Although the patients initially well responded to FLT3 inhibitors such as gilteritinib, the most cases relapsed within a few months after the initiation of treatment...OTS447 possesses cytotoxic activity induced by inhibition of FLT3 signaling pathway and has anti-tumor activity in mouse xenograft model. We are pursuing further optimization of the inhibitor aiming for the effective treatment of AML patients with several types of FLT3-activating mutations.