Abstract 947: The FLT3 inhibitor AC220 (quizartinib) inhibits drug transport by ABCG2 at pharmacologically relevant concentrations and sensitizes cells to ABCG2 substrate chemotherapy drugs.
Authors: Bhullar Jasjeet , Natarajan Karthika , Shukla Suneet , Ambudkar Suresh V. , Baer Maria R. 
Published: August 2014

Abstract
The receptor tyrosine kinase fms-like tyrosine kinase 3 (FLT3) is expressed on acute myeloid leukemia (AML) cells and is mutated, most commonly by internal tandem duplication (ITD), in 30% of cases, resulting in constitutive signaling and short disease-free survival following chemotherapy. FLT3 inhibitors have activity in AML with both FLT3-ITD and wild-type FLT3, but first-generation inhibitors have not had optimal potency, selectivity or pharmacokinetic properties. The second-generation bis-aryl urea FLT3 inhibitor AC220 inhibits FLT3-ITD and wild-type FLT3 at 0.1 and 0.5 μM, respectively, in vivo, has excellent kinase selectivity and pharmacokinetic properties, has shown very good tolerability and activity in single-agent phase I and II trials, and will be tested in combination with chemotherapy. We therefore sought to characterize interactions of AC220 with ATP-binding cassette proteins associated with drug resistance in AML, including ABCB1 (P-glycoprotein) and ABCG2 (breast cancer resistance protein). Uptake of fluorescent substrates and apoptosis were measured by flow cytometry, binding to ABCB1 and ABCG2 by effects on [125I]iodoarylazidoprazosin ([125I]-IAAP) photolabeling, and cell viability by the WST-1 colorimetric assay. AC220 (Selleck Chemicals, Houston, TX) inhibited transport of the fluorescent ABCG2 substrate pheophorbide A in ABCG2-overexpressing drug-selected 8226/MR20 and transfected K562/ABCG2 cells in a concentration-dependent manner, between 100 nM and 5 μM, and inhibited transport of the fluorescent ABCB1 substrate DiOC2(3) in ABCB1-overexpressing drug-selected HL60/VCR and transfected K562/ABCB1 cells in a concentration-dependent manner between 500 nM and 10 μM. AC220 inhibited [125I]-IAAP photolabeling of ABCG2 and ABCB1 with IC50 values of 0.27 μM and 3.3 μM, respectively. Co-incubation with AC220 at 0.1, 0.5 and 1 μM sensitized K562/ABCG2 cells to the ABCG2 substrate chemotherapy drug mitoxantrone 1.6-, 3.3- and 6- fold, respectively, in the cell viability assay. Finally, co-incubation with AC220 at concentrations of 0.5, 1, 5 and 10 μM increased apoptosis of K562/ABCG2 cells induced by fixed concentrations of the ABCG2 substrate chemotherapy drugs mitoxantrone and topotecan by 28, 45, 48 and 60% and 27, 61, 76, and 85%, respectively. We conclude that AC220 is a potent inhibitor of ABCG2-mediated drug transport at pharmacologically relevant concentrations. AC220 inhibition of ABCG2 should sensitize ABCG2-overexpressing AML cells to ABCG2 substrate chemotherapy drugs, but may also cause clinically significant pharmacokinetic interactions. These effects should be considered in the design and evaluation of regimens combining AC220 with chemotherapy drugs.
Citation Format: Jasjeet Bhullar, Karthika Natarajan, Suneet Shukla, Suresh V. Ambudkar, Maria R. Baer. The FLT3 inhibitor AC220 (quizartinib) inhibits drug transport by ABCG2 at pharmacologically relevant concentrations and sensitizes cells to ABCG2 substrate chemotherapy drugs. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 947. doi:10.1158/1538-7445.AM2013-947