Primary outcome analysis of invasive disease-free survival for monarchE: abemaciclib plus adjuvant endocrine therapy for high-risk early breast cancer

Goal monarchE (phase 3, open-label) evaluated abemaciclib+endocrine therapy (ET) vs ET-alone in node-positive, HR+, HER2-, high-risk early breast cancer that resulted in a statistically significant improvement in invasive disease-free survival (IDFS) at a pre-planned interim analysis.

Material and method Following the positive interim analysis, patients continued to be followed for IDFS, distant recurrence and overall survival. 5,637 patients were randomized (1:1) to standard-of-care adjuvant ET with/without abemaciclib (150mg BD for 2 years). Patients with ≥4 positive nodes, or 1-3 nodes and either grade3 disease, tumor size≥5 cm, or central Ki-67≥20% were eligible. We present results of the primary outcome IDFS analysis which was planned after ~390 IDFS events.

Results At the primary outcome analysis, median follow-up of ~19 months (mo) in both arms. With 395 IDFS events observed, abemaciclib+ET continued to demonstrate superior IDFS vs ET-alone (p=.0009; HR=0.713). Two-year IDFS rates were 92.3% (abemaciclib+ET) and 89.3% (ET-alone). With 324 distant relapse-free survival (DRFS) events observed, abemaciclib+ET improved DRFS vs ET-alone (p=.0009; HR=.687). Two-year DRFS rates were 93.8% (abemaciclib+ET) and 90.8% (ET-alone).A key secondary endpoint was efficacy in patients with centrally assessed high Ki-67 (≥20%) (Ki-67H, n=2498). Abemaciclib+ET demonstrated superior IDFS vs ET-alone (p=.0111; HR=.691) and 2-year IDFS rates of 91.6% and 87.1%, respectively.

Summary Abemaciclib+ET demonstrated a clinically meaningful improvement in IDFS in the study population with a statistically significant improvement in IDFS in patients with central Ki-67≥20%.