Background and importance Palbociclib was approved by the EMA for the treatment of hormone receptor positive, human epidermal growth factor receptor 2 (HER2) negative locally advanced or metastatic breast cancer in combination with an aromatase inhibitor or fulvestrant.

Aim and objectives To study the adverse events (AEs) and their impact on dosing and cycle delays in patients treated with palbociclib. To characterise the safety of palbociclib in clinical practice.

Material and methods This was a retrospective observational study (March 2018–July 2019). Patient demographics, clinical and treatment related data and AEs were analysed. Toxicity was classified by common terminology criteria for adverse events.

Results A total of 41 women were included, mean age was 59 (37–78) years and mean number of cycles received was 8.5 (1–18). Thirty-seven patients (90%) presented with AEs. The most common AEs were haematological (68%): neutropenia (58.5%), leucopenia (12%), anaemia (7%) and thrombocytopenia (5%). Among the non-haematological AEs, general disorders (asthenia, fatigue) were the most common (51%) followed by gastrointestinal events (34%), skin and subcutaneous tissue disorders (15%), musculoskeletal and connective tissue disorders (10%), metabolism and nutrition disorders (5%), and hepatobiliary disorders (5%).

In response to treatment related AEs, 17 patients (41%) required dose reduction. In 13 cases (32%) the cause of the modification was neutropenia; other causes were anaemia, fatigue, cholelithiasis and pruritus. Five patients required a second dose reduction and the reasons were the same (4 because of neutropenia and 1 because of fatigue). The mean interval between reductions was 5 cycles (3–10) and currently all are continuing treatment with palbociclib.

As a result of the AEs, 27 patients (67.5%) have required cycle delays. The main cause was neutropenia (50%), followed by anaemia (5%) and fatigue (5%). Other causes were leucopenia, thrombocytopenia, diarrhoea, pruritus and non-treatment reasons.

Ten patients discontinued treatment (24%), 9 due to disease progression and the 1 left because of hypertransaminasaemia produced after the first cycle which triggered early suspension.

Conclusion and relevance Due to proper management of toxicities, the majority of patients did not need to discontinue treatment and palbociclib may be an option in these patients. However, some patients presented AEs which led to delays in the cycles and dose modifications.

References and/or acknowledgements No conflict of interest.