Inclusion Criteria:
1. Signed informed consent form (ICF);
2. Age ≥18 years at the time of signing informed consent;
3. Histologically confirmed small cell lung cancer (SCLC);
4. Extensive-stage SCLC (ES-SCLC) with disease progression or recurrence after prior treatment;
5. At least one measurable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 as assessed by the investigator;
6. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1;
7. Adequate organ and bone marrow function.
8. Women of childbearing potential must use highly effective contraception from screening until 3 months after discontinuation of study treatment and must have a negative pregnancy test (serum or urine) within 7 days before enrollment. Male participants must agree to use appropriate contraception from screening until 3 months after discontinuation of study treatment or have undergone surgical sterilization;
9. No anticancer therapy was received within the specified washout period before the first dose of study treatment;
10. Estimated life expectancy ≥12 weeks.
Exclusion Criteria:
1. Known hypersensitivity to becotatug vedotin;
2. History of another active malignancy requiring treatment or history of allogeneic organ transplantation;
3. Major surgery (excluding diagnostic biopsy) within 4 weeks before the first dose of study treatment;
4. History of drug abuse, chronic alcohol abuse, AIDS, or HIV infection;
5. Active autoimmune disease or history of autoimmune disease with potential risk of recurrence;
6. Current systemic corticosteroid therapy (e.g., prednisone equivalent \>10 mg/day) or use of any other immunosuppressive therapy within 14 days before the first dose of study treatment;
7. Interstitial lung disease (ILD), or previous ILD requiring corticosteroid treatment;
8. History of idiopathic pulmonary fibrosis (IPF), drug-induced pneumonitis, organizing pneumonia (including bronchiolitis obliterans organizing pneumonia), or idiopathic pneumonia;
9. Receipt of live vaccines within 28 days before the first dose of study treatment;
10. Any disease or condition that may contraindicate anticancer therapy, including but not limited to active infection, myocardial infarction within 6 months, symptomatic cardiac disease (including unstable angina, congestive heart failure, or uncontrolled arrhythmia), or ongoing immunosuppressive therapy;
11. Pregnancy or breastfeeding; or male or female participants unwilling to use appropriate contraception when pregnancy is possible;
12. Evidence of hereditary bleeding disorders or coagulation abnormalities;
13. History of malignancy within the previous 3 years, except for adequately treated non-melanoma skin cancer or carcinoma in situ with an expected survival of \>3 years, such as breast cancer in situ, oral carcinoma in situ, or cervical carcinoma in situ;
14. Any other medical history, disease, treatment, or laboratory abnormality that may interfere with study results, prevent full participation in the study, or make the participant unsuitable for enrollment as judged by the investigator.
