HER2-Low Status Does Not Affect Survival Outcomes of Patients with Metastatic Breast Cancer (MBC) Undergoing First-Line Treatment with Endocrine Therapy plus Palbociclib: Results of a Multicenter, Retrospective Cohort Study

Simple Summary Breast cancers (BCs) with a HER2 immunohistochemical score of 1+ or 2+ with negative in situ hybridization are referred as HER2-low BCs. The knowledge about the biological and clinical characteristics of HER2-low BCs is still limited and controversial. Despite that new anti-HER2 antibody-drug conjugates (ADCs) have demonstrated significant activity in HER2-low BCs, no anti-HER2 agents are currently approved for this subgroup in Europe. Therefore, treatment for HER2-low BCs is determined by HR expression status. In this study, we aimed to investigate the prognostic significance of HER2-low status in HR+/HER2 negative (HER2-) metastatic BC (MBC) patients treated with endocrine therapy (ET) plus palbociclib as first line. HR+ MBC patients with HER2-low tumors who received first-line treatment with ET plus palbociclib show similar survival outcomes compared to those HER2-0 disease. Background: Approximately 45-50% of breast cancers (BCs) have a HER2 immunohistochemical score of 1+ or 2+ with negative in situ hybridization, defining the "HER2-low BC" subtype. No anti-HER2 agents are currently approved for this subgroup in Europe, where treatment is still determined by HR expression status. In this study, we investigated the prognostic significance of HER2-low status in HR+/HER2- metastatic BC (MBC) patients treated with endocrine therapy (ET) plus palbociclib as first line. Methods: We conducted a retrospective study including 252 consecutive HR+/HER2- MBC patients who received first-line ET plus palbociclib at six Italian Oncology Units between March 2016 and June 2021. The chi-square test was used to assess differences in the distribution of clinical and pathological variables between the HER-0 and HER2-low subgroups. Survival outcomes, progression-free survival (PFS) and overall survival (OS), were calculated by the Kaplan-Meier method, and the log-rank test was performed to estimate the differences between the curves. Results: A total of 165 patients were included in the analysis: 94 (57%) and 71 (43%) patients had HER2-0 and HER2-low disease, respectively. The median age at treatment start was 64 years. No correlation between patients and tumor characteristics and HER2 status was found. Median PFS (mPFS) for the entire study cohort was 20 months (95% CI,18-25 months), while median OS (mOS) was not reached at the time of analysis. No statistically significant differences, in terms of PFS (p = 0.20) and OS (p = 0.1), were observed between HER2-low and HER2-0 subgroups. Conclusions: In our analysis, HR+ MBC patients with low HER2 expression who received first-line treatment with ET plus Palbociclib reported no statistically different survival outcomes compared to HER2-0 patients. Further prospective studies are needed to confirm the clinical role of HER2 expression level.