Real-world comparison of the efficacy of three CDK4/6 inhibitors (CDK4/6i) in the first-line treatment of endocrine-sensitive advanced breast cancer (aBC): Single institution experience.

e13080 Background: The combination of CDK4/6i and endocrine therapy (ET) is the standard of care in the first-line treatment of hormone receptor positive (HR+), HER2- aBC. In registrational studies, the progression-free survival (PFS) of all three CDK4/6i ranged from 23.8 to 28.12 months and overall survival (OS) from 53.9 to 66.8. In Croatia, both ribociclib and palbociclib were approved for clinical use in August 2018 and abemaciclib in November 2019. Methods: This retrospective study included patients with endocrine-sensitive HR+, HER2- aBC treated with aromatase inhibitor (AI) and CDK4/6i in the first-line setting between August 2018 and October 2023 at University Hospital Centre Zagreb. Ethics committee approval was obtained before the study was commenced. Patient demographics, clinical presentation, tumor characteristics and treatment data were collected. Real-world PFS (rwPFS) and OS analyses were done with the final data cut-off date of December 31, 2023, using type 1 right censoring. Data were analyzed using the Kaplan-Meier method. Results: Of the 274 patients treated with a combination of CDK4/6i and ET in the first-line setting, 172 with endocrine-sensitive tumors (98 de novo metastatic and 74 with treatment-free interval longer than 12 months) were eligible for the study, of which 41.86% (72/172) received ribociclib, 44.19% (76/172) palbociclib, and 13.95% (24/172) abemaciclib. Overall rwPFS was 48 months (95% CI:35-53). In the abemaciclib group, median rwPFS and OS were not reached in the follow-up period. The rwPFS was 48 (95% CI:27-53) for ribociclib and 47 (95% CI:30-49) for palbociclib. The overall OS was 62 months (95% CI:47-62). Median OS in the ribociclib group was 62 months (95% CI:40-62) and in the palbociclib group 52 (95% CI:42-54) months. Conclusions: The PFS and OS results for palbociclib and ribociclib in this retrospective real-world study correlate with results from randomized clinical trials, confirming some inferiority of palbociclib compared to ribociclib in the context of OS. Due to the later availability of the drug, larger sample size and longer follow-up will be needed to make definitive conclusions on the efficacy of abemaciclib.