High estrogen receptor expression and low Ki67 expression are associated with improved time to progression during first-line endocrine therapy with aromatase inhibitors in breast cancer

Background Aromatase inhibitors have played a central role in endocrine therapy for estrogen receptor (ER)-positive breast cancer in postmenopausal women. However, many breast cancer patients with tumors expressing ER are unresponsive to aromatase inhibitors, and all patients with advanced disease eventually develop resistance to the therapy. Methods Twenty-one postmenopausal women with Stage II to IV breast cancer were treated with aromatase inhibitors as first-line endocrine therapy without surgery. Expression levels of ER, progesterone receptor, HER2 and Ki67 were examined by immunohistochemistry, and correlations between response and duration of the therapy and these levels were analyzed. Results Patients whose tumors contained two thirds or more ER-positive cells effectively responded to aromatase inhibitors (P = 0.006) and displayed longer time to progression during first-line endocrine therapy (P = 0.003) and longer time to endocrine therapy failure (P = 0.02). Patients whose tumors showed less than 15% Ki67 labeling index also displayed longer time to progression (P = 0.003). Conclusion High ER expression and low Ki67 expression were associated with improved time to progression with aromatase inhibitors as first-line endocrine therapy. Our findings will be helpful when endocrine therapy is planned in either early stage or advanced breast cancer.