Design, Synthesis, and Biological Evaluation of Small-Molecule PD-1/PD-L1 Inhibitors Bearing a Rigid Benz[d]oxazole Skeleton.

Monoclonal antibodies targeting the programmed cell death protein 1/programmed cell death ligand 1 (PD-1/PD-L1) immune checkpoint have demonstrated clinical efficacy in cancer immunotherapy. However, antibody-based immunotherapies still suffer from unavoidable limitations, such as suboptimal pharmacokinetic properties and potential immunogenicity. In contrast, small-molecule PD-1/PD-L1 inhibitors may complement antibody therapeutics and provide an alternative modality. In this study, we designed and synthesized a series of biphenyl-based inhibitors featuring an oxadiazole motif and a conformationally constrained benzo[d]oxazole framework. SAR analysis revealed that introducing a 1,2,4-oxadiazole ring directly linked to the distal phenyl ring of the biphenyl scaffold, together with cyclization at the ether linkage to construct a rigid benzo[d]oxazole framework connected to the proximal phenyl ring, afforded overall high potency. Notably, compound I-5 emerged as the lead compound, exhibiting an IC50 of 8.04 nM for inhibition of the PD-1/PD-L1 interaction. At the cellular level, I-5 displayed low nonspecific cytotoxicity toward tumor cells, restored T-cell immune function by promoting interferon-γ (IFN-γ) secretion, and markedly enhanced peripheral blood mononuclear cell (PBMC)-mediated cytotoxicity against MDA-MB-231 tumor cells. Collectively, these results suggest that compound I-5 is a promising small-molecule PD-1/PD-L1 inhibitor and warrants further in-depth investigation.