Preclinical anti-tumor activity of small-molecule oral PD-L1 checkpoint inhibitors.

e14558 Background: The PD-1/PD-L1 checkpoint axis is a validated target in oncology, and immunotherapy with antibody approaches have proven efficacy across various tumor types. Oral small-molecule inhibitors of PD-L1 may offer advantages over antibody approaches by enabling tunable on-target engagement and the potential for better tissue penetration and improved efficacy. Here we report the preclinical in vitro activity of small-molecule inhibitors of PD-L1 possessing a novel mechanism of action, favorable pharmacokinetics, and demonstrated in vivo efficacy in a mouse colon adenocarcinoma model. Methods: In vitro activity was assessed in a Jurkat T cell NFAT reporter assay and PD-L1 reduction was confirmed in CHO-K1 cells expressing human PD-L1 (CHO-K1-hPD-L1) and in peripheral blood mononuclear cells (PBMCs) from healthy donors. T cell activation assays were conducted with PBMCs from healthy donors. Non-specific cytokine release was evaluated in human whole blood. Pharmacokinetic (PK) evaluations were conducted in rodents and non-human primates (NHP). In vivo efficacy was evaluated in an MC38 tumor humanized PD-L1 and PD-1 mouse model. Results: Small-molecule inhibitors of PD-L1 were able to disrupt PD-1:PD-L1 and PD-L1:CD80 interactions through the reduction of PD-L1 expression on the cell surface via a novel internalization mechanism, resulting in PD-L1 degradation (EC50s ranged from 1.9 – 24 nM in CHO-K1-hPD-L1 and primary human myeloid cells). Lead PD-L1 inhibitors mediated potent activation of T cells in a NFAT reporter assay (EC50s of 13 and 18 nM) as well as dose responsive elevations in IL-2 production in human PBMCs stimulated with staphylococcal enterotoxin B. Compound treatment did not elicit non-specific cytokine release in human whole blood, supportive of favorable immune safety. PK profiles showed low systemic clearance in rodents and NHP. In an MC38 tumor model, once daily oral administration of compounds at 3, 10, and 30 mg/kg resulted in dose responsive tumor reduction that was associated with T cell activation and infiltration into tumors. Conclusions: Oral small-molecule PD-L1 inhibitors possessing a novel mechanism of action and the ability to mediate T cell activation in primary human immune cell types were identified. These compounds display in vivo anti-tumor efficacy comparable to anti-PD-L1 antibody and possess favorable preclinical profiles for further development, with the potential for all-oral treatment regimens for oncology.