Syntheses, Biological Evaluations, and Mechanistic Studies of Benzo[<i>c</i>][1,2,5]oxadiazole Derivatives as Potent PD-L1 Inhibitors with <i>In Vivo</i> Antitumor Activity

Abstract

A series of novel benzo[<i>c</i>][1,2,5]oxadiazole derivatives were designed, synthesized, and biologically evaluated as inhibitors of PD-L1. Among them, compound <b>L7</b> exhibited 1.8 nM IC₅₀ value in a homogeneous time-resolved fluorescence (HTRF) assay, which was 20-fold more potent than the lead compound <b>BMS-1016</b>. In the surface plasmon resonance (SPR) assay, <b>L7</b> bound to human PD-L1 (hPD-L1) with a <i>K</i>_D value of 3.34 nM, without showing any binding to hPD-1. In the cell-based coculture assay, <b>L7</b> blocked PD-1/PD-L1 interaction with an EC₅₀ value of 375 nM, while <b>BMS-1016</b> had an EC₅₀ value of 2075 nM. Moreover, compound <b>L24</b>, an ester prodrug of <b>L7</b>, was orally bioavailable and displayed significant antitumor effects in tumor models of syngeneic and PD-L1 humanized mice. Mechanistically, <b>L24</b> exhibited significant <i>in vivo</i> antitumor effects probably through promoting antitumor immunity. Together, this series of benzoxadiazole PD-L1 inhibitors holds promise for tumor immunotherapy. Preclinical trials with selected compounds are ongoing in our laboratory.