Inclusion Criteria: - Adult patients, >/= 18 years of age - Histologically or cytologically confirmed adenocarcinoma of the breast with metastatic or locally advanced disease not amenable to curative resection - HER2-positive as assessed by local laboratory on primary or metastatic tumor - At least one measurable lesion and/or non-measurable disease evaluable according to RECIST version 1.1 criteria - Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 - Left ventricular ejection fraction (LVEF) of at least 55% - Life expectancy of at least 12 weeks Exclusion Criteria: - Previous systemic non-hormonal anticancer therapy in the metastatic or locally advanced setting - Previous approved or investigative anti-HER2 agents in any breast cancer treatment setting, except trastuzumab and/or lapatinib in the adjuvant or neoadjuvant setting - Disease progression while receiving trastuzumab and/or lapatinib in the adjuvant or neoadjuvant setting - Disease-free interval from completion of adjuvant or neoadjuvant systemic non-hormonal treatment to recurrent disease of less than 6 months - History of persistent Grade 2 or higher (NCI-CTC Version 4.0) hematological toxicity resulting from previous adjuvant or neoadjuvant therapy - Radiographic evidence of central nervous system (CNS) metastases - Current peripheral neuropathy of Grade 3 or greater - History of other malignancy within the last 5 years, except for carcinoma in situ of the cervix or basal cell carcinoma - Serious uncontrolled concomitant disease that would contraindicate the use of any of the investigational drugs used in this study or would put the patients at high risk for treatment -related complications - Inadequate hematologic, liver or renal function - Uncontrolled hypertension or clinically significant cardiovascular disease - Hepatitis B, hepatitis C or HIV infection - Current chronic daily treatment with corticosteroids (>/= 10 mg/day methylprednisolone or equivalent), excluding inhaled steroids