Abstract 2039: Cellular characterization of small molecule PD-L1 inhibitors reveal their novel mechanisms of action

Abstract Introductions: Immune checkpoint inhibitors, including PD-1/L1, are key regulators of immune response and promising targets in cancer immunotherapy. Like anti-PD-1/L1 antibodies, small molecule PD-L1 inhibitors that have been discovered by us and others could also efficiently block PD-1 and PD-L1 interaction and exhibit anti-tumor efficacy in preclinical and clinically settings. In this study, we explore the cellular mechanism of small molecule PD-L1 inhibitors, unveiling their novel mechanisms of action in the regulation of PD-L1 and its functions. Materials and Methods: The cellular effects of the small molecule PD-L1 inhibitor on PD-L1 protein and RNA levels were assessed through Western blot and real-time qPCR experiments. HTRF assay and cellular luciferase reporter assay were used to evaluate the blockade of PD-1 and PD-L1 interaction in vitro and in cells. Flow cytometry and confocal microscopy were used to investigate the internalization of PD-L1 upon treatment with inhibitors. MSD assay was used to evaluate the alteration of soluble PD-L1 proteins. Results: Small molecule PD-L1 inhibitors discovered by Abbisko Therapeutics potently blocked PD-1/PD-L1 interaction in vitro and in cellular system. It could also rapidly and strongly induce PD-L1 internalization in cells. Further profiling suggested that small molecule PD-L1 inhibitors specifically disrupt PD-L1 N-glycosylation, resulting in aberrant PD-L1 glycosylation. This, in turn, may hinder the proper translocation of newly synthesized PD-L1 to the cell surface, thereby abolishing its interaction with PD-1 and functions in immune regulation. Conclusion: Taken together, these results for the first time revealed the distinctive mechanisms of our small molecule PD-L1 inhibitors. With their multi-layer inhibitory effects stemming from various mechanisms, small molecule PD-L1 inhibitors may offer potentially improved activities and an alternative therapeutic treatment for the cancer patients. Citation Format: Weiling Pan, Yongxian Zhang, Fangfei Qi, Wenqun Xin, Zhigang Feng, Zhui Chen, Haiyan Ying. Cellular characterization of small molecule PD-L1 inhibitors reveal their novel mechanisms of action 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 2039.