Increasing protection after tamoxifen: insights from the extended adjuvant aromatase inhibitor trials

Abstract

For disease-free postmenopausal women with hormone-responsive breast cancer, a risk for relapse remains following 5 years of adjuvant therapy with tamoxifen. Additional therapy with tamoxifen beyond 5 years is not indicated due to a demonstrated lack of efficacy beyond this time frame. Thus, there is a need for other endocrine therapy options in the period beyond 5 years. The third-generation aromatase inhibitors (anastrozole, letrozole, and exemestane) have emerged as at least as effective and somewhat better tolerated alternatives to tamoxifen. Three trials were initiated to evaluate the efficacy and tolerability of aromatase inhibitors in the extended adjuvant setting. Among these, the large, double-blind, randomized, placebo-controlled MA.17 trial has already demonstrated a significant benefit of letrozole when compared with placebo on disease-free survival in postmenopausal women previously treated for 4.5-6 years with tamoxifen. A smaller open-label trial, the Austrian Breast and Colorectal Cancer Study Group 6a has reported a significant benefit for anastrozole on recurrence when used as extended adjuvant therapy when compared with no treatment, and similar results have been seen with extended adjuvant exemestane in the National Surgical Adjuvant Breast and Bowel Project B-33 trial. The results of these trials have important clinical implications for the future of extended adjuvant hormonal therapy for breast cancer.