Association between severe neutropenia and progression-free survival in patients with advanced or recurrent breast cancer treated with palbociclib

This study aimed to clarify the relationship between neutropenia and progression-free survival (PFS) under palbociclib treatment for advanced/recurrent breast cancer and the risk factors for severe neutropenia. We retrospectively identified 37 patients who received palbociclib for advanced breast cancer at Ogaki Municipal Hospital (Ogaki, Japan) between April 2018 and June 2020. Kaplan-Meier log-rank test was used to compare PFS (mild [neutrophil count 1,000-2,000/mm 3 ] versus severe [neutrophil count <500-1,000/mm3]). Multivariate analysis was performed to evaluate the relationships between baseline patient characteristics and severe neutropenia development. There were three, four, 25, and five cases with grade 1, 2, 3, and 4 neutropenia, respectively. Median PFS in patients who developed severe neutropenia (n = 30) and those who did develop mild neutropenia (n = 7) was 176 days (range: 62-894 days) and 91 days (range: 19-384 days), respectively (log-rank test, p = 0.005). Severe neutropenia was independently associated with pre-treatment neutrophil count (odds ratio: 27.700; p =0.007). Severe neutropenia is more likely to occur with a pre-treatment neutrophil count of less than 3,680 mm3. Neutropenia prolongs PFS under palbociclib treatment, suggesting management of AEs and patient education as highly important, especially to prevent drug interruption/dose reduction of palbociclib due to these AEs.