loc_us-gaapIncomeTaxReconciliationOtherAdjustments_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationOtherAdjustments_70" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationOtherAdjustments_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationOtherAdjustments_70" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential" xlink:label="loc_us-gaapIncomeTaxReconciliationForeignIncomeTaxRateDifferential_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationForeignIncomeTaxRateDifferential_70" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential_70" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_IncomeTaxReconciliationOthersReconcilingItems" xlink:label="loc_xbioIncomeTaxReconciliationOthersReconcilingItems_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_xbioIncomeTaxReconciliationOthersReconcilingItems_70" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_EffectiveIncomeTaxRateReconciliationAdjustments" xlink:label="loc_xbioEffectiveIncomeTaxRateReconciliationAdjustments_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_xbioEffectiveIncomeTaxRateReconciliationAdjustments_70" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxSettlementsOther" xlink:label="loc_us-gaapIncomeTaxReconciliationTaxSettlementsOther_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxReconciliationTaxSettlementsOther_70" xlink:type="arc" order="16" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxSettlements" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationTaxSettlements_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationTaxSettlements_70" xlink:type="arc" order="17" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaapIncomeTaxExpenseBenefit_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxExpenseBenefit_70" xlink:type="arc" order="18" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_70" xlink:type="arc" order="19" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/IncomeTaxesDetails-DeferredTaxAssets" xlink:title="999039 - Disclosure - Income Taxes (Details - Deferred Tax Assets)">
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureLineElements" xlink:label="loc_xbioDisclosureLineElements" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureTable" xlink:label="loc_xbioDisclosureTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_xbioDisclosureTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srtStatementGeographicalAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_xbioDisclosureTable" xlink:to="loc_srtStatementGeographicalAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srtSegmentGeographicalDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srtStatementGeographicalAxis" xlink:to="loc_srtSegmentGeographicalDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srtSegmentGeographicalDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_srtStatementGeographicalAxis" xlink:to="loc_srtSegmentGeographicalDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB" xlink:label="loc_countryGB_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtSegmentGeographicalDomain" xlink:to="loc_countryGB_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CH" xlink:label="loc_countryCH_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtSegmentGeographicalDomain" xlink:to="loc_countryCH_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_DE" xlink:label="loc_countryDE_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srtSegmentGeographicalDomain" xlink:to="loc_countryDE_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsForeign" xlink:label="loc_us-gaapDeferredTaxAssetsOperatingLossCarryforwardsForeign_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsOperatingLossCarryforwardsForeign_90" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsCapitalLossCarryforwards" xlink:label="loc_us-gaapDeferredTaxAssetsCapitalLossCarryforwards_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsCapitalLossCarryforwards_90" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic" xlink:label="loc_us-gaapDeferredTaxAssetsOperatingLossCarryforwardsDomestic_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsOperatingLossCarryforwardsDomestic_90" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsInProcessResearchAndDevelopment" xlink:label="loc_us-gaapDeferredTaxAssetsInProcessResearchAndDevelopment_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsInProcessResearchAndDevelopment_90" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost" xlink:label="loc_us-gaapDeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost_90" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch" xlink:label="loc_us-gaapDeferredTaxAssetsTaxCreditCarryforwardsResearch_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsTaxCreditCarryforwardsResearch_90" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseOther" xlink:label="loc_us-gaapDeferredTaxAssetsTaxDeferredExpenseOther_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsTaxDeferredExpenseOther_90" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal" xlink:label="loc_us-gaapDeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsOperatingLossCarryforwardsStateAndLocal_90" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment" xlink:label="loc_us-gaapDeferredTaxAssetsPropertyPlantAndEquipment_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsPropertyPlantAndEquipment_90" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsGross" xlink:label="loc_us-gaapDeferredTaxAssetsGross_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsGross_90" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsValuationAllowance" xlink:label="loc_us-gaapDeferredTaxAssetsValuationAllowance_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsValuationAllowance_90" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsNet" xlink:label="loc_us-gaapDeferredTaxAssetsNet_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxAssetsNet_90" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilities" xlink:label="loc_us-gaapDeferredIncomeTaxLiabilities_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredIncomeTaxLiabilities_90" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxLiabilities" xlink:label="loc_us-gaapDeferredTaxLiabilities_90" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapDeferredTaxLiabilities_90" xlink:type="arc" order="13" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/IncomeTaxesDetailsNarrative" xlink:title="999040 - Disclosure - Income Taxes (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationLineItems" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable" xlink:label="loc_us-gaapEffectiveIncomeTaxRateReconciliationTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapEffectiveIncomeTaxRateReconciliationTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="loc_us-gaapIncomeTaxAuthorityAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationTable" xlink:to="loc_us-gaapIncomeTaxAuthorityAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaapIncomeTaxAuthorityDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapIncomeTaxAuthorityAxis" xlink:to="loc_us-gaapIncomeTaxAuthorityDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="loc_us-gaapIncomeTaxAuthorityDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapIncomeTaxAuthorityAxis" xlink:to="loc_us-gaapIncomeTaxAuthorityDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB" xlink:label="loc_countryGB_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeTaxAuthorityDomain" xlink:to="loc_countryGB_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US" xlink:label="loc_countryUS_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeTaxAuthorityDomain" xlink:to="loc_countryUS_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember" xlink:label="loc_us-gaapStateAndLocalJurisdictionMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeTaxAuthorityDomain" xlink:to="loc_us-gaapStateAndLocalJurisdictionMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_DE" xlink:label="loc_countryDE_60" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeTaxAuthorityDomain" xlink:to="loc_countryDE_60" xlink:type="arc" order="61" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CH" xlink:label="loc_countryCH_80" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeTaxAuthorityDomain" xlink:to="loc_countryCH_80" xlink:type="arc" order="81" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwards" xlink:label="loc_us-gaapOperatingLossCarryforwards_140" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapOperatingLossCarryforwards_140" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_OperatingLossCarryforwardsExpirationDate1" xlink:label="loc_xbioOperatingLossCarryforwardsExpirationDate1_140" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_xbioOperatingLossCarryforwardsExpirationDate1_140" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilityForUncertainTaxPositionsCurrent" xlink:label="loc_us-gaapLiabilityForUncertainTaxPositionsCurrent_140" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapLiabilityForUncertainTaxPositionsCurrent_140" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefits" xlink:label="loc_us-gaapUnrecognizedTaxBenefits_140" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapUnrecognizedTaxBenefits_140" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExaminationPenaltiesAndInterestAccrued" xlink:label="loc_us-gaapIncomeTaxExaminationPenaltiesAndInterestAccrued_140" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEffectiveIncomeTaxRateReconciliationLineItems" xlink:to="loc_us-gaapIncomeTaxExaminationPenaltiesAndInterestAccrued_140" xlink:type="arc" order="4" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/StockholdersEquityDetailsNarrative" xlink:title="999041 - Disclosure - Stockholders&#8217; Equity (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockLineItems" xlink:label="loc_us-gaapClassOfStockLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfStockByClassTable" xlink:label="loc_us-gaapScheduleOfStockByClassTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapScheduleOfStockByClassTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesFinancingTransactionAxis" xlink:label="loc_us-gaapSecuritiesFinancingTransactionAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfStockByClassTable" xlink:to="loc_us-gaapSecuritiesFinancingTransactionAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesFinancingTransactionTypeDomain" xlink:label="loc_us-gaapSecuritiesFinancingTransactionTypeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapSecuritiesFinancingTransactionAxis" xlink:to="loc_us-gaapSecuritiesFinancingTransactionTypeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuritiesFinancingTransactionTypeDomain" xlink:label="loc_us-gaapSecuritiesFinancingTransactionTypeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapSecuritiesFinancingTransactionAxis" xlink:to="loc_us-gaapSecuritiesFinancingTransactionTypeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_CanaccordUnderwritingAgreementMember" xlink:label="loc_xbioCanaccordUnderwritingAgreementMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapSecuritiesFinancingTransactionTypeDomain" xlink:to="loc_xbioCanaccordUnderwritingAgreementMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="loc_us-gaapStatementClassOfStockAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfStockByClassTable" xlink:to="loc_us-gaapStatementClassOfStockAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaapClassOfStockDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapStatementClassOfStockAxis" xlink:to="loc_us-gaapClassOfStockDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain" xlink:label="loc_us-gaapClassOfStockDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapStatementClassOfStockAxis" xlink:to="loc_us-gaapClassOfStockDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesBPreferredStockMember" xlink:label="loc_us-gaapSeriesBPreferredStockMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockDomain" xlink:to="loc_us-gaapSeriesBPreferredStockMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaapCommonStockMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockDomain" xlink:to="loc_us-gaapCommonStockMember_50" xlink:type="arc" order="51" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_SeriesAWarrantsMember" xlink:label="loc_xbioSeriesAWarrantsMember_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockDomain" xlink:to="loc_xbioSeriesAWarrantsMember_70" xlink:type="arc" order="71" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaapAwardTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfStockByClassTable" xlink:to="loc_us-gaapAwardTypeAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_OtherWarrantsMember" xlink:label="loc_xbioOtherWarrantsMember_110" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioOtherWarrantsMember_110" xlink:type="arc" order="111" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_PubliclyTradedWarrantsMember" xlink:label="loc_xbioPubliclyTradedWarrantsMember_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioPubliclyTradedWarrantsMember_150" xlink:type="arc" order="151" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="loc_us-gaapStockIssuedDuringPeriodSharesNewIssues_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapStockIssuedDuringPeriodSharesNewIssues_190" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharePrice" xlink:label="loc_us-gaapSharePrice_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapSharePrice_190" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaapProceedsFromIssuanceOfCommonStock_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapProceedsFromIssuanceOfCommonStock_190" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfStockIssuanceCosts" xlink:label="loc_us-gaapPaymentsOfStockIssuanceCosts_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapPaymentsOfStockIssuanceCosts_190" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="loc_us-gaapPreferredStockSharesAuthorized_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapPreferredStockSharesAuthorized_190" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="loc_us-gaapPreferredStockSharesOutstanding_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapPreferredStockSharesOutstanding_190" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued" xlink:label="loc_us-gaapPreferredStockSharesIssued_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapPreferredStockSharesIssued_190" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:label="loc_us-gaapConvertiblePreferredStockSharesIssuedUponConversion_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapConvertiblePreferredStockSharesIssuedUponConversion_190" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockSharesConverted1" xlink:label="loc_us-gaapConversionOfStockSharesConverted1_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapConversionOfStockSharesConverted1_190" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConversionOfStockSharesIssued1" xlink:label="loc_us-gaapConversionOfStockSharesIssued1_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapConversionOfStockSharesIssued1_190" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightOutstanding" xlink:label="loc_us-gaapClassOfWarrantOrRightOutstanding_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapClassOfWarrantOrRightOutstanding_190" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_WarrantsExpiredDate" xlink:label="loc_xbioWarrantsExpiredDate_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_xbioWarrantsExpiredDate_190" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_190" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:label="loc_us-gaapClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_190" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WarrantsAndRightsOutstandingMaturityDate" xlink:label="loc_us-gaapWarrantsAndRightsOutstandingMaturityDate_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_us-gaapWarrantsAndRightsOutstandingMaturityDate_190" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_WarrantsForfeitedShares" xlink:label="loc_xbioWarrantsForfeitedShares_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_xbioWarrantsForfeitedShares_190" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_WarrantsExercisedShares" xlink:label="loc_xbioWarrantsExercisedShares_190" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapClassOfStockLineItems" xlink:to="loc_xbioWarrantsExercisedShares_190" xlink:type="arc" order="16" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpenseDetails-ShareBasedExpense" xlink:title="999042 - Disclosure - Share-Based Expense (Details - Share based expense)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaapEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaapScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="loc_us-gaapScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaapIncomeStatementLocationAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:to="loc_us-gaapIncomeStatementLocationAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaapIncomeStatementLocationDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapIncomeStatementLocationAxis" xlink:to="loc_us-gaapIncomeStatementLocationDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaapIncomeStatementLocationDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapIncomeStatementLocationAxis" xlink:to="loc_us-gaapIncomeStatementLocationDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaapResearchAndDevelopmentExpenseMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeStatementLocationDomain" xlink:to="loc_us-gaapResearchAndDevelopmentExpenseMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaapGeneralAndAdministrativeExpenseMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapIncomeStatementLocationDomain" xlink:to="loc_us-gaapGeneralAndAdministrativeExpenseMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation" xlink:label="loc_us-gaapShareBasedCompensation_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:to="loc_us-gaapShareBasedCompensation_70" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpenseDetails-AssumptionsEmployee" xlink:title="999043 - Disclosure - Share-Based Expense (Details - Assumptions Employee)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaapAwardTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="loc_us-gaapAwardTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaapEmployeeStockOptionMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaapEmployeeStockOptionMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_50" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_50" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_50" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_50" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExercisePrice_50" xlink:type="arc" order="4" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpenseDetails-EmployeeStockOptionActivity" xlink:title="999044 - Disclosure - Share-Based Expense (Details - Employee stock option activity)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaapAwardTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="loc_us-gaapAwardTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaapEmployeeStockOptionMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaapEmployeeStockOptionMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_70" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_70" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_70" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_70" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_70" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_70" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_70" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExpirationsInPeriodWeightedAverageExercisePrice_70" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_70" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_70" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageRemainingContractualTerm1_70" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_70" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_70" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_70" xlink:type="arc" order="16" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaapSharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2_70" xlink:type="arc" order="17" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_70" xlink:type="arc" order="18" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpenseDetails-Non-vestedEmployeeOptionActivity" xlink:title="999045 - Disclosure - Share-Based Expense (Details - Non-Vested employee Option activity)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaapAwardTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="loc_us-gaapAwardTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_NonVestedEmployeeStockOptionMember" xlink:label="loc_xbioNonVestedEmployeeStockOptionMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioNonVestedEmployeeStockOptionMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares_40" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue_40" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_40" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_40" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_40" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue_40" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_40" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue_40" xlink:type="arc" order="7" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpenseDetails-Non-employeeOptionActivity" xlink:title="999046 - Disclosure - Share-Based Expense (Details - Non-employee option activity)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaapAwardTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="loc_us-gaapAwardTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_NonEmployeeStockOptionsMember" xlink:label="loc_xbioNonEmployeeStockOptionsMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioNonEmployeeStockOptionsMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_70" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_70" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_70" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_70" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_70" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_70" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExpirationsInPeriod_70" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_70" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_70" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_70" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_70" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_70" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_70" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaapSharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2_70" xlink:type="arc" order="16" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_70" xlink:type="arc" order="17" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/Share-basedExpenseDetailsNarrative" xlink:title="999047 - Disclosure - Share-Based Expense (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaapAwardTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:to="loc_us-gaapAwardTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapAwardTypeAxis" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaapEmployeeStockOptionMember_20" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaapEmployeeStockOptionMember_20" xlink:type="arc" order="21" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaapRestrictedStockUnitsRSUMember_50" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_us-gaapRestrictedStockUnitsRSUMember_50" xlink:type="arc" order="51" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_NonEmployeeStockOptionsMember" xlink:label="loc_xbioNonEmployeeStockOptionsMember_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioNonEmployeeStockOptionsMember_70" xlink:type="arc" order="71" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_JSOPMember" xlink:label="loc_xbioJSOPMember_100" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioJSOPMember_100" xlink:type="arc" order="101" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaapAllocatedShareBasedCompensationExpense_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapAllocatedShareBasedCompensationExpense_150" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_150" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_150" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaapStockIssuedDuringPeriodSharesStockOptionsExercised_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapStockIssuedDuringPeriodSharesStockOptionsExercised_150" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_150" xlink:type="arc" order="4" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_150" xlink:type="arc" order="5" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares_150" xlink:type="arc" order="6" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:label="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapSharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_150" xlink:type="arc" order="7" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaapEmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapEmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_150" xlink:type="arc" order="8" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaapEmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapEmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_150" xlink:type="arc" order="9" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_150" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_150" xlink:type="arc" order="11" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExpirations" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExpirations_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExpirations_150" xlink:type="arc" order="12" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted_150" xlink:type="arc" order="13" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_150" xlink:type="arc" order="14" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber" xlink:label="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber_150" xlink:type="arc" order="15" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation" xlink:label="loc_us-gaapShareBasedCompensation_150" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:to="loc_us-gaapShareBasedCompensation_150" xlink:type="arc" order="16" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/EmployeeBenefitPlansDetailsNarrative" xlink:title="999048 - Disclosure - Employee Benefit Plans (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaapDefinedBenefitPlanDisclosureLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="loc_us-gaapScheduleOfDefinedBenefitPlansDisclosuresTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapDefinedBenefitPlanDisclosureLineItems" xlink:to="loc_us-gaapScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanNameAxis" xlink:label="loc_us-gaapRetirementPlanNameAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:to="loc_us-gaapRetirementPlanNameAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanNameDomain" xlink:label="loc_us-gaapRetirementPlanNameDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapRetirementPlanNameAxis" xlink:to="loc_us-gaapRetirementPlanNameDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetirementPlanNameDomain" xlink:label="loc_us-gaapRetirementPlanNameDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapRetirementPlanNameAxis" xlink:to="loc_us-gaapRetirementPlanNameDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_Plan401KMember" xlink:label="loc_xbioPlan401KMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapRetirementPlanNameDomain" xlink:to="loc_xbioPlan401KMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DefinedBenefitPlanContributionsByEmployer" xlink:label="loc_us-gaapDefinedBenefitPlanContributionsByEmployer_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapDefinedBenefitPlanDisclosureLineItems" xlink:to="loc_us-gaapDefinedBenefitPlanContributionsByEmployer_30" xlink:type="arc" order="0" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/CommitmentsAndContingenciesDetailsNarrative" xlink:title="999049 - Disclosure - Commitments and Contingencies (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureLineElements" xlink:label="loc_xbioDisclosureLineElements" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DisclosureTable" xlink:label="loc_xbioDisclosureTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_xbioDisclosureTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis" xlink:label="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_xbioDisclosureTable" xlink:to="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain" xlink:label="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseAxis" xlink:to="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertySubjectToOrAvailableForOperatingLeaseDomain" xlink:label="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseAxis" xlink:to="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_OfficeSpaceMember" xlink:label="loc_xbioOfficeSpaceMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapPropertySubjectToOrAvailableForOperatingLeaseDomain" xlink:to="loc_xbioOfficeSpaceMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseTermOfContract" xlink:label="loc_us-gaapLesseeOperatingLeaseTermOfContract_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapLesseeOperatingLeaseTermOfContract_30" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue" xlink:label="loc_us-gaapLesseeOperatingLeaseLiabilityPaymentsDue_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapLesseeOperatingLeaseLiabilityPaymentsDue_30" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:label="loc_us-gaapLettersOfCreditOutstandingAmount_30" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_xbioDisclosureLineElements" xlink:to="loc_us-gaapLettersOfCreditOutstandingAmount_30" xlink:type="arc" order="2" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xeneticbio.com/role/RelatedPartyTransactionsDetailsNarrative" xlink:title="999050 - Disclosure - Related Party Transactions (Details Narrative)">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="loc_us-gaapRelatedPartyTransactionLineItems" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="loc_us-gaapScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xbrldt:contextElement="segment" xlink:from="loc_us-gaapRelatedPartyTransactionLineItems" xlink:to="loc_us-gaapScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="arc" order="10" xbrldt:closed="true" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:label="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DNaseITechnologyMember" xlink:label="loc_xbioDNaseITechnologyMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="loc_xbioDNaseITechnologyMember_0" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionAxis" xlink:label="loc_us-gaapRelatedPartyTransactionAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="loc_us-gaapRelatedPartyTransactionAxis" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="loc_us-gaapRelatedPartyTransactionDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapRelatedPartyTransactionAxis" xlink:to="loc_us-gaapRelatedPartyTransactionDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionDomain" xlink:label="loc_us-gaapRelatedPartyTransactionDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapRelatedPartyTransactionAxis" xlink:to="loc_us-gaapRelatedPartyTransactionDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_ClinicalTrialServicesMember" xlink:label="loc_xbioClinicalTrialServicesMember_0" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapRelatedPartyTransactionDomain" xlink:to="loc_xbioClinicalTrialServicesMember_0" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="loc_us-gaapRelatedPartyTransactionsByRelatedPartyAxis" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaapScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="loc_us-gaapRelatedPartyTransactionsByRelatedPartyAxis" xlink:type="arc" order="3" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaapRelatedPartyDomain" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaapRelatedPartyTransactionsByRelatedPartyAxis" xlink:to="loc_us-gaapRelatedPartyDomain" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaapRelatedPartyDomain_10" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xbrldt:contextElement="segment" xlink:from="loc_us-gaapRelatedPartyTransactionsByRelatedPartyAxis" xlink:to="loc_us-gaapRelatedPartyDomain_10" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_DrGenkinMember" xlink:label="loc_xbioDrGenkinMember_40" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapRelatedPartyDomain" xlink:to="loc_xbioDrGenkinMember_40" xlink:type="arc" order="41" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="loc_us-gaapResearchAndDevelopmentExpense_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapRelatedPartyTransactionLineItems" xlink:to="loc_us-gaapResearchAndDevelopmentExpense_70" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="loc_us-gaapPrepaidExpenseAndOtherAssetsCurrent_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapRelatedPartyTransactionLineItems" xlink:to="loc_us-gaapPrepaidExpenseAndOtherAssetsCurrent_70" xlink:type="arc" order="1" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent" xlink:label="loc_us-gaapAccountsPayableCurrent_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapRelatedPartyTransactionLineItems" xlink:to="loc_us-gaapAccountsPayableCurrent_70" xlink:type="arc" order="2" />
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProfessionalAndContractServicesExpense" xlink:label="loc_us-gaapProfessionalAndContractServicesExpense_70" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapRelatedPartyTransactionLineItems" xlink:to="loc_us-gaapProfessionalAndContractServicesExpense_70" xlink:type="arc" order="3" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/ecd/role/PvpExecutiveCategoriesOnly" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/ecd/role/PvpAdjustmentsOnly" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" />
    <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/ecd/AwardTypeOnly">
      <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_OtherWarrantsMember" xlink:label="loc_xbioOtherWarrantsMember" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioOtherWarrantsMember" xlink:type="arc" order="0" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_PubliclyTradedWarrantsMember" xlink:label="loc_xbioPubliclyTradedWarrantsMember" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioPubliclyTradedWarrantsMember" xlink:type="arc" order="10" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_NonEmployeeStockOptionsMember" xlink:label="loc_xbioNonEmployeeStockOptionsMember" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioNonEmployeeStockOptionsMember" xlink:type="arc" order="20" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_JSOPMember" xlink:label="loc_xbioJSOPMember" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioJSOPMember" xlink:type="arc" order="30" />
      <link:loc xlink:type="locator" xlink:href="xbio-20251231.xsd#xbio_NonVestedEmployeeStockOptionMember" xlink:label="loc_xbioNonVestedEmployeeStockOptionMember" />
      <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaapShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="loc_xbioNonVestedEmployeeStockOptionMember" xlink:type="arc" order="40" />
    </link:definitionLink>
    <link:definitionLink xlink:type="extended" xlink:role="http://xbrl.sec.gov/ecd/role/TradingArrangementsOnly" />
</link:linkbase>
