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Introduction: Nilogen Oncosystems’ 3D-EXpress platform allows for the detailed characterization offreshly resected patient tumor tissue for inclusion in studies investigating rational combinations of IOtherapeutics. Incorporating data on immune cell composition, tumor cell target expression, mutationalstatus, and HPV infection within tumoroids generated from HNSCC tissues provides tremendous input toidentify candidate tissues for therapeutic testing. The 3D-EXpress platform employed here providesdetailed observations investigating tumor cell viability using confocal microscopy and immune cellactivation indicated by increases in cytokine release. Materials and Methods: 3D tumoroids measuring 150 microns in size were generated from fresh patientHNSCC resection samples collected with proper patient consent and relevant IRB approval. Tumoroidswere never enzymatically dissociated, propagated, or reassembled to maintain the intact tumormicroenvironment (TME). The resulting tumoroids were cryopreserved in the Biorepository, where eachsample had associated patient demographic data, tumor grade, stage, mutational and HPV status inaddition to a detailed tumor immune profile and a FFPE representative sample in TMA format for targetselection. Pooled tumoroids for each patient tumor were treated ex vivo both singly with a selectiveWEE1 inhibitor, AZD1775 and an ATR blocker AZD6738 alone and in combination with a PD-1 immunecheckpoint inhibitor antibody nivolumab for 72h. Results and Summary: To quantify treatment-mediated tumor cell killing we used 3D high-contentconfocal imaging and a proprietary algorithm for data analysis. Additionally, culture supernatantsisolated from treated tumoroids were used for multiplex cytokine detection to monitor changes in theTME upon ex vivo treatment. Responses to treatments were further correlated with the composition ofinnate and adaptive TIL populations within each tumor as assessed by 21-color flow cytometry inaddition to tumor mutational status, HPV infection, smoking status, as well as EGFR and PD-L1expression levels analyzed by multiplex IF on associated TMA slides. Conclusion: These results demonstrate that targeting DNA damage response and immune checkpointblockade may provide a potential combination strategy for the treatment of HNSCC. Furthermore, weshowed that our 3D-EXpress ex vivo tumoroid model provides a unique platform to rapidly assess theefficacy of drugs and drug combinations in fresh patient tumor samples with intact TME. Correlation ofex vivo drug responses with characteristics of each tumor’s immune microenvironment and detailedclinicopathological data may allow to identify clinically relevant biomarkers to enable the most effectivetreatment strategies for individual patients in the clinic. Citation Format: Jared Ehrhart, Seth Currlin, Sharon Camacho, Samantha Hoffman, Angie Rivera, Soner Altiok. 3D-EXpress ex vivo platform using a biorepository of characterized fresh patient tumoroids allows development of rational combinations with drugs targeting DNA damage response and immune checkpoint blockade. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4572.