In Silico Discovery of Androgen Receptor Antagonists with Activity in Castration Resistant Prostate Cancer

Abstract

Previously available androgen receptor (AR) antagonists (bicalutamide, flutamide, and nilutamide) have limited activity against AR in prostate cancers that relapse after castration [castration resistant prostate cancer (CRPC)]. However, recent AR competitive antagonists such as MDV3100, generated through chemical modifications to the current AR ligands, appear to have increased activity in CRPC and have novel mechanisms of action. Using pharmacophore models and a refined homology model of the antagonist-liganded AR ligand binding domain, we carried out in silico screens of small molecule libraries and report here on the identification of a series of structurally distinct nonsteroidal small molecule competitive AR antagonists. Despite their unique chemical architectures, compounds representing each of six chemotypes functioned in vitro as pure AR antagonists. Moreover, similarly to MDV3100 and in contrast to previous AR antagonists, these compounds all prevented AR binding to chromatin, consistent with each of the six chemotypes stabilizing a similar AR antagonist conformation. Additional studies with the lead chemotype (chemotype A) showed enhanced AR protein degradation, which was dependent on helix 12 in the AR ligand binding domain. Significantly, chemotype A compounds functioned as AR antagonists in vivo in normal male mice and suppressed AR activity and tumor cell proliferation in human CRPC xenografts. These data indicate that certain ligand-induced structural alterations in the AR ligand binding domain may both impair AR chromatin binding and enhance AR degradation and support continued efforts to develop AR antagonists with unique mechanisms of action and efficacy in CRPC.