PD-1 Antibody + XELOX in 1st Line Serum A-fetoprotein (AFP)-Elevated Gastric or Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria:

1. Patients must volunteer to participate in the study, signed informed consent, and wereable to comply with the program requirements of visits and related procedures.
2. Age and gender: ≥18 years old and≤75 years old, both men and women.
3. All subjects must have unresectable, local advanced recurrent or metastatic gastricadenocarcinoma (GC) or gastroesophageal junction adenocarcinoma (GEC) confirmed byhistologically.
4. No systematic treatment for advanced or metastatic GC/GEC has been received in thepast. For patients who have received adjuvant or neoadjuvant therapy (includingchemotherapy, radiotherapy and/or radiochemotherapy) for GC/GEC in the past, the lasttreatment must be completed at least six months before the start of study drug.Subjects are allowed to receive palliative radiotherapy, but it must be completed twoweeks before the start of study drug.
5. Serum AFP > 20 ng/ml.
6. All acute toxic reactions caused by previous medication or surgery were alleviated tograde 0-1 (according to NCI-CTCAE version 5.0) or to the level specified by thecriteria for Inclusion/exclusion. The toxicities that do not pose a safety risk topatients determined by investigators are excluded, such as hair loss , etc.
7. Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.
8. Expected survival: ≥12 weeks.
9. Subject must have at least one measurable lesion or evaluable disease by CT or MRI perRECIST 1.1 criteria.
10. The functions of important organs must meet the followingrequirements:(1)Hematological system: Neutrophil count≥1.5×10^9/L; Plateletcount≥80×10^9/L; Hemoglobin≥90g/L;(2)Liver function: Serum albumin≥28g/L; Totalbilirubin (TBI)≤1.5×ULN; Alanine aminotransferase (ALT)≤2.5×ULN (or≤5×ULN if livermetastases are present); Aspartate aminotransferase (AST)≤2.5×ULN (or≤5×ULN if livermetastases are present); (3)Renal function: Serum creatinine≤1.5×ULN or calculatedcreatinine clearance (CrCl) ≥40 mL/min (using the Cockcroft-Gault formula):Female CrCl = (140- age in years) × weight in kg × 0.85/ 72 × serum creatinine in mg/ dL; MaleCrCl = (140- age in years) × weight in kg × 1.00/72 × serum creatinine in mg/dL;(4)Coagulation function: Subjects not receiving anticoagulation therapy: (International Normalized Ratio) INR or activeated partial thromboplastin time (APTT) ≤ 1.5×ULN.
11. Women of childbearing potential (WOCBP) must have a negative serum or urine pregnancytest within 7 days prior to the start of study drug. WOCBP must agree to followinstructions for method(s) of contraception (e.g. intrauterine devices,contraceptives, condoms or abstinence) for the duration of study treatment and 6months after the last dose of study treatment. Subjects must be non-lactating. Maleswho are sexually active with WOCBP must agree to follow instructions for method(s) ofcontraception for the duration of study treatment and 6 months after the last dose ofstudy treatment.

Exclusion Criteria:

1. Known human epidermalgrowth factor receptor-2 (HER2) positive.
2. Currently participating in research and receiving research treatment, or participatingin the research of experimental drugs within four weeks before the start of studydrug, and having received research treatment or used experimental instruments.
3. Major surgery were performed within 4 weeks before the start of the study andincomplete recovery.
4. Existence of any active autoimmune disease or with a history of autoimmune disease (asthe following examples, but not limited to: autoimmune hepatitis, interstitialpneumonia, uveitis, enteritis, hepatitis, pituitary, vasculitis, nephritis,hyperthyroidism; patients with vitiligo; in childhood asthma has been completelyalleviated, adults without any intervention can be included; asthma with medicalintervention could not be included). Substitution therapy is not considered assystemic therapy. Patients with the following diseases are not excluded and mayproceed to further screening: a. Controlled Type I diabetes; b. Hypothyroidism (provided it is managed with hormone replacement therapy only).
5. Any condition that required systemic treatment with either corticosteroids (> 10 mgdaily of prednisone or equivalent) or other immunosuppressive medication ≤ 7 daysbefore randomization.
6. Any active malignancy ≤ 5 years before randomization except for the specific cancerunder investigation in this study and any cured limited tumors (eg, carcinoma in situof the cervix or prostate, basal cell skin cancer).
7. Patients with known central nervous system metastasis (suspected need to be excludedby MRI scans) or a history of hepatic encephalopathy.
8. A history of pneumonia (non-infectious) requiring steroid therapy within 6 months orcurrently suffering from pneumonia (pulmonary infectious).
9. With active infections, fever of unknown origin (≥38.5℃) within 7 days before thestart of study drug; or white blood cell count at baseline > 15×10^9/L); with severechronic or active infections (including tuberculosis infection, etc.) requiringsystemic antibacterial, antifungal or antiviral therapy during screening period,excluding viral hepatitis.
10. With any other disease, metabolic abnormality, abnormal physical examination orabnormal laboratory examination, according to the judgments of the investigators,there is reason to suspect that the patient has a certain disease or condition that isnot suitable for the use of study drugs, or that it will affect the interpretation ofresearch results or put the patient at high risk.
11. Mental or drug abuse disorders known to have an impact on compliance with studyrequirements
12. Congenital or acquired immunodeficiency (e.g. HIV-infected persons).
13. With active hepatitis B virus (HBV) or hepatitis C virus (HCV). Active hepatitis B isdefined as known positive HBsAg results, and HBV-DNA > 2000IU/ml. Active hepatitis Cis defined as known positive hepatitis C antibodies and the quantitative results ofHCV RNA are higher than the lower detection limit of analytical methods. Active HBVwill be allowed if they have HBV DNA < 500 IU/mL (or 2500 copies/mL) at screening.Patients with HBV-DNA < 2000IU/ml through antiviral therapy can be consideredincluded.
14. Was administered a live attenuated vaccine ≤ 4 weeks before randomization, or plan tovaccinate during treatment with against PD-1 monoclonal antibody or within five monthsafter last administration.
15. More than a small amount of pericardial effusion, uncontrolled pleural effusion orclinically obvious peritoneal effusion at screening. It is defined as meeting thefollowing criteria: physical examination at screening can detect pleural andperitoneal effusion, or in screening process, pleural and peritoneal effusion needspuncture and drainage.
16. With history of serious cardiovascular and cerebrovascular diseases: (1) Any historyof heart failure meeting New York Heart Association Classification III or IV or moreserious history of heart disease, myocardial infarction, or cerebrovascular accidentin 3 months before randomization；(2) Left ventricular ejection fraction < 50% by colorDoppler echocardiography；(3) Uncontrollable hypertension; (4) Uncontrolledarrhythmias; (5) Acute coronary syndrome, congestive heart failure, stroke,thromboembolism or other cardiovascular events above grade 3 within 6 months beforethe start of study drug.
17. A history of allergy to anti-PD-1, anti-PD-L1 monoclonal antibody drugs or oxaliplatinor capecitabine.
18. Known dihydropyrimidine dehydrogenase deficiency.
19. Pregnant or lactating women; or female subjects who are expected to conceive duringthe planned trial period (from the start of screening visits to 120 days after thelast administration of the study) or male subjects whose spouses are pregnant.
20. Other situations that the researchers think should be excluded.