Efficacy and Safety of Avapritinib in Treating Unresectable or Metastatic Gastrointestinal Stromal Tumors: A Phase I/II, Open-Label, Multicenter Study

BACKGROUND: Avapritinib is a type 1 kinase inhibitor designed to potently and selectively inhibit oncogenic KIT/PDGFRA mutants by targeting the kinase active conformation. This multicenter, single-arm, open-label, phase I/II bridging study of NAVIGATOR in Chinese patients evaluated the safety and the antineoplastic activity of avapritinib in Chinese patients with unresectable/metastatic gastrointestinal stromal tumors (GIST). METHODS: Phase I comprised dose escalation for safety and phase II dose determination. Phase II comprised dose expansion for safety/efficacy evaluations in patients with PDGFRA D842V mutations or patients having received at least 3 lines of therapy without PDGFRA D842V mutations. The primary endpoints were recommended phase II dose, safety, and Independent Radiology Review Committee (IRRC)-assessed objective response rate (ORR). RESULTS: No dose-limiting toxicities occurred (n = 10); the recommended phase II dose was avapritinib 300 mg once daily orally. Fifty-nine patients initially received avapritinib 300 mg. Common grade ≥3 treatment-related adverse events were anemia, decreased white blood cell count, increased blood bilirubin levels, and decreased neutrophil count. In patients with PDGFRA D842V mutations, IRRC- and investigator-assessed ORRs were 75% and 79%, respectively; clinical benefit rates were both 86%. Median duration of response/progression-free survival were not reached. IRCC- and investigator-assessed ORRs in patients in the fourth- or later-line setting were 22% and 35%, respectively. Median progression-free survivals were 5.6 months for both. Overall survival data were immature and not calculated. CONCLUSION: Avapritinib was generally well tolerated and showed marked anti-tumor activity in Chinese patients with GIST bearing PDGFRA D842V mutations and notable efficacy as fourth- or later-line monotherapy (ClinicalTrials.gov Identifier: NCT04254939).