Background and importance Palbociclib and ribociclib are equivalents in terms of effectiveness in the treatment of metastatic breast cancer (mBC) HER2 negative with positive hormone receptors (RH). The randomised studies PALOMA-2/3 and MONALEESA-2/3 concluded that the most frequent AE is neutropenia of any degree with an incidence of 75.8% and 71.5% for palbociclib and ribociclib, respectively.

Aim and objectives To determine the long term safety profile of palbociclib and ribociclib in real clinical practice.

Material and methods This was an observational, descriptive, retrospective study. All patients diagnosed with mBC HER2 negative and RH positive who started treatment with palbociclib or ribociclib between November 2017 and October 2019 were selected. The main outcomes were percentage of patients that required dose reduction due to AE, causes of AE and time of onset. Other outcomes were percentages of dose delays and their causes. The clinical and analytical data were obtained from the history clinical electronic programme (Diraya) and the treatment data from the prescription programme (OncoFarm).

Results During the study period, 22 patients were treated with palbocicli (4 as firstline therapy) and 44 with ribociclib (22 as firstline therapy). Median duration of treatment was 17.1 months in the palbociclib group and 5.0 months in the ribociclib group. In the palbociclib group, 36% (n=8) of patients the dose was reduced to 100 mg due to neutropenia (6/8), thrombocytopenia (1/8) and unknown cause (1/8); one of these patients required a second dose reduction to 75 mg due to neutropenia 71 days after the first reduction. In the ribociclib group, 6% (n=3) of patients had their dose reduced due to AE, 4% due to neutropenia and 2% to nausea. In 52% of patients treated with palbociclib there were 18 delays: neutropenia(n=11), leucopenia (n=2), thrombocytopenia (n=2), unknown (n=2) and rash (n=1). In the ribociclib group, 6% (n=3) of patients had a dose delay due to AE: 2 due to neutropenia and 1 to nausea and vomiting. At the time of analysis, 7 and 12 patients, respectively, had discontinued treatment for any cause.

Conclusion and relevance In our sample of patients, tolerance, in terms of AE, of ribociclib was better than that of palbociclib. These data are not consistent with previous studies and it cannot be ruled out that the differences were due to differences in the profile of the patients.

References and/or acknowledgements No conflict of interest.