Inclusion Criteria:
1. Voluntarily sign the informed consent and follow the requirements of the protocol;
2. Women aged ≥18 years and ≤75 years at the time of written informed consent;
3. Expected survival time ≥12 weeks;
4. Histologically or cytologically confirmed unresectable, locally recurrent or metastatic HER2-low breast cancer;
5. Provide the latest tumor tissues to the central laboratory for HER2 and HR detection;
6. Meet the treatment requirements in the plan;
7. Must have at least one measurable target lesion that meets the RECIST v1.1 definition;
8. ECOG 0 or 1;
9. Toxicity of previous antineoplastic therapy has returned to ≤ grade 1 defined by NCI-CTCAE v5.0;
10. Organ function level must meet the requirements;
11. For premenopausal women with childbearing potential, a pregnancy test must be performed within 7 days before the initiation of treatment, serum pregnancy must be negative, and must be non-lactating; All enrolled patients (male or female) should use adequate, highly effective contraception for the entire treatment cycle and for 7 months after completion of treatment.
Exclusion Criteria:
1. Received mitomycin C and nitrosourea chemotherapy within 6 weeks before the first dose, and received surgery or radical radiotherapy within 4 weeks before the first dose;
2. Patients who were not suitable to use the control drugs chosen by the researchers because of intolerance to the chemotherapy drugs of the control group or other contraindications;
3. Previous treatment with anti-HER2 drugs;
4. Prior ADC drug therapy with camptothecin derivative as toxin;
5. The history of severe cardiovascular and cerebrovascular diseases in the past six months was screened;
6. Severe impairment of lung function due to concurrent pulmonary diseases;
7. History of ILD/interstitial pneumonia requiring steroid therapy, current ILD/interstitial pneumonia or suspected ILD/interstitial pneumonia;
8. QT prolongation, complete left bundle branch block, III degree atrioventricular block, frequent and uncontrollable arrhythmia;
9. Other primary malignancies diagnosed within 5 years before the first dose;
10. Poorly controlled hypertension;
11. Patients with active central nervous system metastases;
12. Patients with a history of severe allergy to any excipients or components of the study drug;
13. History of autologous or allogeneic stem cell transplantation or organ transplantation;
14. Anthracycline-equivalent cumulative dose of adriamycin \> 360 mg/m2;
15. Human immunodeficiency virus antibody positive, active hepatitis B virus infection, cirrhosis, or hepatitis C virus infection;
16. Serious infection within 4 weeks before the first dose of study drug; Severe infection requiring antibiotic, antiviral or antifungal control at screening;
17. Patients with massive effusions, or effusions with obvious symptoms, or poorly controlled effusions;
18. Carcinomatous lymphangitis;
19. Was receiving \> before randomization; 10mg/d prednisone systemic corticosteroids or equivalent anti-inflammatory active drugs or any form of immunosuppressive therapy;
20. The presence of a severe neurological or mental illness;
21. Subjects with clinically significant bleeding or obvious bleeding tendency within 4 weeks before signing the informed consent;
22. Intestinal obstruction, Crohn's disease, ulcerative colitis or chronic diarrhea;
23. Subjects who are scheduled to receive live vaccine or receive live vaccine within 28 days before the first dose;
24. Patients with other serious physical or laboratory abnormalities or poor compliance that may increase the risk of participating in the study or interfere with the results of the study, and patients who are considered by the investigators to be unsuitable for participating in the study.
