Aromatase Inhibitors for the Treatment and Prevention of Breast Cancer

In a review of current information on aromatase inhibitors (AIs) and their use in breast cancer treatment and prevention, published reports were obtained through a Medline search. Tamoxifen, a selective estrogen receptor modulator, is approved for use in metastatic breast cancer (MBC), the adjuvant treatment of breast cancer, and the prevention of breast cancer in women at high risk. The 50% reduction in breast cancer incidence seen with tamoxifen is significant for women at increased risk but is accompanied by notable toxicities such as thrombotic events and endometrial cancer. Therefore, the development of other effective agents with less toxicity would be a major advance in breast cancer prevention. Aromatase inhibitors, recently approved for the treatment of MBC and in the adjuvant setting, are proving to be slightly more effective than tamoxifen therapy. These drugs, approved for use in only postmenopausal women, inhibit the enzyme aromatase and thereby lower circulating functional estrogen. To date, the most concerning side effect of these agents is an increase in fracture rate. Compared with tamoxifen, thrombotic events and endometrial cancer rates are much lower. Ongoing data from the Arimidex®, Tamoxifen, Alone or in Combination trial continue to favor anastrozole over tamoxifen in the reduction of primary contralateral breast cancers. This information has prompted breast cancer chemoprevention trials with AIs. Although tamoxifen is the gold standard for prevention therapy, results of ongoing studies may indicate a role for AIs in the prevention of breast cancer.