Effect of 17 beta-estradiol-D-17 beta-glucuronide on the rat organic anion transporting polypeptide 2-mediated transport differs depending on substrates

Involvement of organic anion-transporting polypeptides and organic cation transporter in the hepatic uptake of jatrorrhizine

Jatrorrhizine possesses a wide spectrum of pharmacological activities. However, the mechanism underlying hepatic uptake of jatrorrhizine remains unclear.Rat liver slices, isolated rat hepatocytes and human embryonic kidney 293 (HEK293) cells stably expressing human organic anion-transporting polypeptide (OATP) and organic cation transporter (OCT) were used to evaluate the hepatic uptake of jatrorrhizine in this study.Uptake of jatrorrhizine in rat liver slices and isolated rat hepatocytes was significantly inhibited by glycyrrhizic acid (Oatp1b2 inhibitor) and prazosin (Oct1 inhibitor), but not by ibuprofen (Oatp1a1 inhibitor) or digoxin (Oatp1a4 inhibitor). Uptake of jatrorrhizine in OATP1B3 and OCT1-HEK293 cells indicated a saturable process with the K_m of 8.20 ± 1.28 and 4.94 ± 0.55 μM, respectively. However, the transcellular transport of jatrorrhizine in OATP1B1-HEK293 cells was not observed. Rifampicin (OATP inhibitor) for OATP1B3-HEK293 cells and prazosin for OCT1-HEK293 cells could inhibit the uptake of jatrorrhizine with the IC₅₀ of 5.49 ± 1.05 and 2.77 ± 0.72 μM, respectively.The above data indicate that hepatic uptake of jatrorrhizine is involved in both OATP and OCT, which may have important roles in jatrorrhizine liver disposition and potential drug-drug interactions.

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 (K_p,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 K_p,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 K_p,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.

Organic anion-transporting polypeptides

Organic anion-transporting polypeptides or OATPs are central transporters in the disposition of drugs and other xenobiotics. In addition, they mediate transport of a wide variety of endogenous substrates. The critical role of OATPs in drug disposition has spurred research both in academia and in the pharmaceutical industry. Translational aspects with clinical questions are the focus in academia, while the pharmaceutical industry tries to define and understand the role these transporters play in pharmacotherapy. The present overview summarizes our knowledge on the interaction of food constituents with OATPs and on the OATP transport mechanisms. Further, it gives an update on the available information on the structure-function relationship of the OATPs and, finally, covers the transcriptional and posttranscriptional regulation of OATPs.

Interaction of digitalis-like compounds with liver uptake transporters NTCP, OATP1B1, and OATP1B3

Digitalis-like compounds (DLCs) such as digoxin, digitoxin, and ouabain, also known as cardiac glycosides, are among the oldest pharmacological treatments for heart failure. The compounds have a narrow therapeutic window, while at the same time, DLC pharmacokinetics is prone to drug-drug interactions at the transport level. Hepatic transporters organic anion transporting polypeptide (OATP) 1B1, OATP1B3, and Na(+)-dependent taurocholate co-transporting polypeptide (NTCP) influence the disposition of a variety of drugs by mediating their uptake from blood into hepatocytes. The interaction of digoxin, digitoxin, and ouabain with hepatic uptake transporters has been studied before. However, here, we systematically investigated a much wider range of structurally related DLCs for their capability to inhibit or to be transported by these transporters in order to better understand the relation between the activity and chemical structure of this compound type. We studied the uptake and inhibitory potency of a series of 14 structurally related DLCs in Chinese hamster ovary cells expressing NTCP (CHO-NTCP) and human embryonic kidney cells expressing OATP1B1 and OATP1B3 (HEK-OATP1B1 and HEK-OATP1B3). The inhibitory effect of the DLCs was measured against taurocholic acid (TCA) uptake in CHO-NTCP cells and against uptake of β-estradiol 17-β-d-glucuronide (E217βG) in HEK-OATP1B1 and HEK-OATP1B3 cells. Proscillaridin A was the most effective inhibitor of NTCP-mediated TCA transport (IC50 = 22 μM), whereas digitoxin and digitoxigenin were the most potent inhibitors of OATP1B1 and OAPTP1B3, with IC50 values of 14.2 and 36 μM, respectively. Additionally, we found that the sugar moiety and hydroxyl groups of the DLCs play different roles in their interaction with NTCP, OATP1B1, and OATP1B3. The sugar moiety decreases the inhibition of NTCP and OATP1B3 transport activity, whereas it enhances the inhibitory potency against OATP1B1. Moreover, the hydroxyl group at position 12 reinforces the inhibition of NTCP but decreases the inhibition of OATP1B1 and OATP1B3. To investigate whether DLCs can be translocated, we quantified their uptake in transporter-expressing cells by LC-MS. We demonstrated that convallatoxin, ouabain, dihydroouabain, and ouabagenin are substrates of OATP1B3. No transport was observed for the other compounds in any of the studied transporters. In summary, this work provides a step toward an improved understanding of the interaction of DLCs with three major hepatic uptake transporters. Ultimately, this can be of use in the development of DLCs that are less prone to transporter-mediated drug-drug interactions.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

The SLCO (former SLC21) superfamily of transporters

The members of the organic anion transporting polypeptide superfamily (OATPs) are classified within the SLCO solute carrier family. All functionally well characterized members are predicted to have 12 transmembrane domains and are sodium-independent transport systems that mediate the transport of a broad range of endo- as well as xenobiotics. Substrates are mainly amphipathic organic anions with a molecular weight of more than 300Da, but some of the known transported substrates are also neutral or even positively charged. Among the well characterized substrates are numerous drugs including statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, antibiotics, antihistaminics, antihypertensives and anticancer drugs. Based on their amino acid sequence identities, the different OATPs cluster into families (in general with more than 40% amino acid sequence identity) and subfamilies (more than 60% amino acid identity). With the sequencing of genomes from different species and the computerized prediction of encoded proteins more than 300 OATPs can be found in the databases, however only a fraction of them have been identified in humans, rodents, and some additional species important for pharmaceutical research like the rhesus monkey (Macaca mulatta), the dog (Canis lupus familiaris) and the pig (Sus scrofa). These OATPs form 6 families (OATP1-OATP6) and 13 subfamilies. In this review we try to summarize what is currently known about OATPs with respect to endogenous substrates, tissue distribution, transport mechanisms, regulation of expression, structure-function relationship and mutations and polymorphisms.

Clinical significance of organic anion transporting polypeptides (OATPs) in drug disposition: their roles in hepatic clearance and intestinal absorption

Organic anion transporting polypeptide (OATP) family transporters accept a number of drugs and are increasingly being recognized as important factors in governing drug and metabolite pharmacokinetics. OATP1B1 and OATP1B3 play an important role in hepatic drug uptake while OATP2B1 and OATP1A2 might be key players in intestinal absorption and transport across blood-brain barrier of drugs, respectively. To understand the importance of OATPs in the hepatic clearance of drugs, the rate-determining process for elimination should be considered; for some drugs, hepatic uptake clearance rather than metabolic intrinsic clearance is the more important determinant of hepatic clearances. The importance of the unbound concentration ratio (liver/blood), K(p,uu) , of drugs, which is partly governed by OATPs, is exemplified in interpreting the difference in the IC(50) of statins between the hepatocyte and microsome systems for the inhibition of HMG-CoA reductase activity. The intrinsic activity and/or expression level of OATPs are affected by genetic polymorphisms and drug-drug interactions. Their effects on the elimination rate or intestinal absorption rate of drugs may sometimes depend on the substrate drug. This is partly because of the different contribution of OATP isoforms to clearance or intestinal absorption. When the contribution of the OATP-mediated pathway is substantial, the pharmacokinetics of substrate drugs should be greatly affected. This review describes the estimation of the contribution of OATP1B1 to the total hepatic uptake of drugs from the data of fold-increases in the plasma concentration of substrate drugs by the genetic polymorphism of this transporter. To understand the importance of the OATP family transporters, modeling and simulation with a physiologically based pharmacokinetic model are helpful.

Interactions of green tea catechins with organic anion-transporting polypeptides

Organic anion-transporting polypeptides (OATPs) are multispecific transporters that mediate the uptake of numerous drugs and xenobiotics into cells. Here, we examined the effect of green tea (Camellia sinensis) catechins on the function of the four OATPs expressed in human enterocytes and hepatocytes. Uptake of the model substrate estrone-3-sulfate by cells expressing OATP1A2, OATP1B1, OATP1B3, or OATP2B1 was measured in the absence and presence of the four most abundant flavonols found in green tea. Uptake by OATP1A2, OATP1B1, and OATP2B1 was inhibited by epicatechin gallate (ECG) and epigallocatechin gallate (EGCG) in a concentration-dependent way. In contrast, OATP1B3-mediated uptake of estrone-3-sulfate was strongly stimulated by EGCG at low substrate concentrations. The effect of EGCG on OATP1B3 was also studied with additional substrates: uptake of estradiol-17β-glucuronide was unchanged, whereas uptake of Fluo-3 was noncompetitively inhibited. Both ECG and EGCG were found to be substrates of OATP1A2 (K(m) values of 10.4 and 18.8 μM, respectively) and OATP1B3 (34.1 and 13.2 μM, respectively) but not of OATP1B1 or OATP2B1. These results indicate that two of the major flavonols found in green tea have a substantial effect on the function of OATPs expressed in enterocytes and hepatocytes and can potentially alter the pharmacokinetics of drugs and other OATP substrates. In addition, the diverse effects of EGCG on the transport of other OATP1B3 substrates suggest that different transport/binding sites are involved.

1α,25-dihydroxyvitamin D3 on intestinal transporter function: studies with the rat everted intestinal sac

Previous studies have shown that 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) treatment (2.56 nmol/kg i.p. daily×4) increased PepT1, Mrp2, Mrp4, Asbt, but not Mdr1/P-gp in the rat small intestine. In this study, the intestinal everted sac technique, together with various select probes: mannitol (paracellular transport), glycylsarcosine (PepT1), 5(and 6)-carboxy-2',7'-dichlorofluorescein (CDF) diacetate (precursor of CDF for Mrp2), adefovir dipivoxil (precursor of adefovir for Mrp4) and digoxin (P-gp) was used to examine the functional changes of these transporters. After establishing identical permeabilities (Papp) of mannitol for the apical-to-basolateral (A-to-B) and basolateral-to-apical (B-to-A) directions at 20 min in 1,25(OH)2D3-treated vs. vehicle-treated duodenal, jejunal and ileal everted sacs, a significant enhancement of net A-to-B transport of glycylsarcosine in the duodenum, increased B-to-A transport of CDF and A-to-B and B-to-A transport of adefovir in the jejunum were observed with 1,25(OH)2 D3 treatment. However, the A-to-B and B-to-A transport of digoxin in the ileum was unchanged. These changes in transporter function in the rat intestinal everted sac corresponded well to changes in proteins that were observed previously. This study confirms that the rat intestinal PepT1, Mrp2 and Mrp4, but not P-gp are functionally induced by 1,25(OH)2D3 treatment via the vitamin D receptor (VDR).

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.

The blood-brain barrier thyroxine transporter organic anion-transporting polypeptide 1c1 displays atypical transport kinetics

Organic anion-transporting polypeptide (Oatp) 1c1 is a high-affinity T(4) transporter expressed in brain barrier cells. Oatp1c1 transports a variety of additional ligands including the conjugated sterol estradiol 17beta-glucuronide (E(2)17betaG). Intriguingly, published data suggest that E(2)17betaG inhibition of Oatp1c1-mediated T(4) transport exhibits characteristics suggestive of atypical transport kinetics. To determine whether Oatp1c1 exhibits atypical transport kinetics, we first performed detailed T(4) and E(2)17betaG uptake assays using Oatp1c1 stably transfected HEK293 cells and a wide range of T(4) and E(2)17betaG concentrations (100 pm to 300 nm and 27 nm to 200 mum, respectively). Eadie-Hofstee plots derived from these detailed T(4) and E(2)17betaG uptake experiments display a biphasic profile consistent with atypical transport kinetics. These data along with T(4) and E(2)17betaG cis-inhibition dose-response measurements revealed shared high- and low-affinity Oatp1c1 binding sites for T(4) and E(2)17betaG. T(4) and E(2)17betaG recognized these Oatp1c1 binding sites with opposite preferences. In addition, sterols glucuronidated in the 17 or 21 position, exhibited preferential substrate-dependent inhibition of Oatp1c1 transport, inhibiting Oatp1c1-mediated E(2)17betaG transport more strongly than T(4) transport. Together these data reveal that Oatp1c1-dependent substrate transport is a complex process involving substrate interaction with multiple binding sites and competition for binding with a variety of other substrates. A thorough understanding of atypical Oatp1c1 transport processes and substrate-dependent inhibition will allow better prediction of endo- and xenobiotic interactions with the Oatp transporter.

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.

Identification, Ki determination and CoMFA analysis of nuclear receptor ligands as competitive inhibitors of OATP1B1-mediated estradiol-17beta-glucuronide transport

Evidence shows that drug-drug interactions can occur at the level of drug transporters such as the organic anion transporting polypeptides (OATPs), a group of membrane solute carriers that mediate the sodium-independent transport of a wide range of amphipathic organic compounds. The polyspecific OATP1B1 is exclusively expressed at the basolateral membrane of hepatocytes and mediates uptake of amphipathic organic compounds from blood into hepatocytes. Nuclear receptors are ligand-activated transcription factors that play an important role in xenobiotic disposition and human diseases. Quite a few nuclear receptor ligands interact with transport proteins. A high-resolution three-dimensional structure is critical to understand the polyspecificity of OATP1B1 to predict and prevent adverse drug-drug interactions. Unfortunately there are no crystal structures of OATPs/Oatps available to date. Therefore, in this study we attempted to elucidate the characteristics of the substrate binding site of OATP1B1 based on small molecules interacting with it. First, we identified inhibitors of the OATP1B1 model substrate estradiol-17beta-glucuronide from about 40 nuclear receptor ligands. Among them, GW1929, paclitaxel and troglitazone were strong inhibitors, while 5 alpha-androstane, 5 alpha-androstane-3beta, 17beta-diol-17-hexahydrobenzoate and estradiol-3-benzoate were weak inhibitors. Then, we selected 25 compounds and performed inhibition kinetic studies to identify competitive inhibitors and determine their K(i) values which ranged from submicromolar to submillimolar. Finally, we performed CoMFA analysis on the identified competitive inhibitors. The CoMFA results indicate that the substrate binding site of OATP1B1 consists of a large hydrophobic middle part with basic residues at both ends that could be very important for substrate binding.

Xenobiotic transporters of the human organic anion transporting polypeptides (OATP) family

1. The organic anion transporting polypeptides (humans OATP; other species Oatp) belong to the SLCO gene superfamily of transporters and are twelve transmembrane domain glycoproteins expressed in various epithelial cells. Some OATPs/Oatps are expressed in a single organ, while others are expressed ubiquitously. 2. The functionally characterized members mediate sodium-independent transport of a variety of structurally independent, mainly amphipathic organic compounds, including bile salts, hormones and their conjugates, toxins, and various drugs. 3. This review summarizes the general features and the substrates of the eleven human OATPs. Furthermore, it reviews what is known about the mechanism of their multispecificity, their predicted structure, their role in drug-food interactions, and their role in cancer. 4. Finally, some open questions are raised that need to be addressed to advance OATP research in the near future.

Effects of pregnancy on CYP3A and P-glycoprotein activities as measured by disposition of midazolam and digoxin: a University of Washington specialized center of research study

The objectives of the study were to evaluate the effects of pregnancy on CYP3A and P-glycoprotein (P-gp) activities, as measured by disposition of midazolam and digoxin, respectively. Thirteen women received digoxin (0.25 mg p.o.) and midazolam (2 mg p.o.) in random order, separated by 1-2 weeks at 28-32 weeks gestation, and the same order was repeated at 6-10 weeks postpartum. Plasma and urine concentrations were determined by liquid chromatography-mass spectrometry and analyzed by noncompartmental methods. Midazolam CL/F(unbound) (593 +/- 237 l/min vs. 345 +/- 103 l/min; P = 0.007), digoxin CL(Renal, unbound) (272 +/- 45 ml/min vs. 183 +/- 37 ml/min; P < 0.002) and digoxin CL(secretion,) (unbound) (109 +/- 34 ml/min vs. 58 +/- 22 ml/min; P < 0.002) were higher during pregnancy than postpartum. These data are consistent with increased hepatic and/or intestinal CYP3A and renal P-gp activities during pregnancy.

Effect of pregnane X receptor ligands on transport mediated by human OATP1B1 and OATP1B3

The pregnane X receptor is a ligand-activated transcription factor that is abundantly expressed in hepatocytes. Numerous drugs are pregnane X receptor ligands. To bind to their receptor they must cross the sinusoidal membrane. Organic anion transporting polypeptides 1B1 and 1B3 (OATP1B1 and OATP1B3) are polyspecific transporters expressed at the sinusoidal membrane of human hepatocytes. They mediate transport of a variety of drugs including the pregnane X receptor ligands rifampicin and dexamethasone. To test whether additional pregnane X receptor ligands interact with OATP1B1- and 1B3-mediated transport, we developed Chinese Hamster Ovary (CHO) cell lines stably expressing OATP1B1 or 1B3 at high levels. OATP1B1- and 1B3-mediated estradiol-17beta-glucuronide uptake was inhibited by several pregnane X receptor ligands in a concentration dependent way. IC(50) values for rifampicin, paclitaxel, mifepristone, and troglitazone were within their respective pharmacological free plasma concentrations. Kinetic analysis revealed that clotrimazole inhibits OATP1B1-mediated estradiol-17beta-glucuronide transport with a K(i) of 7.7+/-0.3 microM in a competitive way. However, uptake of OATP1B3-mediated estradiol-17beta-glucuronide was stimulated and this stimulation was due to an increased apparent affinity. Transport of estrone-3-sulfate was hardly affected while all other substrates tested were inhibited. Additional azoles like fluconazole, ketoconazole and miconazole did not stimulate OATP1B3-mediated estradiol-17beta-glucuronide transport. In summary, these results demonstrate that pregnane X receptor ligands, by inhibiting or stimulating OATP-mediated uptake, can lead to drug-drug interactions at the transporter level.

Human renal organic anion transporters: Characteristics and contributions to drug and drug metabolite excretion

The kidney is a key organ for promoting the excretion of drugs and drug metabolites. One of the mechanisms by which the kidney promotes excretion is via active secretion. Secretion of drugs and their metabolites from blood to luminal fluid in the nephron is a protein-mediated process that normally involves either the direct or indirect expenditure of energy. Renal transporters for organic anions are located in the proximal tubule segment of the nephron. The primary transporters of organic anions on the basolateral membrane (BLM) of proximal tubule cells are members of the organic anion transporter (OAT) family (mainly OAT1 and OAT3). The sulfate-anion antiporter 1 (SAT-1; hsat-1) may also contribute to organic anion transport at the basolateral membrane. On the apical membrane, the multi-drug resistance-associated protein 2 (MRP2) is an important transport protein to complete the secretion process. However, there are several transport proteins on the basolateral and apical membranes of proximal tubule cells in human kidneys that have not been fully characterized and whose role in the secretion of organic anions remains to be determined. This review will primarily focus on the human renal basolateral and apical membrane transporters for organic anions that may play a role in the excretion of drugs and drug metabolites.

Active efflux across the blood-brain barrier: role of the solute carrier family

The brain uptake of xenobiotics is restricted by the blood-brain brain barrier formed by brain capillary endothelial cells. Active efflux transport systems in the blood-brain barrier work as a detoxification system in the brain by facilitating removal of xenobiotic compounds from the brain. Drugs, acting in the brain, have to overcome such efflux mechanisms to achieve clinically significant concentration in the brain. Multiple transporters are involved in this efflux transport in the brain capillaries. In the past few years, considerable progress has been made in the cloning of these transporters and their functional characterization after heterologous expression. Members of the solute carrier family (SLC) play an important role in the efflux transport, especially for organic anions, which include organic anion transporting polypeptides (OATP/SLCO) and organic anion transporters (OAT/SLC22A). It is believed that coordination of the members of SLC family, and ABC transporters, such as P-glycoprotein, multidrug resistance protein, and breast cancer-resistant protein (BCRP/ABCG2), allows an efficient vectorial transport across the endothelial cells to remove xenobiotics from the brain. In this review, we shall summarize our current knowledge about their localization, molecular and functional characteristics, and substrate and inhibitor specificity.

Flavonoids as a novel class of human organic anion-transporting polypeptide OATP1B1 (OATP-C) modulators

Flavonoids are a class of polyphenolic compounds widely present in the diet and herbal products. The interactions of flavonoids with some major efflux transporters [e.g., P-glycoprotein, multidrug resistance-associated protein 1 (MRP1), and breast cancer resistance protein] have been reported; however, their interactions with uptake transporters are largely unknown. Organic anion-transporting polypeptide OATP1B1 is a liver-specific uptake transporter important in hepatic drug disposition. Our objective was to evaluate the effects of 20 naturally occurring flavonoids, and some of their corresponding glycosides, on the uptake of [3H]dehydroepiandrosterone sulfate (DHEAS) in OATP1B1-expressing and OATP1B1-negative HeLa cells. Many of the tested flavonoids (including biochanin A, genistein, and epigallocatechin-3-gallate) significantly inhibited [3H]DHEAS uptake in a concentration-dependent manner in OATP1B1-expressing cells, with biochanin A being one of the most potent inhibitors with an IC50 of 11.3 +/- 3.22 microM. The flavonoids had negligible or small effects in OATP1B1-negative cells. Four of the eight pairs of tested flavonoids and their glycosides, namely, genistein/genistin, diosmetin/diosmin, epigallocatechin/epigallocatechin-3-gallate, and quercetin/rutin, exhibited distinct effects on [3H]DHEAS uptake. For example, genistin did not inhibit DHEAS uptake, whereas genistein did, and rutin stimulated uptake, whereas quercetin had no effect. [3H]Biochanin A uptake was similar in OATP1B1-expressing and OATP1B1-negative cells, suggesting that it is not a substrate for OATP1B1. A kinetic study revealed that biochanin A inhibited [3H]DHEAS uptake in a noncompetitive manner, with a Ki of 10.2 +/- 1.89 microM. Taken together, these results indicate that flavonoids are a novel class of OATP1B1 modulators, suggesting the potential for diet-drug interactions.

Role of Organic Anion Transporting Polypeptide 2 in Pharmacokinetics of Digoxin and β-Methyldigoxin in Rats

Recently, we found that potent P-glycoprotein (P-gp) inhibitors, such as verapamil and cyclosporin A, markedly modulated the pharmacokinetics of digoxin in rats, whereas they did not affect beta-methyldigoxin pharmacokinetics significantly. Digoxin is also a substrate of rat organic anion transporting polypeptide 2 (Oatp2). Here, we compared the magnitude of Oatp2-mediated drug interaction of digoxin and beta-methyldigoxin using amiodarone as an Oatp2 inhibitor in rats. Amiodarone (20 mg/kg) given intravenously significantly increased plasma levels and decreased biliary excretion, liver distribution, and intestinal distribution of digoxin administered intravenously at a dose of 10 mug/kg. Amiodarone also significantly decreased biliary excretion and liver distribution of beta-methyldigoxin, but the change in plasma levels of beta-methyldigoxin was quite small. These findings may give a clue in selecting these cardiac glycosides in clinical pharmacotherapy for patients receiving multiple drugs towards escape from Oatp2-mediated drug interactions.

Evaluation of drug-drug interaction in the hepatobiliary and renal transport of drugs

Recent studies have revealed the import role played by transporters in the renal and hepatobiliary excretion of many drugs. These transporters exhibit a broad substrate specificity with a degree of overlap, suggesting the possibility of transporter-mediated drug-drug interactions with other substrates. This review is an overview of the roles of transporters and the possibility of transporter-mediated drug-drug interactions. Among the large number of transporters, we compare the Ki values of inhibitors for organic anion transporting polypeptides (OATPs) and organic anion transporters (OATs) and their therapeutic unbound concentrations. Among them, cephalosporins and probenecid have the potential to produce clinically relevant OAT-mediated drug-drug interactions, whereas cyclosporin A and rifampicin may trigger OATP-mediated ones. These drugs have been reported to cause drug-drug interactions in vivo with OATs or OATP substrates, suggesting the possibility of transporter-mediated drug-drug interactions. To avoid adverse consequences of such transporter-mediated drug-drug interactions, we need to be more aware of the role played by drug transporters as well as those caused by drug metabolizing enzymes.

Application of QSAR analysis to organic anion transporting polypeptide 1a5 (Oatp1a5) substrates

Organic anion transporting polypeptide 1a5, Slco1a5 (previously called Oatp3, Slc21a7) is a multispecific transmembrane transport protein that belongs to the OATP/SLCO superfamily of solute carriers. It is expressed in several epithelial barriers such as the small intestine and the choroid plexus where it might play an important role in the disposition of numerous endogenous and exogenous organic compounds. Since the molecular basis of the multispecificity of Oatp1a5 is not known and the three-dimensional structure not solved yet, we used three-dimensional quantitative structure-activity relationship (3D-QSAR) techniques to obtain topological information on the substrate binding site of the protein. We aligned a heterogeneous data set of 18 Oatp1a5 substrates using the Genetic Algorithm Similarity Program (GASP) and performed comparative molecular field analysis (CoMFA) using this alignment. This resulted in a reasonable QSAR model including steric and electrostatic fields with a leave-one-out cross-validated r(cv)2 value of 0.705 and a no-cross-validated regression coefficient r2 value of 0.949. Based on the derived model we identified new potential Oatp1a5 substrates and confirmed their predicted apparent affinity values experimentally.

ASSESSMENT OF DRUG INTERACTIONS IN HEPATOBILIARY TRANSPORT USING RHODAMINE 123 IN SANDWICH-CULTURED RAT HEPATOCYTES

The purpose of the present study was to explore the utility of sandwich-cultured rat hepatocytes as an in vitro tool to examine drug interactions at the hepatic transport level. Rhodamine 123 was used as a model substrate for P-glycoprotein-mediated biliary excretion. Effects of various types of P-glycoprotein modulation on the biliary excretion index (BEI; a relative measure of the extent of biliary excretion) and the in vitro biliary clearance (CL(bile)) were determined. Significant reductions in rhodamine 123 BEI and CL(bile) were noted in the presence of the P-glycoprotein inhibitors verapamil (30-100 microM) and progesterone (100 microM). The P-glycoprotein activator quercetin (10-100 microM) enhanced rhodamine 123 CL(bile) by approximately 4-fold, with only a minor effect on BEI, suggesting that quercetin had a more pronounced effect on uptake at the basolateral membrane rather than excretion across the canalicular membrane. Treatment of hepatocytes for 48 h with dexamethasone (10 microM) resulted in significant enhancement of CL(bile), whereas rifampin (5-50 microM) increased both BEI and CL(bile), indicating that the inducing effects of dexamethasone and rifampin were occurring at the basolateral and canalicular membranes, respectively. Total rhodamine 123 uptake in sandwich-cultured rat hepatocytes was partly saturable and was affected by the presence of typical Oatp1a4 substrates (digoxin, quinine, d-verapamil, 17beta-estradiol-d-17beta-glucuronide). In summary, sandwich-cultured rat hepatocytes are a useful tool to study mechanisms of hepatobiliary drug disposition and to predict the potential for drug interactions in hepatic transport.

Involvement of multispecific organic anion transporter, Oatp14 (Slc21a14), in the transport of thyroxine across the blood-brain barrier

The present study was aimed at investigating the involvement of mouse organic anion transporting polypeptide 14 (mOatp14) in the uptake of T4 across the blood-brain barrier. Functional expression of mOatp14 in HEK293 cells revealed that T4 and rT3 are high affinity substrates of mOatp14 (Michaelis constant, 0.34 and 0.46 microm, respectively), and the specific uptake of T3 was 4-fold less than that of T4 and rT3. Taurocholate, probenecid, and estrone-3-sulfate were moderate inhibitors for mOatp14, whereas digoxin (substrate of Oatp2), benzylpenicillin (substrate of Oat3), and large neutral amino acids had no effect. mOatp14 is widely expressed throughout the brain, except for the cerebellum. The expression of mOatp14 in the isolated brain capillaries and the choroid plexus was shown by Western blot. The uptake clearance of T4 by the cerebral cortex determined using the in situ brain perfusion technique in mice was 580 microl/min.g tissue, 3-fold greater than that by the cerebellum, and a saturable component (Michaelis constant, 1.0 microm) accounts for the major fraction of the total uptake. Taurocholate inhibited the uptake of T4 by the cerebral cortex completely, but the inhibition by estrone-3-sulfate was partial (50%). These results suggest that transporters play a predominant role in the delivery of T4 to the brain, and mOatp14 accounts for estrone-3-sulfate inhibitable fraction, at least partly. The absence of inhibition by digoxin, benzylpenicillin, leucine, and 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid for the uptake of T4 by the cerebral cortex suggests the presence of other unknown transporter for T4 uptake by the brain. Immunohistochemical staining revealed basolateral localization of mOatp14 in the choroid plexus in which it may also play a role in T4 uptake.

Hydroxyurea transport across the blood-brain and blood-cerebrospinal fluid barriers of the guinea-pig

Hydroxyurea is used in the treatment of HIV infection in combination with nucleoside analogues, 2'3'-didehydro-3'deoxythymidine (D4T), 2'3'-dideoxyinosine or abacavir. It is distributed into human CSF and is transported from the CSF to sub-ependymal brain sites, but its movement into the brain directly from the blood has not been studied. This study addressed this by a brain perfusion technique in anaesthetized guinea-pigs. The carotid arteries were perfused with an artificial plasma containing [14C]hydroxyurea (1.6 microm) and a vascular marker, [3H]mannitol (4.6 nm). Brain uptake of [14C]hydroxyurea (8.0 +/- 0.9%) was greater than [3H]mannitol (2.4 +/- 0.2%; 20-min perfusion, n = 8). CSF uptake of [14C]hydroxyurea (5.6 +/- 1.5%) was also greater than [3H]mannitol (0.9 +/- 0.3%; n = 4). Brain uptake of [14C]hydroxyurea was increased by 200 microm hydroxyurea, 90 microm D4T, 350 microm probenecid, 25 microm digoxin, but not by 120 microm hydroxyurea, 16.5-50 microm D4T, 90 microm 2'3'-dideoxyinosine or 90 microm abacavir. [14C]Hydroxyurea distribution to the CSF, choroid plexus and pituitary gland remained unaffected by all these drugs. The metabolic half-life of hydroxyurea was > 15 h in brain and plasma. Results indicate that intact hydroxyurea can cross the brain barriers, but is removed from the brain by probenecid- and digoxin-sensitive transport mechanisms at the blood-brain barrier, which are also affected by D4T. These sensitivities implicate an organic anion transporter (probably organic anion transporting polypeptide 2) and possibly p-glycoprotein in the brain distribution of hydroxyurea and D4T.

Apparent lack of Mrp1-mediated efflux at the luminal side of mouse blood-brain barrier endothelial cells

PURPOSE

The purpose of this work was to determine mrpl-mediated efflux across the luminal membrane of endothelial cells at the blood-brain barrier (BBB) in mice.

METHODS

The transport of radiolabeled etoposide, 17beta-estradiol-D-17beta-glucuronide (E217betaG), vincristine, and doxorubicin across the BBB of mrp1(-/-) and wild-type mice was evaluated by in situ brain perfusion. Etoposide transport was also determined in P-glycoprotein-deficient mdr1a(-/-) mice perfused with both etoposide and mrpl inhibitors like probenecid or MK571. Cerebral vascular volume was determined by co-perfusion with labeled sucrose.

RESULTS

Sucrose perfusion indicated that the vascular space was close to normal in all the studies, indicating that the BBB remained intact. The transport of etoposide, E217betaG, vincristine, and doxorubicin into the brain was not affected by the lack of mrp1. Trans-efflux studies in mrp1-deficient mice with etoposide and E217betaG confirmed that mrpl was not involved in the efflux of these substrates across the BBB. There was also a significant P-gp-mediated efflux of etoposide in studies with P-glycoprotein-deficient mdr1a(-/-) mice. Perfusion of mdr1a(-/-) mice etoposide plus probenecid or MK571 did not affect the brain transport of etoposide.

CONCLUSION

Efflux mediated by mrp1 does not seem to occur across the luminal membrane of the endothelial cells forming the mouse BBB.

Differential effect of acute hepatic failure on in vivo and in vitro P-glycoprotein functions in the intestine

PURPOSE

The expression and function of P-glycoprotein (P-gp) in the intestine in carbon tetrachloride-induced acute hepatic failure (AHF) were evaluated in rats.

METHODS

The expression of P-gp, in vivo absorption and exsorption of P-gp substrates (digoxin and rhodamine 123), and in vitro efflux transport of these P-gp substrates were studied in the absence and presence of a P-gp inhibitor (verapamil or cyclosporin A) using the distal region of small intestine of control and AHF rats.

RESULTS

Western blot analysis revealed that intestinal P-gp expression level remained unchanged, or rather increased, in AHF. The in vivo intestinal P-gp function was significantly lower in AHF, as evaluated by the absorption and exsorption of P-gp substrates. In contrast, in vitro P-gp function was significantly higher in AHF, as evaluated by the efflux transport of P-gp substrates across the everted intestine. Collectively, the intestinal P-gp function was differently affected by AHF between in vivo and in vitro conditions.

CONCLUSIONS

The in vivo intestinal P-gp function was suppressed in AHF, which could not be predicted from in vitro functional studies nor from P-gp expression level. The discrepancy between in vivo and in vitro results may be explained by the presence of endogenous P-gp inhibitors in the plasma of AHF rats.

Retention of transporter activities in cryopreserved, isolated rat hepatocytes

The success of cryopreservation of isolated hepatocytes with existing methodologies is assessed with respect to the retentivity of cell integrity/viability (defined by trypan blue) and metabolic activities upon thawing in comparison to those of freshly prepared cells. But the ability of the cryopreserved cells to transport xenobiotics relative to that of freshly prepared cells has not been investigated. In this study, we optimized our previous methodology for cryopreservation and evaluated the metabolism and transport of thawed hepatocytes. Half of the freshly, isolated rat hepatocytes prepared by collagenase perfusion were immediately used for studies of transport of [(14)C]taurocholate, [(3)H]estrone sulfate and [(3)H]estradiol 17beta-D-glucuronide (1 microM) and metabolism of 7-hydroxy-4-(trifluoromethyl)-coumarin (100 microM), (3,4-difluorobenzyloxy)-5,5-dimethyl-4-(4-methylsulfonylphenyl)-(5H)-furan-2-one (250 microM), bufuralol (100 microM), and tolbutamide (100 microM), probes for UDP-glucuronyl transferase (UGT) and CYP3A, CYP2D, and CYP2C, respectively. The remaining half was cryopreserved using an optimized, programmed-freezing protocol, which was developed to minimize the prolonged release of latent heat during freezing. With the exception of the UGT probe, no significant difference (P > 0.05) was found in both metabolism and transport with freshly isolated versus cryopreserved hepatocytes upon thawing. In conclusion, we have demonstrated for the first time that thawed rat hepatocytes cryopreserved by a programmed-freezing protocol retain drug transport activities.

Mechanisms by which 2',3'-dideoxyinosine (ddI) crosses the guinea-pig CNS barriers; relevance to HIV therapy

The influence of transport mechanisms at the blood-brain barrier (BBB) and blood-CSF barrier (choroid plexus) on the CNS distribution of anti-human immunodeficiency virus (HIV) drugs was examined using guinea-pig brain perfusion and incubated choroid plexus models. 2',3'-dideoxyinosine (ddI) passage across the BBB was demonstrated to be via non-saturable (Kd = 0.22 +/- 0.3 microL/min/g) and saturable (Km = 20.1 +/- 15.0 microm, Vmax = 6.5 +/- 2.1 pmol/min/g) processes. Cross competition studies implicated an equilibrative nucleoside transporter in this influx. The brain distribution of ddI was unchanged in the presence of additional nucleoside reverse transcriptase inhibitors (NRTIs). ddI transport from blood into choroid plexus was demonstrated to involve an organic anion transporting polypeptide 2-like transporter. The NRTIs, abacavir, 3'-azido 3'-deoxythymidine and (-)-beta-L-2',3'-dideoxy-3'-thiacytidine, competed with ddI for transporter binding sites at the choroid plexus, altering the tissue concentration of ddI. This has clinical implications as the choroid plexus is a site of HIV replication, and suboptimal CNS concentrations of anti-HIV drugs could result in neurological complications. Furthermore, this may promote the selection of drug resistant variants of HIV within the CNS, which could re-infect the periphery and lead to HIV therapy failure. This study indicates that understanding drug interactions at the transporter level could prove valuable when selecting drug combinations to treat HIV within the CNS.

The superfamily of organic anion transporting polypeptides

Organic anion transporting polypeptides (Oatps/OATPs) form a growing gene superfamily and mediate transport of a wide spectrum of amphipathic organic solutes. Different Oatps/OATPs have partially overlapping and partially distinct substrate preferences for organic solutes such as bile salts, steroid conjugates, thyroid hormones, anionic oligopeptides, drugs, toxins and other xenobiotics. While some Oatps/OATPs are preferentially or even selectively expressed in one tissue such as the liver, others are expressed in multiple organs including the blood-brain barrier (BBB), choroid plexus, lung, heart, intestine, kidney, placenta and testis. This review summarizes the actual state of the rapidly expanding OATP superfamily and covers the structural properties, the genomic classification, the phylogenetic relationships and the functional transport characteristics. In addition, we propose a new species independent and open ended nomenclature and classification system, which is based on divergent evolution and agrees with the guidelines of the Human Genome Nomenclature Committee.

Interactions of cationic drugs and cardiac glycosides at the hepatic uptake level: studies in the rat in vivo, isolated perfused rat liver, isolated rat hepatocytes and oocytes expressing oatp2

This paper deals with a crucial mechanism for interaction of basic drugs and cardiac glycosides at the hepatic uptake level. Available literature data is provided and new material is presented to picture the differential transport inhibition of bulky (type2) cationic drugs by a number of cardiac glycosides in rat liver. It is shown that the so called organic anion transporting peptide 2 (oatp2) is the likely interaction site: differential inhibition patterns as observed in oocytes expressing oatp2, could be clearly identified also in isolated rat hepatocytes, isolated perfused rat liver and the rat in vivo. The anticipation of transport interactions at the hepatic clearance level should be based on data on the relative affinities of interacting substrates for the transport systems involved along with knowledge on the pharmacokinetics of these agents as well as the chosen dose regimen in the studied species. This review highlights the importance of multispecific tranporter systems such as OATP, accommodating a broad spectrum of organic compounds of various charge, implying potential transport interactions that can affect body distribution and organ clearance.