Background and importance Palbociclib and ribociclib are oral inhibitors of cyclin dependent kinases for firstline and secondline treatment of hormone receptor (HR) positive, HER2 negative locally advanced or metastatic breast cancer (BC). The most common adverse events (AE) described in clinical trials (CT) were haematological, especially neutropenia.

Aim and objectives To compare the safety and incidence of severe neutropenia in patients treated with palbociclib and ribociclib in clinical practice.

Material and methods A descriptive and retrospective study was performed in patients treated with palbociclib and ribociclib in a third level hospital from January 2018 to September 2020. We registered demographic data (sex and age), number of cycles received, discontinuations, dose reductions and absolute neutrophil count (ANC). Demographic and clinical data were obtained from digital clinical history and toxicity grade (G) of neutropenia was classified by CTCAE V.5.0.

Results 62 patients (35 treated with palbociclib and 27 with ribociclib) with a median age of 60 years diagnosed with metastatic HR positive/HER2 negative BC were included. The median number of cycles received was 10 (1–28) and 7 (2–28) in patients treated with palbociclib and ribociclib, respectively. 19 (54.2%) patients treated with palbociclib developed severe neutropenia (3 (8.6%) grade 4 and 16 (45.7%) grade 3) and 10 (37%) patients treated with ribociclib developed severe neutropenia (1 (3.7%) grade 4 and 9 (33.3%) grade 3) after the first 15 days of treatment, leading to a median number of discontinuations of 2 and 3, respectively. 16 (45.7%) patients treated with palbociclib and 8 (29.6%) patients treated with ribociclib required dose reductions. The causes of suspension of treatment were: toxicity (medullary aplasia, severe exanthema, asthenia and anaemia) in 4 (11.4%) patients treated with palbociclib and in 5 (18.5) with ribociclib, interactions (1 patient treated with palbociclib) and disease progression in the rest of the patients.

Conclusion and relevance In clinical practice, the incidence of severe neutropenia in patients treated with palbociclib and ribociclib was higher than in CT, with a lower incidence of neutropenia grade 4 and grade 3 in patients treated with ribociclib. However, severe neutropenia was successfully managed with dose reduction and discontinuation for both treatments, so any patient had to stop treatment due to neutropenia. In contrast, the proportions of patients treated with palbociclib and ribociclib that required dose reductions were the same.

Conflict of interest No conflict of interest