Aromatase Inhibitors

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Aromatase inhibitors stop the production of estrogen in postmenopausal women. Aromatase inhibitors work by blocking the enzyme aromatase, which turns the hormone androgen into small amounts of estrogen in the body. This means that less estrogen is available to stimulate the growth of hormone-receptor-positive breast cancer cells.

There are three aromatase inhibitors:

  • Arimidex (chemical name: anastrozole)
  • Aromasin (chemical name: exemestane)
  • Femara (chemical name: letrozole)

Each is a pill, usually taken once a day. All three are available as generic medicines.

Aromatase inhibitors can't stop the ovaries from making estrogen, so aromatase inhibitors are mainly used to treat postmenopausal women. But because aromatase inhibitors are so much more effective than tamoxifen in postmenopausal women, researchers wondered if there were a way to successfully treat premenopausal women diagnosed with hormone-receptor-positive, early-stage breast cancer with an aromatase inhibitor. Results from the SOFT (Suppression of Ovarian Function Trial) study published in 2015 suggest that premenopausal women diagnosed with hormone-receptor-positive breast cancer can be successfully treated with the aromatase inhibitor Aromasin if their ovarian function is suppressed. If you’re a premenopausal woman willing to take medicine to suppress your ovaries, you may be able to take Aromasin instead of tamoxifen for your hormonal therapy treatment.

You should not take an aromatase inhibitor if you are breastfeeding, pregnant, trying to get pregnant, or if there is any chance that you could be pregnant. Aromatase inhibitors may cause damage to developing embryos. You should use an effective non-hormonal type of birth control -- such as condoms, a diaphragm along with spermicide, or a non-hormonal I.U.D. – while you are taking an aromatase inhibitor. Ask your doctor which type of non-hormonal birth control would be best for you, as well as how long you should use this type of birth control after you stop taking an aromatase inhibitor.

Benefits of aromatase inhibitors

A number of studies have compared aromatase inhibitors with tamoxifen to see which type of medicine was more effective in treating early-stage, hormone-receptor-positive breast cancer in postmenopausal women. Based on the results, most doctors recommend that after initial treatment (surgery and possibly chemotherapy and radiation therapy):

  • an aromatase inhibitor is the best hormonal therapy to start with. When treating early-stage, hormone-receptor-positive breast cancer, aromatase inhibitors have more benefits and fewer serious side effects than tamoxifen.
  • switching to an aromatase inhibitor after taking tamoxifen for 2 to 3 years (for a total of 5 years of hormonal therapy) offers more benefits than 5 years of tamoxifen.
  • taking an aromatase inhibitor for 5 years after taking tamoxifen for 5 years continues to reduce the risk of the cancer coming back, compared to no treatment after tamoxifen.

Side effects of aromatase inhibitors

Aromatase inhibitors tend to cause fewer serious side effects than tamoxifen, such as blood clots, stroke, and endometrial cancer. But aromatase inhibitors can cause more heart problems, more bone loss (osteoporosis), and more broken bones than tamoxifen, at least for the first few years of treatment. If you and your doctor are considering an aromatase inhibitor as part of your treatment plan, you may want to ask your doctor about having a bone density test to see if a bone strengthening medicine might be necessary while you're taking the aromatase inhibitor.

The most common side effects of aromatase inhibitors are joint stiffness or joint pain.

If you're experiencing side effects from taking one aromatase inhibitor medicine, tell your doctor. You may be able to take a different medicine. Arimidex and Femara have similar chemical structures, while Aromasin has a different structure.


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