TY - JOUR T1 - Lysosomal Sequestration Determines Intracellular Imatinib Levels JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.114.097451 SP - mol.114.097451 AU - Herman Burger AU - Xander den Dekker AU - Sandra Segeletz AU - Antonius WM Boersma AU - Peter de Bruijn AU - Maria Debiec-Rychter AU - Takahiro Taguchi AU - Stefan Sleijfer AU - Alex Sparreboom AU - Ron HJ Mathijssen AU - Erik AC Wiemer Y1 - 2015/06/24 UR - http://molpharm.aspetjournals.org/content/early/2015/06/24/mol.114.097451.abstract N2 - The intracellular uptake and retention (IUR) of imatinib is reported to be controlled by the influx transporter SLC22A1 (OCT1). We recently hypothesized that alternative uptake and/or retention mechanisms exist that determine intracellular imatinib levels. Here we systematically investigate the nature of these mechanisms. Imatinib uptake in cells was quantitatively determined by LC-MS-MS. Fluorescent microscopy was used to establish subcellular localization of imatinib. Immunoblotting, cell cycle analyses and apoptosis assays were done to evaluate functional consequences of imatinib sequestration. Uptake experiments revealed high intracellular imatinib concentrations in HEK293, the leukemic cell lines K562, SD-1, and a gastrointestinal stromal tumor cell line GIST-T1. We demonstrated that imatinib IUR is time, dose, temperature and energy dependent and provide evidence that SLC22A1 and other potential imatinib transporters do not substantially contribute to the IUR of imatinib. Prazosin, amantadine, NH4Cl and the V-ATPase inhibitor bafilomycin A1 significantly decreased the IUR of imatinib and likely interfere with lysosomal retention and accumulation of imatinib. Co-staining experiments with Lysotracker Red confirmed lysosomal sequestration of imatinib. Inhibition of the lysosomal sequestration had no effect on the inhibition of c-Kit signaling and imatinib mediated cell cycle arrest but significantly increased apoptosis in imatinib sensitive GIST-T1 cells. We conclude that intracellular imatinib levels are primarily determined by lysosomal sequestration and do not depend on SLC22A1 expression. ER -