This Phase II study is a clinical study exploring the efficacy and safety of BL-M14D1 in combination with Atezolizumab in patients with extensive-stage small cell lung cancer.

Inclusion Criteria:

1. Voluntarily sign the informed consent form and comply with the protocol requirements;
2. No gender restriction;
3. Age: ≥18 years;
4. Expected survival time ≥3 months;
5. Histopathologically and/or cytologically confirmed extensive-stage small cell lung cancer that is incurable or for which there is currently no standard treatment;
6. Agree to provide archived tumor tissue specimens from the primary or metastatic lesion within 3 years, or fresh tissue samples;
7. Must have at least one measurable lesion as defined by RECIST v1.1;
8. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1;
9. Toxicities from prior anti-tumor therapy have recovered to ≤ Grade 1 as defined by NCI-CTCAE v6.0;
10. No severe cardiac dysfunction, with left ventricular ejection fraction (LVEF) ≥50%;
11. Organ function levels must meet the required criteria;
12. Urine protein ≤1+ or ≤1000 mg/24h;
13. For premenopausal women with childbearing potential, a pregnancy test must be performed within 7 days before the start of treatment. Serum pregnancy testing must rule out pregnancy, and the patient must not be breastfeeding. All enrolled trial participants (regardless of gender) should take adequate barrier contraceptive measures throughout the entire treatment period and for 7 months after the end of treatment.

Exclusion Criteria:

1. Use of chemotherapy, biological therapy, immunotherapy, etc., within 4 weeks or 5 half-lives before the first dose;
2. Previous treatment with ADC drugs using topoisomerase I inhibitors as toxins;
3. Small cell carcinoma with non-small cell carcinoma components indicated by pathology must be excluded;
4. Use of immunomodulatory drugs within 2 weeks before the first dose of the study;
5. Receiving long-term systemic corticosteroid therapy at a dose >10 mg/day of prednisone or equivalent before the first dose;
6. History of severe cardiovascular or cerebrovascular diseases;
7. Prolonged QTc interval, complete left bundle branch block, etc.;
8. Active autoimmune diseases and inflammatory diseases;
9. Diagnosis of another malignancy within 5 years before the first dose;
10. Hypertension poorly controlled by two antihypertensive drugs;
11. Patients with poorly controlled blood glucose;
12. History of ILD/interstitial pneumonia treated with corticosteroids, etc.;
13. Concomitant pulmonary diseases leading to clinically severe impairment of respiratory function;
14. Presence of massive serous cavity effusion, or serous cavity effusion with symptoms, etc.;
15. Imaging findings indicating that the tumor has invaded or encased major blood vessels in the chest, neck, pharynx, etc.;
16. Any thrombotic event within 6 months before randomization;
17. Patients with active central nervous system metastases;
18. History of allergy to recombinant humanized antibodies or chimeric human-mouse antibodies, or allergy to any excipient components of the investigational drug, etc.;
19. Previous organ transplantation or allogeneic hematopoietic stem cell transplantation;
20. Cumulative anthracycline dose >360 mg/m² during prior (neo)adjuvant anthracycline therapy;
21. Positive for human immunodeficiency virus antibody, active tuberculosis, active hepatitis B virus infection, or active hepatitis C virus infection;
22. Active infections requiring systemic treatment, or occurrence of severe infection within 4 weeks before informed consent;
23. Receipt of other unapproved clinical study drugs or treatments within 4 weeks before the first dose;
24. Pregnant or breastfeeding women;
25. History of severe neurological or psychiatric disorders;
26. Presence of serious non-healing wounds, ulcers, or fractures within 4 weeks before signing informed consent;
27. Clinically significant bleeding or obvious bleeding tendency within 4 weeks before signing informed consent;
28. History of intestinal obstruction, inflammatory bowel disease, extensive bowel resection, or presence of Crohn's disease, ulcerative colitis, or chronic diarrhea;
29. Trial participants who plan to receive or have received live vaccines within 28 days before the first dose;
30. Other conditions deemed by the investigator to be unsuitable for participation in this clinical trial.