Finasteride (MK-906) is an oral inhibitor of active testosterone 5-alpha-reductase and Ki value is 10 nM.

5α reductase:10. nM(Ki)

After 4, 9, 14, and 21 days, rat prostates were excised to measure androgen and DNA content and to undergo histological and morphological analyses. On day 21, finasteride levels decreased by 65%. Finasteride treatment induced a dose-dependent increase in the incidence of hypospadias (penischisis) in male offspring, with a threshold dose level of approximately 0.1 mg/kg/day and a 100% effect level at 100 mg/kg/day (administered on gestational day 20). Additionally, finasteride reduced the anogenital distance in male offspring rats. After 4 days of treatment with finasteride, DNA content was almost unaffected, but a decrease of up to 52% was observed at 14 days. After prostate sections were stained, 23% of epithelial cells displayed markers of apoptotic cell death on day 14, returning to control levels by day 21. Finasteride caused 16% of epithelial cells to stain for tissue transglutaminase on day 9, with a return to baseline by day 14. Finasteride-induced staining was less intense at 4 days, peaking at 0.7% of epithelial cells and returning to control values by day 9.

Finasteride inhibits the growth rate of prostate lymph node carcinoma cell lines in a dose-dependent manner and significantly suppresses the secretion and expression of prostate-specific antigen in LNCaP cells. It forms a ternary complex with the reduced coenzyme II of type 2 isoenzyme (Ki: 1.19 nM), which then rearranges into a high-affinity complex (E:Ⅰ) with an approximate first-order rate constant of 1.62ms.