A Study to Compare Pertuzumab + Trastuzumab + Vinorelbine vs. Placebo + Trastuzumab + Docetaxel in Previously Untreated HER2-positive Metastatic Breast Cancer

A Study to Compare Pertuzumab + Trastuzumab + Vinorelbine vs. Placebo + Trastuzumab + Docetaxel in Previously Untreated HER2-positive Metastatic Breast Cancer