Risk factors associated with palbociclib-induced neutropenia in patients with metastatic breast cancer

Neutropenia is the most common adverse event with palbociclib, an oral cyclin-dependent kinase 4/6 inhibitor, with grade 3/4 neutropenia occurring in up to 67% of patients in phase III trials evaluating this agent in metastatic breast cancer. This retrospective chart review assessed characteristics of patients on palbociclib to evaluate for risk factors in the development of grade 3/4 neutropenia.

Patients with metastatic breast cancer who received palbociclib were included. Patient demographics collected included age, gender, race, body mass index, breast cancer treatment history, palbociclib starting dose, baseline absolute neutrophil count, baseline platelet count, concomitant hormonal therapy, concomitant use of denosumab, and use of concomitant strong CYP3A4 inhibitors/inducers. Events of interest occurring within 30 days of initiation of palbociclib were also noted including antibiotic and corticosteroid use, mucosal conditions, open wounds, or surgery. The incidence and potential risk factors for grade 3/4 neutropenia in the first 6 months of treatment were analyzed.

A total of 257 patients were included in the analysis with 206 patients (80.2%) and 139 patients (54.1%) experiencing all-grade neutropenia and grade 3/4 neutropenia, respectively. Multivariate analysis found baseline myelosuppression and recent antibiotic use to be independent predictors of grade 3/4 neutropenia. Normal weight patients had an increased risk for grade 3/4 neutropenia compared to obese patients by multivariate analysis.

The results of this study showed baseline myelosuppression and recent antibiotic use within 30 days of palbociclib initiation were predictive of a higher incidence of grade 3/4 neutropenia. Obese patients were less likely to develop grade 3/4 neutropenia compared to normal weight patients.