Impact of HER2-low status on survival outcomes in patients with metastatic breast cancer treated with cdk4/6 inhibitors: A single-center experience

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Aim: Cyclin-dependent kinase (CDK) 4/6 inhibitors combined with endocrine therapy are recommended as the first-line treatment for hormone receptor-positive, HER2-negative metastatic breast cancer without visceral crisis. However, CDK4/6 inhibitor efficacy has been reported to differ between HER2-low and HER2-zero tumors; emerging data suggest that the therapeutic efficacy of CDK4/6 inhibitors may be reduced in HER2-low breast cancer. Therefore, we aimed to evaluate this association in a cohort of patients treated with CDK4/6 inhibitors.Material and Methods: In this single-center retrospective study, 121 patients with hormone receptor-positive, HER2-negative metastatic breast cancer treated with CDK4/6 inhibitors between January 2019 and March 2025 were evaluated. Cases were classified as HER2-low or HER2-zero. Progression-free survival (PFS) and overall survival (OS) were assessed using the Kaplan-Meier method.Results: Among 121 hormone receptor-positive, HER2-negative metastatic breast cancer patients treated with CDK4/6 inhibitors, 53.7% were in the HER2-zero group and 46.3% in the HER2-low group. The median follow-up duration was 28.6 months (range: 5.1-56.4 months) and the median age was 62 years (range: 33-89 years). Ribociclib was used in 68.6% and palbociclib in 31.4% of these cases. The incidence of grade 3-4 toxicities was comparable between the groups (p=0.948). There was no statistically significant difference in median PFS (31.1 vs 34.8 months, p=0.314) or OS (42.3 vs 39.9 months, p=0.946) between the HER2-zero and HER2-low groups. Ribociclib and palbociclib demonstrated similar survival outcomes across HER2 subgroups. Conclusion: The clinical significance of HER2 status in patients treated with CDK4/6 inhibitors remains unclear. In our cohort, we were unable to demonstrate an association between HER2 status and PFS or OS among patients receiving CDK4/6 inhibitor therapy.

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