Inclusion Criteria:

1. The subject must voluntarily participate in this study after being fully informed and must sign a written informed consent form (ICF), and be willing and able to comply with the study procedures and requirements.
2. At the time of signing the ICF, male or female subjects aged ≥65 years, or male or female subjects aged ≥18 years who are assessed by the investigator as not suitable for intensive treatment, defined as: ECOG performance status of 2-3, or comorbidities (CIRS >6 and ≤12), or otherwise judged by the investigator as not suitable for intensive treatment. Intensive treatment is defined as first-line intensive therapy for metastatic small cell lung cancer, i.e., standard chemotherapy with etoposide plus cisplatin or carboplatin combined with immune checkpoint inhibitors and targeted agents.
3. Histologically or cytologically confirmed metastatic small cell lung cancer (SCLC), according to the International Association for the Study of Lung Cancer and the American Joint Committee on Cancer TNM staging system, 9th edition (AJCC 9th).
4. Subjects must not have received prior systemic therapy for metastatic SCLC. Patients who previously received adjuvant, neoadjuvant, or radical chemoradiotherapy for non-metastatic disease are eligible if disease progression occurred >6 months after completion of the last treatment.
5. At least one measurable target lesion according to RECIST v1.1. Lesions previously treated with radiotherapy or other local regional therapy are not considered target lesions unless clear progression is documented after treatment. Lesions must have a longest diameter ≥10 mm on CT or MRI at baseline (lymph nodes must have a short axis ≥15 mm), and must be suitable for repeated accurate measurement per RECIST v1.1.
6. Eastern Cooperative Oncology Group (ECOG) performance status of 0-3.
7. Adequate organ function as defined below, with no blood transfusion or use of hematopoietic growth factors within 14 days prior to testing: Platelets (PLT) ≥80 × 10⁹/L Hemoglobin (HGB) ≥80 g/L Neutrophils (NEUT) ≥1.5 × 10⁹/L Creatinine ≤1.5 × upper limit of normal (ULN) or creatinine clearance (CrCl) ≥30 mL/min (Cockcroft-Gault) ALT and AST ≤2.5 × ULN (≤5 × ULN if liver metastases present) Total bilirubin (TBIL) ≤1.5 × ULN (≤3 × ULN in Gilbert's syndrome) INR or PT ≤1.5 × ULN and APTT ≤1.5 × ULN, or clinically acceptable bleeding risk as assessed by investigator Urine protein <2+ or 24-hour urine protein <1 g Expected survival ≥3 months Women of childbearing potential must agree to use effective contraception during and for 6 months after the study and have a negative serum pregnancy test within 7 days before enrollment and must not be breastfeeding Male subjects must agree to use effective contraception during and for 6 months after the study Willingness to participate and good compliance with follow-up.

Exclusion Criteria:

1. Prior anti-cancer therapy within specified time windows: Anti-PD-1/PD-L1 therapy within 3 years before first dose Anti-angiogenic multi-target TKIs (e.g., anlotinib, apatinib) within 3 years Any anti-cancer therapy (chemotherapy, targeted therapy, immunotherapy, etc.) or investigational drug within 28 days before first dose Traditional Chinese medicines with approved anti-cancer indications within 2 weeks before first dose (e.g., Fufang Banmao capsule, Kang'ai injection, Kanglaite capsule/injection, Aidi injection, Yadanzi oil injection/capsule, Xiaoaiping tablets/injection, Huachansu capsule, etc.)
2. Other primary malignancies, except: Tumors treated by single surgery with complete remission ≥3 years before enrollment Non-melanoma skin cancer or in situ cancers not requiring treatment Prostate cancer requiring only clinical observation
3. Symptomatic or progressing CNS metastases or carcinomatous meningitis. Patients with stable brain metastases may be eligible if: No neurological symptoms No corticosteroid use required and no indication for radiotherapy Largest brain lesion ≤1.5 cm Stable on two brain MRI/CT scans ≥2 weeks apart Additional timing and stability conditions after CNS treatment as specified Off corticosteroids ≥2 weeks if previously treated with CNS radiotherapy ≥4 weeks interval after CNS surgery before first dose
4. Cardiovascular conditions including: NYHA class II or higher heart failure Severe arrhythmia requiring treatment Myocardial infarction, unstable angina, or vascular bypass within 6 months LVEF <40% QTcF prolongation (female >470 ms, male >450 ms) or risk factors for torsades de pointes Uncontrolled hypertension (SBP ≥150 mmHg and/or DBP ≥100 mmHg despite treatment)
5. Thrombotic events within 6 months (e.g., stroke, TIA, DVT, pulmonary embolism), hypertensive crisis, or encephalopathy.
6. History of epilepsy.
7. Superior vena cava syndrome.
8. Active or uncontrolled pulmonary conditions including interstitial lung disease, radiation pneumonitis, immune-related pneumonitis, active tuberculosis, pneumoconiosis, or severe pulmonary impairment (FEV1, DLCO, or DLCO/VA <40%).
9. Severe bone metastasis-related complications (e.g., pathological fracture, spinal cord compression, uncontrolled bone pain).
10. Active uncontrolled infection (≥CTCAE grade 2) or fever >38.5°C of unknown cause.
11. Uncontrolled third-space fluid accumulation (pleural, peritoneal, pericardial effusion), unless stable after drainage.
12. Tumor invasion or unclear boundary with major blood vessels on imaging.
13. Bleeding tendency within 2 months or hemoptysis (>2.5 mL/day) within 2 weeks, or unhealed wounds/ulcers/fractures.
14. Significant gastrointestinal disease affecting drug absorption (e.g., severe ulcers, cirrhosis, bowel obstruction, IBD, surgery affecting absorption, etc.).
15. Live attenuated vaccine within 4 weeks before first dose.
16. Severe hypersensitivity to monoclonal antibody therapies.
17. Active autoimmune disease requiring systemic therapy within 2 years (excluding replacement therapies).
18. Immunodeficiency or ongoing immunosuppressive therapy (>10 mg/day prednisone equivalent) within 2 weeks before dosing.
19. HIV positivity, active hepatitis B or C infection as defined by HBV DNA or HCV RNA thresholds.
20. Active syphilis, dialysis-dependent renal failure.
21. Poorly controlled diabetes (fasting glucose >10 mmol/L).
22. History of organ transplantation or planned transplantation.
23. Major surgery or significant trauma within 4 weeks before first dose.
24. Palliative radiotherapy within 2 weeks before first dose.
25. Unresolved prior treatment toxicities (except alopecia/pigmentation/lab abnormalities not clinically significant); peripheral neuropathy not recovered to ≤Grade 2.
26. Pregnancy or breastfeeding.
27. Severe psychiatric illness, drug abuse, or alcoholism.
28. Known allergy to study drug or any of its components.
29. Any other clinically significant condition that may compromise safety or interfere with study assessments as judged by the investigator.