Background and importance Cyclin dependent kinase inhibitors (CDKs) have demonstrated efficacy in the treatment of hormone receptor positive (RH+) and human epidermal growth factor receptor 2 (HER2-) metastatic breast cancer.

Aim and objectives To evaluate the efficacy and safety of cyclin inhibitor drugs (CDK) in patients with metastatic breast cancer.

Material and methods An 18 month, single centre, retrospective study (January 2019 to June 2020) that included all patients with metastatic breast cancer treated with palbociclib (PB), ribociclib (RB) and abemaciclib (AB) was conducted. The following variables were collected: age, sex, time of disease follow-up, time of treatment with the drug, neoadjuvant/adjuvant and previous hormonal therapy. Efficacy was evaluated by calculating progression free survival (PFS) for each drug, applying the Kaplan–Meier statistic. Progression was analysed according to the radiological criteria response evaluation criteria in solid tumours (RECIST V.1.1). The occurrence of adverse effects (AEs) leading to early dose reduction/suspension of the drug was analysed according to the common terminology criteria for adverse events (CTCAE).

Results 98 women (mean age 62.8 (34–86) years) were included in the study. 53.1% received PB (n=52), 25.5% (n=25) RB and 21.4% (n=21) AB. Mean follow-up time of the disease was 7.5 years. Mean duration of treatment was 9.5 months (0.1–34.3). 59.2% (n=58) received previous adjuvant therapy, 20.4% (n=20) neoadjuvant and 74.5% (n=73) hormonal.

Median PFS was AB vs PB (17.4 vs 11.9 months, p=0.008); AB vs RB (17.4 months vs not reached, p=0.065), PB vs RB (p=0.034). 34.7% (n=34) suspended treatment due to progression and 15.3% (n=15) due to toxicity. 54.1% (n=53) of patients had doses reduced due to AEs during treatment: PB (45.3%, n=24), RB (24.5%, n=13) and AB (30.2%, n=16). The most common AEs were: haematological (69.8%, n=37), gastrointestinal (17%, n=9), skin (9.4%, n=5), asthenic syndrome (9.4%, n=5), renal (5.7%, n=3), hepatic (5.7, n=3) and other (3.8%, n=2).

Conclusion and relevance PFS was more favourable for AB than for PB, but not for RB. Likewise, RB treatment had a higher PFS than PB (p≤0.05). More than 50% of patients had their dose reduced due to the appearance of AEs, with PB being the least tolerated. This highlights the need for close monitoring of these AEs, which impairs long term efficacy.

Conflict of interest No conflict of interest