Direct effects of the selective CYP17 lyase (L) inhibitor, VT-464, on the androgen receptor (AR) and its oral activity in an F876L tumor mouse xenograft model.

Abstract

263 Background: MDV3100 inhibits binding of androgens to AR and abiraterone is known to block androgen production through CYP17 inhibition; both are effective treatments for castration-resistant prostate cancer (CRPC, yet cross-resistance to both agents is problematic. The AR mutations F876L and T877A increase resistance to MDV3100 (M) or abiraterone (A), respectively. A regimen resistance is likely mediated through T877A activation by stimulatory progesterone (P4), pregnenolone (P5), or prednisone. The oral, non-steroidal L-inhibitor, VT-464, in phase 2 CRPC studies without prednisone coadministration, does not increase P4 or P5. Methods: The effects of VT-464, A, and M on AR transcription activity and cell proliferation were assessed and compared using reporter assays, and their oral effects in an M-resistant (F876L) human tumor mouse model were also assessed. Results: VT-464 competitively antagonized (Schild analysis) wild-type AR (IC50 =18 µM) and its potency was not decreased by AR overexpression (VCaP IC50=7.4μM). VT-464 fully antagonized AR T877A (IC50=0.52 μM) and inhibited LNCaP cell (AR T877A mutation) proliferation (IC50=0.70 μM) with potency similar to M. In contrast to M, VT-464 fully inhibited F876L (IC50=2.5 μM). VT-464 did not activate any AR forms up to the highest concentration tested (60 μM). In vivo, greater F876L tumor inhibition was observed with oral VT-464 compared to oral M or abiraterone acetate. Conclusions: VT-464 more potently antagonized mutated AR forms associated with A or M resistance than wild-type; AR antagonism contributed to xenograft activity and appears relevant to its clinical pharmacology. Due to its high L inhibition selectivity and potent mutant AR antagonist activity, VT-464 has a suitable profile for the treatment of early- or late-stage CRPC patients. VT-464 has advanced into phase 2 studies funded by Innocrin (NCT#02012920), the NCI (NCT02130700), and PCF in M- and A-resistant populations.