Defining the Roles of Aromatase Inhibitors in the Adjuvant Treatment of Early-Stage Breast Cancer

The widespread use of tamoxifen has led to significant improvements in survival for postmenopausal women with early-stage hormone receptor-positive breast cancer; however, approximately 30% of patients die despite receiving tamoxifen as adjuvant treatment. In addition, concerns exist regarding tamoxifen-associated side effects, including endometrial cancer and thromboembolic disease. The development of the third-generation aromatase inhibitors (AIs; anastrozole, exemestane, and letrozole) therefore represented a welcome potential alternative to tamoxifen. Several clinical trials have demonstrated the superiority of AIs over tamoxifen in the adjuvant treatment of postmenopausal women with hormone-sensitive breast cancer, but these trials differ in their design and in the characteristics of their patient populations. This review discusses the different designs of the primary adjuvant, switching, extended adjuvant, and sequencing trials that are investigating the use of AIs in the adjuvant treatment of breast cancer and provides direction regarding how the data from these trials could be used to guide treatment choice. This review also demonstrates why one should not extrapolate results from clinical trials to clinical situations that differ from the clinical trial or from clinical trials investigating a particular AI to clinical situations involving another AI.