In vitro methods to support transporter evaluation in drug discovery and development

This white paper addresses current approaches and knowledge gaps concerning methods to assess the role of transport proteins in drug/metabolite disposition in humans. The discussion focuses on in vitro tools to address key questions in drug development, including vesicle- and cell-based systems. How these methods can be used to assess the liability of compounds for transporter-based drug-drug interactions (DDIs) in vivo is also explored. Existing challenges and approaches to examine the involvement of transporters in drug disposition are discussed

Requirement of the N-Terminal Extension for Vacuolar Trafficking and Transport Activity of Yeast Ycf1p, an ATP-binding Cassette Transporter

Ycf1p is the prototypical member of the yeast multidrug resistance-associated protein (MRP) subfamily of ATP-binding cassette (ABC) transporters. Ycf1p resides in the vacuolar membrane and mediates glutathione-dependent transport processes that result in resistance to cadmium and other xenobiotics. A feature common to many MRP proteins that distinguishes them from other ABC transporters is the presence of a hydrophobic N-terminal extension (NTE), whose function is not clearly established. The NTE contains a membrane spanning domain (MSD0) with five transmembrane spans and a cytosolic linker region (L0). The goal of this study was to determine the functional significance of the NTE of Ycf1p by examining the localization and functional properties of Ycf1p partial molecules, expressed either singly or together. We show that MSD0 plays a critical role in the vacuolar membrane trafficking of Ycf1p, whereas L0 is dispensable for localization. On the other hand, L0 is required for transport function, as determined by monitoring cadmium resistance. We also examine an unusual aspect of Ycf1p biology, namely, the posttranslational proteolytic processing that occurs within a lumenal loop of Ycf1p. Processing is shown to be Pep4p dependent and thus serves as a convenient marker for proper vacuolar localization. The processed fragments associate with each other, suggesting that these natural cleavage products contribute together to Ycf1p function