FLT3 inhibitors in acute myeloid leukemia

Design and Synthesis of New Sulfonamides-Based Flt3 Inhibitors

Background:

Flt3 is an oncogenic kinase involved in different leukemias. It is most prominently associated with acute myeloid leukemia (AML). Flt3-specific inhibitors have shown promising results in interfering with AML.

Methods:

The crystallographic structures of two inhibitors complexed within Flt3, namely, quizartinib and F6M, were used to guide the synthesis of new sulfonamide-based Flt3 inhibitors.

Results:

One of the prepared compounds showed low micromolar anti-Flt3 bioactivity, and interestingly, low micromolar bioactivity against the related oncogenic kinase VEGFR2.

Conclusion:

Sulfonamides were successfully used as privileged scaffolds for the synthesis of novel Flt3 inhibitors of micromolar potencies.

FLT3 activating mutations display differential sensitivity to multiple tyrosine kinase inhibitors

Fms-like tyrosine kinase-3 (FLT3) is a receptor tyrosine kinase that normally functions in hematopoietic cell survival, proliferation and differentiation. Constitutively activating mutations of FLT3 map predominately to the juxtamembrane domain (internal tandem duplications; ITD) or the activation loop (AL) of the kinase domain and are detected in about 1/3 of de novo acute myeloid leukemia (AML) patients. Small molecule tyrosine kinase inhibitors (TKI) effectively target FLT3/ITD mutations, but some activating mutations, particularly those on the AL, are relatively resistant to many FLT3 TKI. We reproduced many of the AL or other non-ITD activating mutations and tested 13 FLT3 TKI for their activity against these and wild-type FLT3. All 13 TKI tested inhibited BaF3/ITD cell proliferation in a concentration-dependent manner as reported, but most TKI exhibited a wide range of differential activity against AL and other point mutants. Western blotting results examining inhibition of FLT3 autophosphorylation and signaling pathways indicate that many AL mutations reduce TKI binding. Most FLT3 TKI effectively target wild-type FLT3 signaling. As a demonstration of this differential activity, treatment of BaF3 D835Y cells transplanted in BALB/c mice with sorafenib showed no effect in vivo against this mutant whereas lestaurtinib proved effective at reducing disease burden. Thus, while FLT3 TKI have been selected based on their ability to inhibit FLT3/ITD, the selection of appropriate TKI for AML patients with FLT3 AL and other activating point mutations requires personalized consideration.

Fluvastatin inhibits FLT3 glycosylation in human and murine cells and prolongs survival of mice with FLT3/ITD leukemia

FLT3 is frequently mutated in acute myeloid leukemia (AML), but resistance has limited the benefit of tyrosine kinase inhibitors (TKI). We demonstrate that statins can impair FLT3 glycosylation, thus leading to loss of surface expression and induction of cell death, as well as mitigation of TKI resistance. Immunofluorescence microscopy confirms a reduction in surface localization and an increase in intracellular FLT3/internal tandem duplication (ITD) accumulation. This aberrant localization was associated with increased STAT5 activation but inhibition of both MAPK and AKT phosphorylation. Growth inhibition studies indicate that FLT3/ITD-expressing cells were killed with an IC(50) within a range of 0.2-2μM fluvastatin. Several mechanisms of resistance could be circumvented by fluvastatin treatment. An increase in the IC(50) for inhibition of phosphorylated FLT3/ITD by lestaurtinib caused by exogenous FLT3 ligand, resistance to sorafenib caused by the D835Y or FLT3/ITD N676K mutations, and activation of the IL-3 compensatory pathway were all negated by fluvastatin treatment. Finally, fluvastatin treatment in vivo reduced engraftment of BaF3 FLT3/ITD cells in Balb/c mice. These results demonstrate that statins, a class of drugs already approved by the US Food and Drug Administration, might be repurposed for the management of FLT3 mutant acute myeloid leukemia cases either alone or in conjunction with FLT3 TKI.

Mutations of FLT3/ITD confer resistance to multiple tyrosine kinase inhibitors

FMS-like tyrosine kinase 3 (FLT3) normally functions in the survival/proliferation of hematopoietic stem/progenitor cells, but its constitutive activation by internal tandem duplication (ITD) mutations correlates with a poor prognosis in AML. The development of FLT3 tyrosine kinase inhibitors (TKI) is a promising strategy, but resistance that arises during the course of treatment caused by secondary mutations within the mutated gene itself poses a significant challenge. In an effort to predict FLT3 resistance mutations that might develop in patients, we used saturation mutagenesis of FLT3/ITD followed by selection of transfected cells in FLT3 TKI. We identified F621L, A627P, F691L and Y842C mutations in FLT3/ITD that confer varying levels of resistance to FLT3 TKI. Western blotting confirmed that some FLT3 TKI were ineffective at inhibiting FLT3 autophosphorylation and signaling through MAP kinase, STAT5 and AKT in some mutants. Balb/c mice transplanted with the FLT3/ITD Y842C mutation confirmed resistance to sorafenib in vivo but not to lestaurtinib. These results indicate a growing number of FLT3 mutations that are likely to be encountered in patients. Such knowledge, combined with known remaining sensitivity to other FLT3 TKI, will be important to establish as secondary drug treatments that can be substituted when these mutants are encountered.