Targeting rapid <scp>TKI</scp> ‐induced <scp>AXL</scp> upregulation overcomes adaptive <scp>ERK</scp> reactivation and exerts antileukemic effects in <i>FLT3</i> / <scp>ITD</scp> acute myeloid leukemia

Acute myeloid leukemia (AML) patients with the FMS-related receptor tyrosine kinase 3 internal tandem duplication (FLT3/ITD) mutation have a poorer prognosis, and treatment with FLT3 tyrosine kinase inhibitors (TKIs) has been hindered by resistance mechanisms. One such mechanism is known as adaptive resistance, in which downstream signaling pathways are reactivated after initial inhibition. Past work has shown that FLT3/ITD cells undergo adaptive resistance through the reactivation of extracellular signal-regulated kinase (ERK) signaling within 24 h of sustained FLT3 inhibition. We investigated the mechanism(s) responsible for this ERK reactivation and hypothesized that targeting tyrosine-protein kinase receptor UFO (AXL), another receptor tyrosine kinase that has been implicated in cancer resistance, may overcome the adaptive ERK reactivation. Experiments revealed that AXL is upregulated and activated in FLT3/ITD cell lines mere hours after commencing TKI treatment. AXL inhibition combined with FLT3 inhibition to decrease the ERK signal rebound and to exert greater anti-leukemia effects than with either treatment alone. Finally, we observed that TKI-induced AXL upregulation occurs in patient samples, and combined inhibition of both AXL and FLT3 increased efficacy in our in vivo models. Taken together, these data suggest that AXL plays a role in adaptive resistance in FLT3/ITD AML and that combined AXL and FLT3 inhibition might improve FLT3/ITD AML patient outcomes.