Flt3 Inhibitors: Recent Advances and Problems for Clinical Application

FLT3, a type III receptor tyrosine kinase, expresses on most acute leukemia cells as well as normal hematopoietic stem/progenitor cells. Mutation in the FLT3 gene is the most frequent genetic alteration in acute myeloid leukemia (AML) and is well known as an important driver mutation for the development of myeloid malignancies. FLT3 mutation is a strong poor prognostic factor for the long-term survival in AML patients, while neither high-dose chemotherapy nor allogeneic hematopoietic stem cell transplantation can overcome a poor prognosis. Development of an FLT3 inhibitor is, therefore, much awaited. To date, several potent FLT3 inhibitors have been developed and some of them were evaluated for efficacy in clinical trials, although no FLT3 inhibitor has been yet approved. Moreover, several problems for clinical use, such as adverse effects, blood concentration and resistance have been apparent. Recently developed AC220 is a highly selective and sensitive FLT3 inhibitor. In Phase I and II trials, AC220 so far showed the best efficacy of AML cells harboring FLT3 mutation among clinically evaluated FLT3 inhibitors, while severe bone marrow suppression and QTc prolongation should be resolved for the clinical use. In this review, I summarize the characteristics of FLT3 inhibitors in clinical development and discuss important issues to be resolved for clinical use.