FIGURE 40–6 Control of androgen secretion and activity and some sites of action of antiandrogens: (1) competitive inhibition of GnRH receptors; (2) stimulation (+, pulsatile administration) or inhibition via desensitization of GnRH receptors (–, continuous administration); (3) decreased synthesis of testosterone in the testis; (4) decreased synthesis of dihydrotestosterone by inhibition of 5α-reductase; (5) competition for binding to cytosol androgen receptors.
approved for this use in the United States. The dosage is 5 mg/d. Dutasteride is a similar orally active steroid derivative with a slow onset of action and a much longer half-life than finasteride. It is approved for treatment of benign prostatic hyperplasia at a dosage of 0.5 mg daily. These drugs are not approved for use in women or children, although finasteride has been used successfully in the treatment of hirsutism in women and is approved for treatment of early male pattern baldness in men (1 mg/d).
Flutamide, a substituted anilide, is a potent antiandrogen that has been used in the treatment of prostatic carcinoma. Although not a steroid, it behaves like a competitive antagonist at the androgen receptor. It is rapidly metabolized in humans. It frequently causes mild gynecomastia (probably by increasing testicular estrogen production) and occasionally causes mild reversible hepatic toxicity. Administration of this compound causes some improvement in most patients with prostatic carcinoma who have not had prior endocrine therapy. Preliminary studies indicate that flutamide is also useful in the management of excess androgen effect in women.
Bicalutamide, nilutamide, and enzalutamide are potent orally active antiandrogens that can be administered as a single daily dose and are used in patients with metastatic carcinoma of the prostate. Studies in patients with carcinoma of the prostate indicate that these agents are well tolerated. Bicalutamide is recommended (to reduce tumor flare) for use in combination with a GnRH analog and may have fewer gastrointestinal side effects than flutamide. A dosage of 150–200 mg/d (when used alone) is required to reduce prostate-specific antigen levels to those achieved by castration, but, in combination with a GnRH analog, 50 mg/d may be adequate. Nilutamide is administered in a dosage of 300 mg/d for 30 days followed by 150 mg/d. The dosage of enzalutamide is 160 mg/d orally.
Cyproterone and cyproterone acetate are effective anti-androgens that inhibit the action of androgens at the target organ. The acetate form has a marked progestational effect that suppresses the feedback enhancement of LH and FSH, leading to a more effective antiandrogen effect. These compounds have been used in women to treat hirsutism and in men to decrease excessive sexual drive and are being studied in other conditions in which the reduction of androgenic effects would be useful. Cyproterone acetate in a dosage of 2 mg/d administered concurrently with an estrogen is used in the treatment of hirsutism in women, doubling as a contraceptive pill; it has orphan drug status in the United States.
Spironolactone, a competitive inhibitor of aldosterone (see Chapter 15), also competes with dihydrotestosterone for the androgen receptors in target tissues. It also reduces 17α-hydroxylase activity, lowering plasma levels of testosterone and androstenedione. It is used in dosages of 50–200 mg/d in the treatment of hirsutism in women and appears to be as effective as finasteride, flutamide, or cyproterone in this condition.