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.