A Study of A166 Versus Trastuzumab Emtansine (T-DM1) in Patients With HER2-Positive Unresectable or Metastatic Breast Cancer Previously Treated With Trastuzumab and Taxane Therapy

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. Disease progression after receiving a trastuzumab-based regimen (or a commercially available trastuzumab biosimilar or inetetamab) in the advanced or metastatic setting, or disease progression/recurrence within 12 months during or after (neo)adjuvant therapy (with a trastuzumab-based regimen or commercially available trastuzumab biosimilar). 4. Have previously received taxanes. 5. Patients must have experienced disease progression or intolerance during or after the most recent treatment prior to randomization. 6. 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 , T-DM1 (trastuzumab emtansine) 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