Highlights

A novel FLT3 and FLT3/D835Y inhibitor, compound 36 was discovered through the optimization of indirubin N-oxime derivatives.

Compound 36 showed potent FLT3 and FLT3/D835Y potency with IC₅₀ values of 0.87 nM and 0.32 nM, respectively.

In the docking study, 36 was interacted in the ATP binding site of FLT3 active conformations as a Type 1 kinase inhibitor.

36 showed potent anti-AML efficacy at MV4;11 and FLT3/D835Y expressed MOLM14 cells (IC₅₀ = 1.0 and 1.87 nM, respectively).

In the xenograft animal model, 36 completely suppressed the tumor progression by oral administration of 20 mg/kg for 21 days.

Abstract

FMS-like receptor tyrosine kinase-3 (FLT3) is expressed on acute leukemia cells and is implicated in the survival, proliferation and differentiation of hematopoietic cells in most acute myeloid leukemia (AML) patients. Despite recent achievements in the development of FLT3-targeted small-molecule drugs, there are still unmet medical needs related to kinase selectivity and the progression of some mutant forms of FLT3. Herein, we describe the discovery of novel orally available type 1 FLT3 inhibitors from structure-activity relationship (SAR) studies for the optimization of indirubin derivatives with biological and pharmacokinetic profiles as potential therapeutic agents for AML. The SAR exploration provided important structural insights into the key substituents for potent inhibitory activities of FLT3 and in MV4-11 cells. The profile of the most optimized inhibitor (36) showed IC₅₀ values of 0.87 and 0.32 nM against FLT3 and FLT3/D835Y, respectively, along with potent inhibition against MV4-11 and FLT3/D835Y expressed MOLM14 cells with a GI₅₀ value of 1.0 and 1.87 nM, respectively. With the high oral bioavailability of 42.6%, compound 36 displayed significant in vivo antitumor activity by oral administration of 20 mg/kg once daily dosing schedule for 21 days in a mouse xenograft model. The molecular docking study of 36 in the homology model of the DFG-in conformation of FLT3 resulted in a reasonable binding mode in type 1 kinases similar to the reported type 1 FLT3 inhibitors Crenolanib and Gilteritinib.