Medicines to Reduce Breast Cancer Risk

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Aromatase inhibitors

What are aromatase inhibitors?

Aromatase inhibitors are newer drugs that are sometimes used to treat breast cancer or help keep breast cancer from coming back after surgery. The drugs in this class include:

Aromatase inhibitors work differently from tamoxifen and raloxifene. Instead of blocking the estrogen receptors, they stop a key enzyme (called aromatase) from changing other hormones into estrogen. This lowers estrogen levels in the body, taking away the fuel that estrogen receptor-positive breast cancers need to grow.

These drugs are only useful in women whose ovaries aren’t making estrogen (such as those who have already gone through menopause).

What are the benefits and risks of taking aromatase inhibitors?

Studies have shown that aromatase inhibitors are better than tamoxifen for treating advanced breast cancer. For keeping breast cancer from coming back after surgery, several studies have found that aromatase inhibitors (used instead of or after tamoxifen) are slightly better than tamoxifen alone.

Some short-term effects of aromatase inhibitors are much like those caused by tamoxifen and raloxifene, including hot flashes and vaginal dryness. Muscle and joint pain and headaches happen more often. Unlike tamoxifen and raloxifene, aromatase inhibitors tend to speed up osteoporosis (bone thinning), which can lead to broken bones.

Aromatase inhibitors seem much less likely to cause serious blood clots. Based on the studies done so far, they do not seem to raise the risk of cancer of the uterus, like tamoxifen and raloxifene do.

Because these drugs have been available for a shorter period of time, less is known about other possible long-term effects they may have, such as on the risk of heart disease. Future research will help define these effects.

Are aromatase inhibitors approved for use in reducing breast cancer risk?

No. At this time, aromatase inhibitors are not approved to be used to reduce breast cancer risk. They are used either to treat advanced breast cancer or given after surgery (instead of or after tamoxifen) to help prevent breast cancer from coming back. The FDA has not approved any of these drugs to reduce the risk of developing breast cancer.

But one of these drugs has been shown to lower breast cancer risk in a clinical trial. The MAP3 study compared exemestane to placebo (sugar pill) in a group of 4,560 post-menopausal women who had an increased risk of breast cancer.

After an average of about 3 years on the study, there were 32 cases of invasive breast cancer in the women on placebo, but only 11 cases in the 2,285 women taking exemestane. This is a 65% lower risk in the exemestane group compared to the placebo group. Exemestane did not have a strong effect on the risk of non-invasive or pre-invasive cancer, such as ductal carcinoma in situ (DCIS).

Side effects of exemestane were usually not severe, the most common being hot flashes and joint pain. The women in the group treated with exemestane did not have more fractures (broken bones) during the 3-year study follow-up. The women did not report more problems with osteoporosis, but the study did not look for these problems, either. (Other studies have shown that women on the drug lose more bone over time than women not taking the drug.)

Other studies are looking at the effect of aromatase inhibitors on breast cancer risk. The British IBIS-II study is comparing anastrozole to placebo for 5 years in 6,000 post-menopausal women who are at increased risk of breast cancer. Recruiting for the study was completed in January 2012. Women will take the drug for 5 years, so it will be some time before results are available. Smaller studies are also being done with letrozole.

Aromatase inhibitors to reduce breast cancer risk: More research is needed

Like raloxifene, aromatase inhibitors may someday prove to be as good as or even better than tamoxifen in reducing breast cancer risk, but more study results will be needed to show this. Much less is known about the possible long-term effects of these drugs. Even if they are shown to reduce risk, each woman and her doctor will still need to weigh the possible benefits and risks when deciding if one of them is right for her.


Last Medical Review: 06/04/2013
Last Revised: 07/17/2013