This trial is a registered phase III, randomized, open-label, multicenter study to evaluate the efficacy and safety of BL-B01D1 in patients with recurrent small cell lung cancer after failure of anti-PD-1/PD-L1 Monoclonal Antibodies and platinum-based chemotherapy.

Inclusion Criteria:

1. Voluntarily sign the informed consent and follow the requirements of the protocol;
2. Age ≥18 years old;
3. Expected survival time ≥3 months;
4. Patients with recurrent small-cell lung cancer after failure of anti-PD-1/PD-L1 monoclonal antibodies and platinum-based chemotherapy;
5. Consent to provide archival tumor tissue samples or fresh tissue samples of primary or metastatic lesions within 3 years;
6. Must have at least one measurable lesion according to RECIST v1.1 definition;
7. ECOG 0 or 1;
8. Toxicity of previous antineoplastic therapy has returned to ≤ grade 1 defined by NCI-CTCAE v5.0;
9. No severe cardiac dysfunction, left ventricular ejection fraction ≥50%;
10. The organ function level must meet the requirements on the premise that blood transfusion is not allowed within 14 days before the screening period, and no cell growth factor drugs are allowed;
11. A serum pregnancy test must be performed within 7 days before the start of treatment for premenopausal fertile women, and the result must be negative and must not be lactating; All enrolled patients should take adequate barrier contraception during the entire treatment cycle and for 6 months after the end of treatment.

Exclusion Criteria:

1. The patient has histological or cytologic evidence of non-small cell lung cancer or mixed components of small cell lung cancer/non-small cell lung cancer;
2. Prior to randomization, chemotherapy, targeted therapy, or biological therapy were used within 4 weeks or 5 half-lives, small molecule targeted therapy was used within 5 days, or palliative radiotherapy was used within 2 weeks;
3. Patients with recurrent small cell lung cancer who are eligible for curative local therapy;
4. Received chemotherapy with TOP I inhibitor;
5. Received anti-EGFR and/or HER3 antibody /ADC drugs;
6. History of severe heart disease or cerebrovascular disease;
7. Unstable thrombotic events requiring therapeutic intervention within 6 months before screening; Infusion-related thrombosis was excluded;
8. Complete left bundle branch block, III degree atrioventricular block, frequent and uncontrollable arrhythmia;
9. Diagnosis of active malignancy within 3 years before randomization;
10. Hypertension poorly controlled by two antihypertensive drugs;
11. Patients with poor glycemic control;
12. A history of ILD requiring steroid therapy, or current ILD or grade ≥2 radiation pneumonitis;
13. Complicated pulmonary diseases leading to clinically severe respiratory function impairment;
14. Patients with active central nervous system metastases;
15. Severe infection within 4 weeks before randomization; There was evidence of pulmonary infection or active pulmonary inflammation requiring clinical intervention within 2 weeks before randomization;
16. Patients with massive or symptomatic effusions or poorly controlled effusions;
17. Imaging examination showed that the tumor had invaded or enveloped the large blood vessels in the abdomen, chest, neck, and pharynx;
18. Severe unhealed wound, ulcer, or fracture within 4 weeks before signing the informed consent;
19. Subjects with clinically significant bleeding or obvious bleeding tendency within 4 weeks before signing the informed consent;
20. Patients with inflammatory bowel disease, extensive bowel resection history, immune enteritis history, intestinal obstruction, chronic diarrhea or Gilbert's syndrome;
21. Patients with a history of allergy to recombinant humanized antibodies or to any of the excipients of BL-B01D1;
22. Human immunodeficiency virus antibody positive, active hepatitis B virus infection or hepatitis C virus infection;
23. A history of severe neurological or mental illness;
24. Subjects who were scheduled to receive live vaccine or received live vaccine within 28 days before study randomization;
25. Other circumstances that were assessed by the investigator as inappropriate for participation in the trial.