This trial is a registered phase III, randomized, open-label, multicenter study to evaluate the efficacy and safety of BL-B01D1 in patients with unresectable locally advanced, recurrent, or metastatic HR+HER2- breast cancer after failure of at least one prior line of chemotherapy.

Inclusion Criteria:

1. Voluntarily sign the informed consent and follow the requirements of the protocol;
2. No gender limit;
3. Age ≥18 years old;
4. expected survival time ≥3 months;
5. Patients with unresectable locally advanced, recurrent metastatic HR+HER2- breast cancer;
6. The subjects had received 1-2 lines of chemotherapy regimens in the unresectable locally advanced recurrence or metastasis stage, and had been treated with endocrine, CDK4/6 inhibitors, and taxanes;
7. Documented radiographic disease progression;
8. Consent to provide archival tumor tissue samples or fresh tissue samples of primary or metastatic lesions within 3 years;
9. Must have at least one measurable lesion according to RECIST v1.1 definition;
10. ECOG score 0 or 1;
11. Toxicity of previous antineoplastic therapy has returned to ≤ grade 1 defined by NCI-CTCAE v5.0;
12. No severe cardiac dysfunction, left ventricular ejection fraction ≥50%;
13. No blood transfusion, no use of cell growth factors and/or platelet raising drugs within 14 days before screening, and the organ function level must meet the requirements;
14. Urine protein ≤2+ or < 1000mg/24h;
15. 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 it must be non-lactating; All enrolled patients (male or female) were advised to use adequate barrier contraception throughout the treatment cycle and for 6 months after the end of treatment.

Exclusion Criteria:

1. Prior receipt of an ADC drug with a topoisomerase I inhibitor as a toxin;
2. Prior receipt of an ADC or antibody drug targeting EGFR and/or HER3;
3. Chemotherapy, biological therapy, immunotherapy, etc., have been used within 4 weeks or 5 half-lives before the first dose, small molecule targeted therapy has been used within 5 days, palliative radiotherapy, modern Chinese medicine preparations approved by NMPA for anti-tumor therapy, etc., have been used within 2 weeks;
4. anthracycline equivalent cumulative dose of adriamycin > 360 mg/m2;
5. History of severe cardiovascular or cerebrovascular disease;
6. Unstable deep vein thrombosis, arterial thrombosis, and pulmonary embolism requiring medical intervention within 6 months before screening; Infusion-related thrombosis was excluded;
7. QT prolongation, complete left bundle branch block, III degree atrioventricular block, frequent and uncontrollable arrhythmia;
8. Other malignant tumors diagnosed within 3 years before the first dose;
9. Hypertension poorly controlled by two antihypertensive drugs; Patients with poor glycemic control;
10. A history of interstitial lung disease (ILD) requiring steroid therapy, current ILD or grade ≥2 radiation pneumonitis, or suspicion of such disease on imaging during screening;
11. Complicated pulmonary diseases leading to clinically severe respiratory function impairment;
12. Patients with active central nervous system metastases;
13. Patients with massive or symptomatic effusions or poorly controlled effusions;
14. Imaging examination showed that the tumor had invaded or wrapped around the large blood vessels in the abdomen, chest, neck, and pharynx;
15. Severe infection within 4 weeks before randomization; Evidence of pulmonary infection or active pulmonary inflammation within 2 weeks before randomization;
16. Was receiving &gt before randomization; Long-term systemic corticosteroid therapy with 10mg/ day prednisone or equivalent anti-inflammatory active drugs or any form of immunosuppressive therapy;
17. Severe unhealed wound, ulcer, or fracture within 4 weeks before signing the informed consent;
18. Subjects with clinically significant bleeding or obvious bleeding tendency within 4 weeks before signing the informed consent;
19. Patients with inflammatory bowel disease, extensive bowel resection history, immune enteritis history, intestinal obstruction or chronic diarrhea;
20. Have a history of allergy to recombinant humanized antibodies or to BL-B01D1 and any excipients; A history of autologous or allogeneic stem cell transplantation;
21. Human immunodeficiency virus antibody positive, active hepatitis B virus infection or hepatitis C virus infection;
22. A history of severe neurological or psychiatric illness;
23. Received other unmarketed investigational drug or treatment within 4 weeks before the first dose; A live vaccine dose within 28 days before the planned dose or the first dose;
24. Any complications or other circumstances deemed by the investigator to preclude participation in the trial.