Subcellular localization of several structurally different tyrosine kinase inhibitors

Sa？etak Protein tyrosine kinases form an important target for a new class of anticancer drugs, the tyrosine kinase inhibitors (TKIs). Recently we demonstrated that sunitinib, an inhibitor of the membrane-associated vascular endothelial growth factor receptor (VEGFR), is trapped in lysosomes which isolates the drug from its intended target. Therefore we investigated whether this also holds for other TKIs, targeted against different protein kinases. For this purpose we used the ProteoExtractR kit, which enables a subcellular extraction separating cellular proteins into four distinct fractions covering the cytosol, membranes and membrane organelles (including lysosomes), nuclear proteins and the cytoskeleton. Since TKIs are 98-100 % protein bound we used this property to study their subcellular distribution and used Caco-2 cells as a model. As expected after 2 hours exposure sunitinib was trapped in cytosol (58 %) and organelles (42 % including lysosomes). Crizotinib, an inhibitor of ALK-EML4, showed a similar distribution. However, erlotinib, an inhibitor of the epidermal growth factor receptor (EGFR) showed a very low cellular accumulation and was limited to the organelle fraction. In contrast, the other EGFR inhibitor, gefitinib was predominantly located in the cytosolic (39 %) and membrane fraction (44 %). Sorafenib, another VEGFR inhibitor was predominantly located in the organelle fraction (85 %) and cytosol (15 %) after 2 hours, while after 24 hours distribution decreased (9.9 fold) with a slight shift. Dasatinib, an inhibitor of BCR-Abl was located only in the cytosol (100 %). In general localization after 24 hours was comparable, albeit several small changes were seen. In conclusion protein fractionation with the ProteoExtractR Subcellular Proteome Extraction kit demonstrated large differences in TKI levels in various cellular organelles, with a pattern in agreement with lysosomal accumulation of sunitinib