Baseline Characteristics, Treatment Patterns, and Outcomes in Patients with HER2-Positive Metastatic Breast Cancer by Hormone Receptor Status from SystHERs.

Purpose: We report treatments and outcomes in a contemporary patient population with HER2-positive metastatic breast cancer (MBC) by hormone receptor (HR) status from the Systemic Therapies for HER2-positive Metastatic Breast Cancer Study (SystHERs).

Experimental Design: SystHERs (NCT01615068) was an observational, prospective registry study of U.S.-based patients with newly diagnosed HER2-positive MBC. Endpoints included treatment patterns and clinical outcomes.

Results: Of 977 eligible patients (enrolled from 2012 to 2016), 70.1% ( = 685) had HR-positive and 29.9% ( = 292) had HR-negative disease. Overall, 59.1% (405/685) of patients with HR-positive disease received any first-line endocrine therapy (with or without HER2-targeted therapy or chemotherapy); 34.9% (239/685) received HER2-targeted therapy + chemotherapy + sequential endocrine therapy. Patients with HR-positive versus HR-negative disease had longer median overall survival (OS; 53.0 vs 43.4 months; hazard ratio, 0.70; 95% confidence interval, 0.56-0.87). Compared with patients with high HR-positive staining (10%-100%, = 550), those with low HR-positive staining (1%-9%, = 60) received endocrine therapy less commonly (64.2% vs 33.3%) and had shorter median OS (53.8 vs 40.1 months). Similar median OS (43.4 vs 40.1 months) was observed in patients with HR-negative versus low HR-positive tumors (1%-9%).

Conclusions: Despite evidence that first-line HER2-targeted therapy, chemotherapy, and sequential endocrine therapy improves survival in patients with HR-positive, HER2-positive disease, only 34.9% of patients in this real-world setting received such treatment. Patients with low tumor HR positivity (1%-9%) had lower endocrine therapy use and worse survival than those with high tumor HR positivity (10%-100%).