Discovery and Structure-Based Optimization of Novel Atg4BInhibitors for the Treatment of Castration-Resistant Prostate Cancer

Autophagy inhibition is an attractive target for cancer therapy. In this study, we discovered inhibitors of Atg4B essential forautophagosome formation and evaluated their potential as therapeuticsfor prostate cancer. Seventeen compounds were identified as candidatesafterin silicoscreening and a thermal shift assay. Among them,compound17showed the most potent Atg4B inhibitory activity,inhibited autophagy induced by anti-castration-resistant prostate cancer(CRPC) drugs, and significantly enhanced apoptosis. Although17hasbeen known as a phospholipase A2(PLA2) inhibitor, other PLA2inhibitors had no effect on Atg4B and autophagy. We then performedstructural optimization based on molecular modeling and succeeded indeveloping21f(by shortening the alkyl chain of17), which was a potentcompetitive inhibitor for Atg4B (Ki= 3.1 mu M) with declining PLA2inhibitory potency. Compound21fenhanced the anticancer activity of anti-CRPC drugsviaautophagy inhibition. These findings suggest that21fcan be used as an adjuvant drug for therapy with anti-CRPC drugs.