S-1 and Cisplatin (3 Weekly) Versus S-1 and Oxaliplatin Combination Chemotherapy for Advanced Gastric Cancer

Inclusion Criteria: 1. Written informed consent before the enrollment 2. Age ≥18 years old 3. Histologically/cytologically confirmed recurrent or metastatic gastric or esophagogastric junctional adenocarcinoma 4. Eastern Cooperative Oncology Group (ECOG) performance status 0-2 5. Patients able to swallow food and drugs 6. At least one measurable or evaluable lesion according to RECIST criteria version 1.1 7. Adequate bone, hepatic, and renal function as assessed by the following laboratory requirements to be conducted within 7 days prior to first administration of study drugs * Absolute neutrophil count (ANC) ≥ 1,500/ uL, platelet ≥ 100,000/ uL, haemoglobin (Hb) ≥ 9.0 g/dl, * Serum creatinine ≤ 1.5 mg/dL (If serum creatinine is greater than 1.5 mg/dL, creatinine clearance \[Ccr\] should be 60 mL/min or greater. Ccr is calculated by Cockcroft-Gault formula or 24hr urine collection) * Total bilirubin ≤ 1.5 x upper limit of normal (ULN), AST/ALT levels ≤ 3.0 x ULN (AST/ALT levels ≤ 5.0 x ULN for patients with liver involvement of their cancer) 8. In patients who received adjuvant or neoadjuvant chemotherapy, completion of systemic chemotherapy 6 months before the study enrollment, and no previous administration of platinum derivatives 9. Estimated life expectancy of more than 3 months Exclusion Criteria: 1. Other histologic types than adenocarcinoma 2. Recurrence within 24 weeks following completion of adjuvant chemotherapy 3. R1 gastrectomy (i.e., microscopic residual disease) 4. History of another malignancy within the last five years from the day of written informed consent except cured basal cell carcinoma of skin and cured carcinoma in situ of uterine cervix 5. Radiotherapy within 4 weeks after randomization 6. History of significant neurologic or psychiatric disorders, and presence or history of CNS metastasis 7. Major surgery within 4 weeks before study entry, or insufficient recovery from major surgery (except the patients who received only open and closure or biopsy) 8. Other serious illness or medical conditions as follows; * Any following conditions occurred within 6 months before study entry: myocardial infarction, severe/unstable angina, bypass surgery for coronary artery/peripheral artery, congestive heart failure (NYHA class III or IV), cerebral infarction or transient ischemic attack * Conduction abnormality such as 2nd degree or greater AV block or severe arrhythmia that requires medical treatments (right bundle branch block (RBBB) is eligible, but left bundle branch block (LBBB) is not.) * Uncontrolled hypertension * Liver cirrhosis (Child Pugh Class B or greater) * Interstitial pneumonia, pulmonary fibrosis * Active viral hepatitis B * Uncontrolled diabetes mellitus * Uncontrolled ascites or pleural effusion * Uncontrolled active infection or sepsis 9. Administration of medications which may have potentially pharmacokinetic interaction with S-1, cisplatin, and oxaliplatin * Flucytosine, a fluorinated pyrimidine antifungal agent * Anti-viral agents, such as sorivudine, and brivudine, or chemical similar drugs * Warfarin (except, low dose warfarin for the purpose of prophylaxis), phenprocoumon * Phenytoin * Allopurinol 10. Participation to other clinical trials or administration of other investigational drugs within 30 days before the randomisation 11. Pregnant or lactating women 12. Women or men of child bearing potential not employing adequate contraception during study treatments or until the 3 months after the end of study treatments 13. Ineligible for the study at the discretion of investigators