Article: Letrozole

Letrozole (Femara®) is an oral non-steroidal aromatase inhibitor that has been introduced for the adjuvant treatment of hormonally-responsive breast cancer.

Estrogens are produced by the conversion of androgens through the activity of the aromatase enzyme. Letrozole blocks production of estrogens in this way by competitive, reversible binding to the heme of its cytochrome p450 unit. The action is specific, and letrozole does not reduce production of mineralo- or corticosteroids. In contrast, the antiestrogenic action of Tamoxifen, the major medical therapy prior to the arrival of aromatase inhibitors, is due to its interfering with the estrogen receptor, rather than inhibiting estrogen production.

FDA-approved use

Letrozole is approved by the Food and Drug Administration (FDA) for the treatment of local or metastatic breast cancer that is hormone receptor positive or has an unknown receptor status in postmenopausal women. Side effects include signs and symptoms of hypoestrogenism. There is concern that long term use may lead to osteoporosis.

Off-label use

It has been in use for the use of ovulation induction by fertility doctors since 2001; having less side-effects than Clomiphene Citrate (Clomid) for the patient. A Canadian study presented at the American Society of Reproductive Medicine 2005 Conference suggests that it may increase the risk of birth defects compared with a control group, however a more detailed follow-up study found no basis for concern when letrozole is used for ovulation induction.^[1]

The manufacturer, Novartis, has issued letters to doctors in Canada and the United States reiterating that it is not approved for such a use and is not safe to use with pregnant women or women who may become pregnant.

Letrozole has shown to reduce estrogen levels by 98 percent while raising your testosterone levels. This high percentage point allows athletes and bodybuilders to use Letrozole after the completion of a steroid cycle to destroy the gynecomastia related breast tissue that is a side effect of some anabolic steroids that are used as part of a steroid cycle. Abuse of the product (usage above 5 mg./day is known to temporarily kill sex drive.)

A related agent is anastrozole.