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.

As of June 4, 2024, this study is no longer accepting any newly screened participants.

Inclusion Criteria:

• 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 (prior to neoadjuvant therapy): 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 pre-operative 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

Exclusion Criteria:

• Stage IV breast cancer
• An overall response of disease progression according to the investigator at the conclusion of preoperative systemic therapy
• Prior treatment with T-DM1, or atezolizumab, or other immune checkpoint inhibitors
• History of exposure to various cumulative doses of anthracyclines
• History of other malignancy within 5 years prior to screening, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or ductal carcinoma in situ (DCIS)
• Current grade >=2 peripheral neuropathy
• History of idiopathic pulmonary fibrosis, organizing pneumonia, or pneumonitis
• History of or active autoimmune disease or immune deficiency
• Treatment with immunostimulatory or immunosuppressive agents
• Cardiopulmonary dysfunction
• Any known active liver disease

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing