Abstract 83: Differential inhibitory effects of galeterone, abiraterone, orteronel and ketoconazole on CYP17A1 and steroidogenesis.

Abstract

Abstract Prostate cancer growth is dependent on androgens. Because the CYP17A1 enzyme plays a central role in converting progestogens to androgens, inhibitors of CYP17A1 have been developed to treat castration-resistant prostate cancer (CRPC). CYP17A1 has both hydroxylase and lyase catalytic functions. However, hydroxylase inhibition causes build-up of progestogens and mineralocorticoids while also reducing cortisol. This syndrome, referred to secondary mineralocorticoid excess, causes edema, hypokalemia and hypertension and is treated clinically with concomitant steroids. We analyzed four inhibitors for their effects on CYP17A1 hydroxylase and lyase activities, and more globally on the steroidogenic pathway. Human CYP17A1 expressed in yeast microsomes was incubated with pregnenolone or 17α-hydroxypregnenolone and products quantitated using LC/MS/MS. Galeterone, abiraterone, orteronel or ketoconazole were added, and hydroxylase or lyase IC50 values calculated. In a separate experiment, H295R human adrenocortical carcinoma cells were incubated with a range of drug concentrations between 0.316 nM and 10 μM for 24 hours and media was analyzed for steroid production by LC/MS/MS. We observed that all drugs inhibited CYP17A1. However, potency and selectivity for hydroxylase and lyase varied significantly. Galeterone was the most potent CYP17A1 lyase inhibitor that also exhibited selectivity (3x more selective for lyase than hydroxylase). In contrast, abiraterone most selectively inhibited CYP17A1 hydroxylase (by 1.7-fold), while orteronel and ketoconazole were less potent lyase inhibitors than galeterone. In the cell-based assay, all drugs at 1 μM inhibited testosterone synthesis by ≥94%. However, while galeterone produced only moderate changes in cortisol (decreased 14%) and other intermediate precursors, evidence of hydroxylase inhibition for the other inhibitors was supported by significant elevation of progesterone (increased 293-fold by abiraterone), mineralocorticoids (orteronel increased DOC 74-fold), or reductions in cortisol (abiraterone, orteronel, ketoconazole reduced by 91%, 70% and 94%, respectively). Steroid changes across a range of drug concentrations from 0.316 nM to 10 μM showed evidence of different potencies for CYP17 hydroxylase and lyase, as well as potential off-target effects on other enzymes in the steroidogenic pathway. In summary, we found that galeterone is a selective and potent CYP17A1 lyase inhibitor that also antagonizes and degrades the AR, and exhibits minimal evidence of deleterious steroid changes associated with secondary mineralocorticoid excess. These experimental data recapitulate the Phase 1 clinical experience where no mineralocorticoid excess was observed, or prednisone required in CRPC patients receiving galeterone therapy. ARMOR2, a phase 2 clinical study in CRPC, is being initiated in 2012. Citation Format: Douglas Jacoby, Martin Williams. Differential inhibitory effects of galeterone, abiraterone, orteronel and ketoconazole on CYP17A1 and steroidogenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 83. doi:10.1158/1538-7445.AM2013-83