Imatinib Dose Escalation to 800 mg/Day in Korean Patients With Metastatic or Unresectable GIST Harboring KIT Exon 9 Mutation: KENEDI

Inclusion Criteria: * Age 18 or older * Histologically confirmed metastatic or unresectable GIST with CD117(+), DOG-1 (+), or KIT mutation * ECOG PS(Eastern Cooperative Oncology Group Performance Status) 0\~2 * Primary mutation at KIT exon 9 * Imatinib treatment for less than 4 weeks from the first dose at 400 mg per day * No prior use of tyrosine kinase inhibitors ((but, patients who have recurrence 6 months after completion of adjuvant imatinib at a dose of 400 mg per day can be enrolled in this study) * At least one evaluable disease by RECIST v1.0 * Resolution of all toxic effects of prior treatments (chemotherapy, surgery, RFA(radiofrequency ablation), radiotherapy, and/or TACE) * Adequate bone marrow function as defined by platelets ≥ 75 x 109/L and neutrophils ≥ 1.5 x 109/L (within 1 week prior to the first dose of imatinib at 400 mg per day) * Adequate renal function, with serum creatinine \< 1.5 x ULN (within 1 week prior to the first dose of imatinib at 400 mg per day) * Adequate hepatic function with serum total bilirubin \< 1.5 x ULN, alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \< 2.5 x ULN in the absence of liver metastases, or \< 5 x UNL in the presence of liver metastases (within 1 week prior to the first dose of imatinib at 400 mg per day) * No other malignant disease apart from non-melanotic skin cancer or carcinoma in situ of the uterine cervix or any other cancer except where treated with curative intent \> 5 years previously without evidence of relapse * Provision of a signed written informed consent Exclusion Criteria: * Severe co-morbid illness and/or active infections * Pregnant or lactating women * History of other malignancies except basal cell carcinoma and carcinoma in situ of uterine cervix * CNS metastasis * Clinically significant bleeding in GI tract * GI obstruction or malabsorption * Known hypersensitivity to imatinib