Adjuvant Therapy With Aromatase Inhibitors for Postmenopausal Women With Early Breast Cancer: Evidence and Ongoing Controversy

Results of five major randomized trials have increased our understanding of the role of aromatase inhibitors in the adjuvant setting. Two of these trials, the Anastrozole or Tamoxifen Alone or in Combination (ATAC) trial and the International Breast Cancer Study Group’s BIG 1-98 trial compared an aromatase inhibitor versus tamoxifen as initial hormonal therapy. Three other trial were designed as cross-over studies; the Intergroup Exemestane Study (IES) and the Austrian Breast and Colorectal Cancer Study Group (ABCSG) Trial 8/German ARNO 95 trial compared a crossover from tamoxifen to an aromatase inhibitor versus continued tamoxifen in women who had completed 2 to 3 years of tamoxifen. The MA-17 trial compared the use of letrozole with placebo for 5 years, following 5 years of tamoxifen. Based on the results of these studies, the use of an aromatase inhibitor for the adjuvant treatment of postmenopausal women with hormone receptor–positive early breast cancer has largely replaced the previous standard of 5 years of tamoxifen. Still unanswered, however, are questions regarding optimal sequencing, selection of aromatase inhibitor, and duration of treatment. This review will provide an overview of the major studies with an emphasis on these important questions.