Survival Outcomes in Patients With Hormone Receptor–Positive Metastatic Breast Cancer With Low or No ERBB2 Expression Treated With Targeted Therapies Plus Endocrine Therapy

IMPORTANCE: Approximately 45% to 60% of hormone receptor (HR)–positive metastatic breast cancer (mBC) shows a low-level expression of ERBB2. Low ERBB2 expression is defined as ERBB2 immunohistochemical expression of 1+ or 2+ with a negative ERBB2 amplification by in situ hybridization. The efficacy of the antibody-drug conjugate trastuzumab deruxtecan in low-ERBB2, HR-positive mBC has been practice changing. However, there are conflicting data on the prognostic value of low ERBB2 expression in HR-positive mBC and whether low ERBB2 expression is a separate entity. OBJECTIVE: To examine whether outcomes differ by immunohistochemical analysis for patients with HR-positive mBC with low ERBB2 expression vs those without ERBB2 expression when treated with targeted therapy (TT) plus endocrine therapy (ET). DESIGN, SETTING, AND PARTICIPANTS: This single-institution cohort study used prospectively collected electronic data from the MD Anderson Cancer Center for patients with a diagnosis of HR-positive mBC treated with ET in combination with a TT (cyclin-dependent kinase 4/6 inhibitors [CDK4/6is], everolimus, or alpelisib) between January 1, 2010, and December 31, 2021. EXPOSURE: HR-positive mBC with either low or no ERBB2 expression. MAIN OUTCOME AND MEASURES: The main outcomes were median progression-free survival and overall survival. Data on demographic characteristics, estrogen and progesterone receptor status, ERBB2 status, histologic subtype, menopausal status, treatment duration, and survival status were collected. RESULTS: A total of 1585 women (median [range] age, 51 [24-92] years) were included in the study. Of these women, 1013 (63.9%) had mBC with low ERBB2 expression and 572 (36.1%) had mBC with no ERBB2 expression. A total of 1084 (68.4%) were treated with a CDK4/6i (912 patients were treated in the first line and 172 were treated in the second line); 475 (30.0%) received everolimus and 26 (1.6%) received alpelisib. In the patients who received a first-line CDK4/6i, 618 (67.8%) received an aromatase inhibitor as their ET backbone and 265 (29.1%) received fulvestrant. With a median follow-up time of 17.9 months (range, 1-111 months), progression-free survival and overall survival were not statistically different between the patients with low and no ERBB2 expression treated with TT plus ET. CONCLUSIONS AND RELEVANCE: In this cohort study of patients with HR-positive mBC treated with TT plus ET, low ERBB2 expression did not have a significant association with prognosis.