Efficacy and safety analysis of Chinese patients in monarchE: Abemaciclib combined with adjuvant endocrine therapy for high risk HR+, HER2- early breast cancer.

522 Background: In monarchE, abemaciclib (oral CDK4&6 inhibitor) plus endocrine therapy (ET) as adjuvant treatment for HR+, HER2- high risk early breast cancer (EBC), demonstrated a statistically significant improvement in invasive disease-free survival (IDFS) compared to ET alone. Here we present the efficacy and safety analysis of Chinese patients from monarchE. Methods: The overall study design was reported previously. Eligible patients were randomized to receive abemaciclib (150 mg BID for 2 years) combined with standard adjuvant ET or ET alone. The primary endpoint was IDFS per STEEP criteria. Secondary endpoints included distant relapse-free survival (DRFS), overall survival, and safety. Exploratory subgroup analyses were conducted among Chinese patients enrolled from Mainland China, Hong Kong, and Taiwan in the intent-to-treat (ITT) population. Results: A total of 501 Chinese patients were randomized to receive abemaciclib plus ET (259 patients) or ET alone (242 patients). At the time of data cutoff (July 8, 2020), 356 (71.1%) patients were still in the 2-year treatment period. A total of 26 IDFS events were observed (11 and 15 events in abemaciclib plus ET and ET arm, respectively). Comparing to ET alone, abemaciclib combined with ET reduced the risk of developing invasive disease or death by 34.3% (HR: 0.657, 95% CI: 0.301, 1.435) for Chinese patients, together with a clinically meaningful improvement in the 2-year IDFS rate (95.6% vs 92.1%). The addition of abemaciclib to ET also resulted in an improvement in DRFS (HR: 0.601, 95% CI: 0.245, 1.477) for Chinese patients, with the 2-year DRFS rate at 96.7% (ET alone: 93.4%). In the abemaciclib arm, the most frequent treatment-emergent adverse events (TEAEs) and grade ≥3 TEAEs: diarrhea (90.3% and 5.0%), leukopenia (76.8% and 21.2%), and neutropenia (76.4% and 23.9%), respectively. Conclusions: Abemaciclib combined with adjuvant ET demonstrated clinically meaningful IDFS and DRFS benefits among Chinese patients with HR+, HER2-, high risk EBC, which was consistent with the ITT population as reported previously. The safety profile of abemaciclib in Chinese EBC patients was consistent with global population and also with that observed in Chinese metastatic breast cancer patients. Clinical trial information: NCT03155997.