- Participants must have metastatic, unresectable locally advanced, or locally recurrent HER2-positive breast cancer. For the phase II portion of the study, it is required that participants have measurable disease, as defined by RECIST 1.1, which can be accurately evaluated on computerized tomography (CT) or magnetic resonance (MRI). Measurable disease is defined as: at least one lesion of >10 mm in the longest diameter for a non-lymph node or >15 mm in the short-axis diameter for a lymph node which is serially measurable according to RECIST 1.1.criteria.1