5 α-Reductase Inhibitor

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Chapter: Essential pharmacology : Androgens and Drugs for Erectile Dysfunction

Finasteride : A competitive inhibitor of the enzyme 5 αreductase which converts testosterone into more active dihydrotestosterone responsible for androgen action in many tissues including prostate gland and hair follicles.


5 α-REDUCTASE INHIBITOR

 

Finasteride

 

A competitive inhibitor of the enzyme 5 αreductase which converts testosterone into more active dihydrotestosterone responsible for androgen action in many tissues including prostate gland and hair follicles. It is relatively selective for 5 αreductase type 2 isoenzyme which predominates in male urogenital tract. Circulating and prostatic dihydrotestosterone concentration are lowered but plasma LH and testosterone levels are unchanged because testosterone itself mediates feedback pituitary LH inhibition.

 

Treatment with finasteride has resulted in decreased prostate size and increased peak urinary flow rate in ~50% patients with symptomatic benign hypertrophy of prostate (BHP). The beneficial effects are typically delayed needing ~6 months for maximum symptomatic relief. Patients with large prostate (volume > 40 ml) obtain greater relief than those with smaller gland; but it is the only drug which can retard disease progression.

 

Withdrawal of the drug results in regrowth of prostate, but with continued therapy benefit is maintained for 3 years or more. The relief of obstructive symptoms, however, is less marked compared to surgery and α1 blockers. It primarily reduces the static component of obstruction, while α1 blockers overcome the dynamic component. Concurrent treatment with both produces greater symptomatic relief.

 

Finasteride has also been found effective in male pattern baldness. In such subjects it promotes hair growth and prevents further hair loss. Observable response takes 3 or more months therapy and benefit is reversed within 1 year of treatment cessation. However, 20–30% cases do not improve.

 

Finasteride is effective orally, extensively metabolized in liver—metabolites are excreted in urine and faeces; plasma t½ 4–8 hours (elderly 6–15 hours). It is well tolerated by most patients; side effects are decreased libido, impotence and decreased volume of ejaculate (each in 3–4% patients), skin rashes, swelling of lips.

 

Dose for BHP 5 mg OD, review after 6 months; for male pattern baldness 1 mg/day.

 

FINCAR, FINAST, FINARA 5 mg tab; FINPECIA, ASTIFINE 1 mg tab.

 

Dutasteride

 

This newer congener of finasteride inhibits both type 1 and type 2 5αreductase and reduces dihydrotestosterone levels. It is metabolized by CYP3A4 and is very long-acting (t½ is ~ 9 weeks). It is approved for use in BHP and is being tested in baldness and for prevention of prostate carcinoma. Interactions with CYP3A4 inducers and inhibitors are possible.

 

Dose: 0.5 mg OD; DUPROST, DURIZE 0.5 mg tab.

 

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