This is a single-arm, phase II study aiming to evaluate the feasibility and efficacy of sintilimab (PD-1 inhibitor) in combination of apatinib and two-drug chemotherapy (S-1 plus nab-paclitaxel) as conversion therapy in patients with stage IV gastric cancer in China.

Inclusion Criteria:

• gastric adenocarcinoma confirmed by gastroscopy and pathology (histologically/cytologically ) ;

• life expectancy of ≥3-month；

• unresectable patients who were initially diagnosed as stage IV (clinical stage, American Joint Committee on Cancer 8th edition);

• Eastern Cooperative Oncology Group performance status: 0-1;

• must have at least 1 of the following unresectable factors indicated by CT, MRI or positron emission tomography(PET)-CT:

  1. N3 lymphatic metastasis;
  2. Extensive or bulky lymph nodes;
  3. T4b;
  4. Hepatic metastasis: ≤5 lesions, total diameter of ≤8cm;
  5. Peritoneal metastasis (CY1, P1);
  6. Kukernburg tumor;

• adequate organ function;

• pregnant test negative of females of childbearing potential , and willing to use adequate contraception;

• written Informed Consensus Form;

Exclusion Criteria:

• prior use of any checkpoint inhibitor treatment, including PD-1, programmed cell death ligand-1(PDL-1), CTLA4 etc;
• Her-2 positive with willing to use herceptin treatment;
• prior active autoimmune disease or history of autoimmune disease;
• clinically significant cardiovascular and cerebrovascular diseases, including but not limited to severe acute myocardial infarction within 6 months before enrollment, unstable or severe angina, or coronary artery bypass surgery, Congestive heart failure (New York heart association (NYHA) class > 2), ventricular arrhythmia which need medical intervention, left ventricular ejection fraction(LVEF) < 50%;
• not controlled hypertension;
• prior systemic treatment to metastatic disease；
• previous digestive tract bleeding history within 3 months or evident gastrointestinal bleeding tendency;
• history of immunodeficiency including seropositivity for human immunodeficiency virus (HIV), or other acquired or congenital immune-deficient disease, or active hepatitis ;
• patients who may receive vaccination during study period;
• mental disorders history, or psychotropic drug abuse history;
• unable to orally administration;