FLC-8, a novel covalent FLT3 inhibitor, targets Cys807 to suppress mutant AML growth

Targeting FLT3 is a validated strategy for Acute Myeloid Leukemia (AML), a severe blood cancer. However, current FLT3 inhibitors frequently face resistance due to mutations, limiting their long-term efficacy. This creates an urgent need for new therapeutic approaches that can overcome these resistance mechanisms. This research explores a previously unexploited nucleophilic residue, Cys807, within the FLT3 kinase domain, as a potential site for covalent inhibition to address the challenge of drug resistance in FLT3-mutant AML.

Researchers developed a series of 6-methylisoxazolo[5,4-b]pyridin-3-amines, identifying FLC-8 as a lead compound. They evaluated its potency against FLT3-WT and clinically relevant mutants (G697R, N676D) using IC50 assays. Covalent engagement of Cys807 was confirmed via mass spectrometry, peptide mapping, and C807S mutation analysis. The study assessed FLC-8's impact on STAT5, AKT, and ERK signaling, and apoptosis in AML cells. Kinome profiling determined selectivity. In vivo, FLC-8 was tested in MV4-11 xenograft models at 10-50 mg/kg to evaluate tumor growth inhibition and toxicity.

FLC-8 demonstrated potent inhibition of FLT3. It inhibited FLT3-WT with an IC50 of 10.2 nM, and clinically relevant mutants G697R and N676D with IC50 values of 11.6 nM and 24.1 nM, respectively. Covalent binding to Cys807 was definitively confirmed by mass spectrometry and peptide mapping, with activity loss upon C807S mutation. FLC-8 effectively suppressed FLT3-mediated STAT5, AKT, and ERK signaling pathways and induced apoptosis in AML cells, maintaining low-nanomolar potency over 72 h. Kinome profiling revealed a narrow inhibition spectrum, suggesting high selectivity. In vivo, FLC-8 significantly inhibited tumor growth:

FLC-8 inhibited MV4-11 xenograft growth with a Tumor Growth Inhibition (TGI) of 136-178% at 10-50 mg/kg without overt toxicity.