Adenocarcinoma of Small Intestine

To explore the safety and efficacy of Surufatinib and Sintilimab in Combination With Capecitabine in Patients With Previously Treated Metastatic Adenocarcinoma of Small Intestine or Appendix Carcinoma : a Single-arm, a Single-center , Phase 2 Trial. Meanwhile, Exploring the maximum tolerant dose or recommended II research dose of Surufatinib combined with a fixed dose of Sintilimab and Capecitabine using 3 + 3 dose climbing experiment.