Identification of new small molecule inhibitors of PD-1/PD-L1 interaction by a comprehensive ligand fishing system.

The development of programmed death 1 (PD-1) checkpoint/programmed death-ligand 1 (PD-L1) interaction inhibitors has opened a new front in the treatment of carcinoma. An increasing amount of research is devoted to small-molecule compounds that target this PD-1/PD-L1 interaction. In this article, we report the discovery of three new PD-1/PD-L1 inhibitors from Scutellaria baicalensis Georgi and Sophora flavescens Aiton herbal extracts, namely baicalin, maackiain, and oxysophocarpine, using a comprehensive ligand fishing system, which integrates a dual-target surface plasmon resonance biosensor and a magnetic beads method. These three compounds were also confirmed in the serum in vivo, validated by binding affinity evaluation, molecular docking, and competitive enzyme-linked immunosorbent assay (ELISA) assay to act upon the interface of PD-1/PD-L1. Thus, these three ingredients could be potential PD-1/PD-L1 inhibitors and may serve as hit compounds for immunotherapeutic drug discovery. These results also highlight the efficiency of the dual-target surface plasmon resonance (SPR) and magnetic beads ligand fishing system in drug screening for disease treatment.