Key Inclusion Criteria:

          1. Diagnosed with 1 of the following advanced mastocytosis diagnoses by Eligibility
             Committee:

               1. Aggressive Systemic Mastocytosis (ASM)

               2. Systemic Mastocytosis with an Associated Hematologic Neoplasm (SM-AHN)

               3. Mast Cell Leukemia (MCL)

          2. Measurable disease according to modified IWG-MRT-ECNM criteria

          3. ECOG (0 to 3)

          4. Have clinically acceptable local laboratory screening results (clinical chemistry,
             hematology) within certain limits

        Key Exclusion Criteria:

          1. Persistent toxicity from previous therapy for Advanced Systemic Mastocytosis that has
             not resolved to ≤ Grade 1

          2. Associated hematologic neoplasm requiring immediate antineoplastic therapy

          3. Clinically significant cardiac disease

          4. Known positivity for the FIP1L1 PDGFRA fusion (Patients with eosinophilia without
             detectable KIT D816V mutation must also lack the PDGFRA fusion mutation prior to
             enrollment)

          5. Seropositive for human immunodeficiency virus (HIV) 1 or 2, or positive for hepatitis
             B surface antigen or hepatitis C virus (HCV) antibody

          6. History of clinically significant bleeding event within 30 days before the first dose
             of study drug or need for therapeutic anticoagulation on study

          7. Diagnosed with or treated for malignancy other than the disease under study within the
             prior 3 years before enrollment

          8. Received any cytoreductive therapy or any investigational agent less than 14 days, and
             for cladribine, interferon alpha, pegylated interferon, and any antibody therapy less
             than 28 days, before screening bone marrow biopsy

          9. Received hematopoietic growth factor support within 14 days before the first dose of
             study drug

         10. Received strong CYP3A4 inhibitors or inducers before the first dose of study drug

         11. Need for treatment with steroids