Background and importance Palbociclib is a selective cyclin-dependent kinase inhibitor approved for the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative locally advanced or metastatic breast cancer (LA/MBC) in combination with an aromatase inhibitor as first-line treatment, or with fulvestrant in previously treated patients. Real-world data regarding its safety and tolerability when prescribed as first-line treatment are still scarce.

Aim and objectives To determine the long-term safety profile of palbociclib when prescribed as first-line treatment for HR-positive, HER2-negative LA/MBC.

Material and methods An observational, retrospective, descriptive study was performed at a tertiary hospital. All patients who started palbociclib as first-line treatment for HR-positive, HER2-negative LA/MBC between January 2018 and August 2019 were included. Adverse events (AE) were graded according to CTCAE v5.0 criteria. Frequency and causes of dose delays, reductions or permanent treatment discontinuations were collected. Clinical and analytical data were obtained from electronical clinical records, and treatment data from the dispensing electronic program. External reference data were used from the PALOMA-2 trial to compare the real-world data.

Results A total of 49 women were studied, median follow-up 33 (1–44) months. All patients had an AE of any grade and 39 (79.6%) presented grade 3 or 4 AE. Grade 3 or 4 AE included neutropenia (69.4%), leukopenia (34.7%) and thrombocytopenia (6.1%). Toxicity led to a dose delay in 39 (79.6%) patients; neutropenia (82.1%), thrombocytopenia (7.7%) and fatigue (7.7%) were the AE most frequently involved. A first dose reduction was necessary in 31 (63.3%) of the patients, while 13 (26.5%) required further reductions. Neutropenia (70.4%) was the main cause of dose reduction. Permanent treatment interruption was mandatory in 6 (12.8%) patients, because of neutropenia (50.0%), pneumonitis (16.6%), asthenia (16.6%) and blurred vision and dizziness (16.6%). Two (4.3%) deaths ocurred due to pneumonitis and progressive multifocal leukoencephalopathy. More cycle delays, dose reductions and dose interruptions were reported in comparision with the PALOMA-2 trial (79.6% vs 67.0%, 63.3% vs 36.0% and 12.8 vs 9.7%, respectively).

Conclusion and relevance The real clinical practice toxicity profile of palbociclib as first-line treatment for HR-positive, HER2-negative LA/MBC is similar to that previously reported in PALOMA-2, although more treatment modifications were necessary.

Conflict of interest No conflict of interest