Abstract 6554: Characterization of the tumor immune infiltrate in a colorectal syngeneic model dosed with anti PD1 in combination with radiotherapy or PARP inhibitor

Abstract Introduction: Immune checkpoint inhibitors (ICIs) have revolutionized the treatment of many cancers, significantly improving survival outcomes. However, for some indications, phase I and II studies have not demonstrated substantial antitumor activity when ICIs are used as monotherapy. Combining ICIs with poly(ADP ribose) polymerase (PARP) inhibitors (PARPis) or radiotherapy are currently being explored in numerous clinical trials. Therefore, understanding the impact of these therapies on the tumor immune microenvironment (TIME) and the responses after combining radiotherapy, PARPis and ICIs in preclinical tumor models is crucial for guiding drug discovery research. Methods and Experimental Procedures: The murine colorectal carcinoma cell line MC38 was subcutaneously implanted into C57BL/6 mice. The response of the MC38 cancer model to anti PD1, fractionated radiotherapy, and the PARPi Niraparib was assessed by measuring tumor growth with calipers three times per week. Mice receiving radiotherapy were administered targeted X rays using an Xstrahl CIX3 cabinet irradiator, with animals shielded by lead panels to expose only the tumor for localized treatment. Changes to the TIME following these treatments were evaluated using multi color flow cytometry, and plasma cytokines were assessed by Luminex xMAP technology. Data Treatment with anti PD1 resulted in a mixed response, with tumor regressions observed in a subset of treated animals. The combination of anti PD1 with fractionated radiotherapy significantly enhanced therapeutic efficacy, whereas combining anti PD1 with Niraparib did not yield additional benefits. Characterization of the tumor immune microenvironment (TIME) following these treatments revealed modulation of the intra-tumoral immune infiltrate. Notably, a significant decrease in tumor-associated macrophages and an increase in CD8 cytotoxic T cells were observed in tumors treated with the combination of anti PD1 and fractionated radiotherapy. These results were consistent with the increased efficacy observed in the combination group. However, combining Niraparib with anti PD1 did not show any advantages over the anti PD1 monotherapy which was also reflected by the changes seen when TIME was characterized. Conclusion: In this study we have characterized and established effective dosing regimens for anti PD1, fractionated radiotherapy, and Niraparib, both as monotherapies and in combination. Additionally, we evaluated changes to the tumor immune microenvironment (TIME) in a syngeneic mouse cancer model. The differences in efficacy and TIME findings with the combinations evaluated demonstrate the value of these models for immuno-oncology (IO) drug discovery research. Citation Format: Theoni (Tania) Katopodi, Charlotte Bell, Pablo Binder, Lyndsey Hanson, Emily Wright, Amy Cantrell, Stewart Brown, Paul Farrington, Tobias Bunday, Nick Moore, Lorraine Mooney, Jane Kendrew. Characterization of the tumor immune infiltrate in a colorectal syngeneic model dosed with anti PD1 in combination with radiotherapy or PARP inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6554.