Inclusion Criteria:

- Female ≥ 18 years of age

- Histologically or cytologically confirmed invasive breast cancer with distant
metastasis(ses) (designated as Stage IV or metastatic breast cancer)

- Diagnosis with Stage IV or metastatic disease at either primary diagnosis or
recurrence

- Not received prior systemic or local treatment (e.g., chemotherapy, endocrine or
radiotherapy) for Stage IV/metastatic breast cancer

- Prior adjuvant and/or neo-adjuvant therapy is permitted

- Documentation of HER2 overexpression or gene amplification, in the invasive component
of either a metastatic disease site or primary tumor, defined as: 3+ by IHC and/or
HER2/neu gene amplification by fluorescence, chromogenic or silver in situ
hybridization [FISH, CISH or SISH; >6 HER2/neu gene copies per nucleus or a FISH,
CISH or SISH test ratio (HER2 gene copies to chromosome 17 signals) of ≥2.0]

- Documentation by the central laboratory of positive p95HER2 expression in the
invasive component of either a metastatic disease site (preferred) or primary tumor

- No history of CNS metastases (including leptomeningeal involvement) or stable CNS
metastases (defined as asymptomatic and off steroids for ≥ 3 months)

- Baseline Left Ventricular Ejection Fraction (LVEF) ≥50% measured by echocardiography
(ECHO) or multi-gated acquisition scan (MUGA)

- Recovered or stabilized from all adverse events associated with prior anti-cancer
therapies, including radiotherapy, at the time of screening

- Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2

- Have adequate marrow and organ function as defined as:

SYSTEM LABORATORY VALUES Hematologic ANC ≥1.5 x 109/L Hemoglobin ≥9 g/dL (after
transfusion if needed) Platelets ≥100 x 109/L Hepatic Albumin ≥ 2.5
g/dL Serum bilirubin ≤1.5 x ULN unless due to Gilbert's syndrome AST and ALT ≤3 x ULN
Renal Calculated creatinine clearance ≥ 40 mL/min Serum Creatinine ≤1.5 mg/dL or
132.6µmol/L (Abbreviations: ANC, absolute neutrophil count; ULN, upper limit of normal;
AST, aspartate aminotransferase; ALT, alanine aminotransferase)

- Women of childbearing potential, including women whose last menstrual period was <12
months ago (unless surgically sterile) must have a negative serum pregnancy test and
agree to use effective contraception, as defined in protocol

- Signed Informed Consent Form

Exclusion Criteria:

- History of other malignancy. Exception: Subjects who have been disease-free for 5
years or subjects with a history of completely resected non-melanoma skin cancer
(basal or squamous) are eligible

- Concurrent anti-cancer treatment or concurrent treatment with an investigational drug

- Administration of an investigational drug within 30 days or 5 half-lives, whichever
is longer, preceding the first dose of study treatment

- Prior treatment with anti-HER2 therapy, except trastuzumab or lapatinib (time from
last dose of trastuzumab or lapatinib to randomization must be ≥3 months)

- Serious cardiac illness or medical condition including but not confined to:

- Uncontrolled arrhythmias

- Uncontrolled or symptomatic angina

- History of congestive heart failure (CHF)

- Documented myocardial infarction <6 months from study entry

- Current active hepatic or biliary disease (with exception of Gilbert's syndrome,
asymptomatic gallstones, liver metastases or stable chronic liver disease per
investigator assessment)

- Concurrent disease or condition, or any pre-existing medical disorder that in the
opinion of the investigator may interfere with the subject's safety, obtaining
informed consent or compliance to the study procedures

- Pregnant or lactating female

- Any clinically significant gastrointestinal abnormalities that may alter absorption
such as malabsorption syndrome or major resection of the stomach or bowels (consult
with GSK Medical Monitor if uncertain about eligibility)

- Known immediate or delayed hypersensitivity reaction or idiosyncrasy to drugs
chemically related to any of the study drugs or their excipients that, in the opinion
of the investigator contra-indicates participation