Abstract 1353: Potentiating salvage radiotherapy in radiorecurrent prostate cancer through anti-CTLA4 therapy: Implications from a syngeneic model

Abstract Background: Advanced prostate cancer (PCa) is a leading cause in cancer death and can elicit significant morbidity and mortality. A common treatment modality for advanced PCa is radiation therapy (RT). Currently, salvage of local disease recurrence after RT is a major clinical problem. Emerging evidence indicates the importance of the immune system in governing RT response. In support of this, immune checkpoint inhibitors (ICIs), which enhance immune activation, have demonstrated clinical therapeutic promise in combination with RT in certain advanced cancers. Irradiation (IR) combined with ICIs may prime the immune system to recognize and target recurrent cancer. Purpose: Investigate the therapeutic efficacy of ICIs in combination with RT for radiorecurrent PCa in a syngeneic pre-clinical model. Methods: TRAMP-C2 cells were treated with 10 Gy of radiation over 5 fractions (similar to clinical hypofractionated (HF) schedule) to generate TRAMP-C2 HF cells. Immune-competent mice were transplanted subcutaneously with TRAMP-C2 HF cells. Once tumors reached palpable size, mice were administered either ICIs (anti-PDL1 or anti-CTLA4) alone or in combination with RT. Tumor volume was monitored to determine the treatment effect. Correlative studies on excised tumors and secondary lymphoid organs included flow cytometry and NanoString gene expression panel to evaluate immunological mechanisms that contribute to anti-tumor effects. Results: TRAMP-C2 HF cells were validated for radiation resistance and exhibited a more aggressive phenotype (reduction in senescence, increase in clonogenicity) similar to clinically recurrent PCa. Radiation resistance of TRAMP-C2 HF tumors was validated in vivo. Administration of anti-PDL1 and anti-CTLA4 as monotherapy or in combination did not achieve significant tumor growth delay compared to control. IR alone produced an observable tumor growth delay compared to ICIs alone. The combination of anti-PDL1 and IR did not yield additional growth delay compared to IR alone. Strikingly, significant tumor growth delay was seen with the combination of IR and anti-CTLA4 compared to IR alone, while also resulting in complete cure in a third of the mice. mice. Lymph nodes and tumors from mice treated with IR and anti-CTLA4 vs. IR and isotype control demonstrated differential expression of genes in T cell functions and enrichment in both CD4+ and CD8+ T cell populations. Conclusion: We generated the first syngeneic radiorecurrent PCa model and demonstrated that combining anti-CTLA4 and IR results in synergistic tumor response. Combined therapy resulted in augmented immune response, most notably enhancement of CD8 T cell activity. Significance: These findings contribute to our understanding of immunological events associated with RT and ICIs in the context of radiorecurrent PCa and support new avenues for salvage therapy in clinical trials. Citation Format: Hanzhi Wang, Linsey Gong, Xiaoyong Huang, Stephanie D. White, Hanz T. Chung, Danny Vesprini, Tera N. Petchiny, Emmanouil Fokas, Hansen He, Robert S. Kerbel, Stanley K. Liu. Potentiating salvage radiotherapy in radiorecurrent prostate cancer through anti-CTLA4 therapy: Implications from a syngeneic model abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 1353.