Activating mutations of the protein tyrosine kinase (PTK) FLT3 can be found in approximately 30% of patients with acute myeloid leukemia (AML) thereby representing the most frequent genetic alterations in AML. Activating internal tandem duplication (ITD) mutations in the FLT3 juxtamembrane domain have been shown to be an independent negative prognostic factor for survival. BAY 43-9006 is a novel dual action RAF kinase and VEGFR inhibitor that targets tumor cell proliferation and tumor angiogenesis. We show that BAY 43-9006 potently inhibits FLT3 wild type (FLT3-WT) and FLT3-ITD enzymatic activity in cell free assays. BAY 43-9006 at single digit nanomolar concentrations inhibited the proliferation of MV4;11 and EOL-1 leukemic cells that express FLT3-ITD in vitro, whereas the growth of the FLT-3 independent RS4-11 leukemic cell line was only weakly inhibited (IC50 = 12 μM). Cell cycle arrest and induction of apoptosis were observed upon treatment with BAY 43-9006 in MV4;11 and EOL-1 cells. In HEK293 cells stably transfected with FLT3-WT or FLT3-ITD, BAY 43-9006 at nanomolar concentrations (5-50 nM) blocked basal and ligand (FL) dependent FLT3-mediated tyrosine autophosphorylation as well as ERK1/2 and Stat5 transphosphorylation. Inhibition of FLT3-mediated signaling was also observed in MV4;11 cells treated with nanomolar doses of BAY 43-9006. The anti-tumor efficacy of BAY 43-9006 was evaluated against the MV4;11 leukemia grown subcutaneously in NCr nu/nu mice. Doses of 3 and 10 mg/kg administered orally for 14 days induced complete regression of 60% and 90% of MV4;11 tumors, respectively. Modeling of the binding of BAY 43-9006 to FLT3 showed that it interacts with FLT3 in a similar binding mode as reported for BRAF. The demonstration that BAY 43-9006 exhibits potent target inhibition and efficacy in FLT3-ITD models suggests that this compound may have biologic activity in AML patients with FLT3 mutations.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]