Finasteride is a competitive and specific inhibitor of Type II 5-α-reductase. Finasteride has no affinity for the androgenic, antiandrogenic, estrogenic, antiestrogenic, or progestational effect. Inhibition of this enzyme blocks the peripheral conversion of testosterone to the androgen dihydrotestosterone (DHT), resulting in significant decreases in serum and tissue DHT concentrations. Circulating levels of testosterone were increased by approximately 10-15% compared with placebo, yet remained within the physiologic range. Finasteride produces a rapid reduction in serum DHT concentration, reaching significant suppression within 24 hours of dosing. In Benign Prostatic Hyperplasia (BPH), enlargement of the prostate gland is dependent upon the conversion of testosterone to DHT within the prostate. Finasteride is effective in reducing circulating and intraprostatic DHT.
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