Study of Low-Dose Radiotherapy Concurrent Chemotherapy With Serplulimab for Patients With ES-SCLC

Serplulimab will be administered by intravenous infusion at a dose of 4.5mg/kg on Day 1 of each 21-day cycle until unacceptable toxicity or loss of clinical benefit as determined by the investigator after an integrated assessment of radiographic and biochemical data, and clinical status.

Cisplatin will be administered as intravenous infusion at a dose of 75 mg per meter squared (75 mg/m\^2) after completion of serplulimab on Day 1 of each 21-day cycle during the induction phase (Cycles 1-4).

Carboplatin will be administered as intravenous infusion at a dose of area under the concentration-time curve (AUC) of 5 mg/mL/min on Day 1 of each 21-day cycle during the induction phase (Cycles 1-4).

Etoposide will be administered intravenously at a dose of 100 mg/m\^2 on Days 1, 2 and 3 of each 21-day cycle during the induction phase (Cycles 1-4).

Participants will receive concurrent low-dose radiotherapy treatment, in once daily fractions, 3 Gy per fraction, to a target dose of 15 Gy in 5 fractions from Day 1-Day 5 in the first cycle, third cycle（ For patients with primary lung lesions (intrathoracic lesions) evaluated as small PR (tumor shrinkage \< 80%)/SD/PD），forth cycle（for subjects evaluated as PD/SD/PR with extrathoracic residual metastases).