A Study of YL201 in Combination With Serplulimab in Participants With Treatment-naïve Extensive-stage Small Cell Lung Cancer

Inclusion Criteria: 1. Voluntarily sign the written informed consent form and comply with the protocol requirements 2. Age ≥18 years. 3. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1. 4. Histologically or cytologically confirmed extensive-stage small cell lung cancer (ES-SCLC). 5. No prior systemic treatment for ES-SCLC. 6. At least one extracranial measurable lesion according to RECIST v1.1. 7. Adequate organ function. 8. Life expectancy ≥3 months. Exclusion Criteria: 1. Any histological types of transformed SCLC or combined SCLC. 2. History of immune-related adverse events (irAEs) of CTCAE Grade ≥3 during prior immunotherapy, or unresolved adverse events from previous antitumor therapy. 3. Major surgery (excluding diagnostic procedures) or severe trauma within 4 weeks prior to randomization or planned major surgery during the study period. 4. History of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation. 5. Presence of active brain metastases, brainstem metastases, or leptomeningeal metastases. 6. Presence of severe and uncontrolled cardiovascular or cerebrovascular disease. 7. History of interstitial lung disease (ILD) /pneumonitis requiring steroid treatment, or current diagnosis of ILD/pneumonitis, or concurrent pulmonary disease leading to clinically severe impairment of respiratory function. 8. Active autoimmune or inflammatory diseases within 2 years prior to randomization. 9. Severe infection within 4 weeks prior to randomization, or active infection requiring intravenous anti-infective therapy within 2 weeks or oral anti-infective therapy within 1 week prior to randomization. 10. Known active tuberculosis or active syphilis infection. 11. History of immunodeficiency or positive for human immunodeficiency virus (HIV) antibodies test. 12. Active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection. Participants with inactive HBV infection must receive antiviral therapy throughout the study. 13. History of other primary malignancies within 5 years prior to randomization. 14. Known hypersensitivity to any component of the investigational product. 15. Females who are pregnant or breastfeeding, or who plan to become pregnant or breastfeed during the study period. 16. Any condition that, in the opinion of the investigator, would make the participant unsuitable for study participation.