The organic cation transporter 2 (OCT2) provides an important pathway for the uptake of cationic compounds in the kidney, which is the essential step in their elimination from the organism. Although many drugs have been identified which interact with human OCT2, structural elements required for an interaction with OCT2 are not well defined. To address this issue, HEK293 cells stably expressing human OCT2 were generated. IC(50) values of commonly used drugs for inhibition of [(3)H]MPP(+) uptake were determined and correlated with physicochemical descriptors. We found only a significant correlation between the topological polar surface area (TPSA) and IC(50) values (r = 0.71, p < 0.0001). Structural alignment of most potent inhibitor drugs of OCT2-mediated MPP(+) uptake was used to construct a two-point pharmacophore consisting of an ion-pair interaction site and a hydrophobic aromatic site separated by 5.0 A. Taken together, our data identify structural determinants for inhibitor interactions with OCT2.

ABSTRACT:The organic cation transporter 2 (OCT2, SLC22A2) plays an important role for renal drug elimination. Recent clinical studies indicate an impact of the frequent nonsynonymous c.808G>T (p.270Ala>Ser) polymorphism on renal clearance of metformin and the extent of the metformin-cimetidine interaction. The role of this polymorphism for renal disposition of endogenous compounds and drugs other than metformin has not been investigated. In addition, it is unclear whether the observed genotype dependence of an OCT2-mediated drug-drug interaction might occur also with other OCT inhibitors. To address these issues, we generated human embryonic kidney cells stably expressing wild-type OCT2 or the p.270Ala>Ser variant. No differences in protein expression levels and membrane incorporation pattern were observed between the two cell lines. The p.270Ala>Ser variant significantly impaired uptake kinetics of 1-methyl-4-phenylpyridinium, dopamine, norepinephrine, and propranolol. V max values were significantly reduced for uptake of all four compounds mediated by the p.270Ala>Ser variant compared with wild-type OCT2. In addition, a significant difference in the affinity to wild-type and mutant OCT2 was observed for dopamine (K m dopamine: 932 ؎ 77 versus 1285 ؎ 132 M). Moreover, out of a set of 27 compounds p.270Ala>Ser OCT2 was significantly less sensitive to inhibition by cimetidine, flurazepam, metformin, mexiletine, propranolol, and verapamil than wild-type OCT2 (e.g., for propranolol: IC 50 wild type versus p.270Ala>Ser 189 versus 895 M, P < 0.001). Our results indicate that the common OCT2 c.808G>T single nucleotide polymorphism significantly alters uptake of endogenous compounds and drugs. Moreover, for selected compounds the extent of OCT2-mediated drug interactions could depend on OCT2 c.808G>T genotype.

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Adenosine triphosphate-binding cassette (ABC) transporters are involved in energy-dependent transport of substrates across biological membranes. We hypothesized that their expression is altered during human heart failure, suggesting a pathophysiologic basis. Messenger ribonucleic acid quantification of all known ABC transporters revealed multiple alterations in ABC transporter expression in failing human hearts (New York Heart Association classification III-IV) compared to nonfailing controls. These include a loss of ABCC7 chloride channels and an increased expression of the K(ATP) channel regulatory subunits ABCC8. Moreover, ABCG2, an efflux pump for xenobiotics/drugs, was expressed at much higher levels in failing hearts compared to nonfailing control hearts. ABCG2 was found in cardiac capillary endothelial cells and cardiomyocytes. Experiments in cells stably transfected with human ABCG2 revealed that the peroxisome proliferator-activated receptor-gamma agonist rosiglitazone was transported by ABCG2 but also inhibited the export of the prototypical ABCG2 substrate pheophorbide A (IC(50) 25 microM). These results suggest that altered ABC transporter expression in failing hearts might contribute to impaired channel conductance or might affect the cardiac disposition of drugs.

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