Multiple transporters affect the disposition of atorvastatin and its two active hydroxy metabolites: application of in vitro and ex situ systems

The role of BCS (biopharmaceutics classification system) and BDDCS (biopharmaceutics drug disposition classification system) in drug development

Biopharmaceutics Classification System and Biopharmaceutics Drug Distribution Classification System are complimentary, not competing, classification systems that aim to improve, simplify, and speed drug development. Although both systems are based on classifying drugs and new molecular entities into four categories using the same solubility criteria, they differ in the criterion for permeability and have different purposes. Here, the details and applications of both systems are reviewed with particular emphasis of their role in drug development.

Interaction of the antiviral drug telaprevir with renal and hepatic drug transporters

Telaprevir is a new, direct-acting antiviral drug that has been approved for the treatment of chronic hepatitis C viral infection. First data on drug-drug interactions with co-medications such as cyclosporine, tacrolimus and atorvastatin have been reported recently. Drug transporting proteins have been shown to play an important role in clinically observed drug-drug interactions. The aim of this study was therefore to systematically investigate the potential of telaprevir to inhibit drug transporting proteins. The effect of telaprevir on substrate uptake mediated by drug transporters located in human kidney and liver was investigated on a functional level in HEK293 cell lines that over-express single transporter. Telaprevir was shown to exhibit significant inhibition of the human renal drug transporters OCT2 and MATE1 with IC(50) values of 6.4 μM and 23.0 μM, respectively, whereas no inhibitory effect on OAT1 and OAT3 mediated transport by telaprevir was demonstrated. Liver drug transporters were inhibited with an IC(50) of 2.2 μM for OATP1B1, 6.8 μM for OATP1B3 and 20.7 μM for OCT1. Our data show that telaprevir exhibited significant potential to inhibit human drug transporters. In view of the inhibitory potential of telaprevir, clinical co-administration of telaprevir together with drugs that are substrates of renal or hepatic transporters should be carefully monitored.

Evaluation of organic anion-transporting polypeptide 1B1 and CYP3A4 activities in primary human hepatocytes and HepaRG cells cultured in a dynamic three-dimensional bioreactor system

The long-term stability of liver cell functions is a major challenge when studying hepatic drug transport, metabolism, and toxicity in vitro. The aim of the present study was to investigate organic anion-transporting polypeptide (OATP) 1B1 and CYP3A4 activities in fresh primary human hepatocytes and differentiated cryopreserved HepaRG cells when cultured in a three-dimensional (3D) bioreactor system. OATP1B1 activity was determined by loss from media experiments of [(3)H]estradiol-17β-D-glucuronide and atorvastatin acid (ATA) for up to 7 days in culture. ATA metabolite formation was determined at days 3 to 4 to evaluate CYP3A4 activity. Overall, the results showed that freshly isolated human hepatocytes inoculated in the bioreactor retained OATP1B1 activity for at least 7 days, whereas in HepaRG cells no OATP1B1 activity was observed beyond day 2. The activity data were in agreement with immunohistochemical stainings, which showed that OATP1B1 protein expression was preserved for at least 9 days in fresh human hepatocytes, whereas OATP1B1 was expressed markedly lower in HepaRG cells after 9 days in culture. Fresh human hepatocytes and HepaRG cells exhibited similar CYP3A4 activity in bioreactor culture, and immunohistochemical stainings supported these findings. Activity and mRNA expression of OATP1B1 and CYP3A4 in primary human hepatocytes compared with HepaRG cells in fresh suspensions were in agreement with data obtained in bioreactor culture. In conclusion, freshly isolated human hepatocytes cultured in a 3D bioreactor system preserve both OATP1B1 and CYP3A4 activities, allowing long-term in vitro studies on drug disposition and toxicity.

Assessment and modulation of forsythiaside absorption with MDCKII cells and validation with in situ intestinal experiment

Forsythiaside was characterized by low intestinal absorption by in situ rat experiment and Caco-2 cells. The mechanisms behind this low absorption had not yet been elucidated. The purpose of this study was to investigate the role of efflux transporters in the intestinal absorption of forsythiaside as a potential mechanism for its low small-intestinal absorption following oral administration. Polarized MDCKII cell lines stably transfected with human or murine complementary DNA encoding for various efflux transporters (P-gp/MDR1, MRP2 and Bcrp1) were used to study transepithelial transport of forsythiaside and compare results with the MDCKII-Wild type cells. The transportation inhibitors GF120918, MK571 and Ko143 were used to investigate the transport mechanism. The active transport of forsythiaside was found in MDCKII-WT cells. The MDCKII-MRP2 and MDCKII-Bcrp1 cells significantly increased forsythiaside efflux ratio compared with the parental cells due to the apically directed transport by MRP2 and Bcrp1, respectively. The efflux ratios in MRP2 and Bcrp1 transfected cell lines were greatly decreased in the presence of MK-571 and Ko143, respectively, which indicated that forsythiaside efflux by MRP2 and Bcrp1 were significantly inhibited by their selective inhibitors. MDCKII-MDR1 cells did not exhibit a significant reduction in the forsythiaside efflux compared with the parental cells, indicating that it was not a good substrate for MDR1. And the results were then validated by the in situ experiment. This study presents direct evidence that forsythiaside is effluxed by both MRP2 and Bcrp1, which may contribute to its poor oral bioavailability.

Assessment of extracts from red yeast rice for herb-drug interaction by in-vitro and in-vivo assays

Red yeast rice (RYR) is made by fermenting the yeast Monascus purpureus over rice. It is a source of natural red food colorants, a food garnish and a traditional medication. Results of the current study demonstrated that polar fractions of the RYR preparations contained herbal-drug interaction activity, which if left unremoved, enhanced P-glycoprotein activity and inhibited the major drug metabolizing cytochromes P450, i,e, CYP 1A2, 2C9 and 3A4. The data from Caco-2 cell absorption and animal model studies further demonstrated that the pharmacokinetic modulation effect by RYR preparations containing the polar fractions ("untreated" preparation) was greater than that from RYR preparations with the polar fractions removed ("treated" preparation). The data indicates a potential for herb-drug interactions to be present in RYR commonly sold as nutritional supplements when the polar fractions are not removed and this should be taken into consideration when RYR is consumed with medications, including verapamil.

Lead optimisation of pyrazoles as novel FPR1 antagonists

Optimisation of a series of pyrazole inhibitors of the human FPR1 receptor has been achieved. The use of an in vitro media loss assay was utilised to identify sub-series with more robust DMPK profiles. These were subsequently improved to generate analogues with attractive overall profiles.

Understanding the interplay of drug transporters involved in the disposition of rosuvastatin in the isolated perfused rat liver using a physiologically-based pharmacokinetic model

The role of hepatic uptake (Oatp1a1 and Oatp1b4) and efflux (Bcrp and Mrp2) transporters in the disposition of rosuvastatin were investigated using the isolated perfused rat liver (IPRL). A simple physiologically-based pharmacokinetic model was developed to quantitatively determine the interplay between the individual transporters. Uptake and elimination of rosuvastatin in the IPRL was rapid and extensive. In the presence of rifamycin (an equipotent inhibitor of both Oatp1a1 and Oatp1a4) the perfusate clearance of rosuvastatin was reduced, but rifampicin (a potent inhibitor of Oatp1a4) had no effect upon the perfusate clearance. This might indicate a limited role for Oatp1a4, but it is possible that Oatp1a1 (or other uptake transporters) may have redundancy in their affinity for rosuvastatin. In the presence of GF120918 (a potent inhibitor of Bcrp) and in the Wistar TR- rat (a naturally occurring mutant not expressing Mrp2) the biliary clearance was reduced and virtually abolished in the TR- pre-incubated GF120918. Bcrp and Mrp2 appear to represent the primary efflux mechanisms for rosuvastatin in the rat. Rosuvastatin disposition in the IPRL is mediated in part by Oatp1a1 and efflux is almost entirely by Mrp2 and Bcrp. Other uptake processes may be involved.

BDDCS applied to over 900 drugs

Here, we compile the Biopharmaceutics Drug Disposition Classification System (BDDCS) classification for 927 drugs, which include 30 active metabolites. Of the 897 parent drugs, 78.8% (707) are administered orally. Where the lowest measured solubility is found, this value is reported for 72.7% (513) of these orally administered drugs and a dose number is recorded. The measured values are reported for percent excreted unchanged in urine, LogP, and LogD (7.4) when available. For all 927 compounds, the in silico parameters for predicted Log solubility in water, calculated LogP, polar surface area, and the number of hydrogen bond acceptors and hydrogen bond donors for the active moiety are also provided, thereby allowing comparison analyses for both in silico and experimentally measured values. We discuss the potential use of BDDCS to estimate the disposition characteristics of novel chemicals (new molecular entities) in the early stages of drug discovery and development. Transporter effects in the intestine and the liver are not clinically relevant for BDDCS class 1 drugs, but potentially can have a high impact for class 2 (efflux in the gut, and efflux and uptake in the liver) and class 3 (uptake and efflux in both gut and liver) drugs. A combination of high dose and low solubility is likely to cause BDDCS class 4 to be underpopulated in terms of approved drugs (N = 53 compared with over 200 each in classes 1-3). The influence of several measured and in silico parameters in the process of BDDCS category assignment is discussed in detail.

Effects of uremic toxins on transport and metabolism of different biopharmaceutics drug disposition classification system xenobiotics

Chronic kidney disease (CKD) is recognized to cause pharmacokinetic changes in renally excreted drugs; however, pharmacokinetic changes are also reported for drugs that are nonrenally eliminated. Few studies have investigated how uremic toxins may affect drug transporters and metabolizing enzymes and how these may result in pharmacokinetic/metabolic changes in CKD. Here, we investigated the effects of uremic toxins and human uremic serum on the transport of the prototypical transporter substrate [(3) H]-estrone sulfate and three Biopharmaceutics Drug Disposition Classification System (BDDCS) drugs, propranolol, losartan, and eprosartan. We observed a significant decrease in [(3) H]-estrone sulfate, losartan, and eprosartan uptake with some uremic toxins in both transfected cells and rat hepatocytes. The uptake of losartan was decreased in rat and human hepatocytes (28% and 48%, respectively) in the presence of hemodialysis (HD) serum. Time-course studies of losartan showed a 27%, 65%, and 68% increase in area under the curve (AUC) in the presence of HD serum, rifampin, and sulfaphenazole, respectively. Intracellular losartan AUC decreased significantly in the treatment groups, and the metabolite AUC decreased by 41% and 26% in rifampin- and sulfaphenazole-treated group, respectively. The intracellular AUC of eprosartan increased 190% in the presence of HD serum. These studies indicate that the uremic toxins contained in HD serum play an important role in drug disposition through drug transporters, and that there would be differential effects depending on the BDDCS classification of the drug.

Alteration of hepatic but not renal transporter expression in diet-induced obese mice

Drug pharmacokinetics can be altered in obese and diabetic subjects. In consideration of the prevalence of obesity and diabetes, characterization of transporter expression in mouse models of diabetes and obesity may be a useful tool to aid in prediction of altered drug pharmacokinetics or adverse drug reactions. It has been reported that ob/ob mice, which display a severe obesity and diabetes phenotype, exhibit multiple changes in drug transporter expression in liver and kidney. In the present study, the mRNA and protein expression of major drug transporters was determined in livers and kidneys of diet-induced obese (DIO) C57BL/6J male mice. The mice were fed a high-fat diet (HFD) (60% fat) from 6 weeks of age and display obesity, fatty liver, and mild hyperglycemia. The HFD diet increased expression of multidrug resistance-associated proteins Abcc3 and 4 mRNA and protein in liver by 3.4- and 1.4-fold, respectively, compared with that detected in control mice fed a low-fat diet (LFD). In contrast, Abcc1 mRNA and protein decreased by 50% in livers of DIO mice compared with those in livers to lean mice. The HFD did not alter transporter expression in kidney compared with the LFD. In summary, unlike ob/ob and db/db mice, DIO mice exhibited a selective induction of efflux transporter expression in liver (i.e., Abcc3 and 4). In addition, diet-induced obesity affects transporter expression in liver but not kidney in the C57BL/6J mouse model. These data indicate that hepatic transporter expression is only slightly altered in a model of mild diabetes and nonalcoholic fatty liver disease and obesity.

Assessment and modulation of phillyrin absorption by P-gp using Caco-2 cells and MDR1-MDCKII cells

The study was to investigate the absorption mechanism and transport modulation of phillyrin by P-gp in Caco-2 cells and MDR1-MDCKII cells. Three concentrations of phillyrin were tested in transport studies. The absorptive transports of phillyrin in the two cell models were not concentration-dependent which indicated passive diffusion as the dominating process in the test concentrations. The absorptive P (app) were 7.15, 6.39 and 10.03 × 10(-6) cm s(-1), respectively, for different concentrations (2.2, 4.8 and 8.4 μg ml(-1)) in Caco-2 cells. And the low absorptive P (app) was consistent with the low oral bioavailability of phillyrin observed in pharmacokinetic experiments. In transport inhibition experiment, the efflux inhibitors, verapamil and GF120918 can increase the absorption of phillyrin in Caco-2 cells which suggested the involvement of efflux transporters. In the further inhibition experiment in MDR1-MDCKII cells, the absorption was greatly increased and the efflux of phillyrin was competitively inhibited by verapamil and GF120918, which confirmed the involvement of P-gp in the efflux of phillyrin.

No clinically relevant drug-drug interactions when dalcetrapib is co-administered with atorvastatin

OBJECTIVES

Dalcetrapib, which targets cholesteryl ester transfer protein, is in clinical development for prevention of cardiovascular events and is likely to be used concomitantly with statins. Two studies investigated co-administration of dalcetrapib with atorvastatin and any effects of the timing of atorvastatin on the pharmacokinetics of dalcetrapib.

RESEARCH DESIGN AND METHODS

Two crossover studies were performed in healthy subjects: a two-period study of dalcetrapib 900 mg concurrently with atorvastatin (concurrent dosing study) and a three-period study of dalcetrapib 600 mg (dose chosen for Phase III) with atorvastatin concurrently or serially 4 h after dalcetrapib (interval dosing study).

MAIN OUTCOME MEASURES

The primary pharmacokinetic end points were AUC(0 - 24) and C(max); lipid effects and tolerability were secondary end points.

RESULTS

In the concurrent study (n = 26), co-administration reduced dalcetrapib AUC(0 - 24) and C(max) and caused small changes in AUC(0 - 24) and C(max) of atorvastatin and its active metabolites. In the interval study (n = 52), serial and concurrent co-administration of atorvastatin resulted in similar reductions in dalcetrapib exposure that were comparable to those observed in the concurrent dosing study. Co-administration did not decrease the efficacy of dalcetrapib or atorvastatin and was generally well tolerated.

CONCLUSIONS

These results indicate no clinically relevant interactions for co-administration of dalcetrapib with atorvastatin.

Intestinal absorption of HMG-CoA reductase inhibitor pitavastatin mediated by organic anion transporting polypeptide and P-glycoprotein/multidrug resistance 1

The purpose of this study was to investigate the involvement of organic anion transporting polypeptide (OATP/Oatp) and P-glycoprotein (P-gp)/multidrug resistance 1 (MDR1/Mdr1) in intestinal absorption of pitavastatin. Pitavastatin was found to be a substrate for human OATP1A2, OATP2B1, and MDR1 and rat Oatp1a5, Oatp2b1, and Mdr1a in experiments using transporter-expressing Xenopus oocytes and LLC-PK1 cell systems. Naringin inhibited Oatp1a5- and Mdr1a-mediated transport of pitavastatin with IC(50) values of 18.5 and 541 µM, respectively. The difference in the IC(50) values of naringin for Oatp1a5 and Mdr1a-mediated pitavastatin transport may account for the complex concentration-dependent effect of naringin on the intestinal absorption of pitavastatin. Rat intestinal permeability of pitavastatin measured by the in situ closed-loop perfusion method was indeed significantly reduced by 200 µM naringin, but was significantly increased by 1000 µM naringin. Furthermore, the permeability was significantly increased by elacridar, a potent inhibitor of Mdr1, while the permeability was significantly decreased in the presence of both elacridar and naringin, suggesting that Oatp1a5 and Mdr1a are both involved in intestinal absorption of pitavastatin. Our present results indicate that OATP/Oatp and MDR1/Mdr1 play roles in the intestinal absorption of pitavastatin as influx and efflux transporters, respectively.

Clearance (née Rowland) concepts: a downdate and an update

A number of experimental observations in the late 1960s, early 1970s could not be explained by the pharmacokinetic theory available at that time. For example rats receiving phenobarbital as an enzyme inducing agent exhibited increased elimination of phenylbutazone in vitro in liver microsomes and in vivo in whole animals compared to that observed in non-induced animals. However, for desipramine, although phenobarbital increased elimination in microsomes, no change in plasma disappearance was noted in vivo for this drug between rats induced with phenobarbital and control rats. Similar in vitro-in vivo discordancies were seen with changes in protein binding. The introduction of clearance concepts in the early 1970s by Professor Rowland and others provided the scientific rationale for these apparently contradictory findings and the recognition that clearance, not half-life, was the measure of the body's ability to eliminate drugs and most importantly that changes in pathology and physiology could be correlated with measures of clearance. Up to that time half-life was well recognized in terms of basic chemical principles as an appropriate measure of the rate of elimination and reflective of changes in the rate of elimination. The difference between chemistry and pharmacokinetics, however, is that in chemistry the volume in which the reaction occurs does not change. In contrast, in pharmacokinetics, disease states and differences in physiology can change the space available in which the drug may distribute in the body. Thus, it was necessary to develop a pharmacokinetic measure of volume that was independent of elimination, i.e., V(ss). Now, the relationship between V(ss) and clearance led to a unique measure of time of drug in the body, the mean residence time. Although this parameter is calculated in all PK programs, very few pharmaceutical scientists know how it can be useful. Very recently, we have shown that the concepts of accumulation, prediction of which is the clinically relevant use for half-life and mean residence time, are flawed and that the appropriate time dependent parameter to predict accumulation has not been previously correctly identified. Finally, when clearance concepts were developed our understanding of the importance of drug transporters was nonexistent. A critical, and generally unrecognized assumption (which is only explicitly stated in Professor Rowland's seminal 1973 paper), in the development of the theory of clearance is that the unbound drug concentration in the organ of elimination is in a constant equilibrium with the unbound drug concentration in the systemic circulation, where drug concentration measurements are made. Transporter drug-drug and disease interactions may, in fact, change this equilibrium and potentially what we consider as intrinsic clearance, may not be independent of an eliminating organ volume parameter, contrary to what we have been teaching for the past 37 years.

Organic anion transporting polypeptide 1a/1b-knockout mice provide insights into hepatic handling of bilirubin, bile acids, and drugs

Organic anion transporting polypeptides (OATPs) are uptake transporters for a broad range of endogenous compounds and xenobiotics. To investigate the physiologic and pharmacologic roles of OATPs of the 1A and 1B subfamilies, we generated mice lacking all established and predicted mouse Oatp1a/1b transporters (referred to as Slco1a/1b-/- mice, as SLCO genes encode OATPs). Slco1a/1b-/- mice were viable and fertile but exhibited markedly increased plasma levels of bilirubin conjugated to glucuronide and increased plasma levels of unconjugated bile acids. The unexpected conjugated hyperbilirubinemia indicates that Oatp1a/1b transporters normally mediate extensive hepatic reuptake of glucuronidated bilirubin. We therefore hypothesized that substantial sinusoidal secretion and subsequent Oatp1a/1b-mediated reuptake of glucuronidated compounds can occur in hepatocytes under physiologic conditions. This alters our perspective on normal liver functioning. Slco1a/1b-/- mice also showed drastically decreased hepatic uptake and consequently increased systemic exposure following i.v. or oral administration of the OATP substrate drugs methotrexate and fexofenadine. Importantly, intestinal absorption of oral methotrexate or fexofenadine was not affected in Slco1a/1b-/- mice. Further analysis showed that rifampicin was an effective and specific Oatp1a/1b inhibitor in controlling methotrexate pharmacokinetics. These data indicate that Oatp1a/1b transporters play an essential role in hepatic reuptake of conjugated bilirubin and uptake of unconjugated bile acids and drugs. Slco1a/1b-/- mice will provide excellent tools to study further the role of Oatp1a/1b transporters in physiology and drug disposition.

From drug delivery systems to drug release, dissolution, IVIVC, BCS, BDDCS, bioequivalence and biowaivers

This is a summary report of the conference on drug absorption and bioequivalence issues held in Titania Hotel in Athens (Greece) from the 28(th) to the 30(th) of May 2009. The conference included presentations which were mainly divided into three sections. The first section focused on modern drug delivery systems such as polymer nanotechnology, cell immobilization techniques to deliver drugs into the brain, nanosized liposomes used in drug eluting stents, encapsulation of drug implants in biocompatible polymers, and application of differential scanning calorimetry as a tool to study liposomal stability. The importance of drug release and dissolution were also discussed by placing special emphasis on camptothecins and oral prolonged release formulations. The complexity of the luminal environment and the value of dissolution in lyophilized products were also highlighted. The second session of the conference included presentations on the Biopharmaceutics Classification Scheme (BCS), the Biopharmaceutics Drug Disposition Classification System (BDDCS), and the role of transporters in the classification of drugs. The current status of biowaivers and a modern view on non-linear in vitro-in vivo (IVIVC) correlations were also addressed. Finally, this section ended with a special topic on biorelevant dissolution media and methods. The third day of the conference was dedicated to bioequivalence. Emphasis was placed on high within-subject variability and its impact on study design. Two unresolved issues of bioequivalence were also discussed: the use of generic antiepileptic drugs and the role of metabolites in bioequivalence assessment. Finally, the conference closed with a presentation of the current regulatory status of WHO and EMEA.

The drug transporter-metabolism alliance: uncovering and defining the interplay

Two decades ago the importance of transporter-enzyme interplay and its effects on drug bioavailability and hepatic disposition were first recognized. Here we review the history of uncovering and defining this interplay with a primary emphasis on studies from our laboratory. We review the early 1990s oral bioavailability studies that found that the highly lipophilic, poorly water-soluble cyclosporine formulation on the market at that time did not have an absorption problem, but rather a gut metabolism problem. This led to studies of the interactive nature of CYP3A and P-glycoprotein in the intestine, and investigations of this interplay using cellular systems and isolated perfused rat organ studies. Studies investigating uptake transporter-enzyme interactions using cellular, perfused rat liver and intact rats are reviewed, followed by the human transporter-enzyme interaction studies. Work characterizing the rate limiting processes in the drug transporter-metabolism alliance is then addressed, ending with a review of areas of the interplay that require further studies and analysis.

Interplay of transporters and enzymes in drug and metabolite processing

This review highlights the "interplay" between enzymes and transporters, essential components of eliminating organs for drug removal. The understanding of the interplay is important in terms of deciphering the change of one eliminatory pathway on compensatory mechanisms in drug disposal, and, ultimately, their importance in drug-drug interactions. Controversy existed on the explanation underlying the interplay between transporters and enzymes in the Caco-2 cell monolayer or cell culture systems, but less so on eliminating organs such as the intestine and liver. For the Caco-2 system, the increase in the mean residence time (MRT) accompanying increased secretion had been construed as the basis for increased metabolism. We hold the opposite view and assert that increased secretion should evoke a decrease in metabolism due to the competition between the enzyme and apical efflux transporter for the drug within the cell. To illustrate this point, simulations on the MRT, fraction of dose metabolized (f(met)) and the extraction ratio (ER) as defined by various investigators under linear and nonlinear metabolic conditions were compared to observed data and the trends upon induction/inhibition of secretion. The conclusion is that the f(met) is the more appropriate index to reflect the extent of metabolism in transporter-enzyme interplay, since the parameter captures drug metabolism in the cell when its contents in the apical, cell, and basolateral compartments or the entire dose is considered to be available for metabolism. This parameter for metabolism (f(met)) bears a reciprocal relationship to the secretory intrinsic clearance and is in concordance with the notion that both the enzyme and apical transporter compete for the cellular substrate within. For the liver and intestine, several physiologically based pharmacokinetic (PBPK) models that contain transporters and enzymes were utilized, together with the solved equations for the area under the curve (AUC), metabolic, excretory, and total clearance (CL) to shed meaningful insight of how the inhibition of one pathway can result in a higher AUC and therefore a reduced total clearance for drug, but a higher apparent clearance of the alternate pathway; induction of the same pathway would lead to an increased total clearance but decreased drug AUC, and reduced clearance of the alternate pathway. The use of an increased MRT to explain increased extents of metabolism upon increased apical excretion is not tenable in these organs or "open systems" since the MRT of drug in the cell is reduced with irreversible loss from biliary excretion or hastened gastrointestinal transit of the secreted drug in the lumen. Data in the literature for the Caco-2 system, knockout animals and organ perfusion systems were discussed in relation to these concepts on clearance based on fundamental, pharmacokinetic theory. The shortcomings in data interpretation were discussed. The general conclusion is that a reciprocal relationship exists between the clearances related to enzymes and apical transporters due to their competition for the substrate within the cell, and is a relationship independent of the MRT of drug in the system.

Hepatic OATP1B transporters and nuclear receptors PXR and CAR: interplay, regulation of drug disposition genes, and single nucleotide polymorphisms

Drug uptake transporters are now increasingly recognized as clinically relevant determinants of variable drug responsiveness and unexpected drug-drug interactions. Emerging evidence strongly suggests members of the organic anion transporting polypeptide (OATP) family appear to be particularly important to the disposition of many drugs in clinical use today. Specifically, the liver-enriched OATP1B subfamily members OATP1B1 and OATP1B3 exhibit broad substrate specificity and the ability to transport drugs which are ligands for xenobiotic sensing nuclear receptors such as the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Accordingly, OATP1B transporters may indirectly regulate expression of drug metabolism genes via modulation of the intracellular concentration of PXR and CAR ligands. Moreover, a number of functionally important single nucleotide polymorphisms (SNPs) in OATP1B transporters have been described. In this review, a brief summary of known SNPs in PXR and CAR will be followed by an in-depth outline of OATP1B1 and OATP1B3 transporters particularly in relation to the known SNPs in these OATPs and the interplay between OATP1B transporters with PXR and CAR, both in vitro and in vivo.

Predicting drug disposition via application of a Biopharmaceutics Drug Disposition Classification System

A Biopharmaceutics Drug Disposition Classification System (BDDCS) was proposed to serve as a basis for predicting the importance of transporters in determining drug bioavailability and disposition. BDDCS may be useful in predicting: routes of drug elimination; efflux and absorptive transporters effects on oral absorption; when transporter-enzyme interplay will yield clinically significant effects (e.g. low drug bioavailability and drug-drug interactions); and transporter effects on post-absorptive systemic drug levels following oral and i.v. dosing. For highly soluble, highly permeable Class 1 compounds, metabolism is the major route of elimination and transporter effects on drug bioavailability and hepatic disposition are negligible. In contrast for the poorly permeable Class 3 and 4 compounds, metabolism only plays a minor role in drug elimination. Uptake transporters are major determinants of drug bioavailability for these poorly permeable drugs and both uptake and efflux transporters could be important for drug elimination. Highly permeable, poorly soluble, extensively metabolized Class 2 compounds present the most complicated relationship in defining the impact of transporters due to a marked transporter-enzyme interplay. Uptake transporters are unimportant for Class 2 drug bioavailability, (ensure space after,) but can play a major role in hepatic and renal elimination. Efflux transporters have major effects on drug bioavailability, absorption, metabolism and elimination of Class 2 drugs. It is difficult to accurately characterize drugs in terms of the high permeability criteria, i.e. > or =90% absorbed. We suggest that extensive metabolism may substitute for the high permeability characteristic, and that BDDCS using elimination criteria may provide predictability in characterizing drug disposition profiles for all classes of compounds.

Multiple efflux pumps are involved in the transepithelial transport of colchicine: combined effect of p-glycoprotein and multidrug resistance-associated protein 2 leads to decreased intestinal absorption throughout the entire small intestine

The purpose of this study was to thoroughly characterize the efflux transporters involved in the intestinal permeability of the oral microtubule polymerization inhibitor colchicine and to evaluate the role of these transporters in limiting its oral absorption. The effects of P-glycoprotein (P-gp), multidrug resistance-associated protein 2 (MRP2), and breast cancer resistance protein (BCRP) inhibitors on colchicine bidirectional permeability were studied across Caco-2 cell monolayers, inhibiting one versus multiple transporters simultaneously. Colchicine permeability was then investigated in different regions of the rat small intestine by in situ single-pass perfusion. Correlation with the P-gp/MRP2 expression level throughout different intestinal segments was investigated by immunoblotting. P-gp inhibitors [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), verapamil, and quinidine], and MRP2 inhibitors [3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK571), indomethacin, and p-aminohippuric acid (p-AH)] significantly increased apical (AP)-basolateral (BL) and decreased BL-AP Caco-2 transport in a concentration-dependent manner. No effect was obtained by the BCRP inhibitors fumitremorgin C (FTC) and pantoprazole. P-gp/MRP2 inhibitors combinations greatly reduced colchicine mucosal secretion, including complete abolishment of efflux (GF120918/MK571). Colchicine displayed low (versus metoprolol) and constant permeability along the rat small-intestine. GF120918 significantly increased colchicine permeability in the ileum with no effect in the jejunum, whereas MK571 augmented jejunal permeability without changing the ileal transport. The GF120918/MK571 combination caused an effect similar to that of MK571 alone in the jejunum and to that of GF120918 alone in the ileum. P-gp expression followed a gradient increasing from proximal to distal segments, whereas MRP2 decreased from proximal to distal small intestinal regions. Overall, it was revealed that the combined effect of P-gp and MRP2, but not BCRP, dominates colchicine transepithelial transport, leading to complete coverage of the entire small intestine, and makes the efflux transport dominate the intestinal permeability process.

Prediction of the pharmacokinetics of atorvastatin, cerivastatin, and indomethacin using kinetic models applied to isolated rat hepatocytes

The disposition of atorvastatin, cerivastatin, and indomethacin, established substrates of rat hepatic basolateral uptake transporters, has been evaluated in suspended rat hepatocytes. Cell and media concentration-time data were simultaneously fitted to a model incorporating active uptake, permeation, binding, and metabolism. Use of the model to estimate the ratio of intracellular to extracellular steady-state free drug concentrations demonstrated the strong influence of active uptake on the kinetics of atorvastatin (18:1) and cerivastatin (8:1), in comparison with indomethacin (3.5:1). Indomethacin, however, was shown to have a higher uptake clearance (599 +/- 101 microl/min/10(6) cells) than atorvastatin (375 +/- 45 microl/min/10(6) cells) and cerivastatin (413 +/- 47 microl/min/10(6) cells). The high passive permeability of indomethacin (237 +/- 63 microl/min/10(6) cells) clearly negated the effect of the active transport on the overall disposition. An analogous physiological model was constructed that allowed prediction of the in vivo pharmacokinetics, including the free intracellular concentration in liver. Hepatic clearance was well predicted by the model, in contrast to predictions based on standard methods. Volume of distribution was well predicted for indomethacin and predicted reasonably for atorvastatin and cerivastatin and higher than might be expected for an acid compound. Furthermore, the terminal half-life predictions for all three compounds were within 2-fold of the observed values. The ability to estimate the free-intracellular hepatic concentration of uptake substrates has major benefits in terms of predicting pharmacokinetics, potential CYP-mediated drug-drug interactions, and efficacy of hepatically targeted therapeutics.

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.

The impact of host pharmacogenetics on antiretroviral drug disposition

The ability to predict efficacy and toxicity during antiretroviral therapy for HIV would be of obvious advantage. The substantial variability between patients in terms of bioavailability and distribution of current regimens is likely driven by genetic and environmental factors. Protease inhibitors and nucleoside/nucleotide reverse transcriptase inhibitors are metabolized by cytochrome P450 enzymes. Their bioavailability and excretion may also be affected by variability in drug transporters of the ABC and SLC families. In pharmacokinetics and efficacy studies, issues are complicated by multiple loci effects (driven by the large number of proteins contributing to disposition) and heterogeneity in both study populations and the virus (ie, the target). Some of these issues are now being tackled, but studies need to be sufficiently powered and the phenotype carefully characterized. This review aims to summarize the current understanding of pharmacogenetic determinants of antiretroviral disposition.

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.

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.

Saturable uptake of lipophilic amine drugs into isolated hepatocytes: mechanisms and consequences for quantitative clearance prediction

The hepatic uptake of quinine, fluvoxamine, and fluoxetine (0.1-10 microM) was investigated with freshly isolated rat hepatocytes. The cell-to-medium concentration ratios (K(p)) were concentration-dependent: the mean maximum K(p) values (at 0.1 microM) were 150 (quinine), 500 (fluvoxamine), and 2000 (fluoxetine). There was also a large capacity site that was not saturable over the concentration range used (possibly partition into the phospholipid component of membranes); representing this site, the mean minimum K(p) values (at 10 microM) were 30 (quinine), 200 (fluvoxamine), and 500 (fluoxetine). To eliminate concomitant metabolism, cells were pretreated with the irreversible P450 inhibitor, aminobenzotriazole. The saturable uptake was substantially eliminated after exposure to carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (ATP inhibitor). The difference between the maximum and minimum K(p) for these three amine drugs, as well as for dextromethorphan, propranolol, and imipramine, was within a limited range of 3-fold, indicating a common magnitude of saturable uptake. Basic, permeable drugs are expected to be sequestered into lysosomes, which actively maintain their low internal pH (approximately 5) using ATP, and this process is predictable from the combined effects of pH-driven ion accumulation and unsaturable binding representing partition into membranes. The resultant predicted maximum K(p) correlated strongly with the observed maximum K(p). Thus, at low substrate concentrations, the fraction of drug unbound in the hepatocyte incubation (critical for assessing drug clearance and drug-drug interaction potential) may be dependent upon saturable as well as unsaturable binding, and for lipophilic, basic drugs, this can be readily estimated assuming a common degree of uptake into lysosomes.

SLCO1B1 521T-->C functional genetic polymorphism and lipid-lowering efficacy of multiple-dose pravastatin in Chinese coronary heart disease patients

AIMS

Pravastatin is a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, which is widely used both in primary and secondary prevention of coronary heart disease (CHD). Pravastatin is not subject to metabolism by cytochrome P450s, but it is actively transported from blood into target tissues (e.g. hepatocytes in the liver) by the organic anion transporting polypeptide 1B1 (OATP1B1), encoded by SLCO1B1. The aim of the present study was to evaluate the impact of SLCO1B1 521T-->C (Val174Ala) functional genetic polymorphism on the lipid-lowering efficacy of multiple-dose pravastatin in Chinese patients with CHD.

METHODS

Forty-five hospitalized patients with CHD prospectively received pravastatin as a single-agent therapy (20 mg day(-1) p.o.) for 30 days. Serum triglycerides, total cholesterol, low-density lipoprotein-cholesterol and high-density lipoprotein-cholesterol concentrations were determined before and after pravastatin treatment.

RESULTS

Pravastatin treatment significantly decreased plasma lipids in all patients (P < 0.001). Importantly, we showed an attenuated pravastatin pharmacodynamic effect on total cholesterol in patients with 521TC heterozygote genotype (from 5.52 +/- 0.51 mmol l(-1) to 4.70 +/- 0.35 mmol l(-1), % change -14.5 +/- 6.6%, N = 9) compared with 521TT homozygote genotype (from 5.47 +/- 1.15 mmol l(-1) to 4.21 +/- 0.89 mmol l(-1), % change -22.4 +/- 10.3%, N = 36) (mean +/- SD, P = 0.03, two-tailed test with alpha set at 5%). SLCO1B1 521T-->C functional polymorphism did not significantly influence pravastatin pharmacodynamics on other plasma lipids (P > 0.05).

CONCLUSIONS

The 521T-->C polymorphism of SLCO1B1 appears to modulate significantly the total cholesterol-lowering efficacy of pravastatin in Chinese patients with CHD. Further studies are warranted to determine the extent to which SLCO1B1 genetic variation may contribute to resistance to pravastatin in Asian patients treated with standard doses of pravastatin.

effect of OATP1B transporter inhibition on the pharmacokinetics of atorvastatin in healthy volunteers

The inhibition of hepatic uptake transporters, such as OATP1B1, on the pharmacokinetics of atorvastatin is unknown. Here, we investigate the effect of a model hepatic transporter inhibitor, rifampin, on the kinetics of atorvastatin and its metabolites in humans. The inhibitory effect of a single rifampin dose on atorvastatin kinetics was studied in 11 healthy volunteers in a randomized, crossover study. Each subject received two 40-mg doses of atorvastatin, one on study day 1 and one on study day 8, separated by 1 week. One intravenous 30-min infusion of 600 mg rifampin was administered to each subject on either study day 1 or study day 8. Plasma concentrations of atorvastatin and metabolites were above the limits of quantitation for up to 24 h after dosing. Rifampin significantly increased the total area under the plasma concentration-time curve (AUC) of atorvastatin acid by 6.8+/-2.4-fold and that of 2-hydroxy-atorvastatin acid and 4-hydroxy-atorvastatin acid by 6.8+/-2.5- and 3.9+/-2.4-fold, respectively. The AUC values of the lactone forms of atorvastatin, 2-hydroxy-atorvastatin and 4-hydroxy-atorvastatin, were also significantly increased, but to a lower extent. An intravenous dose of rifampin substantially increased the plasma concentrations of atorvastatin and its acid and lactone metabolites. The data confirm that OATP1B transporters represent the major hepatic uptake systems for atorvastatin and its active metabolites. Inhibition of hepatic uptake may have consequences for efficacy and toxicity of drugs like atorvastatin that are mainly eliminated by the hepatobiliary system.

Elucidating the Effect of Final-Day Dosing of Rifampin in Induction Studies on Hepatic Drug Disposition and Metabolism

Because rifampin (RIF) induces hepatic enzymes and inhibits uptake transporters, dosing a drug that is a dual substrate of enzymes and uptake transporters on the final day of an inducing regimen should exhibit less inductive effect than dosing on the following day in the absence of RIF, since RIF decreases drug uptake into liver. In vitro and in vivo rat studies were conducted using digoxin as a model substrate. Digoxin was administered to an uninduced control group to obtain baseline values. The second group (induced with dexamethasone) received digoxin alone, mimicking administration of a test drug 1 day following completion of an induction regimen, whereas the third group (induced) received digoxin with RIF mimicking the concomitant dosing on the final day of an induction regimen. Results from hepatocyte concentration-time course studies showed that compared with uninduced control (26.9 +/- 1.3 microM . min/mg), digoxin area under the time-concentration curve (AUC) in induced cells when no RIF is present decreased significantly (13.7 +/- 0.9 microM . min/mg; p < 0.01), suggesting induction of Cyp3a. However, digoxin AUC for induced cells in the presence of RIF (27.3 +/- 0.9 microM . min/mg) matched the control. Rat pharmacokinetic studies showed that compared with digoxin clearance in uninduced controls (7.08 +/- 1.57 ml/min/kg), digoxin clearance in induced rats increased 2-fold (15.6 +/- 3.7 ml/min/kg; p < 0.001), but when RIF was coadministered in the induced rats, digoxin clearance (7.14 +/- 1.24 ml/min/kg) overlapped with control. That is, concomitant dosing of RIF and digoxin masked the inductive effect. To observe full inductive effects, test drugs should be administered 1 day after final dosing of RIF to minimize potential organic anion transporting polypeptide inhibition effects.

Pharmacokinetic and pharmacodynamic alterations of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors: drug-drug interactions and interindividual differences in transporter and metabolic enzyme functions

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) are widely used for the treatment of hypercholesterolemia. Their efficacy in preventing cardiovascular events has been shown by a large number of clinical trials. However, myotoxic side effects, sometimes severe, including myopathy or rhabdomyolysis, are associated with the use of statins. In some cases, such toxicity is associated with pharmacokinetic alterations. In this review, the pharmacokinetic aspects and physicochemical properties of statins are reviewed in order to understand the mechanism governing their pharmacokinetic alterations. Among the statins, simvastatin, lovastatin and atorvastatin are metabolized by cytochrome P450 3A4 (CYP3A4) while fluvastatin is metabolized by CYP2C9. Cerivastatin is subjected to 2 metabolic pathways mediated by CYP2C8 and 3A4. Pravastatin, rosuvastatin and pitavastatin undergo little metabolism. Their plasma clearances are governed by the transporters involved in the hepatic uptake and biliary excretion. Also for other statins, which are orally administered as open acid forms (i.e. fluvastatin, cerivastatin and atorvastatin), hepatic uptake transporter(s) play important roles in their clearances. Based on such information, pharmacokinetic alterations of statins can be predicted following coadministration of other drugs or in patients with lowered activities in drug metabolism and/or transport. We also present a quantitative analysis of the effect of some factors on the pharmacokinetics of statins based on a physiologically based pharmacokinetic model. To avoid a pharmacokinetic alteration, we need to have information about the metabolizing enzyme(s) and transporter(s) involved in the pharmacokinetics of statins and, along with such information, model-based prediction is also useful.

IN VITRO AND IN VIVO CORRELATION OF HEPATIC TRANSPORTER EFFECTS ON ERYTHROMYCIN METABOLISM: CHARACTERIZING THE IMPORTANCE OF TRANSPORTER-ENZYME INTERPLAY

The effects of hepatic uptake and efflux transporters on erythromycin (ERY) disposition and metabolism were examined by comparing results from rat hepatic microsomes, freshly isolated hepatocytes, and in vivo studies. Uptake studies carried out in freshly isolated rat hepatocytes showed that ERY and its metabolite (N-demethyl-ERY) are substrates of Oatp1a4 and Oatp1b2. Whereas rifampin and GG918 [GF120918: N-{4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)-ethyl]-phenyl}-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamine] exerted minimal effects on metabolism in microsomes, rifampin (2.5 microM) and GG918 (0.5 microM) significantly decreased and increased ERY metabolism in hepatocytes, respectively. Concentration-time course studies further demonstrated that, compared with the intracellular N-demethyl-ERY control area under the curve (AUC) (0.795 +/- 0.057 microM . min), a decreased AUC (0.513 +/- 0.028 microM . min, p < 0.005) was observed when ERY was coincubated with rifampin, and an increased AUC (2.14 +/- 0.21 microM . min, p < 0.05) was found when GG918 was present. The results of the i.v. bolus studies showed that, compared with the ERY clearance of the controls (47.2 +/- 12.5 ml/min/kg for the rifampin group and 42.1 +/- 5.7 for the GG918 group), a decreased blood clearance, 29.8 +/- 6.1 ml/min/kg (p < 0.05) and 21.7 +/- 9.0 ml/min/kg (p < 0.01), was observed when rifampin or GG918, respectively, was coadministered. When either inhibitor was codosed with ERY, volume of distribution at steady state was unchanged, but t1/2 and mean residence time significantly increased compared with the controls. Hepatic uptake and efflux transporters modulate intracellular concentrations of ERY, thereby affecting metabolism. The interplay of transporters and enzymes must be considered in evaluating potential drug-drug interactions.