Abstract 2151: Combining PD1 inhibition with radiotherapy enhances anti tumor responses in the CT26 colorectal mouse model

Abstract Introduction In recent years, therapies that enhance anti cancer immune responses, like immune checkpoint inhibitors (ICIs), have transformed cancer treatment. Although these therapies can induce lasting responses across various cancer types, not all patients benefit from them. To maximize the potential of immuno oncology (IO) agents, researchers are exploring combinations with other treatments, such as radiotherapy, in both preclinical models and clinical trials. Understanding how IO therapies interact with radiotherapy to influence the tumor immune microenvironment (TIME), and other biomarkers is crucial for advancing drug discovery research. Methods and experimental procedures CT26 cells were exposed to various irradiation doses in vitro, and DNA damage response biomarkers were assessed using the Jess automated western blot system. Additionally, to establish a tolerated radiotherapy dosing regimen in vivo, different irradiation doses were tested in Balbc CT26 tumor bearing mice. The most effective and tolerated dose regimen was then combined with PD1 dosing. The response of the CT26 cancer model to PD1, with or without radiotherapy, was evaluated. Mice receiving radiotherapy were administered targeted X rays localized only to the tumor area. Changes to the tumor immune microenvironment (TIME) following these therapies were assessed using flow cytometry, while changes in splenic immune populations were analyzed by flow cytometry and ELISPOT. Serum cytokines at study termination were assessed using the Luminex x map technology. Data Treatment with radiotherapy or PD1 as monotherapies led to a mixed response, with tumor regressions obtained in a fraction of the animals treated. The combination of PD1 with radiotherapy significantly increased therapeutic efficacy. Characterizing the TIME following treatment with PD1 in combination with radiotherapy revealed modulation of the intra-tumoral immune infiltrate that reflected the increased therapeutic efficacy. Notably, a significant decrease in the frequency of tumor associated macrophages and an increase in T, NK, and B cells, was observed in tumors treated with the combination of anti PD1 and radiotherapy. Conclusion We have identified and established effective dosing regimens for radiotherapy in vivo in combination with PD1 therapy. Additionally, we have assessed changes to the tumor and spleen immune populations in a syngeneic mouse cancer model. These findings highlight the utility of these models in immuno-oncology (IO) drug discovery research. Citation Format: Charlotte Bell, Theoni (Tania) Katopodi, Pablo Binder, Tobias Bunday, Emily Wright, Amy Cantrell, Nick Moore, Stewart Brown, Paul Farrington, Shannon Sharman, Lyndsey Hanson, Lorraine Mooney, Jane Kendrew. Combining PD1 inhibition with radiotherapy enhances anti tumor responses in the CT26 colorectal mouse model [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 2151.