5 α-Reductase Inhibitor
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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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