Real‐world benefit of combination palbociclib and endocrine therapy for metastatic breast cancer and correlation with neutropenia

BACKGROUND: Combination CDK4/6 inhibitor and endocrine therapy has been shown to significantly improve progression‐free survival (PFS) in patients with hormone receptor (HR)‐positive, HER2‐negative metastatic breast cancer (mBC). The aim of this retrospective study was to evaluate the real‐world benefit of first‐line combination therapy in this cohort and to correlate treatment efficacy with neutropenia, a common toxicity of CDK4/6 inhibitors. METHODS: This study included HR‐positive, HER2‐negative advanced or mBC patients who were treated with palbociclib plus endocrine therapy, mainly letrozole, between 1 January 2015 and 1 March 2018. Progression‐free survival (PFS) was determined using Kaplan–Meier analysis. The predictive value of absolute neutrophil count (ANC) and neutrophil‐to‐lymphocyte ratio (NLR) for PFS were explored using Cox regression models. Both ANC and NLR were used as a time‐dependent variable. RESULTS: In total, 165 patients were included with median PFS of 24.19 months (95% CI 18.93–NR). Median PFS for patients with bone‐only metastases (n = 54) was not reached (95% CI 18.21–NR). Among patients with all other metastases (n = 111), median PFS was 24.19 months (95% CI 16.33–33.82). Lower ANC was correlated with decreased risk of progression (HR 0.84, 95% CI 0.71–0.97, p = 0.008). There was no significant association between NLR and the risk of disease progression (HR 1.07, 95% CI 0.97–1.18, p = 0.203). CONCLUSION: The effectiveness of palbociclib and endocrine therapy in the treatment of HR‐positive, HER2‐negative mBC in the real‐world setting is similar to the efficacy reported in the PALOMA‐2 trial. Patients with lower neutrophil count may have a lower risk of early disease progression.