Abstract 3619: Inhibition of FLT3 autophosphorylation and downstream signaling both in vitro and in vivo by AC220, a second generation potent and selective FLT3 inhibitor

Fms-like tyrosine kinase 3 (FLT3) is thought to be a major driver in the pathogenesis of acute myeloid leukemia (AML). FLT3-activating mutations are found in ∼ 30% of AML patients and are associated with poor outcome in this patient population. AC220 is a small molecule kinase inhibitor with potent and selective FLT3 inhibitory activity. Here we characterize the effect of AC220 on FLT3 autophosphorylation, activation of downstream signaling pathways (STAT5, ERK and AKT), cell cycle distribution, and apoptosis endpoints in multiple leukemia cell lines expressing either wild type FLT3 (FLT3-WT, SEM-K2 cells (FLT3-WT overexpression) and RS4;11 cells (FLT3-WT)) and/or ITD-mutated FLT3 (FLT3-ITD, MV4-11 (FLT3-ITD), MOLM-14 (FLT3-ITD/FLT3-WT)). Across each of these parameters we also compare the effects of AC220 to those of other known FLT3 inhibitors including, sorafenib, sunitinib, lestaurtinib, and midostaurin. AC220 inhibited FLT3 autophosphorylation in both FLT3-WT and FLT3-ITD cells with roughly equal potency, while other FLT3 inhibitors showed differential pFLT3 inhibition across several cell lines. Although AC220-mediated inhibition of FLT3 autophosphorylation was observed in all cell lines, subsequent cell death was only detected in the cell lines with activated FLT3. This contrasts with other FLT3 inhibitors, where potent inhibition of cell proliferation in the non-FLT3 activated RS4;11 cells was observed, highlighting the lack of FLT3 selectivity of these compounds. Consistent with FLT3 inhibition, AC220 also inhibited constitutively activated STAT5, ERK and AKT in these cell lines, suggesting that one or more of these pathways play a role in FLT3-mediated cell growth and survival. Furthermore, the inhibition of these downstream markers correlated with the inhibition of FLT3 across each of the FLT3 inhibitors tested. Lastly, in an MV4-11 murine xenograft model, AC220 administration resulted in the reduction of phosphorylated FLT3 and downstream signaling molecules concomitant with tumor reduction. Together, these data suggest that selective FLT3 inhibition is sufficient to inhibit the growth and induce cell death of FLT3-driven leukemia cells. AC220 is currently being evaluated in a phase II clinical trials in relapsed or refractory AML patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; C