JSKN003 Versus Trastuzumab Emtansine (T-DM1) for HER2-Positive, Advanced Breast Cancer

Inclusion Criteria: * 1\. Voluntarily agree to participate in the study and sign the informed consent. * 2.Age≥18 years old. * 3.Patients with unresectable locally advanced or metastatic breast cancer confirmed by histology or cytology. * 4.Confirmed to be HER2 positive (HER2-positive is defined as IHC 3+ or IHC 2+ with ISH positive) by the pathology department of participating study center. * 5.Have received treatment regimen including trastuzumab (allowed marketed trastuzumab biosimilars) or inetetamab with radiologic or pathologic progression/ relapse during the advanced stage, during neoadjuvant or adjuvant therapy, or within 12 months after treatment. * 6.Previously treated with taxanes. * 7.Had radiologic and/or pathologic progression or intolerance of the latest systemic anti-tumor therapy. * 8.At least one extracranial measurable lesion at baseline according to RECIST 1.1 criteria. * 9.ECOG PS of 0 - 1. * 10.Patients with adequate organ and bone marrow functions. * 11.Expected survival ≥ 3 months. * 12.Female and male patients of childbearing age agree to take adequate contraceptive measures during and upon completion of the study for 7 months after the last dose of JSKN003 or T-DM1. Exclusion Criteria: * 1\. Have previously been treated with an anti-HER2 ADC loaded with topoisomerase I inhibitors or medenosin derivative 1 (DM1) or have relapsed after receiving such therapy during or within 12 months after the adjuvant/neo-adjuvant setting or in the advanced stage. * 2.History of any other malignant tumors within three years before randomization. * 3.With uncontrollable serous effusion within 14 days before randomization, which requires frequent drainage or medical intervention. * 4.Known contraindication to T-DM1or not suitable to receive JSKN003 or T-DM1 by investigator. * 5.Has not recovered from adverse reactions caused by previous anti-tumor treatments to ≤ Grade 1 (refer to NCI CTCAE 5.0) or baseline (excluding grade 2 alopecia, hyperpigmentation, simple laboratory test abnormalities, and other toxicity for a non-safety risk by investigators). * 6.Received immunotherapy, macromolecular targeted therapy or other anti-tumor biological therapy within 4 weeks before randomization, or received palliative radiotherapy, endocrine therapy, cytotoxic drug chemotherapy and small molecular targeted drug therapy within 2 weeks before randomization, or received traditional Chinese medicine preparations with anti-tumor indications within 2 weeks before randomization. * 7.Major organ surgery within 28 days before randomization. * 8.Untreated (including baseline findings) or unstable cerebral parenchymal metastasis, spinal cord metastasis or compression, and cancerous meningitis. * 9.The cumulative amount of previous exposure to anthracyclines has reached the pre-specified dosage. * 10.History of LVEF \< 40% during prior anti-HER2 drug therapy or symptomatic congestive heart failure (CHF). * 11.Serious or uncontrolled cardiovascular disease. * 12.History of (non-infectious) interstitial lung disease/pneumonitis requiring therapy or grade ≥3 interstitial lung disease/ pneumonitis during previous anti-tumor treatments. * 13.Active infections requiring intravenous antibiotics, antivirals, or antifungals within 14 days before randomization. * 14\. Active hepatitis B or hepatitis C. * 15.History of immunodeficiency or HIV antibody test positive at screening. * 16.Received a potent inhibitor of CYP3A4 within 14 days prior to randomization or during study treatment. * 17.Pregnant or nursing females; * 18.Other reasons enrolled in this clinical trial as considered unsuitable by the investigator.