Purpose: The aim of this study was to evaluate the antileukemia activity of a novel FLT3 kinase inhibitor, FI-700. Experimental Design:The antileukemia activity of FI-700 was evaluated in human leukemia cell lines, mutant or wild-type (Wt)-FLT3^expressing mouse myeloid precursor cell line, 32D and primary acute myeloid leukemia cells, and in xenograft or syngeneic mouse leukemia models. Results: FI-700 showed a potent IC 50 value against FLT3 kinase at 20 nmol/L in an in vitro kinase assay.

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... 00 showed selective growth inhibition against mutant FLT3-expressing leukemia cell lines and primary acute myeloid leukemia cells, whereas it did not affect the FLT3 ligand (FL)d riven growth ofWt-FLT3^expressing cells.These antileukemia activities were induced by the significant dephosphorylations of mutant FLT3 and STAT5, which resultedin G 1 arrest of the cell cycle. Oral administration of FI-700 induced the regression of tumors in a s.c. tumor xenograft model and increased the survival of mice in an i.v. transplanted model. Furthermore, FI-700 treatment eradicated FLT3/ITD-expressing leukemia cells, both in the peripheral blood and in the bone marrow. In this experiment, the depletion of FLT3/ITD-expressing cells by FI-700 was more significant than that of Ara-C, whereas bone marrow suppression by FI-700 was lower than that byAra-C. Conclusions: FI-700 is a novel and potent FLT3 inhibitor with promising antileukemia activity.