This study is to evaluate the antitumor activity (i.e. progression free survival, overall survival, response rate, etc), safety and quality of life in patient with recurrent or metastatic gastric cancer during 4-regimen (i.e. SP, FL/Tax, FL/Doc, FOLFOX) based chemotherapy. In addition, it is to evaluate efficacy and adverse events of anticancer agents according to the results of pharmacogenetic study.

Inclusion Criteria:

• Histologically confirmed adenocarcinoma of the stomach
• Unresectable metastatic disease or recurred AGC
• Age ≥ 20 years old
• Eastern Cooperative Oncology Group performance status 0-2
• Estimated life expectancy > 12weeks
• Patients with pre-treatment(radiotherapy, chemotherapy) is not carried out. (However, adjuvant chemotherapy using one kind of medication is allowed. It must passed more than 6 months since end of the treatment.) Conventional radiation therapy should be performed before more than 2 weeks. And it is not allowed radiation therapy on primary tumor, measurable lesions, and abdominal
• According to RECIST ver.1.1, using imaging techniques (CT or MRI), measurement possible disease or not measurable, but assessable disease
• Screening laboratory values within the following limits : Hemoglobin ≥ 9 g/dL, Absolute neutrophil count ≥ 1.5 x 10³/L, Platelet count ≥ 75 x 10³/L, Serum creatinine ≤ 1.5 x UNL, Creatinine clearance ≥ 50 mL/min, AST/ALT ≤ 5.0 x UNL
• The patient or protector have to sign a consent form. And he should be able to understand the right to withdraw consent without disadvantage.

Exclusion Criteria:

• Prior chemotherapy (however, previous (neo-)adjuvant chemotherapy allowed if finished at least 6month ago except S-1, Paclitaxel, Docetaxel, Oxaliplatin . Allowed 5-FU, Cisplatin if finished 6month ago)

• Patients with oral intake is impossible or with malabsorption syndrome

• Patients with medically uncontrolled severe complications or infection

• Patients who have central nervous system disorders, mental disorders, or evidence of CNS metastases

• Patient with diagnosed active overlapping cancer within the past 5years except adequately treated carcinoma insitu of the cervix, basal or squamous cell carcinoma of skin

• The possibility(will) of pregnancy , or pregnant and lactating women

• Patients with clinically significant heart disease

• Progression of gastric lesions is not possible to evaluate

• During clinical trials, patient receiving of systemic chemotherapy or other clinical trials drugs or radiotherapy (However, it is possible that palliative radiotherapy treatment for pain relief on primary lesion after more than 2 weeks)

• Peripheral neuropathy of Grade 2 or greater.

• Patients who taking drugs to change of pharmacological activity of S-1 medicine(Warfarin, phenytoin, allopurinol etc.). Except regular treatment of corticosteroids(or equivalent drugs). However, it is allowed the purposes for pre-treatment of study drugs and the control of nausea and vomiting. (However, it is allowed the treatment for acute allergic reactions, or the treatment with low doses (methylprednisolone 20mg or less, or the equivalent drug) from about 6 months)

• Other cases

  • Concurrent disease or condition that would make the subject inappropriate for study participation or any serious medical disorder that would interfere with the subject's safety.
  • Psychological condition or dementia that would not permit the subject to complete the study or sign informed consent
  • Patients with dropout sure in clinical trials or cannot be regular follow-up following reasons. For example, psychological, social, family, geographic reasons or a difficult condition of compliance with clinical trial and proper follow up.
  • Poorly controlled chronic liver disease or diabetes mellitus
  • Else, in the investigator's opinion, should exclude the patient from the study.