Real-world Effectiveness of CDK 4/6 Inhibitors in Estrogen-positive Metastatic Breast Cancer

Background: Initial treatment for advanced ER-positive/HER2-negative breast cancer involves a CDK 4/6 inhibitor (CDK 4/6i). Recent overall survival (OS) analyses led the Danish Medical Council to exclude palbociclib as preferred option. This study aimed to evaluate the real-world effectiveness of abemaciclib, palbociclib, and ribociclib in a Danish context. Additionally, to compare the inhibitors to identify potential endpoint differences.

Material And Methods: Patients undergoing first or second line CDK 4/6i treatments from January 1st, 2017, until December 31st, 2021 were included. The primary endpoint was progression free survival (PFS).

Results: Among 2069 Danish patients, 1554 received first line treatment, 515 received second line treatment. In first line, abemaciclib's median PFS was unreached; palbociclib had a median PFS of 32.0 months (95% CI: 28.9-35.3); ribociclib 42.4 months (95% CI: 35.1-52.9). First-line median OS was 37.8 months (95% CI: 32.5-NA); 49.7 months (95% CI: 44.7-54.1); and 54.4 months (95% CI: 47.9-NA) for abemaciclib, palbociclib and ribociclib, respectively. No significant differences in OS were observed, nor in PFS in second line.

Conclusion: This study confirms first-line CDK 4/6i effectiveness, with abemaciclib and ribociclib showing prolonged PFS vs. palbociclib. This study could not confirm a ranking of the three CDK 4/6i.