Design, synthesis and biological evaluation of isoxazole-containing biphenyl derivatives as small-molecule inhibitors targeting the programmed cell death-1/ programmed cell death-ligand 1 immune checkpoint

Monoclonal antibodies targeting the programmed cell death-1/ programmed cell death-ligand 1 (PD-1/PD-L1) immune checkpoint have achieved enormous success in cancer immunotherapy. But the antibody-based immunotherapies carry a number of unavoidable deficiencies such as poor pharmacokinetic properties and immunogenicity. Small-molecule PD-1/PD-L1 inhibitors offer the superiority of complementarity with monoclonal antibodies and represent an appealing alternative. A novel series of isoxazole-containing biphenyl compounds were designed, synthesized and evaluated as PD-1/PD-L1 inhibitors in this paper. The structure–activity relationship of the novel synthesized compounds indicated that the ring-closure strategy of introducing isoxazole could be employed and the 3-cyanobenzyl group was significant for the inhibitory activity against the PD-1/PD-L1 protein–protein interactions. Molecular docking studies were performed to help understand the binding mode of the small-molecule inhibitor with the PD-L1 dimer. In particular, compound II-12 was a promising anti-PD-1/PD-L1 inhibitor with the IC50 value of 23.0 nM, providing valuable information for future drug development.