Real-world analysis of HER2-ultralow in HR+/HER2- metastatic breast cancer: prevalence and first-line chemotherapy outcomes.

HER2-ultralow is an emerging subgroup of metastatic breast cancer (mBC). However, despite an increasing interest, limited data exist on its prevalence and outcomes, especially among patients receiving standard first-line chemotherapy. This study assessed the prevalence and outcomes of HER2-ultralow mBC in a real-world cohort of hormone receptor-positive (HR+)/HER2-negative patients receiving first-line standard-of-care (SOC) chemotherapy. A retrospective, single-center cohort study. We included HR+/HER2-negative mBC patients treated with SOC between January 2016 and February 2023. Patient data were reviewed from electronic health records. HER2-zero tumors (immunohistochemistry 0) were rescored by expert pathologists using the American Society of Clinical Oncology/College of American Pathologists guidelines to distinguish HER2-ultralow from HER2-null cases. Real-world progression-free survival (rwPFS) and real-world overall survival (rwOS) were estimated using Kaplan-Meier and multivariable Cox regression models. Among 320 patients (median age, 62.4 years), 72.8% had visceral metastases, and 17.5% had bone-only disease. Previous CDK4/6 inhibitor treatment was reported in 43.4%. Rescoring identified 15.6% with HER2-ultralow, 61.9% with HER2-low, and 22.5% with HER2-null tumors. The median follow-up was 39.4 months (95% confidence interval (CI), 37.1-47.6). Median rwPFS was 7.9 months (95% CI, 4.6-19.0), 7.3 months (95% CI, 6.4-9.1), and 6.2 months (95% CI, 4.6-8.9) for HER2-ultralow, HER2-low, and HER2-null groups, respectively. Median rwOS was 19.5 months (95% CI, 10.7-33.4), 21.5 months (95% CI, 19.0-25.8), and 16.6 months (95% CI, 11.9-23.6). In patients previously treated with CDK4/6 inhibitors, median rwPFS was 4.6 months (95% CI, 2.7-21.4), 6.1 months (95% CI, 5.5-7.3), and 5.6 months (95% CI, 3.8-8.7). HER2-ultralow accounts for 15.6% of HR+/HER2-negative mBC cases and demonstrates outcomes comparable to HER2-low with SOC chemotherapy.