SYD985 vs. Physician's Choice in Participants With HER2-positive Locally Advanced or Metastatic Breast Cancer

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.