Quizartinib Resistance Mutations and Treatment Outcomes in Relapsed or Refractory FLT3-ITD-Positive AML.

Quizartinib is a FMS-like tyrosine kinase 3 (FLT3) inhibitor indicated for FLT3 internal tandem duplication (FLT3-ITD)-positive acute myeloid leukemia (AML). We aimed to evaluate quizartinib resistance mechanisms, in addition to efficacy and safety outcomes, in patients with relapsed or refractory FLT3-ITD-positive AML. This multicenter, single-arm study in Japan (jRCTs071200015) enrolled 18 patients between May 2020 and December 2022. Of these, 15 patients received oral quizartinib (up to 53 mg once daily) for up to 12 cycles of 28 days each, then were followed for 12 months. The primary endpoint was to evaluate the type and rate of quizartinib resistance mutations; secondary endpoints included composite complete remission (CRc) rate, overall response rate (ORR), hematopoietic stem cell transplantation (HSCT) rate, relapse-free survival (RFS), overall survival (OS), and adverse events (AEs). Among seven evaluable patients, acquired mutations were detected in four patients (NF1 [R2616X], CSF3R [Q754X], NRAS [G13R], and FLT3 [D835Y] in one patient each), while loss of FLT3-ITD was observed in two patients. In efficacy analyses (n = 15), CRc rate was 66.7% (95% confidence interval [CI], 38.4-88.2), ORR was 73.3% (44.9-92.2), and median OS was 13.6 months (5.4-not evaluable). Three patients (20.0%) received HSCT directly after quizartinib; in these patients, median RFS was 8.5 months (95% CI, 6.2-not evaluable). Grade ≥ 3 non-hematologic AEs and grade 1 QT prolongation were each reported in three patients (20.0%). These data offer additional information on potential resistance mechanisms in patients with relapsed or refractory FLT3-ITD-positive AML. Trial Registration: Japan Registry of Clinical Trials (jRCTs071200015).