5α-reductase catalyzes the NADPH-dependent reduction of Δ4,5 double bonds in several steroid substrates, including testosterone , which is converted to dihydrotestosterone, the primary mediator of prostate growth. The 5α-reductase enzymes responsible for the reduction of testosterone to dihydrotestosterone exist as two forms: type I, which occurs in the skin, liver, and ventral prostate and type II, which is expressed in ventral prostate, epididymis, and other reproductive tissues. Dutasteride is a time-dependent, dual inhibitor of 5α-reductase (apparent Kis = 17 and 4.3 nM at 10 and 30 minute reaction times, respectively) that is frequently used in the treatment of benign prostatic hyperplasia. Compared to finasteride , a selective inhibitor of type II 5α-reductase, dutasteride has a greater terminal half-life (1 vs. 14 hours in rat, respectively) and is more effective at decreasing serum levels of dihydrotestosterone at single doses > 10 mg. Dutasteride has also been used as a strategy to reduce androgen levels related to prostate tumor burden on androgen receptor activity.
