6ER-031 Effectiveness of abemaciclib, palbociclib and ribociclib in metastatic breast cancer

<h3>Background and Importance</h3> Cyclin-dependent kinase 4/6 inhibitors (CDK4/6 inhibitors): abemaciclib-(ABE), palbociclib-(PAL), and ribociclib-(RIB), have shown efficacy in treating metastatic hormone receptor-positive (HR+) breast cancer (MBC), as demonstrated in the MONARCH-3, PALOMA-2, and MONALEESA-2 trials, respectively. <h3>Aim and Objectives</h3> To evaluate and compare the effectiveness of CDK4/6 inhibitors in routine clinical practice. <h3>Material and Methods</h3> This was a 7-year retrospective observational study (September 2017 to May 2024) including all patients treated with CDK4/6 inhibitors for HR+/HER2- MBC. Effectiveness was assessed by analysing median progression-free survival (PFS) and overall survival (OS) for each treatment. Comparisons of these medians were performed using a log-rank test, with Kaplan-Meier analysis conducted via SPSS v26. <h3>Results</h3> A total of 164 patients were included, with a mean age of 64.7 years (31–90), ECOG 0–2. Bone metastases were present in 72% of patients, visceral metastases in 43.9%, and central nervous system involvement in 4.9%. Abemaciclib was given to 29.9% (n=49) of patients (75.5% in 1st-line), ribociclib to 31.7% (n=52) (88.5% in 1st-line), and palbociclib to 38.4% (n=63) (74.6% in 1st-line). In 8.5% of 1st-line cases, prior CDK4/6 inhibitors therapy had been discontinued due to toxicity. Median PFS for 1st-line therapy was 10.4 months with ABE (95% CI: 4.9–15.9), 18.1 months with RIB (95% CI: 2.9–33.3), and 15.4 months with PAL (95% CI: 11.9–18.9). Significant differences were observed between ABE-RIB (p=0.014) and between ABE-PAL (p=0.035), but not between RIB-PAL (p=0.193). Median OS was 12.2 months with ABE (95% CI: 0–28.4), 9.2 months with RIB (95% CI: 0–36.2), and 20.2 months with PAL (95% CI: 14.5–25.9), with no statistically significant differences between ABE-RIB (p=0.204), ABE-PAL (p=0.312), or RIB-PAL (p=0.464). <h3>Conclusion and Relevance</h3> The median PFS and OS observed were lower than those reported in pivotal trials, likely due to the observational study design, smaller sample size, and patient characteristics in routine clinical practice. Larger prospective studies are needed to confirm the observed PFS differences between ABE and other CDK4/6 inhibitors. <h3>References and/or Acknowledgements</h3> <h3>Conflict of Interest</h3> No conflict of interest