Acyclic nucleotide phosphonates (adefovir, cidofovir, and tenofovir) are eliminated predominantly into the urine, and renal failure is their dose-limiting toxicity, particularly for adefovir and cidofovir. In this study, we examined the involvement of multidrug resistance-associated protein (MRP)4 (ABCC4) in their luminal efflux in the kidney. ATP-dependent uptake of adefovir and tenofovir but not cidofovir was observed only in the membrane vesicles expressing MRP4. The ATP-dependent uptake of adefovir and tenofovir by MRP4 was not saturated at 1 mM. The ATP-dependent uptake of adefovir by membrane vesicles expressing MRP4 was osmotic-sensitive. No ATP-dependent uptake of either agent was observed in the membrane vesicles expressing human MRP2 or breast cancer resistance protein. These nucleotide analogs were given to mice by constant intravenous infusion, and the plasma, urine, and tissue concentrations were determined. The kidney accumulation of adefovir and tenofovir was significantly greater in Mrp4 knockout mice (130 versus 66 and 191 versus 87 pmol/g tissue, respectively); thus, the renal luminal efflux clearance was estimated to be 37 and 46%, respectively, of the control. There was no difference in the fraction of mono-and diphosphorylated forms of adefovir in the kidney between wild-type and Mrp4 knockout mice. In mice, cidofovir was also eliminated via the urine by tubular secretion as well as glomerular filtration. There was no change in the kinetic parameters of cidofovir in Mrp4 knockout mice. Our results suggest that MRP4 is involved in the luminal efflux of both adefovir and tenofovir, but it makes only a limited contribution to the urinary excretion of cidofovir.Acyclic nucleotide phosphonates adefovir, cidofovir, and tenofovir (Fig.

A clinical drug-drug interaction between famotidine (a H 2 receptor antagonist) and probenecid has not been reproduced in rats. The present study hypothesized that the species-dependent probenecid sensitivity is due to a species difference in the contribution of renal organic anion and cation transporters. The transport activities of the H 2 receptor antagonists (cimetidine, famotidine, and ranitidine) by rat and human basolateral organic anion and cation transporters [human organic anion transporter (hOAT) 1, hOAT2, r/hOAT3, rat organic cation transporter (rOct) 1, and r/hOCT2] were compared using their cDNA transfectants. The transport activities (V max /K m ) of famotidine (K m , 345 M) by rOat3 were 8-and 15-fold lower than those of cimetidine (K m , 91 M) and ranitidine (K m , 155 M), respectively, whereas the activity by hOAT3 (K m , 124 M) was 3-fold lower than that of cimetidine (K m , 149 M) but similar to that of ranitidine (K m , 234 M). Comparison of the relative transport activity with regard to that of cimetidine suggests that famotidine was more efficiently transported by hOAT3 than rOat3, and vice versa, for ranitidine. Only ranitidine was efficiently transported by hOAT2 (K m , 396 M). rOct1 accepts all of the H 2 receptor antagonists with a similar activity, whereas the transport activities of ranitidine and famotidine (K m , 61/56 M) by r/hOCT2 were markedly lower than that of cimetidine (K m , 69/73 M). Probenecid was a potent inhibitor of r/OAT3 (K i , 2.6 -5.8 M), whereas it did not interact with OCTs. These results suggest that, in addition to the absence of OCT1 in human kidney, a species difference in the transport activity by hOAT3 and rOat3 accounts, at least in part, for the species difference in the drug-drug interaction between famotidine and probenecid.The kidney plays important roles in the detoxification of xenobiotics and endogenous wastes as well as maintaining the correct balance of ions and nutrients in the body. Urinary excretion is the major detoxification mechanism in the kidney, and this is governed by glomerular filtration, tubular secretion across the proximal tubules, and reabsorption. The renal uptake of organic anions and cations on the basolateral membrane of the proximal tubules has been characterized by multispecific organic anion and cation transporters (OAT/ SLC22 and OCT/SLC22), respectively (Lee and Kim, 2004;Wright and Dantzler, 2004;Shitara et al., 2005).Molecular cloning of basolateral transporters from different species allows examination of differences in their substrate specificities and transport activities, leading to a better understanding of the molecular mechanisms of species differences in drug disposition. For OCTs, the isoform expressed in the kidney differs between rodents and humans. Both Oct1 (Slc22a1) and Oct2 (Slc22a2) are involved in the renal uptake of organic cations on the basolateral membrane of the proximal tubules in rodents, whereas OCT2 is the predominant isoform in the human kidney (Koepsell, 2004;Lee and Kim, 2004;Wright a...

Edoxaban (the free base of DU-176b), an oral direct factor Xa inhibitor, is mainly excreted unchanged into urine and feces. Because active membrane transport processes such as active renal secretion, biliary excretion, and/or intestinal secretion, and the incomplete absorption of edoxaban after oral administration have been observed, the involvement of drug transporters in the disposition of edoxaban was investigated. Using a bidirectional transport assay in human colon adenocarcinoma Caco-2 cell monolayers, we observed the vectorial transport of [ 14 C]edoxaban, which was completely inhibited by verapamil, a strong P-glycoprotein (P-gp) inhibitor. In an in vivo study, an increased distribution of edoxaban to the brain was observed in Mdr1a/1b knockout mice when compared with wild-type mice, indicating that edoxaban is a substrate for P-gp.However, there have been no observations of significant transport of edoxaban by renal or hepatic uptake transporters, organic anion transporter (OAT)1, OAT3, organic cation transporter (OCT)2, or organic anion transporting polypeptide (OATP)1B1. Edoxaban exhibited no remarkable inhibition of OAT1, OAT3, OCT1, OCT2, OATP1B1, OATP1B3, or P-gp up to 30 mM; therefore, the risk of clinical drug-drug interactions due to any edoxaban-related transporter inhibition seems to be negligible. Our results demonstrate that edoxaban is a substrate of P-gp but not of other major uptake transporters tested. Because metabolism is a minor contributor to the total clearance of edoxaban and strong P-gp inhibitors clearly impact edoxaban transport, the P-gp transport system is a key factor for edoxaban's disposition.