Comparing Regorafenib Combined With Envafolimab to Physician's Choice in Patients With Metastatic Gastrointestinal Stromal Tumors Harboring KIT Exon 17 Mutations Refractory to Standard Treatment

Inclusion Criteria: * Aged ≥ 18 years, regardless of gender; * Confirmed as gastrointestinal stromal tumor by histopathological examination; * Having at least one measurable target lesion meeting the criteria of mRECIST v1.1 (non-lymph node lesions with a long axis ≥ 1.0 cm or a long axis ≥ the thickness of 2 slides); imaging assessment should be performed within 14 days before the first medication; * Disease progression or intolerance after treatment with imatinib, sunitinib, regorafenib, or ripretinib; * Primary or secondary KIT exon 17 mutation detected by genetic testing; * Sufficient organ and bone marrow function, defined as follows: Blood routine: Absolute neutrophil count (ANC) ≥ 1.5×10⁹/L; platelet count (PLT) ≥ 75×10⁹/L; hemoglobin (HGB) ≥ 9.0 g/dL. No use of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), red blood cell transfusion, or platelet transfusion within 14 days before the examination; Liver and kidney function: For subjects without liver metastasis, serum total bilirubin (TBIL) ≤ 1.5×upper limit of normal (ULN); alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5×ULN. For subjects with liver metastasis: TBIL ≤ 1.5×ULN; ALT and AST ≤ 5×ULN. Renal function: Serum creatinine (Scr) ≤ 1.5×ULN; Sufficient coagulation function, defined as international normalized ratio (INR) ≤ 1.5 or prothrombin time (PT) ≤ 1.5×ULN; if the subject is receiving anticoagulant therapy, it is acceptable…