Table 1 Eligibility criteria

● Patient ≥ 18 years at the day of consenting to the study

● Provision of informed consent prior to any study specific procedures

● Untreated localized gastric or GEJ adenocarcinoma considered resectable (clinical stage ≥ cT2 and/or cN + and no metastasis)

● Histologically confirmed adenocarcinoma

● ECOG performance status score ≤ 1

● Tumor tissue must be provided for biomarker analyses (fresh or archival with an FFPE tissue block)

● All subjects must consent to allow the acquisition of blood samples for performance of correlative studies

● Screening laboratory values must meet the following criteria:

WBC ≥ 2000/ mm³, Neutrophils ≥ 1500/ mm³, Platelets ≥ 100 000/ mm³, Hemoglobin ≥ 9.0 g/dL, Bilirubin ≤ 1.5 × ULN, AST and ALT ≤ 3 × ULN, measured or calculated creatinine ≥ 50 ml/min clearance (CrCl) (using the Cockcroft-Gault formula), Potassium ≥ LLN, Magnesium ≥ LLN and Calcium ≥ LLN

● Female subject of childbearing potential must have a negative urine or serum pregnancy test within 72 h before study start

● Subject in reproductive age must be willing to use adequate contraception during the study and at least 9 months in men and 12 months in women after the last dose of investigational drug. In addition, given the toxicities observed on the male reproductive system, a conservation of gametes will be proposed for men

● Subject affiliated to a social security regimen

● Subject with any distant metastasis

● Subject with no recovering from the effects of major surgery or significant traumatic injury within 14 days before inclusion

● Documented significant cardiovascular disease within the past 6 months before the first dose of study treatment, including: history of congestive heart failure (defined as NYHA III or IV), myocardial infarction, unstable angina, coronary angioplasty, coronary stenting, coronary artery bypass graft, cerebrovascular accident or hypertensive crisis

● History of anterior organ transplant

● Pneumonitis or interstitial lung disease

● History of other malignancy within the previous 3 years (except for appropriately treated in-situ cervix carcinoma and non-melanoma skin carcinoma)

● Active, known, or suspected autoimmune disease

● Subject with a condition requiring systemic treatment with either corticosteroids (> 10 mg daily prednisone equivalent) or other immunosuppressive medications within 14 days of start of study treatment

● Known history of HIV or HBV infection, history of active tuberculosis, active HCV infection

● Vaccination with live vaccine within 30 days before the first dose of study treatment

● Prior treatment with an anti-PD-1, anti-PD-L1, anti-PD-L2 or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways

● Recent or concomitant treatment with brivudine

● Prior anticancer therapy for the current malignancy

● Known hypersensitivity to any of the study drugs or their excipients

● Chronic inflammable gastro-intestinal disease

● Uracilemia ≥ 16 ng/ml

● QT/QTc > 450 ms for men and > 470 ms for women

● Peripheral neuropathy ≥ Grade II

● Uncontrolled diabetes

● Active infection requiring systemic therapy

● Participation in another therapeutic clinical study

● Patient deprived of liberty or placed under the authority of a tutor

● Patient assessed by the investigator to be unable or unwilling to comply with the requirements of the protocol