Poster Presentation Clinical Oncology Society of Australia Annual Scientific Meeting 2026

monarchE: Subgroup analysis of adjuvant abemaciclib + endocrine therapy (ET) for HR+, HER2-, high-risk early breast cancer (EBC) by nodal status (146109)

Javier Cortes 1 , Stephen Johnston 2 , Miguel Martin 3 , Sara M. Tolaney 4 , Matthew Goetz 5 , Hope Rugo 6 , Joohyuk Sohn 7 , Ozgur Ozyilkan 8 , Miguel Henriques Abreu 9 , Matthias Zaiss 10 , Karina Maia Vianna 11 , Hanne Melgaard Nielsen 12 , Arlene Chan 13 , Haralambos Kalofonos 14 , Serafin Morales 15 , Belen San Antonio 16 , Maria Munoz Fernandez 16 , Sasha Amdur 16 , Patrick Neven 17 , Brenda Grimes 16 , Frances Boyle 18
  1. International Breast Cancer Center (IBCC), Quironsalud Group, Barcelona, Spain
  2. Breast Unit, Royal Mars den Hospital, London, UK
  3. Hospital General Universitario Gregorio Marañon, Universidad Complutense, CIBERONC, GEICAM, Madrid, Spain
  4. Dana-Farber Cancer Institute, Brigham and Women's Hospital, Boston, MA, USA
  5. Mayo Clinic, Rochester, MN, USA
  6. City of Hope Comprehensive Cancer Center, Duarte, California, USA
  7. Yonsei Cancer Center, Yonsei University, Korea
  8. Baskent University, Turkey
  9. Department of Medical Oncology, Portuguese Institute of Oncology of Porto, Porto, Portugal
  10. Praxis für interdisziplinäre Onkologie und Hämatologie, Freiburg, Germany
  11. CIONC- Centro Integrado de Oncologia de Curitiba, Brazil
  12. Department of Oncology, Aarhus University Hospital, Denmark
  13. Curtin Medical School, Faculty of Health Sciences, Perth, Western Australia
  14. Department of Medicine, Division of Oncology, University Hospital of Patras, Patras, Greece
  15. Hospital Universitari Arnau de Vilanova, Lleida, Spain
  16. Eli Lilly and Company, Indianapolis, IN, USA
  17. Leuven Cancer Institute, Universitaire Ziekenhuizen, Louvain, Belgium
  18. North Sydney’s Mater Hospital, Sydney, NSW, Australia

Aims: An updated efficacy analysis for node positive subgroups in monarchE Cohort 1 (C1) is presented.

Methods: monarchE is a phase 3 trial in high-risk HR+, HER2- EBC. Patients were randomized 1:1 to ET for ≥5 years +/- abemaciclib for 2 years. High-risk EBC was defined as either 1-3 ALN (N1) with grade-3 disease and/or tumor≥5 cm or, ≥4 ALN (N2: 4-9, N3: ≥10) (C1). A smaller group of patients (C2) were enrolled with N1, Grade ≤2, tumor size <5 cm, and central Ki-67 ≥20%. IDFS/DRFS/OS were assessed in C1 nodal subgroups.

Results: In C1 (N=5120), 1761 (34.4%) patients had N1, 2223 (43.4%) had N2, and 1123 (21.9%) had N3 disease. With median follow-up of 76.8 months in C1, in the ET arm, a comparable number of patients with N1 and N2 disease had IDFS events at 6-years (21.6%/25.4%), while event number was higher in N3 (38.1%). A similar trend was observed in DRFS/OS. Adding abemaciclib to ET reduced the risk of developing an IDFS event vs ET alone across subgroups: N1 HR=0.75 [0.61,0.94], N2 HR=0.69 [0.57,0.82] and N3 HR=0.73 [0.59,0.89]. Risk of developing DRFS events was reduced vs ET alone: N1 HR=0.75 [0.59,0.95], N2 HR=0.69 [0.57,0.84], and N3 HR=0.74 [0.60,0.92]. Abemaciclib+ET consistently reduced the risk of death vs ET alone across ALN subgroups: N1 HR=0.88 [0.65,1.18], N2 HR=0.85 [0.66,1.09], and N3 HR=0.73 [0.56,0.96].

Conclusions: In the ET arm, patients with N1 ≥1 additional risk factor had recurrence and death risks comparable to patients with N2, while patients with N3 had poorer prognosis. Addition of adjuvant abemaciclib + ET led to a clinically meaningful reduction in the risk of recurrence and death regardless of nodal burden, confirming the consistent and sustained benefit of 2-years of treatment with abemaciclib and supporting its use in eligible patients with node-positive high-risk EBC.