Switching From Tamoxifen to Exemestane Extends Survival in Early Breast Cancer

Researchers from the Intergroup Exemestane Study have shown for the first time that postmenopausal women with early-stage breast cancer who switched to the drug exemestane (Aromasin) after taking tamoxifen for 2 to 3 years following initial treatment had a 15% lower risk of dying than those who continued to take tamoxifen. Exemestane, an aromatase inhibitor, also reduced the risk of breast cancer recurrence and metastasis, and the development of cancer in the opposite breast. The two drugs were associated with different side effects. The study results were presented at the 2006 annual meeting of the American Society of Clinical Oncology in Atlanta.

Different Mechanisms

"These results show that switching to exemestane after 2 to 3 years of tamoxifen is safe and can improve the cure rate in postmenopausal women with breast cancer," said lead author Raoul C. Coombes, MD, PHD, professor of medical oncology and head of the department of oncology at the Imperial College of London. "Both drugs can be an important part of therapy for these patients."

Tamoxifen is given for 5 years to women with breast cancer to reduce the risk of cancer recurrence. Exemestane is another drug used to reduce breast cancer recurrence, but it works through a different mechanism: It blocks estrogen production by inhibiting aromatase, an enzyme necessary for estrogen to be produced.

Study Protocol and Results

In this study, which began in 1998, overall survival, breast cancer recurrence and metastasis, and the incidence of cancer in the opposite breast were compared between 2,352 postmenopausal women with early-stage breast cancer who were randomly assigned to take exemestane after 2 to 3 years of taking tamoxifen, and 2,372 women who continued to take tamoxifen. In both cases, the goal was to take tamoxifen or tamoxifen followed by exemestane for 5 years.

After a median follow-up of 4.8 years since randomization, women in the exemestane group had a 15% lower risk of dying than those who took only tamoxifen. These women also had a 24% lower risk of experiencing any first event (such as breast cancer recurrence, a new breast cancer, or dying from something other than breast cancer), a 17% reduction in the risk of metastasis, and a 44% lower incidence of cancer in the opposite breast.

Women who took tamoxifen alone were more likely to develop blood clots or gynecologic problems (such as uterine cancer or polyps and vaginal bleeding) than the exemestane group, while women in the exemestane group had slightly more fractures (and should have bone mineral density monitoring before and during therapy). There were no significant differences in the incidence of heart attack, angina, or stroke between the two groups.