Inclusion Criteria:

- Histologically documented HER2-positive locally advanced or metastatic breast cancer

- Tumor tissue blocks or 15-20 unstained tissue slides for confirmatory central
laboratory HER2 status testing and other exploratory assessments

- Prior trastuzumab in any line of therapy

- No prior T-DM1 or pertuzumab therapy

- Measurable disease

- For women of childbearing potential, agreement to use an effective form of
contraception and to continue its use for the duration of the study

- Life expectancy >= 90 days

Exclusion Criteria:

- Less than 21 days since the last anti-tumor therapy, including chemotherapy,
biologic, experimental, immune, hormonal or radiotherapy for the treatment of breast
cancer, with the following exceptions: hormone-replacement therapy or oral
contraceptives; palliative radiation therapy involving =< 25% of marrow-bearing bone
if administered >= 14 days prior to first study treatment

- History of intolerance or hypersensitivity to trastuzumab and/or adverse events
related to trastuzumab that resulted in trastuzumab being permanently discontinued

- Peripheral neuropathy of Grade >= 2

- History of clinically significant cardiac dysfunction

- Current severe, uncontrolled systemic disease (e.g., clinically significant
cardiovascular, pulmonary, or metabolic disease)

- Brain metastases that are untreated, progressive, or have required any type of
therapy to control symptoms from brain metastases within 60 days of the first study
treatment

- History of other malignancy within the last 5 years, except for appropriately treated
carcinoma in situ of the cervix, non-melanoma skin carcinoma, or other malignancy
with a similar expected curative outcome