This is a single-arm, open-lable exploratory study of Lurbinectedin in combination with Serplulimab as maintenance therapy in participants with extensive-stage small-cell lung cancer (ES-SCLC) after first-line induction therapy with carboplatin, etoposide, and Serplulimab. The study consists of 2 phases: an induction phase and a maintenance phase. Participants need to have an ongoing response or stable disease per the Response Evaluation Criteria in Solid Tumor (RECIST) v1.1 criteria after completion of 4 cycles of carboplatin, etoposide, and Serplulimab induction treatment in order to be considered for eligibility screening for the maintenance phase. Eligible participants will receive lurbinectedin plus Serplulimab in the maintenance phase.

Inclusion Criteria for the Induction Phase:

• Fully informed about the study and voluntarily signed a written informed consent form, and able to comply with the requirements and restrictions listed in the informed consent form;
• Male or female with age ≥ 18 years;
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0-1;
• Histologically or cytologically confirmed diagnosis of extensive-stage small cell lung cancer (ES-SCLC) and has not received any systemic treatment for ES-SCLC;
• At least one measurable lesion (according to RECIST 1.1 criteria);
• Having adequate bone marrow, hepatic, renal and metabolic function, meaning the functional level of the organs meets the following requirements:
• Platelet count (PLT) ≥ 100×10^9/L; Hemoglobin (Hb) ≥ 90 g/L; Absolute neutrophil count (ANC) ≥ 2.0×10^9/L;
• Regardless of whether liver metastasis is present, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3.0×upper limit of normal (ULN);
• Alkaline phosphatase (ALP) ≤ 5×ULN;
• Total bilirubin (TBIL) ≤ 1.5×ULN, and direct bilirubin ≤ 1.0×ULN;
• Serum creatinine ≤ 1.5×ULN or creatinine clearance rate ≥ 30 mL/min (calculated using the Cockcroft-Gault formula);
• Creatine phosphokinase (CPK) ≤ 2.5×ULN;
• Albumin ≥ 3.0 g/dL.
• Women of childbearing potential(WOCBP) must have a negative serum pregnancy test before enrollment. WOCBP must use effective contraceptive measure during the trial drug treatment and for 6 months after the last administration. Male patients (with partners of WOCBP) must use effective contraceptive measure during the trial drug treatment and for 4 months after the last administration;
• Human immunodeficiency virus (HIV)-negative, with no active hepatitis B virus (HBV) or hepatitis C virus (HCV) infection.

Exclusion Criteria for the Induction Phase:

• Have a history of central nervous system (CNS) metastasis or related history;
• Have a history of active autoimmune disease or immunodeficiency, or related history;
• Have a history of malignancies other than SCLC within 5 years before enrollment;
• Previous treatment with immune checkpoint inhibitors or lurbinectedin;
• Have a history of idiopathic pulmonary fibrosis or pneumonia, or active pneumonia detected in CT screening;
• Treatment with any other investigational product within 28 days before enrollment.

Inclusion Criteria for the Maintenance Phase:

• Have a continuous response or stable disease according to RECIST 1.1 criteria after 4 cycles of induction therapy;
• Toxicity from the induction therapy phase has recovered to ≤ Grade 1;
• Have sufficient bone marrow and organ function.

Exclusion Criteria for the Maintenance Phase:

• Have a history of central nervous system (CNS) metastasis or related history;
• Has received chest consolidation radiotherapy;
• Severe infection within 2 weeks before enrollment.

Individual participant data (IPD) will not be shared