Phase 1b/2 Study of U3-1287 in Combination With Trastuzumab Plus Paclitaxel in Newly Diagnosed Metastatic Breast Cancer (MBC)

This is a Phase 1b/2 study. In Phase 1b portion, subjects will know the treatment they are receiving . Subjects will receive U3-1287 with trastuzumab plus paclitaxel . The phase 1b portion will determine if adding U3-1287 to trastuzumab plus paclitaxel will be safe in subjects with metastatic breast cancer. In phase 2 portion, subjects will be blinded to the treatments they are receiving . Subjects will receive either trastuzumab plus paclitaxel with U3-1287 or trastuzumab plus paclitaxel and placebo.The phase 2 portion will determine if adding U3-1287 to trastuzumab plus paclitaxel will be safe and improve survival in subjects with metastatic breast cancer.