Background and Importance Palbociclib is a selective cyclin-dependent kinase 4/6 (CDK4/6) inhibitor approved for the treatment of hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2−) locally advanced or metastatic breast cancer (LA/MBC). Neutropenia is the most common adverse event. In contrast to neutropenia induced by chemotherapy agents, neutropenia resulting from CDK4/6 inhibitors is reversible and dose reductions and modifications are recommended.

Aim and Objectives The aim of this study was to evaluate the neutropenia due to palbociclib and to analyse how modifications in treatments are made in clinical practice.

Material and Methods We conducted a descriptive, observational and retrospective study (April 2016-July 2022) of patients treated with Palbociclib in a third level hospital. The data were obtained from the electronic medical records of the patients and the Farmatools Management programme . The parameters analysed were: demographic information, menopausal status, prior lines of therapy to palbociclib, frequency and grades of neutropenia, time from first dose to first episode onset, doses reductions, cycles delays, use of human granulocyte colony stimulating factor (G-CSF), changes to other CDK4/6 inhibitor and discontinuation treatment. Data were processed by Microsoft Excel software

Results 50 women with HR+/HER2− MBC were treated with palbociclib. Median age was 62 years. 92%(46/50) was postmenopausal. 80%(40/50) received prior therapy to palbociclib and 58%(23/40) was in the context of MBC. 54%(27/50) received Palbociclib as first-line treatment. Starting dose were: 82%(41/50) 125 mg; 12%(6/50) 100 mg; 6%(3/50) 75 mg.

The frequency of neutropenia (all-grade) was 74%(37/50); 27%(10/37) was grade 1-2; 73%(27/37) was grade 3-4. Time from first dose to first episode onset (cycles) was reported in: 8,1%(3/37) first-cycle; 56,7%(21/37) second-cycle; 13,5%(5/37) three-cylcle; 21,6%(8/37) ≥ fourth-cycle. Neutropenia led to dose reduction in 54%(20/37) of patients; 32%(12/37) required a dose reduction; 21,6%(8/37) required two doses reductions. Cycles delays occurred in 78%(29/37) of patients. 19%(7/37) was treated with G-CSF as supportive therapy. 5,4%(2/37) needed to change to another CDK4/6 inhibitor. 10,8%(4/37) discontinued treatment.

Conclusion and Relevance The frequency of neutropenia in our population was similar to clinical trials.¹ In clinical practice this toxicity can be managed with dose reduction and cycles delays without lead to discontinuation treatment (only four patients) as it is described in guidelines.²

References and/or Acknowledgements 1. Pivotal study PALOMA-3

2. https://cima.aemps.es/cima/dochtml/ft/1161147003/FT_1161147003.html

Conflict of Interest No conflict of interest