Background: Several classification models of sensitivity and resistance to endocrine therapies have been proposed for the clinical course of initial endocrine therapy for metastatic breast cancer. However, the efficacy and safety of secondary endocrine therapy in cases with a poor response to the initial endocrine therapy have not been examined.
Methods: A multicenter prospective observational study is planned with the objectives of demonstrating the efficacy and safety of secondary endocrine therapy in estrogen receptor (ER)-positive, human epidermal growth factor receptor type2 (HER2)-negative postmenopausal metastatic breast cancer for which initial endocrine therapy did not have a favorable clinical effect (i.e., low sensitivity to initial endocrine therapy). The subjects are patients with breast cancer with low sensitivity to initial endocrine therapy, and are defined as cases with recurrence during 5 years of adjuvant therapy or those with metastatic breast cancer that showed progression within 9 months after initial endocrine therapy. The efficacy and safety of current endocrine therapeutic agents selected by physicians and patients will be examined using outcomes including clinical benefit, progression-free survival, overall survival, time to treatment failure, time to chemotherapy, response, health-related quality of life, and adverse events.
Conclusion: Evaluation of the efficacy and safety of secondary endocrine therapy for breast cancer with low sensitivity to initial endocrine therapy will provide information for evidence-based selection of appropriate secondary endocrine therapy. The results will also clarify the remaining clinical issues to be resolved and provide a foundation for planning of future clinical research.