Standard Versus Reduced CDK4/6 Inhibitor Therapy in Elderly Patients with Metastatic Hormone Receptor-Positive, HER2-Negative Breast Cancer: An Observational Multicenter Study

Background: Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors are the standard of care for hormone receptor (HR)+/human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer in combination with endocrine therapy. However, the real-world efficacy and safety of standard versus reduced doses in elderly patients remain unclear. This study aims to compare the clinical outcomes of standard versus reduced doses of CDK4/6 inhibitors in elderly patients with metastatic breast cancer. Methods: This multicenter retrospective cohort study included 158 patients aged >= 70 years diagnosed with HR+/HER2-negative metastatic breast cancer who received either standard or reduced doses of CDK4/6 inhibitors (Ademaciclib, Ribociclib, Palbociclib) as first-line therapy. Progression-free survival (PFS), overall survival (OS), and adverse events (AEs) were evaluated. PFS and OS were estimated using the Kaplan-Meier method, and comparisons between groups were performed using a log-rank test. Results: Of the total population, 108 patients (68.4%) received the standard dose, and 50 patients (31.6%) received a reduced dose. The standard-dose group had significantly longer median PFS compared to the reduced-dose group (21.3 vs. 15.2 months, p = 0.014), while the median OS did not differ significantly (37.2 vs. 37.2 months, p = 0.103). Subgroup analyses revealed no significant differences in PFS or OS between standard and reduced doses for Ademaciclib and Ribociclib, while Palbociclib at standard dose showed superior PFS (21.9 vs. 12.7 months, p = 0.029) and OS (50.5 vs. 28.6 months, p = 0.026). The incidence of Grade 2-4 AEs was higher in the standard-dose group (74.2% vs. 56.8%, p = 0.044). Conclusions: Dose reduction of CDK4/6 inhibitors, particularly Ademaciclib and Ribociclib, is a viable option in elderly patients, maintaining comparable OS outcomes to standard dosing while reducing the risk of adverse events. Palbociclib at standard dose may offer superior outcomes. These findings support personalized dosing strategies to optimize efficacy and tolerability in frail or elderly patients.