Switching trial of adjuvant tamoxifen with an aromatase inhibitor in postmenopausal patients with breast cancer

Tamoxifen is currently a standard of care for postmenopausal patients with breast cancer with hormone receptor-positive tumors who are candidates for adjuvant endocrine therapy. This treatment is highly effective and relatively safe. However, a significant proportion of women are constitutively resistant or become resistant to tamoxifen, despite having hormone receptor-positive tumors. Moreover, the prolonged exposure to tamoxifen can produce severe and life-threatening side effects like endometrial carcinoma or thromboembolic disease. Aromatase inhibitors (AI) have been shown to be quite effective in advanced disease, and also have promising efficacy in early-stage breast cancer as alternatives to tamoxifen. This article reviews the results achieved by AI following 2-3 years or 5 years of tamoxifen. At least 3 trials indicate that switching to an AI following 2-3 years of tamoxifen can produce a substantial benefit. A fourth trial indicates that additional benefit can be achieved by a few years of treatment with an AI after 5 years of tamoxifen. The updated results of previous trials and ongoing trials will soon establish criteria for selecting patients who might be better candidates for sequencing, and to fine tune strategies that are more appropriate.