Phase Ib part: To evaluate safety and tolerability in combination of Regorafenib, Nivolumab and chemotherapy in patient with unresectable advanced/recurrent gastric/ gastroesophageal junction/esophageal adenocarcinoma and to determine recommended dose in phase II part.~Phase II part: To evaluate safety and potential efficacy in combination of Regorafenib, Nivolumab and chemotherapy in patients in expanded arm.~The protocol treatment in this study is regorafenib and nivolumab plus CapeOX (Cohort A) / FOLFOX (Cohort B).~Regorafenib (the initial dose is 90 mg/dose) is orally administered daily for 21 days, followed by a 7-day washout period. Nivolumab is administered intravenously at a dose of 360 mg every 3 weeks (Cohort A) or 240 mg every 2 weeks (Cohort B).~Cohort A:CapeOX~Capecitabine 1,000 mg/m^2 administered orally, twice daily (Days 1 to 14 continuous dosing of CapeOX therapy)~Oxaliplatin 130 mg/m^2 administered intravenously (Day 1 of CapeOX therapy) *Repeat every 3 weeks as CapeOX therapy~Cohort B:FOLFOX~Leucovorin 400 mg/m^2 administered intravenously (Day 1 of FOLFOX therapy)~Fluorouracil 400 mg/m^2 administered intravenously (Day 1 of FOLFOX therapy) and 1,200 mg/m2 administered intravenously (Days 1 to 2 of FOLFOX therapy)~Oxaliplatin 85 mg/m^2 administered intravenously (Day 1 of FOLFOX therapy) *Repeat every 2 weeks as FOLFOX therapy
This is an open-label, single-arm, single-center Phase Ib/II study to exploratorily evaluate the tolerability, safety, and efficacy of regorafenib and nivolumab plus chemotherapy in patients with unresectable advanced/recurrent gastric/ gastroesophageal junction/esophageal adenocarcinoma.