Abstract PS1-11-03: Real-world treatment patterns and overall survival in patients with HR+/HER2- metastatic breast cancer treated with chemotherapy following endocrine therapy in the US

Abstract Background: Current therapeutic approaches for patients with HR+/HER2-low metastatic breast cancer (mBC) do not differ from patients with HR+/HER2- mBC, with systemic chemotherapy remaining the standard of care (SOC) following progression on multiple lines of endocrine therapy (ET)-based treatment or in cases of primary endocrine resistance. Given the scarcity of real-world data on treatment patterns and real-world overall survival (rwOS) in this population, further research is essential to inform decision making and better understand the effectiveness of SOC treatments. Methods: A cohort of adults (≥18 years old) treated with chemotherapy following ET for HR+/HER2- mBC in the United States (US) was identified retrospectively in the Komodo Research Database (1/1/2016-4/30/2024). A 12-month washout period was used to identify patients with newly diagnosed mBC based on the presence of ≥2 medical claims with diagnosis codes for the primary BC followed by ≥2 claims with diagnosis codes for secondary neoplasms, excluding breast, skin, or lymph nodes. HR+/HER2- statuses were inferred from treatments received based on expert’s input. As such, HER2 expression profile below the threshold for positivity was not further stratified. Patients were included if they initiated chemotherapy after ≥2 prior lines of ET-based treatment (with or without targeted agents) or within 6 months of starting first-line ET + CDK4/6i (primary endocrine resistance) in the mBC setting. Treatment sequences in the mBC setting were described. rwOS was assessed from chemotherapy initiation to follow-up end among patients who initiated chemotherapy between 1/1/2017-10/31/2022 (i.e., had ≥18 months of potential observation, based on expert clinical input) using the Kaplan-Meier method. Results: The study identified 1,371 patients receiving chemotherapy for HR+/HER2- mBC. Median age at chemotherapy initiation was 59 years, 98.0% were female, 71.3% were White, 12.6% were Black or African American, 9.2% were Hispanic or Latino, and 3.8% were Asian or Pacific Islander. Most patients were commercially insured (67.8%), followed by Medicare (24.4%) and Medicaid (7.8%). The most common comorbidities were liver disease (30.3%), diabetes (21.4%), cardiovascular disease (20.6%) and chronic obstructive pulmonary disease (15.8%). Median time from initiation of mBC treatment to follow-up end was 31.6 months. Among the 1,371 patients, 25.1% initiated chemotherapy after one line, 49.1% after two lines, and 25.8% after ≥3 lines of mBC treatment. Most common regimens at chemotherapy initiation were capecitabine-based (60.2%), paclitaxel-based (17.3%) and albumin-bound paclitaxel-based (10.4%). Most patients (53.6%) received a next line of therapy, 37.7% remained on initial chemotherapy, and 8.7% discontinued all treatments after chemotherapy initiation. The most common next regimens included additional chemotherapy (49.1%), ET + other targeted therapy (excluding CDK4/6i; 10.9%), and ET + CDK4/6i (4.1%). Among 996 patients with sufficient potential observation to assess rwOS (≥18 months), median rwOS was 18.9 months (95% confidence interval [CI]: 17.4, 20.0 months), and rwOS rates at 12, 24, and 36 months were 65.0% (95% CI: 61.9%, 67.8%), 40.5% (95% CI: 37.4%, 43.6%) and 24.1% (95% CI: 21.2%, 27.1%), respectively. Conclusions: Real-world outcomes for patients with HR+/HER2- mBC treated with chemotherapy following progression on multiple lines of ET-based treatment or with primary endocrine resistance are poor, with median rwOS of approximately 1.5 years. These results highlight a need for more effective therapies with tolerable safety profiles to improve outcomes among patients with HR+ and HER2- or HER2-low mBC expression profiles who progress on ET-based treatments. Citation Format: T. A. Traina, C. Rossi, M. Levesque-Leroux, A. Tardif-Samson, P. Gagnon-Sanschagrin, A. Guérin, S. Vlassak, V. Guan, J. Salcedo. Real-world treatment patterns and overall survival in patients with HR+/HER2- metastatic breast cancer treated with chemotherapy following endocrine therapy in the US [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS1-11-03.