A Study of Trastuzumab Emtansine (T-DM1) Plus Pertuzumab/Pertuzumab Placebo Versus Trastuzumab [Herceptin] Plus a Taxane in Participants With Metastatic Breast Cancer (MARIANNE)

Inclusion Criteria:

- Adult participants >/=18 years of age

- HER2-positive breast cancer

- Histologically or cytologically confirmed adenocarcinoma of the breast with locally
recurrent or metastatic disease, and be a candidate for chemotherapy. Participants
with locally advanced disease must have recurrent or progressive disease, which must
not be amenable to resection with curative intent.

- Participants must have measurable and/or non-measurable disease which must be
evaluable per Response Evaluation Criteria in Solid Tumors (RECIST) 1.1

- Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or 1

- Adequate organ function as determined by laboratory results

Exclusion Criteria:

- History of prior (or any) chemotherapy for metastatic breast cancer or recurrent
locally advanced disease

- An interval of <6 months from the last dose of vinca-alkaloid or taxane cytotoxic
chemotherapy until the time of metastatic diagnosis

- Hormone therapy <7 days prior to randomization

- Trastuzumab therapy and/or lapatinib (neo- or adjuvant setting) <21 days prior to
randomization

- Prior trastuzumab emtansine or pertuzumab therapy