This trial is a registrational Phase III, randomized, open-label, multicenter study to compare the efficacy and safety of BL-B01D1 in combination with tislelizumab versus platinum-based chemotherapy in combination with tislelizumab in first-line patients with extensive-stage small cell lung cancer.

Inclusion Criteria:

1. Voluntarily sign the informed consent form and comply with the protocol requirements;
2. Age ≥ 18 years;
3. Expected survival time ≥ 3 months;
4. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1;
5. Patients with histopathologically and/or cytologically confirmed extensive-stage small cell lung cancer;
6. Agree to provide archived tumor tissue specimens from the primary or metastatic lesions within 3 years, or fresh tissue samples;
7. Must have at least one measurable lesion as defined by RECIST v1.1;
8. Toxicity from prior anti-tumor therapy must have recovered to ≤ Grade 1 as defined by NCI-CTCAE v5.0;
9. No severe cardiac dysfunction, with left ventricular ejection fraction ≥ 50%;
10. Organ function levels must meet the requirements;
11. Urinary protein ≤ 2+ or < 1000 mg/24h;
12. For premenopausal women of childbearing potential, a pregnancy test must be performed within 7 days before starting treatment, and the serum pregnancy test must be negative; patients must not be breastfeeding. All enrolled patients (regardless of male or female) must use adequate barrier contraception throughout the entire treatment period and for 6 months after the end of treatment.

Exclusion Criteria:

1. Pathology indicates small cell carcinoma containing non-small cell carcinoma components;
2. Patients who have previously received systemic treatment;
3. Previous treatment with ADC drugs where the small molecule toxin is a topoisomerase I inhibitor;
4. Use of immunomodulatory drugs within 14 days prior to the first dose of the study drug;
5. History of severe heart disease or cerebrovascular disease;
6. Receiving long-term systemic corticosteroid therapy at a dose >10 mg/day of prednisone or equivalent prior to the first dose;
7. Active autoimmune diseases and inflammatory diseases;
8. Unstable thrombotic events requiring therapeutic intervention within 6 months prior to screening;
9. Prolonged QT interval, complete left bundle branch block, etc.;
10. Diagnosis of active malignancy within 3 years prior to study randomization;
11. Hypertension inadequately controlled with two antihypertensive medications;
12. Poorly controlled diabetes mellitus;
13. History of interstitial lung disease (ILD)/pneumonitis requiring steroid therapy, etc.;
14. Concurrent pulmonary disease resulting in clinically severe impairment of respiratory function;
15. Patients with active central nervous system (CNS) metastases;
16. Severe infection within 4 weeks prior to study randomization;
17. Presence of large serous cavity effusions, or serous cavity effusions with symptoms, etc.;
18. Imaging findings indicating tumor invasion or encasement of major blood vessels in the abdomen, chest, neck, or pharynx;
19. Severe, non-healing wound, ulcer, or bone fracture within 4 weeks prior to signing informed consent;
20. Subjects with clinically significant bleeding or obvious bleeding tendency within 4 weeks prior to signing informed consent;
21. Patients with inflammatory bowel disease, history of extensive bowel resection, history of immune-related enteritis, intestinal obstruction, or chronic diarrhea;
22. History of allergy to recombinant humanized antibodies or any excipient component of BL-B01D1;
23. History of autologous or allogeneic stem cell transplantation;
24. Positive for human immunodeficiency virus antibody, active hepatitis B virus infection, or hepatitis C virus infection;
25. History of severe neurological or psychiatric disorders;
26. Receipt of other unapproved clinical study drugs or treatments within 4 weeks prior to study randomization;
27. Subjects planning to receive or having received live vaccines within 28 days prior to study randomization;
28. Other conditions deemed by the investigator to make the subject unsuitable for participation in this clinical trial due to complications or other circumstances.