Abstract 416: HEC201625, a novel oral small molecule inhibitor of PD-L1, demonstrated potent anti-tumor activities both in vitro and in vivo .

Abstract Background: Despite the clinical success of PD-1 and PD-L1 antibodies, there remains a need for improvement, including the requirement for intravenous administration, suboptimal response rates, and immune-related toxicities. In contrast, orally bioavailable small molecule inhibitors of PD-L1 may address these issues by offering treatment convenience, better tissue distribution, and easier adverse events management in combination therapies. Herein, we report the discovery of HEC201625, a novel small molecule inhibitor of PD-L1, to enhance antitumor immune response by blocking the PD-1/PD-L1 pathway, providing an alternative for cancer immunotherapy. Experimental Methods: In vitro efficacy was tested in HTRF binding and NFAT-driven reporter assays. Functional immune activation was evaluated in co-cultures of human primary T cells with tumor cells. In vivo efficacy was assessed in syngeneic MC38-hPDL1 models and PBMC humanized xenograft models with A375, NCI-H358, or MDA-MB-231 cells. Combination studies were performed with 5-FU, anti-VEGF, and KRAS G12C inhibitors. PK and toxicity profiles were evaluated both in vitro and in animal studies. Results: HEC201625 binding induced PD-L1 dimerization and blocked PD-1/PD-L1 interaction potently and selectively with an IC50 of 2 nM. In the PD-1/PD-L1 NFAT blockade bioassay, HEC201625 effectively reversed PD-1/PD-L1 interaction and restored Jurkat T cell activity. In the co-culture assay with human primary T cells and tumor cells, HEC201625 enhanced human primary T cell activation in a dose-dependent manner. In vivo, HEC201625 showed antitumor activities comparable to those of the anti-PD-L1 antibody atezolizumab in the MC38-hPDL1 murine colon carcinoma model. HEC201625 also demonstrated equivalent or superior efficacy to atezolizumab across multiple humanized immune system tumor models, including A375, NCI-H358, and MDA-MB-231. Synergistic antitumor effects were observed when HEC201625 was combined with 5-FU, anti-VEGF, or KRAS G12C inhibitors. In contrast, no activity was detected in the immunodeficient NCG mice treated with HEC201625, confirming its immune-dependent mechanism. Moreover, HEC201625 demonstrated good bioavailability across multiple animal species, no significant inhibition against 87 off-target receptors, and a favorable safety profile in animal toxicology studies. Conclusion: HEC201625 is a novel oral small molecule inhibitor of PD-L1 demonstrating potent and selective immune activation and favorable pharmacokinetics and preclinical safety profiles. These results support its further development as a clinical candidate, with IND-enabling studies ongoing and Phase I trials planned. Citation Format: Bing Liu, Mingyu Guan, Shaoyan Li, Ning Kang, Ming Li, Ying Zeng, Jing Li, Cliff Cheng, Yingjun Zhang, Kai Lin. HEC201625, a novel oral small molecule inhibitor of PD-L1, demonstrated potent anti-tumor activities both in vitro and in vivo abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 416.