Abiraterone treatment in castration-resistant prostate cancer selects for progesterone responsive mutant androgen receptors

Abstract

Purpose: The CYP17A1 inhibitor abiraterone markedly reduces androgen precursors and is thereby effective in castration-resistant prostate cancer (CRPC). However, abiraterone increases progesterone, which can activate certain mutant androgen receptors (AR) identified previously in flutamide-resistant tumors. Therefore, we sought to determine if CYP17A1 inhibitor treatment selects for progesterone-activated mutant ARs.

Experimental design: AR was examined by targeted sequencing in metastatic tumor biopsies from 18 patients with CRPC who were progressing on a CYP17A1 inhibitor (17 on abiraterone, 1 on ketoconazole), alone or in combination with dutasteride, and by whole-exome sequencing in residual tumor in one patient treated with neoadjuvant leuprolide plus abiraterone.

Results: The progesterone-activated T878A-mutant AR was present at high allele frequency in 3 of the 18 CRPC cases. It was also present in one focus of resistant tumor in the neoadjuvant-treated patient, but not in a second clonally related resistant focus that instead had lost one copy of PTEN and both copies of CHD1. The T878A mutation appeared to be less common in the subset of patients with CRPC treated with abiraterone plus dutasteride, and transfection studies showed that dutasteride was a more potent direct antagonist of the T878A versus the wild-type AR.

Conclusions: These findings indicate that selection for tumor cells expressing progesterone-activated mutant ARs is a mechanism of resistance to CYP17A1 inhibition.

Conflict of interest statement

Conflict of Interest Statement: The authors do not have any conflicts of interest to report.

©2014 American Association for Cancer Research.

Figures

Figure 1. Activation of T878A mutant AR by progesterone and reduced metabolite

A, C4-2 cells (expressing endogenous T878A AR) cultured in steroid depleted medium containing charcoal-dextran serum were stimulated for 24 hours with progesterone (Pg) at 0.1 – 10 nM, with the addition of dutasteride at 10 μM (black bars) or vehicle control (DMSO, white bars). Expression of the indicated mRNA was then assessed by real time quantitative RT-PCR, with all results being normalized to basal levels. Values shown for all panels are average and S.D. for triplicate PCR reactions and are representative of at least 3 independent experiments. B, C4-2 cells in steroid depleted medium were stimulated with 0.1 – 10 nM progesterone (Pg, black bars) or pregnan-3,20-dione (Pd, grey bars) for 24 hours and mRNA levels were then assessed.

Figure 2. Dutasteride is more potent antagonist of the T878A mutant versus wildtype AR

A, C4-2 cells in steroid depleted medium were stimulated for 24 hours with DHT (0.1 – 10 nM) with the addition of dutasteride at 10 μM (black bars) or vehicle control (DMSO, white bars). B, C4-2 cells in steroid depleted medium were stimulated for 24 hours with vehicle (EtOH, white bars), DHT (1 nM, black bars), or progesterone (1 nM, grey bars), in the presence of dutasteride (Duta, 10 μM), bicalutamide (Bic, 10 μM), or vehicle (Veh). C, VCaP cells (amplified wildtype AR) in steroid depleted medium were stimulated for 24 hours with vehicle (EtOH, white bars) or DHT (1 nM, black bars), in the presence of dutasteride (Duta, 10 μM), bicalutamide (Bic, 10 μM), or vehicle (Veh). D, PC-3 cells (AR negative) that were stably transfected with wild-type (WT) or T878A AR were cultured in steroid depleted medium and stimulated for 24 hours with vehicle (EtOH, white bars) or DHT (1 nM, black bars), alone or in the presence of dutasteride (Duta, 10 μM) or bicalutamide (Bic, 10 μM). Expression of the AR regulated endogenous FKBP5 was then assessed. AR protein levels are shown on left.