FLT3-mutant allelic burden and clinical status are predictive of response to FLT3 inhibitors in AML

We examined 6 different FMS-like tyrosine kinase-3 (FLT3) inhibitors (lestaurtinib, midostaurin, AC220, KW-2449, sorafenib, and sunitinib) for potency against mutant and wild-type FLT3, as well as for cytotoxic effect against a series of primary blast samples obtained from patients with acute myeloid leukemia (AML) harboring internal tandem duplication (FLT3/ITD) mutations. We found that inhibition of FLT3 autophosphorylation in a FLT3/ITD specimen does not always induce cell death, suggesting that some FLT3/ITD AML may not be addicted to FLT3 signaling. Relapsed samples and samples with a high mutant allelic burden were more likely to be responsive to cytotoxicity from FLT3 inhibition compared with the samples obtained at diagnosis or those with a low mutant allelic burden. These FLT3 inhibitors varied to a considerable degree in their selectivity for FLT3, and this selectivity influenced the cytotoxic effect. These results have important implications for the potential therapeutic use of FLT3 inhibitors in that patients with newly diagnosed FLT3-mutant AML might be less likely to respond clinically to highly selective FLT3 inhibition. (Blood. 2010; 115: 1425-1432)