Aromatase is the enzyme responsible for converting androgens to estrogens. Letrozole by
inhibiting aromatase reduces the production of estrogens. This results in enhanced
stimulation of gonadotropin releasing hormone (GnRH) which in turn leads to increased
secretion of follicle stimulating hormone (FSH)& luteinizing hormone (LH). The outcome
is normal ovarian follicular development & ovulation.
Letrozole is rapidly absorbed after oral administration. Absorption is not affected by the
food. The maximum plasma levels reached about 1 hour after ingestion. Letrozole has a
bioavailability of 99.9 %. Plasma protein binding of Letrozole is approximately 60% mainly
to albumin (55%). It is metabolized by CPY2A6 and CYP3A4 to inactive carbinol metabolite
(4, 4′-methanolbisbenzonitrile) which is excreted in urine. The elimination half-life is
about40-42 hours.
The usual dose is 2.5 mg to be taken for 5 days from day 3 to day 7 of menstrual cycle.
Higher doses up to 5 mg may be used in women who do not get desired results with lower
dose.
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