Abstract #1787: Characterization of the biochemical and cellular activity of the second-generation FLT3 inhibitor AC220

AC220 is a novel, second-generation FLT3 inhibitor with a unique combination of excellent potency, selectivity and pharmacokinetic properties compared to first-generation compounds. Activating mutations in FLT3 occur in approximately 30 % of patients with acute myeloid leukemia (AML). FLT3 furthermore is important for dendritic cell growth and differentiation, and therefore is an important target for kinase inhibitor therapy in AML and certain autoimmune diseases. AC220 has been explicitly optimized for the combination of properties believed to be required for the successful treatment of FLT3-dependent AML, in contrast to the first-generation inhibitors CEP-701 (lestaurtinib), PKC-412 (midostaurin), and MLN-518 (tandutinib), none of which were originally developed as a FLT3 inhibitor, as well as sunitinib and sorafenib, which have been explored as inhibitors of FLT3. AC220 is the most potent among FLT3 inhibitors tested in a biochemical binding assay (K d = 1.6 nM), as well as in cellular assays (IC 50 = 1.1 nM for autophosphorylation of FLT3 with an internal tandem duplication (ITD) mutation, 4.2 nM for wild type FLT3, and 0.56 nM for FLT3-dependent cellular proliferation). A biochemical KinomeScan screen against almost 80 % of human protein kinases, representing the most comprehensive evaluation of selectivity of any kinase inhibitor to date, revealed a highly selective interaction pattern. FLT3 was the highest affinity target identified. The only targets with affinity for AC220 within 10-fold of FLT3 were closely related class III receptor tyrosine kinases (RTKs) (KIT, PDGFR, RET, CSF1R), and the only targets with affinity within 100-fold were four additional RTKs (FLT1, FLT4, DDR1, VEGFR2). In cellular assays, AC220 inhibited the autophosphorylation of KIT (IC 50 = 15 nM in H526 cells), CSF1R (IC 50 = 145 nM in M-NFS-60 cells), RET (80 % inhibition at 1 µM in TT cells) and PDGFR (IC 50 = 145 nM in MG-63 cells) with lower potency than FLT3. These results confirm FLT3 as the primary and most potently inhibited target of AC220 in cells. They also demonstrate significant cellular activity of AC220 against KIT, CSF1R, RET and PDGFR. The pharmacokinetics of AC220 in animals demonstrate that free plasma concentrations sufficient for inhibition of FLT3, and possibly other class III RTKs, can be achieved. The maximum concentration of AC220 (C max ) was 3.8 µM after a single oral 10 mg/kg dose in mice, with dose proportional exposure from 0.1 to 100 mg/kg. AC220 is being evaluated in a phase I/II clinical trial in AML. Preliminary clinical results suggest concentrations of AC220 can be achieved at very well tolerated doses that may potentially be sufficient to inhibit some of the secondary targets. A phase I/II study is planned in GIST and melanoma, as well as phase II studies in AML. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 1787.