Substituting the aromatase inhibitor exemestane after 2 to 3 years on tamoxifen reduces the risk of breast cancer by about 32%, when compared to the standard 5-year tamoxifen course in postmenopausal women with primary, estrogen-receptorpositive breast cancer, according to a randomized trial involving almost 5,000 women from 37 countries.
After an average of about 30 months, 183 women in the exemestane group developed local or metastatic recurrence or contralateral breast cancer or died, versus 266 in the tamoxifen group. Overall survival, however, was not significantly different in the two groups: 93 deaths occurred in the exemestane group and 106 occurred in the tamoxifen group.
Severe toxic effects in the exemestane group were rare but the drug was linked with more arthralgia, diarrhea, and fractures than tamoxifen. Tamoxifen, on the other hand, was associated with more gynecologic symptoms, including vaginal bleeding and muscle cramps, with more thromboembolic events, and with a greater occurrence of a second non-breast cancer before a distant relapse.
Despite several positive reports on the benefits of aromatase inhibitors and given tamoxifen's long and successful track record, however, an accompanying NEJM editorial suggests that at present exemestane be limited to women at high risk of recurrence who understand the limitations of current data.
Coombes RC, Hall E, Gibson LJ, et al. A randomized trial of exemestane after two to three years of tamoxifen therapy in postmenopausal women with primary breast cancer. N Engl J Med. 2004;350:1081-1092.