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March 26, 2016
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1 Introduction
aromatase inhibitor



Aromatase inhibitors ( AI ) are a class of drug s used in the treatment of breast cancer and ovarian cancer in postmenopausal women.

Some cancers require estrogen to grow. Aromatase is an enzyme that synthesizes estrogen. Aromatase inhibitors block the synthesis of estrogen. This lowers the estrogen level, and slows the growth of cancers.

AIs are categorized into two types:

  • Irreversible steroidal inhibitors such as exemestane form a permanent bond with the aromatase enzyme complex.

  • Non-steroidal inhibitors (such as anastrozole, letrozole) inhibit the enzyme by reversible competition.

Aromatase inhibitors work by inhibiting the action of the enzyme aromatase, which converts androgens into estrogens by a process called aromatization. As breast tissue is stimulated by estrogens, decreasing their production is a way of suppressing recurrence of the breast tumor tissue


In contrast to pre-menopausal women, in whom most of the estrogen is produced in the ovaries , in post-menopausal women most of the body's estrogen is produced in the adrenal gland from the conversion of androgens. The other main source of estrogen post-menopausally is adipose tissue . Because some breast cancers respond to estrogen, lowering the estrogen level in post-menopausal women using aromatase inhibitors has been proven to be effective in breast cancer treatment.

Aromatase inhibitors are generally not used to treat breast cancer in premenopausal women. Since most of the circulating estrogen is produced by the ovaries, not by conversion of androgens to estrogen, blocking the enzyme aromatase does not significantly decrease the production of estrogen. When aromatase inhibitors are used in premenopausal women, the decrease in estrogen activates the hypothalamus and pituitary axis to increase gonadotropin secretion, which in turn stimulates the ovary to increase androgen production. This counteracts the effect of the aromatase inhibitor.

An ongoing area of clinical research is optimizing adjuvant hormonal therapy in postmenopausal women with breast cancer. Tamoxifen has been standard treatment, however the ATAC trial has shown that clinical results are superior with an AI in postmenopausal women with localized breast cancer that is estrogen receptor positive. Further studies of various AIs are ongoing.

Other uses and sources of aromatase inhibitors

Investigations are ongoing to look for other applications. Researchers are studying aromatase inhibitors to stimulate ovulation (in a manner similar to, but not exactly the same as, clomiphene citrate) or suppress estrogen production, ie in endometriosis.

AIs have also been used experimentally in the treatment of adolescents whose predicted adult height is low.

Bodybuilders who take anabolic steroids may also take AIs to prevent the steroids from being converted to estrogen; an increase in estrogen levels has undesirable consequences for a bodybuilder, such as gynecomastia. This is often the case when a natural aromatase inhibitor 4-OHAD has itself been inhibited.

4-OHAD is a metabolite of testosterone, which can mean 4-OHAD remains inhibited whilst aromatase levels are allowed high.

In one recent study, aromatase inhibitors were found to be no more successful at treating pubertal gynecomastia than a placebo.

Aromatase inhibitors have also been shown to reverse age-related declines in testosterone, as well as primary hypogonadism.

Extracts of certain mushrooms have been shown to inhibit aromatase when evaluated by enzyme assays, the white mushroom has shown the greatest ability to inhibit the enzyme.

Some of the aromatase inhibitors in use include:


  • Aminoglutethimide

  • Testolactone (Teslac)


  • Anastrozole (Arimidex)

  • Letrozole (Femara)

  • Exemestane (Aromasin)

  • Vorozole (Rivizor)

  • Formestane (Lentaron)

  • Fadrozole (Afema)


  • 4-androstene-3,6,17-trione ("6-OXO", marketed as a nutritional supplement for athletes and weight lifters)

  • 1,4,6-androstatrien-3,17-dione (ATD)

  • 4-hydroxyandrostenedione

??? An increase in joint disorders (including arthritis, arthrosis and arthralgia).

??? An increase in the incidence of osteoporosis and fractures (specifically fractures of spine, hip and wrist). A hip fracture often results in prolonged hospitalization and long term care in a post-menopausal woman.

??? Hypercholesterolemia (especially without 5-alpha-reductase inhibitor)

Bisphosphonates are sometimes prescribed to prevent the osteoporosis induced by aromatase inhibitors but have another serious side effect, osteonecrosis of the jaw. Since statins have a bone strengthening effect, combining a statin with an aromatase inhibitor may avoid both fractures and possible cardiovascular risks without jaw osteonecrosis. In one study of women with breast cancer taking anastrozole, statin use was associated with a 38% reduced fracture risk, or approximately the equivalent of 10 mg Fosamax daily.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "aromatase inhibitor".

Last Modified:   2010-11-25

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