Nivolumab Adding on Gemcitabine/S-1 in Metastatic Pancreatic Cancer

Inclusion Criteria: 1. histologically proven pancreatic adenocarcinoma 2. newly diagnosed, stage IV pancreatic cancer with limited metastases and tumor burden 3. no previous radiotherapy, chemotherapy, targeted therapy, curative surgery, local therapy (eg. radiofrequency ablation, irreversible electroporation, etc.), immunotherapy, cell therapy (autologous or allogenic) used for pancreatic cancer 4. presence of at least one measurable lesion at the pancreas and at least one measurable metastatic lesion 5. age between 20 and 75 years at registration 6. ECOG performance status of 0 or 1 7. adequate major organ functions 8. baseline CA 19-9 \> upper limit of normal 9. Glasgow prognostic score of 0 (ie. albumin ≥ 3.5 g/dL and CRP ≤ 1 mg/dL) 10. ability to take study medication (S-1) orally 11. no clinically significant abnormal ECG findings within 28 days prior to registration 12. Women of childbearing potential (including women with chemical menopause or no menstruation for other medical reasons) must agree to use contraception from the time of informed consent until 5 months or more after the last dose of investigational products. Also, women must agree not to breastfeed from the time of informed consent until 5 months or more after the last dose of the investigational product. 13. Men must agree to use contraception from the start of study treatment until 7 months or more after the last dose of the investigational product. 14. Sign written informed consent Exclusion Criteria: 1. interstitial lung disease that is symptomatic or may interfere with the detection or management of suspected treatment-related pulmonary toxicity within 28 days prior to registration 2. presence of diarrhea ≥ CTCAE v.5.0 grade 2 3. concomitant systemic infection requiring treatment 4. clinically significant co-morbid medical conditions, including cardiovascular disease known autoimmune disease 5. concurrent autoimmune disease or history of chronic or recurrent autoimmune disease 6. prior organ allograft or allogeneic bone marrow transplantation 7. received systemic corticosteroids (except for temporary use, e.g., for examination or prophylaxis of allergic reactions) or immunosuppressants within 28 days before registration 8. HBV (positive HBsAg or HBV DNA) or HCV carrier (positive anti-HCV or HCV RNA) 9. known history of testing positive for human immunodeficiency virus (HIV) or known acquired immunodeficiency syndrome 10. moderate or severe ascites, pleural effusion, or pericardial effusion requiring treatment 11. central nervous system metastasis 12. prior or concurrent malignancies within the last 3 years, with the exception of carcinoma in situ of the cervix, or basal type skin cancer 13. concomitant treatment with flucytosine, phenytoin or warfarin 14. any major surgery within 4 weeks of study treatment. Participants must have recovered from the effects of major surgery or significant traumatic injury at least 14 days before the first dose of study treatment. 15. transfusion from 72 hours prior to registration to the first dose of study drug administration 16. pregnant women or nursing mothers, or positive pregnancy tests 17. severe mental disorder 18. treatment with botanical preparations (eg, herbal supplements or traditional Chinese medicines) intended for general health support or to treat the disease under study within 2 weeks prior to registration 19. vaccine therapies for prevention of infectious diseases within 4 weeks of study drug administration except inactivated seasonal influenza vaccine 20. any condition requiring anti-platelet or anticoagulant therapy within 12 weeks prior to registration 21. oral or iv antibiotic use within 2 weeks prior to registration 22. uncontrollable pain caused by a tumor 23. receiving antineoplastic agents within 28 days before registration 24. patients judged by the principal investigator or subinvestigators to be inappropriate as subjects of this study