Phase I/II Study of SOL for Untreated Metastatic Colorectal Cancer

Inclusion Criteria: * Written informed consent. * Adequate oral intake. * Histologically proved adenocarcinoma (colorectal cancer). * Unresectable, recurrent and advanced colorectal cancer. * At least 4 weeks since prior major surgery * Age: 20 - 74 at enrollment. * Performance Status (Eastern Cooperative Oncology Group \[ECOG\] scale): 0 - 2. * No prior treatment (ex. radiation therapy, chemotherapy, hormonal therapy) to advanced disease. Patients who received adjuvant chemotherapy more than 180 days before enrollment can be allowed but those who received S-1 or Oxaliplatin containing treatment shall be excluded. * Proved presence of measurable lesions within 30 days before enrollment. * Patients with the following function of bone marrow, liver and kidney based on the laboratory values measured within 15 days before enrollment. * Hemoglobin \>\_ 9.0g/dL * WBC: LLN - 12,000/mm3 * Absolute granulocyte count \>\_ 1,500/mm3 * Platelets \>\_ 100,000/mm3 * Total Bilirubin \<\_ 1.5mg/dL * AST(GOT) and ALT(GPT) \<\_ 100U/L * ALP \<\_ 600U/L * Creatinine \<\_ 1.0mg/dl * Expected to survive more than 90 days after enrollment. Exclusion Criteria: * Patients who not received S-1 and Oxaliplatin as neoadjuvant or adjuvant chemotherapies before enrollment. * Contraindication to S-1, and serious hypersensitivity to oral Leucovorin. * No other concurrent investigational therapy. * Patients did who not received blood transfusion and hematogenesis stimulating agents (ex. Granulocyte-Colony Stimulating Factor). * Serious drug hypersensitivity. * Prior history of peripheral neuropathy. * Marked infection or inflammation (ex. patients with a fever of 38.0 degrees or higher). * Poorly-controlled diabetes, hypertension or hypercalcemia. * Severe (inpatient care is needed) arrhythmia, heart disease or its history. * Severe (inpatient care is needed) lung disease (ex. interstitial pneumonia or pulmonary fibrosis). * Psychiatric disorder requiring to receive treatment with antipsychotic drug. * Fresh bleeding from the gastrointestinal tract. * Requiring to drainage massive ascites, pleural effusion retention or pericardial fluid. * Extensive bone metastasis. * Clinically suspicious brain metastasis or brain metastasis. * Diarrhea (watery diarrhea). * Simultaneously active double cancer. * Pregnancy or nursing female patients and possibility (intent) to bear baby. Male patients with intent to impregnate. * Other patients evaluated to be inadequate to participate in the study by (sub) investigators.