Inclusion Criteria:
* Pathologically confirmed small cell lung cancer (SCLC), staged as extensive-stage SCLC (ES-SCLC) according to the VALG staging system;
* No prior systemic treatment received;
* At least a 6-month interval between the last treatment (radiation therapy and chemotherapy) and diagnosis of extensive-stage SCLC;
* Showing at least one target lesion (RECIST 1.1) that has not been previously irradiated;
* Male or female patients aged ≥18 and ≤70 years;
* ECOG performance status (PS) score of 0 or 1;
* Life expectancy of ≥12 weeks;
* Adequate organ function: (1) Hematologic: WBC ≥ 3.0 × 10⁹/L, ANC ≥ 1.5 × 10⁹/L, PLT ≥ 100 × 10⁹/L, HGB ≥ 9.0 g/dL.(2) Hepatic function: AST ≤ 2.5 × ULN, ALT ≤ 2.5 × ULN, liver metastases allowed if ALT and AST ≤ 5 × ULN, TBIL ≤ 1.5 × ULN (except for Gilbert's syndrome, where total bilirubin ≤ 3.0 mg/dL),(3) Renal function: Cr ≤ 1.5 × ULN or CrCl ≥ 50 mL/min, (4) Coagulation: INR ≤ 1.5 × ULN, APTT ≤ 1.5 × ULN;
* Agreement to use an appropriate contraceptive method from the first dose of the study treatment until 6 months after the last dose of the study treatment. Women of childbearing potential must have a negative pregnancy test within 7 days prior to enrollment;
* The patient must have fully understood the study and voluntarily consent to participate by signing the informed consent form (ICF).
Exclusion Criteria:
* Histologically or cytologically confirmed mixed small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC);
* Previous treatment with immune checkpoint inhibitors, or treatment with irinotecan or other DNA topoisomerase inhibitors;
* Use of a strong CYP3A4 inducer within 2 weeks prior to the first dose of the investigational drug, or a strong CYP3A4 inhibitor or UGT1A1 inhibitor within 1 week prior to the first dose;
* Presence of clinically symptomatic brain metastasis, leptomeningeal metastasis, or spinal cord compression;
* Hematologic disorders, including but not limited to lymphoma, acute or chronic leukemia, multiple myeloma, aplastic anemia, myelodysplastic syndrome, etc;
* Clinically symptomatic third-space fluid accumulation, such as pericardial effusion, pleural effusion, or ascites that cannot be controlled by drainage or other treatments;
* Active, known, or suspected autoimmune disease;
* Use of corticosteroids (doses\> 10 mg/day of prednisone or equivalent) or other immunosuppressive agents within 14 days prior to the first dose of the study drug;
* Receipt of live vaccines or planned vaccination with a live vaccine within 4 weeks prior to the first dose of the study drug;
* Interstitial lung disease, drug-induced pneumonia, radiation-induced pneumonia requiring steroid treatment, or clinically symptomatic active pneumonia, or severe pulmonary dysfunction;
* Active tuberculosis or a history of active tuberculosis within 48 weeks prior to screening, regardless of whether it was treated;
* Any toxicity from previous anticancer therapy, that has not resolved to grade ≤1 (according to CTCAE v5.0) prior to the first dose of the investigational drug;
* Underwent a minor surgery (including catheter placement) within 48 hours prior to the first dose of the study drug;
* Presence of uncontrolled cardiovascular symptoms or diseases;
* Hypersensitivity to the investigational drug or its excipients;
* A history of any other malignancy within 5 years prior to the first dose of the study drug, except for adequately treated non-melanoma skin cancer or in situ carcinoma;
* A history of psychiatric disorders, alcohol abuse, drug abuse, or substance misuse;
* HBsAg-positive and HBV DNA levels exceeding the upper limit of normal, or HCV-positive (HCV RNA or HCV Ab indicating acute or chronic infection); a history of HIV-positive status or acquired immunodeficiency syndrome (AIDS);
* Received any other investigational drug or participated in another interventional clinical trial within 4 weeks prior to signing the informed consent form (ICF);
* A history of allogeneic bone marrow transplantation or solid organ transplantation;
* Any other factors as determined by the investigator.
