The primary objective is to assess clinical benefit in patients with KIT exon 11-mutant GIST (Cohort A) defined as clinical benefit rate (CBR), which is the composite of complete response (CR), partial response (PR) and stable disease (SD) lasting ≥16 weeks per modified response evaluation criteria in solid tumors (RECIST 1.1) as a measure of disease control. The secondary objective is to assess clinical benefit in patients with GIST that lacks an activating KIT exon 11 mutation (Cohort B) and in the total patient population. The efficacy assessments are tumor response using Response Criteria in Solid Tumors (RECIST) Version 1.1, modified for GIST and assessment of progression-free survival (PFS) and overall survival (OS). The safety assessments include routine physical and laboratory evaluations, electrocardiograms (ECGs), echocardiograms (ECHOs), and adverse event (AE) monitoring. Other assessments include optional 18F fluorodeoxyglucose positron emission tomography (FDG-PET); optional pre- and post-treatment tumor biopsy for pharmacodynamic studies; and pharmacokinetics (PK). It is estimated that accrual will be complete within 1 year; the total estimated duration of the study is 3 years.