A Phase II Randomized Study of Gemcitabine and Nab-paclitaxel in Combination With S- 1/LV (GASL) or Oxaliplatin (GAP) as First-line Treatment for Metastatic Pancreatic Cancer

Inclusion Criteria: * A.Cytology or histology confirmed pancreatic adenocarcinoma with evidence of distant metastasis.Mixed component of other cell type (eg, adenosquamous, adenocarcinoma with neuroendocrine differentiation) is eligible but should notify PI before registration. * B.No history of prior chemotherapy for pancreatic cancer, except adjuvant chemotherapy that completed at least 6 months before documentation of recurrence by imaging study. * C.Patients with prior radiotherapy are eligible if there are other measurable target lesions that is not irradiated. * D.At least one measurable lesion according to RECIST version 1.1 * E.Ability to understand and willingness to sign a written informed consent document. * F.ECOG performance status 0-1 * G.Age of 20 years or above * H.Adequate organ function as defined by the following criteria: absolute neutrophil count (ANC) ≥ 1,500/mm3 hemoglobin level ≥ 9 g/dL platelet count ≥ 100,000/mm3 total bilirubin ≤ 2 ULN or ≤5 ULN if caused by biliary obstruction and achieving adequate drainage judged by investigator aspartate aminotransferase (AST) / alanine aminotransferase (ALT) ≤ 3 x ULN or ≤ 5.0×ULN in the presence of liver metastases creatinine clearance rate (CCr) ≥ 50 mL/min (24-hour urine collection or calculated by Cockroft-Gault formula; male: \[(140 - age) × weight (kg)\]/\[72 × serum creatinine(mg/dL)\];female=male x 0.85 -I.Patients with childbearing potential shall have effective contraception for both the patient and his or her partner during the study. Exclusion Criteria: * A. Other malignancy within the past 5 years except for adequately treated localized cancer or carcinoma in situ;All patients with other malignancy within the past 5 years should notify PI before registration. * B.Active or uncontrolled infection; * C.Significant medical conditions that is contraindicated to study medication or render patient at high risk from treatment complications at physician discretion * D.Pregnant women or nursing mothers, or positive pregnancy test for women of childbearing potential. * E.History of active autoimmune disease within 3 years or use of steroid more than prednisolone 10mg/day.