• Gerhardt Attard, Alison H M Reid, Richard J Auchus, Beverly A Hughes, Amy Mulick Cassidy, Emilda Thompson, Nikhil Babu Oommen, Elizabeth Folkerd, Mitch Dowsett, Wiebke Arlt, and Johann S de Bono.
• The Institute of Cancer Research, Sutton, Surrey SM2 5PT, UK. Gerhardt.attard@icr.ac.uk
• J. Clin. Endocrinol. Metab. 2012 Feb 1; 97 (2): 507-16.

ContextAbiraterone acetate is a small-molecule cytochrome P450 17A1 (CYP17A1) inhibitor that is active in castration-resistant prostate cancer.ObjectiveOur objective was to determine the impact of abiraterone with and without dexamethasone treatment on in vivo steroidogenesis.Design And MethodsWe treated 42 castrate, castration-resistant prostate cancer patients with continuous, daily abiraterone acetate and prospectively collected blood and urine before and during abiraterone treatment and after addition of dexamethasone 0.5 mg daily.ResultsTreatment with single-agent abiraterone acetate was associated with accumulation of steroids with mineralocorticoid properties upstream of CYP17A1. This resulted in side effects, including hypertension, hypokalemia, and fluid overload, in 38 of 42 patients that were generally treated effectively with eplerenone. Importantly, serum and urinary androgens were suppressed by more than 90% from baseline. Urinary metabolites of 17-hydroxypregnenolone and 17-hydroxyprogesterone downstream of 17α-hydroxylase remained unchanged. However, 3α5α-17-hydroxypregnanolone, which can be converted via the backdoor pathway toward 5α-dihydrotestosterone, increased significantly and correlated with levels of the major 5α-dihydrotestosterone metabolite androsterone. In contrast, urinary metabolites of 11-deoxycortisol and active glucocorticoids declined significantly. Addition of dexamethasone to abiraterone acetate significantly suppressed ACTH and endogenous steroids, including 3α5α-17-hydroxypregnanolone.ConclusionCYP17A1 inhibition with abiraterone acetate is characterized by significant suppression of androgen and cortisol synthesis. The latter is associated with a rise in ACTH that causes raised mineralocorticoids, leading to side effects and incomplete 17α-hydroxylase inhibition. Concomitant inhibition of 17,20-lyase results in diversion of 17-hydroxyprogesterone metabolites toward androgen synthesis via the backdoor pathway. Addition of dexamethasone reverses toxicity and could further suppress androgens by preventing a rise in substrates of backdoor androgen synthesis.

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