Main Inclusion Criteria:

          -  Female patients with histologically-confirmed, unresectable locally advanced or
             metastatic breast cancer;

          -  Patients should have had either progression during or after at least two
             HER2-targeting treatment regimens for locally advanced or metastatic disease or
             progression during or after (ado-)trastuzumab emtansine treatment for locally advanced
             or metastatic disease;

          -  HER2-positive tumor status;

          -  Patients must have measurable or non-measurable disease that is evaluable per RECIST
             1.1;

          -  Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2;

          -  Estimated life expectancy > 12 weeks at randomization;

          -  Adequate organ function and blood cell counts.

        Main Exclusion Criteria:

          -  Current or previous use of a prohibited medication as listed in the protocol;

          -  History of infusion-related reactions and/or hypersensitivity to trastuzumab,
             (ado-)trastuzumab emtansine;

          -  History of keratitis;

          -  Severe, uncontrolled systemic disease at screening;

          -  Left Ventricular Ejection Fraction (LVEF) < 50%, or a history of clinically
             significant decrease in LVEF during previous treatment with trastuzumab or
             (ado-)trastuzumab emtansine;

          -  Cardiac troponin value above the Upper Limit of Normal (ULN);

          -  History of clinically significant cardiovascular disease;

          -  Untreated brain metastases, symptomatic brain metastases, brain metastases requiring
             steroids to manage symptoms, or treatment for brain metastases within 8 weeks prior to
             randomization;

          -  History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis
             obliterans), drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of
             active pneumonitis on screening chest CT scan.