Simple SummarySecond-line treatment strategy after the first-line CDK4/6 inhibitor with aromatase inhibitor is considered by the behavior of hormone receptor positive human epidermal growth factor receptor-2 negative (HR+HER2−) metastatic breast cancer (MBC). Progression free survival 2 was one of the association factors for overall survival of HR+HER2− MBC. Therefore, the second-line treatment strategy was important to improve prognosis in patients with HR+/HER2− MBC.AbstractBackground: We analyzed real-world practice of second-line treatment in hormone receptor (HR)+ human epidermal growth factor receptor-2 (HER2)− metastatic breast cancer (MBC) following the first-line CDK4/6 inhibitor with letrozole. In addition, we evaluated the relationship between second-line treatment strategies and survival outcome. Methods: Using the clinical data warehouse, clinical information including MBC diagnosis, treatment and survival outcomes were collected. Results: In total, 305 patients were treated with the first-line palbociclib plus letrozole, and we evaluated 166 patients who were treated with second-line treatment. Of the 166 patients, 28.5% were treated with capecitabine (C), followed by exemestane with everolimus (EE) (27.3%) or cytotoxic chemotherapy other than capecitabine (T) (18.8%) and fulvestrant-based treatment or endocrine monotherapy (F) (12.7%). Eighteen patients (10.9%) were enrolled in clinical trials (CT). With regard to treatment strategies, and the median progression-free survival of second-line treatment in a metastatic setting (PFS2) was 7.4 months with C, 5.2 months with EE, 4.8 months with T, 3.6 months with F, and 3.6 months with CT (p= 0.066). In patients with visceral organ disease progression, C (31.3%) or T(31.3%) was the most common second-line treatment followed by EE (21.9%). Most of the 47 patients with bone metastasis alone were treated with EE (38.2%), followed by C (23.4%) and F (21.3%) (p= 0.008). The median overall survival of second-line treatment in a metastatic setting (OS2) was 42.3 months with C, 35.7 months with F, 30.7 months with EE, and 23.1 months with T. The median OS2 for those in CT was not reached (p= 0.064). ER driven BC, disease progression site and PFS2 were associated with OS and OS2 in HR+HER2− MBC (ps< 0.05). Conclusions: We suggested the second line treatment strategy was important to improve prognosis in patients with HR+/HER2− MBC, especially given the recent standardization of first-line treatment and the many available second-line options.