Influence of Molecular Structure on Substrate Binding to the Human Organic Cation Transporter, hOCT1

Abstract

Organic cation transporters play a critical role in the elimination of therapeutic compounds in the liver and the kidney. We used computational quantitative structure activity approaches to predict molecular features that influence interaction with the human ortholog of the organic cation transporter (hOCT1). [ 3 H]tetraethylammonium uptake in HeLa cells stably expressing hOCT1 was inhibited to varying extents by a diverse set of 30 molecules. A subset of 22 of these was used to produce, using Catalyst, a pharmacophore that consisted of three hydrophobic features and a positive ionizable feature. The correlation coefficient of observed versus predicted IC 50 was 0.86 for this training set, which was superior to calculated logP alone (r ϭ 0.73) as a predictor of hOCT1 inhibition. A descriptor-based quantitative structure-activity relationship study using Cerius 2 resulted in an equation relating five molecular descriptors to log IC 50 with a correlation coefficient of 0.95. Furthermore, a group of phenylpyridinium and quinolinium compounds were used to investigate the spatial limitations of the hOCT1 binding site. The affinity for hOCT was higher for 4-phenylpyridiniums Ͼ 3-phenylpyridiniums Ͼ quinolinium, indicating that substrate affinity was influenced by the distribution of hydrophobic mass. In addition, supraplanar hydrophobic mass was found to increase the affinity for binding hOCT1. These results indicate how a combination of computational and in vitro approaches may yield insight into the binding affinity of transporters and may be applicable to predicting these properties for new therapeutics.The transepithelial transport of organic cations (OCs) plays an important role in the excretion of xenobiotic compounds from the body by means of the liver and kidney, and from the cerebrospinal fluid via the choroid plexus (Pritchard and Miller, 1993). The proteins involved in the translocation of OCs transport a broad range of substrates. These substrates include naturally occurring plant alkaloids and synthetic drugs such as cimetidine and procainamide. These chemicals are not only therapeutically diverse but show remarkable structural diversity as well. A characterization of the structural parameters of substrates translocated by organic cation transporters may provide insight into the molecular determinants of substrate specificity (Ullrich, 1999). Because the elimination of many therapeutic drugs is significantly influenced by the interaction of these compounds with OC transporters, information pertinent to predicting the kinetics of such interactions may be useful in estimating the pharmacokinetics of a broad range of pharmaceuticals.The organic cation transporter,

Determination and modelling of stereoselective interactions of ligands with drug transporters: A key dimension in the understanding of drug disposition

“…It is of interest to note that the model developed in this latter study is similar to the previously reported pharmacophore which contained three hydrophobic sites and a positive ionizable site where the calculated distances between the positive ion site and the three hydrophobic sites were 4.2, 5.1 and 5.3 Å (Bednarczyk et al 2003). If the three hydrophobic sites postulated in the earlier work are considered to be a single hydrophobic area, then the major differences between the two models are the added hydrogen bonding areas.…”

“…The identification and characterization of interactions with the hOCT1 has been primarily accomplished using cellular uptake and trans -stimulation studies (Zhang et al 1999; Bednarczyk et al 2003), which are complicated and variable. The development of a CMAC(hOCT1) column has recently been reported as an alternative approach to the determination of hOCT1 binding interactions (Moaddel et al 2005b).…”

“…However, the stereochemical aspects of substrate–transporter and inhibitor–transporter interactions were not addressed in these papers. Indeed, chiral compounds were used in the development of the Pgp models (Ekins et al 2002a, 2002b) and hOCT1 (Bednarczyk et al 2003) but the chirality of the compounds was ignored and/or racemic mixtures were used instead of the single enantiomer. The result is that the models were unable to provide an accurate three-dimensional view of the transporter and the binding interactions.…”

“…An hOCT1 pharmacophore model containing three hydrophobic pockets and one positive ionizable site has been recently reported (Bednarczyk et al 2003). The model was generated using a training set composed of structurally diverse organic cations but did not include sets of stereoisomers and, therefore, did not explore the stereochemical relationship between these sites.…”

“…Mapping of R - and S -propranolol to the human organic cation transporter pharmacophore reported by Bednarczyk et al (2003), where: (A) the mapping of R -propranolol, and (B) the mapping of S -propranolol. Reprinted from Moaddel et al (2007b).…”