In humans, the CYP17A1 gene is largely associated with endocrine effects and steroid hormone metabolism.    Furthermore, mutations in the CYP17A1 gene are associated with rare forms of congenital adrenal hyperplasia, in particular 17α-hydroxylase deficiency/17,20-lyase deficiency and isolated 17,20-lyase deficiency. Overall, CYP17A1 is an important target for inhibition in the treatment of prostate cancer because it produces androgen that is required for tumor cell growth.   Currently, the FDA has approved only one CYP17A1 inhibitor, abiraterone, which contains a steroidal scaffold that is similar to the endogenous CYP17A1 substrates. Abiraterone is structurally similar to the substrates of other cytochrome P450 enzymes involved in steroidogenesis, and interference can pose a liability in terms of side effects. Using nonsteroidal scaffolds is expected to enable the design of compounds that interact more selectively with CYP17A1.  Potent inhibitors of the CYP17A1 enzyme provide a last line defense against ectopic androgenesis in advanced prostate cancer. 
Anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin) are members of the third generation of aromatase inhibitors that has now replaced aminoglutethimide (Cytadren), the progestins, and tamoxifen (Nolvadex) as the hormonal therapy of choice in estrogen-receptor-positive, postmenopausal, metastatic breast cancer. This article will review the role of aromatase in the pathogenesis of breast cancer and the results of recent studies that have established the role of its inhibitors in estrogen-receptor-positive breast cancer. We will also briefly outline the rationale and design of ongoing studies. [ONCOLOGY 15(8):965-979, 2001]
Recent postmarketing reports and a US Food and Drug Administration analysis have documented uncommon persistent sexual and nonsexual side-effects in a subset of younger men who have taken finasteride 1 mg for androgenic alopecia. While the mechanisms of the sexual side-effects in humans is incompletely understood, one study found lower cerebrospinal fluid concentrations of dihydrotestosterone, progesterone, dihydroprogesterone and allopregnanolone, and higher levels of testosterone, 5α-androstane-3α,17β-diol and pregnenolone. Another study found up-regulation of the androgen receptor in the human foreskin with a mean of 5 years after finasteride discontinuation.