This trial is a registrational Phase III, randomized, open-label, multicenter study to evaluate the efficacy and safety of BL-M07D1 combined with Pertuzumab versus docetaxel plus Trastuzumab and Pertuzumab in patients with first-line HER2-positive recurrent or metastatic breast cancer.

In this trial, the treatment group receives BL-M07D1 and pertuzumab, while the control group receives trastuzumab, pertuzumab, and docetaxel.

Inclusion Criteria:

1. Voluntarily sign the informed consent form and comply with the protocol requirements;
2. Female patients aged ≥18 and ≤75 years at the time of signing the informed consent form;
3. Expected survival time ≥12 weeks;
4. Patients with histologically or cytologically confirmed, previously untreated, unresectable recurrent or metastatic HER2-positive breast cancer;
5. Clear hormone receptor (HR) status;
6. Agree to provide eligible tumor tissue specimens;
7. Have at least one measurable target lesion as defined by RECIST v1.1;
8. ECOG performance status score of 0 or 1;
9. Toxicity from prior anti-tumor therapy must have recovered to ≤ Grade 1 as defined by NCI-CTCAE v5.0;
10. Organ function levels must meet the requirements;
11. For premenopausal women with childbearing potential, a pregnancy test must be performed within 7 days before starting treatment, with a negative serum pregnancy result, and must be non-lactating; all enrolled patients must use adequate and highly effective contraceptive measures throughout the entire treatment period and for 7 months after treatment completion.

Exclusion Criteria:

1. Received surgical treatment, radical radiotherapy, immunotherapy, etc. within 4 weeks or 5 half-lives prior to the first dose.
2. Previously received ADC drug therapy with camptothecin derivatives as toxins.
3. History of severe cardiovascular or cerebrovascular disease within six months before screening.
4. Concomitant pulmonary disease resulting in severely impaired lung function.
5. History of interstitial lung disease (ILD)/interstitial pneumonia requiring corticosteroid therapy, etc.
6. QT interval prolongation, complete left bundle branch block, third-degree atrioventricular block, or frequent and uncontrolled arrhythmias.
7. Diagnosed with another primary malignancy within 5 years before the first dose.
8. Newly developed deep vein thrombosis within 14 days before screening.
9. Hypertension poorly controlled by antihypertensive medications.
10. Patients with active central nervous system metastases.
11. History of severe allergic reactions to recombinant humanized antibodies or any excipient or component of BL-M07D1.
12. History of autologous or allogeneic stem cell transplantation or organ transplantation.
13. Previously received anthracycline therapy exceeding the prescribed dose limit.
14. Positive for human immunodeficiency virus antibody, active hepatitis B virus infection, cirrhosis, or hepatitis C virus infection.
15. Severe infection within 4 weeks prior to the first use of the study drug, etc.
16. Patients with large serous cavity effusions, serous cavity effusions with obvious symptoms, or poorly controlled serous cavity effusions.
17. Receiving systemic corticosteroid therapy >10 mg/day prednisone or equivalent prior to randomization, etc.
18. Presence of severe neurological or psychiatric disorders.
19. Subjects with clinically significant bleeding or obvious bleeding tendency within 4 weeks prior to signing informed consent.
20. Intestinal obstruction, Crohn's disease, ulcerative colitis, or chronic diarrhea, etc.
21. Subjects planning to receive or having received live vaccines within 28 days before the first dose.
22. Presence of other serious physical conditions, abnormal laboratory findings, or poor compliance that may increase the risk of participating in the study, interfere with study results, or make the patient unsuitable for participation in the study in the investigator's opinion.