|Systematic (IUPAC) name|
|Licence data||US FDA:|
|Protein binding||60%, mainly to albumin|
|Metabolism||pharmacologically-inactive carbinol metabolite (4,4΄-methanol-bisbenzonitrile)|
|Mol. mass||285.303 g/mol|
|(what is this?)|
Letrozole is approved by the United States 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.
Letrozole has been used for ovarian stimulation by fertility doctors since 2001 because it has fewer side-effects than clomiphene (Clomid) and less chance of multiple gestation. A Canadian study presented at the American Society of Reproductive Medicine 2005 Conference suggested that letrozole may increase the risk of birth defects. A more detailed ovulation induction follow-up study found that letrozole, compared with a control group of clomiphene, had significantly lower congenital malformations and chromosomal abnormalities at an overall rate of 2.4% (1.2% major malformations) compared with clomiphene 4.8% (3.0% major malformations). Despite this, India banned the usage of letrozole in 2011, citing potential risks to infants. In 2012, an Indian parliamentary committee said that the drug controller office colluded with letrozole's makers to approve the drug for infertility in India and also stated that letrozole's use for infertility was illegal worldwide; however, such off-label uses are legal in many countries such as the US and UK.
The anti-estrogen action of letrozole has been shown to be useful in pretreatment for termination of pregnancy, in combination with misoprostol. It can be used in place of mifepristone, which is expensive and unavailable in many countries.
Letrozole is sometimes used as a treatment for gynecomastia, although it is probably most effective at this if caught in an early stage (such as in users of anabolic steroids).[unreliable source?]
Letrozole has also been shown to delay the fusing of the growth plates in mice. When used in combination with growth hormone, letrozole has been shown effective in one adolescent boy with a short stature.
Mechanism of action
Letrozole prevents the aromatase from producing estrogens 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.
Generally, side effects include signs and symptoms of hypoestrogenism. There is concern that long term use may lead to osteoporosis, which is in certain patient populations such as post-menopausal women or osteoporotics, bisphosphonates may also be prescribed.
Comparison with tamoxifen
Tamoxifen is also used to treat hormonally-responsive breast cancer, but it does so by interfering with the estrogen receptor. However, letrozole is effective only in post-menopausal women, in whom estrogen is produced predominantly in peripheral tissues (i.e. in adipose tissue, like that of the breast) and a number of sites in the brain. In pre-menopausal women, the main source of estrogen is from the ovaries not the peripheral tissues, and letrozole is ineffective.
See also: (according to art of drug synthesis book)
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