Inclusion Criteria:

          1. Histologically or cytologically-confirmed advanced or metastatic GIST with a D842V
             mutation in the PDGFRA gene as determined by central laboratory testing

          2. Measurable disease as per modified RECIST 1.1

             • A lesion in an area that was previously treated with local therapy (e.g. radiation,
             surgery, or cryotherapy) can be considered measurable disease as long as there is
             objective evidence of progression of the lesion prior to randomization

          3. Subjects (male or female) ≥ 18 years of age

          4. Female subjects with reproductive potential must have negative serum or urine
             pregnancy test

          5. Eastern Cooperative Oncology Group (ECOG) performance status of ≤ 2

        Exclusion Criteria:

          1. Severe liver disease (e.g. cirrhosis, non-alcoholic steatohepatitis, sclerosing
             cholangitis)

          2. Known, active infection with hepatitis B virus (HBV) or hepatitis C virus (HCV)

          3. Female subject who is pregnant or breastfeeding, or planning to become pregnant within
             30 days after ending treatment

          4. Systemic anti-cancer treatment (e.g. chemotherapy, tyrosine kinase inhibitors,
             immunotherapy, or investigational agents) or investigational device within 3 weeks or
             5 half-lives (if the drug's half-life in subject is known) prior to randomization,
             whichever is shorter