5-alpha-reductase inhibitor

5α-reductase inhibitors (or 5-alpha-reductase inhibitors) are a group of drugs with antiandrogenic activity, used in the treatment of benign prostatic hyperplasia and androgenic (or androgenetic) alopecia. These drugs decrease the levels of available 5α-reductase prior to testosterone binding with the enzyme, thus reducing levels of dihydrotestosterone that derives from such a bond.

Indications
5α-reductase inhibitors are clinically used in the treatment of conditions which are exacerbated by dihydrotestosterone. Specifically, these indications may include: (Rossi, 2004)
 * mild-to-moderate benign prostatic hyperplasia
 * androgenic (or androgenetic) alopecia

Adverse drug reactions
Adverse drug reactions (ADRs) experienced with 5α-reductase inhibitors are generally dose-dependent. Common ADRs include impotence, decreased libido, decreased ejaculate volume. Rare ADRs include: breast tenderness and enlargement, and allergic reaction. (Rossi, 2004)

Pharmacology
The enzyme 5α-reductase is involved in the conversion of testosterone to the active form dihydrotestosterone by reducing the Δ4,5 double-bond. In benign prostatic hyperplasia, dihydrotestosterone acts as a potent cellular androgen and promotes prostate growth - inhibiting the enzyme reduces the excessive prostate growth. In alopecia, pattern-baldness is one of the effects of androgenic receptor activation. Reducing the levels of dihydrotestosterone thus reduces alopecia.

Examples

 * finasteride (Proscar, Propecia)
 * dutasteride (Avodart)
 * FCE 28260, an experimental compound