<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019e1d03-69ae-7b32-8e8c-95ca6fbb1d65,g:91e273d3-32c1-4f8c-bcdf-e00ac9ca2858-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.summitplc.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="smmt-20260630.xsd#CondensedConsolidatedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019e1d03-6a7b-7148-bdef-c047e24733e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019e1d03-6a7b-709b-a6a6-8826bbacae34" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019e1d03-6a7b-7148-bdef-c047e24733e7" xlink:to="loc_us-gaap_StatementTable_019e1d03-6a7b-709b-a6a6-8826bbacae34" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019e1d03-6a7b-7e75-a987-7d94b651dc87" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019e1d03-6a7b-709b-a6a6-8826bbacae34" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019e1d03-6a7b-7e75-a987-7d94b651dc87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-7e75-a987-7d94b651dc87_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e1d03-6a7b-7e75-a987-7d94b651dc87" xlink:to="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-7e75-a987-7d94b651dc87_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-709a-b728-ef04afab70c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019e1d03-6a7b-7e75-a987-7d94b651dc87" xlink:to="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-709a-b728-ef04afab70c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019e1d03-6a7b-7dc3-b28e-4a8a263cd18e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-709a-b728-ef04afab70c4" xlink:to="loc_us-gaap_CommonStockMember_019e1d03-6a7b-7dc3-b28e-4a8a263cd18e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019e1d03-6a7b-7427-8773-fe5248b77549" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-709a-b728-ef04afab70c4" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019e1d03-6a7b-7427-8773-fe5248b77549" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e1d03-6a7b-7a95-9a63-ba9d34dc52ab" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-709a-b728-ef04afab70c4" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019e1d03-6a7b-7a95-9a63-ba9d34dc52ab" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019e1d03-6a7b-7154-856b-a78b1a68aa2b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019e1d03-6a7b-709a-b728-ef04afab70c4" xlink:to="loc_us-gaap_RetainedEarningsMember_019e1d03-6a7b-7154-856b-a78b1a68aa2b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019e1d03-6a7b-7148-bdef-c047e24733e7" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e1d03-6a7b-7397-8e27-7ea114479045" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019e1d03-6a7b-7397-8e27-7ea114479045" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e1d03-6a7b-7fb1-a43b-9058b24d7a69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_StockholdersEquity_019e1d03-6a7b-7fb1-a43b-9058b24d7a69" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e1d03-6a7b-7c77-a3f5-bc03a2fa8348" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019e1d03-6a7b-7c77-a3f5-bc03a2fa8348" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e1d03-6a7b-7b54-b282-feb42adc46c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019e1d03-6a7b-7b54-b282-feb42adc46c7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019f5b4e-c199-7f74-b26e-647ac3737289" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019f5b4e-c199-7f74-b26e-647ac3737289" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019f5b4e-84c7-7d8e-8918-3b2c93a7e567" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_019f5b4e-84c7-7d8e-8918-3b2c93a7e567" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019f5ac7-a278-76db-a497-db93f94fb452" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesNewIssues_019f5ac7-a278-76db-a497-db93f94fb452" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019f5ac9-5b4c-7b9c-bc01-9b62c336611a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueNewIssues_019f5ac9-5b4c-7b9c-bc01-9b62c336611a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e1d03-6a7b-7255-96c9-993c4f8b493f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019e1d03-6a7b-7255-96c9-993c4f8b493f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e1d03-6a7b-7185-ac4f-1ac4cba4a3b4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_019e1d03-6a7b-7185-ac4f-1ac4cba4a3b4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e1d03-6a7b-720c-9441-16480a6984a4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019e1d03-6a7b-7ade-8529-bf857efb6a0a" xlink:to="loc_us-gaap_NetIncomeLoss_019e1d03-6a7b-720c-9441-16480a6984a4" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019e1d03-6a7b-7dbe-a94f-7a9ae1fb7fa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019e1d03-6a7b-78c0-a737-b6eee823a8c1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/SegmentReportingScheduleofSignificantExpensesoftheCODMFunctionDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#SegmentReportingScheduleofSignificantExpensesoftheCODMFunctionDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/SegmentReportingScheduleofSignificantExpensesoftheCODMFunctionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e1d03-6a7b-78a4-9d86-55e9c72c822d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e1d03-6a7b-78a4-9d86-55e9c72c822d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019e1d03-6a7b-7f65-b9cb-2c10033b6005" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_019e1d03-6a7b-78a4-9d86-55e9c72c822d" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019e1d03-6a7b-7f65-b9cb-2c10033b6005" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e1d03-6a7b-7f65-b9cb-2c10033b6005_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e1d03-6a7b-7f65-b9cb-2c10033b6005" xlink:to="loc_us-gaap_SegmentDomain_019e1d03-6a7b-7f65-b9cb-2c10033b6005_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019e1d03-6a7b-7eb9-8a63-2c251a17315b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019e1d03-6a7b-7f65-b9cb-2c10033b6005" xlink:to="loc_us-gaap_SegmentDomain_019e1d03-6a7b-7eb9-8a63-2c251a17315b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ReportableSegmentMember_019e1d03-6a7b-73d3-88e6-4c93b7979638" xlink:href="smmt-20260630.xsd#smmt_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019e1d03-6a7b-7eb9-8a63-2c251a17315b" xlink:to="loc_smmt_ReportableSegmentMember_019e1d03-6a7b-73d3-88e6-4c93b7979638" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019e1d03-6a7b-7528-834f-1e9d439e4875" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019e1d03-6a7b-7528-834f-1e9d439e4875" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_LaborAndRelatedExpensesExcludingStockBasedCompensation_019e1d03-6a7b-72c1-a4b7-ba509f2ecf7c" xlink:href="smmt-20260630.xsd#smmt_LaborAndRelatedExpensesExcludingStockBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_smmt_LaborAndRelatedExpensesExcludingStockBasedCompensation_019e1d03-6a7b-72c1-a4b7-ba509f2ecf7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e1d03-6a7b-7300-82ce-fb179b8d4b6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e1d03-6a7b-7300-82ce-fb179b8d4b6f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_019e1d03-6a7b-7071-982b-aca672a746f0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_019e1d03-6a7b-7071-982b-aca672a746f0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_019e1d03-6a7b-76f7-a125-9157da767617" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_us-gaap_OperatingExpenses_019e1d03-6a7b-76f7-a125-9157da767617" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonoperatingIncomeExpense_019e1d03-6a7b-74a1-bac9-489f06102521" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_us-gaap_NonoperatingIncomeExpense_019e1d03-6a7b-74a1-bac9-489f06102521" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019e1d03-6a7b-7576-ad4c-c656dafb2336" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_019e1d03-6a7b-7f76-ae09-5990f3b54fc5" xlink:to="loc_us-gaap_NetIncomeLoss_019e1d03-6a7b-7576-ad4c-c656dafb2336" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/AkesoLicenseandCollaborationAgreementDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#AkesoLicenseandCollaborationAgreementDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/AkesoLicenseandCollaborationAgreementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_019e1d03-6a7b-7819-9b7c-a9bf5580b5e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_019e1d03-6a7b-7819-9b7c-a9bf5580b5e7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_019e1d03-6a7b-7105-9883-d17bddd0c5db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_019e1d03-6a7b-7819-9b7c-a9bf5580b5e7" xlink:to="loc_us-gaap_TypeOfArrangementAxis_019e1d03-6a7b-7105-9883-d17bddd0c5db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019e1d03-6a7b-7105-9883-d17bddd0c5db_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019e1d03-6a7b-7105-9883-d17bddd0c5db" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019e1d03-6a7b-7105-9883-d17bddd0c5db_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019e1d03-6a7b-7de0-8fa1-a2b473220e00" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019e1d03-6a7b-7105-9883-d17bddd0c5db" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019e1d03-6a7b-7de0-8fa1-a2b473220e00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_019e1d03-6a7b-7240-9737-60a1db45ad7a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019e1d03-6a7b-7de0-8fa1-a2b473220e00" xlink:to="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_019e1d03-6a7b-7240-9737-60a1db45ad7a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementUpfrontPaymentOneMember_019e1d03-6a7b-7dab-a187-fcc01f1df325" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementUpfrontPaymentOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_019e1d03-6a7b-7240-9737-60a1db45ad7a" xlink:to="loc_smmt_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementUpfrontPaymentOneMember_019e1d03-6a7b-7dab-a187-fcc01f1df325" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementUpfrontPaymentTwoMember_019e1d03-6a7b-7462-b3b9-79187a472e86" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementUpfrontPaymentTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementMember_019e1d03-6a7b-7240-9737-60a1db45ad7a" xlink:to="loc_smmt_CollaborativeArrangementTransactionWithPartyToCollaborativeArrangementUpfrontPaymentTwoMember_019e1d03-6a7b-7462-b3b9-79187a472e86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementRightsAndObligationsUpfrontPayment_019e1d03-6a7b-7580-9d8a-9273bd86779e" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementRightsAndObligationsUpfrontPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CollaborativeArrangementRightsAndObligationsUpfrontPayment_019e1d03-6a7b-7580-9d8a-9273bd86779e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CashPaymentForCollaborativeArrangementUpfrontPayment_019e1d03-6a7b-72a2-bd99-433379e9991f" xlink:href="smmt-20260630.xsd#smmt_CashPaymentForCollaborativeArrangementUpfrontPayment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CashPaymentForCollaborativeArrangementUpfrontPayment_019e1d03-6a7b-72a2-bd99-433379e9991f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementRightsAndObligationsUpfrontPaymentSharesOfCommonStockIssuedInLieuOfCash_019e1d03-6a7b-70b0-94cc-7e3b767adbf0" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementRightsAndObligationsUpfrontPaymentSharesOfCommonStockIssuedInLieuOfCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CollaborativeArrangementRightsAndObligationsUpfrontPaymentSharesOfCommonStockIssuedInLieuOfCash_019e1d03-6a7b-70b0-94cc-7e3b767adbf0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e1d03-6a7b-7ca2-b328-a2a59b7d71bd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e1d03-6a7b-7ca2-b328-a2a59b7d71bd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementRightsAndObligationsUpfrontPaymentPaidInShares_019e1d03-6a7b-705d-93a1-7f2e7fb6ff11" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementRightsAndObligationsUpfrontPaymentPaidInShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CollaborativeArrangementRightsAndObligationsUpfrontPaymentPaidInShares_019e1d03-6a7b-705d-93a1-7f2e7fb6ff11" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_PaymentsForCollaborativeArrangement_019e1d03-6a7b-7a26-acd6-9fed3a20c955" xlink:href="smmt-20260630.xsd#smmt_PaymentsForCollaborativeArrangement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_PaymentsForCollaborativeArrangement_019e1d03-6a7b-7a26-acd6-9fed3a20c955" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementRightsAndObligationsAdditionalPotentialCommercialMilestonePayments_019e1d03-6a7b-76c1-9ce5-2a1fb51f86fd" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementRightsAndObligationsAdditionalPotentialCommercialMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CollaborativeArrangementRightsAndObligationsAdditionalPotentialCommercialMilestonePayments_019e1d03-6a7b-76c1-9ce5-2a1fb51f86fd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementRightsAndObligationsMaximumAggregateMilestonePayments_019e1d03-6a7b-7e57-a93e-60472dd6c497" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementRightsAndObligationsMaximumAggregateMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CollaborativeArrangementRightsAndObligationsMaximumAggregateMilestonePayments_019e1d03-6a7b-7e57-a93e-60472dd6c497" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementRightsAndObligationsPotentialRegulatoryMilestonePayments_019e1d03-6a7b-73a7-950b-9f50029cf038" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementRightsAndObligationsPotentialRegulatoryMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CollaborativeArrangementRightsAndObligationsPotentialRegulatoryMilestonePayments_019e1d03-6a7b-73a7-950b-9f50029cf038" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CollaborativeArrangementRightsAndObligationsPotentialCommercialMilestonePayments_019e1d03-6a7b-7119-b4cd-c32f54906f98" xlink:href="smmt-20260630.xsd#smmt_CollaborativeArrangementRightsAndObligationsPotentialCommercialMilestonePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_019e1d03-6a7b-7457-bab1-8ee40483412e" xlink:to="loc_smmt_CollaborativeArrangementRightsAndObligationsPotentialCommercialMilestonePayments_019e1d03-6a7b-7119-b4cd-c32f54906f98" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/OtherIncomeNetDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#OtherIncomeNetDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/OtherIncomeNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_smmt_OtherIncomeNetLineItems_019f856c-c95f-7fa8-9e33-4e262d79d8b9" xlink:href="smmt-20260630.xsd#smmt_OtherIncomeNetLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_OtherIncomeNetTable_019f856c-c95f-798a-9fe1-ec740872738a" xlink:href="smmt-20260630.xsd#smmt_OtherIncomeNetTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_smmt_OtherIncomeNetLineItems_019f856c-c95f-7fa8-9e33-4e262d79d8b9" xlink:to="loc_smmt_OtherIncomeNetTable_019f856c-c95f-798a-9fe1-ec740872738a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019f856c-c95f-7bb3-904c-5bc10ad33d30" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_smmt_OtherIncomeNetTable_019f856c-c95f-798a-9fe1-ec740872738a" xlink:to="loc_dei_LegalEntityAxis_019f856c-c95f-7bb3-904c-5bc10ad33d30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f856c-c95f-7bb3-904c-5bc10ad33d30_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019f856c-c95f-7bb3-904c-5bc10ad33d30" xlink:to="loc_dei_EntityDomain_019f856c-c95f-7bb3-904c-5bc10ad33d30_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019f856d-50de-7a4c-8891-2b3fce0e2bd7" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019f856c-c95f-7bb3-904c-5bc10ad33d30" xlink:to="loc_dei_EntityDomain_019f856d-50de-7a4c-8891-2b3fce0e2bd7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ThirdPartyMember_019f856d-50de-7ffa-b6bb-bd0da770088d" xlink:href="smmt-20260630.xsd#smmt_ThirdPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019f856d-50de-7a4c-8891-2b3fce0e2bd7" xlink:to="loc_smmt_ThirdPartyMember_019f856d-50de-7ffa-b6bb-bd0da770088d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SaleOfIntangibleAssetContingentConsiderationRegulatoryMilestonesLiability_019f5c25-08b8-7e7c-b2c0-2343c4376195" xlink:href="smmt-20260630.xsd#smmt_SaleOfIntangibleAssetContingentConsiderationRegulatoryMilestonesLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_smmt_OtherIncomeNetLineItems_019f856c-c95f-7fa8-9e33-4e262d79d8b9" xlink:to="loc_smmt_SaleOfIntangibleAssetContingentConsiderationRegulatoryMilestonesLiability_019f5c25-08b8-7e7c-b2c0-2343c4376195" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SaleOfIntangibleAssetContingentConsiderationSalesMilestonesLiability_019f5c25-d5d9-7d26-bcb0-4e2b7d66a483" xlink:href="smmt-20260630.xsd#smmt_SaleOfIntangibleAssetContingentConsiderationSalesMilestonesLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_smmt_OtherIncomeNetLineItems_019f856c-c95f-7fa8-9e33-4e262d79d8b9" xlink:to="loc_smmt_SaleOfIntangibleAssetContingentConsiderationSalesMilestonesLiability_019f5c25-d5d9-7d26-bcb0-4e2b7d66a483" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/NetLossperShareScheduleofPotentiallyDilutiveSecuritiesExcludedFromtheComputationofLossPerShareDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#NetLossperShareScheduleofPotentiallyDilutiveSecuritiesExcludedFromtheComputationofLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/NetLossperShareScheduleofPotentiallyDilutiveSecuritiesExcludedFromtheComputationofLossPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e1d03-6a7b-7340-acb9-4c92accb7d72" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e1d03-6a7b-7c44-b4c6-0dcf2e6af2eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e1d03-6a7b-7340-acb9-4c92accb7d72" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e1d03-6a7b-7c44-b4c6-0dcf2e6af2eb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e1d03-6a7b-7b0f-ac8b-18f96314834f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019e1d03-6a7b-7c44-b4c6-0dcf2e6af2eb" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e1d03-6a7b-7b0f-ac8b-18f96314834f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e1d03-6a7b-7b0f-ac8b-18f96314834f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e1d03-6a7b-7b0f-ac8b-18f96314834f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e1d03-6a7b-7b0f-ac8b-18f96314834f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e1d03-6a7b-7c65-8d47-c7c991051b76" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019e1d03-6a7b-7b0f-ac8b-18f96314834f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e1d03-6a7b-7c65-8d47-c7c991051b76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e1d03-6a7b-7d7f-b0e0-89baf36d9204" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e1d03-6a7b-7c65-8d47-c7c991051b76" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e1d03-6a7b-7d7f-b0e0-89baf36d9204" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-733a-a31c-4dd9b115fc89" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e1d03-6a7b-7c65-8d47-c7c991051b76" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-733a-a31c-4dd9b115fc89" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e1d03-6a7b-7865-bee0-2a9f62ac81fa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019e1d03-6a7b-7c65-8d47-c7c991051b76" xlink:to="loc_us-gaap_EmployeeStockMember_019e1d03-6a7b-7865-bee0-2a9f62ac81fa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e1d03-6a7b-7ff1-b5fa-fb6fdc12a068" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019e1d03-6a7b-7340-acb9-4c92accb7d72" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019e1d03-6a7b-7ff1-b5fa-fb6fdc12a068" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/FairValueMeasurementsandShortTermInvestmentsScheduleofAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#FairValueMeasurementsandShortTermInvestmentsScheduleofAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/FairValueMeasurementsandShortTermInvestmentsScheduleofAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e1d03-6a7b-744d-bcf3-ce87750bb04f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e1d03-6a7b-7f39-a0fb-533aa634a237" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e1d03-6a7b-744d-bcf3-ce87750bb04f" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e1d03-6a7b-7f39-a0fb-533aa634a237" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e1d03-6a7b-769d-884a-b5e12af744da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e1d03-6a7b-7f39-a0fb-533aa634a237" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e1d03-6a7b-769d-884a-b5e12af744da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e1d03-6a7b-769d-884a-b5e12af744da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e1d03-6a7b-769d-884a-b5e12af744da" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e1d03-6a7b-769d-884a-b5e12af744da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e1d03-6a7b-7307-a897-3a47e6bcd902" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019e1d03-6a7b-769d-884a-b5e12af744da" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e1d03-6a7b-7307-a897-3a47e6bcd902" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019e1d03-6a7b-77ec-976c-b782dfcb699c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e1d03-6a7b-7307-a897-3a47e6bcd902" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019e1d03-6a7b-77ec-976c-b782dfcb699c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019e1d03-6a7b-7b5d-8318-2b9e1cd2c926" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019e1d03-6a7b-7307-a897-3a47e6bcd902" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019e1d03-6a7b-7b5d-8318-2b9e1cd2c926" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019e1d03-6a7b-76aa-b1a4-060e35b707d1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e1d03-6a7b-7f39-a0fb-533aa634a237" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019e1d03-6a7b-76aa-b1a4-060e35b707d1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e1d03-6a7b-76aa-b1a4-060e35b707d1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e1d03-6a7b-76aa-b1a4-060e35b707d1" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e1d03-6a7b-76aa-b1a4-060e35b707d1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e1d03-6a7b-77e0-88ca-247fc3966839" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019e1d03-6a7b-76aa-b1a4-060e35b707d1" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e1d03-6a7b-77e0-88ca-247fc3966839" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019e1d03-6a7b-7cce-9c6c-7b9a643b506b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e1d03-6a7b-77e0-88ca-247fc3966839" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019e1d03-6a7b-7cce-9c6c-7b9a643b506b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e1d03-6a7b-790e-bfd8-574256347d5d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019e1d03-6a7b-77e0-88ca-247fc3966839" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e1d03-6a7b-790e-bfd8-574256347d5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019e1d03-6a7b-72e3-9f79-9f331b749108" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019e1d03-6a7b-7f39-a0fb-533aa634a237" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019e1d03-6a7b-72e3-9f79-9f331b749108" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e1d03-6a7b-72e3-9f79-9f331b749108_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e1d03-6a7b-72e3-9f79-9f331b749108" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e1d03-6a7b-72e3-9f79-9f331b749108_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e1d03-6a7b-74ca-a79f-c0661ae31bdc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019e1d03-6a7b-72e3-9f79-9f331b749108" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e1d03-6a7b-74ca-a79f-c0661ae31bdc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasurySecuritiesMember_019e1d03-6a7b-7c8c-b7f5-e4c9278f4a50" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_USTreasurySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e1d03-6a7b-74ca-a79f-c0661ae31bdc" xlink:to="loc_us-gaap_USTreasurySecuritiesMember_019e1d03-6a7b-7c8c-b7f5-e4c9278f4a50" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019e1d03-6a7b-7525-85f0-d806dee47a27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019e1d03-6a7b-74ca-a79f-c0661ae31bdc" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019e1d03-6a7b-7525-85f0-d806dee47a27" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss_019f6736-721b-7c27-a9c9-b5ced225425b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e1d03-6a7b-744d-bcf3-ce87750bb04f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss_019f6736-721b-7c27-a9c9-b5ced225425b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e1d03-6a7b-73bc-bc42-74887e2d4805" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e1d03-6a7b-744d-bcf3-ce87750bb04f" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019e1d03-6a7b-73bc-bc42-74887e2d4805" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e1d03-6a7b-7737-9359-ec5f99e70560" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e1d03-6a7b-744d-bcf3-ce87750bb04f" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019e1d03-6a7b-7737-9359-ec5f99e70560" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest_019e1d03-6a7b-7847-b72a-1b3fe14f7f04" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e1d03-6a7b-744d-bcf3-ce87750bb04f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest_019e1d03-6a7b-7847-b72a-1b3fe14f7f04" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019e1d03-6a7b-70ec-8a61-17ebc38c999d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019e1d03-6a7b-744d-bcf3-ce87750bb04f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019e1d03-6a7b-70ec-8a61-17ebc38c999d" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/StockholdersEquityDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#StockholdersEquityDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/StockholdersEquityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CapitalUnitLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfCapitalUnitsTable_019e1d03-6a7b-76f1-bb0b-d33252b1bbd4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfCapitalUnitsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_ScheduleOfCapitalUnitsTable_019e1d03-6a7b-76f1-bb0b-d33252b1bbd4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-70d9-9e48-18533d409758" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCapitalUnitsTable_019e1d03-6a7b-76f1-bb0b-d33252b1bbd4" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-70d9-9e48-18533d409758" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-70d9-9e48-18533d409758_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-70d9-9e48-18533d409758" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-70d9-9e48-18533d409758_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-7c24-a9df-35826da089ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-70d9-9e48-18533d409758" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-7c24-a9df-35826da089ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019e1d03-6a7b-70ea-8d8d-632f6fdf7220" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-7c24-a9df-35826da089ae" xlink:to="loc_us-gaap_PrivatePlacementMember_019e1d03-6a7b-70ea-8d8d-632f6fdf7220" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_OriginalDistributionAgreementMember_019e1d03-6a7b-728b-be46-320ab2b43a71" xlink:href="smmt-20260630.xsd#smmt_OriginalDistributionAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-7c24-a9df-35826da089ae" xlink:to="loc_smmt_OriginalDistributionAgreementMember_019e1d03-6a7b-728b-be46-320ab2b43a71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_DistributionAgreementMember_019e1d03-6a7b-7fd3-ad01-306ab51fcc13" xlink:href="smmt-20260630.xsd#smmt_DistributionAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-7c24-a9df-35826da089ae" xlink:to="loc_smmt_DistributionAgreementMember_019e1d03-6a7b-7fd3-ad01-306ab51fcc13" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7bd9-aba2-f140d91bee48" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfCapitalUnitsTable_019e1d03-6a7b-76f1-bb0b-d33252b1bbd4" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7bd9-aba2-f140d91bee48" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7bd9-aba2-f140d91bee48_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7bd9-aba2-f140d91bee48" xlink:to="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7bd9-aba2-f140d91bee48_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-74f3-bf80-c747c33e9fe3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7bd9-aba2-f140d91bee48" xlink:to="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-74f3-bf80-c747c33e9fe3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AffiliatedEntityMember_019e1d03-6a7b-7949-afe4-1ba193c23b10" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_AffiliatedEntityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-74f3-bf80-c747c33e9fe3" xlink:to="loc_srt_AffiliatedEntityMember_019e1d03-6a7b-7949-afe4-1ba193c23b10" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019e1d03-6a7b-7830-bd4b-72ec98139b7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-74f3-bf80-c747c33e9fe3" xlink:to="loc_us-gaap_RelatedPartyMember_019e1d03-6a7b-7830-bd4b-72ec98139b7b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonrelatedPartyMember_019e1d03-6a7b-7641-9434-1f66b83131a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NonrelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-74f3-bf80-c747c33e9fe3" xlink:to="loc_us-gaap_NonrelatedPartyMember_019e1d03-6a7b-7641-9434-1f66b83131a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019e1d03-6a7b-717f-8935-b4c35937a3a5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019e1d03-6a7b-717f-8935-b4c35937a3a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019e1d03-6a7b-7ab6-9f04-67734b5f28db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019e1d03-6a7b-7ab6-9f04-67734b5f28db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_019e1d03-6a7b-7641-9973-a8877670221d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_PreferredStockSharesIssued_019e1d03-6a7b-7641-9973-a8877670221d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019e1d03-6a7b-7853-8bfb-b76a36a2bf9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019e1d03-6a7b-7853-8bfb-b76a36a2bf9e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019e1d03-6a7b-768b-a003-593c7cc29d9e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019e1d03-6a7b-768b-a003-593c7cc29d9e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019e1d03-6a7b-7e9e-ac2e-47917d4c9010" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019e1d03-6a7b-7e9e-ac2e-47917d4c9010" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e1d03-6a7b-7de8-80a8-e6be19048913" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019e1d03-6a7b-7de8-80a8-e6be19048913" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019e1d03-6a7b-7e21-8cb4-0859d69445e4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019e1d03-6a7b-7e21-8cb4-0859d69445e4" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019e1d03-6a7b-7c59-a83e-8456be48b65b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_019e1d03-6a7b-7c59-a83e-8456be48b65b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019e1d03-6a7b-7750-b936-a6bbf1ce9448" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019e1d03-6a7b-7750-b936-a6bbf1ce9448" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SaleOfStockAggregateOfferingPrice_019e1d03-6a7b-7b50-a127-79d8a36f8067" xlink:href="smmt-20260630.xsd#smmt_SaleOfStockAggregateOfferingPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_smmt_SaleOfStockAggregateOfferingPrice_019e1d03-6a7b-7b50-a127-79d8a36f8067" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfStockIssuanceCosts_019e1d03-6a7b-75b1-b6f2-c46046cf6d93" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfStockIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_PaymentsOfStockIssuanceCosts_019e1d03-6a7b-75b1-b6f2-c46046cf6d93" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SaleOfStockAmountRemainingUnderTheOffering_019e1d03-6a7b-78f7-8436-62acf86215b2" xlink:href="smmt-20260630.xsd#smmt_SaleOfStockAmountRemainingUnderTheOffering"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_smmt_SaleOfStockAmountRemainingUnderTheOffering_019e1d03-6a7b-78f7-8436-62acf86215b2" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019e1d03-6a7b-7ae1-8839-8ce8b73e4381" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_us-gaap_ClassOfWarrantOrRightOutstanding_019e1d03-6a7b-7ae1-8839-8ce8b73e4381" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ClassOfWarrantOrRightExercisesInPeriod_019e1d03-6a7b-7291-a9e9-437e07070b1c" xlink:href="smmt-20260630.xsd#smmt_ClassOfWarrantOrRightExercisesInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_smmt_ClassOfWarrantOrRightExercisesInPeriod_019e1d03-6a7b-7291-a9e9-437e07070b1c" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ClassOfWarrantOrRightWeightedAverageExercisePriceExercisesInPeriod_019e1d03-6a7b-7e2e-91cb-2363d845beaf" xlink:href="smmt-20260630.xsd#smmt_ClassOfWarrantOrRightWeightedAverageExercisePriceExercisesInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CapitalUnitLineItems_019e1d03-6a7b-7c33-9b33-c60ddc5765c9" xlink:to="loc_smmt_ClassOfWarrantOrRightWeightedAverageExercisePriceExercisesInPeriod_019e1d03-6a7b-7e2e-91cb-2363d845beaf" xlink:type="arc" order="15"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7334-bad0-494a275ac9c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7334-bad0-494a275ac9c3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019e1d03-6a7b-77c9-b66f-6570848c2fca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7334-bad0-494a275ac9c3" xlink:to="loc_us-gaap_PlanNameAxis_019e1d03-6a7b-77c9-b66f-6570848c2fca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e1d03-6a7b-77c9-b66f-6570848c2fca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019e1d03-6a7b-77c9-b66f-6570848c2fca" xlink:to="loc_us-gaap_PlanNameDomain_019e1d03-6a7b-77c9-b66f-6570848c2fca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019e1d03-6a7b-7947-8cfc-32b54c251987" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019e1d03-6a7b-77c9-b66f-6570848c2fca" xlink:to="loc_us-gaap_PlanNameDomain_019e1d03-6a7b-7947-8cfc-32b54c251987" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_A2020StockIncentivePlanMember_019e1d03-6a7b-7985-8b2d-74489e979584" xlink:href="smmt-20260630.xsd#smmt_A2020StockIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e1d03-6a7b-7947-8cfc-32b54c251987" xlink:to="loc_smmt_A2020StockIncentivePlanMember_019e1d03-6a7b-7985-8b2d-74489e979584" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_A2020IncrementalPoolMember_019e1d03-6a7b-7b28-bc9c-501e228b935c" xlink:href="smmt-20260630.xsd#smmt_A2020IncrementalPoolMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e1d03-6a7b-7947-8cfc-32b54c251987" xlink:to="loc_smmt_A2020IncrementalPoolMember_019e1d03-6a7b-7b28-bc9c-501e228b935c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_A2024InducementPoolMember_019e1d03-6a7b-7996-b188-b400cb41fb48" xlink:href="smmt-20260630.xsd#smmt_A2024InducementPoolMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019e1d03-6a7b-7947-8cfc-32b54c251987" xlink:to="loc_smmt_A2024InducementPoolMember_019e1d03-6a7b-7996-b188-b400cb41fb48" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7e62-97c0-f744f397f45a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7334-bad0-494a275ac9c3" xlink:to="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7e62-97c0-f744f397f45a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7e62-97c0-f744f397f45a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7e62-97c0-f744f397f45a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7e62-97c0-f744f397f45a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7785-9608-9cd74ed40877" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7e62-97c0-f744f397f45a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7785-9608-9cd74ed40877" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_EmployeeStockOptionPerformanceBasedMember_019e1d03-6a7b-74ec-b332-ff55dd93f615" xlink:href="smmt-20260630.xsd#smmt_EmployeeStockOptionPerformanceBasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7785-9608-9cd74ed40877" xlink:to="loc_smmt_EmployeeStockOptionPerformanceBasedMember_019e1d03-6a7b-74ec-b332-ff55dd93f615" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAllowableUnderThePlan_019e1d03-6a7b-7758-a331-34c5cac83b08" xlink:href="smmt-20260630.xsd#smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAllowableUnderThePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAllowableUnderThePlan_019e1d03-6a7b-7758-a331-34c5cac83b08" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019e1d03-6a7b-7fcf-8469-e15e1f5f1685" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019e1d03-6a7b-7fcf-8469-e15e1f5f1685" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e1d03-6a7b-7074-9612-855c1f7bcbf4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_019e1d03-6a7b-7074-9612-855c1f7bcbf4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAcceleratedVestingNumber_019e1d03-6a7b-7a22-851b-d5ae8eb406c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAcceleratedVestingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAcceleratedVestingNumber_019e1d03-6a7b-7a22-851b-d5ae8eb406c2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019e1d03-6a7b-7c2c-a839-17bbf0566e51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost_019e1d03-6a7b-7c2c-a839-17bbf0566e51" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ShareBasedPaymentArrangementCostNotYetRecognizedDueToAcceleratedVestingAmount_019e1d03-6a7b-70ee-aaff-bf126b4b9c2a" xlink:href="smmt-20260630.xsd#smmt_ShareBasedPaymentArrangementCostNotYetRecognizedDueToAcceleratedVestingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_smmt_ShareBasedPaymentArrangementCostNotYetRecognizedDueToAcceleratedVestingAmount_019e1d03-6a7b-70ee-aaff-bf126b4b9c2a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ShareBasedPaymentArrangementWeightedAverageRecognitionPeriod_019e1d03-6a7b-7794-857a-50d80c2ba0e7" xlink:href="smmt-20260630.xsd#smmt_ShareBasedPaymentArrangementWeightedAverageRecognitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-751b-ba94-6a9c17d21a5c" xlink:to="loc_smmt_ShareBasedPaymentArrangementWeightedAverageRecognitionPeriod_019e1d03-6a7b-7794-857a-50d80c2ba0e7" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/StockBasedCompensationScheduleofStockOptionActivityDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#StockBasedCompensationScheduleofStockOptionActivityDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/StockBasedCompensationScheduleofStockOptionActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7e4e-ae72-98c1b7f32ac2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7e4e-ae72-98c1b7f32ac2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7644-bd9c-cf984a1b7156" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7e4e-ae72-98c1b7f32ac2" xlink:to="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7644-bd9c-cf984a1b7156" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7644-bd9c-cf984a1b7156_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7644-bd9c-cf984a1b7156" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7644-bd9c-cf984a1b7156_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73d8-be20-4dd8fe13db44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7644-bd9c-cf984a1b7156" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73d8-be20-4dd8fe13db44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_EmployeeStockOptionTimeBasedMember_019e1d03-6a7b-7f8a-b0aa-8487ec74915a" xlink:href="smmt-20260630.xsd#smmt_EmployeeStockOptionTimeBasedMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73d8-be20-4dd8fe13db44" xlink:to="loc_smmt_EmployeeStockOptionTimeBasedMember_019e1d03-6a7b-7f8a-b0aa-8487ec74915a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e1d03-6a7b-74b5-a488-2993837cb1da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e1d03-6a7b-74b5-a488-2993837cb1da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e1d03-6a7b-7196-9436-5b28b066d347" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e1d03-6a7b-74b5-a488-2993837cb1da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e1d03-6a7b-7196-9436-5b28b066d347" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019e1d03-6a7b-7dcd-b414-a8271f168644" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e1d03-6a7b-74b5-a488-2993837cb1da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_019e1d03-6a7b-7dcd-b414-a8271f168644" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_019e1d03-6a7b-75e9-95b5-65a7ab51330d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e1d03-6a7b-74b5-a488-2993837cb1da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_019e1d03-6a7b-75e9-95b5-65a7ab51330d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e1d03-6a7b-78e8-904e-df2b108f734c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019e1d03-6a7b-74b5-a488-2993837cb1da" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019e1d03-6a7b-78e8-904e-df2b108f734c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019e1d03-6a7b-77f0-88ff-9e8304b6d307" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019e1d03-6a7b-725a-a61e-1a7264a2b340" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019e1d03-6a7b-725a-a61e-1a7264a2b340" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e1d03-6a7b-76ac-99f7-31d5649d4d2d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019e1d03-6a7b-76ac-99f7-31d5649d4d2d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e1d03-6a7b-79f5-81d1-becdd6153ffc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e1d03-6a7b-79f5-81d1-becdd6153ffc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e1d03-6a7b-7691-b4d7-d8d04c51385a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e1d03-6a7b-79f5-81d1-becdd6153ffc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e1d03-6a7b-7691-b4d7-d8d04c51385a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e1d03-6a7b-7a3a-8c3e-06250d97ec1f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e1d03-6a7b-79f5-81d1-becdd6153ffc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019e1d03-6a7b-7a3a-8c3e-06250d97ec1f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_019e1d03-6a7b-7adf-b17e-5f2f368070eb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e1d03-6a7b-79f5-81d1-becdd6153ffc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_019e1d03-6a7b-7adf-b17e-5f2f368070eb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019e1d03-6a7b-72d6-b3ca-f65957ece01e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019e1d03-6a7b-79f5-81d1-becdd6153ffc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019e1d03-6a7b-72d6-b3ca-f65957ece01e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019e1d03-6a7b-72e2-8dc5-79c094935428" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019e1d03-6a7b-7217-a6b6-e70219cf3385" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019e1d03-6a7b-7217-a6b6-e70219cf3385" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019e1d03-6a7b-7985-b598-bc6afce2b497" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019e1d03-6a7b-7985-b598-bc6afce2b497" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_019e1d03-6a7b-7e12-8de1-09ed2ad0ab5b" xlink:href="smmt-20260630.xsd#smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-7b76-a8a7-b444e0897488" xlink:to="loc_smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_019e1d03-6a7b-7e12-8de1-09ed2ad0ab5b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019e1d03-6a7b-742e-b81e-fd33d25e95db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_019e1d03-6a7b-7e12-8de1-09ed2ad0ab5b" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019e1d03-6a7b-742e-b81e-fd33d25e95db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019e1d03-6a7b-777e-823e-70c745afcaf3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_019e1d03-6a7b-7e12-8de1-09ed2ad0ab5b" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019e1d03-6a7b-777e-823e-70c745afcaf3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e1d03-6a7b-71d2-b3e6-7e2819d59377" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_smmt_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTermAbstract_019e1d03-6a7b-7e12-8de1-09ed2ad0ab5b" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019e1d03-6a7b-71d2-b3e6-7e2819d59377" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/StockBasedCompensationScheduleofRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#StockBasedCompensationScheduleofRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/StockBasedCompensationScheduleofRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-706a-86e6-84209314a5d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7a57-ad0d-99eab5d4eac6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-706a-86e6-84209314a5d4" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7a57-ad0d-99eab5d4eac6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-74a3-95c6-62c43a910fda" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019e1d03-6a7b-7a57-ad0d-99eab5d4eac6" xlink:to="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-74a3-95c6-62c43a910fda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-74a3-95c6-62c43a910fda_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-74a3-95c6-62c43a910fda" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-74a3-95c6-62c43a910fda_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-725e-ada2-214d3774897b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-74a3-95c6-62c43a910fda" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-725e-ada2-214d3774897b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-7cdf-9590-939551c5db21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-725e-ada2-214d3774897b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-7cdf-9590-939551c5db21" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e1d03-6a7b-77d8-8f70-244f753ca560" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-706a-86e6-84209314a5d4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e1d03-6a7b-77d8-8f70-244f753ca560" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e1d03-6a7b-7b3c-a221-f82970cb6a2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e1d03-6a7b-77d8-8f70-244f753ca560" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e1d03-6a7b-7b3c-a221-f82970cb6a2c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e1d03-6a7b-7bc9-bae2-e8ca92d04c33" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019e1d03-6a7b-77d8-8f70-244f753ca560" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019e1d03-6a7b-7bc9-bae2-e8ca92d04c33" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019e1d03-6a7b-7ab6-a16c-0426ab68b76f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e1d03-6a7b-7b50-b331-cf6bfea010c3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019e1d03-6a7b-706a-86e6-84209314a5d4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e1d03-6a7b-7b50-b331-cf6bfea010c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e1d03-6a7b-7d28-9f5d-fcdd6d6eb13d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e1d03-6a7b-7b50-b331-cf6bfea010c3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e1d03-6a7b-7d28-9f5d-fcdd6d6eb13d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e1d03-6a7b-7804-ad7c-63f953443d38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019e1d03-6a7b-7b50-b331-cf6bfea010c3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019e1d03-6a7b-7804-ad7c-63f953443d38" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019e1d03-6a7b-7440-9af1-cda024829d4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e1d03-6a7b-7e1e-b555-6ca8d48260bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e1d03-6a7b-74e5-9d61-416c9ba7bfaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e1d03-6a7b-7e1e-b555-6ca8d48260bf" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e1d03-6a7b-74e5-9d61-416c9ba7bfaa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e1d03-6a7b-757d-be84-be4d88c9843b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e1d03-6a7b-74e5-9d61-416c9ba7bfaa" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_019e1d03-6a7b-757d-be84-be4d88c9843b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e1d03-6a7b-74e5-9d61-416c9ba7bfaa" xlink:to="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7a77-8aa6-036000611f85_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7a77-8aa6-036000611f85_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e1d03-6a7b-7fc3-bdf1-97ecc1082d73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e1d03-6a7b-7fc3-bdf1-97ecc1082d73" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-7579-a0ca-13f1a171c993" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-7579-a0ca-13f1a171c993" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e1d03-6a7b-76f5-8c4a-ceaede8e04b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:to="loc_us-gaap_EmployeeStockMember_019e1d03-6a7b-76f5-8c4a-ceaede8e04b5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e1d03-6a7b-7640-9934-04b8793cd55f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e1d03-6a7b-7e1e-b555-6ca8d48260bf" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e1d03-6a7b-7640-9934-04b8793cd55f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails_1" xlink:type="simple" xlink:href="smmt-20260630.xsd#StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails_1"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e1d03-6a7b-7e1e-b555-6ca8d48260bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e1d03-6a7b-74e5-9d61-416c9ba7bfaa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e1d03-6a7b-7e1e-b555-6ca8d48260bf" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e1d03-6a7b-74e5-9d61-416c9ba7bfaa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019e1d03-6a7b-74e5-9d61-416c9ba7bfaa" xlink:to="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7a77-8aa6-036000611f85_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-7a77-8aa6-036000611f85_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019e1d03-6a7b-7a77-8aa6-036000611f85" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019e1d03-6a7b-7fc3-bdf1-97ecc1082d73" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019e1d03-6a7b-7fc3-bdf1-97ecc1082d73" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-7579-a0ca-13f1a171c993" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019e1d03-6a7b-7579-a0ca-13f1a171c993" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019e1d03-6a7b-76f5-8c4a-ceaede8e04b5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019e1d03-6a7b-73af-af2e-120e57efd5fc" xlink:to="loc_us-gaap_EmployeeStockMember_019e1d03-6a7b-76f5-8c4a-ceaede8e04b5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e1d03-6a7b-7640-9934-04b8793cd55f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019e1d03-6a7b-7e1e-b555-6ca8d48260bf" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e1d03-6a7b-7640-9934-04b8793cd55f" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/RelatedPartyTransactionsDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#RelatedPartyTransactionsDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/RelatedPartyTransactionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e1d03-6a7b-7460-8206-7f5690f159aa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e1d03-6a7b-7460-8206-7f5690f159aa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7eb0-9a58-481cf84f045a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e1d03-6a7b-7460-8206-7f5690f159aa" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7eb0-9a58-481cf84f045a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7eb0-9a58-481cf84f045a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7eb0-9a58-481cf84f045a" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7eb0-9a58-481cf84f045a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7eb0-9a58-481cf84f045a" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SubleaseAgreementFirstAmendmentMember_019e1d03-6a7b-768c-9077-c0ac8fb89906" xlink:href="smmt-20260630.xsd#smmt_SubleaseAgreementFirstAmendmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_SubleaseAgreementFirstAmendmentMember_019e1d03-6a7b-768c-9077-c0ac8fb89906" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SubleaseAgreementFirstAndThirdAmendmentMember_019e1d03-6a7b-73b7-977b-9fb81a3e33dd" xlink:href="smmt-20260630.xsd#smmt_SubleaseAgreementFirstAndThirdAmendmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_SubleaseAgreementFirstAndThirdAmendmentMember_019e1d03-6a7b-73b7-977b-9fb81a3e33dd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SubleaseAgreementSecondAmendmentMember_019e1d03-6a7b-7bcf-8247-c05e8227dd6c" xlink:href="smmt-20260630.xsd#smmt_SubleaseAgreementSecondAmendmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_SubleaseAgreementSecondAmendmentMember_019e1d03-6a7b-7bcf-8247-c05e8227dd6c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_GeniusSubleaseAgreementMember_019e1d03-6a7b-7d97-b4ff-d71be48b3a7f" xlink:href="smmt-20260630.xsd#smmt_GeniusSubleaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_GeniusSubleaseAgreementMember_019e1d03-6a7b-7d97-b4ff-d71be48b3a7f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_InvestmentsResearchSubleaseAgreementMember_019e1d03-6a7b-70a9-b86b-fd8aafe8d9c2" xlink:href="smmt-20260630.xsd#smmt_InvestmentsResearchSubleaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_InvestmentsResearchSubleaseAgreementMember_019e1d03-6a7b-70a9-b86b-fd8aafe8d9c2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_GeniusSubleaseAgreementAndInvestmentsResearchSubleaseAgreementMember_019e1d03-6a7b-79ad-bc61-787510da604c" xlink:href="smmt-20260630.xsd#smmt_GeniusSubleaseAgreementAndInvestmentsResearchSubleaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_GeniusSubleaseAgreementAndInvestmentsResearchSubleaseAgreementMember_019e1d03-6a7b-79ad-bc61-787510da604c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_SubleaseAgreementThirdAmendmentMember_019e1d03-6a7b-7769-8e34-7cc0d5babb95" xlink:href="smmt-20260630.xsd#smmt_SubleaseAgreementThirdAmendmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_SubleaseAgreementThirdAmendmentMember_019e1d03-6a7b-7769-8e34-7cc0d5babb95" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_AkesoSupplyAgreementMember_019e1d03-6a7b-74e5-8dc6-6b97ae68de77" xlink:href="smmt-20260630.xsd#smmt_AkesoSupplyAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-75c9-98a6-eab9afbdd602" xlink:to="loc_smmt_AkesoSupplyAgreementMember_019e1d03-6a7b-74e5-8dc6-6b97ae68de77" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7246-b472-80f7dc0ef453" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e1d03-6a7b-7460-8206-7f5690f159aa" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7246-b472-80f7dc0ef453" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7246-b472-80f7dc0ef453_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7246-b472-80f7dc0ef453" xlink:to="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7246-b472-80f7dc0ef453_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7add-8c82-5072c5f56103" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-7246-b472-80f7dc0ef453" xlink:to="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7add-8c82-5072c5f56103" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AffiliatedEntityMember_019e1d03-6a7b-7668-a3e6-15498864b636" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_AffiliatedEntityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7add-8c82-5072c5f56103" xlink:to="loc_srt_AffiliatedEntityMember_019e1d03-6a7b-7668-a3e6-15498864b636" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyMember_019e1d03-6a7b-7636-9481-477e673037ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7add-8c82-5072c5f56103" xlink:to="loc_us-gaap_RelatedPartyMember_019e1d03-6a7b-7636-9481-477e673037ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ExecutiveOfficerMember_019f8651-6228-785a-81d0-6dc301144611" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7add-8c82-5072c5f56103" xlink:to="loc_srt_ExecutiveOfficerMember_019f8651-6228-785a-81d0-6dc301144611" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_CoChiefExecutiveOfficerMember_019e1d03-6a7b-7b28-b2d1-84349863bbeb" xlink:href="smmt-20260630.xsd#smmt_CoChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7add-8c82-5072c5f56103" xlink:to="loc_smmt_CoChiefExecutiveOfficerMember_019e1d03-6a7b-7b28-b2d1-84349863bbeb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_WilsonSonsiniGoodrichRosatiP.C.WSGRMember_019e1d03-6a7b-7442-af23-8cac033acd93" xlink:href="smmt-20260630.xsd#smmt_WilsonSonsiniGoodrichRosatiP.C.WSGRMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7add-8c82-5072c5f56103" xlink:to="loc_smmt_WilsonSonsiniGoodrichRosatiP.C.WSGRMember_019e1d03-6a7b-7442-af23-8cac033acd93" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-7b6d-a6a0-56d5ffcb9b2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubsidiarySaleOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_019e1d03-6a7b-7460-8206-7f5690f159aa" xlink:to="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-7b6d-a6a0-56d5ffcb9b2f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-7b6d-a6a0-56d5ffcb9b2f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-7b6d-a6a0-56d5ffcb9b2f" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-7b6d-a6a0-56d5ffcb9b2f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-79b9-9643-ff5e88386dc8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsidiarySaleOfStockAxis_019e1d03-6a7b-7b6d-a6a0-56d5ffcb9b2f" xlink:to="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-79b9-9643-ff5e88386dc8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrivatePlacementMember_019e1d03-6a7b-74d5-8ea0-6f0fcf26eccb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PrivatePlacementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SaleOfStockNameOfTransactionDomain_019e1d03-6a7b-79b9-9643-ff5e88386dc8" xlink:to="loc_us-gaap_PrivatePlacementMember_019e1d03-6a7b-74d5-8ea0-6f0fcf26eccb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_AreaOfPremisesSubleased_019e1d03-6a7b-76f1-9bb8-7e6a0d69dd84" xlink:href="smmt-20260630.xsd#smmt_AreaOfPremisesSubleased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_AreaOfPremisesSubleased_019e1d03-6a7b-76f1-9bb8-7e6a0d69dd84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_RelatedPartyTransactionSubleaseExtensionTerm_019e1d03-6a7b-7624-8fe3-971af29f5b9e" xlink:href="smmt-20260630.xsd#smmt_RelatedPartyTransactionSubleaseExtensionTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_RelatedPartyTransactionSubleaseExtensionTerm_019e1d03-6a7b-7624-8fe3-971af29f5b9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_RelatedPartyTransactionPaymentsToRelatedParty_019e1d03-6a7b-7018-9e82-2c1ebb1e2f64" xlink:href="smmt-20260630.xsd#smmt_RelatedPartyTransactionPaymentsToRelatedParty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_RelatedPartyTransactionPaymentsToRelatedParty_019e1d03-6a7b-7018-9e82-2c1ebb1e2f64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_LessorOperatingLeaseNumberOfContracts_019e1d03-6a7b-7e54-991d-2b90b638773c" xlink:href="smmt-20260630.xsd#smmt_LessorOperatingLeaseNumberOfContracts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_LessorOperatingLeaseNumberOfContracts_019e1d03-6a7b-7e54-991d-2b90b638773c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_LessorOperatingLeaseAreaOfPremisesLeased_019e1d03-6a7b-7eb7-910f-b74db0ce2a23" xlink:href="smmt-20260630.xsd#smmt_LessorOperatingLeaseAreaOfPremisesLeased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_LessorOperatingLeaseAreaOfPremisesLeased_019e1d03-6a7b-7eb7-910f-b74db0ce2a23" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LessorOperatingLeaseTermOfContract_019e1d03-6a7b-73e2-8a35-d548a401d877" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LessorOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_LessorOperatingLeaseTermOfContract_019e1d03-6a7b-73e2-8a35-d548a401d877" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LessorOperatingLeasePaymentsToBeReceived_019e1d03-6a7b-77be-91b4-22330d6ed148" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LessorOperatingLeasePaymentsToBeReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_LessorOperatingLeasePaymentsToBeReceived_019e1d03-6a7b-77be-91b4-22330d6ed148" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubleaseIncome_019e1d03-6a7b-7c2b-83b3-253b256a57b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SubleaseIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_SubleaseIncome_019e1d03-6a7b-7c2b-83b3-253b256a57b3" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_LesseeOperatingLeaseProportionOfAmountPayablePercentage_019e1d03-6a7b-740b-b879-2dabd2c9bc95" xlink:href="smmt-20260630.xsd#smmt_LesseeOperatingLeaseProportionOfAmountPayablePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_LesseeOperatingLeaseProportionOfAmountPayablePercentage_019e1d03-6a7b-740b-b879-2dabd2c9bc95" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019e1d03-6a7b-727c-a2af-60e3be32c741" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019e1d03-6a7b-727c-a2af-60e3be32c741" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_019e1d03-6a7b-7df8-aedb-9e112b1ed49b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_019e1d03-6a7b-7df8-aedb-9e112b1ed49b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019e1d03-6a7b-7357-8ecc-222873d0d65c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction_019e1d03-6a7b-7357-8ecc-222873d0d65c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockPricePerShare_019f671f-3c95-7fb8-a98d-77a239768cff" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockPricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_SaleOfStockPricePerShare_019f671f-3c95-7fb8-a98d-77a239768cff" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019f671f-3c95-72b1-90c0-2afecd96c95f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SaleOfStockConsiderationReceivedOnTransaction"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_SaleOfStockConsiderationReceivedOnTransaction_019f671f-3c95-72b1-90c0-2afecd96c95f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ClassOfWarrantOrRightExercisesInPeriod_019e1d03-6a7b-78e8-b1e0-ed02ab9a04d9" xlink:href="smmt-20260630.xsd#smmt_ClassOfWarrantOrRightExercisesInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_ClassOfWarrantOrRightExercisesInPeriod_019e1d03-6a7b-78e8-b1e0-ed02ab9a04d9" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ClassOfWarrantOrRightWeightedAverageExercisePrice_019e1d03-6a7b-7c51-a03c-d75e06874741" xlink:href="smmt-20260630.xsd#smmt_ClassOfWarrantOrRightWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_ClassOfWarrantOrRightWeightedAverageExercisePrice_019e1d03-6a7b-7c51-a03c-d75e06874741" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ClassOfWarrantOrRightWeightedAverageExercisePriceExercisesInPeriod_019e1d03-6a7b-7ffe-a0df-c38098a164c7" xlink:href="smmt-20260630.xsd#smmt_ClassOfWarrantOrRightWeightedAverageExercisePriceExercisesInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_smmt_ClassOfWarrantOrRightWeightedAverageExercisePriceExercisesInPeriod_019e1d03-6a7b-7ffe-a0df-c38098a164c7" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LegalFees_019e1d03-6a7b-74f8-a1ed-0df7e8f6cb18" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LegalFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_019e1d03-6a7b-7e07-8eb5-281b91c0000e" xlink:to="loc_us-gaap_LegalFees_019e1d03-6a7b-74f8-a1ed-0df7e8f6cb18" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.summitplc.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="smmt-20260630.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.summitplc.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019e1d03-6a7b-7627-8205-1c12ae5dc21b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019e1d03-6a7b-7697-8278-729be684aa44" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e1d03-6a7b-7627-8205-1c12ae5dc21b" xlink:to="loc_us-gaap_LossContingenciesTable_019e1d03-6a7b-7697-8278-729be684aa44" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e1d03-6a7b-7107-99cf-c1f17be39cd4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e1d03-6a7b-7697-8278-729be684aa44" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e1d03-6a7b-7107-99cf-c1f17be39cd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e1d03-6a7b-7107-99cf-c1f17be39cd4_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e1d03-6a7b-7107-99cf-c1f17be39cd4" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e1d03-6a7b-7107-99cf-c1f17be39cd4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e1d03-6a7b-7ef4-a0fd-3c08a0936bf4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis_019e1d03-6a7b-7107-99cf-c1f17be39cd4" xlink:to="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e1d03-6a7b-7ef4-a0fd-3c08a0936bf4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_PrincetonOfficeLeaseMember_019e1d03-6a7b-746a-b8e8-e4ce27cb000e" xlink:href="smmt-20260630.xsd#smmt_PrincetonOfficeLeaseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RealEstateAndAccumulatedDepreciationNameOfPropertyDomain_019e1d03-6a7b-7ef4-a0fd-3c08a0936bf4" xlink:to="loc_smmt_PrincetonOfficeLeaseMember_019e1d03-6a7b-746a-b8e8-e4ce27cb000e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7afd-8639-ab13704e329e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e1d03-6a7b-7697-8278-729be684aa44" xlink:to="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7afd-8639-ab13704e329e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7afd-8639-ab13704e329e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7afd-8639-ab13704e329e" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7afd-8639-ab13704e329e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7b32-90ba-2b3ea1935f1c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionAxis_019e1d03-6a7b-7afd-8639-ab13704e329e" xlink:to="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7b32-90ba-2b3ea1935f1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_DugganFebruaryNoteMember_019e1d03-6a7b-720e-bb55-c59b47c86944" xlink:href="smmt-20260630.xsd#smmt_DugganFebruaryNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7b32-90ba-2b3ea1935f1c" xlink:to="loc_smmt_DugganFebruaryNoteMember_019e1d03-6a7b-720e-bb55-c59b47c86944" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ZanganehNoteMember_019e1d03-6a7b-7c3f-937b-55d0c85e3665" xlink:href="smmt-20260630.xsd#smmt_ZanganehNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7b32-90ba-2b3ea1935f1c" xlink:to="loc_smmt_ZanganehNoteMember_019e1d03-6a7b-7c3f-937b-55d0c85e3665" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_DugganSeptemberNoteMember_019e1d03-6a7b-7c6d-b673-4e9647a62c99" xlink:href="smmt-20260630.xsd#smmt_DugganSeptemberNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionDomain_019e1d03-6a7b-7b32-90ba-2b3ea1935f1c" xlink:to="loc_smmt_DugganSeptemberNoteMember_019e1d03-6a7b-7c6d-b673-4e9647a62c99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-796e-aec6-f095892b794c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019e1d03-6a7b-7697-8278-729be684aa44" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-796e-aec6-f095892b794c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-796e-aec6-f095892b794c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-796e-aec6-f095892b794c" xlink:to="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-796e-aec6-f095892b794c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7307-a507-15dcaf8e80e3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RelatedPartyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_019e1d03-6a7b-796e-aec6-f095892b794c" xlink:to="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7307-a507-15dcaf8e80e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_019e1d03-6a7b-7a1f-b0cc-8d520e11e29e" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7307-a507-15dcaf8e80e3" xlink:to="loc_srt_ChiefExecutiveOfficerMember_019e1d03-6a7b-7a1f-b0cc-8d520e11e29e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_ChiefExecutiveOfficerAndPresidentMember_019e1d03-6a7b-703c-82c8-2c23ad51e8cd" xlink:href="smmt-20260630.xsd#smmt_ChiefExecutiveOfficerAndPresidentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_019e1d03-6a7b-7307-a507-15dcaf8e80e3" xlink:to="loc_smmt_ChiefExecutiveOfficerAndPresidentMember_019e1d03-6a7b-703c-82c8-2c23ad51e8cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AreaOfRealEstateProperty_019e1d03-6a7b-7bd9-a3ad-c0d2e2c9ca0a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e1d03-6a7b-7627-8205-1c12ae5dc21b" xlink:to="loc_us-gaap_AreaOfRealEstateProperty_019e1d03-6a7b-7bd9-a3ad-c0d2e2c9ca0a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_LesseeOperatingLeaseAverageAnnualLeasePayments_019e1d03-6a7b-7136-b1ec-9b368e01a876" xlink:href="smmt-20260630.xsd#smmt_LesseeOperatingLeaseAverageAnnualLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e1d03-6a7b-7627-8205-1c12ae5dc21b" xlink:to="loc_smmt_LesseeOperatingLeaseAverageAnnualLeasePayments_019e1d03-6a7b-7136-b1ec-9b368e01a876" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_IncreaseInAreaOfRealEstateProperty_019e1d03-6a7b-7857-b131-ff5876edf630" xlink:href="smmt-20260630.xsd#smmt_IncreaseInAreaOfRealEstateProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e1d03-6a7b-7627-8205-1c12ae5dc21b" xlink:to="loc_smmt_IncreaseInAreaOfRealEstateProperty_019e1d03-6a7b-7857-b131-ff5876edf630" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_smmt_OtherCommitmentPeriodForWhichTheMajorityOfCommitmentsAreToBePaid_019e1d03-6a7b-72e3-8439-c41eb3f2098b" xlink:href="smmt-20260630.xsd#smmt_OtherCommitmentPeriodForWhichTheMajorityOfCommitmentsAreToBePaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e1d03-6a7b-7627-8205-1c12ae5dc21b" xlink:to="loc_smmt_OtherCommitmentPeriodForWhichTheMajorityOfCommitmentsAreToBePaid_019e1d03-6a7b-72e3-8439-c41eb3f2098b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NotesPayable_019e1d03-6a7b-7558-b75f-0aeaebed6745" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NotesPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019e1d03-6a7b-7627-8205-1c12ae5dc21b" xlink:to="loc_us-gaap_NotesPayable_019e1d03-6a7b-7558-b75f-0aeaebed6745" xlink:type="arc" order="4"/>
  </link:definitionLink>
</link:linkbase>
