D4A is specifically an inhibitor of CYP17A1 (17α-hydroxylase/17,20-lyase), 3β-HSD, and 5α-reductase.[1] In addition, it has also been found to act as a competitive antagonist of the androgen receptor (AR), with potency reportedly comparable to that of enzalutamide.[1] However, the initial 5α-reduced metabolite of D4A, 3-keto-5α-abiraterone, is an agonist of the AR, and has been found to stimulate prostate cancer progression.[2] The formation of this metabolite can be blocked by the coadministration of dutasteride, a selective and highly potent 5α-reductase inhibitor, and the addition of this medication may improve the effectiveness of abiraterone acetate in the treatment of prostate cancer.[2]