Key Inclusion Criteria:  Diagnosed with 1 of the following diagnoses according to the 2016 World Health Organization (WHO) classification for systemic mastocytosis (SM):  Indolent systemic mastocytosis (ISM) Smoldering systemic mastocytosis (SSM) Moderate-to-severe symptoms based on a disease-specific PRO and after establishing a stable regimen of at least 2 antimediator therapies over a 14-day eligibility period Eastern Cooperative Oncology Group (ECOG) Performance Status 0 to 2 For patients receiving corticosteroids, the dose must be ≤10 mg/day of prednisone or equivalent  Key Exclusion Criteria:  Diagnosed with any of the following WHO SM classifications: bone marrow mastocytosis, advanced systemic mastocytosis including SM with associated hematologic neoplasm, aggressive SM, mast cell leukemia; or mast cell sarcoma Diagnosed with mastocytosis of the skin without systemic involvement Received prior treatment with any targeted KIT inhibitor Received prior cytoreductive therapy or investigational agent for <14 days or 5 half- lives of the drug and for cladribine, interferon alpha, pegylated interferon, or antibody therapy <28 days or 5 half-lives of the drug (whichever is longer), before starting screening assessments Received radiotherapy or psoralen and ultraviolet A therapy <14 days before starting screening assessments Received any hematopoietic growth factor support <14 days before starting screening assessments History of clinically significant bleeding event within 30 days before the first dose of study drug or need for therapeutic anticoagulation on study Need for treatment of corticosteroids at >10 mg/day of prednisone or equivalent