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Adjuvant Sequencing of Tamoxifen and Anastrozole Is Superior to Tamoxifen Alone in Postmenopausal Women with Low Proliferating Breast Cancer

Purpose: To assess the predictive value of Ki67 expression in postmenopausal hormone receptor–positive early-breast cancer patients, who were either treated with adjuvant tamoxifen (TAM) alone or with TAM followed by anastrozole (ANA). Experimental Design: Expression of Ki67 was determined centrally by immunohistochemistry on whole tissue sections of postmenopausal endocrine-responsive breast cancers from patients who had been enrolled in the prospectively randomized Austrian Breast and Colorectal Cancer Study Group Trial 8, and had received TAM for 5 years, or TAM for 2 years followed by ANA for 3 years. Ki67 expression was evaluated both as a continuous variable and dichotomized to low (≤10%) and high (>10%). Recurrence-free survival (RFS) and overall survival (OS) were analyzed using Cox models adjusted for clinical and pathologic parameters. Results: Patients with a high Ki67 expression (394/1,587; 23%) had a significantly shorter RFS (adjusted HR for recurrence = 1.90, 95% CI: 1.37–2.64, P = 0.0001) and OS (adjusted HR for death = 1.78, 95% CI: 1.18–2.70, P = 0.006). In women with breast tumors expressing medium or high ER levels (n = 1,438), the interaction between Ki67 and adjuvant endocrine treatment was significant for RFS (P = 0.03). TAM followed by ANA was superior to TAM alone in patients with low Ki67 (adjusted HR = 0.53, 95% CI: 0.34–0.83, P = 0.005) but not in high Ki67 disease (adjusted HR = 1.18, 95% CI: 0.66–1.89, P = 0.68). Conclusions: Adjuvant sequencing of TAM and ANA is superior to TAM alone, particularly in postmenopausal women with medium or high ER expressing, low proliferating breast cancer. Clin Cancer Res; 17(24); 7828–34. ©2011 AACR.