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Adding abemaciclib to fulvestrant extended median overall survival by over 7 months compared to placebo in HR+/HER2– advanced breast cancer patients who had already progressed on a CDK4/6 inhibitor — but the difference didn't reach statistical significance. The final OS analysis of the phase 3 postMONARCH trial showed a promising numerical trend, keeping the door open for continued CDK4/6 inhibition strategies in this tough-to-treat population.
The final overall survival (OS) results from the phase 3 postMONARCH trial are in, and while abemaciclib (Verzenio) plus fulvestrant didn't cross the statistical significance threshold, the data tell an encouraging story. Patients with HR+/HER2– advanced breast cancer who had already progressed on a CDK4/6 inhibitor lived a median of 32.1 months on the combo vs. 24.6 months on placebo plus fulvestrant — a 7.5-month difference that fell just short of significance (HR, 0.80; P = .11).
Investigators pointed to early excess deaths in the abemaciclib arm during the first 6 months as a likely contributor to the statistical miss, noting the survival curves clearly separated after 12 months. The trial was also primarily powered for progression-free survival (PFS), not OS, and widespread access to later-line therapies likely diluted the survival signal.
By the Numbers:
Why it matters: These results add to a growing body of evidence supporting continued CDK4/6 inhibition even after disease progression — a strategy that's gaining traction with the recent FDA approval of imlunestrant plus abemaciclib in a related setting.
