Advanced systemic mastocytosis (AdvSM), a clonal hematologic neoplasm driven predominantly by D816V-mutant KIT, is often characterized by organ damage. Associated hematologic neoplasms (AHN; 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 (NCT03580655) study. Endpoints 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 95% CI, 63%-83%). Median DOR was 58 months, PFS 51 months, and OS 62 months. Disease progression occurred in 21/107 patients, 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, avapritinib-treated patients with AdvSM experienced deep and durable responses and a favorable benefit-risk profile.
Gotlib et al. (Wed,) studied this question.