Among postmenopausal women with hormone receptor-positive breast cancer, use of an aromatase inhibitor at some point in the course of adjuvant (post-surgery) treatment results in a lower risk of cancer recurrence than use of tamoxifen only. Based on these results, updated guidelines from the American Society of Clinical Oncology recommend consideration of aromatase inhibitor therapy for this group of patients.
Each year roughly 200,000 U.S. women are diagnosed with breast cancer. Many of these breast cancers will be hormone receptor-positive, meaning that they are stimulated to grow by the circulating female hormones estrogen and/or progesterone.
Treatment of hormone receptor-positive breast cancer often involves hormonal therapies that suppress or block the action of estrogen. These therapies include tamoxifen as well as agents known as aromatase inhibitors. Tamoxifen acts by blocking estrogen receptors, whereas aromatase inhibitors suppress the production of estrogen in postmenopausal women. Aromatase inhibitors include Arimidex® (anastrozole), Femara® (letrozole), and Aromasin® (exemestane).
Several large trials comparing aromatase inhibitors to tamoxifen have established the efficacy of aromatase inhibitors for the treatment of hormone receptor-positive breast cancers among postmenopausal women. Whether aromatase inhibitors are used as initial hormonal therapy or sequentially with tamoxifen, use of an aromatase inhibitor at some point during the course of adjuvant treatment has been shown to reduce the risk of breast cancer recurrence compared with tamoxifen alone.
Based on these results, the American Society of Clinical Oncology recently updated its hormonal therapy guidelines. As recommended in the latest guideline, the panel recommends that “postmenopausal women with hormone receptor-positive breast cancer consider incorporating [aromatase inhibitor] therapy at some point during adjuvant treatment, either as up-front therapy or as sequential treatment after tamoxifen. The optimal timing and duration of endocrine therapy treatment remain unresolved.”
Potential side effects of aromatase inhibitors that will need to be considered by the patient and her physician include bone loss and joint pain. Aromatase inhibitors also appear to be more likely than tamoxifen to cause increases in blood pressure and cholesterol levels.
 Burstein HJ, Prestrud AA, Seidenfeld J et al. American Society of Clinical Oncology Clinical Practice Guidelines: Update on adjuvant endocrine therapy for women with hormone receptor-positive breast cancer. Journal of Clinical Oncology [early online publication]. July 12, 2010.