Involvement of human organic cation transporter 1 in the hepatic uptake of 1-(2-methoxyethyl)-2-methyl-4,9-dioxo-3-(pyrazin-2-ylmethyl)-4,9-dihydro-1H-naphtho[2,3-d]imidazolium bromide (YM155 monobromide), a novel, small molecule survivin suppressant.

1-(2-Methoxyethyl)-2-methyl-4,9-dioxo-3-(pyrazin-2-ylmethyl)-4,9-dihydro-1H-naphtho[2,3-d]imidazolium bromide (YM155 monobromide), which is a hydrophilic and cationic compound, exhibits antitumor activity in experimental human hormone refractory prostate carcinoma models. Urinary excretion was 18.3 to 28.6% of the dose in the clinical phase I study, and nonrenal elimination may be explained by the biliary excretion of YM155 in its unchanged form. Because the penetration through the sinusoidal membrane of the hepatocytes is the first step and an important part of biliary excretion, we evaluated the uptake of [(14)C]YM155 into human cryopreserved hepatocytes. YM155 was taken up into hepatocytes in a temperature- and concentration-dependent manner. The saturable uptake component was much higher than the nonsaturable passive diffusion component. In vitro hepatic uptake clearance was consistent with the in vivo hepatic intrinsic clearance calculated using clinical study data. Hepatic uptake of YM155 was inhibited by organic cation transporter (OCT) inhibitors, and the IC(50) values for YM155 uptake were comparable to those reported for human OCT1-mediated transport. The interaction of YM155 with candidate transporter, OCT1, was also characterized using S2 cells stably expressing human OCT1 (OCT1-S2) cells. In OCT1-expressing S2 cells, YM155 inhibited the OCT1-mediated uptake of a typical OCT1 substrate, [(14)C]tetraethylammonium. In addition, YM155 was taken up into OCT1-S2 cells These results indicated that OCT1 was the predominant transporter for the hepatic uptake of YM155, and the transporter-mediated uptake clearance observed in vitro may account for the in vivo intrinsic hepatic clearance.

Human organic cation transporter 3 mediates the transport of antiarrhythmic drugs.

Antiarrhythmic drugs have been considered to be transported by the organic cation transport system. The purpose of this study was to elucidate the molecular mechanism underlying the transport of antiarrhythmic drugs using cells from the second segment of the proximal tubule (S2) cells of mice expressing human-organic cation transporter 3 (S2 human-OCT3). The antiarrhythmic drugs tested were cibenzoline, disopyramide, lidocaine, mexiletine, phenytoin, pilsicanide, procainamide and quinidine. Human-OCT3 mediated a time- and dose-dependent uptake of quinidine and lidocaine, with Km values of 216 and 139 microM, respectively. Human-OCT3 also mediated the uptake of disopyramide and procainamide but not that of phenytoin. All antiarrhythmic drugs tested inhibited histamine uptake mediated by human-OCT3 in a dose-dependent manner. The IC50 values of antiarrhythmic drugs for human-OCT3 ranged between 0.75 and 656 microM. Kinetic analysis revealed that disopyramide, lidocaine, procainamide and quinidine inhibited histamine uptake mediated by human-OCT3 in a competitive manner. In conclusion, these results suggest that human-OCT3 mediates the transport of antiarrhythmic drugs, which may be the mechanism underlying the distribution and the elimination of these drugs.

Characterization of the renal tubular transport of zonampanel, a novel alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist, by human organic anion transporters.

Zonampanel monohydrate (YM872; [2,3-dioxo-7-(1H-imidazol-1-yl)-6-nitro-1,2,3,4-tetrahydro-1-quinoxalinyl]acetic acid monohydrate) is a novel alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist. The major elimination route for zonampanel has been reported to be by urine via the kidneys. The purpose of this study is to elucidate the molecular mechanism of the renal excretion of zonampanel using cells stably expressing human organic anion transporters (hOAT) 1, hOAT2, hOAT3, and hOAT4, as well as human organic cation transporters (hOCT) 1 and hOCT2. Another AMPA receptor antagonist, YM90K [6-(1H-imidazol-1-yl)-7-nitro-2,3(1H,4H)-quinoxalinedione monohydrochloride], a decarboxymethylated form of zonampanel, was also used for comparing the substrate specificity. Zonampanel inhibited the uptake of prototypical organic anion substrates, [14C]para-aminohippurate in hOAT1 and [3H]estrone sulfate in hOAT3 and hOAT4, in a competitive manner. A time- and concentration-dependent increase in [14C]zonampanel uptake was observed in cells expressing hOAT1, hOAT3, and hOAT4. The Km values of zonampanel uptake by hOAT1, hOAT3, and hOAT4 were 1.4, 7.7, and 215 microM, respectively. Considering the localization of each transporter, results suggest that zonampanel is taken up via hOAT1 and hOAT3 from the blood into proximal tubular cells and then effluxed into the lumen via hOAT4. Probenecid and cimetidine competitively inhibited [14C]zonampanel uptake by the hOATs (hOAT1, hOAT3, and hOAT4 for probenecid; hOAT3 for cimetidine). YM90K inhibited the uptake of the prototypical substrate via hOAT3 competitively, but the uptake via hOAT1 noncompetitively. These findings suggest that the prototypical organic anion substrates (para-aminohippurate and estrone sulfate), cimetidine, probenecid, and zonampanel share binding specificity in each hOAT, whereas YM90K does not in hOAT1, possibly due to it being the decarboxymethylated form.

Interactions of human- and rat-organic anion transporters with pravastatin and cimetidine.

We have elucidated the interactions of human and rat organic anion transporters (hOATs and rOATs) with pravastatin and cimetidine. Pravastatin inhibited hOAT1/rOAT1, hOAT2/rOAT2, hOAT3/rOAT3, and hOAT4. The mode of inhibition was noncompetitive for hOAT1 and hOAT2, whereas it was competitive for hOAT3 and hOAT4. Cimetidine also inhibited hOAT1/rOAT1, hOAT3/rOAT3, and hOAT4. The mode of inhibition was a combination of competitive and noncompetitive manners for hOAT1, whereas it was competitive for hOAT3. The effects of OAT inhibitors on OAT1, OAT2, and OAT3 exhibited some but not so remarkable interspecies differences between humans and rats. In conclusion, we have characterized pravastatin and cimetidine as OAT inhibitors.

Evidence for a role of human organic anion transporters in the muscular side effects of HMG-CoA reductase inhibitors.

The purpose of this study was to elucidate the role of human organic anion transporters (human OATs) in the induction of drug-induced skeletal muscle abnormalities. 3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors have been clinically used for lowering plasma cholesterol levels, and are known to induce various forms of skeletal muscle abnormalities including myopathy and rhabdomyolysis. Immunohistochemical analysis revealed that human OAT1 and human OAT3 are localized in the cytoplasmic membrane of the human skeletal muscles. The activities of human OATs were measured using mouse cell lines from renal proximal tubules stably expressing human OATs. Human OAT3, but not human OAT1, mediates the transport of pravastatin. Fluvastatin inhibited organic anion uptake mediated by human OAT1 in a mixture of competitive and noncompetitive manner, whereas simvastatin and fluvastatin noncompetitively inhibited the organic anion uptake mediated by human OAT3. In conclusion, the organic anion transporters OAT1 and OAT3 are localized in the cytoplasmic membrane of human skeletal muscles. Pravastatin, simvasatin, and fluvasatin inhibit human OATs activity. These results suggest that muscle organic anion transporters play a role in the muscular side effects of HMG-CoA reductase inhibitors.

Interactions of human organic anion transporters with diuretics.

The tubular secretion of diuretics in the proximal tubule has been shown to be critical for the action of drugs. To elucidate the molecular mechanisms for the tubular excretion of diuretics, we have elucidated the interactions of human organic anion transporters (hOATs) with diuretics using cells stably expressing hOATs. Diuretics tested were thiazides, including chlorothiazide, cyclothiazide, hydrochlorothiazide, and trichlormethiazide; loop diuretics, including bumetanide, ethacrynic acid, and furosemide; and carbonic anhydrase inhibitors, including acetazolamide and methazolamide. These diuretics inhibited organic anion uptake mediated by hOAT1, hOAT2, hOAT3, and hOAT4 in a competitive manner. hOAT1 exhibited the highest affinity interactions for thiazides, whereas hOAT3 did those for loop diuretics. hOAT1, hOAT3, and hOAT4 but not hOAT2, mediated the uptake of bumetanide. hOAT3 and hOAT4, but not hOAT1 mediated the efflux of bumetanide. hOAT1 and hOAT3, but not hOAT2 and hOAT4 mediated the uptake of furosemide. In conclusion, it was suggested that hOAT1 may play an important role in the basolateral uptake of thiazides, and hOAT3 in the uptake of loop diuretics. In addition, it was also suggested that bumetanide taken up by hOAT3 and/or hOAT1 is excreted into the urine by hOAT4.

Interaction of human and rat organic anion transporter 2 with various cephalosporin antibiotics.

Cephalosporin antibiotics are thought to be excreted into the urine via organic anion transporters (OATs) and OAT can mediate nephrotoxicity by cephalosporins, particularly by cephaloridine. The purpose of this study was to elucidate the interaction of human-OAT2 and rat-OAT2 with cephalosporin antibiotics using proximal tubule cells stably expressing human-OAT2 and rat-OAT2. Human-OAT2 is localized to the basolateral side of the proximal tubule, whereas rat-OAT2 is localized to the apical side of the proximal tubule. Cephalosporins tested were cephalothin, cefoperazone, cefazolin, ceftriaxone, cephaloridine, cefotaxime, cefadroxil and cefamandole. These cephalosporins dose-dependently inhibited organic anion uptake mediated by human-OAT2 and rat-OAT2. There was no species difference observed for the effects of OAT2 with cephalosporins between human and rat transporters. Kinetic analysis revealed that the inhibitory effects for human-OAT2 were competitive. Cephaloridine significantly decreased the viability of cells stably expressing human-OAT2, human-OAT1, human-OAT3 and human-OAT4. The decreased viability of cells stably expressing human-OAT1, human-OAT3 and human-OAT4 but not human-OAT2 was reversed by probenecid. In conclusion, human-OAT2 interacts with cephalosporins, and thus, human-OAT2 may mediate the uptake of cephalosporins on the basolateral side of the proximal tubule. The interaction of human-OAT2 with cephalosporins was the weakest among the basolateral human-OATs tested. In addition, it is suggested that human-OATs mediate cephaloridine-induced nephrotoxicity.

Expression of human organic anion transporters in the choroid plexus and their interactions with neurotransmitter metabolites.

The purpose of the present study was to elucidate the expression of human organic anion transporter 1 (hOAT1) and hOAT3 in the choroid plexus of the human brain and their interactions with neurotransmitter metabolites using stable cell lines. Immunohistochemical analysis revealed that hOAT1 and hOAT3 are expressed in the cytoplasmic membrane and cytoplasm of human choroid plexus. Neurotransmitter metabolites, namely, 5-methoxyindole-3-acetic acid (5-MI-3-AA), homovanillic acid (HVA), vanilmandelic acid (VMA), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindole-3-acetic acid (5-HI-3-AA), N-acetyl-5-hydroxytryptamine (NA-5-HTT), melatonin, 5-methoxytryptamine (5-MTT), 3,4-dihidroxymandelic acid (DHMA), 5-hydroxytryptophol, and 5-methoxytryptophol (5-MTP), but not methanephrine (MN), normethanephrine (NMN), and 3-methyltyramine (3-MT), at 2 mM, inhibited para-aminohippuric acid uptake mediated by hOAT1. On the other hand, melatonin, 5-MI-3-AA, NA-5-HTT, 5-MTT, 5-MTP, HVA, 5-HI-3-AA, VMA, DOPAC, 5-hydroxytryptophol, and MN, but not 3-MT, DHMA, and NMN, at 2 mM, inhibited estrone sulfate uptake mediated by hOAT3. Differences in the IC(50) values between hOAT1 and hOAT3 were observed for DHMA, DOPAC, HVA, 5-HI-3-AA, melatonin, 5-MI-3-AA, 5-MTP, 5-MTT, and VMA. HOAT1 and hOAT3 mediated the transport of VMA but not HVA and melatonin. These results suggest that hOAT1 and hOAT3 are involved in the efflux of various neurotransmitter metabolites from the cerebrospinal fluid to the blood across the choroid plexus.

Interactions of human organic anion transporters and human organic cation transporters with nonsteroidal anti-inflammatory drugs.

The purpose of this study was to elucidate the interactions of human organic anion transporters (hOATs) and human organic cation transporters (hOCTs) with nonsteroidal anti-inflammatory drugs (NSAIDs) using cells stably expressing hOATs and hOCTs. NSAIDs tested were acetaminophen, acetylsalicylate, salicylate, diclofenac, ibuprofen, indomethacin, ketoprofen, mefenamic acid, naproxen, piroxicam, phenacetin, and sulindac. These NSAIDs inhibited organic anion uptake mediated by hOAT1, hOAT2, hOAT3, and hOAT4. By comparing the IC(50) values of NSAIDs for hOATs, it was found that hOAT1 and hOAT3 exhibited higher affinity interactions with NSAIDs than did hOAT2 and hOAT4. HOAT1, hOAT2, hOAT3, and hOAT4 mediated the uptake of either ibuprofen, indomethacin, ketoprofen, or salicylate, but not acetylsalicylate. Although organic cation uptake mediated by hOCT1 and hOCT2 was also inhibited by some NSAIDs, hOCT1 and hOCT2 did not mediate the uptake of NSAIDs. In conclusion, hOATs and hOCTs interacted with various NSAIDs, whereas hOATs but not hOCTs mediated the transport of some of these NSAIDs. Considering the localization of hOATs, it was suggested that the interactions of hOATs with NSAIDs are associated with the pharmacokinetics and the induction of adverse reactions of NSAIDs.

Characterization of methotrexate transport and its drug interactions with human organic anion transporters.

Life-threatening drug interactions are known to occur between methotrexate and nonsteroidal anti-inflammatory drugs (NSAIDs), probenecid, and penicillin G. The purpose of this study was to characterize methotrexate transport, as well as to determine the site and the mechanism of drug interactions in the proximal tubule. Mouse proximal tubule cells stably expressing basolateral human organic anion transporters (hOAT1 and hOAT3) and apical hOAT (hOAT4) were established. The K(m) values for hOAT1-, hOAT3-, and hOAT4-mediated methotrexate uptake were 553.8 microM, 21.1 microM, and 17.8 microM, respectively. NSAIDs (salicylate, ibuprofen, ketoprofen, phenylbutazone, piroxicam, and indomethacin), probenecid, and penicillin G dose dependently inhibited methotrexate uptake mediated by hOAT1, hOAT3, and hOAT4. Kinetic analysis of inhibitory effects of these drugs on hOAT3-mediated methotrexate uptake revealed that these inhibitions were competitive. The K(i) values for the effects of salicylate, phenylbutazone, indomethacin, and probenecid on hOAT3-mediated methotrexate uptake were comparable with therapeutically relevant plasma concentrations of unbound drugs. In addition, in the presence of human serum albumin, the K(i) values were comparable with therapeutically relevant total plasma concentrations of drugs. In conclusion, these results suggest that methotrexate is taken up via hOAT3 and hOAT1 at the basolateral side of the proximal tubule and effluxed or taken up at the apical side via hOAT4. In addition, hOAT1, hOAT3, and hOAT4 are the sites of drug interactions between methotrexate and NSAIDs, probenecid, and penicillin G. Furthermore, it was predicted that hOAT3 is the site of drug interactions between methotrexate and salicylate, phenylbutazone, indomethacin, and probenecid in vivo.

Role of human organic anion transporter 4 in the transport of ochratoxin A.

The purpose of this study was to investigate the characteristics of ochratoxin A (OTA) transport by multispecific human organic anion transporter 4 (hOAT4) using mouse proximal tubule cells stably transfected with hOAT4 (S(2) hOAT4). Immunohistochemical analysis revealed that hOAT4 protein was localized to the apical side of the proximal tubule. S(2) hOAT4 expressed hOAT4 protein in the apical side as well as basolateral side and the cells were cultured on the plastic dish for experiments. S(2) hOAT4 exhibited a time- and concentration-dependent, and a saturable increase in OTA uptake, with an apparent K(m) value of 22.9+/-2.44 microM. The OTA uptakes were inhibited by several substrates for the OATs. Probenecid, piroxicam, octanoate and citrinin inhibited OTA uptake by hOAT4 in a competitive manner (K(i)=44.4-336.4 microM), with the following order of potency: probenecid > piroxicam > octanoate >citrinin. The efflux of OTA by S(2) hOAT4 was higher than that by mock. Addition of OTA resulted in slight decrease in viability of S(2) hOAT4 compared with mock. These results indicate that hOAT4 mediates the high-affinity transport of OTA on the apical side of the proximal tubule, whereas the transport characteristics of OTA are distinct from those by basolateral OATs.

Involvement of rat organic anion transporter 3 (rOAT3) in cephaloridine-induced nephrotoxicity: in comparison with rOAT1.

This study was performed to elucidate the possible involvement of organic anion transporter 3 (OAT3) in cephaloridine (CER)-induced nephrotoxicity and compare the substrate specificity between rOAT3 and rat OAT1 (rOAT1) for various cephalosporin antibiotics, using proximal tubule cells stably expressing rOAT3 (S2 rOAT3) and rOAT1 (S2 rOAT1). S2 rOAT3 exhibited a CER uptake and a higher susceptibility to CER cytotoxicity than did mock, which was recovered by probenecid. Various cephalosporin antibiotics significantly inhibited both estrone sulfate uptake in S2 rOAT3 and para-aminohippuric acid uptake in S2 rOAT1. The Ki values of CER, cefoperazone, cephalothin and cefazolin for rOAT3- and rOAT1-mediated organic anion transport ranged from 0.048 to 1.14 mM and from 0.48 to 1.32 mM, respectively. These results suggest that rOAT3, at least in part, mediates CER uptake and CER-induced nephrotoxicity as rOAT1. There was some difference of affinity between rOAT3 and rOAT1 for cephalosporin antibiotics.

Interaction of human organic anion transporters 2 and 4 with organic anion transport inhibitors.

The organic anion transport system is involved in the tubular excretion and reabsorption of various drugs and substances. The purpose of this study was to characterize the effects of various organic anion transport inhibitors on renal organic anion transport using proximal tubule cells stably expressing human organic anion transporter 2 (hOAT2) and hOAT4. Immunohistochemical analysis revealed that hOAT2 is localized to the basolateral side of the proximal tubule in the kidney. hOAT2 mediated a time- and concentration-dependent increase in prostaglandin F(2alpha) (PGF(2alpha)) uptake. The organic anion transport inhibitors used for this study were probenecid, 8-(noradamantan-3-yl)-1,3-dipropylxanthine (KW-3902), betamipron, and cilastatin. Probenecid, but not KW-3902, betamipron, and cilastatin, significantly inhibited hOAT2-mediated PGF(2alpha) uptake. In contrast, probenecid, KW-3902, and betamipron, but not cilastatin, inhibited hOAT4-mediated estrone sulfate (ES) uptake. Kinetic analyses revealed that these inhibitions were competitive. The K(i) value of probenecid for hOAT2 was 766 microM, whereas those of probenecid, KW-3902, and betamipron for hOAT4 were 54.9, 20.7, and 502 microM, respectively. These results suggest that probenecid, KW-3902, and betamipron could inhibit hOAT4-mediated ES uptake in vitro, whereas probenecid alone could inhibit the hOAT2-mediated PGF(2alpha) uptake. Comparing the K(i) values with the therapeutically relevant concentrations of unbound inhibitors in the plasma, probenecid alone was predicted to inhibit hOAT4-mediated organic anion transport in vivo.

Human organic anion transporters and human organic cation transporters mediate renal transport of prostaglandins.

Prostaglandin E(2) (PGE(2)) and prostaglandin F(2 alpha) (PGF(2 alpha)) have been used for the induction of labor and the termination of pregnancy. Renal excretion is shown to be an important pathway for the elimination of PGE(2) and PGF(2 alpha). The purpose of this study was to elucidate the molecular mechanism of renal PGE(2) and PGF(2 alpha) transport using cells stably expressing human organic anion transporter (hOAT) 1, hOAT2, hOAT3, and hOAT4, and human organic cation transporter (hOCT) 1 and hOCT2. A time- and dose-dependent increase in PGE(2) and PGF(2 alpha) uptake was observed in cells expressing hOAT1, hOAT2, hOAT3, hOAT4, hOCT1, and hOCT2. The K(m) values of PGE(2) uptake by hOAT1, hOAT2, hOAT3, hOAT4, hOCT1, and hOCT2 were 970, 713, 345, 154, 657, and 28.9 nM, respectively, whereas those of PGF(2 alpha) uptake by hOAT1, hOAT3, hOAT4, hOCT1, and hOCT2 were 575, 1092, 692, 477, and 334 nM, respectively. PGE(2) and PGF(2 alpha) significantly inhibited organic anion uptake by hOATs and organic cation uptake by hOCTs. In conclusion, considering the localization of these transporters, the results suggest that PGE(2) and PGF(2 alpha) transport in the basolateral membrane of the proximal tubule is mediated by multiple pathways including hOAT1, hOAT2, hOAT3, and hOCT2, whereas that in the apical side is mediated by hOAT4.

Interaction of human organic anion transporters with various cephalosporin antibiotics.

Cephalosporin antibiotics are thought to be excreted into the urine via organic anion transporters (OATs). The purpose of this study was to elucidate the interaction of human-OATs with various cephalosporin antibiotics, using proximal tubule cells stably expressing human-OAT1, human-OAT3 and human-OAT4. Human-OAT1 and human-OAT3 are localized to the basolateral side of the proximal tubule, whereas human-OAT4 is localized to the apical side. The cephalosporin antibiotics tested were cephalothin, cefoperazone, cefazolin, ceftriaxone, cephaloridine, cefotaxime, cefadroxil and cefamandole. All of these cephalosporin antibiotics significantly inhibited organic anion uptake mediated by human-OAT1, human-OAT3 and human-OAT4. Kinetic analysis revealed that these inhibitions were competitive. The inhibition constant (K(i)) values of cefoperazone, cefazolin, ceftriaxone and cephaloridine for human-OAT1 were much lower than those for human-OAT3 and human-OAT4, whereas the K(i) values of cephalothin and cefotaxime for human-OAT3 were much lower than those for human-OAT1 and human-OAT4. Human-OAT4 mediated the bidirectional transport of estrone sulfate, an optimal substrate for human-OAT4. These results suggest that human-OAT1, human-OAT3 and human-OAT4 interact with various cephalosporin antibiotics, and that human-OAT1 and human-OAT3 play a distinct role in the basolateral uptake of cephalosporin antibiotics. Since the K(i) value of cephaloridine for human-OAT4-mediated organic uptake was much higher than that for human-OAT1, the results indicate the possibility that human-OAT4 limits the efflux of cephaloridine, leading to the accumulation of cephaloridine and the induction of nephrotoxicity.

Human organic anion transporters mediate the transport of tetracycline.

The purpose of this study was to elucidate the molecular mechanism for renal tetracycline transport by human organic anion transporters (hOATs) using proximal tubular cells stably expressing hOATs. The cells stably expressing hOAT1, hOAT2, hOAT3 and hOAT4 exhibited a higher amount of [3H]tetracycline uptake compared with mock cells. The apparent Km values for hOAT2-, hOAT3- and hOAT4-mediated tetracycline uptakes were 439.9 +/- 23.0, 566.2 +/- 28.4 and 122.7 +/- 16.0 microM, respectively. Tetracycline significantly inhibited the organic anion uptake by hOAT1, hOAT2 and hOAT4, but not hOAT3. In addition, oxytetracycline, minocycline and doxycycline inhibited the organic anion uptake by hOAT1, whereas oxytetracycline, minocycline but not doxycycline inhibited the organic anion uptake by hOAT2. In contrast, oxytetracycline, minocycline and doxycycline exhibited no significant inhibitory effects on the organic anion uptake by hOAT3 and hOAT4. HOAT1 and hOAT4 mediated the efflux of tetracycline, but hOAT2 and hOAT3 did not. These results suggest that hOAT1, hOAT2 and hOAT3 mediate the basolateral uptake and/or efflux of tetracycline, whereas hOAT4 is responsible for the reabsorption as well as the efflux of tetracycline in the apical side of the proximal tubule. These pharmacological characteristics of hOATs may be significantly related to events associated with the development of tetracycline-induced nephrotoxicity in the human kidney.

Human organic anion transporters and human organic cation transporters mediate renal antiviral transport.

Renal excretion is an important elimination pathway for antiviral agents, such as acyclovir (ACV), ganciclovir (GCV), and zidovudine (AZT). The purpose of this study was to elucidate the molecular mechanisms of renal ACV, GCV, and AZT transport using cells stably expressing human organic anion transporter 1 (hOAT1), hOAT2, hOAT3, and hOAT4, and human organic cation transporter 1 (hOCT1) and hOCT2. Time- and concentration-dependent uptake of ACV and GCV was observed in hOAT1- and hOCT1-expressing cells. In contrast, uptake of valacyclovir, L-valyl ester of ACV, was observed only in hOAT3-expressing cells. On the other hand, AZT uptake was observed in hOAT1-, hOAT2-, hOAT3-, and hOAT4-expressing cells. The Km values of ACV uptake by hOAT1 and hOCT1 were 342.3 and 151.2 microM, respectively, whereas those of GCV uptake by hOAT1 and hOCT1 were 895.5 and 516.2 microM, respectively. On the other hand, the Km values of AZT uptake by hOAT1, hOAT2, hOAT3, and hOAT4 were 45.9, 26.8, 145.1, and 151.8 microM, respectively. In addition, probenecid weakly inhibited the hOAT1-mediated ACV uptake. In conclusion, these results suggest that hOAT1 and hOCT1 mediate renal ACV and GCV transport, whereas hOAT1, hOAT2, hOAT3, and hOAT4 mediate renal AZT transport. In addition, L-valyl ester appears to be important in differential substrate recognition between hOAT1 and hOAT3. hOAT1 may not be the molecule responsible for the drug interaction between ACV and probenecid.