Discovery of benzo[d]isothiazole derivatives as novel scaffold inhibitors targeting the programmed cell death-1/programmed cell death-ligand 1 (PD-1/PD-L1) interaction through "ring fusion" strategy.

The inhibition of programmed cell death-1/programmed cell death-ligand 1 (PD-1/PD-L1) interaction by monoclonal antibodies (mAbs) has achieved promising outcomes in cancer immunotherapy. Due to the inherent deficiencies of mAbs drugs, such as high cost of treatment, immunogenicity, poor pharmacokinetics and penetration of solid tumors, researchers are encouraged to develop small molecule inhibitors, to overcome mAbs drugs' deficiencies and change the situation where small molecule drugs are not available on the market. Herein, we reported a series of benzo[d]isothiazole derivatives targeting the PD-1/PD-L1 interaction through "ring fusion" strategy using BMS-202 as a starting point. Among them, compound D7 exhibited the best inhibitory activity with an IC value of 5.7 nM by homogeneous time-resolved fluorescence (HTRF) binding assay. In immunotoxicity analysis, D7 showed low cytotoxicity to Jurkat T cells in CCK-8 assay compared to BMS-202. The binding mode between D7 and PD-L1 protein was explored by molecular docking and molecular dynamics (MD) simulations, which revealed crucial chemical groups, such as biphenyl group interacting with Ile54, Tyr56, Met115, Ala121, Ile54, Met115, Ala121 and Tyr123 by hydrophobic interactions, bromobenzene moiety forming π-π stacking interaction with Tyr56, as well as l-serine moiety forming hydrogen bond (H-bond) and salt bridge interactions with Asp122 and Lys124. Furthermore, molecular modeling studies showed that D7 is likely to bind to the FA8 (fatty acid 8) binding site of human serum albumin (HSA). Taken together, D7 significantly inhibits the PD-1/PD-L1 interaction with low cytotoxicity, indicating that D7 is a promising starting point for further drug development in cancer immunotherapy.