(PIONEER) Study to Evaluate Efficacy and Safety of Avapritinib (BLU-285), A Selective KIT Mutation-targeted Tyrosine Kinase Inhibitor, Versus Placebo in Patients With Indolent Systemic Mastocytosis

Key Inclusion Criteria: * 1\. Patient must have SM, confirmed by Central Pathology Review of BM biopsy, and central review of B- and C-findings by WHO diagnostic criteria. * 2\. Patient must have moderate-to-severe symptoms based on minimum mean total symptom score (TSS) of the ISM Symptom Assessment Form (ISM-SAF) over the 14-day eligibility screening period. * 3\. Patient must have failed to achieve adequate symptom control for 1 or more Baseline symptoms. * 4\. For patients receiving corticosteroids, the dose must be ≤ 20 mg/d prednisone or equivalent, and the dose must be stable for ≥ 14 days. * 5\. Patient must have an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 to 2. Key Exclusion Criteria: * 1\. Patient has been diagnosed with any of the following WHO SM subclassifications: cutaneous mastocytosis only, smoldering SM, SM with associated hematologic neoplasm, aggressive SM, mast cell leukemia, or mast cell sarcoma. * 2\. Patient must not have received prior treatment with avapritinib. * 3\. Patient must not have had any cytoreductive therapy including but not limited to masitinib and midostaurin, or investigational agent for \< 14 days or 5 half-lives of the drug (whichever is longer), and for cladribine, interferon alpha, pegylated interferon, or antibody therapy \< 28 days or 5 half-lives of the drug (whichever is longer), before beginning the 14-day ISM-SAF eligibility TSS assessment. * 4\. Patient must not have received radiotherapy or psoralen and ultraviolet A (PUVA) therapy \< 14 days before beginning the 14-day ISM-SAF eligibility TSS assessment. * 5\. Patient must not have received any hematopoietic growth factor the preceding 14 days before beginning the 14-day ISM-SAF eligibility TSS assessment. * 6\. Patient must not have a QT interval corrected using Fridericia's formula (QTcF) of \> 480 msec.