Description_x000D_ Biochem/physiol Actions_x000D_ D4A (δ4-Abiraterone; 3-Pyridinyl)-androsta-4,16-dien-3-one) is an abiraterone metabolite found to be more active than abiraterone. Abiraterone is a potent, selective, and orally bioavailable inhibitor of CYP17A1 (CYP450c17), an enzyme that catalyzes two key serial reactions (17 alpha 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 can stop the production of androgens in both places. It is approved for the treatment of metastatic castration-resistant prostate cancer. D4A blocks not only CYP17A1, but also blocks two other enzymes, 3?-hydroxysteroid dehydrogenase (3?HSD) and steroid-5?-reductase (SRD5A), which are involved in producing the androgen 5?-dihydrotestosterone (DHT), while also blocking the androgen receptor.