HM43239 is a novel potent small molecule FLT3 inhibitor that selectively inhibits not only FLT3 wild type, ITD mutants or TKD mutations, but also FLT3 ITD/TKD double mutations. IC50s’ of HM43239 against FLT3 WT, FLT3 ITD and FLT3 D835Y kinases are 1.1 nM, 1.8 nM and 1.0 nM, respectively.