Transporters as a determinant of drug clearance and tissue distribution

Nanovehicular intracellular delivery systems

This article provides an overview of principles and barriers relevant to intracellular drug and gene transport, accumulation and retention (collectively called as drug delivery) by means of nanovehicles (NV). The aim is to deliver a cargo to a particular intracellular site, if possible, to exert a local action. Some of the principles discussed in this article apply to noncolloidal drugs that are not permeable to the plasma membrane or to the blood-brain barrier. NV are defined as a wide range of nanosized particles leading to colloidal objects which are capable of entering cells and tissues and delivering a cargo intracelullarly. Different localization and targeting means are discussed. Limited discussion on pharmacokinetics and pharmacodynamics is also presented. NVs are contrasted to micro-delivery and current nanotechnologies which are already in commercial use. Newer developments in NV technologies are outlined and future applications are stressed. We also briefly review the existing modeling tools and approaches to quantitatively describe the behavior of targeted NV within the vascular and tumor compartments, an area of particular importance. While we list "elementary" phenomena related to different level of complexity of delivery to cancer, we also stress importance of multi-scale modeling and bottom-up systems biology approach.

In vitro biliary clearance of angiotensin II receptor blockers and 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors in sandwich-cultured rat hepatocytes: comparison with in vivo biliary clearance

Previous reports have indicated that in vitro biliary clearance (Cl(biliary)) determined in sandwich-cultured hepatocytes correlates well with in vivo Cl(biliary) for limited sets of compounds. This study was designed to estimate the in vitro Cl(biliary) in sandwich-cultured rat hepatocytes (SCRHs) of angiotensin II receptor blockers and HMG-CoA reductase inhibitors that undergo limited metabolism, to compare the estimated Cl(biliary) values with published in vivo Cl(biliary) data in rats, and to characterize the mechanism(s) of basolateral uptake and canalicular excretion of these drugs in rats. The average biliary excretion index (BEI) and in vitro Cl(biliary) values of olmesartan, valsartan, pravastatin, rosuvastatin, and pitavastatin were 15, 19, 43, 45, and 20%, respectively, and 1.7, 3.2, 4.4, 46.1, and 34.6 ml/min/kg, respectively. Cl(biliary) predicted from SCRHs, accounting for plasma unbound fraction, correlated with reported in vivo Cl(biliary) for these drugs. The rank order of Cl(biliary) values predicted from SCRHs was consistent with in vivo Cl(biliary) values. Bromosulfophthalein inhibited the uptake of all drugs. BEI and Cl(biliary) values of olmesartan, valsartan, pravastatin, and rosuvastatin, known multidrug resistance-associated protein (Mrp) 2 substrates, were reduced in SCRHs from Mrp2-deficient (TR(-)) compared with wild-type (WT) rats. Although Mrp2 plays a minor role in pitavastatin biliary excretion, pitavastatin BEI and Cl(biliary) were reduced in TR(-) compared with WT SCRHs; Bcrp expression in SCRHs from TR(-) rats was decreased. In conclusion, in vitro Cl(biliary) determined in SCRHs can be used to estimate and compare in vivo Cl(biliary) of compounds in rats and to characterize transport proteins responsible for their hepatic uptake and excretion.

Quantitative atlas of membrane transporter proteins: development and application of a highly sensitive simultaneous LC/MS/MS method combined with novel in-silico peptide selection criteria

PURPOSE

To develop an absolute quantification method for membrane proteins, and to construct a quantitative atlas of membrane transporter proteins in the blood-brain barrier, liver and kidney of mouse.

METHODS

Mouse tissues were digested with trypsin, and mixed with stable isotope labeled-peptide as a quantitative standard. The amounts of transporter proteins were simultaneously determined by liquid chromatography-tandem mass spectrometer (LC/MS/MS).

RESULTS

The target proteins were digested in-silico, and target peptides for analysis were chosen on the basis of the selection criteria. All of the peptides selected exhibited a detection limit of 10 fmol and linearity over at least two orders of magnitude in the calibration curve for LC/MS/MS analysis. The method was applied to obtain the expression levels of 34 transporters in liver, kidney and blood-brain barrier of mouse. The quantitative values of transporter proteins showed an excellent correlation with the values obtained with existing methods using antibodies or binding molecules.

CONCLUSION

A sensitive and simultaneous quantification method was developed for membrane proteins. By using this method, we constructed a quantitative atlas of membrane transporter proteins at the blood-brain barrier, liver and kidney in mouse. This technology is expected to have major implications for various fields of biomedical science.

Genetic variations and gene expression of transporters in drug disposition and response

BACKGROUND

The importance of transporters in drug disposition and response has led to increasing interest in genetic variations and expression differences of their genes.

OBJECTIVE

This review summarizes: i) genetic variations in transporters and associated drug response; and ii) a pharmacogenomic approach to correlate transporter expression and drug response.

METHODS

Several transporters in ATP-binding cassette family and solute carrier family are discussed.

CONCLUSION

The field of transporter pharmacogenomics is in its early stage. Transporter expression at mRNA levels could be more directly related to their functions and more practical to be assayed in high throughput. Correlating microarray expression of transporters with anticancer drug activity in the NCI-60 panel has provided an approach for identifying drug-transporter relationships and predicting drug response.

Interindividual variability of methadone response: impact of genetic polymorphism

Methadone, an opioid analgesic, is used clinically in pain therapy as well as for substitution therapy in opioid addiction. It has a large interindividual variability in response and a narrow therapeutic index. Genetic polymorphisms in genes coding for methadone-metabolizing enzymes, transporter proteins (p-glycoprotein; P-gp), and mu-opioid receptors may explain part of the observed interindividual variation in the pharmacokinetics and pharmacodynamics of methadone. Cytochrome P450 (CYP) 3A4 and 2B6 have been identified as the main CYP isoforms involved in methadone metabolism. Methadone is a P-gp substrate, and, although there are inconsistent reports, ABCB1 genetic polymorphisms also contribute slightly to the interindividual variability of methadone kinetics and influence dose requirements. Genetic polymorphism is the cause of high interindividual variability of methadone blood concentrations for a given dose; for example, in order to obtain methadone plasma concentrations of 250 ng/mL, doses of racemic methadone as low as 55 mg/day or as high as 921 mg/day can be required in a 70-kg patient without any co-medication. The clinician must be aware of the pharmacokinetic properties and pharmacological interactions of methadone in order to personalize methadone administration. In the future, pharmacogenetics, at a limited level, can also be expected to facilitate individualized methadone therapy.

Functional consequences of active hepatic uptake on cytochrome P450 inhibition in rat and human hepatocytes

A series of cytochrome P450 (P450) inhibition experiments were conducted with four hepatic uptake substrates (AZ3, AZ25, atorvastatin, and pitavastatin) using hepatocytes and recombinant P450s. The uptake was shown to be temperature-dependent and was inhibited by estrone sulfate, signifying an active component. At the lowest concentrations tested, the inhibitors concentrated up to 1000-fold in rat hepatocytes, but demonstrated only 5-fold greater P450 inhibition relative to recombinant rat P450s, indicating high intracellular binding. Inhibitor accumulation was considerably lower in human hepatocytes and an increase in inhibitory potency relative to recombinant human P450s was not obvious. This study highlights several technical and conceptual issues in the study of P450 inhibition mediated by compounds actively transported across the basolateral hepatocyte membrane. Primarily, the incubation medium concentration once the inhibitor has fully accumulated into the hepatocytes rather than the starting medium concentration, along with the extent of intracellular binding, must be considered as a foundation for in vitro-in vivo extrapolations. Additionally, it is suggested that if the K(m) value for the active uptake process is close to the P450 inhibition K(i), hepatocytes may be used only to establish the free drug accumulation ratio at a clinically relevant drug concentration, and this information, along with the (recombinant P450) K(i) value, may be used to simulate the likely impact of active hepatic uptake on P450 inhibition in vivo.

ABCG2/BCRP decreases the transfer of a food-born chemical carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in perfused term human placenta

We have studied the role of ATP binding cassette (ABC) transporters in fetal exposure to carcinogens using 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) a known substrate for ABC transporters as a model compound. In perfusion of human term placenta, transfer of (14)C-PhIP (2 microM) through the placenta resulted in fetal-to-maternal concentration ratio (FM ratio) of 0.72+/-0.09 at 6 h. The specific ABCG2 inhibitor KO143 increased the transfer of (14)C-PhIP from maternal to fetal circulation (FM ratio 0.90+/-0.08 at 6 h, p<0.05) while the ABCC1/ABCC2 inhibitor probenecid had no effect (FM ratio at 6 h 0.75+/-0.10, p=0.84). There was a negative correlation between the expression of ABCG2 protein in perfused tissue and the FM ratio of (14)C-PhIP (R=-0.81, p<0.01) at the end of the perfusion. The expression of ABCC2 protein did not correlate with FM ratio of PhIP (R: -0.11, p=0.76). In addition, PhIP induced the expression of ABC transporters in BeWo cells at mRNA level. In conclusion, our data indicates that ABCG2 decreases placental transfer of (14)C-PhIP in perfused human placenta. Also, PhIP may modify ABC transporter expression in choriocarcinoma cells.

Rate-limiting steps in hepatic drug clearance: comparison of hepatocellular uptake and metabolism with microsomal metabolism of saquinavir, nelfinavir, and ritonavir

The intrinsic metabolic clearance of saquinavir, nelfinavir, and ritonavir was determined over a range of concentrations (0.02-20 microM) in both rat liver microsomes and fresh isolated rat hepatocytes in suspension. Clearance values were found to be concentration dependent for both systems, and at low concentrations, microsomal clearance was much greater (7-14-fold) than in hepatocytes. Kinetic parameters showed substantially lower microsomal K(m) values (5-42 nM) compared with suspended rat hepatocytes (34-270 nM) but similar scaled V(max) values (2-26 nmol/min/g liver). In the absence of metabolism (achieved by pretreating hepatocytes with a mechanism-based inhibitor of cytochrome P450), saquinavir, nelfinavir, and ritonavir were actively and rapidly taken up into hepatocytes (cell/medium concentration ratios of 306-3352), and intracellular unbound drug concentrations between 5- and 12-fold higher than extracellular unbound concentrations were achieved. Comparison of the rate of uptake into hepatocytes with the rate of metabolism in hepatocytes and microsomes indicates that the former is the rate-limiting step at low concentrations. The rate of metabolism saturates at lower concentrations (100-400-fold) than the rate of uptake; hence, at the high concentrations metabolic rate-limited clearance occurs. In conclusion, the clearance of saquinavir, nelfinavir, and ritonavir is extremely rapid, and it is proposed that in the case of hepatocytes and by inference in vivo, the rate of uptake limits the metabolic clearance of these three drugs.

Physiology, structure, and regulation of the cloned organic anion transporters

1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.

Polymorphism in human organic cation transporters and metformin action

Considerable interindividual variabilities in clinical efficacy and adverse events are sometimes recognized in the treatment of Type 2 diabetes mellitus with oral antihyperglycemic drugs. Metformin is the most commonly used biguanide in clinical practice, and also improves insulin resistance and reduces cardiovascular risk. However, certain patients taking metformin do not respond sufficiently. The molecular reasons for the variability in response to metformin are not clear. However, it has been recently suggested that genetic factors may be responsible for the variability. Metformin is not metabolized but is transported by at least two organic cation transporters (OCT), OCT1 and OCT2. Recently, genetic polymorphisms in OCT 1 and OCT2 have been found to be associated with changes in pharmacokinetic/pharmacodynamic responses to substrate drugs. This review focuses on the impact of the genetic polymorphism of organic cation transporters on transport activity, and the implications for the clinical efficacy of metformin.

Comparison of immortalized Fa2N-4 cells and human hepatocytes as in vitro models for cytochrome P450 induction

Fa2N-4 cells have been proposed as a tool to identify CYP3A4 inducers. To evaluate whether Fa2N-4 cells are a reliable surrogate for cryopreserved human hepatocytes, we assessed the basal mRNA expression of 64 drug disposition genes in Fa2N-4 cells. Significant differences were found in the expression of major drug-metabolizing enzymes, nuclear receptors, and transporters between both cell types. Importantly, the expression of constitutive androstane receptor (CAR) and several hepatic uptake transporters was significantly lower (>50-fold) in Fa2N-4 cells, whereas the expression of pregnane X-receptor (PXR) and aryl hydrocarbon receptor (AhR) was similar between Fa2N-4 cells and human hepatocytes. By using an optimized induction assay for Fa2N-4 cells, CYP3A4 induction by rifampicin, the prototypical PXR activator, increased from 1.5- to 7-fold at the level of functional activity. With nine selected compounds, which are known inducers of CYP3A4 either via activation of PXR, CAR, or both, we evaluated CYP3A4 and CYP2B6 mRNA induction using Fa2N-4 cells and human hepatocytes. No response was observed in Fa2N-4 cells treated with the selective CAR activators 6-(4-chlorophenyl)imidazo[2,1-b][1,3]-thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime and artemisinin. CYP3A4 and CYP2B6 induction in Fa2N-4 cells were also low for phenytoin, phenobarbital, and efavirenz, which are dual activators of PXR/CAR. This finding was in agreement with the lack of expression of CAR. The EC(50) value for rifampicin-mediated CYP3A4 induction was 10-fold higher than that in human hepatocytes. This result could be attributed to the low expression of hepatic organic anion-transporting polypeptides OATP1B1 and OATP1B3 in Fa2N-4 cells. In summary, our findings identify limitations of Fa2N-4 cells as a predictive induction model.

Prediction and identification of drug interactions with the human ATP-binding cassette transporter multidrug-resistance associated protein 2 (MRP2; ABCC2)

The chemical space of registered oral drugs was explored for inhibitors of the human multidrug-resistance associated protein 2 (MRP2; ABCC2), using a data set of 191 structurally diverse drugs and drug-like compounds. The data set included a new reference set of 75 compounds, for studies of hepatic drug interactions with transport proteins, CYP enzymes, and compounds associated with liver toxicity. The inhibition of MRP2-mediated transport of estradiol-17beta-D-glucuronide was studied in inverted membrane vesicles from Sf9 cells overexpressing human MRP2. A total of 27 previously unknown MRP2 inhibitors were identified, and the results indicate an overlapping but narrower inhibitor space for MRP2 compared with the two other major ABC efflux transporters P-gp (ABCB1) and BCRP (ABCG2). In addition, 13 compounds were shown to stimulate the transport of estradiol-17beta-D-glucuronide. The experimental results were used to develop a computational model able to discriminate inhibitors from noninhibitors according to their molecular structure, resulting in a predictive power of 86% for the training set and 72% for the test set. The inhibitors were in general larger and more lipophilic and presented a higher aromaticity than the noninhibitors. The developed computational model is applicable in an early stage of the drug discovery process and is proposed as a tool for prediction of MRP2-mediated hepatic drug interactions and toxicity.

The effect of P-gp (Mdr1a/1b), BCRP (Bcrp1) and P-gp/BCRP inhibitors on the in vivo absorption, distribution, metabolism and excretion of imatinib

Imatinib is transported by P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP), however, the exact impact of these transporters on absorption, distribution, metabolism and excretion (ADME) of imatinib is not fully understood due to incomplete data. We have performed a comprehensive ADME study of imatinib given as single agent or in combination with the well known BCRP/P-gp inhibitors, elacridar and pantoprazole, in wild-type and P-gp and/or BCRP knockout mice. The absence of P-gp and BCRP together resulted in a significantly higher area under the plasma concentration-time curve (AUC) after i.v. administration, whereas the AUC after oral dosing was unaltered. Both elacridar and pantoprazole significantly increased the AUC of orally administered imatinib in wild-type but also in P-gp/BCRP knockout mice. This lower clearance was not due to a (further) reduction in biliary excretion. Fecal excretion was significantly reduced in P-gp and P-gp/BCRP knockout but not in BCRP knockout mice, whereas the brain penetration was significantly higher in P-gp/BCRP knockout mice compared to single P-gp or BCRP knockout or wild-type mice. In conclusion, P-gp and BCRP have only a modest effect on the ADME of imatinib in comparison to metabolic elimination. P-gp is the most prevalent factor for systemic clearance and limiting the brain penetration. The considerable drug-drug interaction observed with elacridar or pantoprazole is only partly mediated by inhibition of P-gp and BCRP and far more by the inhibition of other elimination pathways.

Role of OATP transporters in the disposition of drugs

During recent years, it has become increasingly recognized that drug transporters play important roles in drug absorption and disposition. Organic anion transporting polypeptides (OATPs) are membrane transporters critically involved in the cellular uptake of drugs in tissues important for pharmacokinetics, such as the intestine, liver and kidneys. Recent advances in the pharmacogenomics of OATP1B1 have revealed that OATP transporters can play important roles in explaining interindividual variability in drug pharmacokinetics, and thus contribute to interindividual as well as interethnic variability in drug response. This article will provide an up-to-date review of human OATPs and their substrates, and a current compilation of their DNA sequence variations.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

Predicting drug disposition, absorption/elimination/transporter interplay and the role of food on drug absorption

The ability to predict drug disposition involves concurrent consideration of many chemical and physiological variables and the effect of food on the rate and extent of availability adds further complexity due to postprandial changes in the gastrointestinal (GI) tract. A system that allows for the assessment of the multivariate interplay occurring following administration of an oral dose, in the presence or absence of meal, would greatly benefit the early stages of drug development. This is particularly true in an era when the majority of new molecular entities are highly permeable, poorly soluble, extensively metabolized compounds (BDDCS Class 2), which present the most complicated relationship in defining the impact of transporters due to the marked effects of transporter-enzyme interplay. This review evaluates the GI luminal environment by taking into account the absorption/transport/elimination interplay and evaluates the physiochemical property issues by taking into account the importance of solubility, permeability and metabolism. We concentrate on the BDDCS and its utility in predicting drug disposition. Furthermore, we focus on the effect of food on the extent of drug availability (F), which appears to follow closely what might be expected if a significant effect of high fat meals is inhibition of transporters. That is, high fat meals and lipidic excipients would be expected to have little effect on F for Class 1 drugs; they would increase F of Class 2 drugs, while decreasing F for Class 3 drugs.

A possible role of multidrug resistance-associated protein 2 (Mrp2) in hepatic excretion of PCB126, an environmental contaminant: PBPK/PD modeling

3,3',4,4',5'-Pentachlorobiphenyl (PCB126) is a carcinogenic environmental pollutant and its toxicity is mediated through binding with aryl hydrocarbon receptor (AhR). Earlier, we found that PCB126 treated F344 rats had 110-400 times higher PCB126 concentration in the liver than in the fat. Protein binding was suspected to be a major factor for the high liver concentration of PCB126 despite its high lipophilicity. In this research, we conducted a combined pharmacokinetic/pharmacodynamic study in male F344 rats. In addition to blood and tissue pharmacokinetics, we use the development of hepatic preneoplastic foci (glutathione-S-transferase placental form [GSTP]) as a pharmacodynamic endpoint. Experimental data were utilized for building a physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) model. PBPK/PD modeling was consistent with the experimental PK and PD data. Salient features of this model include: (1) bindings between PCB126 and hepatic proteins, particularly the multidrug resistance-associated protein (Mrp2), a protein transporter; (2) Mrp2-mediated excretion; and (3) a relationship between area under the curve of PCB126 in the livers and % volume of GSTP foci. Mrp2 involvement in PCB126 pharmacokinetics is supported by computational chemistry calculation using a three-dimensional quantitative structure-activity relationship model of Mrp2 developed by S. Hirono et al. (2005, Pharm. Res. 22, 260-269). This work, for the first time, provided a plausible role of a versatile hepatic transporter for drugs, Mrp2, in the disposition of an important environmental pollutant, PCB126.

Kinetic identification of membrane transporters that assist P-glycoprotein-mediated transport of digoxin and loperamide through a confluent monolayer of MDCKII-hMDR1 cells

A robust screen for compound interaction with P-glycoprotein (P-gp) has some obvious requirements, such as a cell line expressing P-gp and a probe substrate that is transported solely by P-gp and passive permeability. It is actually difficult to prove that a particular probe substrate interacts only with P-gp in the chosen cell line. Using a confluent monolayer of MDCKII-hMDR1 cells, we have determined the elementary rate constants for the P-gp efflux of amprenavir, digoxin, loperamide, and quinidine. For amprenavir and quinidine, transport was fitted with just P-gp and passive permeability. For digoxin and loperamide, fitting required a basolateral transporter (p < 0.01), which was inhibited by the P-gp inhibitor N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918). This means that when digoxin is used as a probe substrate and a compound is shown to inhibit digoxin flux, it could be that the inhibition occurs at the basolateral transporter rather than at P-gp. Digoxin basolateral>apical efflux also required an apical importer (p < 0.05). We propose that amprenavir and quinidine are robust probe substrates for assessing P-gp interactions using the MDCKII-hMDR1 confluent cell monolayer. Usage of another cell line, e.g., LLC-hMDR1 or Caco-2, would require the same kinetic validation to ensure that the probe substrate interacts only with P-gp. Attempts to identify the additional digoxin and loperamide transporters using a wide range of substrates/inhibitors of known epithelial transporters (organic cation transporters, organic anion transporters, organic ion-transporting polypeptide, uric acid transporter, or multidrug resistance-associated protein) failed to inhibit the digoxin or loperamide transport through their basolateral transporter.

In vitro-in vivo extrapolation of hepatic clearance involving active uptake: theoretical and experimental aspects

The importance of hepatic uptake transporters in drug clearance is well recognized. The subject is reviewed with the intention of providing an overview of the concepts in order to link the increasing knowledge of transporter-mediated uptake into established models of hepatic clearance. In order to understand and quantify their impact, models of hepatic elimination that incorporate permeability barriers are required. Models that include both active and passive uptake into hepatocytes are discussed and simulations of the influence of active uptake and passive diffusion on hepatic clearance are presented. The advantages and weaknesses of a number of in vitro assays of hepatic uptake are described, and their ability to predict hepatic clearance is reviewed.

Efficient delivery of siRNA using dendritic poly(L-lysine) for loss-of-function analysis

RNA interference (RNAi) is a valuable tool for the validation of gene identification and functional genomics. Previously, it was reported that 6th generation dendritic poly(L-lysine) (KG6) transfected DNA into several cultivated cell lines with high efficiency and without any cytotoxic effects. In this study, the potential of KG6 to be an efficient siRNA carrier is investigated. KG6 showed effective knockdown of GAPDH with low cytotoxicity in combination with the weak-base amphiphilic peptide, Endo-Porter. In addition, the knockdown of PEPCK, which is the rate-limiting enzyme for gluconeogenesis, led to a reduction in glucose production in rat hepatoma H4IIEC3 cells. Knockdown of organic cation transporter 1 (OCT1), which is thought to be the gene that influences metformin action, was shown to successfully diminish the ability of metformin to inhibit gluconeogenesis in H4IIEC3 cells. In conclusion, using a combination of KG6 and Endo-Porter, a model system in which genes that influence metformin action can be identified was successfully constructed.

Role of transporters in the disposition of the selective phosphodiesterase-4 inhibitor (+)-2-[4-({[2-(benzo[1,3]dioxol-5-yloxy)-pyridine-3-carbonyl]-amino}-methyl)-3-fluoro-phenoxy]-propionic acid in rat and human

The role of transporters in the disposition of (+)-2-[4-({[2-(benzo[1,3]dioxol-5-yloxy)-pyridine-3-carbonyl]-amino}-methyl)-3-fluoro-phenoxy]-propionic acid (CP-671,305), an orally active inhibitor of phosphodiesterase-4, was examined. In bile duct-exteriorized rats, a 7.4-fold decrease in the half-life of CP-671,305 was observed, implicating enterohepatic recirculation. Statistically significant differences in CP-671,305 pharmacokinetics (clearance and area under the curve) were discernible in cyclosporin A- or rifampicin-pretreated rats. Considering that cyclosporin A and rifampicin inhibit multiple uptake/efflux transporters, the interactions of CP-671,305 with major human hepatic drug transporters, multidrug resistance protein 1 (MDR1), multidrug resistance-associated protein 2 (MRP2), breast cancer resistant protein (BCRP), and organic anion-transporting polypeptide (OATPs) were evaluated in vitro. CP-671,305 was identified as a substrate of MRP2 and BCRP, but not MDR1. CP-671,305 was a substrate of human OATP2B1 with a high affinity (Km = 4 microM) but not a substrate for human OATP1B1 or OATP1B3. Consistent with these results, examination of hepatobiliary transport of CP-671,305 in hepatocytes indicated active uptake followed by efflux into bile canaliculi. Upon examination as a substrate for major rat hepatic Oatps, CP-671,305 displayed high affinity (Km = 12 microM) for Oatp1a4. The role of rat Mrp2 in the biliary excretion was also examined in Mrp2-deficient rats. The observations that CP-671,305 pharmacokinetics were largely unaltered suggested that compromised biliary clearance of CP-671,305 was compensated by increased urinary clearance. Overall, these studies suggest that hepatic transporters play an important role in the disposition and clearance of CP-671,305 in rat and human, and as such, these studies should aid in the design of clinical drug-drug interaction studies.

Pyrazole inhibitors of HMG-CoA reductase: An attempt to dramatically reduce synthetic complexity through minimal analog re-design

An extraordinarily potent and hepatoselective class of HMG-CoA reductase inhibitors containing a pyrazole core was recently reported; however, its development was hampered by a long and difficult synthetic route. We attempted to circumvent this obstacle by preparing closely related analogs wherein the key dihydroxyheptanoic acid sidechain was tethered to the pyrazole core via an oxygen linker ('oxypyrazoles'). This minor change reduced the total number of synthetic steps from 14 to 7. Although the resulting analogs maintained much of the in vitro and cell activity of the pyrazoles, inferior in vivo activity precluded further development. Caco-2 cell permeability data suggest that enhanced cellular efflux of the oxypyrazoles relative to the pyrazoles may be responsible for the poor in vivo activity.

Involvement of breast cancer resistance protein (ABCG2) in the biliary excretion mechanism of fluoroquinolones

Fluoroquinolones are effective antibiotics for the treatment of bile duct infections. It has been shown that the biliary excretion of grepafloxacin is partly accounted for by multidrug resistance-associated protein 2 (MRP2/ABCC2), whereas neither MRP2 nor P-glycoprotein is involved in the biliary excretion of ulifloxacin. In the present study, we examined the involvement of breast cancer resistance protein (BCRP/ABCG2) in the biliary excretion of fluoroquinolones (grepafloxacin, ulifloxacin, ciprofloxacin, and ofloxacin). In Madin-Darby canine kidney II cells expressing human BCRP or mouse Bcrp, the basal-to-apical transport of grepafloxacin and ulifloxacin was greater than that of the mock control, which was inhibited by a BCRP inhibitor, 3-(6-isobutyl-9-methoxy-1,4-dioxo-1,2,3,4,6,7,12,12a-octahydropyrazino[1',2':1,6]pyrido[3,4-b]indol-3-yl)-propionic acid tert-butyl ester (Ko143). Plasma and bile concentrations of fluoroquinolones were determined in wild-type and Bcrp(-/-) mice after i.v. bolus injection. The cumulative biliary excretion of fluoroquinolones was significantly reduced in Bcrp(-/-) mice, resulting in a reduction of the biliary excretion clearances to 86, 50, 40, and 16 of the control values, for ciprofloxacin, grepafloxacin, ofloxacin, and ulifloxacin, respectively. Preinfusion of sulfobromophthalein significantly inhibited the biliary excretion of grepafloxacin in Bcrp(-/-) mice. There was no change in the tissue/plasma concentration ratios of fluoroquinolones in the liver or brain, whereas those in the kidney were increased 3.6- and 1.5-fold for ciprofloxacin and grepafloxacin, respectively, in Bcrp(-/-) mice but were unchanged for ofloxacin and ulifloxacin. The present study shows that BCRP mediates the biliary excretion of fluoroquinolones and suggests that it is also involved in the tubular secretion of ciprofloxacin and grepafloxacin.

Organic anion transporter 3 (Oat3/Slc22a8) knockout mice exhibit altered clearance and distribution of penicillin G

The interaction of renal basolateral organic anion transporter 3 (Oat3) with commonly used pharmacotherapeutics (e.g., NSAIDs, beta-lactams, and methotrexate) has been studied extensively in vitro. However, the in vivo role of Oat3 in drug disposition, in the context of other transporters, glomerular filtration, and metabolism, has not been established. Moreover, recent investigations have identified inactive human OAT3 polymorphisms. Therefore, this investigation was designed to elucidate the in vivo role of Oat3 in the disposition of penicillin G and prototypical substrates using an Oat3 knockout mouse model. Oat3 deletion resulted in a doubling of penicillin's half-life (P < 0.05) and a reduced volume of distribution (P < 0.01), together yielding a plasma clearance that was one-half (P < 0.05, males) to one-third (P < 0.001, females) of that in wild-type mice. Inhibition of Oat3 abolished the differences in penicillin G elimination between genotypes. Hepatic accumulation of penicillin was 2.3 times higher in male knockouts (P < 0.05) and 3.7 times higher in female knockouts (P < 0.001). Female knockouts also exhibited impaired estrone-3-sulfate clearance. Oat3 deletion did not impact p-aminohippurate elimination, providing correlative evidence to studies in Oat1 knockout mice that suggest Oat1 governs tubular uptake of p-aminohippurate. Collectively, these findings are the first to indicate that functional Oat3 is necessary for proper elimination of xenobiotic and endogenous compounds in vivo. Thus Oat3 plays a distinct role in determining the efficacy and toxicity of drugs. Dysfunctional human OAT3 polymorphisms or instances of polypharmacy involving OAT3 substrates may result in altered systemic accumulation of beta-lactams and other clinically relevant compounds.

Multiple Inhibition Mechanisms and Prediction of Drug–Drug Interactions: Status of Metabolism and Transporter Models as Exemplified by Gemfibrozil–Drug Interactions

PURPOSE

To assess the consequences of multiple inhibitors and differential inhibition mechanisms on the prediction of 12 gemfibrozil drug-drug interactions (DDIs). In addition, qualitative zoning of transporter-related gemfibrozil and cyclosporine DDIs was investigated.

METHODS

The effect of gemfibrozil and its acyl-glucuronide on different enzymes was incorporated into a metabolic prediction model. The impact of CYP2C8 time-dependent inhibition by gemfibrozil acyl-glucuronide was assessed using repaglinide, cerivastatin, loperamide, rosiglitazone and pioglitazone DDIs. Gemfibrozil and cyclosporine inhibition data obtained in human embryonic kidney cells expressing OATP1B1 and hepatic input concentration ([I]in) were used for qualitative zoning of 14 transporter-mediated DDIs.

RESULTS

Incorporation of time-dependent inhibition by gemfibrozil glucuronide showed no significant improvement in the prediction, as CYP2C8 contributed <65% to the overall elimination of the victim drugs investigated. Qualitative zoning of OATP1B1 DDIs resulted in no false negative predictions; yet the magnitude of observed interactions was significantly over-predicted.

CONCLUSIONS

Time-dependent inhibition by gemfibrozil glucuronide is only important for victim drugs eliminated predominantly (>80%) via CYP2C8. Qualitative zoning of OATP1B1 inhibitors based on [I]in/K (i) is valid in drug screening to avoid false negatives. Refinement of the transporter model by incorporating the fraction of drug transported by a particular transporter is recommended.

Projection of Exposure and Efficacious Dose Prior to First-in-Human Studies: How Successful Have We Been?

PURPOSE

Preclinical and clinical data for 35 proprietary Bristol-Myers Squibb discovery compounds (years 1997 to 2005) were collected and analyzed. In each case, exposure and efficacy in human subjects were projected at the time of nomination (for development) prior to first-in-human dosing.

MATERIALS AND METHODS

Projections of area under the plasma concentration-time curve (AUC) in humans involved the use of one or more methods: (1) allometric scaling of animal pharmacokinetic data; (2) clearance projection employing in vitro data (liver microsomes and hepatocytes); (3) chimpanzee as an animal model; (4) the species-invariant time method; and (5) the Css-mean residence time or "Css-MRT" method. Whenever possible, prior clinical experience with lead compounds enabled the selection of the most appropriate method(s). Multiple approaches were also available at the time of the human efficacious dose projections: (1) efficacious exposure from animal efficacy models; (2) in vitro potency; and (3) prior experience with clinical leads.

RESULTS

Over the 8 year period described, AUC in humans was projected within 2-fold (20 out of 35 compounds; 57%), greater than 2-fold to 4-fold (11 out of 35 compounds; 32%), and greater than 4-fold (4 out of 35 compounds; 11%) of the observed value. At the time of writing, clinical efficacy data were available for 10 compounds only. In this instance, the efficacious doses were also projected within 2-fold (7 out of 10 compounds; 70%), greater than 2-fold to 4-fold (2 out of 10 compounds; 20%), and greater than 4-fold (1 out of 10 compounds; 10%) of the actual clinical dose.

CONCLUSION

Overall, it was possible to project human exposure and efficacious dose within 4-fold of observed clinical values for about 90% of the compounds.

Solution equilibria of copper(II) complexation with N,N′-(2,2′-azanediylbis(ethane-2,1-diyl))dipicolinamide: A bio-distribution and dermal absorption study

The protonation equilibria of a pentadentate ligand, N,N'-(2,2'-azanediylbis(ethane-2,1-diyl))dipicolinamide ([H(2)(5555)-N]) and the complexation of this ligand with Cu(II) Ca(II), Zn(II) and Ni(II) have been studied by pH-potentiometry, (1)H NMR spectroscopy and UV-vis spectrophotometry. (1)H NMR detected the protonation of the pyridyl groups and formation of Cu[H(2)(5555)-N]H species at low pH, while amide group deprotonation at higher pH resulted in the formation of Cu[H(2)(5555)-N]H(-1) and Cu[H(2)(5555)-N]H(-2) species in solution. Potentiometric detection of protonated species was limited by the acidic nature of the pyridyl nitrogen donors. From UV-vis spectroscopy it is suggested that the amide nitrogens are coordinated. This conclusion is supported by Molecular Mechanics calculations. Water-octanol partition coefficients for the Cu(II)-[H(2)(5555)-N] system indicated that although the Cu[H(2)(5555)-N]H(-1) species is largely hydrophilic, approximately 54% of the complex goes into the organic phase. This percentage is able to promote dermal absorption of copper with a calculated penetration rate of 1.92x10(-1)cmh(-1). This was confirmed by dermal absorption studies which illustrate the role of hydrophobicity in promoting percutaneous drug administration.

Primary hepatocytes: current understanding of the regulation of metabolic enzymes and transporter proteins, and pharmaceutical practice for the use of hepatocytes in metabolism, enzyme induction, transporter, clearance, and hepatotoxicity studies

This review brings you up-to-date with the hepatocyte research on: 1) in vitro-in vivo correlations of metabolism and clearance; 2) CYP enzyme induction, regulation, and cross-talk using human hepatocytes and hepatocyte-like cell lines; 3) the function and regulation of hepatic transporters and models used to elucidate their role in drug clearance; 4) mechanisms and examples of idiosyncratic and intrinsic hepatotoxicity; and 5) alternative cell systems to primary human hepatocytes. We also report pharmaceutical perspectives of these topics and compare methods and interpretations for the drug development process.

Expression of ABC-transporters in human corneal tissue and the transformed cell line, HCE-T

PURPOSE

The aim of this study was to elucidate the expression pattern of transport proteins relevant to drug absorption in human cornea and to assess the human corneal epithelial cell line, HCE-T, regarding its use as an in vitro model for drug-absorption studies.

METHODS

Human corneal tissue and HCE-T cells were examined for the expression of P-glycoprotein (P-gp/MDR1), multidrug resistance-associated protein 1 (MRP1), multidrug resistance-associated protein 2 (MRP2), lung resistance-related protein (LRP), and breast cancer-resistance protein (BCRP), using reverse transcriptase-polymerase chain reaction and immunofluorescence microscopy. Moreover, transporter activity was measured by bi-directional flux studies across excised human cornea and HCE-T cell layers using a P-gp/MDR1 substrate, rhodamine 123 (Rh123).

RESULTS

Transport studies of Rh123 revealed no significant differences in fluxes in the apical-to-basolateral and basolateral-to-apical directions across excised human corneas or HCE-T cell layers, suggesting the absence or insignificant, if any, participation of P-gp/MDR1 to Rh123 fluxes. Of all the transporter proteins under investigation, only LRP was found in human cornea. By contrast, a signal for LRP was not found in HCE-T, but the expression of MRP1, MRP2, and BCRP could be confirmed. Of note is the lack of P-gp/MDR1 expression in any of the specimens we examined.

CONCLUSIONS

Only a limited array of ABC-transporters is functionally expressed in human cornea. The expression pattern of HCE-T cells appears to be widely different from that of the native corneal tissue. Hence, the in vitro model of human cornea, HCE-T, should be used with much caution when predicting transport rates across the human corneal epithelial barrier in vivo.

Modulation of the cellular accumulation and intracellular activity of daptomycin towards phagocytized Staphylococcus aureus by the P-glycoprotein (MDR1) efflux transporter in human THP-1 macrophages and madin-darby canine kidney cells

P-glycoprotein (P-gp; MDR1), a major efflux transporter, recognizes various antibiotics and is present in macrophages. We have examined its effect on the modulation of the intracellular accumulation and activity of daptomycin towards phagocytized Staphylococcus aureus (ATCC 25923) in human THP-1 macrophages, in comparison with MDCK epithelial cells (wild type and MDCK-MDR1 overexpressing P-gp; the bulk of the protein was immunodetected at the surface of all three cell types). Daptomycin displayed concentration-dependent intracellular activity (Hill equation pattern) in THP-1 and MDCK cells with (i) 50% effective drug extracellular concentration (EC(50); relative potency) and static concentrations at 9 to 10 times the MIC and (ii) maximal efficacy (E(max); CFU decrease at infinite extracellular drug concentration) at 1.6 to 2 log compared to that of the postphagocytosis inoculum. Verapamil (100 microM) and elacridar (GF 120918; 0.5 microM), two known inhibitors of P-gp, decreased daptomycin EC(50) (about threefold) in THP-1 and MDCK cells without affecting E(max). Daptomycin EC(50) was about three- to fourfold higher and accumulation in MDCK-MDR1 commensurately lower than in wild-type cells. In THP-1 macrophages, (i) verapamil and ATP depletion increased, and ouabain (an inducer of mdr1 [the gene encoding P-gp] expression) decreased the accumulation of daptomycin in parallel with that of DiOC(2) (a known substrate of P-gp); (ii) silencing mdr1 with duplex human mdr1 siRNAs reduced the cell content in immunoreactive P-gp to 15 to 30% of controls and caused an eight- to 13-fold increase in daptomycin accumulation. We conclude that daptomycin is subject to efflux from THP-1 macrophages and MDCK cells by P-gp, which reduces its intracellular activity against phagocytized S. aureus.

Use of hepatocytes to assess the contribution of hepatic uptake to clearance in vivo

The wealth of information that has emerged in recent years detailing the substrate specificity of hepatic transporters necessitates an investigation into their potential role in drug elimination. Therefore, an assay in which the loss of parent compound from the incubation medium into hepatocytes ("media loss" assay) was developed to assess the impact of hepatic uptake on unbound drug intrinsic clearance in vivo (CL(int ub in vivo)). Studies using conventional hepatocyte incubations for a subset of 36 AstraZeneca new chemical entities (NCEs) resulted in a poor projection of CL(int ub in vivo) (r2 = 0.25, p = 0.002, average fold error = 57). This significant underestimation of CL(int ub in vivo) suggested that metabolism was not the dominant clearance mechanism for the majority of compounds examined. However, CL(int ub in vivo) was described well for this dataset using an initial compound "disappearance" CL(int) obtained from media loss assays (r2 = 0.72, p = 6.3 x 10(-11), average fold error = 3). Subsequent studies, using this method for the same 36 NCEs, suggested that the active uptake into human hepatocytes was generally slower (3-fold on average) than that observed with rat hepatocytes. The accurate prediction of human CL(int ub in vivo) (within 4-fold) for the marketed drug transporter substrates montelukast, bosentan, atorvastatin, and pravastatin confirmed further the utility of this assay. This work has described a simple method, amenable for use within a drug discovery setting, for predicting the in vivo clearance of drugs with significant hepatic uptake.

Interaction and transport characteristics of mycophenolic acid and its glucuronide via human organic anion transporters hOAT1 and hOAT3

The immunosuppressant mycophenolate mofetil (MMF) is frequently administered with calcineurin inhibitors and corticosteroids to recipients of organ transplantations. However, the renal handling of the active metabolite mycophenolic acid (MPA) and 7-O-MPA-glucuronide (MPAG) has been unclear. The purpose of the present study was to assess the interaction of MPA and MPAG with the human renal organic anion transporters hOAT1 (SLC22A6) and hOAT3 (SLC22A8), by conducting uptake experiments using HEK293 cells stably expressing these transporters. MPA and MPAG inhibited the time-dependent uptake of p-[(14)C]aminohippurate by hOAT1 and that of [(3)H]estrone sulfate by hOAT3. The apparent 50% inhibitory concentration (IC(50)) of MPA for hOAT1 and hOAT3 was estimated at 10.7 and 1.5 microM, respectively. In the case of MPAG, the IC(50) values were calculated at 512.3 microM for hOAT1 and 69.1 microM for hOAT3. Eadie-Hofstee plot analyses showed that they inhibited hOAT1 noncompetitively and hOAT3 competitively. No inhibitory effects of tacrolimus, cyclosporin A and azathioprine on transport of p-[(14)C]aminohippurate by hOAT1 and of [(3)H]estrone sulfate by hOAT3 were observed. No transport of MPA by these transporters was observed. On the other hand, the uptake of MPAG into cells was stimulated by the expression of hOAT3, but not hOAT1. These findings propose the possibility that the administration of MMF decreases the renal clearance of drugs which are substrates of hOAT1 and hOAT3. Present data suggest that hOAT3 contributes to the renal tubular secretion of MPAG.

Pharmacogenetics and pharmacogenomics of cholesterol-lowering therapy

PURPOSE OF REVIEW

To summarize recent findings on pharmacokinetics, pharmacodynamics, drug-drug interactions and influence of lifestyle heterogeneity on adverse events in cholesterol-lowering therapy

RECENT FINDINGS

The prevention of cardiovascular disease is critically dependent on lipid-lowery therapy, including statins, cholesterol absorption inhibitors, fibrates and nicotinic acid. Statins are the most prescribed drugs in lipid lowering therapy with variability in response and almost one third of the patients do not meet their treatment goals. The severe adverse effects of treatment with cerivastatin stimulated the search for new genes and gene variations affecting pharmacokinetics, drug-drug interactions and pharmacodynamics. Moreover, instead of monotherapy, combined therapy of statins with ezetemibe and niacin was considered. This led to the identification of CD13, NPC1L1 and HM74A as new targets and CYP2C8 and glucuronidation enzymes as potential targets for drug-drug interactions. Moreover multiple polymorphic sites and pleiotrophic gene targets were reinvestigated in larger cohorts and the relevant pathogenetic factors start to evolve.

SUMMARY

Statin therapy is widely used and well tolerated by the majority of patients. To further reduce potential adverse effects and to increase efficacy, combined therapy concepts with ezetimibe or niacin are underway.

Involvement of carrier-mediated transport system in uptake of tetrodotoxin into liver tissue slices of puffer fish Takifugu rubripes

Although puffer fish contain tetrodotoxin (TTX) at a high concentration mainly in liver, the underlying mechanism remains to be elucidated. In the present study, uptake of TTX into the liver tissue slices of puffer fish Takifugu rubripes was investigated by in vitro incubation experiment. When T. rubripes liver slices were incubated with 0-2000microM TTX at 20 degrees C for 60min, the uptake rates exhibited non-linearity, suggesting that the TTX uptake into T. rubripes liver is carrier-mediated. The TTX uptake was composed of a saturable component (V(max) 47.7+/-5.9pmol/min/mg protein and K(m) 249+/-47microM) and a non-saturable component (P(dif) 0.0335+/-0.0041microL/min/mg protein). The uptake of TTX was significantly decreased to 0.4 and 0.6 fold by the incubation at 5 degrees C and the replacement of sodium-ion by choline in the buffer, respectively, while it was not affected by the presence of 1mM l-carnitine, p-aminohippurate, taurocholate or tetraethylammonium. The TTX uptake by black scraper Thamnaconus modestus liver slices was much lower than that of T. rubripes and independent of the incubation temperature, unlike T. rubripes. These results reveal the involvement of carrier-mediated transport system in the TTX uptake by puffer fish T. rubripes liver slices.

Myotoxicity associated with lipid-lowering drugs

PURPOSE OF REVIEW

Lipid-lowering drugs are associated with myotoxicity, which ranges in severity from myalgias to rhabdomyolysis resulting in renal failure and death. Although rhabdomyolysis is rare, muscle symptoms and serum creatine kinase elevations are sufficiently frequent during the course of lipid-lowering drug therapy to pose diagnostic challenges for the clinician. Progress in our understanding of this form of myotoxicity is reviewed.

RECENT FINDINGS

Muscle pain and weakness are the cardinal symptoms and often interfere with vigorous exercise. These symptoms may occur with or without serum creatine kinase elevations. The risk of myotoxicity is increased by combination statin-fibrate therapy as well as by factors that elevate tissue levels of the lipid-lowering drug, including the dose, drug-drug interactions, and host factors. Underlying neuromuscular diseases may become clinically apparent during statin therapy and may predispose to myotoxicity. The pathophysiology of myotoxicity most probably involves metabolic effects of the statins on the isoprenoid pool and on mitochondrial function.

SUMMARY

Management of myotoxicity requires an evaluation of risk factors prior to prescribing lipid-lowering drugs, attention to muscle symptoms, and withdrawal of drug in the event of significant abnormalities.

Pharmacogenetics of Opioids

Opioids are used for acute and chronic pain and dependency. They have a narrow therapeutic index and large interpatient variability in response. Genetic factors regulating their pharmacokinetics (metabolizing enzymes, transporters) and pharmacodynamics (receptors and signal transduction elements) are contributors to such variability. The polymorphic CYP2D6 regulates the O-demethylation of codeine and other weak opioids to more potent metabolites with poor metabolizers having reduced antinociception in some cases. Some opioids are P-glycoprotein substrates, whereas, ABCB1 genotypes inconsistently influence opioid pharmacodynamics and dosage requirements. Single-nucleotide polymorphisms in the mu opioid receptor gene are associated with increasing morphine, but not methadone dosage requirements and altered efficacy of mu opioid agonists and antagonists. As knowledge regarding the interplay between genes affecting opioid pharmacokinetics including cerebral kinetics and pharmacodynamics increases, our understanding of the role of pharmacogenomics in mediating interpatient variability in efficacy and side effects to this important class of drugs will be better informed. Opioid drugs as a group have withstood the test of time in their ability to attenuate acute and chronic pain. Since the isolation of morphine in the early 1800s by Friedrich Sertürner, a large number of opioid drugs beginning with modification of the 4,5-epoxymorphinan ring structure were developed in order to improve their therapeutic margin, including reducing dependence and tolerance, ultimately without success.

P-Glycoprotein (P-gp) expressed in a confluent monolayer of hMDR1-MDCKII cells has more than one efflux pathway with cooperative binding sites

The multidrug resistance transporter P-glycoprotein (P-gp) effluxes a wide range of substrates and can be affected by a wide range of inhibitors or modulators. Many studies have presented classifications for these binding interactions, within either the context of equilibrium binding or the Michaelis-Menten enzyme analysis of the ATPase activity of P-gp. Our approach is to study P-gp transport and its inhibition using a physiologically relevant confluent monolayer of hMDR1-MDCKII cells. We measure the elementary rate constants for P-gp efflux of substrates and study inhibition using pairwise combinations with a different unlabeled substrate acting as the inhibitor. Our current kinetic model for P-gp has only a single binding site, because a previous study proved that the mass-action kinetics of efflux of a single substrate were not sensitive to whether there are one or more substrate-binding and efflux sites. In this study, using this one-site model, we found that, with "high" concentrations of either a substrate or an inhibitor, the elementary rate constants fitted independently for each of the substrates alone quantitatively predicted the efflux curves, simply applying the assumption that binding at the "one site" was competitive. On the other hand, at "low" concentrations of both the substrate and inhibitor, we found no inhibition of the substrate efflux, despite the fact that both the substrate and inhibitor were being well-effluxed. This was not an effect of excess "empty" P-gp molecules, because the competitive efflux model takes site occupancy into account. Rather, it is quantitative evidence that the substrate and inhibitor are being effluxed by multiple pathways within P-gp. Remarkably, increasing the substrate concentration above the "low" concentration, caused the inhibition to become competitive; i.e., the inhibitor became effective. These data and their analysis show that the binding of these substrates must be cooperative, either positive or negative.

The pivotal role of hepatocytes in drug discovery

This review promotes the value of isolated hepatocytes in modern Drug Discovery programmes and outlines how increased understanding, particularly in the area of in vitro-in vivo extrapolation (IVIVE), has led to more widespread use. The importance of in vitro metabolic intrinsic clearance data for predicting in vivo clearance has been acknowledged for several years and the greater utility of hepatocytes, compared with hepatic microsomes and liver slices, for this application is discussed. The application of hepatocytes in predicting drug-drug interactions (DDIs) resulting from reversible and irreversible (time-dependent) inhibition is relatively novel but affords the potential to study both phase I and phase II processes together with any impact of drug efflux and/or uptake (cellular accumulation). Progress in this area is reviewed along with current opinions on the comparative use of primary hepatocytes and higher throughput reporter gene-based systems for studying cytochrome P450 (CYP) induction. The appreciation of the role of transporter proteins in drug disposition continues to evolve. The study of hepatic uptake using isolated hepatocytes and the interplay between drug transport and metabolism with respect to both clearance and DDIs and subsequent IVIVE is also considered.

Impact of transporter-mediated drug absorption, distribution, elimination and drug interactions in antimicrobial chemotherapy

A comprehensive list of drug transporters has recently become available as a result of extensive genome analysis. Membrane transporters play important roles in determining the pharmacokinetic aspects of intestinal absorption, tissue distribution, and the urinary and biliary excretions of a wide variety of therapeutic drugs. The identification and characterization of transporters responsible for the transfer of nutrients and xenobiotics, including drugs, is expected to provide a scientific basis for understanding drug disposition, as well as the molecular mechanisms of drug-drug/drug-food/drug-protein interactions and inter-individual/inter-species differences. This review focuses on the influence of transporters on the pharmacokinetics of beta-lactam antibiotics, new quinolones, and other antimicrobial agents, as well as focusing on the drug-drug interactions associated with transporter-mediated uptake from the small intestine and transporter-mediated elimination from the kidney and liver.

Olfactory mucosa-expressed organic anion transporter, Oat6, manifests high affinity interactions with odorant organic anions

We have characterized the expression of organic anion transporter 6, Oat6 (slc22a20), in olfactory mucosa, as well as its interaction with several odorant organic anions. In situ hybridization reveals diffuse Oat6 expression throughout olfactory epithelium, yet olfactory neurons laser-capture microdissected from either the main olfactory epithelium (MOE) or the vomeronasal organ (VNO) did not express Oat6 mRNA. These data suggest that Oat6 is expressed in non-neuronal cells of olfactory tissue, such as epithelial and/or other supporting cells. We next investigated interaction of Oat6 with several small organic anions that have previously been identified as odortype components in mouse urine. We find that each of these compounds, propionate, 2- and 3-methylbutyrate, benzoate, heptanoate, and 2-ethylhexanoate, inhibits Oat6-mediated uptake of a labeled tracer, estrone sulfate, consistent with their being Oat6 substrates. Previously, we noted defects in the renal elimination of odortype and odortype-like molecules in Oat1 knockout mice. The finding that such molecules interact with Oat6 raises the possibility that odorants secreted into the urine through one OAT-mediated mechanism (Eraly et al., JBC 2006) are transported through the olfactory mucosa through another OAT-mediated mechanism. Oat6 might play a direct or indirect role in olfaction, such as modulation of the availability of odorant organic anions at the mucosal surface for presentation to olfactory neurons or facilitation of delivery to a distal site of chemosensation, among other possibilities that we discuss.