Efficacy and safety of avapritinib in advanced systemic mastocytosis: 4-year follow-up of the PATHFINDER study

ABSTRACT: Advanced systemic mastocytosis (AdvSM), a clonal hematologic neoplasm driven predominantly by D816V-mutant KIT, is often characterized by organ damage. Associated hematologic neoplasms (AHNs; usually myeloid) are often present, leading to poor survival. We report on the oral, highly selective, potent KIT D816V inhibitor avapritinib (200 mg once daily, starting dose) with >4 years follow-up from the fully enrolled PATHFINDER study. End points included overall response rate (ORR; primary), duration of response (DOR), progression-free survival (PFS), overall survival (OS), changes in objective biomarkers of disease, and safety (all secondary). Of 107 patients with AdvSM (including 71 [66%] with SM-AHN; overall population median follow-up, 49 months), 83 were response-evaluable. ORR was 73% (95% confidence interval, 63-83). Median DOR was 58 months; PFS, 51 months, and OS, 62 months. Disease progression occurred in 21 of 107 patients (20%), predominantly in SM-AHN and largely driven by the AHN. Reductions in objective biomarkers of disease were observed. Most frequent (≥30% patients) treatment-emergent adverse events (TEAEs; any grade; grade ≥3) were thrombocytopenia (58%; 31%); periorbital edema (57%; 6%), anemia (54%; 33%), peripheral edema (48%; 2%), and diarrhea (36%; 5%). Adverse events of special interest were cognitive effects (34%; 8%) and intracranial bleeds (4%; 2%). Eleven (10%) patients experienced TEAEs leading to death, of which 1 was deemed related to avapritinib by the principal investigator. With 4-year follow-up, patients with AdvSM treated with avapritinib experienced deep and durable responses and a favorable benefit-risk profile. This trial was registered at www.ClinicalTrials.gov as #NCT03580655.