AC220 Is a Uniquely Potent and Selective Second-Generation FLT3 Inhibitor

Activating mutations in the receptor tyrosine kinase FLT3 are present in up to approximately 30 % of acute myeloid leukemia (AML) patients, implicating FLT3 as a potential target for kinase inhibitor therapy. While several compounds have been evaluated in the clinic as FLT3 inhibitors, including CEP-701 (lestaurtinib), PKC-412 (midostaurin), MLN-518 (tandutinib) and most recently sorafenib, none of these was originally developed as a FLT3 inhibitor. We report here the characterization of AC220, a novel compound that has been expressly optimized as a selective FLT3 inhibitor. We demonstrate that this second generation FLT3 inhibitor has a unique combination of high potency, exceptional selectivity, bioavailability, and pharmacokinetic properties compatible with once a day oral dosing. In an in vitro binding assay AC220 interacted with FLT3 with high affinity (K_d = 1.6 nM). In the FLT3 dependent human leukemia cell line MV4-11, which harbors a homozygous activating FLT3 internal tandem duplication (ITD) mutation, AC220 inhibited FLT3 autophosphorylation and cellular proliferation with subnanomolar potency. Inhibition of proliferation of a FLT3 independent cell line was several hundred-fold less potent, demonstrating cellular selectivity for FLT3. A biochemical screen against a panel of 402 kinase assays representing almost 80 % of human protein kinases revealed a highly focused and selective interaction pattern. The only targets with affinity for AC220 within 10-fold that for FLT3 were closely related class III receptor tyrosine kinases (KIT, PDGFR, RET, CSF1R), and the only targets with affinity within 100-fold that for FLT3 were four additional receptor tyrosine kinases (FLT1, FLT4, DDR1, VEGFR2). When orally administered to mice at a dose of 10 mg/kg, AC220 achieved a peak plasma concentration (C_max) of 3.8 μM (2,100 ng/mL) within two hours of dosing. When corrected for plasma protein binding, the concentration of AC220 in plasma remained above the cellular IC₅₀ for FLT3 inhibition 24 hours after dosing. Total exposure (AUC_{0-24 h}) as well as C_max increased proportionally with the administered dose from 0.1 to approximately 30 mg/kg. At higher doses, both C_max and AUC_{0-24 h} continued to increase, approaching a plateau above 100 mg/kg. In a FLT3-ITD-dependent MV4-11 tumor xenograft model, AC220 showed substantial, dose dependent efficacy when dosed at 1, 3 and 10 mg/kg orally once a day for 28 days. Tumors regressed at 3 and 10 mg/kg, and remained static at 1 mg/kg. In a follow-on study at the 10 mg/kg oral dose, tumor size was monitored for an additional 60 days after dosing was discontinued. By the end of the study eight complete responses and two partial responses were observed in the ten animals treated with AC220. AC220 also had activity in a leukemia tumor model at doses as low as 1 mg/kg given orally once a day. A direct comparison of AC220 with the first generation FLT3 inhibitors CEP-701, MLN-518, PKC-412, sorafenib and sunitinib revealed that the combination of potency, selectivity and pharmacokinetic properties is unique to AC220. AC220 is a second generation FLT3 inhibitor that has been explicitly optimized for the combination of properties believed to be required for the successful treatment of FLT3-dependent AML, and specifically to test the hypothesis that selective FLT3 inhibition will result in clinical benefit. AC220 is currently being evaluated in a phase I clinical trial in relapsed or refractory AML patients.