Finasteride is an orally active testosterone 5-alpha-reductase inhibitor (Ki= 10 nM). Target: 5-alpha ReductaseApproved: 1992Finasteride, a synthetic 4-azasteroid antiandrogen compound, is a specific inhibitor of steroid Type II 5α-reductase, an intracellular enzyme that converts the androgen testosterone into 5α-dihydrotestosterone (DHT). Finasteride is used in the treatment of prostate cancer, benign prostatic hyperplasia, and androgenetic alopecia (male pattern baldness). In benign prostatic hyperplasia, finasteride inhibits 5alpha-reductase activity in epithelium for Ki of 10 nM, significantly lower than in stroma (Ki = 33nM) [1].

References:
[1]. Weisser, H. and M. Krieg, In vitro inhibition of androstenedione 5alpha-reduction by finasteride in epithelium and stroma of human benign prostatic hyperplasia. J Steroid Biochem Mol Biol, 1998. 67(1): p. 49-55.