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 K m values of ACV uptake by hOAT1 and hOCT1 were 342.3 and 151.2 M, respectively, whereas those of GCV uptake by hOAT1 and hOCT1 were 895.5 and 516.2 M, respectively. On the other hand, the K m values of AZT uptake by hOAT1, hOAT2, hOAT3, and hOAT4 were 45.9, 26.8, 145.1, and 151.8 M, 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.

Human organic anion transporter 2 (hOat2[SLC22A7]) is highly expressed in the human liver. Although localization, gene expression, substrate specificity and transport mechanisms of other human Oat isoforms such as human Oat1 (hOat1), human Oat3 (hOat3) and human Oat4 (hOat4) have been elucidated, information concerning human Oat2 (hOat2) is less defined. The objective of this study was to provide further information on the transport mechanism and substrate specificity of hOat2. When expressed in Xenopus laevis oocytes, the transport of organic compounds mediated by hOat2 was not affected by the replacement of extracellular sodium with lithium, choline and mannitol. The uptake of estrone sulfate (ES) in hOat2-expressing oocytes was significantly trans-stimulated by preloading the oocytes with fumarate and succinate, but not glutarate. Moreover, we observed that hOat2 mediates the transport of bumetanide, ES, glutarate, dehydroepiandrosterone sulfate, allopurinol, prostaglandin E2, 5-fluorouracil, paclitaxel and L-ascorbic acid. These compounds are identified for the first time as hOat2 substrates. A wide range of structurally unrelated organic compounds inhibited the hOat2-mediated uptake of tetracycline, except for sulfobromophthalein. All of these findings indicate that hOat2 is a sodium-independent multi-specific organic anion/dimethyldicarboxylate exchanger. Our present findings thus provide further insights into the role of hOat2 in hepatic drug transport.

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 2␣ (PGF 2␣ ) 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 2␣ 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 M, whereas those of probenecid, KW-3902, and betamipron for hOAT4 were 54.9, 20.7, and 502 M, 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 2␣ 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.Various organic anion transport inhibitors are used experimentally and clinically. Probenecid is a conventional and standard organic anion transport inhibitor experimentally, but it is used as a uricosuric drug clinically. In addition, KW-3902, developed as an adenosine A1 receptor antagonist , was also shown to inhibit organic anion transport in the basolateral membrane of opossum kidney cells derived from the American opossum kidney (Nagai et al., 1999). On the other hand, betamipron and cilastatin are administered in combination with carbapenem antibiotics, panipenem, and imipenem, respectively (Birnbaum et al., 1985;Shiba et al., 1991). Betamipron inhibits the uptake of panipenem and imipenem into proximal tubule cells (Hirouchi et al., 1994). On the other hand, imipenem is degraded by human renal dehydropeptidase-I and therefore must be administered in combination with cilastatin, a dehydropeptidase-I inhibitor, to prevent loss of antimicrobial activity in urine and limit potential nephrotoxicity associated with renal metabolism (Craig, 1997).In our previous study, we elucidated the interaction of human organic anion transporter 1 (hOAT1) and hOAT3 with organic anion transport inhibitors including probenecid, KW-3902, betamipron, and cilastatin (Takeda et al., 2001). Thus, the purpose of this study was to characterize the interaction of hOAT2 and hOAT4 with these organic anion transport inhibitors using cells derived from the second portion of the pr...

We isolated cDNA encoding a multispecific organic anion transporter 2 (OAT2) from the mouse kidney cDNA library. Isolated mouse OAT2 (mOAT2) consisted of 1623 base pairs that encoded a 540-amino acid residue protein with 12 putative membrane-spanning domains, and the amino acid sequence was 87% identical to that of rat OAT2 (rOAT2). The gene coding for mOAT2, Slc22a7, is found on chromosome 17C. Northern blot analysis revealed that the mOAT2 mRNA is abundantly expressed in the male mouse kidney, whereas it was predominantly expressed in both the liver and kidney of female mice. When expressed in Xenopus laevis oocytes, mOAT2 mediated the high affinity transport of glutarate (K m ϭ 15.8 Ϯ 3.2 M) and prostaglandin E 2 (K m ϭ 5.2 Ϯ 0.5 nM) in a sodium-independent manner. mOAT2-expressing oocytes also mediated the uptake of ␣-ketoglutarate, glutarate, prostaglandin E 2 , p-aminohippuric acid, methotrexate, ochratoxin A, valproate, and allopurinol. However, we did not observe mOAT2-mediated uptake of salicylate. A wide range of structurally unrelated organic anions inhibited mOAT2-mediated glutarate uptake especially erythromycin, a potent inhibitor. These results indicate that isolated mOAT2 is a multispecific organic anion transporter having some differences in substrate specificity compared with rOAT2. In addition, we found that there exists a sex-and speciesrelated differential gene expression of the OAT2 isoform.Organic anion transporters play central roles in the elimination of a wide range of endogenous and exogenous anionic compounds including drugs, environmental toxicants, and their metabolites. Current extensive molecular studies have identified several families of multispecific organic anion transporters that are involved in the elimination of various organic anions (Moller and Sheikh, 1983;Boyer et al

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