Drug-drug interactions involving membrane transporters in the human kidney

Transport of the dipeptidyl peptidase-4 inhibitor sitagliptin by human organic anion transporter 3, organic anion transporting polypeptide 4C1, and multidrug resistance P-glycoprotein

Sitagliptin, a selective dipeptidyl peptidase 4 inhibitor recently approved for the treatment of type 2 diabetes, is excreted into the urine via active tubular secretion and glomerular filtration in humans. In this report, we demonstrate that sitagliptin is transported by human organic anion transporter hOAT3 (Km=162 microM), organic anion transporting polypeptide OATP4C1, and multidrug resistance (MDR) P-glycoprotein (Pgp), but not by human organic cation transporter 2 hOCT2, hOAT1, oligopeptide transporter hPEPT1, OATP2B1, and the multidrug resistance proteins MRP2 and MRP4. Our studies suggested that hOAT3, OATP4C1, and MDR1 Pgp might play a role in transporting sitagliptin into and out of renal proximal tubule cells, respectively. Sitagliptin did not inhibit hOAT1-mediated cidofovir uptake, but it showed weak inhibition of hOAT3-mediated cimetidine uptake (IC50=160 microM). hOAT3-mediated sitagliptin uptake was inhibited by probenecid, ibuprofen, furosemide, fenofibric acid, quinapril, indapamide, and cimetidine with IC50 values of 5.6, 3.7, 1.7, 2.2, 6.2, 11, and 79 microM, respectively. Sitagliptin did not inhibit Pgp-mediated transport of digoxin, verapamil, ritonavir, quinidine, and vinblastine. Cyclosporine A significantly inhibited Pgp-mediated transport of sitagliptin (IC50=1 microM). Our data indicate that sitagliptin is unlikely to be a perpetrator of drug-drug interactions with Pgp, hOAT1, or hOAT3 substrates at clinically relevant concentrations. Renal secretion of sitagliptin could be inhibited if coadministered with OAT3 inhibitors such as probenecid. However, the magnitude of interactions should be low, and the effects may not be clinically meaningful, due to the high safety margin of sitagliptin.

Membrane localization and pH-dependent transport of a newly cloned organic cation transporter (PMAT) in kidney cells

Plasma membrane monoamine transporter (PMAT) is a novel membrane transporter recently cloned and characterized in our laboratory. We previously demonstrated that PMAT functions as a polyspecific organic cation transporter and efficiently transports many organic cations such as monoamine neurotransmitters and 1-methyl-4-phenylpyridinium (MPP(+)). In this study, we explored the role of PMAT in the renal handling of organic cations. Using a polyclonal antibody generated toward the NH(2)-terminal 66 amino acid residues of human PMAT, we showed that the PMAT protein (approximately 55 kDa) is expressed in the human kidney and is primarily targeted to the apical membranes when expressed in polarized Madin-Darby canine kidney (MDCK) cells. Using MDCK cells stably expressing human PMAT, we showed that PMAT-mediated MPP(+) uptake is strongly dependent on extracellular pH. Lowering extracellular pH from 7.4 to 6.6 greatly stimulated PMAT-mediated MPP(+) uptake, whereas elevating extracellular pH to 8.2 abolished transporter activity. Kinetic analysis revealed that the apparent V(max) at pH 6.6 is about fourfold higher than that at pH 7.4, whereas the apparent K(m) values were not statistically different at these two conditions. Under acidic conditions (pH 6.6), the proton ionophore, carbonyl cyanide p-trifluormethoxyphenylhydrazone, drastically reduced PMAT-mediated MPP(+) uptake, suggesting that the stimulatory effect of proton may be due to transporter coupling with a proton gradient. Taken together, our data suggest that PMAT is expressed on the apical membranes of renal epithelial cells and may use luminal proton gradient to drive organic cation reabsorption in the kidney.