Inclusion Criteria:

- Histologically or cytologically documented breast cancer

- Locally advanced or metastatic breast cancer

- HER2-positive breast cancer documented as FISH-positive, IHC 3 + or CISH-positive by
local laboratory assessment

- Histologically or cytologically confirmed invasive breast cancer: incurable,
unresectable, locally advanced breast cancer previously treated with multimodality
therapy or metastatic breast cancer

- Prior treatment for breast cancer in the adjuvant, unresectable, locally advanced, or
metastatic setting must include both: a taxane, alone or in combination with another
agent, and Trastuzumab, alone or in combination with another agent in the adjuvant,
unresectable, locally advanced, or metastatic setting

- Documented progression of incurable unresectable, locally advanced, or metastatic
breast cancer during their most recent treatment regimen

- Progression must occur during or after most recent treatment for locally
advanced/metastatic breast cancer or within 6 months after completing adjuvant
therapy

- Adequate hematologic and end organ function

- Agreement to use an effective form of birth control throughout the study

- Life expectancy ≥ 90 days as assessed by the investigator

Exclusion Criteria:

- Less than 14 days from the first study treatment since the last anti-cancer therapy,
including chemotherapy, biologic, experimental, immune, hormonal or endocrine therapy

- Prior T-DM1 therapy

- History of exposure to cumulative doses of select anthracyclines

- History of intolerance or hypersensitivity to trastuzumab, murine proteins, or any of
the excipients, that resulted in trastuzumab being permanently discontinued

- Brain metastases that are untreated or progressive or currently require any type of
therapy, including radiation, surgery, and/or steroids to control symptoms from brain
metastases within 30 days before the first study treatment

- Peripheral neuropathy of Grade ≥ 3 per National Cancer Institute Common Terminology
Criteria for Adverse Events (NCI CTCAE), version 4.0, at the time of the first study
treatment

- History of clinically significant cardiac dysfunction

- Current known active infection with HIV, hepatitis B virus, or hepatitis C virus

- Current severe, uncontrolled systemic disease

- Major surgical procedure or significant traumatic injury within 28 days prior to
first study treatment

- Pregnancy or lactation

NOTE: The site selection process has been completed. Patients can enroll at participating
sites.