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Merck & Co., Inc.

Drug Names(s): finasteride

Description: Administration of propecia decreases scalp and serum DHT concentrations in men with male pattern hair loss (androgenetic alopecia). Propecia is a competitive and specific inhibitor of Type II 5a-reductase, an intracellular enzyme that converts the androgen testosterone into DHT. By this mechanism, propecia appears to interrupt a key factor in the development of androgenetic alopecia in those patients genetically predisposed.

In humans, the mechanism of action of propecia is based on its preferential inhibition of the Type II isozyme. Using native tissues (scalp and prostate), in vitro binding studies examining the potential of propecia to inhibit either isozyme revealed a 100-fold selectivity for the human Type II 5a-reductase over Type I isozyme. For both isozymes, the inhibition by propecia is accompanied by reduction of the inhibitor to dihydrofinasteride and adduct formation with NADP+.

Inhibition of Type II 5a-reductase blocks the peripheral conversion of testosterone to DHT, resulting in significant decreases in serum and tissue DHT concentrations.

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