Real-world treatment patterns and outcomes in hormone receptor–positive, HER2-low metastatic breast cancer, 2018–2023: a retrospective, observational, US cohort study

PURPOSE: To investigate real-world demographics, clinical characteristics, treatment patterns, and outcomes in US patients with ≥ 1 line of therapy (LOT) for hormone receptor-positive (HR+) HER2-low metastatic breast cancer (mBC). METHODS: This retrospective cohort study used a US-based electronic health record-derived de-identified database of patients with mBC diagnosed in 2018-2023. Patient demographics, clinical characteristics at date of mBC diagnosis, treatment patterns, and real-world overall survival (rwOS) and progression-free survival (rwPFS) were analyzed. RESULTS: Of 2662 patients, 49.4% had recurrent mBC. The median (Q1-Q3) number of LOTs was 2 (1-3). Endocrine therapy (ET) plus a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) was the most common first-line, second-line, and third-line therapy. In LOT1, 86.2% of patients received ET-containing regimens; consecutive use of ET-containing regimens in subsequent lines constituted 66.0%, 46.0%, 28.3%, and 19.3% of patients in LOT2-5, respectively. Of 830 patients who received chemotherapy, a median (Q1-Q3) of 1 (0-2) prior lines of ET-containing regimens were received. Median (95% CI) rwOS from mBC diagnosis was 42.4 (40.7, 45.0) months; 5-year survival was 36.9%. Median (95% CI) rwPFS decreased from 16.3 (15.2, 17.3) months in LOT1 to 9.1 (8.3, 10.0), 6.2 (5.6, 7.2), 5.3 (4.6, 6.1), and 3.8 (3.3, 4.9) months in LOT2-5, respectively. CONCLUSIONS: Treatment of HR+, HER2-low mBC was characterized by progressive endocrine exhaustion followed by later lines of chemotherapy, with outcomes worsening at each subsequent treatment line. Data highlight the need for new, effective treatment options in this setting.