Multiple Mechanisms of Ligand Interaction with the Human Organic Cation Transporter, OCT2

Abstract

OCT2 is the entry step for organic cation secretion by renal proximal tubules. Although many drugs inhibit OCT2 activity, neither the mechanistic basis of their inhibition nor their transport status is generally known. Using representatives of several structural classes of OCT2 inhibitory ligands described by Kido et al, we determined the kinetic basis of their inhibition of 1-methyl-4- phenylpyridinium (MPP) transport into CHO cells that stably expressed hOCT2. The 'Cluster II' inhibitors, metformin and cimetidine, interacted competitively with MPP. However, other Cluster II compounds, including tetraethylammonium (TEA), diphenidol and phenyltoloxamine, were mixed-type inhibitors of MPP transport. Cluster III representative, adrenosterone, and Cluster I representative, carvedilol, displayed noncompetitive inhibitory profiles. Competitive Counterflow (CCF) was used to determine if the inhibitory ligands served as substrates of hOCT2. Carvedilol and adrenosterone did not support CCF, consistent with the prediction that members of these structural classes are likely to be nontransported inhibitors of OCT2. Cluster II representatives MPP, metformin, cimetidine and TEA all supported CCF, consistent with independent assessments of their OCT2-mediated transport. Other Cluster II representatives, diphenidol and phenyltoloxamine, failed to support CCF, suggesting that neither compound is transported by OCT2. The results underscore the caution required for development of predictive models of ligand interaction with multidrug transporters.