Phase II Evaluation of the Tyrosine Kinase Inhibitor MLN518 in Patients with Acute Myeloid Leukemia (AML) Bearing a FLT3 Internal Tandem Duplication (ITD) Mutation.

MLN518 is a small molecule inhibitor of FLT3, PDGFR and c-Kit that is currently being evaluated as a therapy for AML. Previous phase I evaluation of MLN518 showed that it inhibits the phosphorylation of both wild-type and ITD-mutated FLT3 in patients’ leukemic blasts with an IC₉₀ in the range of 100–175 ng/mL. Anti-leukemic activity was also observed, with decreases in both peripheral and bone marrow blasts. Dose-limiting toxicity, consisting of reversible general muscular weakness and/or fatigue was associated with trough plasma MLN518 concentrations > 1000 ng/mL. We are now conducting a phase II study of MLN518 in patients with relapsed or refractory AML and in untreated patients with AML considered unfit for standard AML therapies. Eligibility requires demonstration of the FLT3 ITD mutation in the patient’s blasts. All patients are treated with MLN518 at an initial dose of 525 mg po bid, with provision for dose reduction if MLN518-associated weakness occurs. Twenty patients have been treated with MLN518 in this study, eighteen of whom are currently evaluable (2 patients have recently started therapy). Toxicities associated with MLN518 therapy have included weakness/fatigue, QTc prolongation (relationship to MLN518 uncertain), and nausea and vomiting. MLN518 plasma concentration-time data for the first fourteen patients demonstrates that all patients achieved steady-state trough plasma concentrations > 150 ng/mL. Both inter- and intra-subject variability (%CV) in trough steady-state concentrations were < 30%. Assessment of total and phosphorylated FLT3 in leukemic blasts isolated from peripheral blood was possible in 4 patients. Western blots from blasts obtained before and after MLN518 dosing demonstrated either partial or complete inhibition of FLT3 phosphorylation with MLN518 plasma concentrations > 130 ng/mL. Of the eighteen evaluable patients, response could not be assessed in three because intercurrent illness and/or MLN518-associated toxicity precluded adequate treatment with MLN518 (≥ 14 days). Seven patients experienced progressive AML without evidence of any anti-leukemic effect. Two patients had stable disease for ≥ 50 days and subsequently underwent bone marrow transplantation. Although no complete or partial remissions have been observed, 6 patients have demonstrated evidence of an anti-leukemic effect with decreases in both peripheral and bone marrow blasts of 1-3 months duration. In these 6 patients the mean decrease in the absolute peripheral blast count was 92%, with a range of 85–100%. The mean decrease in the bone marrow blast percentage was 62%, with a range of 44–94%. We conclude that MLN518 has anti-leukemic activity in FLT3 ITD-mutated AML and should be further evaluated as a component of remission-induction and/or maintenance therapy in this disease.