Anticancer activity of the selective CYP17A1 inhibitor, VT-464, in preclinical models of castrate-resistant prostate cancer.

Abstract

104 Background: Castrate resistant prostate cancer (CRPC) continues to be sensitive to anti-androgen therapy as evidenced by the recent successes of abiraterone acetate (AA) and enzalutamide (ENZ). VT-464 is a novel, non-steroidal, small molecule CYP17A1 inhibitor with selectivity for the lyase activity of this dual enzyme. The objective of this study was to evaluate the anti-cancer activity of VT-464 compared to AA in CRPC in vitro models that are ENZ-responsive and ENZ-resistant and in an ENZ-resistant xenograft model. Methods: In vitro studies used the human CRPC, C4-2, and ENZ-resistant cell lines, MR49C and MR49F cells, in androgen-free media. AR transcriptional activity was assessed by probasin luciferase. AR-related and steroidogenesis pathways were assessed by western blot and/or qRT-PCR. A MR49F xenograft model in castrate mice compared oral VT-464 treatment to vehicle and AA. Steroid concentrations were measured using LC-MS chromatography. Results: VT-464 demonstrated a greater decrease in AR transactivation compared to AA in C4-2 and both ENZ-resistant cell lines. A greater decrease in AR-dependent gene transcription occurred with VT-464 treatment compared to AA in all cell lines. Prostate-specific antigen (PSA) protein levels in vitro were also lower with VT-464. Gene transcripts StAR, CYP17A1, HSD17B3 and SRD5A1 increased following treatment with VT-464 both in vitro and in vivo. A greater increase was seen with VT-464 treatment compared to AA. In vivo results demonstrated greater tumor growth inhibition and decreased serum PSA levels in mice treated with oral VT-464 compared to AA. Steroid analysis revealed lower testosterone (T) and dihydrotestosterone (DHT) concentrations in C4-2 cells with VT-464 treatment compared to AA. In vivo, the intra-tumoral DHT and T levels were significantly lower in response to VT-464 or AA compared to vehicle, with the greatest decrease seen with VT-464. Conclusions: The selective CYP17 inhibitor VT-464 demonstrated anti-cancer activity in pre-clinical models of CRPC, lowering intratumoral T and DHT concentrations significantly in castrate mice. These results suggest greater androgen suppression and inhibition of AR axis signaling by VT-464 than by AA.