Predominant Contribution of Rat Organic Anion Transporting Polypeptide-2 (Oatp2) to Hepatic Uptake of β-Lactam Antibiotics

Inflammation-Induced Attenuation of Prostaglandin D2 Elimination across Rat Blood-Brain Barrier: Involvement of the Downregulation of Organic Anion Transporter 3 and Multidrug Resistance-Associated Protein 4

Prostaglandin (PG) D₂ is a lipid mediator, and in the brain, overproduction of PGD₂ is reportedly involved in the progression and exacerbation of neuroinflammation. The objective of this study was to elucidate PGD₂ efflux transport, under normal and inflammatory conditions, across the blood-brain barrier (BBB), which is formed by brain capillaries. Elimination of [³H]PGD₂ across the BBB of normal and lipopolysaccharide (LPS)-induced inflammatory rats was examined by the intracerebral microinjection technique. After intracerebral injection, the percentage of [³H]PGD₂ remaining in the ipsilateral cerebrum decreased with time, with a half-life of 13 min. This [³H]PGD₂ elimination across the BBB was significantly inhibited by the co-administration of unlabeled PGD₂, which suggests carrier-mediated PGD₂ efflux transport at the BBB. In isolated rat brain capillaries, mRNA expression of organic anion transporter (Oat) 3, organic anion-transporting polypeptide (Oatp) 1a4, and multidrug resistance-associated protein (Mrp) 4 was observed. In addition, co-administration of substrates/inhibitors for Oat3, Oatp1a4, and/or Mrp4, such as benzylpenicillin and cefmetazole, reduced [³H]PGD₂ elimination across the BBB. Data suggest that Oat3 and Mrp4, but not Oatp1a4 are involved in PGD₂ elimination across the BBB, as Oatp1a4-expressing Xenopus (X.) oocytes did not show the significant [³H]PGD₂ uptake compared with water-injected X. oocytes. In LPS-treated rats, [³H]PGD₂ elimination across the BBB and mRNA expression levels of Oat3 and Mrp4 were significantly decreased. Our data suggest that Oat3- and Mrp4-mediated PGD₂ elimination across the BBB is attenuated under inflammatory conditions.

Interaction of swine organic anion transporting polypeptide 1a2 with tetracycline, macrolide and β-lactam antibiotics

Organic anion-transporting polypeptides (human OATPs; animals Oatps; gene symbol SLCO/Slco) are integral membrane proteins that mediate the sodium-independent transport of a wide range of endogenous compounds as well as many xenobiotics. Antibiotics, antidiabetic drugs, anti-inflammatory drugs, antifungals, antivirals, antihistamines, antihypertensives, fibrates, statins, cardiac glycosides, immunosuppressants, and anticancer drugs are among the substrates transported by OATPs. Because of the broad substrate specificity, wide tissue distribution and the involvement of drug-drug interaction, human OATPs have been extensively recognized as key determinants for drug absorption, distribution and excretion. In a previous study, we cloned a functional orthologue of human OATP1A2 from the pig liver and designated it as swine Oatp1a2 (sOatp1a2) based on sequence analysis and phylogenic study. In the present study, transport capability of swine Oatp1a2 for tetracyclines, macrolides and β-lactam antibiotics was investigated. It was found that most of the tested antibiotics, including the tetracycline family members such as tetracycline, doxycycline, oxytetracycline and chlortetracycline as well as the β-lactam antibiotics such as penicillin, amoxicillin and cefquinome are directly transported by sOatp1a2; while macrolides such as tylosin, tilmicosin, clarithromycin and erythromycin may only inhibit uptake function of the transporter. As a group of well-known inhibitors of OATP family members, the effect of flavonoids on sOatp1a2 function was evaluated and it was found that all the flavonoids tested are inhibitors of the swine transporter as well.

Organic Anion Transporting Polypeptide 1a4 is Responsible for the Hepatic Uptake of Cardiac Glycosides in Mice

Among organic anion transporting polypeptide (Oatp) family transporters expressed in the rodent liver, such as Oatp1a1, Oatp1a4, Oatp1b2, and Oatp2b1, Oatp1a4 has a unique character to recognize neutral cardiac glycosides as a substrate in addition to organic anions. The relative contribution of Oatp1a4 to the substrate uptake into hepatocytes has not been clarified. In this study, we investigated the importance of Oatp1a4 in the hepatic uptake of its substrate drugs using Slco1a4-/- mice. The hepatic mRNA expression of Slco1a4 was decreased significantly in Slco1a4-/- mice, whereas no differences were seen in other hepatic transporters between wild-type and Slco1a4-/- mice. We determined the plasma concentrations and liver-to-plasma concentration ratios (Kp,liver) of Oatp1a4 substrates, including ouabain, digoxin, BQ-123, fexofenadine, rosuvastatin, pravastatin, nafcillin, and telmisartan, after continuous intravenous infusion. The plasma concentrations of ouabain and rosuvastatin were 2.1-fold and 1.7-fold higher in Slco1a4-/- mice, and Kp,liver of ouabain and digoxin were 13.4-fold and 4.3-fold lower in Slco1a4-/- mice, respectively. Furthermore, the biliary clearance of ouabain and digoxin with regard to plasma concentration were 21.9-fold and 4.1-fold lower in Slco1a4-/- mice, respectively, accompanied with a marked reduction in their Kp,liver, whereas the systemic clearance of ouabain, but not digoxin, was reduced significantly in Slco1a4-/- mice. These results suggest that Oatp1a4 plays a major role in the hepatic accumulation of cardiac glycosides in mice.

Successful Prediction of In Vivo Hepatobiliary Clearances and Hepatic Concentrations of Rosuvastatin Using Sandwich-Cultured Rat Hepatocytes, Transporter-Expressing Cell Lines, and Quantitative Proteomics

We determined whether in vivo transporter-mediated hepatobiliary clearance (CL) and hepatic concentrations of rosuvastatin (RSV) in the rat could be predicted by transport activity in sandwich-cultured rat hepatocytes (SCRHs) and/or transporter-expressing cell lines scaled by differences in transporter protein expression between SCRHs, cell lines, and rat liver. The predicted hepatobiliary CLs and hepatic concentrations of RSV were compared with our previously published positron emission tomography imaging data. Sinusoidal uptake CL ([Formula: see text]) and efflux (canalicular and sinusoidal) CLs of [3H]-RSV in SCRHs were evaluated in the presence and absence of Ca2+ and in the absence and presence of 1 mM unlabeled RSV (to estimate passive diffusion CL). [Formula: see text] of RSV into cells expressing organic anion transporting polypeptide (Oatp) 1a1, 1a4, and 1b2 was also determined. Protein expression of Oatps in SCRHs and Oatp-expressing cells was quantified by liquid chromatography tandem mass spectrometry. SCRHs well predicted the in vivo RSV sinusoidal and canalicular efflux CLs but significantly underestimated in vivo [Formula: see text]. Oatp expression in SCRHs was significantly lower than that in the rat liver. [Formula: see text], based on RSV [Formula: see text] into Oatp-expressing cells (active transport) plus passive diffusion CL in SCRHs, scaled by the difference in protein expression in Oatp cells versus SCRH versus rat liver, was within 2-fold of that observed in SCRHs or in vivo. In vivo hepatic RSV concentrations were well predicted by Oatp-expressing cells after correcting [Formula: see text] for Oatp protein expression. This is the first demonstration of the successful prediction of in vivo hepatobiliary CLs and hepatic concentrations of RSV using transporter-expressing cells and SCRHs.

Liver uptake of cefditoren is mediated by OATP1B1 and OATP2B1 in humans and Oatp1a1, Oatp1a4, and Oatp1b2 in rats

OATPs and Oatps mediated liver uptake of cefditoren in humans and in rats.

Appropriate risk criteria for OATP inhibition at the drug discovery stage based on the clinical relevancy between OATP inhibitors and drug-induced adverse effect

DDI could be caused by the inhibition of OATP-mediated hepatic uptakes. The aim of this study is to set the risk criteria for the compounds that would cause DDI via OATP inhibition at the drug discovery stage. The IC₅₀ values of OATP inhibitors for human OATP-mediated atorvastatin uptake were evaluated in the expression system. In order to set the risk criteria for OATP inhibition, the relationship was clarified between OATP inhibitory effect and severe adverse effects of OATP substrates, rhabdomyolysis, hyperbilirubinemia and jaundice. Rhabdomyolysis would be caused in the atorvastatin AUC more than 9-fold of that at a minimum therapeutic dose. The atorvastatin AUC was 6- to 9-fold increased with the OATP inhibitors of which IC₅₀ values were ≤1 μmol/L. Hyperbilirubinemia and jaundice would be caused with the OATP inhibitors of which IC₅₀ values were ≤6 μmol/L. This investigation showed that the compounds with IC₅₀ of ≤1 μmol/L would have high risk for OATP-mediated DDI that would cause severe side effects. Before the detailed analysis based on the dosage, unbound fraction in blood and effective concentration to evaluate the clinical DDI potency, this criteria enable high throughput screening and optimize lead compounds at the drug discovery stage.

Effect of transporter inhibition on the distribution of cefadroxil in rat brain

BACKGROUND

Cefadroxil, a cephalosporin antibiotic, is a substrate for several membrane transporters including peptide transporter 2 (PEPT2), organic anion transporters (OATs), multidrug resistance-associated proteins (MRPs), and organic anion transporting polypeptides (OATPs). These transporters are expressed at the blood-brain barrier (BBB), blood-cerebrospinal fluid barrier (BCSFB), and/or brain cells. The effect of these transporters on cefadroxil distribution in brain is unknown, especially in the extracellular and intracellular fluids within brain.

METHODS

Intracerebral microdialysis was used to measure unbound concentrations of cefadroxil in rat blood, striatum extracellular fluid (ECF) and lateral ventricle cerebrospinal fluid (CSF). The distribution of cefadroxil in brain was compared in the absence and presence of probenecid, an inhibitor of OATs, MRPs and OATPs, where both drugs were administered intravenously. The effect of PEPT2 inhibition by intracerebroventricular (icv) infusion of Ala-Ala, a substrate of PEPT2, on cefadroxil levels in brain was also evaluated. In addition, using an in vitro brain slice method, the distribution of cefadroxil in brain intracellular fluid (ICF) was studied in the absence and presence of transport inhibitors (probenecid for OATs, MRPs and OATPs; Ala-Ala and glycylsarcosine for PEPT2).

RESULTS

The ratio of unbound cefadroxil AUC in brain ECF to blood (Kp,uu,ECF) was ~2.5-fold greater during probenecid treatment. In contrast, the ratio of cefadroxil AUC in CSF to blood (Kp,uu,CSF) did not change significantly during probenecid infusion. Icv infusion of Ala-Ala did not change cefadroxil levels in brain ECF, CSF or blood. In the brain slice study, Ala-Ala and glycylsarcosine decreased the unbound volume of distribution of cefadroxil in brain (Vu,brain), indicating a reduction in cefadroxil accumulation in brain cells. In contrast, probenecid increased cefadroxil accumulation in brain cells, as indicated by a greater value for Vu,brain.

CONCLUSIONS

Transporters (OATs, MRPs, and perhaps OATPs) that can be inhibited by probenecid play an important role in mediating the brain-to-blood efflux of cefadroxil at the BBB. The uptake of cefadroxil in brain cells involves both the influx transporter PEPT2 and efflux transporters (probenecid-inhibitable). These findings demonstrate that drug-drug interactions via relevant transporters may affect the distribution of cephalosporins in both brain ECF and ICF.

Pharmacological significance of prostaglandin E2 and D2 transport at the brain barriers

Prostaglandin (PG) E2 and PGD2, which are biosynthesized from arachidonic acid generated by enzymatic cleavage of membrane phospholipid in response to various stimuli, play key roles in multiple brain pathophysiological processes, including modulation of synaptic plasticity, neuroinflammation, and sleep promotion. Concentrations of PGE2 and PGD2 in brain interstitial fluid (ISF) and cerebrospinal fluid (CSF) are maintained at appropriate levels for normal brain function by regulatory systems. The blood-brain barrier (BBB) and the blood-CSF barrier (BCSFB) possess ISF/CSF-to-blood efflux transport systems that are the primary cerebral clearance pathways for PGE2 and PGD2. However, regulatory dysfunction at the brain barriers may seriously affect brain function. In a mouse inflammation model, significant reduction of PGE2 efflux transport at the BBB has been observed. Several kinds of cephalosporin antibiotics and nonsteroidal anti-inflammatory drugs inhibit the BBB- and BCSFB-mediated efflux transport of PGE2 and PGD2. Especially, drugs that inhibit multidrug resistance-associated protein 4 (MRP4)-mediated PGE2 transport are capable of reducing PGE2 efflux at the BBB. Thus, it might be important in the treatment of inflammatory and infectious diseases to use drugs that do not inhibit clearance of PGE2 at the brain barriers, in order to avoid unexpected adverse CNS effects. Further, considering that PGD2 in CSF is a natural sleep-promoting factor, changes in the activity of the PGD2 efflux transport system at the BCSFB may modify the PGD2 level in CSF, thus affecting physiological sleep. These findings indicate that the efflux transport systems at the brain barriers play key roles in the pathophysiology and pharmacology of PGE2 and PGD2.

OATP and MRP2-mediated hepatic uptake and biliary excretion of eprosartan in rat and human

BACKGROUND

Eprosartan is an angiotensin II receptor antagonist, used in the treatment of hypertension and heart failure in clinical patients. The objective of this study was to clarify the mechanism underlying hepatic uptake and biliary excretion of eprosartan in rats and humans.

METHODS

Perfused rat liver in situ, rat liver slices, isolated rat hepatocytes and human organic anion-transporting polypeptide (OATP)-transfected cells in vitro were used in this study.

RESULTS

Extraction ratio of eprosartan was decreased by rifampicin in perfused rat livers. Uptake of eprosartan in rat liver slices and isolated rat hepatocytes was significantly inhibited by Oatp modulators such as ibuprofen, digoxin, rifampicin and cyclosporine A, but not by tetraethyl ammonium or p-aminohippurate. Uptake of eprosartan in rat hepatocytes indicated a saturable process. Although uptake of eprosartan in OATP1B3-human embryonic kidney cells (HEK) 293 cells was not observed, significant differences in cellular accumulations of eprosartan between vector- and OATP1B1-Madin-Darby canine kidney strain (MDCK) II cells were found in transcellular transport study. Moreover, cumulative biliary excretion rate of eprosartan in the presence of probenecid (Multidrug resistance-associated protein 2 (Mrp2) inhibitor) was significantly decreased in perfused rat livers. Vectorial basal-to-apical transport of eprosartan was also observed in OATP1B1/MRP2 double transfectants.

CONCLUSIONS

Eprosartan was transported by multiple Oatps (at least Oatp1a1 and Oatp1a4)/Mrp2 in rat and OATP1B1/MRP2, at least, in human.

Involvement of organic anion-transporting polypeptides in the hepatic uptake of dioscin in rats and humans

The objective of this study was to clarify the mechanism underlying hepatic uptake of dioscin (diosgenyl 2,4-di-O-a-L-rhamnopyranosyl-p-D-glucopyranoside), an herbal ingredient with antihepatitis activity, in rats and humans. The liver uptake index (LUI) in vivo, perfused rat liver in situ, rat liver slices, isolated rat hepatocytes, and human organic anion-transporting polypeptide (OATP)-transfected cells in vitro were used to evaluate hepatic uptake of dioscin. Values of 11.9% ± 1.6% and 15.0% ± 0.9% of dose for uptake of dioscin were observed by LUI in vivo and perfused rat livers in situ, respectively. The time course of dioscin uptake by rat liver slices was temperature-dependent. Uptake of dioscin by rat liver slices and isolated rat hepatocytes was inhibited significantly by Oatp modulators, such as ibuprofen (Oatp1a1 inhibitor), digoxin (Oatp1a4 substrate), and glycyrrhizic acid (Oatp1b2 inhibitor), but not by TEA or p-aminohippurate. Uptake of dioscin in rat hepatocytes and OATP1B3-human embryonic kidney (HEK) 293 cells indicated a saturable process with a Km of 3.75 ± 0.51 μM and 2.08 ± 0.27 μM, respectively. (-)-Epigallocatechin gallate, cyclosporin A, rifampicin, and telmisartan inhibited transport of dioscin in OATP1B3-HEK293 cells. However, transcellular transport of dioscin in OATP1B1- or OATP1B1/multidrug resistance-associated protein 2-Madin-Darby canine kidney strain II cells was not observed. These results indicate that hepatic uptake of dioscin is involved in OATP1B3 in humans, and multiple Oatps might participate in this process in rats.

Relevance of PepT1 in the intestinal permeability and oral absorption of cefadroxil

PURPOSE

To determine the contribution of intestinal PepT1 on the permeability and oral absorption of the β-lactam antibiotic drug cefadroxil.

METHODS

The effective permeability (P eff ) of cefadroxil was evaluated in wild-type and PepT1 knockout mice following in situ single-pass intestinal perfusions. The plasma concentration-time profiles of cefadroxil were also examined after oral gavage.

RESULTS

The P eff (cm/s) of cefadroxil in wild-type mice was 0.49 × 10(-4) in duodenum, 0.80 × 10(-4) in jejunum, 0.88 × 10(-4) in ileum and 0.064 × 10(-4) in colon. The P eff (cm/s) in PepT1 knockout mice was significantly reduced in small intestine, but not in colon, as shown by values of 0.003 × 10(-4), 0.090 × 10(-4), 0.042 × 10(-4) and 0.032 × 10(-4), respectively. Jejunal uptake of cefadroxil was saturable (Km = 2-4 mM) and significantly attenuated by the sodium-proton exchange inhibitor 5-(N,N-dimethyl)amiloride. Jejunal permeability of cefadroxil was not affected by L-histidine, glycine, cephalothin, p-aminohippurate or N-methylnicotinamide. In contrast, cefadroxil permeability was significantly reduced by glycylproline, glycylsarcosine, or cephalexin. Finally, PepT1 ablation resulted in 23-fold reductions in peak plasma concentrations and 14-fold reductions in systemic exposure of cefadroxil after oral dosing.

CONCLUSIONS

The findings are definitive in demonstrating that PepT1 is the major transporter responsible for the small intestinal permeability of cefadroxil as well as its enhanced oral drug performance.

Primary hepatocyte cultures as prominent in vitro tools to study hepatic drug transporters

Before any drug can be placed on the market, drug efficacy and safety must be ensured through rigorous testing. Animal models are used for this purpose, though currently increasing attention goes to the use of alternative in vitro systems. In particular, liver-based testing platforms that allow the prediction of pharmacokinetic (PK) and pharmacotoxicological properties during the early phase of drug development are of interest. They also enable the screening of potential effects on hepatic drug transporters. The latter are known to affect drug metabolism and disposition, thereby possibly underlying drug-drug interactions, which, in turn, may result in liver toxicity. Clearly, stable in vivo-like functional expression of drug transporters in hepatic in vitro settings is a prerequisite to be applicable in routine PK and pharmacotoxicological testing. In the first part of the article, an updated overview of hepatic drug transporters is provided, followed by a state-of-the-art review of drug-transporter production and activity in primary hepatocyte cultures (PHCs), being the gold-standard in vitro system. Specific focus is hereby put on strategies to maintain long-term functional expression, in casu of drug transporters, in these systems. In the second part, the use of PHCs to assess hepatobiliary transport and transporter-mediated interactions is outlined.

Species differences in biliary clearance and possible relevance of hepatic uptake and efflux transporters involvement

From a search of the available literature, a database of 22 drugs of all charge types and several different therapeutic classes was compiled to compare rat and human biliary clearance data. Dog biliary excretion data were also found for nine of the drugs. For 19 of the 22 drugs (86%), rat unbound biliary clearance values, when normalized for body weight, exceeded those for humans by factors ranging from 9 to over 2500-fold, whereas human/dog differences were much less dramatic. It was possible to define hepatic uptake and efflux transporter involvement for many of the drugs. On the basis of the findings, it is postulated that regardless of the biliary efflux transporters implicated, when drugs do not require active hepatic uptake to access the liver there may be fairly insignificant differences in rat, dog, and human biliary clearance. Conversely, when the organic anion-transporting polypeptide drug transporters are involved, one may expect at least a 10-fold discrepancy in rat to human biliary clearance normalized for body weight and corrected for plasma protein binding.

In vivo biliary clearance should be predicted by intrinsic biliary clearance in sandwich-cultured hepatocytes

It has been reported that in vivo biliary clearance can be predicted using sandwich-cultured rat and human hepatocytes. The predicted apparent biliary clearance (CL(bile, app)) from sandwich- cultured rat hepatocytes (SCRH) based on medium concentrations correlates to in vivo CL(bile, app) based on plasma concentrations of angiotensin II receptor blockers (ARBs), HMG-CoA reductase inhibitors (statins), β-lactam antibiotics, and topotecan. However, the predicted biliary clearance from SCRH was 7- to 300-fold lower than in vivo biliary clearance. We speculated that the process of biliary excretion might not have been evaluated using sandwich-cultured hepatocytes. To evaluate this issue, intrinsic biliary clearance (CL(bile, int)) based on intracellular compound concentrations was evaluated to investigate the in vitro-in vivo correlation of this process among ARBs, statins, β-lactam antibiotics, and topotecan. Intrinsic biliary clearance in SCRH correlated to in vivo values obtained by constant intravenous infusion of six compounds, but not rosuvastatin and cefmetazole, to rats. Moreover, differences between SCRH and in vivo CL(bile, int) (0.7-6-fold) were much smaller than those of CL(bile, app) (7-300-fold). Therefore, in vivo CL(bile, int) is more accurately reflected using SCRH than CL(bile, app). In conclusion, to predict in vivo biliary clearance more accurately, CL(bile, int) should be evaluated instead of CL(bile, app) between SCRH and in vivo.

Quantitative time-lapse imaging-based analysis of drug-drug interaction mediated by hepatobiliary transporter, multidrug resistance-associated protein 2, in sandwich-cultured rat hepatocytes

There is increasing interest in developing efficient screening platforms to predict drug-induced liver injury. Therefore, we explored a microscope-based analysis to quantitatively evaluate interaction of drugs with multidrug resistance-associated protein 2 (MRP2), essential for hepatic excretion of drugs in sandwich-cultured rat hepatocytes (SCRHs), using 5 (and 6)-carboxy-2',7'-dichlorofluorescein (CDF) diacetate, which is intracellularly hydrolyzed to the fluorescent substrate CDF. Drug-MRP2 interactions were evaluated by measuring the fluorescence change in bile canaliculi in SCRHs in the presence or absence of MRP2 inhibitors using quantitative time-lapse imaging (QTLI) analysis. Fluorescence was negligible in SCHs from rat (r) Mrp2-deficient Eisai hyperbilirubinemic rat, suggesting that Mrp2 is primarily responsible for CDF accumulation. According to QTLI, rifampicin, cyclosporine, and 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK-571) attenuated CDF accumulation in a concentration-dependent manner, with IC₅₀ values (IC₅₀, QTLI)) of 3.02, 1.63, and 2.87 μM, respectively. The ratios of IC₅₀ values obtained from the biliary excretion index over the IC(₅₀, QTLI) were 1.34, 1.94, and 1.94, but ratios over IC₅₀ values in CDF uptake by Mrp2-expressing membrane vesicles varied more: 6.69, 3.07, and 2.43 for rifampicin, cyclosporine, and MK-571, respectively. When the IC(₅₀, QTLI) of rifampicin was corrected for the hepatocyte/medium distribution ratio, the relative ratio of IC(₅₀, VES)/IC(₅₀, QTLI) was reduced to 2.25 from 6.69 (20.2/3.02) and was close to the ratio for MK-571 (2.43, 6.96/2.87), which is thought to cross the plasma membrane by passive diffusion. Our results indicate that QTLI is a suitable method to evaluate drug-MRP2 interaction at the bile canalicular membrane, when the hepatocyte/medium distribution ratio in SCRHs is taken into account.

An improved nonlinear model describing the hepatic pharmacokinetics of digoxin: evidence for two functionally different uptake systems and saturable binding

PURPOSE

To develop a semi-distributed liver model for the evaluation of saturable sinusoidal uptake and binding kinetics of the Oatp1a4 substrate digoxin.

METHODS

In the perfused rat liver, two successive digoxin doses of 42 and 125 microg were administered, and the outflow concentration was determined by LC/MS/MS. [14C]-sucrose was used as vascular reference. The data were analyzed simultaneously by a population approach using sucrose to determine the sinusoidal mixing of digoxin.

RESULTS

The results suggest the existence of a high-affinity, low-capacity system, and a low-affinity, high-capacity system for sinusoidal uptake with apparent Michaelis constants (K(M)) of 0.24 and 332 microg/ml, respectively. Incorporation of saturable sinusoidal binding of digoxin considerably improved the fit, and the parameter estimates were consistent with those of binding to hepatic Na,K-ATPase. Simpler models that neglect the concentration gradient in flow direction failed to describe the outflow data in the high dose range.

CONCLUSION

The semi-distributed liver model with saturable uptake should be useful for a functional characterization of transporters in the in situ rat liver.

Involvement of multidrug resistance-associated protein 4 in efflux transport of prostaglandin E(2) across mouse blood-brain barrier and its inhibition by intravenous administration of cephalosporins

Prostaglandin E(2) (PGE(2)) acts as a modulator of synaptic signaling and excitability in the brain. Because PGE(2) is barely inactivated enzymatically in adult brain, its brain level is considered to be controlled by efflux transport across the blood-brain barrier (BBB). The purpose of the present study was to clarify the efflux transport of PGE(2) at the BBB and the interaction of various drugs with this process. [(3)H]PGE(2) was eliminated from brain across the BBB with a half-life of 16.3 min, and the elimination was inhibited by 3 mM unlabeled PGE(2). Multidrug resistance-associated protein 4 (MRP4/ABCC4) was reported to be localized at the luminal membrane of the BBB. MRP4-expressing membrane vesicles showed significant uptake of [(3)H]PGE(2) and the uptake was inhibited by cefmetazole with an IC(50) value of 10.2 microM. At the concentration of 20 microM, several drugs, including cefazolin, cefotaxime, ceftriaxone, and ketoprofen, significantly inhibited [(3)H]PGE(2) uptake into MRP4-expressing membrane vesicles. Using the brain efflux index method, preadministration of cefmetazole, cefazolin, ceftriaxone, and cefotaxime was found to inhibit [(3)H]PGE(2) efflux from brain across the BBB. Furthermore, intravenous administration of the cefmetazole dose dependently reduced [(3)H]PGE(2) elimination across the BBB (ID(50) = 120 mg/kg). These results indicate that PGE(2) is eliminated from the brain by MRP4-mediated efflux transport at the BBB, and peripheral administration of cefmetazole decreases the efflux transport of PGE(2) at the BBB; this interaction may influence brain function.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Organic anion transporter OAT1 is involved in renal handling of citrulline

Because citrulline plasma concentration is elevated in kidney failure, citrulline could be a biomarker of renal insufficiency, although the mechanism regulating its disposition in the kidney has not been clarified. In rat kidney slices, citrulline uptake was apparently Na+ dependent, saturable with Km 556 μM, and significantly inhibited by anionic (PAH) and cationic (TEA) compounds, but not by probenecid at 1 mM. Preincubation of kidney slices with glutarate increased citrulline uptake, while such an increase was not observed after preincubation of the slices in Na+-free buffer. This result suggested that a sodium-dependent dicarboxylate cotransporter is involved in citrulline uptake by rat kidney slices. In studies using transporter-overexpressing cells, human organic anion transporter 1 (OAT1) and rat Oat1 exhibited citrulline transport activity with Km values of 238 and 373 μM, respectively, while other OATs and organic cation transporters (OCTs) did not transport citrulline. Based on the relative activity factor method, the contribution of rat Oat1 to the overall uptake of citrulline in rat kidney slices was ∼70%. Moreover, the interaction among citrulline, PAH, and probenecid uptakes via rat Oat1 suggested that there are multiple functional sites on Oat1 and that the citrulline site may be distinct from the PAH and probenecid site. Thus OAT1/Oat1 appears to be one of the major contributors to renal basolateral uptake of citrulline, and impaired activities of these transporters may contribute substantially to the increase in plasma citrulline in renal failure. Accordingly, citrulline may be useful for diagnosis of kidney function as is creatinine.

PEPT1 involved in the uptake and transepithelial transport of cefditoren in vivo and in vitro

Cefditoren is a third-generation cephalosporin developed by Meiji Seika Kaisha Ltd. Many beta-lactam antibiotics are transported by the H+/peptide symporters PEPT1 and PEPT2 that are preferentially expressed in the luminal membrane of the intestine and kidney, respectively. In this study, we employed everted small intestinal preparations, in situ jejunal perfusion, and Caco-2 cells as models to determine the effects of glycylsarcosine (Gly-Sar) and clonidine on uptake and transport of cefditoren. In vivo, rats were administered cefditoren (10 mg/kg) intravenously, in the absence and presence of Gly-Sar (10 mg/kg). The effects of Gly-Sar coadministration on biliary and urinary cefditoren excretion were investigated. Gly-sar significantly decreased cefditoren uptake and transport in the three in vitro and in situ models. A kinetic study showed that cefditoren transport by PEPT1 in Caco-2 cells had K(m) and V(max) values of 0.94+/-0.11 mM and 0.49+/-0.09 nmol/mg protein/5 min, respectively. Clonidine induced a 50% increase of cefditoren absorption across the intestinal mucosa after intravenous infusion, in the jejunal perfusion model. In vivo, biliary and urinary excretions over 6 h were an average of 34% and 4% of the administered cefditoren respectively. Gly-Sar coadministration increased the renal clearance of cefditoren by 200% and values of CL(urine) for cefditoren in the presence of 50 mM Gly-Sar were significantly higher than the controls. Biliary excretion was unchanged, however, compared to cefditoren alone. This study provides the first in vitro and in vivo evidence that cefditoren is a substrate of PEPT1.

Roles of inner blood-retinal barrier organic anion transporter 3 in the vitreous/retina-to-blood efflux transport of p-aminohippuric acid, benzylpenicillin, and 6-mercaptopurine

The purpose of the present study was to characterize rat organic anion transporter (Oat) 3 (Oat3, Slc22a8) in the efflux transport at the inner blood-retinal barrier (BRB). Reverse transcription-polymerase chain reaction analysis showed that rat (r) Oat3 mRNA is expressed in retinal vascular endothelial cells (RVECs), but not rOat1 and rOat2 mRNA. The expression of Oat3 in the retina and human cultured retinal endothelial cells was further confirmed by Western blot analysis. Immunohistochemical staining in RVECs showed that rOat3 is colocalized with glucose transporter 1, but not P-glycoprotein, suggesting that rOat3 is possibly located at the abluminal membrane of the RVEC. The contribution of rOat3 to the efflux of [(3)H]p-aminohippuric acid ([(3)H]PAH), [(3)H]benzylpenicillin ([(3)H]PCG), and [(14)C]6-mercaptopurine ([(14)C]6-MP), substrates of rOat3, from the vitreous humor/retina to the circulating blood across the inner BRB was evaluated using the microdialysis method. [(3)H]PAH, [(3)H]PCG, [(14)C]6-MP, and [(14)C] or [(3)H]d-mannitol, a bulk flow marker, were biexponentially eliminated from the vitreous humor after vitreous bolus injection. The elimination rate constant of [(3)H]PAH, [(3)H]PCG, and [(14)C]6-MP during the terminal phase was approximately 2-fold greater than that of d-mannitol. This efflux transport was reduced in the retinal presence of probenecid, PAH, and PCG, whereas it was not inhibited by digoxin. In conclusion, rOat3 is expressed at the inner BRB and involved in the vitreous humor/retina-to-blood transport of PAH, PCG, and 6-MP. This transport system is one mechanism to limit the retinal distribution of PAH, PCG, and 6-MP.

Identification and species similarity of OATP transporters responsible for hepatic uptake of beta-lactam antibiotics

The present study was designed to identify the organic anion transporting polypeptide (OATP) molecule(s) responsible for the uptake of beta-lactam antibiotics in human liver, using cryopreserved hepatocytes, as well as Xenopus oocytes and cultured cells expressing human OATPs. Nafcillin uptake by human hepatocytes was saturable with a Km of 533 microM. In vitro uptake studies revealed that OATP1B3 and OATP1B1 transported nafcillin with Km values of 74 microM and 11 mM, respectively. Analysis by the relative activity factor method suggested that OATP1B3 contributes mainly to nafcillin uptake and OATP1B1 contributes moderately. This conclusion was supported by the results of a study with selective inhibitors. Furthermore, OATP1B3 transported six other beta-lactam antibiotics, and their uptake clearances by OATP1B3 correlated well with those mediated by rat Oatp1a4, which is the predominant contributor to basolateral uptake of nafcillin by rat hepatocytes. These findings suggest that OATP1B3 plays a major role in the hepatic uptake of beta-lactam antibiotics in humans, and probably corresponds functionally to Oatp1a4 in rat liver.

Effect of plasma protein binding on in vitro-in vivo correlation of biliary excretion of drugs evaluated by sandwich-cultured rat hepatocytes

In the present study, we examined in vitro biliary clearance of several compounds in sandwich-cultured rat hepatocytes (SCRH) and compared it with that observed in vivo in rats; the effect of plasma protein binding on in vitro-in vivo correlation of biliary excretion was also assessed. The in vitro biliary excretion was determined by differential cumulative uptake of compounds in SCRH preincubated in the presence and absence of Ca(2+)/Mg(2+). The cumulative uptake study of radiolabeled substrates revealed that the function of canalicular efflux transporters such as bile salt export pump, multidrug resistance-associated protein 2, breast cancer resistance protein, and multidrug resistance 1 was adequately maintained in SCRH. Unlabeled test compounds, pravastatin, rosuvastatin, valsartan, cefmetazole, and cefoperazone exhibited varying degrees of in vitro biliary excretion in the cumulative uptake study using SCRH. In vivo biliary excretions of these compounds were measured in common bile duct-cannulated rats. Whereas their biliary excretion ratios were all more than 60% of the dose, the in vivo intrinsic biliary clearances varied from 10.5 to 1787.2 ml/min/kg. The in vitro intrinsic biliary clearances of test compounds were well correlated with their corresponding in vivo intrinsic clearances calculated on the basis of the plasma unbound concentration (r(2) = 0.984), whereas less correlation was observed when they were calculated on the basis of plasma total concentration (r(2) = 0.217). These results indicate that SCRH is a useful in vitro model for predicting in vivo intrinsic biliary clearance in rats. In addition, for an accurate prediction, it is necessary to evaluate the in vivo intrinsic biliary clearance based on plasma unbound concentration but not total concentration.

Involvement of multidrug resistance-associated protein 2 (Abcc2) in molecular weight-dependent biliary excretion of beta-lactam antibiotics

In the present study, we attempted to identify the membrane permeation process(es) primarily involved in the molecular-weight-dependent biliary excretion of beta-lactam antibiotics. A search of the literature indicated that the molecular weight threshold operates mainly in the transport process across bile canalicular membranes. We confirmed that biliary clearance of the model biliary-excretion-type cephalosporin cefoperazone was reduced to 10% of the control in Eisai hyperbilirubinemic rats, which are genetically deficient in multidrug resistance-associated protein (Mrp) 2, indicating that Mrp2 plays a major role as an efflux transporter on the canalicular membranes. ATP-dependent uptake of several cephalosporins including cefoperazone, cefbuperazone, cefpiramide, and ceftriaxone, all of which are mainly excreted into bile, was confirmed in membrane vesicles from Sf9 cells transfected with rat Mrp2. Both the inhibitory potency of the cephalosporins for Mrp2-mediated transport and the uptake of cephalosporins by Mrp2-expressing vesicles were molecular weight-dependent, suggesting that Mrp2 is one of the major transporters involved in molecular weight-dependent biliary excretion. An uptake study in membrane vesicles of Sf9 cells transfected with breast cancer resistance protein (Bcrp) revealed that Bcrp accepts cefoperazone, cefbuperazone, cefpiramide, cefotetan, ceftriaxone, cefotiam, cefamandole, and cefazolin as substrates, and Bcrp-mediated transport was also molecular weight-dependent, suggesting that Bcrp also contributes to molecular weight-dependent biliary excretion of beta-lactam antibiotics in rats.