Blueprint Medicines announced initial data from the Phase 2 Pioneer clinical trial of avapritinib in patients with indolent systemic mastocytosis. Initial data from the dose-finding Part 1 of trial showed "rapid and robust reductions" in serum tryptase, a measure of mast cell burden, in patients treated with 25 mg, 50 mg or 100 mg of avapritinib once daily, the company said. All dose levels of avapritinib tested were well-tolerated, and no patients discontinued treatment due to an adverse event, it added.. The results will be presented today in a poster presentation at the American Society of Hematology Annual Meeting. At baseline, all patients had symptomatic disease despite best available therapy. Median Indolent SM Symptom Assessment Form total symptom score was 52. Patients were taking a median of three medications to treat their disease. Mean serum tryptase was 84 micrograms per liter. Across all avapritinib dose cohorts, reductions in serum tryptase were robust, occurred rapidly and were sustained in patients treated up to 30 weeks. The placebo cohort showed no change in serum tryptase at 12 weeks. All doses of avapritinib tested were well-tolerated, and most reported AEs were Grade 1 or 2, according to Blueprint. Across all avapritinib cohorts, five patients had Grade 3 AEs, and no patients had serious AEs. In patients treated with placebo, two patients had Grade 3 AEs, and two patients had serious AEs. There was one Grade 3 cognitive effect reported in the 100 mg cohort. The event resolved following dose modification, and the patient remained on therapy as of the data cutoff date. No patients discontinued treatment due to an AE. Blueprint plans to submit a new drug application to the FDA for avapritinib for the treatment of patients with advanced SM in Q1 of 2020.