A retrospective, observational cohort study of treatment patterns and outcomes among patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative metastatic breast cancer in the United States following endocrine therapy.

PURPOSE: To describe patient characteristics, treatment patterns and real-world survival in patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) metastatic breast cancer (mBC) who received chemotherapy in the post-endocrine therapy (ET) setting where cytotoxic agents are the mainstay of treatment. METHODS: This retrospective, observational cohort study utilized data from the US nationwide Flatiron Health Research Database. Women aged ≥ 18 years with HR+/HER2 - mBC who received chemotherapy post-ET from January 1, 2015, to December 31, 2024, were included. Patient characteristics, treatment patterns, real-world overall survival (rwOS), and time to next treatment or death (rwTTNTD) were described post-ET. RESULTS: 5041 patients were included; median age was 63 years at mBC diagnosis. Most were non-Hispanic White (63%) and treated in the community setting (82%) and had Eastern Cooperative Oncology Group performance status 0-1 (59%) at the start of first-line treatment (1L) post-ET, recurrent mBC (68%), and ≥ 2 prior lines of ET-based treatment in the mBC setting (65%). Median time from mBC diagnosis to 1L post-ET was 19 months. Most patients (97%) received chemotherapy 1L post-ET. Median (95% CI) rwOS from start of 1L post-ET was 13.5 (13.0-14.1) months. 30% of patients who initiated 1L post-ET did not survive to the next treatment line; there was a decrease in rwOS with each subsequent treatment. Similar findings were observed for rwTTNTD. CONCLUSIONS: These findings reinforce the high unmet need in patients with HR+/HER2 - mBC whose disease has become resistant to ET, highlighting the need for effective treatments in this setting.