Tislelizumab Plus FOLFOX Versus POF in the Treatment of Locally Advanced: a Multicenter, Open-label, Randomized Phase III Studyunresectable or Metastatic Gastric/Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria:

* Patients with advanced unresectable, histologically confirmed adenocarcinoma of the gastric or gastroesophageal junction.
* 18-70yeas.
* ECOG PS 0-1.
* No previous chemotherapy (perioperative chemotherapy, six months after fluorouracil alone or one year after oxaliplatin or a combination of taxoid and fluorouracil can be included), radiotherapy or immunotherapy.
* With normal marrow, liver and renal function: a hemoglobin (HGB) of ≥100g/L (without blood transfusion during 14 days); a leucopenia count of ≥4.0×109/L; a platelet count of ≥100×109/L; a total bilirubin (TBil) of ≤1.5 upper normal limitation (UNL); a creatinine (Cr) of ≤ 1.5 UNL; a creatinine clearance rate ≥ 50ml/min (Cockcroft-Gault); a alanine aminotransferase (ALAT) and aspartate aminotransferase (ASAT) of ≤2.5 UNL or ≤5 UNL in case of liver metastasis.
* Life expectancy ≥3 months.
* With normal electrocardiogram results and no history of congestive heart failure.
* With normal coagulation function: activated partial thromboplastin time (APTT), prothrombin time (PT) and INR, each ≤ 1.5 x ULN.
* Female subjects of child-bearing potential must agree to use contraceptive measures starting 1 week before the administration of the first dose of Tislelizumab until 8 weeks after discontinuing study drug. Male subjects must agree to use contraceptive measures during the study and 8 weeks after last dose of study drug.
* With written informed consent signed voluntarily by patients themselves or their supervisors witted by doctors.
* Have a known PD-L1 CPS/MMR (or MSI) /HER2(FISH) test result, or have sufficient samples for relevant testing.

Exclusion Criteria:

* Patients with a history of another neoplastic disease within the past three years, excluding basal cell carcinoma of the skin, cervical carcinoma in situ, or nonmetastatic prostate cancer.
* Patients with brain or central nervous system metastases, including leptomeningeal disease.
* Pregnant (positive pregnancy test) or breast feeding.
* Serious, non-healing wound, ulcer, or bone fracture.
* Significant cardiac disease as defined as: unstable angina, New York Heart Association (NYHA) grade II or greater, congestive heart failure, history of myocardial infarction within 6 months Evidence of bleeding diathesis or coagulopathy.
* History of a stroke or CVA within 6 months.
* Clinically significant peripheral vascular disease.
* HIV-positive, active hepatitis B or C (HBV, HCV);
* Inability to comply with study and/or follow-up procedures.
* Patients with any other medical condition or reason, in that investigator's opinion, makes the patient unstable to participate in a clinical trial.
* Her2-positive (IHC 3+ or 2+/FISH+) patients.