Finasteride, strictly considered, is not an antiandrogen, in that it does not act at the androgen receptor. It is a competitive inhibitor of the type 2 isoenzyme of 5a-reductase, the enzyme responsible for conversion of testosterone to the more potent dihydrotestosterone. This drug has been approved for treatment of benign prostatic hyperplasia and male pattern baldness.
Increased 5a-reductase activity in the skin is considered to be the major pathogenetic mechanism of "idiopathic" hirsutism (i.e., excessive hair growth occurring in the absence of increased serum androgens and with ovulatory cycles). Selective enzyme inhibition has been proposed as a rational medical approach to this condition. However, because 5a-reductase plays a key role in the andro-genic regulation of hair growth, enzyme inhibition may be potentially effective in all forms of hirsut-ism, irrespective of specific pathogenic mechanisms. In the skin, the type 1 isoenzyme of 5a-reductase isoenzymes appears to be primarily expressed; the importance of the type 2 isoenzyme in hair growth is still unclear. However, the isoenzyme selectivity of finasteride is not absolute at therapeutic doses. Studies assessing finasteride efficacy in women with hirsutism consistently indicated positive results, without noticeable side effects. Patients with hirsutism given the drug only occasionally reported a decrease in libido. Finasteride has been used in a dose of 5 mg/day in most published trials, although similar effects on skin androgens were found with 1 mg. A recent uncontrolled study reported similar results with continuous 2.5 mg/day or intermittent 2.5 mg every 3 days of finasteride.
In the controlled comparative study mentioned previously (18), the efficacy of finasteride in hirsute women was similar to that of spironolactone or flutamide. Consistent results were also obtained in other studies comparing finasteride, spironolactone, or flutamide efficacy. Alternatively, other studies showed less improvement of hirsutism with finasteride than with spironolactone or flutamide. Interestingly, preliminary data suggest that the addition of finasteride might increase the efficacy of spironolactone in women with hirsutism (25).
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