A Study to Evaluate the Efficacy and Safety of Trastuzumab Emtansine Versus the Combination of Trastuzumab Plus Docetaxel in Patients With HER2-positive Breast Cancer

Inclusion Criteria:

* Age \>/= 18 years
* HER2-positive disease, as defined by an immunohistochemistry test score of 3+ and/or in situ hybridization positivity, prospectively confirmed by a Sponsor-designated central laboratory prior to enrollment
* Histologically or cytologically confirmed adenocarcinoma of the breast with locally recurrent or metastatic disease appropriate for chemotherapy
* Patients must have measurable and/or non-measurable disease that is evaluable per the Response Evaluation Criteria in Solid Tumors (RECIST) criteria, version 1.1
* Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
* Adequate organ function
* For women of childbearing potential and men with partners of childbearing potential, agreement by the patient and/or partner to use two adequate non-hormonal forms of contraception during treatment and for at least 6 months after the last dose of study drug

Exclusion Criteria:

* Pregnancy or lactation
* Current severe, uncontrolled systemic disease (e.g., clinically significant cardiovascular, pulmonary, or metabolic disease; wound healing disorders; ulcers; bone fractures, except bone fractures because of disease under study)
* Currently known active infection with HIV, hepatitis B virus (HBV), or hepatitis C virus (HCV)
* Major surgical procedure or significant traumatic injury within approximately 28 days prior to randomization or anticipation of the need for major surgery during the course of study treatment
* Current peripheral neuropathy Grade \>/= 2 per National Cancer Institute Common Terminology Criteria for Adverse Events, Version 4.0 (NCI CTCAE, v4.0)
* History of systemic anti-cancer therapy after the diagnosis of metastatic breast cancer (MBC) or for recurrent locally advanced disease, with the exception of prior hormonal regimens for recurrent locally advanced disease or MBC
* An interval of \< 12 months after the last dose of vinca alkaloid or taxane chemotherapy (i.e., for treatment of early stage, non-metastatic disease)
* Hormonal therapy \< 7 days prior to randomization
* Trastuzumab \< 21 days prior to randomization
* Lapatinib \</= 14 days prior to randomization
* Prior trastuzumab emtansine therapy
* Treatment with any other anti-cancer therapy/investigational drug (not defined above) within 21 days prior to randomization
* History of other malignancy within the last 5 years, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or other malignancies with an expected curative outcome
* Current chronic daily treatment with corticosteroids (dose \> 10 mg/day methylprednisone equivalent)
* History of intolerance (including Grade 3 or 4 infusion reaction) or hypersensitivity to trastuzumab, murine proteins, docetaxel or paclitaxel
* Known hypersensitivity any of the study drugs, including excipients, or any drugs formulated in polysorbate 80