 This is an open-label, multi-center, non-randomized study of the safety and tolerability of the combination of T-DM1 plus docetaxel for the treatment of participants with metastatic breast cancer MBC and of T-DM1 plus docetaxel with or without pertuzumab, for the treatment of participants with locally advanced breast cancer LABC. The study comprises an initial dose finding feasibility part to determine the maximum tolerated dose MTD of T-DM1 and docetaxel, followed by an extension part aiming to consolidate the safety and efficacy of the recommended docetaxelT-DM1 combination regimen. 