The objective of this study is to assess the safety, efficacy, pharmacokinetics, and immunogenicity of MRG002 in patients with HER2 positive breast cancer with liver metastases.

Inclusion Criteria:

1. Willing to sign the informed consent form and follow the requirements specified in the protocol.
2. Aged 18 to 75 (including 18 and 75), both genders; Life expectancy ≥ 12 weeks;
3. The score of ECOG for performance status is 0 or 1
4. Patients with HER2-positive breast cancer confirmed by central laboratory and with evidence of liver metastasis by imaging;
5. Archival or biopsy tumor specimens should be provided (primary or metastatic) for HER2 testing;
6. Patients must have measurable lesions according to the Response Criteria in Solid Tumors (RECISTv1.1);
7. Organ functions must meet the basic requirements.
8. Patients of childbearing potential are willing to take effective contraceptive measures from the time of signing the informed consent form to 6 months after last administration of the study drug.

Exclusion Criteria:

1. Previous history of other primary malignancies;
2. Presence of peripheral neuropathy ≥ grade 2 (according to CTCAE V5.0);
3. Previously received antibody-drug conjugates, investigational drugs, anti-tumor vaccines or drugs, endocrine therapy for breast cancer, radiotherapy, CYP3A4 inhibitors or inducers, anthracyclines and other treatments;
4. Central nervous system metastasis and/or neoplastic meningitis;
5. History of decompensated cirrhosis, or liver metastases with a single lesion ≥ 10 cm in longest diameter;
6. Pleural or peritoneal effusion with combined clinical symptoms, which seriously endangers the life safety of subjects or urgently requires clinical treatment; Or pericardial effusion with combined clinical symptoms;
7. Any serious or uncontrolled systemic disease judged by the investigator;
8. Uncontrolled cardiac disease;
9. Evidence of active infection;
10. Pulmonary embolism or deep venous thrombosis within 3 months prior to study treatment;
11. History of interstitial pneumonia, severe chronic obstructive pulmonary disease, severe pulmonary dysfuction, symptomatic bronchospasm, etc.;
12. Patients with active autoimmune disease or a history of autoimmune diseases who are receiving immunosuppressive agents or systemic hormone therapy, and are still using them within 2 weeks prior to enrollment;
13. History of hypersensitivity to any component of MRG002 or known history of hypersensitivity of ≥ Grade 3 to trastuzumab injection;
14. Uncontrolled tumor-related bone pain or urgent spinal cord compression;
15. Other conditions inappropriate for participation in this study, as deemed by the investigator.