A Phase III, Randomized, Double-Blind, Placebo-Controlled Clinical Trial to Evaluate the Efficacy and Safety of Adjuvant Atezolizumab or Placebo and Trastuzumab Emtansine for HER2-Positive Breast Cancer at High Risk of Recurrence Following Preoperative Therapy


This is a Phase III, two-arm, randomized, double-blind placebo-controlled study in participants with HER2-positive primary breast cancer who have received preoperative chemotherapy and HER2-directed therapy, including trastuzumab followed by surgery, with a finding of residual invasive disease in the breast and/or axillary lymph nodes.

Study Arms

  • Active Comparator: Arm A: Placebo + Trastuzumab Emtansine
  • Experimental: Arm B: Atezolizumab + Trastuzumab Emtansine


  • Histologically confirmed invasive breast carcinoma
  • Centrally-confirmed human epidermal growth factor receptor 2 (HER2)-positive invasive breast cancer
  • Centrally confirmed PD-L1 and hormone receptor status
  • Clinical stage at disease presentation: cT4/anyN/M0, any cT/N2-3/M0, or cT1-3/N0-1/M0 (participants with cT1mi/T1a/T1b/N0 are not eligible)
  • Completion of preoperative systemic chemotherapy including at least 9 weeks of taxane and 9 weeks of trastuzumab (anthracycline and/or additional HER2-targeted agents are permitted)
  • <=12 weeks between primary surgery and randomization
  • Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1
  • Screening left ventricular ejection fraction (LVEF) >= 50% and no decrease in LVEF by >15% from the pre-chemotherapy LVEF. If no pre-chemotherapy LVEF, screening LVEF >= 55%
  • Life expectancy >= 6 months
  • Adequate hematologic and end-organ function