The 5-alpha reductase inhibitors are a family of agents used to treat benign prostatic hypertrophy. They act by inhibiting the steroid 5-alpha reductase which catalyzes the conversion of testosterone to dihydrotestosterone. Dihydrotestosterone is an important prostatic growth factor and its inhibition causes gradual shrinkage of prostate. Both of the available agents lower serum concentrations of dihydrotestosterone without affecting serum testosterone and usually with minimal antiandrogenic effects.
The 5-alpha reductase inhibitors in clinical use in the United States include two agents of similar chemical structure (azasteroids) and activity: finasteride (Proscar: 1992) and dutasteride (Avodart: 2001). Both agents cause a reduction in the size of the prostate, improve urinary flow and suppress prostate-specific antigen (PSA) levels. Both finasteride and dutasteride are associated with a low rate (<1%) of serum enzyme elevations during chronic therapy, which are usually mild-to-moderate in severity, self-limited in course, and rarely require dose modification or discontinuation. Neither of the 5-alpha reductase inhibitors in current use has been linked to clinically apparent acute liver injury.
The 5-alpha reductase inhibitors used for benign prostatic hypertrophy are discussed individually, with references on their hepatotoxicity given together at the end of this introductory section.