ANTIANDROGENS (androgen antagonists) are a class of drugs that are hormone antagonists. Some drugs act directly to prevent the actions of the male sex hormone, testosterone, at receptors on its target tissues (e.g. cyproterone). Others act indirectly by preventing the formation of androgens by inhibiting the enzyme 5α-reductase (e.g. flutamide). Finally, some agents act indirectly by inhibiting the release of androgens (e.g. buserelin).

Cyproterone is used in high doses as an ANTICANCER AGENT for cancer of the prostate gland. It is also used in relatively moderate doses, for the treatment of precocious puberty in males, and for hypersexuality or sexual deviation in men (in whom the drug causes a condition of reversible sterility through a reduction in the production of sperm and a decrease in libido). It works by being a derivative of progesterone with weak progestogenic activity. Thus it is a partial agonist at androgen receptors, competing with dihydrotestosterone for receptors in androgen-sensitive target tissues. By an effect on the hypothalamus it decreases the synthesis of gonadotrophins. It can also be used (orally at low dose, and in a preparation containing oestrogen) to treat acne, and excess body hair (hirsutism) in women. Flutamide is used orally as an anticancer agent for the treatment of prostate cancer. It inhibits the enzyme 5α-reductase, which converts 4-ene-oxysteroids (e.g. testosterone) irreversibly to the corresponding 5α-3-oxysterone in vivo (e.g. dihydrotestosterone). The latter has a greater affinity for androgen receptors, which then regulate specific gene expression. Inhibitors such as finasteride, which inhibit this enzyme, do not themselves bind to androgen receptors or have any direct hormonal actions, and do not inhibit the formation of other steroids, and so do not affect spermatogenesis. The main use of 5α-reductase inhibitors in men is to treat benign prostatic hyperplasia (BPH). In women they may have a role in treating hirsutism, male-pattern baldness and acne. Trials are now being conducted to examine a possible role in prophylaxis against prostate cancer. See 5α-REDUCTASE INHIBITORS.

Buserelin is an analogue of the hypothalamic hormone, gonadotrophin-releasing hormone (gonadorelin). In chronic use it reduces pituitary secretion of gonadotrophin, which results in reduced secretion of sex hormones by the ovaries or testes. Buserelin is used to treat endometriosis, and also as an anticancer agent for cancer of the prostate gland. It is also used prior to in vitro fertilization.