Inclusion Criteria:
1. Sign the informed consent form voluntarily and follow the protocol requirements;
2. Gender is not limited;
3. Age ≥18 years old and ≤75 years old;
4. Expected survival time for 3 months or more;
5. Patients with histologically and/or cytologically confirmed unresectable locally advanced or metastatic HER2-positive breast cancer;
6. Consent to provide archived tumor tissue samples or fresh tissue samples from the primary or metastatic lesions;
7. At least one measurable lesion meeting the RECIST v1.1 definition was required;
8. Physical condition score ECOG 0 or 1 ;
9. The toxicity of previous antineoplastic therapy has returned to ≤ grade 1 as defined by NCI-CTCAE v5.0;
10. No severe cardiac dysfunction, left ventricular ejection fraction ≥50%;
11. No blood transfusion, no colony-stimulating factor, and no albumin are allowed within 14 days before the first use of the study drug, and the organ function level must meet the requirements;
12. Blood coagulation function: international standardization ratio (INR) 1.5 or less, and the part activated clotting time (APTT) live enzymes acuities were 1.5 x ULN;
13. Urinary protein ≤2+ or ≤1000mg/24h;
14. Fertile female subjects, or male subjects with fertile partners, must use highly effective contraception from 7 days before the first dose until 7 months after the end of the dose. Female subjects of childbearing potential had to have a negative serum pregnancy test within 7 days before the first dose.
Exclusion Criteria:
1. Received chemotherapy with mitomycin C and nitrosourea within 6 weeks before the first dose, received surgery within 4 weeks before the first dose, and received endocrine therapy within 2 weeks before the first dose;
2. Patients with locally advanced or metastatic disease who have received previous systemic therapy;
3. Had received prior ADC drug therapy with camptothecin derivative as toxin;
4. Screening within the first half of the serious heart, cerebrovascular disease;
5. Complicated with pulmonary diseases leading to severe impairment of lung function;
6. A history of ILD/interstitial pneumonia requiring steroid therapy, current ILD/interstitial pneumonia, or suspected ILD;
7. QT prolongation, complete left bundle branch block, III degree atrioventricular block, frequent and uncontrollable arrhythmia;
8. Other primary malignancies diagnosed within 5 years before the first dose;
9. Poorly controlled hypertension;
10. Patients with active central nervous system metastases;
11. Need treatment intervention of unstable thrombotic events, except infusion related thrombosis;
12. Patients with a history of allergy to recombinant humanized antibodies or to any excipients of the trial drug;
13. Had received more than the following cumulative doses of anthracyclines;
14. Systemic corticosteroids or immunosuppressive agents were required within 2 weeks before study dosing;
15. Patients with massive or symptomatic effusions or poorly controlled effusions;
16. Severe systemic infection within 4 weeks before screening;
17. Active autoimmune and inflammatory diseases;
18. Human immunodeficiency virus antibody positive, active hepatitis B virus infection or hepatitis C virus infection;
19. Previous history of allogeneic stem cell, bone marrow or organ transplantation;
20. A history of severe neurological or psychiatric illness;
21. Pregnancy or lactation women;
22. Patients who were deemed by the investigator to be ineligible for participation in the study.
