This is a randomized, open-label study of Serplulimab plus chemotherapy (Carboplatin-Etoposide) in comparison with Atezolizumab plus chemotherapy in previously untreated US patients with ES-SCLC.

Subjects in this study will be randomized to arm A or B at 1:1 ratio as follows:

• Arm A (Serplulimab): Serplulimab + chemotherapy (carboplatin-etoposide)
• Arm B (control): Atezolizumab + chemotherapy (carboplatin-etoposide)

Inclusion Criteria:

Voluntary participation in clinical studies.

Male or female aged ≥ 18 years at the time of signing the ICF.

Histologically or cytologically diagnosed with ES-SCLC (according to the Veterans Administration Lung Study Group staging system).

No prior systemic therapy for ES-SCLC.

At least one measurable lesion as assessed according to RECIST 1.1 within 4 weeks prior to randomization.

Major organs are functioning well.

Every effort should be made to provide tumor tissues for the determination of PD-L1 expression.

An ECOG PS score of 0 or 1.

An expected survival ≥ 12 weeks.

Subjects with prior denosumab use that can and agree to switch to bisphosphonate therapy for bone metastases starting prior to randomization and throughout treatment.

Participant must keep contraception.

Exclusion Criteria:

Histologically or cytologically confirmed mixed SCLC.

Known history of severe allergy to any monoclonal antibody.

Known hypersensitivity to carboplatin or etoposide.

Patients with myocardial infarction within half a year before the first dose of the study drug, poorly controlled arrhythmia.

Pregnant or breastfeeding females.

Patients with a known history of psychotropic drug abuse or drug addiction.

Patients who have other factors that could lead to the early termination of this study based on the investigator's judgment.