Harvard Women's Health Watch

Another drug prevents breast cancer in postmenopausal women

A large international trial of exemestane (Aromasin), a drug that reduces the risk of breast cancer recurrence, has found that it can also help prevent breast cancer from developing in the first place. That makes exemestane a third option for preventing breast cancer in postmenopausal women who are at elevated risk for the disease. Two other drugs, tamoxifen (Nolvadex, generic) and raloxifene (Evista), are already approved for prevention, but few women take them for that purpose because they can have serious (although rare) side effects such as stroke and blood clots. Exemestane appears to have less frightening side effects — for example, hot flashes, joint pain, and loss of bone density.

All three of these drugs target estrogen, which fuels the growth of most breast cancers, but exemestane works by a different mechanism than the other two. Tamoxifen and raloxifene are selective estrogen-receptor modulators, which bind to estrogen receptors in the breast and block their interaction with estrogen. Exemestane belongs to a different class of drugs, called aromatase inhibitors, which work by blocking the body's production of estrogen. Previous studies have shown that aromatase inhibitors are more effective than tamoxifen in preventing breast cancer from recurring. The study, funded by the drug's maker, Pfizer, and conducted under the auspices of the National Cancer Institute's clinical trials unit, looked at whether exemestane could reduce the likelihood of a first occurrence of breast cancer. Results were presented at the American Society of Clinical Oncology meeting in Chicago on June 4, 2011, and simultaneously published online in The New England Journal of Medicine.

The study. The trial enrolled 4,560 healthy postmenopausal women with an elevated risk for breast cancer because of age (60 or over) and other factors. Half were given exemestane, the other half a placebo. The two groups were similar in age, race, body mass index, and risk factors for breast cancer. Participants underwent a physical exam and answered questions about their symptoms and quality of life at the start of the study, after six months, after one year, and annually for a total of three years. The women also received annual mammograms.

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