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.
Aromatase inhibitors can't stop the ovaries from making estrogen, so aromatase inhibitors only work in postmenopausal women.
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. Arimidex and Femara are available as generic medicines.
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.
Joint pain from taking an aromatase inhibitor can be troubling. But a 2008 British study suggests that women who experienced joint pain while taking hormonal therapy medicine were less likely to have the breast cancer come back (recur). Knowing that this side effect might indicate a reduced risk of the cancer coming back may help some people stick with treatment despite the side effects.
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.