Real-World Experience of Palbociclib-Induced Adverse Events and Compliance With Complete Blood Count Monitoring in Women With Hormone Receptor-Positive/HER2-Negative Metastatic Breast Cancer.

Background: The cyclin-dependent kinase 4/6 inhibitor palbociclib has emerged as a novel therapeutic agent in metastatic breast cancer. Neutropenia is commonly observed, and thus stringent treatment guidelines regarding complete blood count (CBC) monitoring have been developed. The aim of this study was to provide a real-world experience of the toxicities associated with palbociclib therapy and to evaluate compliance with CBC monitoring.

Patients And Methods: We performed a retrospective single-center audit of hormone receptor-positive metastatic breast cancer patients treated with palbociclib over a 6-month period in an Irish tertiary referral hospital.

Results: A total of 64 patients were included in the analysis. Palbociclib was most commonly used in combination with letrozole (n = 40). A total of 28 patients (44%; 95% confidence interval, 31.2-56.2) had treatment deferrals due to neutropenia, with a median time to first deferral of 4 weeks. Fifteen patients (23%; 95% confidence interval, 15.4-37.7) required dose adjustments; however, there was no association with an increased risk of progressive disease (P = .56). Only 3 patients discontinued treatment as a result of poor tolerance. Adverse events were as expected; however, 7 venous thromboembolic events were reported.

Conclusion: Compliance was good with existing CBC monitoring guidelines. We observed an 11% incidence of venous thromboembolic events, a significant increase from 2% reported in the PALOMA-3 trial. Further studies are recommended to determine if prophylactic anticoagulation may benefit these patients.