Cabozantinib Is Well Tolerated in Acute Myeloid Leukemia and Effectively Inhibits the Resistance-Conferring FLT3/Tyrosine Kinase Domain/F691 Mutation

BACKGROUND: Cabozantinib, a tyrosine kinase inhibitor of FMS-like tyrosine kinase 3 (FLT3), MET, AXL, vascular endothelial growth factor receptor, and KIT, is approved for use in multiple malignancies. We assessed the safety and tolerability of cabozantinib in AML, given up-regulation of multiple relevant pathways. METHODS: Adults were eligible if they were 18 years old or older with relapsed/refractory AML or if they were 70 years old or older with newly diagnosed AML but were ineligible for conventional therapy. Cabozantinib was administered in 28-day cycles, and dose escalation occurred via cohorts. A pharmacodynamic evaluation of serial plasma samples via a plasma inhibitory assay (PIA) was used to assess FLT3-inhibitory activity in FLT3-mutant cell lines. RESULTS: Among 18 patients enrolled, 5 were found to harbor FLT3/ITD mutations. Sixteen patients (89%) had relapsed/refractory AML, and most were treated with 2 or more lines of prior treatment. No dose-limiting toxicities (DLTs) were detected at the first dose level (40mg daily), but 2 patients experienced DLTs at the next level (60mg daily). The remaining patients were then dosed at 40mg daily, the maximum tolerated dose (MTD). Additional grade 2 or higher toxicities, possibly/ probably related to cabozantinib, included fatigue, nausea, transaminitis, and electrolyte imbalance. No patients had a marrow response according to formal criteria, but 4 had peripheral blast reductions; 2 of these 4 patients transiently cleared circulating blasts. One patient experienced a reduction in marrow blasts, and 1 had stable disease. The FLT3-inhibitory activity of plasma samples, as assessed with the PIA, revealed potent and sustained inhibition in FLT3/ ITD and, notably, F691 tyrosine kinase domain (TKD)-mutant cells. CONCLUSIONS: Cabozantinib is well tolerated in AML patients at an MTD of 40mg daily and is a potent inhibitor of FLT3/ ITD-and F691 TKD-altered tyrosine kinases. (C) 2017 American Cancer Society.