Finding: Treatment with the aromatase inhibitor exemestane reduced the development of invasive breast cancer by 65%.
Approach: A randomized, double-blind trial of 4,560 postmenopausal women compared exemestane to placebo over 3 years.
Impact: Significant reduction in breast cancer risk may be achieved with a class of drugs that has fewer side effects than the traditional SERMs.
The current standard for chemoprevention of breast cancer involves selective estrogen-receptor modulators (SERM), including tamoxifen and raloxifene. Tamoxifen has been shown clinically to reduce breast cancer risk significantly, but its use is associated with serious side effects due to antiestrogenic actions on tissues aside from the breast, especially endometrial cancer and venous thromboembolism. Exemestane belongs to the class of drugs called aromatase inhibitors, which also antagonize the actions of estrogen but do so by blocking its synthesis in peripheral tissues, such as the breast. In a randomized, double-blind, placebo-controlled trial, Goss and colleagues showed that exemestane reduced the annual incidence of invasive breast cancer by 65%, without any associated serious adverse events and only minimal quality-of-life differences compared to placebo. A total of 4,560 postmenopausal women who were at moderately increased risk for breast cancer—meaning they had at least 1 study-specific risk factor—were treated with either exemestane or placebo over the course of 3 years. Symptoms and adverse effects were similar between the 2 groups, with menopausal symptoms slightly more frequent in those treated with drug. In addition to the relative reduction in invasive breast cancer in the treatment group, exemestane also reduced the risk of known breast cancer precursor lesions and the more aggressive HER2-positive tumors; a previous study showed an exemestane-related reduction in contralateral breast cancer as well. Overall, the results of the study suggest that the risk-benefit profile of exemestane may be superior to the more traditional SERMs in the primary prevention of breast cancer, and follow-up of treated women may continue to confirm the long-term efficacy of aromatase inhibitors.
Note: Research Watch is written by Cancer Discovery Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.
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