eMonarcHER: A Randomized, Double Blind, Placebo-Controlled Phase 3 Study of Abemaciclib Plus Standard Adjuvant Endocrine Therapy in Participants With High-Risk, Node-Positive, HR+, HER2+ Early Breast Cancer Who Have Completed Adjuvant HER2-Targeted Therapy


The purpose of this study is to measure how well abemaciclib works in participants with early breast cancer who are taking hormone therapy after surgery. Participants must have breast cancer that is hormone receptor positive (HR+) and human epidermal receptor 2 positive (HER2+). Your participation could last up to 10 years depending on how you and your tumor respond.

Study Arms

  • Experimental: Abemaciclib + Standard Adjuvant Endocrine Therapy
  • Active Comparator: Placebo + ET


  • Have confirmed HR+, HER2+ in initial diagnostic tissue, early invasive breast cancer without evidence of disease recurrence or distant metastases
  • Have undergone definitive surgery of the primary breast tumor(s)
  • Have tumor tissue from breast (preferred) or lymph node
  • Have received a minimum of four cycles of chemotherapy in either the neoadjuvant or adjuvant setting per standard of care therapy
  • Have completed approximately nine months to one year of standard HER2-targeted therapy without evidence of disease recurrence (neoadjuvant/adjuvant combined duration)
  • Have received one of the following eligible HER2-targeted adjuvant regimens AND be randomized within 12 weeks of completing the regimen:
    • For participants treated with neoadjuvant therapy (chemotherapy administered with HER2-targeted therapy): Single agent adjuvant ado-trastuzumab emtansine or
    • For participants not treated with neoadjuvant therapy: Adjuvant pertuzumab with trastuzumab
  • Have high risk disease, defined by one of the following:
    • For participants treated with neoadjuvant therapy (as defined above): Pathologically detected axillary nodal disease in the surgical specimen
    • For participants not treated with neoadjuvant therapy: Axillary node positive disease meeting one of the following criteria:
      • Pathological tumor involvement in greater than or equal to (≥) four ipsilateral axillary lymph nodes OR
      • Pathological tumor involvement in one to three ipsilateral axillary lymph node(s) and at least 1 of the following criteria:
        • Histological Grade 3
        • Primary invasive tumor size ≥5 centimeters determined pathologically