Patient‐derived Immunocompetent Tumor Organoids: A Platform for Chemotherapy Evaluation in the Context of T‐cell Recognition**

Abstract Most of the anticancer compounds synthesized by chemists are primarily evaluated for their direct cytotoxic effects at the cellular level, often overlooking the critical role of the immune system. In this study, we developed a patient‐derived, T‐cell‐retaining tumor organoid model that allows us to evaluate the anticancer efficacy of chemical drugs under the synergistic paradigm of antigen‐specific T‐cell‐dependent killing, which may reveal the missed drug hits in the simple cytotoxic assay. We evaluated clinically approved platinum (Pt) drugs and a custom library of twenty‐eight Pt IV compounds. We observed low direct cytotoxicity of Pt drugs, but variable synergistic effects in combination with immune checkpoint inhibitors (ICIs). In contrast, the majority of Pt IV compounds exhibited potent tumor‐killing capabilities. Interestingly, several Pt IV compounds went beyond direct tumor killing and showed significant immunosynergistic effects with ICIs, outstanding at sub‐micromolar concentrations. Among these, Pt‐19, Pt IV compounds with cinnamate axial ligands, emerged as the most therapeutically potent, demonstrating pronounced immunosynergistic effects by promoting the release of cytotoxic cytokines, activating immune‐related pathways and enhancing T cell receptor (TCR) clonal expansion. Overall, this initiative marks the first use of patient‐derived immunocompetent tumor organoids to explore and study chemotherapy, advancing their path toward more effective small molecule drug discovery.