Key Inclusion Criteria:  Diagnosed with 1 of the following advanced mastocytosis diagnoses by Eligibility Committee:  Aggressive Systemic Mastocytosis (ASM) Systemic Mastocytosis with an Associated Hematologic Neoplasm (SM-AHN) Mast Cell Leukemia (MCL) Measurable disease according to modified IWG-MRT-ECNM criteria ECOG (0 to 3) Have clinically acceptable local laboratory screening results (clinical chemistry, hematology) within certain limits  Key Exclusion Criteria:  Persistent toxicity from previous therapy for Advanced Systemic Mastocytosis that has not resolved to ≤ Grade 1 Associated hematologic neoplasm requiring immediate antineoplastic therapy Clinically significant cardiac disease 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) Seropositive for human immunodeficiency virus (HIV) 1 or 2, or positive for hepatitis B surface antigen or hepatitis C virus (HCV) antibody History of clinically significant bleeding event within 30 days before the first dose of study drug or need for therapeutic anticoagulation on study Diagnosed with or treated for malignancy other than the disease under study within the prior 3 years before enrollment 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 Received hematopoietic growth factor support within 14 days before the first dose of study drug Received strong CYP3A4 inhibitors or inducers before the first dose of study drug Need for treatment with steroids