Comparing the Efficacy of Sintilimab Plus Nab-POF Regimen, Sintilimab Plus XELOX Chemotherapy, and Lenvatinib, Sintilimab Plus XELOX Chemotherapy in the Treatment of HER2-negative, Metastatic Gastric Cancer

Inclusion Criteria: 1. Males or females aged over than 18 years 2. Histologically or cytologically confirmed gastric adenocarcinoma (including GEJ adenocarcinoma) 3. Presence of measurable lesions(RECIST 1.1) 4. Newly diagnosed patients; or patients with recurrence after surgery, who have been off adjuvant chemotherapy for at least 6 months and have not received any anti-cancer treatment within the past 6 months 5. ECOG PS score: 0-1 6. Expected survival over than 3 months 7. Normal major organ function or reserve, meeting the following criteria (determined by laboratory test data within 7 days (inclusive) before screening): HB ≥90 g/dL, ANC ≥1.5×109/L, PLT ≥100×109/L; BIL\<1.5×ULN, ALT and AST \<2.5×ULN, if with metastases to liver, then ALT, AST \<5×ULN; serum Cr≤1×ULN, endogenous creatinine clearance \>50 mL/min (calculated by CK formula), international normalized ratio (INR) or prothrombin time (PT) ≤1.5×ULN, if the subject is receiving anticoagulant therapy, PT should be within the intended range of the anticoagulant drug 8. Subjects who voluntarily participate in this study, sign the informed consent form, have good compliance, and cooperate with follow-up Exclusion Criteria: 1. Histopathological assessment of HER2 positivity (immunohistochemistry 3+ or immunohistochemistry 2+ and FISH positive) 2. Known dMMR/MSI-H 3. History of other malignant neoplasms within 3 years prior to enrollment, except for cured cervical carcinoma in situ or basal cell carcinoma 4. Presence of brain metastases to meninges 5. Malignant pleural or peritoneal effusion 6. Presence of gastrointestinal obstruction, gastrointestinal haemorrhage (fecaloccult blood +++ or higher), or perforation 7. Prior treatment with anti-PD-1, anti-PD-L1, or anti-PD-L2, CD137, CTLA-4 antibodies, or any other antibody or drug specifically targeting T-cell co-stimulatory or checkpoint pathways 8. Subjects with active or history of autoimmune diseases that might relapse (e.g., systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, autoimmune thyroid disorder, multiple sclerosis, vasculitis, glomerulonephritis, etc.), or high-risk patients (e.g., those requiring immunosuppression after organ transplant), except for those with vitiligo, psoriasis, alopecia, or Graves' disease that have not required systemic treatment within the past 2 years, or hypothyroidism requiring only thyroid hormone replacement therapy, or Type I diabetes mellitus requiring only insulin replacement therapy 9. Current interstitial lung disorder or pneumonia, pulmonary fibrosis, acute lung disorder, or radiation pneumonitis 10. Participation in other drug clinical studies within 4 weeks prior to the first dose (based on the use of investigational drug), except observational (non-interventional) clinical studies 11. Use of immunosuppressant drugs within 4 weeks prior to the first dose of study treatment, excluding nasal, inhaled, or other topical glucocorticoids or physiological doses of systemic glucocorticoids (i.e., no more than 10 mg/day of prednisone or equivalent dose of other glucocorticoids), or short-term (no more than 7 days) use of glucocorticoids for the prevention or treatment of non-autoimmune allergic diseases 12. Receipt of live attenuated vaccines within 4 weeks prior to the first dose of study treatment or planned immunization during the study period. Note: Inactivated seasonal influenza vaccines for injection are allowed within 4 weeks prior to the first dose; however, live attenuated influenza vaccines are not allowed. 13. Major surgery (craniotomy, thoracotomy, or laparotomy) within 4 weeks prior to the first dose of study treatment or anticipated need for major surgery (not related to this study) during the study treatment period. 14. History of human immunodeficiency virus (HIV) infection (i.e., HIV antibody positive), or other acquired or congenital immunodeficiency diseases, or history of organ transplant or stem cell transplant 15. Active chronic hepatitis B or active hepatitis C; hepatitis B virus carriers, but patients with stable hepatitis B viral loads after drug therapy (HBV DNA ≤ 200 IU/mL or copy number \< 1000 copies/mL), and patients who have cleared hepatitis C virus (HCV RNA negative) are eligible for enrollment 16. Known active pulmonary tuberculosis 17. Patients with severe infection within 4 weeks before the first dose, or active infection requiring oral or vein antibiotic therapy within 2 weeks before the first dose 18. Symptomatic congestive cardiac failure (New York Heart Association Class II-IV) or symptomatic or poorly controlled arrhythmia 19. Uncontrolled hypertension (blood pressure systolic ≥160 mmHg or blood pressure diastolic ≥100 mmHg) despite standard treatment 20. Any arterial embolism event within 6 months before enrollment, including myocardial infarction, angina unstable, cerebrovascular accident, or transient ischemic attack 21. History of deep vein thrombosis, pulmonary embolism, or any other severe embolism within 3 months before enrollment (thrombosis originating from implanted venous access port or catheter, or superficial vein thrombosis is not considered as "severe" thromboembolism) 22. History of definite neurological or mental disorder: such as epilepsy, dementia, poor compliance, or peripheral nervous system disorders 23. Alcohol dependence or history of drug addiction or drug abuse within the past year 24. Pregnant or lactating women,women of childbearing potential who are not taking adequate contraception 25. Other acute or chronic disease, mental disorder, or laboratory test abnormality that might lead to the following results: increased risk associated with study participation or study drug administration, or interference with the interpretation of study results, and the investigator judges the patient to be ineligible for participation in this study.