A Randomized, Double-Blind, Phase III Trial of Paclitaxel/Trastuzumab/Pertuzumab With Atezolizumab or Placebo in First-Line HER2-Positive Metastatic Breast Cancer


To determine whether the addition of atezolizumab to a regimen of paclitaxel, pertuzumab, and trastuzumab will improve the progression-free survival (PFS), as assessed by investigator using Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria, relative to a regimen of paclitaxel, pertuzumab, trastuzumab, and placebo in patients with newly documented HER2-positive measurable metastatic breast cancer.

Study Arms

  • Experimental: Pertuzumab, Trastuzumab, Paclitaxel, Atezolizumab)
  • Placebo Comparator: Pertuzumab, Trastuzumab, Paclitaxel, Placebo


  • Histologically confirmed adenocarcinoma of the breast with locally recurrent, unresectable disease or metastatic disease confirmed as described below; eligible patients include those with either:
    • De novo metastatic disease presenting without prior history of HER2-positive breast cancer:
      • Diagnosis should have been made from a biopsy of a metastatic disease site, but biopsy from the breast primary or involved regional lymph nodes is acceptable if biopsy of the metastatic sites was thought to carry excessive risk for the patient
    • Locally recurrent or metastatic disease following prior therapy for early breast cancer:
      • Diagnosis must have been made from the biopsy of the locally recurrent or metastatic disease
      • There must be an interval of >= 6 months between completion of neoadjuvant/adjuvant HER2-targeted therapy and documentation of locally recurrent or metastatic HER2-positive disease by biopsy
  • Patients must have measurable disease based on RECIST 1.1, as determined by the site, to be eligible
  • The tumor specimen obtained at the time of diagnosis of locally recurrent or metastatic disease must have been determined to be HER2-positive based on central testing according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines (Wolff 2018); HER2 status will initially be assessed using a Food and Drug Administration (FDA)-cleared IHC assay; positive is defined as IHC 3+ staining intensity; if HER2 IHC results are equivocal (2+), then HER2 status will be determined using a FDA-cleared HER2 in situ hybridization (ISH) test according to ASCO/CAP guidelines; sites can send biopsy specimens for central testing which have been determined to be HER2-positive or initially equivocal by either IHC or ISH on local testing
  • The tumor specimen obtained at the time of diagnosis used for HER2 testing must also have central testing for PD-L1 status; patients will be eligible irrespective of PD-L1 testing result including PD-L1 indeterminant
  • The tumor specimen obtained at the time of diagnosis used for HER2 and PD-L1 testing should also have central testing for estrogen receptor (ER) and progesterone receptor (PgR) according to current ASCO/CAP guideline recommendations for hormone receptor testing; patients with, 1% ER and PgR staining by IHC will be classified as negative; if sufficient material for central confirmation of ER and PgR is unavailable, local testing results for ER and PgR may be used for eligibility
  • Localized palliative radiation therapy is allowed for symptom management if completed >= 14 days prior to randomization
  • Patients must have imaging of the chest/abdomen/pelvis, preferably with a computed tomography (CT) scan, and a bone scan within 4 weeks prior to randomization; (NOTE: if a patient is unable to receive CT contrast, a magnetic resonance imaging [MRI] of the abdomen/pelvis and non-contrast chest CT should be performed; positron emission tomography/computed tomography [PET/CT] is not an acceptable alternative)
  • MRI of the brain (or contrast CT scan of the brain if patients are unable to undergo MRI) must be obtained in patients with symptoms suggesting possible central nervous system (CNS) metastatic disease; neuroimaging is recommended but not required in asymptomatic patients