A Study of A166 in Patients With HER2-Positive Unresectable or Metastatic Breast Cancer

Inclusion Criteria: 1. Male or female patient ≥ 18 years and ≤ 75 years when signing the informed consent form. 2. Breast cancer patients by histopathology and/or cytology documented. 3. In the advanced or metastatic stage, having received no more than 5 lines of systemic therapy, one of which must include a HER2 ADC with a topoisomerase inhibitor payload. 4. Patients must have experienced disease progression or intolerance during or after the most recent treatment prior to randomization. 5. At least one measurable lesion according to RECIST 1.1 criteria. Exclusion Criteria: 1. Previous treatment with A166 or any HER2-targeted antibody-drug conjugate (ADC) with a microtubule inhibitor payload. 2. Known history of severe hypersensitivity to other monoclonal antibodies, or allergy to A166 or their components. 3. Permanent discontinuation of trastuzumab or its biosimilars due to any toxicity in prior treatments. 4. Presence of severe corneal epithelial disease at baseline; or inability to perform daily activities without contact lenses. 5. Presence of spinal cord compression or clinically active central nervous system (CNS) metastases. 6. Other conditions considered by the investigator to make the patient unsuitable for participation in the study.