Perioperative Treatment in Resectable Gastric Cancer With Spartalizumab (PDR001) in Combination With Fluorouracil, Leucovorin, Oxaliplatin, and Docetaxel (FLOT)

Inclusion Criteria: * Age ≥ 18 years * Untreated localized gastric or GEJ adenocarcinoma considered resectable (clinical stage ≥cT2 and/or cN+ and no metastasis) * Histologically confirmed adenocarcinoma * ECOG performance status score of 0 or 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 x ULN, AST and ALT ≤ 3 x ULN * Measured or calculated creatinine ≥ 50 ml/min clearance (CrCl) (using the Cockcroft-Gault formula) * Potassium ≥ LLN * Magnesium ≥ LLN * Calcium ≥ LLN * Female subject of childbearing potential must have a negative urine or serum pregnancy test within 72h 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, as usually in routine practice * Subject affiliated to a social security regimen * Patient has signed informed consents obtained before any trial related activities and according to local guidelines Exclusion Criteria: 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, including stem cell allograft * 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) * Subject with 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 GASPAR Protocol - EUDRACT number: 2020-004497-21 - version 1.3 / 2021-01-18 Page 8 sur 44 * Known history of HIV or HBV infection * Known active HCV infection * Known history of active tuberculosis * 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 (herpes virostatic) * 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 msec for men and \> 470 msec 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