Tuspetinib (HM43239) hydrate is an orally active and selective FLT3 inhibitor with IC 50 s of 1.1 nM, 1.8 nM and 1.0 nM for FLT3 WT, FLT3 internal tandem duplication (ITD) and FLT3 D835Y kinases, respectively. Tuspetinib hydrate inhibits the kinase activity of FLT3 as a reversible type I inhibitor and modulates p-STAT5, p-ERK, SYK , JAK1/2, and TAK1. Tuspetinib hydrate inhibits the proliferation and induces the apoptosis of leukemic cells.