Abstract
BACKGROUND: Cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitors combined with endocrine therapy have revolutionized the treatment landscape of hormone receptor (HR)-positive advanced breast cancer. However, reliable biomarkers for CDK4/6 inhibitors remain limited. This study investigated dynamic peripheral blood indicators as potential prognostic biomarkers for survival outcomes and therapeutic efficacy. METHODS: This post-hoc analysis included patients with HR-positive, HER2-negative metastatic breast cancer treated with bireocilib or placebo with fulvestrant from BRIGHT-2 trial. Peripheral blood samples collected at baseline and on-treatment were used to calculate absolute lymphocyte count (ALC), neutrophil-to-lymphocyte ratio (NLR), monocyte-to-lymphocyte ratio (MLR), platelet-to-lymphocyte ratio (PLR), systemic immune-inflammation index (SII) and prognostic nutritional index (PNI). Kaplan-Meier analysis and Cox proportional hazards models assessed survival outcomes, while dynamic changes in indicators were compared using paired Wilcoxon signed-rank test. RESULTS: A total of 305 patients was included in this analysis. Bireociclib significantly improved PFS versus placebo regardless of MLR, PLR, and PNI levels (all p < 0.05). In multivariate Cox analysis, NLR, MLR, SII and PNI were independent prognostic factors for both PFS and overall survival (OS). None of the hematological indicators demonstrated significant predictive value across different treatment arms. In experimental group, PLR remained stable during treatment in the response group but increased significantly in the non-response group (p = 0.003). In control group, none of the indicators demonstrated significant changes from baseline to on-treatment point. CONCLUSIONS: Peripheral blood-derived indicators are prognostic markers for HR-positive, HER2-negative metastatic breast cancer. Elevated PLR during treatment indicates poor response to CDK4/6 inhibitors.