Galeterone is an inhibitor of CYP17A1 (CYP450c17), an enzyme that catalyzes two key serial reactions (17-α hydroxylase and 17,20 lyase) in androgen and estrogen biosynthesis resulting in the formation of DHEA and androstenedione, which may ultimately be metabolized into testosterone. CYP17 is the key enzyme for androgen biosynthesis in both the testes and adrenals, so its inhibition should stop the production of androgens in both places. Galeterone, moreover, is a potent blocker of the androgen receptor and causes down-regulation of androgen receptor (AR) protein expression in vitro and in vivo.