Abstract IA012: Sulindac modulates the response of proficient MMR colorectal cancer to immune checkpoint inhibitors

Abstract Immune checkpoint inhibitors (ICIs) therapy has been widely used to treat different human cancers, particularly advanced solid tumors. However, clinical studies have reported that ICI immunotherapy benefits only ~15% of colorectal cancer (CRC) patients, specifically those with tumors characterized by microsatellite instability (MSI), a molecular marker of defective DNA mismatch repair (dMMR). For the majority of CRC patients who carry proficient MMR (pMMR), ICIs have shown little clinical benefit. In this study, we examined the efficacy of sulindac to enhance the response of pMMR CRC to anti-PD-L1 immunotherapy. We first utilized CT26 and CMT93 syngeneic mouse tumor models to compare the inhibitory effects of PD-L1 antibody (Ab), sulindac, and their combination on pMMR CRC tumor growth. We found that mice treated with combination therapy showed a significant reduction in tumor volume, along with increased infiltration of CD8+ T lymphocytes in the tumor tissues. We also demonstrated that sulindac could downregulate PD-L1 by blocking NF-κB signaling, which in turn led to a decrease in exosomal PD-L1. Notably, PD-L1 Ab can be bound and consumed by exosomal PD-L1 in the blood circulation. Therefore, in combination therapy, sulindac downregulating PD-L1 leads to increased availability of PD-L1 Ab, which potentially improves the overall efficacy of anti-PD-L1 therapy. We validated all these results using humanized Patient-Derived Xenografts (PDX) animal models. Moreover, we demonstrated the safety of low-dose sulindac as it did not have a systemic inhibitory effect on prostaglandin E2 (PGE2). In conclusion, our findings provide unique insights into the mechanism of action and efficacy for sulindac as an immunomodulatory agent for the ICI therapy and immunoprevention of pMMR CRC. Citation Format: Yaguang Xi. Sulindac modulates the response of proficient MMR colorectal cancer to immune checkpoint inhibitors [abstract]. In: Proceedings of the Second Biennial NCI Meeting: Translational Advances in Cancer Prevention Agent Development (TACPAD); 2022 Sep 7-9. Philadelphia (PA): AACR; Can Prev Res 2022;15(12 Suppl_2): Abstract nr IA012.