Efficacy and Synergy of Small Molecule Inhibitors Targeting FLT3-ITD(+) Acute Myeloid Leukemia

SIMPLE SUMMARY: FLT3-ITD mutations belong to the most frequent yet most detrimental genetic alterations in AML. Next-generation FLT3 inhibitors are potent therapeutics and often effective in AML patients carrying the FLT3-ITD driver kinase. However, AML cells are particularly quick in acquiring resistance to FLT3 kinase blockers. We aimed to identify novel therapeutic options for FLT3-ITD(+) AML, to investigate possible emerging resistance mechanisms to FLT3 inhibitors and to explore alternative targeting strategies. We applied a kinase-focused drug screen to find alternative therapeutics. We identified ispinesib, a kinesin spindle blocker, and kinase blockers WS6, ponatinib and cabozantinib, as very efficacious agents against FLT3-ITD(+) AML cells. Importantly, we identify the combination of cabozantinib and ispinesib as particularly potent against FLT3-ITD(+) AML. We suggest that a combinatorial treatment with these drugs could overcome resistance mechanisms and kill FLT3-ITD(+) AML blasts. ABSTRACT: Constitutive activation of FLT3 by ITD mutations is one of the most common genetic aberrations in AML, present in ~1/3 of cases. Patients harboring FLT3-ITD display worse clinical outcomes. The integration and advancement of FLT3 TKI in AML treatment provided significant therapeutic improvement. However, due to the emergence of resistance mechanisms, FLT3-ITD(+) AML remains a clinical challenge. We performed an unbiased drug screen to identify 18 compounds as particularly efficacious against FLT3-ITD(+) AML. Among these, we characterized two investigational compounds, WS6 and ispinesib, and two approved drugs, ponatinib and cabozantinib, in depth. We found that WS6, although not yet investigated in oncology, shows a similar mechanism and potency as ponatinib and cabozantinib. Interestingly, ispinesib and cabozantinib prevent activation of AXL, a key driver and mechanism of drug resistance in FLT3-ITD(+) AML patients. We further investigated synergies between the selected compounds and found that combination treatment with ispinesib and cabozantinib or ponatinib shows high synergy in FLT3-ITD(+) AML cell lines and patient samples. Together, we suggest WS6, ispinesib, ponatinib and cabozantinib as novel options for targeting FLT3-ITD(+) AML. Whether combinatorial tyrosine kinase and kinesin spindle blockade is effective in eradicating neoplastic (stem) cells in FLT3-ITD(+) AML remains to be determined in clinical trials.