A Study Comparing QLC5508 in Combination With QL2107 Versus Tislelizumab Plus Platinum-based Chemotherapy in Participants Wih Treatment-naïve Extensive-Stage Small Cell Lung Cancer

Inclusion Criteria:

1. Age ≥18 years.
2. Participants with histologically or cytologically confirmed Extensive-Stage Small Cell Lung Cancer (ES-SCLC) per the Veterans Administration Lung Cancer Study Group (VALG) staging system.
3. No prior systemic treatment for ES-SCLC.
4. At least one extracranial measurable lesion according to RECIST v1.1.
5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1, without deterioration within 2 weeks prior to the first dose of study treatment.
6. Life expectancy ≥12 weeks.
7. Has adequate organ function.
8. Male and female participants of reproductive/childbearing potential must agree to avoid pregnancy.
9. Voluntarily sign the written informed consent form and comply with the protocol requirements.

Exclusion Criteria:

1. Has received or undergoing any of the following treatment:

   Previous or current treatment with B7-H3 targeted therapy. Previous or current treatment with topoisomerase I inhibitors.

2. Participants with transformed Non-Small Cell Lung Cancer (NSCLC), epidermal growth factor receptor (EGFR) mutation-positive NSCLC that has transformed to SCLC, or mixed SCLC-NSCLC histology.
3. Presence with active brain metastases, leptomeningeal metastasis, brainstem metastasis or spinal cord compression.
4. Radiotherapy with a limited field of radiation within 2 weeks prior to the study treatment; more than 30% of the bone marrow irradiation or large-scale radiotherapy within 4 weeks prior to study treatment.
5. Major surgery within 4 weeks prior to the study treatment.
6. Unresolved AEs ≥ Grade 2 (CTCAE v6.0) from prior therapy.
7. Previous or concurrent primary malignancies.
8. History of severe heart disease or cerebrovascular disease.
9. History of Severe or uncontrolled hypertension or diabetes mellitus.
10. Severe infection within 4 weeks prior to study treatment; or uncontrolled active infection at screening.
11. Known or suspected interstitial lung disease/non-infectious pneumonitis; or other moderate to severe pulmonary diseases that significantly impair respiratory function and may interfere with the detection or management of drug-related pulmonary toxicity.
12. Known history of pre-existing or newly diagnosed autoimmune disease.
13. History of severe neuropathy or mental disorders.
14. History of severe hypersensitivity reaction, severe infusion reaction or idiosyncrasy to drugs chemically related to study drugs or any components of the study drugs.
15. Unlikely to comply with study procedures and requirements in the opinion of the investigator.
16. Any disease or condition that, in the opinion of the investigator, would compromise participant safety or interfere with study assessments.