Risk Factors and the Clinical Impact of Palbociclib Early Dose Reduction in Patients with Hormone Receptor-Positive Metastatic Breast Cancer

Dose reduction owing to neutropenia is frequently required during palbociclib treatment for metastatic breast cancer. In this study, we investigated the risk factors associated with early palbociclib dose reduction and its impact on treatment efficacy in Japanese patients. Patients who received palbociclib were classified into two groups based on whether dose reduction occurred within the first 56 days of treatment. The background characteristics and time to treatment failure (TTF) were compared between the two groups. Out of the 68 patients included in the analysis, 37 (54％) required early dose reduction. Multivariate logistic regression analysis identified the co-administration of proton pump inhibitors or potassium-competitive acid blockers (odds ratio [OR]: 3.33, 95％ confidence interval [CI]: 1.08 – 10.30, P = 0.036) and a baseline neutrophil count <3,779/μL (OR: 3.14, 95％ CI: 1.09 – 9.08, P = 0.034) as significant factors associated with early dose reduction. The median TTF was 15.6 months in the early dose reduction group and 7.6 months in the standard dose group, with no statistically significant difference observed (P = 0.627). Because early dose reduction does not affect the therapeutic efficacy of palbociclib, these findings suggest that palbociclib treatment should be continued by appropriately implementing dose interruptions or reductions in accordance with standard protocols for managing neutropenia.