A Novel Drug Candidate that Selectively Targets the Critical Androgen Receptor-ELK1 Growth Axis in Advanced and Drug-Resistant Prostate Cancer

Androgen receptor (AR)-dependent prostate cancer (PCa) cells require co-activation of ELK1 by AR to activate a critical set of cell cycle and mitosis genes, regardless of hormone - sensitivity. A small molecule antagonist (KCI807) that inhibits AR-dependent growth by selectively binding to AR and blocking its association with ELK1 is limited as a drug by auto-induced metabolism. Using structure-activity data, consistent with modeling a physically mapped KCI807 binding pocket, we developed a new class of compounds with a different core structure comprising 5-Hydroxy-2-(3-hydroxyphenyl)-1-methylquinolin-4(1H)-one (KCI830), with variable N- substituents. The compound with a N-2,2,2-trifluoroethyl substitution (KCI838) was the fastest acting and most potent inhibitor of AR-dependent cell growth and colony formation in PCa model cells, including exclusively AR splice variant-dependent and other enzalutamide-resistant cells, without affecting growth of AR-negative cell lines. Critical tests were conducted to establish that KCI838 recapitulates the previously elucidated mode of action of KCI807. KCI838 selectively inhibited ELK1-dependent vs. androgen response element (ARE)-driven promoter and gene activation by AR. KCI838 blocked AR binding to ELK1 in situ tested by BRET assay. Increasing the total cellular AR by [~]2-fold using ectopic AR expression caused the predicted change in drug dose-response profile for growth, implicating AR as the exclusive target for the activity of KCI838. KCI838s molecular scaffold conferred reduced enzyme induction in primary human hepatocytes and weakened interactions with human UGT1A1 and CYP1A2. In mice bearing an aggressive, enzalutamide-resistant patient-derived PCa tumor xenograft characteristically overexpressing prostatic acid phosphatase, daily bolus injections of a soluble 3phosphate monoester prodrug of KCI838 (KCI838PME) progressively inhibited tumor growth, concomitant with tumor accumulation of active hydrophobic drug, without significant toxicity. Additionally, ALZET osmotic pumps were used to establish proof-of-concept for reversible in vivo anti-tumor activity of KCI838PME administered in a low dose, controlled release mode. The results warrant investigation of KCI838PME in a controlled-release formulation, to treat PCa that is resistant to current AR-targeted therapies while obviating the need for testosterone suppression.