A new study has found that aromatase inhibitors are more effective treatments for postmenopausal women with hormone receptor positive breast cancer than tamoxifen in preventing recurrence and mortality. Both patients who were started on tamoxifen from the outset and those who were initially treated with tamoxifen and then switched to an aromatase inhibitor after two to three years were studied.

Compared to tamoxifen, at the five-year mark, aromatase inhibitor therapy was found to be associated with a 2.9% reduction in breast cancer recurrence (9.6% for aromatase inhibitors versus 12.6% for tamoxifen), as well as a 1.1% decline in breast cancer mortality (4.8% for aromatase inhibitors versus 5.9% for tamoxifen) which was not statistically significant. In the second group, after three years from treatment divergence (i.e., approximately five years after beginning hormonal treatment), aromatase inhibitor therapy was found to be associated with a 3.1% reduction in recurrence (5.0% for aromatase inhibitor versus 8.1% for tamoxifen since divergence) and a 0.7% reduction in death from breast cancer (1.7% for aromatase inhibitor versus 2.4% for tamoxifen since divergence). These results were not changed when participants were grouped by age, lymph node status, tumor grade, or progesterone receptor status. The authors conclude that aromatase inhibitors produce significantly lower breast cancer recurrence rates compared with tamoxifen, either as initial hormonal therapy or after two to three years of tamoxifen. Additional follow-up is required to obtain clearer information concerning the comparative impacts of the treatments on long-term survival.

Tamoxifen and aromatase inhibitors both better than no hormonal treatment

This study confirms a growing body of evidence that aromatase inhibitors produce better outcomes for many eligible breast cancer survivors than tamoxifen. In addition, there is no apparent advantage to using tamoxifen followed by an aromatase inhibtor compared to starting treatment with an aromatase inhibitor from the outset. Aromatase inhibitors are designed to inhibit the action of the enzyme aromatase, which converts adrenal androgens into estrogens by a process called aromatization. Aromatase inhibitors, including Femara (letrozole), Arimidex (anastrozole), and Aromasin (exemestane), generally are used for estrogen receptor positive (ER+) breast cancer in postmenopausal women. Since the growth of ER+ breast cancer is promoted by estrogen, decreasing the production of estrogen in the body might suppress recurrence. Whereas aromatase inhibitors block the production of estrogen, tamoxifen works by interfering with a tumor's ability to use estrogen.

Both types of treatment reduce recurrence and improve survival. However, up to half of eligible breast cancer survivors discontinue tamoxifen or aromatase inhibitor treatment before the end of five years (the treatment period often recommended). This is mainly because of the side effects associated with both types of hormonal treatment. However, it has been found that a given woman can have different degrees of side effects depending on the treatment. Therefore, it may be worthwhile to try several different aromatase inhibitors or even to switch to tamoxifen if no aromatase inhibitor can be tolerated rather than completely abandoning hormonal treatment.