Abemaciclib plus endocrine therapy for hormone receptor-positive, HER2-negative, node-positive, high-risk early breast cancer (monarchE): results from a preplanned interim analysis of a randomised, open-label, phase 3 trial

Background: Adjuvant abemaciclib plus endocrine therapy previously showed a significant improvement in invasive disease-free survival and distant relapse-free survival in hormone receptor-positive, human epidermal growth factor receptor 2 (HER2; also known as ERBB2)-negative, node-positive, high-risk, early breast cancer. Here, we report updated results from an interim analysis to assess overall survival as well as invasive disease-free survival and distant relapse-free survival with additional follow-up. Methods: In monarchE, an open-label, randomised, phase 3 trial, adult patients (aged ≥18 years) who had hormone receptor-positive, HER2-negative, node-positive, early breast cancer at a high risk of recurrence with an Eastern Cooperative Oncology Group performance status of 0 or 1 were recruited from 603 sites including hospitals and academic and community centres in 38 countries. Patients were randomly assigned (1:1) by means of an interactive web-based response system (block size of 4), stratified by previous chemotherapy, menopausal status, and region, to receive standard-of-care endocrine therapy of physician's choice for up to 10 years with or without abemaciclib 150 mg orally twice a day for 2 years (treatment period). All therapies were administered in an open-label manner without masking. High-risk disease was defined as either four or more positive axillary lymph nodes, or between one and three positive axillary lymph nodes and either grade 3 disease or tumour size of 5 cm or larger (cohort 1). A smaller group of patients were enrolled with between one and three positive axillary lymph nodes and Ki-67 of at least 20% as an additional risk feature (cohort 2). This was a prespecified overall survival interim analysis planned to occur 2 years after the primary outcome analysis for invasive disease-free survival. Efficacy was assessed in the intention-to-treat population. Safety was assessed in all treated patients. The study is registered with ClinicalTrials.gov, NCT03155997, and is ongoing. Findings: Between July 17, 2017, and Aug 12, 2019, 5637 patients were randomly assigned (5601 [99·4%] were women and 36 [0·6%] were men). 2808 were assigned to receive abemaciclib plus endocrine therapy and 2829 were assigned to receive endocrine therapy alone. At a median follow-up of 42 months (IQR 37–47), median invasive disease-free survival was not reached in either group and the invasive disease-free survival benefit previously reported was sustained: HR 0·664 (95% CI 0·578–0·762, nominal p<0·0001). At 4 years, the absolute difference in invasive disease-free survival between the groups was 6·4% (85·8% [95% CI 84·2–87·3] in the abemaciclib plus endocrine therapy group vs 79·4% [77·5–81·1] in the endocrine therapy alone group). 157 (5·6%) of 2808 patients in the abemaciclib plus endocrine therapy group died compared with 173 (6·1%) of 2829 patients in the endocrine therapy alone group (HR 0·929, 95% CI 0·748–1·153; p=0·50). The most common grade 3–4 adverse events were neutropenia (in 548 [19·6%] of 2791 patients receiving abemaciclib plus endocrine therapy vs 24 [0·9%] of 2800 patients in the endocrine therapy alone group), leukopenia (318 [11·4%] vs 11 [0·4%]), and diarrhoea (218 [7·8%] vs six [0·2%]). Serious adverse events occurred in 433 (15·5%) of 2791 patients receiving abemaciclib plus endocrine therapy versus 256 (9·1%) of 2800 receiving endocrine therapy. There were two treatment-related deaths in the abemaciclib plus endocrine therapy group (diarrhoea and pneumonitis) and none in the endocrine therapy alone group. Interpretation: Adjuvant abemaciclib reduces the risk of recurrence. The benefit is sustained beyond the completion of treatment with an absolute increase at 4 years, further supporting the use of abemaciclib in patients with high-risk hormone receptor-positive, HER2-negative early breast cancer. Further follow-up is needed to establish whether overall survival can be improved with abemaciclib plus endocrine therapy in these patients. Funding: Eli Lilly.