Postoperative Immunotherapy vs Standard Chemotherapy for Gastric Cancer With High Risk for Recurrence

Inclusion Criteria: * Histologically proven gastric, lower esophageal or GE-junction adenocarcinoma (Siewert I-III) * Subjects must have completed pre-operative chemotherapy with a fluoropyrimidine-platinum containing regimen and macroscopically complete surgery prior to randomization * Minimal duration of neoadjuvant chemotherapy should be 6 weeks, maximum 12 weeks. * Total or distal gastrectomy with D2 lymphadenectomy according to ESMO guidelines should have been completed for gastric and junctional Siewert type III cancers. Ivor Lewis or McKeown oesophagectomy with two field lymphadenectomy should have been performed for junctional Siewert type I cancers. For Siewert type II cancers either total gastrectomy with D2-lymphadenectomy or oesophagectomy with two field lymphadenectomy should have been completed. Open, minimal invasive or hybrid surgical approaches are acceptable as long as the requirements above are fulfilled. * Regardless of the type of surgery a minimum of 15 lymph nodes should have been resected and examined. * Recovered from surgery and fit for study treatment as assessed by a multidisciplinary team. Surgery should have been completed 2 to 3 months before randomization. * ypN1-3 status according to current (8th) version of TNM classification system. In case of an ypN0 status patients must meet the inclusion criterion of R1 resection. * R0 or R1 resection according to current (8th) version of TNM classification system. In case of R0 resection, patients must meet the inclusion criterion of ypN1-3 * WHO performance status score of 0 or 1 * Age ≥ 18 years * Adequate organ function assessed within 7 days before randomization: * White blood cell count (WBC) \> 2 x 109/L * Absolute neutrophil count (ANC) \> 1.5 x 109/L * Platelets ≥ 100 x 109/L * Hemoglobin ≥ 9 g/dL * Measured/calculated creatinine clearance ≥ 60 mL/min (according to Cockroft-Gault formula). * Total bilirubin within normal limits (if the patient has documented Gilbert's disease ≤ 1.5 \* ULN or direct bilirubin ≤ ULN) * Aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 1.5ULN * Cardiac assessment by 12 Lead ECG and if clinically indicated, cardiac function assessment (using either echocardiography or MUGA scan) * All toxicities (exception alopecia) attributed to prior anti-cancer therapy must have resolved to grade 1 (NCI CTCAE version 4) or baseline before administration of study drug. * Women of childbearing potential (WOCBP\*) must have a negative serum or urine pregnancy test (minimum sensitivity 25 IU/L or equivalent units of human chorionic gonadotropin (HCG)) within 24 hours prior to randomization * Men who are sexually active with an WOCBP must adhere to contraception (condom) during the study and for a period of 7 months after the last dose of the study treatment in the experimental arm and 6 months in the control arm. * Patients of childbearing / reproductive potential should use highly effective method of birth control measures during the study treatment period and for at least 5 months after the last study treatment. A highly effective method of birth control is defined as those which result in low failure rate (i.e. less than 1% per year) when used consistently and correctly. * Female subjects who are breast feeding should discontinue nursing prior to the first dose of study treatment and until 6 months after the last study treatment. * Absence of any psychological, familial, sociological or geographical condition potentially hampering compliance with the study protocol and follow-up schedule; those conditions should be discussed with the patient before registration in the trial * Before patient registration/randomization, written informed consent must be given according to ICH/GCP, and national/local regulations. Exclusion Criteria: * R2 resection status * M1 stage according to current (8th) version of TNM classification system * Patients who have undergone complete resection of metastases * Impaired renal, hepatic, cardiac, pulmonary or endocrine status that compromises the eligibility of the patient for postoperative chemotherapy or immunotherapy * Clinically significant (that is, active) cardiovascular disease: cerebral vascular accident/stroke (\< 6 months prior to enrollment), myocardial infarction (\< 6 months prior to enrollment), unstable angina pectoris, congestive heart failure (New York Heart Association Classification Class ≥ II), or serious cardiac arrhythmia requiring medication * Subjects with previous malignancies are excluded unless a complete remission or complete resection was achieved at least 5 years prior to study entry. Adequately treated cervical carcinoma in situ, and localized non-melanoma skin cancer are no exclusion criteria, regardless of timepoint of diagnosis. * Subjects with active, known, or suspected infectious or autoimmune disease * Patients who have received antibiotics within the last 14 days before randomization are excluded. * Subjects with Type I diabetes mellitus, residual hypothyroidism due to autoimmune thyroiditis only requiring hormone replacement, skin disorders (such as vitiligo, psoriasis, or alopecia) not requiring systemic treatment are permitted to enroll * Subjects with a condition requiring systemic treatment with either corticosteroids (≥ 10 mg daily prednisone or equivalent) or other immunosuppressive medications within 14 days of study drug administration * Subjects with interstitial lung disease that is symptomatic or may interfere with the detection or management of suspected drug-related pulmonary toxicity * Subjects with \> Grade 1 peripheral neuropathy * Prior treatment with an anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti-CTLA-4 antibody, or any other antibody or drug specifically targeting T-cell co-stimulation or immune checkpoint pathways * Prior or concomitant treatment with radiotherapy/radiochemotherapy * Any positive test result for HBV or HCV indicating acute or chronic infection * Known history of HIV or known AIDS and, if required by local practice or positive HIV testing at screening * Known uncontrollable hypersensitivity to the components of cisplatin/oxaliplatin, fluorouracil (5-FU) or capecitabine, epirubicine or docetaxel * Known dihydropyrimidine dehydrogenase (DPD) deficiency * Ongoing or concomitant use of the antiviral drug sorivudine or its chemically related analogs, such as brivudine.