Testing the Drug Atezolizumab or Placebo With Usual Therapy in First-Line HER2-Positive Metastatic Breast Cancer

This randomized phase III trial studies how well paclitaxel, trastuzumab, and pertuzumab with or without atezolizumab works in treating patients with breast cancer that has spread to other parts of the body (metastatic). Chemotherapy drugs, such as paclitaxel, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Trastuzumab is a form of "targeted therapy" because it works by attaching itself to specific molecules (receptors) on the surface of cancer cells, known as HER2 receptors. When trastuzumab attaches to HER2 receptors, the signals that tell the cells to grow are blocked and the cancer cell may be marked for destruction by the body's immune system. Monoclonal antibodies, such as pertuzumab, may interfere with the ability of cancer cells to grow and spread. Immunotherapy with monoclonal antibodies, such as atezolizumab, may induce changes in body's immune system and may interfere with the ability of tumor cells to grow and spread. It is not yet known whether giving paclitaxel, trastuzumab, and pertuzumab with or without atezolizumab may kill more tumor cells. \*NOTE: This study has a central confirmation step. The purpose of this step is to confirm by central testing that the patient's tumor has specific receptors. If the patient meets all the study requirements, the patient will join the study and begin therapy for breast cancer while the tumor is being tested.

PRIMARY OBJECTIVE: I. To determine whether the addition of atezolizumab to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel) will improve the progression-free survival (PFS), assessed by investigator using Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria, relative to the addition of a placebo to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel) in patients with newly documented HER2-positive measurable metastatic breast cancer. SECONDARY OBJECTIVES: I. To determine whether the addition of atezolizumab to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel) will improve the overall survival (OS) relative to the addition of placebo to a regimen of pertuzumab and trastuzumab, combined with a taxane (paclitaxel or docetaxel). II. To determine whether the addition of atezolizumab to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel) will improve the overall objective response (OR) in the cohort of patients randomized 6 months or more prior to study accrual closure assessed by investigator using RECIST 1.1 criteria, relative to the addition of placebo to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel). III. To determine the immune-related toxicity profile of the two treatment regimens. IV. To determine the cardiac safety profile of the two treatment regimens. EXPLORATORY OBJECTIVES: I. To determine whether the addition of atezolizumab to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel) will decrease the incidence of subsequent brain metastases in patients without known brain metastases at study entry relative to the addition of a placebo to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel). II. To determine the utility of PD-L1 immunohistochemistry (IHC) staining as a predictive and prognostic biomarker associated with clinical response, as assessed by investigator using RECIST 1.1 criteria, to atezolizumab in combination with trastuzumab and pertuzumab combined with a taxane (paclitaxel or docetaxel). III. To identify potential biomarkers that can predict benefit from the addition of atezolizumab in patients with newly documented HER2-positive measurable metastatic breast cancer treated with a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel). IV. To explore the toxicity profile of the two treatment regimens using patient-reported symptomatic adverse events in addition to standard adverse event reports. V. To determine the feasibility and added value of frequent assessment of toxicity using Patient Reported Outcomes (PRO)-Common Terminology Criteria for Adverse Events (CTCAE) with electronic(e)PRO reporting. VI. To explore whether the addition of atezolizumab to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel) will contribute to increased patient-reported fatigue in comparison to the addition of placebo to a regimen of pertuzumab and trastuzumab combined with a taxane (paclitaxel or docetaxel). OUTLINE: STEP 1: Patients receive pertuzumab intravenously (IV) over 30-60 minutes on days 1 and 22, trastuzumab IV over 30-90 minutes on days 1 and 22, and paclitaxel IV over 60 minutes on days 1, 8, 15, 22, 29, and 36 or docetaxel IV over 60 minutes on days 1 and 22. Treatment continues for 6 weeks in the absence of disease progression or unacceptable toxicity. Patients may undergo a biopsy at baseline. Patients undergo computed tomography (CT), magnetic resonance imaging (MRI), and/or bone scan at baseline. STEP 2: Patients are randomized to 1 of 2 arms. ARM I: Patients receive pertuzumab IV over 30-60 minutes on days 1 and 22 of each cycle, trastuzumab IV over 30-90 minutes on days 1 and 22 of each cycle, and paclitaxel IV over 60 minutes on days 1, 8, 15, 22, 29, and 36 of each cycle or docetaxel IV over 60 minutes on days 1 and 22 of each cycle. Cycles repeat every 6 weeks in the absence of disease progression or unacceptable toxicity. Patients also receive atezolizumab IV over 30-60 minutes on day 22 of cycle 1 and days 1 and 22 of subsequent cycles. Cycles repeat every 6 weeks for 2 years in the absence of disease progression or unacceptable toxicity. Patients undergo a CT or MRI at baseline, prior to weeks 10, 19, 28 and then at 9, 12, 15, 18, 21, and 24 months from study entry, every 3 months through year 3, every 6 months for years 4 and 5, and then every 6 months for years 6 through 10 from study entry. Patients undergo a bone scan at baseline, every 6 months from study entry, every 6 months through year 3, and every 12 months for years 4 and 5. ARM II: Patients receive pertuzumab, trastuzumab, and paclitaxel or docetaxel as in Arm I. Patients also receive placebo IV 30-60 minutes on day 22 of cycle 1 and days 1 and 22 of subsequent cycles. Cycles repeat every 6 weeks for 2 years in the absence of disease progression or unacceptable toxicity. Patients undergo a CT or MRI at baseline, prior to weeks 10, 19, 28 and then at 9, 12, 15, 18, 21, and 24 months from study entry, every 3 months through year 3, every 6 months for years 4 and 5, and then every 6 months for years 6 through 10 from study entry. Patients undergo a bone scan at baseline, every 6 months from study entry, every 6 months through year 3, and every 12 months for years 4 and 5. After completion of study treatment, patients are followed up at 30 days and every 3 months for 3 years and then every 6 months for 5 years.