Poster Presentation Clinical Oncology Society of Australia Annual Scientific Meeting 2026

monarchE: Primary overall survival (OS) results of adjuvant abemaciclib + endocrine therapy (ET) for HR+, HER2-, high-risk early breast cancer (EBC) (146102)

Stephen Johnston 1 , Miguel Martin 2 , Joyce O’Shaughnessy 3 , Roberto Hegg 4 , Sara M. Tolaney 5 , Valentina Guarneri 6 7 , Lucia Del Mastro 8 9 , Mario Campone 10 , Javier Cortes 11 , Hope S. Rugo 12 13 , Matthew P. Goetz 14 , Erika P. Hamilton 15 , Elzbieta Senkus 16 17 , Laura Testa 18 , Patrick Neven 19 , Jens Huober 20 , Ran Wei 21 , Maria Munoz 21 , Priya Rastogi 22 , Nadia Harbeck 23 , Nina Ditsch 24 , Frances Boyle 25
  1. The Royal Marsden NHS Foundation Trust, London, UK
  2. Hospital General Universitario Gregorio Marañon, Universidad Complutense, CIBERONC, GEICAM, Madrid, Spain
  3. Baylor University Medical Center, Texas Oncology, US Oncology, Dallas, TX, USA
  4. University of São Paulo and Pérola Centro de Pesquisa em Oncologia, São Paulo, Brazil
  5. Dana-Farber Cancer Institute, Boston, MA, USA
  6. Department of Surgery, Oncology and Gastroenterology, University of Padova, Padova, Italy
  7. Division of Oncology 2, Istituto Oncologico Veneto IRCCS, Padova, Italy
  8. Department of Internal Medicine and Medical Specialties (DIMI), School of Medicine, University of Genova, Genova, Italy
  9. Clinical Oncology Unit, IRCCS Ospedale Policlinico San Martino, Genova, Italy
  10. Nantes Université, Inserm, CNRS, CRCI(2)NA, Nantes, France
  11. International Breast Cancer Center (IBCC), Quironsalud Group, Barcelona, Spain
  12. City of Hope Comprehensive Cancer Center, Duarte, California, USA
  13. USCF Helen Diller Family Comprehensive Cancer Center, University of California, San Francisco, CA, USA
  14. Mayo Clinic, Rochester, MN, USA
  15. Sarah Cannon Research Institute, Nashville, TN, USA
  16. Medical University of Gdańsk, Gdańsk, Poland
  17. Breast Unit, University Clinical Center, Gdańsk, Poland
  18. D'Or Institute for Research and Education (IDOR), São Paulo, Brazil
  19. University Hospitals Leuven, Louvain, Belgium
  20. HCOH Kantonsspital St Gallen, St Gallen, Switzerland
  21. Eli Lilly and Company, Indianapolis, IN, USA
  22. UPMC Hillman Cancer Center and NSABP Foundation, Pittsburgh, PA, USA, USA
  23. Brustzentrum, Frauenklinik and CCC Munich, LMU University Hospital, Munich, Germany
  24. Department of Obstetrics and Gynecology, University Hospital of Augsburg, Augsburg, Germany
  25. North Sydney’s Mater Hospital, Sydney, NSW, Australia

Aims: We report primary OS (secondary endpoint) and updated IDFS and DRFS from the monarchE trial.

Methods: monarchE is an open-label, phase 3 trial in patients with HR+, HER2-, high-risk EBC. Patients were randomized 1:1 to ET for ≥5 years ± abemaciclib for the first 2 years. High-risk EBC was defined as either ≥4 positive axillary lymph nodes (ALN), or 1-3 ALN + either Grade 3 disease and/or tumor ≥5 cm (Cohort 1). Cohort 2 enrolled patients with 1-3+ ALN and central Ki67 ≥20%. The intent-to-treat (ITT) population consisted of Cohorts 1 (n=5120) and 2 (n=517). Primary OS analysis was triggered by approx. 650 deaths in ITT population.

Results: In the ITT population (median follow-up: 6.3 years) 301 patients in the abemaciclib+ET and 360 patients in the ET arm had died. Adding abemaciclib to ET reduced the risk of death by 15.8% vs ET (HR=0.84;95%CI:0.72–0.98; P=0.027), meeting the prespecified boundary for significance. OS benefit was consistent across prespecified subgroups. IDFS and DRFS benefit persisted up to 7-years (HR=0.73;95%CI:0.66-0.82 and 0.75;0.66-0.84, respectively). At 7-years, IDFS was 77.4% with abemaciclib+ET vs 70.9% with ET, and DRFS was 80.0% vs 74.9% (absolute benefit:6.5% and 5.1%, respectively). In Cohort 1, IDFS/DRFS/OS were consistent with the ITT population. Long-term safety data did not support concerns of delayed toxicities.

Conclusions: Adding 2-years of adjuvant abemaciclib to ET resulted in statistically significant and clinically meaningful improvement in OS over ET in patients with HR+, HER2−, node-positive, high-risk EBC. At 7-years, abemaciclib+ET demonstrated a sustained IDFS and DRFS benefit.

  1. Data from: Johnston, S., et al. Annals of Oncology (2025). https://doi.org/10.1016/j.annonc.2025.10.005