XParTS II: Capecitabine/CDDP(XP) and S-1/CDDP(SP) as the First-line Treatment for Advanced Gastric Cancer

Inclusion Criteria: 1. Histologically confirmed gastric adenocarcinoma with unresectable metastatic or recurrent disease 2. Lesions confirmed on imaging within 28 days before registration (not required measurable lesions as defined in RECIST version 1.1) 3. No previous chemotherapy or radiotherapy. However, adjuvant chemotherapy is allowed the case of more than 6 months from the end of adjuvant chemotherapy 4. ECOG Performance Status of 0 to 2 5. Life expectancy of at least 3 months after registration 6. Written informed consent 7. Age of 20 to 74 years with either gender 8. Adequate Major organ functions within 14 days before registration Exclusion Criteria: 1. Positive HER2 status 2. Previous history of fluoropyrimidines therapy within 6 months prior to registration 3. Previous treatment with platinum agents 4. Previous history of serious hypersensitivity to fluoropyrimidines or platinum agents 5. Previous history of adverse reactions suggestive of dihydropyrimidine dehydrogenase (DPD) deficiency 6. More than one cancer at the same time or more than one cancer at different times separated by a 5-year disease-free interval. However, multiple active cancers do not include carcinoma in situ or skin cancer which is determined to have been cured as a result of treatment. 7. Obvious infection or inflammation (pyrexia ≥ 38.0˚C) 8. Active hepatitis 9. Heart disease that is serious or requires hospitalization, or history of such disease within past year 10. Having complication that is serious or requires hospitalization (intestinal paralysis, intestinal obstruction, interstitial pneumonia or pulmonary fibrosis, poorly controlled diabetes mellitus, renal failure, liver disorders, or hepatic cirrhosis) 11. Being treated or in need of treatment with flucytosine, phenytoin or warfarin potassium 12. Chronic diarrhea (watery stool or ≥4 times/day) 13. Active gastrointestinal bleeding 14. Body cavity fluids requiring drainage or other treatment 15. Clinical suspicion or previous history of metastasis to brain or meninges 16. Women who are pregnant, breastfeeding, or potentially (hoping to become) pregnant 17. Unwillingness to practice contraception 18. Poor oral intake 19. Psychiatric disorders which are being or may need to be treated with psychotropics 20. Otherwise determined by investigators or site principal investigators to be unsuitable for participation in study