Role of transport proteins in drug absorption, distribution and excretion

A higher dose requirement of tacrolimus in active Crohn's disease may be related to a high intestinal P-glycoprotein content

Tacrolimus, a relatively new therapeutic option for patients with corticosteroid-refractory Crohn's disease or ulcerative colitis, is a substrate for the apically directed efflux transporter P-glycoprotein (P-gp). Duodenal biopsy specimens obtained from a patient with corticosteroid-refractory Crohn's disease and with significantly higher-than-average tacrolimus dose requirements were analyzed for P-gp by Western blot. The P-gp content in this patient was more than double that in specimens obtained from 9 of 10 healthy subjects. Elevated intestinal P-gp could have resulted in decreased tacrolimus absorption, thereby leading to decreased blood concentration and decreased efficacy in this patient. The cause and prevalence of this phenomenon are unknown.

Genetic polymorphisms of ATP-binding cassette transporters ABCB1 and ABCG2: therapeutic implications

Pharmacogenomics, the study of the influence of genetic factors on drug action, is increasingly important for predicting pharmacokinetics profiles and/or adverse reactions to drugs. Drug transporters, as well as drug metabolism play pivotal roles in determining the pharmacokinetic profiles of drugs and their overall pharmacological effects. There is an increasing number of reports addressing genetic polymorphisms of drug transporters. However, information regarding the functional impact of genetic polymorphisms in drug transporter genes is still limited. Detailed functional analysis in vitro may provide clear insight into the biochemical and therapeutic significance of genetic polymorphisms. This review addresses functional aspects of the genetic polymorphisms of human ATP-binding cassette transporters, ABCB1 and ABCG2, which are critically involved in the pharmacokinetics of drugs.

Evaluation of the role of P-glycoprotein in the uptake of paroxetine, clozapine, phenytoin and carbamazapine by bovine retinal endothelial cells

Expression of the drug transport proteins, including P-glycoprotein (Pgp), in the brain vascular endothelium represents a challenge for the effective delivery of drugs for the treatment of several central nervous system (CNS) disorders including depression, schizophrenia and epilepsy. It has been hypothesized that Pgp plays a major role in drug efflux at the blood-brain barrier, and may be an underlying factor in the variable responses of patients to CNS drugs. However, the role of Pgp in the transport of many CNS drugs has not been directly demonstrated. To explore the role of Pgp in drug transport across an endothelial cell barrier derived from the central nervous system, the expression and activity of Pgp in bovine retinal endothelial cells (BRECs) and the effects of representative CNS drugs on Pgp activity were examined. Significant Pgp expression in BRECs was demonstrated by western analyses, and expression was increased by treatment of the cells with hydrocortisone. Intracellular accumulation of the well-characterized Pgp-substrate Taxol was markedly increased by the non-selective transporter inhibitor verapamil and the Pgp-selective antagonist PGP-4008, demonstrating that Pgp is active in these endothelial cells. In contrast, neither verapamil nor PGP-4008 affected the intracellular accumulation of [3H]paroxetine, [14C]phenytoin, [3H]clozapine or [14C]carbamazapine, indicating that these drugs are not substrates for Pgp. Paroxetine, clozapine and phenytoin were shown to be Pgp inhibitors, while carbamazapine did not inhibit Pgp at any concentration tested. These results indicate that Pgp is not likely to modulate patient responses to these drugs.

Use ofin vitrotransporter assays to understand hepatic and renal disposition of new drug candidates

Hepatic and renal transporters contribute to the uptake, secretion and reabsorption of endogenous compounds, xenobiotics and their metabolites and have been implicated in drug-drug interactions and toxicities. Characterising the renal and hepatic disposition of drug candidates early in development would lead to more rational drug design, as chemotypes with 'ideal' pharmacokinetic characteristics could be identified and further refined. Because transporters are often organ specific, 'custom' transporter panels need to be identified for each major organ and chemotype to be evaluated, and appropriate studies planned. This review outlines the major renal and hepatic transporters and some of the in vitro transporter reagents, assays and processes that can be used to evaluate the renal and hepatic disposition of new chemical entities during drug discovery and development.

Effects of Ketoconazole on Glucuronidation by UDP-Glucuronosyltransferase Enzymes

PURPOSE

Ketoconazole has been shown to inhibit the glucuronidation of the UGT2B7 substrates zidovudine and lorazepam. Its effect on UGT1A substrates is unclear. A recent study found that coadministration of irinotecan and ketoconazole led to a significant increase in the formation of SN-38 (7-ethyl-10-hydroxycamptothecine), an UGT1A substrate. This study investigates whether ketoconazole contributes to the increase in SN-38 formation by inhibiting SN-38 glucuronidation.

EXPERIMENTAL DESIGN

SN-38 glucuronidation activities were determined by measuring the rate of SN-38 glucuronide (SN-38G) formation using pooled human liver microsomes and cDNA-expressed UGT1A isoforms (1A1, 1A7 and 1A9) in the presence of ketoconazole. Indinavir, a known UGT1A1 inhibitor, was used as a positive control. SN-38G formation was measured by high-performance liquid chromatograph.

RESULTS

Ketoconazole competitively inhibited SN-38 glucuronidation. Among the UGT1A isoforms screened, ketoconazole showed the highest inhibitory effect on UGT1A1 and UGT1A9. The K(i) values were 3.3 +/- 0.8 micromol/L for UGT1A1 and 31.9 +/- 3.3 micromol/L for UGT1A9.

CONCLUSIONS

These results show that ketoconazole is a potent UGT1A1 inhibitor, which seems the basis for increased exposure to SN-38 when coadministered with irinotecan.

Development of stably transfected monolayer overexpressing the human apical sodium-dependent bile acid transporter (hASBT)

PURPOSE

The human apical sodium-dependent bile acid transporter (hASBT) represents a potential target for prodrug design to increase oral drug absorption. Unfortunately, available monolayer cell culture models do not reliably express hASBT, and nonpolarized cells only allow for uptake assessment, which limits prodrug development efforts. The objective of this study was to develop and characterize a stably transfected hASBT-MDCK cell line.

METHODS

cDNA encoding hASBT was cloned into pcDNA3.1-V5-polyHis-B to generate an expression plasmid that was then transfected into MDCK-II cells. Clonal populations were chosen based on high hASBT activity and monolayer integrity. Western blot confirmed the expression of the recombinant hASBT; functionality was characterized using taurocholic acid.

RESULTS

In the selected clone, hASBT-mediated taurocholate permeability across hASBT-MDCK monolayers was almost 25-fold higher with sodium, than without sodium where hASBT is not functional. In the presence of sodium, taurocholate and mannitol permeabilities were 23.0x10(-6) cm/sec and 2.60x10(-6) cm/s, respectively, indicating high hASBT functionality and monolayer integrity. hASBT-MDCK monolayer properties were stable over 6 months and demonstrated low within-day variability. Taurocholate uptake and inhibition kinetic parameters from hASBT-MDCK were similar to those obtained from hASBT-COS7 model, confirming hASBT functionality in hASBT-MDCK.

CONCLUSIONS

Results indicate that the developed hASBT-MDCK system is a competent, high-expression, stable assay for hASBT transport and inhibition studies.

Plasma pharmacokinetics and synovial fluid concentrations after oral administration of single and multiple doses of celecoxib in Greyhounds

OBJECTIVE

To determine the plasma pharmacokinetics and synovial fluid concentrations after oral administration of single and multiple doses of celecoxib in Greyhounds.

ANIMALS

7 adult Greyhounds.

PROCEDURES

Dogs received celecoxib (median dose, 11.8 mg/kg [range, 11.5 to 13.6 mg/kg], PO, q 24 h) for 10 days. Blood samples were collected prior to administration of celecoxib and serially for 24 hours after the 1st and 10th doses were administered. A synovial joint catheter was placed into a stifle joint in each dog for collection of synovial fluid samples. Concentrations of celecoxib in plasma and synovial fluid were quantified by use of a validated liquid chromatography/mass spectrometry method. Identification of hydroxy- and carboxyl-celecoxib in plasma and synovial fluid was also performed. Pharmacokinetic parameters were determined by use of noncompartmental analysis.

RESULTS

Administration of multiple doses of celecoxib resulted in a significant decrease (40%) in median area under the curve (AUC) values and a corresponding decrease in median maximum concentrations (Cmax; 2,620 to 2,032 ng/mL) between the 1st and 10th doses. Synovial fluid concentrations were less than the corresponding plasma concentrations at all times except 24 hours after administration of the 10th dose of celecoxib.

CONCLUSIONS AND CLINICAL RELEVANCE

Celecoxib distributes into the synovial fluid of Greyhounds. Although the exact mechanism for the decreases in AUC and Cmax is not known, results suggested that the plasma pharmacokinetics of celecoxib are different after administration of multiple doses in Greyhounds. These findings warrant further investigation on the absorption, distribution, metabolism, and elimination of celecoxib in Greyhounds and other breeds of dogs.

Determination of phase I metabolic enzyme activities in liver microsomes of Mrp2 deficient TR− and EHBR rats

The canalicular multispecific organic anion transporter/multidrug resistance protein 2 (cMOAT/Mrp2) plays a major role in the transport of anionic xenobiotics across the bile canalicular membrane. Transport deficient rats (TR-) and Eisai-hyperbilirubinemic rats (EHBR), defective in Mrp2, are mutants of Wistar and Sprague Dawley (SD) rats, respectively. In this study, Phase I metabolic enzyme activities in liver microsomes prepared from these mutant male and female rats were compared to their corresponding non-mutant rats. The total cytochrome P450 contents and NADPH-cytochrome P450 reductase activity in male and female TR- rats were significantly higher than in Wistar rats. In male TR- rats, ethoxyresorufin O-deethylation (EROD), pentoxyresorufin O-deethylation (PROD), testosterone 2alpha, 7alpha and 16 alpha-hydroxylase activities were higher, but testosterone 6beta-hydroxylase activity and the rate of androstenedione formation were lower than in Wistar rats. Female TR- rats had higher 7alpha-hydroxylase activity, but EROD activity was lower in female Wistar rats. Similar studies conducted in EHBR versus SD rats demonstrated increased total cytochrome P450 content in male and female EHBR rats; NADPH-cytochrome P450 reductase activity was not significantly affected. Decreased PROD activity and the rate of androstenedione formation were observed in male and female EHBR rats. Furthermore, testosterone 6beta-hydroxylase activity was lower in male EHBR rats than in male SD rats while testosterone 7alpha-hydroxylase activity was significantly higher in male and female EHBR rats. Thus, in addition to Mrp2 deficiency, differential expression of CYP isoforms and their potential impact on the metabolism and pharmacokinetics of compounds should be considered when interpreting data from these rat strains.

HIV/AIDS: a minority health issue

HIV infection among racial and ethnic minorities is an ongoing health crisis. The disproportionate impact of HIV infection on racial and ethnic minorities has affected communities already struggling with many social and economic challenges, such as poverty, substance abuse, homelessness,unequal access to health care, and unequal treatment once in the health care system. Superimposed on these challenges is HIV infection, the transmission of which is facilitated by many of these factors. Although the epidemic is disproportionately affecting all racial and ethnic minorities, within these minority populations women are particularly affected. The care and management of racial and ethnic minorities who have HIV infection has been complicated by the unequal access to health care and the unequal treatment once enrolled in health care. Health insurance status, lack of concordance between the race of the patient and the provider, and satisfaction with the quality of their care all impact on treatment outcomes in this population. In addition, the provider must be aware of the many comorbid conditions that may affect the delivery of care to minority patients living with HIV infection: depression, substance and alcohol abuse, and posttraumatic stress disorders. The impact of these comorbid conditions on the therapeutic relationship, including treatment and adherence, warrants screening for these disorders and treating them when identified. Because the patient provider relationship has been repeatedly identified as a predictor of higher adherence, developing and maintaining a strong therapeutic alliance is critical. Participation of racial and ethnic minorities in HIV clinical trials, as in other disease states, has been very poor. Racial and ethnic minorities have been chronically underrepresented in HIV clinical trials, despite their overrepresentation in the HIV epidemiology. This underrepresentation seems to be the result of a combination of factors including (1) provider bias in referring to clinical trials, (2) mistrust of clinical research, (3) past poor experience with the health care system, and (4) the conspiracy theories of HIV disease. The paucity of minority health care professionals and minority investigators in HIV research further affects minority participation in clinical research. To improve racial and ethnic minority participation in clinical trials a sustained effort is necessary at multiple levels. Increased recruitment and retention is an ongoing need, and one that will not be satisfactorily addressed until there are better community-academic and research partner-ships, and the research questions posed also address issues of concern and significance to the affected community. Reduction in barriers to participation in clinical trials, especially given the many competing needs of racial and ethnic minority patients, is also needed. Multidisciplinary HIV care teams and research staff with training in cultural competency and cultural sensitivity may also be helpful. Prevention of HIV infection remains essential, especially among those seeking care for HIV infection. Despite several published recommendations for the inclusion of HIV prevention in the clinical care setting, studies have documented how few providers actually achieve this goal, especially those who care for disadvantaged patients. Although there are many barriers to discussing HIV risk behaviors and prevention strategies in an office visit,including time constraints and potential provider discomfort in discussing these matters, clinical visits represent an important opportunity to reinforce HIV prevention and possibly decrease further HIV transmission.

The bioavailability of polyphenols is highly governed by the capacity of the intestine and of the liver to secrete conjugated metabolites

BACKGROUND

After ingestion of a complex meal containing foods and beverages of plant origin, different polyphenols are likely to be simultaneously present in the intestine. However, almost nothing is known about their interactions and possible consequences on their bioavailability.

AIM OF THE STUDY

The present study deals with the intestinal absorption and splanchnic metabolism of three polyphenols, genistein, hesperetin and ferulic acid (FA),when perfused in the small intestine alone or in combination, at different doses (15 and 120 microM).

METHODS

The fate of polyphenols in the small intestine was studied using a rat in situ intestinal perfusion model. Polyphenols were analysed in perfusate, bile and plasma by HPLC.

RESULTS

Whatever the perfused dose, the efficiency of the net transfer towards the enterocyte was similar for the three polyphenols and not significantly modified by any association between these molecules. However, FA largely differed from the two flavonoids by its low intestinal secretion of conjugates. When perfused at 15 microM, the secretion of conjugates back to the lumen represented 6.2% of the net transfer into the enterocytes for FA compared to 25.5 and 20 % for genistein and hesperetin respectively. Intestinal conjugation and secretion of conjugates back to the gut lumen varied with the dose of flavonoids: saturation of conjugation was observed for the highest dose or when a high dose of a second flavonoid was perfused simultaneously. Intensity of the biliary secretion substantially differed among tested polyphenols: 7.7% of the net transfer for FA vs 50% for genistein or hesperetin. The extent of the enterohepatic cycling of these polyphenols was proportional to the perfused dose and unaffected by the simultaneous presence of different compounds in the intestine.

CONCLUSION

Genistein and hesperetin appeared less available than FA for peripheral tissues because of a high intestinal and biliary secretion of their conjugates. Moreover, data suggest that a high polyphenol intake may improve their bioavailability due to saturation of the intestinal secretion of conjugates.

Fromin vivotoin vitro/in silicoADME: progress and challenges

High-throughput screening technologies in biological sciences of large libraries of compounds obtained via combinatorial or parallel chemistry approaches, as well as the application of design rules for drug-likeness, have resulted in more hits to be evaluated with respect to their ADME or drug metabolism and pharmacokinetic properties. The traditional in vivo methods using preclinical species, such as rat, dog or monkey, are no longer sufficient to cope with this demand. This editorial discusses the changes towards medium- to high-throughput in vitro and in silico ADME screening. In addition, much more attention is now put on early safety and risk assessment of promising lead series and potential clinical candidates.

Transporters and their impact on drug disposition

OBJECTIVE

To review the recent advances in knowledge about human transporters and their effect on drug disposition.

DATA SOURCES

A MEDLINE search (1996-March 2005) was performed to identify pertinent literature on human transporters and their impact on drug disposition. Additional articles were identified from a manual search of the references of retrieved articles.

STUDY SELECTION AND DATA EXTRACTION

Based on the identified studies, data were extracted on the impact of transporters on drug absorption, distribution, and elimination.

DATA SYNTHESIS

The pharmacokinetic disposition of drugs is known to be influenced by metabolic enzymes, kidney function, and transporters. Recent research on human transporters has greatly advanced our understanding of their diversity and importance in drug disposition. In particular, members of the multidrug resistance family of transporters (MDR, MRP) are present in organs and tissues throughout the body and are known to significantly affect the absorption, distribution, and elimination of commonly prescribed drugs. A growing number of studies now demonstrate that alterations in transporter function as a result of drug interactions or genetic polymorphisms may explain a significant portion of the variability in treatment response for certain drugs.

CONCLUSIONS

Human transporters contribute significantly to the pharmacokinetic disposition of drugs. Knowledge of substrates, inducers, and inhibitors of these transporters is necessary to ensure optimal patient outcomes.

Metabolism-based drug-drug interactions: what determines individual variability in cytochrome P450 induction?

Individual variability in cytochrome P450 (P450) induction comprises an important component contributing to the difficulties in assessing and predicting metabolism-based drug-drug interactions in humans. In this article, we outline the major factors responsible for the individual variability in P450 induction, including variable transporter activity and metabolism of inducers in vivo, genetic variations of P450 genes and their regulatory regions, genetic variations of receptors and regulatory proteins required for induction, and different physiological and environmental elements. With a better understanding of the major determinants in P450 induction and a profile of the phenotypes of these determinants in each individual, it is believed that the individual variability in induction-mediated drug-drug interactions can be adequately evaluated.

Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases

The blood-brain barrier (BBB) serves as a protective mechanism for the brain by preventing entry of potentially harmful substances from free access to the central nervous system (CNS). Tight junctions present between the brain microvessel endothelial cells form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the barrier properties of the brain capillary endothelial cells. Because of these properties, the BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for drug efflux transporters such as P-glycoprotein (Pgp), multidrug resistance proteins (MRPs) or organic anion transporting polypeptides (OATPs) that are expressed at brain capillary endothelial cells and/or astrocytic end-feet and are key elements of the molecular machinery that confers the special permeability properties to the BBB. The combined action of these carrier systems results in rapid efflux of xenobiotics from the CNS. The objective of this review is to summarize transporter characteristics (cellular localization, specificity, regulation, and potential inhibition) for drug efflux transport systems identified in the BBB and blood-cerebrospinal fluid (CSF) barrier. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on brain access of therapeutic drugs also are described and critically discussed. The potential impact of efflux transport on the pharmacodynamics of agents acting in the CNS is illustrated. Furthermore, the current knowledge about drug efflux transporters as a major determinant of multidrug resistance of brain diseases such as epilepsy is reviewed. Finally, we summarize strategies for modulating or by-passing drug efflux transporters at the BBB as novel therapeutic approaches to drug-resistant brain diseases.

Species differences in the disposition of the CCR5 antagonist, UK-427,857, a new potential treatment for HIV

UK-427,857 (4, 4-difluoro-N-[(1S)-3-[exo-3-(3-isopropyl-5-methyl-4H-1,2,4-triazol-4-yl)-8-azabicyclo[3.2.1]oct-8-yl]-1-phenylpropyl]cyclohexanecarboxamide) is a novel CCR5 antagonist undergoing investigation for use in the treatment of human immunodeficiency virus (HIV) infection. Pharmacokinetic and metabolism studies have been performed in mouse, rat, dog, and human after single and multiple administration by oral and intravenous routes. The compound has physicochemical properties that are borderline for good pharmacokinetics, being moderately lipophilic (log D(7.4) 2.1) and basic (pK(a) 7.3), possessing a number of H-bonding functionalities, and with a molecular weight of 514. The compound was incompletely absorbed in rat (approximately 20-30%) but well absorbed in dog (>70%). Based on in vitro studies in Caco-2 cells, UK-427,857 has relatively poor membrane permeability, and transcellular flux is enhanced in the presence of inhibitors of P-glycoprotein. Further evidence for the involvement of P-glycoprotein in restricting the oral absorption of UK-427,857 was obtained in P-glycoprotein null mice (mdr1a/mdr1b knockout). In these animals, AUC after oral administration was 3-fold higher than in control animals. In oral dose escalation studies in humans, the compound demonstrated nonlinear pharmacokinetics, with increased dose-normalized exposure with increased dose size, consistent with saturation of P-glycoprotein. The oral dose-exposure relationship of UK-427,857 in humans was not reflected in either rat or dog. In animal species and humans, UK-427,857 undergoes some metabolism, with parent compound the major component present in the systemic circulation and excreta. Elimination of radioactive dose was primarily via the feces. In rat, parent compound was secreted via bile and directly into the gastrointestinal tract. Metabolites were products of oxidative metabolism and showed a high degree of structural consistency across species.

Drug transporters: their role and importance in the selection and development of new drugs

Drug transporters expressed in various tissues play a significant role in drug disposition. By regulating the function of such transporters, it may be possible to eventually develop drugs with ideal pharmacokinetic profiles. In this article, we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug development process. The ability to manipulate transporter function offers the opportunity of being able to deliver a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side-effects), controlling the elimination process, and/or improving oral bioavailability. During drug development, it would be very useful to be able to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The use of specific inhibitors of transporters is also an attractive approach to controlling drug disposition, leading to improved efficacy. Currently, optimizing the pharmacokinetic properties of a drug during the early stages of its development is widely accepted as being of great importance. High-throughput screening systems using transporter gene transfected cells or computational (in silico) approaches are efficient tools for assessing transport activity during the early stage of drug development. In addition, drug-drug interactions involving drug transporters and functional genetic polymorphisms of drug transporters are also described. It would also be extremely valuable to be able to quantitatively predict inter-individual pharmacokinetic differences caused by transporter polymorphisms or pharmacokinetic changes caused by drug-drug interactions involving transporters during drug development.

The use of pharmacokinetic and pharmacodynamic data in the assessment of drug safety in early drug development

The pharmaceutical industry continues to look for ways to reduce drug candidate attrition throughout the drug discovery and development process. A significant cause of attrition is due to safety issues arising either as a result of animal toxicity testing or in the clinical programme itself. A factor in the assessment of safety during early drug development is the pharmacokinetic profile of the compound. This allows safety data to be considered in the light of systemic drug exposure and therefore permits a quantitative assessment. This is particularly applicable when assessing the risk of a new chemical entity (NCE) in relation to safety parameters such as QT interval prolongation, where free plasma concentrations have been shown to be predictive of this property in relation to potency in preclinical testing. Prior to actual human exposure it is therefore important to be able to predict reliably the pharmacokinetic behaviour of an NCE in order to place such safety findings into a quantitative risk context. The emerging science of pharmacogenetics is likely to further our ability to assess the risk of NCEs to populations and individuals due to genetic variance. The drug metabolizing enzyme CYP2D6 has been recognized as providing the potential to result in widely differing systemic drug exposure in the patient population due to polymorphic expression. Further knowledge is likely to add to our understanding of population differences in exposure and response and aid in the identification of risk factors. One potential strategy for improving the effectiveness of the drug discovery process is to obtain clinical pharmacokinetic data more rapidly in order to assess more accurately the potential for both efficacy and safety of an NCE. Whilst procedures and technologies are available that allow this on the microdose scale, it is important that we recognize potential limitations of these approaches in order that they can be applied beneficially.

Do men and women differ in proximal small intestinal CYP3A or P-glycoprotein expression?

The higher systemic clearance of some CYP3A4 [whether also P-glycoprotein (P-gp)] drug substrates in women versus men is attributed in part to a higher hepatic CYP3A4 content in women. This, combined with the general paucity of reported sex differences in the apparent oral clearance of CYP3A4 substrates, suggested a sex-dependent expression of CYP3A4 in the intestine, but in a pattern opposite to that in the liver. Accordingly, duodenal biopsies obtained from healthy men (n = 46) and women (n = 45) were analyzed, by Western blot, for relative CYP3A4, as well as for CYP3A5 and P-gp, expression levels. Among all subjects, CYP3A4 and P-gp varied 8- and 10-fold, respectively. CYP3A5, which was readily detected in 27% of these predominantly white individuals, varied 7-fold. For all three proteins, a sex difference was not detected (p >/= 0.55). The lack of a difference remained for CYP3A4 and P-gp when the analysis was restricted to white individuals (n = 74) or to individuals with undetectable CYP3A5. Comparing the 21 premenopausal women (all were aged <45 years) with the 43 men aged <45 years, again no sex differences were detected in CYP3A4 and P-gp. Comparing the pre- with postmenopausal women, mean CYP3A4 content was 20% lower in the postmenopausal individuals (p = 0.01). The lack of a sex-dependent difference in proximal intestinal CYP3A4 could account, in part, for the lack of reported sex differences in the oral, relative to systemic, clearance of some CYP3A4 substrates. Ramifications of lower intestinal CYP3A4 content in post- versus premenopausal women require further investigation.

Which in vitro Screens Guide the Prediction of Oral Absorption and Volume of Distribution?

The development of medium to high-throughput in vitro screening of ADME (Absorption, Distribution, Metabolism, Excretion) properties has been the reply to higher demands on drug metabolism scientists to cope with progress in chemistry and biology. Two areas will be discussed here, namely screens for oral absorption and for volume of distribution. The prediction of these human pharmacokinetic parameters can be based on proper combination of simple physicochemical measurements. In the future in vitro screens most likely will be combined with in silico assessments of various ADME properties leading to the concept of in combo screening in drug discovery.

Pharmacogenomics of drug transporters: a new approach to functional analysis of the genetic polymorphisms of ABCB1 (P-glycoprotein/MDR1)

In the 21st century, emerging genomic technologies (i.e., bioinformatics, functional genomics, and pharmacogenomics) are shifting the paradigm of drug discovery research and improving the strategy of medical care for patients. In order to realize the personalized medicine, it is critically important to understand molecular mechanisms underlying inter-individual differences in the drug response, namely, pharmacological effect vs. side effect. Evidence is now accumulating to strongly suggest that drug transporters are one of the determinant factors governing the pharmacokinetic profile of drugs. Effort has been made to identify genetic variation in drug transporter genes. In particular, genetic variations of the human ABCB1 (P-glycoprotein/MDR1) gene have been most extensively studied. Hitherto more than fifty single nucleotide polymorphisms (SNPs) and insertion/deletion polymorphisms in the ABCB1 gene have been reported. However, at the present time, information is still limited with respect to the actual effect of those genetic polymorphisms on the function of ABCB1. In this context, we have undertaken functional analyses of ABCB1 polymorphisms. To quantify the impact of genetic polymorphisms on the substrate specificity of ABCB1, we have developed a high-speed screening system and a new structure-activity relationship (SAR) analysis method. This review addresses functional aspects of the genetic polymorphism of ABCB1 and provides the standard method to evaluate the effect of polymorphisms on the function.

Pharmacokinetic Considerations Relating to Tacrolimus Dosing in the Elderly

Relaxation of the upper age limits for solid organ transplantation coupled with improvements in post-transplant survival have resulted in greater numbers of elderly patients receiving immunosuppressant drugs such as tacrolimus. Tacrolimus is a potent agent with a narrow therapeutic window and large inter- and intraindividual pharmacokinetic variability. Numerous physiological changes occur with aging that could potentially affect the pharmacokinetics of tacrolimus and, hence, patient dosage requirements. Tacrolimus is primarily metabolised by cytochrome P450 (CYP) 3A enzymes in the gut wall and liver. It is also a substrate for P-glycoprotein, which counter-transports diffused tacrolimus out of intestinal cells and back into the gut lumen. Age-associated alterations in CYP 3A and P-glycoprotein expression and/or activity, along with liver mass and body composition changes, would be expected to affect the pharmacokinetics of tacrolimus in the elderly. However, interindividual variation in these processes may mask any changes caused by aging. More investigation is needed into the impact aging has on CYP and P-glycoprotein activity and expression. No single-dose, intense blood-sampling study has specifically compared the pharmacokinetics of tacrolimus across different patient age groups. However, five population pharmacokinetic studies, one in kidney, one in bone marrow and three in liver transplant recipients, have investigated age as a co-variate. None found a significant influence for age on tacrolimus bioavailability, volume of distribution or clearance. The number of elderly patients included in each study, however, was not documented and may have been only small. It is likely that inter- and intraindividual pharmacokinetic variability associated with tacrolimus increase in elderly populations. In addition to pharmacokinetic differences, donor organ viability, multiple co-morbidity, polypharmacy and immunological changes need to be considered when using tacrolimus in the elderly. Aging is associated with decreased immunoresponsiveness, a slower body repair process and increased drug adverse effects. Elderly liver and kidney transplant recipients are more likely to develop new-onset diabetes mellitus than younger patients. Elderly transplant recipients exhibit higher mortality from infectious and cardiovascular causes than younger patients but may be less likely to develop acute rejection. Elderly kidney recipients have a higher potential for chronic allograft nephropathy, and a single rejection episode can be more devastating. There is a paucity of information on optimal tacrolimus dosage and target trough concentration in the elderly. The therapeutic window for tacrolimus concentrations may be narrower. Further integrated pharmacokinetic-pharmacodynamic studies of tacrolimus are required. It would appear reasonable, based on current knowledge, to commence tacrolimus at similar doses as those used in younger patients. Maintenance dose requirements over the longer term may be lower in the elderly, but the increased variability in kinetics and the variety of factors that impact on dosage suggest that patient care needs to be based around more frequent monitoring in this age group.

The Role of P-Glycoprotein and Organic Anion-Transporting Polypeptides in Drug Interactions

The use of polytherapy in clinical practice necessitates an appreciation and understanding of the potential for drug interactions. Recent publications provide insight into the role of the active transport systems P-glycoprotein (P-gp) and human organic anion-transporting polypeptides (OATPs) in drug interactions. Active drug transporters influence the bioavailability of a number of drugs by controlling their movement into, and out of, cells. The active transport systems P-gp and OATP play an important role in drug elimination. The activity of these transport systems is controlled, in part, by genetic factors; however, drugs and foods also influence the activity of these systems. It appears that interference with P-gp or OATP, either as upregulation or inhibition, may affect plasma drug concentrations by altering intestinal absorption, proximal renal-tubular excretion or biliary excretion. Overall, the net bioavailability of a drug or substance is affected by the relative contributions of cellular efflux (P-gp) and influx (OATP) mechanisms and to what extent these systems are active during phases of uptake and absorption versus removal and excretion from the body. Many of the drugs and foods that affect active drug transport activity are known to interact with the cytochrome P450 enzyme system; therefore, the net effect of concomitant drug administration is complex. One must now consider the impact of metabolism (CYP-mediated drug biotransformation), P-gp-mediated drug efflux and OATP-mediated uptake when making assessments of drug absorption and distribution.

Transepithelial Transport of Telmisartan in Caco-2 Monolayers

Telmisartan is the most recently marketed angiotensin II type 1 receptor antagonist. Drug-drug interactions involving transporters can directly affect the therapeutic safety and efficacy of many important drugs. In clinical practice, telmisartan is coadministered with many kinds of drugs. However, little is known about the contribution of transporters to the intestinal transport of telmisartan. The aim of this study was to determine the transport mechanism of telmisartan across intestinal epithelial cells. In the presence of an inwardly directed proton gradient, the apical-to-basal transport of telmisartan was greater than basal-to-apical transport. Thus, we focused on the uptake mechanism of telmisartan across brush-border membranes. The uptake of telmisartan by Caco-2 cells was shown to be energy- and proton-dependent. Although some monocarboxylates inhibited the uptake of telmisartan, L-lactic acid, which is a typical substrate of the monocarboxylate transporter (MCT) 1-MCT4, did not affect the uptake of telmisartan. Preloading of acetic acid enhanced the uptake of telmisartan, showing a trans-stimulation effect. These results suggest that the carrier-mediated transport system is involved in the uptake of telmisartan by Caco-2 cells and that the apical-localized transport system is similar to MCTs, but not MCT1-MCT4. It is possible that telmisartan reduce the absorption of coadministered drugs by sharing the MCTs. Since MCTs have an important role in the intestinal absorption of pharmacologically active compounds, it is important to be aware of the potential of telmisartan-drug interactions involving MCTs and to act in order to prevent undesirable and harmful consequences.

Directional trans-epithelial transport of organic anions in porcine LLC-PK1 cells that co-express human OATP1B1 (OATP-C) and MRP2

The transcellular transport of many compounds, which cannot readily cross the lipid bilayer, is mediated by drug uptake and efflux transporters. Human OATP1B1 and MRP2 have been implicated in the hepato-biliary transport of many endogenous and exogenous compounds. Here, we have established epithelial porcine kidney LLC-PK1 derived cell lines, that express both transporters in a polarized fashion, as a model to predict hepato-biliary transport. Immunological identification of OATP1B1 in the recombinant cell lines was greatly facilitated by its C-terminal tagging with a peptide sequence derived from hemagglutinin (HA) avoiding the generation of OATP1B1 specific antibodies. Importantly, the tag did not interfere with the functionality of the transporter. Compared to LLC-PK1 cells and cells which expressed only OATP1B1, the cell line that co-expressed MRP2 and OATP1B1 displayed high directional basolateral-to-apical transport of 17 beta-estradiol-17 beta-glucuronide and estrone-3-sulfate. Dehydroepiandrosterone sulfate already displayed a significant basolateral-to-apical transport in the parental cell line, which was further stimulated upon expression of both transporters. Transcellular flux of all steroid conjugates in the opposite direction (apical-to-basolateral) was much lower. By employing this cellular model we were able to demonstrate for the first time that OATP1B1 together with MRP2 mediates the trans-cellular transport of rifampicin. It is anticipated that the models established herein will greatly facilitate the identification of transporters involved in the disposition of novel drug candidates.

CYP3A activity measured by the midazolam test is not related to 3435 C >T polymorphism in the multiple drug resistance transporter gene

A recent study with 69 Japanese liver transplants treated with tacrolimus found that the MDR13435 C >T polymorphism, but not the MDR12677 G >T polymorphism, was associated with differences in the intestinal expression level of CYP3A4 mRNA. In the present study, over 6 h, we measured the kinetics of a 75 microg oral dose of midazolam, a CYP3A substrate, in 21 healthy subjects genotyped for the MDR13435 C >T and 2677 G >T polymorphism. No statistically significant differences were found in the calculated pharmacokinetic parameters between the three 3435 C >T genotypes (TT, CT and CC group, respectively: Cmax (mean +/- SD: 0.30 +/- 0.08 ng/ml, 0.31 +/- 0.09 ng/ml and 0.31 +/- 0.11 ng/ml; Apparent clearance: 122 +/- 29 l/h, 156 +/- 92 l/h and 111 +/- 35 l/h; t1/2: 1.9 +/- 1.1 h, 1.6 +/- 0.90 h and 1.7 +/- 0.7 h). In addition, the 30-min 1'OH midazolam to midazolam ratio, a marker of CYP3A activity, determined in 74 HIV-positive patients before the introduction of antiretroviral treatment, was not significantly different between the three 3435 C >T genotypes (mean ratio +/- SD: 3.65 +/- 2.24, 4.22 +/- 3.49 and 4.24 +/- 2.03, in the TT, CT and CC groups, respectively). Similarly, no association was found between the MDR12677 G >T polymorphism and CYP3A activity in the healthy subjects or in the HIV-positive patients. The existence of a strong association between the activity of CYP3A and MDR13435 C >T and 2677 G >T polymorphisms appears unlikely, at least in Caucasian populations and/or in the absence of specific environmental factors.

Pharmacogenomics of MDR and MRP subfamilies

Drug-metabolizing enzymes, drug transporters and drug targets play significant roles as determinants of drug efficacy and toxicity. Their genetic polymorphisms often affect the expression and function of their products and are expected to become surrogate markers to predict the response to drugs in individual patients. With the sequencing of the human genome, it has been estimated that approximately 500–1200 genes code for drug transporters and, recently, there have been significant and rapid advances in the research on the relationships between genetic polymorphisms of drug transporters and interindividual variation of drug disposition. At present, the clinical studies of multi-drug resistance protein 1 (MDR1, P-glycoprotein, ABCB1), which belongs to the ATP-binding cassette (ABC) superfamily, are the most comprehensive among the ABC transporters, but clinical investigations on other drug transporters are currently being performed around the world. MDR1 can be said to be the most important drug transporter, since clinical reports have suggested that it regulates the disposition of various types of clinically important drugs, but in vitro investigations or animal experiments have strongly suggested that the members of the multi-drug resistance-associated protein (MRP) subfamily can also become key molecules for pharmacotherapy. In addition to those, breast cancer resistance protein (BCRP, ABCG2), another ABC transporter, is well known as a key molecule of multi-drug resistance to several anticancer agents. However, this review focuses on the latest information on the pharmacogenetics of the MDR and MRP subfamilies, and its impact on pharmacotherapy is discussed.

Herbal modulation of P-glycoprotein

P-glycoprotein (Pgp) is a 170 kDa phosphorylated glycoprotein encoded by human MDR1 gene. It is responsible for the systemic disposition of numerous structurally and pharmacologically unrelated lipophilic and amphipathic drugs, carcinogens, toxins, and other xenobiotics in many organs, such as the intestine, liver, kidney, and brain. Like cytochrome P450s (CYP3A4), Pgp is vulnerable to inhibition, activation, or induction by herbal constituents. This was demonstrated by using an ATPase assay, purified Pgp protein or intact Pgp-expressing cells, and proper probe substrates and inhibitors. Curcumin, ginsenosides, piperine, some catechins from green tea, and silymarin from milk thistle were found to be inhibitors of Pgp, while some catechins from green tea increased Pgp-mediated drug transport by heterotropic allosteric mechanism, and St. John's wort induced the intestinal expression of Pgp in vitro and in vivo. Some components (e.g., bergamottin and quercetin) from grapefruit juice were reported to modulate Pgp activity. Many of these herbal constituents, in particular flavonoids, were reported to modulate Pgp by directly interacting with the vicinal ATP-binding site, the steroid-binding site, or the substrate-binding site. Some herbal constituents (e.g., hyperforin and kava) were shown to activate pregnane X receptor, an orphan nuclear receptor acting as a key regulator of MDR1 and many other genes. The inhibition of Pgp by herbal constituents may provide a novel approach for reversing multidrug resistance in tumor cells, whereas the stimulation of Pgp expression or activity has implication for chemoprotective enhancement by herbal medicines. Certain natural flavonols (e.g., kaempferol, quercetin, and galangin) are potent stimulators of the Pgp-mediated efflux of 7,12-dimethylbenz(a)-anthracene (a carcinogen). The modulation of Pgp activity and expression by these herb constituents may result in altered absorption and bioavailability of drugs that are Pgp substrates. This is exemplified by increased oral bioavailability of phenytoin and rifampin by piperine and decreased bioavailability of indinavir, tacrolimus, cyclosporine, digoxin, and fexofenadine by coadministered St. John's wort. However, many of these drugs are also substrates of CYP3A4. Thus, the modulation of intestinal Pgp and CYP3A4 represents an important mechanism for many clinically important herb-drug interactions. Further studies are needed to explore the relative role of Pgp and CYP3A4 modulation by herbs and the mechanism for the interplay of these two important proteins in herb-drug interactions.

Influence of CYP3A5 and MDR1 (ABCB1) polymorphisms on the pharmacokinetics of tacrolimus in renal transplant recipients

BACKGROUND

A body-weight-based dose of tacrolimus often results in marked individual diversity of blood drug concentration. Tacrolimus is a substrate for cytochrome P450 (CYP) 3A5 and p-glycoprotein encoded by CYP3A5 and MDR1 (ABCB1), respectively, having multiple single nucleotide polymorphisms. In this study, we genotyped CYP3A5 A6986G, MDR1 G2677(A/T), and C3435T polymorphisms and investigated the association between these polymorphisms and the pharmacokinetics of tacrolimus in renal transplant recipients.

METHODS

Thirty consecutive recipients were enrolled in this study. The pharmacokinetics of tacrolimus was analyzed on day 28 after transplant, when the daily dose was adjusted to the target trough level of 10-15 ng/mL. The polymerase chain reaction-restriction fragment length polymorphism and direct sequence method were used for genotyping the CYP3A5 and MDR1 polymorphisms, respectively.

RESULTS

The single tacrolimus dose per body weight was significantly higher in CYP3A5 *1 carriers than CYP3A5 *3/*3 carriers (0.143+/-0.050 vs. 0.078+/-0.031 mg/kg, P<0.001). The dose-adjusted trough level and the area under the concentration-time curve (AUC0-12) were significantly lower in CYP3A5 *1 carriers than CYP3A5 *3/*3 carriers (0.040+/-0.014 vs. 0.057+/-0.024 ng/mL/mg/kg, P=0.015 and 0.583+/-0.162 vs. 0.899+/-0.319 ng.hr/mL/mg/kg, P=0.004), respectively. The MDR1 polymorphism was not associated with any pharmacokinetic parameters.

CONCLUSIONS

Kidney transplant recipients with the CYP3A5 *1 allele required a higher daily tacrolimus dose compared with those with the CYP3A5 *3/*3 genotype to maintain both the target trough level and AUC0-12, suggesting that this polymorphism is useful for determining the appropriate dose of tacrolimus.

The ABC Transporters MDR1 and MRP2: Multiple Functions in Disposition of Xenobiotics and Drug Resistance

ATP-binding cassette (ABC) transporters comprise one of the largest membrane bound protein families. They are involved in transport of numerous compounds. These proteins transport substrates against a concentration gradient with ATP hydrolysis as a driving force across the membrane. Mammalian ABC proteins have important physiological, pharmacological and toxicological functions including the transport of lipids, bile salts, drugs, toxic and environmental agents. The efflux pumps serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs. In general terms, the transporters remove xenobiotics from the cellular environment. For example, in cancer cells, over expression of these molecules may confer to multidrug resistance against cytostatic drugs. In addition, based on diverse structural characteristics and a broad substrate specifity, ABC transport proteins alter the intracellular concentration of a variety of therapeutically used compounds and toxicologically relevant agents. We review the function of the human multidrug resistance protein MDR1, (P-glycoprotein, ABCB1) and the multidrug resistance protein MRP2 (ABCC2). We focus on four topics namely 1) structure and physiological functions of these transporters, 2) substrates e.g., drugs, xenotoxins, and environmental toxicants including their conjugates, 3) drug-drug interactions, and the role of chemosensitizers which may be able to reverse drug resistance, and 4) pharmacologically and toxicologically relevant genetic polymorphisms in transport proteins and their clinical implications.

Pharmacogenetics‐Based New Therapeutic Concepts

Pharmacogenetics, one of the fields of clinical pharmacology, studies how genetic factors influence drug response. If hereditary traits are taken into account appropriately before starting drug treatment, the type of drug and its dosage can be tailored to the individual patient's needs. Pharmacogenetics adds a considerable amount of stringency to the doctor's therapeutic approach. Today, it is the relationship between dosage requirements and genetic variations in drug metabolizing enzymes like cytochrome P450 (CYP) 2D6 and CYP2C19, or in drug transporters like p-glycoprotein, that is substantiated best. A standard dose will bring about more adverse effects than usual if enzymatic activity is lacking or feeble. Sometimes, however, therapeutic response might be better due to higher concentrations: proton pump inhibitors for eradication of Helicobacter pylori are more efficacious in carriers of a deficient CYP2C19 variant. The drug's interaction with its target (e.g. receptor) also depends on genetic factors. In some cases genetic tests can help distinguish between responders and non-responders of a specific drug treatment. The first pharmacogenetic tests are already on the market.

Safety of statins: focus on clinical pharmacokinetics and drug interactions

Statin monotherapy is generally well tolerated, with a low frequency of adverse events. The most important adverse effects associated with statins are myopathy and an asymptomatic increase in hepatic transaminases, both of which occur infrequently. Because statins are prescribed on a long-term basis, however, possible interactions with other drugs deserve particular attention, as many patients will typically receive pharmacological therapy for concomitant conditions during the course of statin treatment. This review summarizes the pharmacokinetic properties of statins and emphasizes their clinically relevant drug interactions.

New Insights into Drug Absorption

Sirolimus is a recently marketed immunosuppressant that, in common with cyclosporine and tacrolimus, exhibits a low average oral bioavailability (approximately 20%). Likewise, sirolimus is a substrate for the major drug-metabolizing enzyme cytochrome P450 3A4 (CYP3A4) and the efflux transporter P-glycoprotein (P-gp), both of which are expressed in close proximity in epithelial cells lining the small intestine. Using CYP3A4-expressing Caco-2 cell monolayers, we examined the interplay between metabolism and transport on the intestinal first-pass extraction of sirolimus. Modified Caco-2 cells metabolized [14C]sirolimus to the same CYP3A4-mediated metabolites as human small intestinal and liver microsomes. [14C]Sirolimus also degraded to the known ring-opened product, seco-sirolimus. A ring-opened dihydro species (M2) was, surprisingly, the major product detected in cells at all sirolimus concentrations examined (2-100 micromol/L) and in incubations with human liver and intestinal homogenates but not in corresponding microsomes. M2 formation was NADPH-dependent but unaffected by prototypical CYP3A4 inhibitors. Although M2 was formed from purified seco-sirolimus (20 micromol/L) in the homogenates, it was not detected in cells when seco-sirolimus was added to the apical compartment because seco-sirolimus was essentially impermeable to the apical membrane. Sirolimus, seco-sirolimus (basolaterally dosed), and M2 were all secreted across the apical membrane, and secretion of each was inhibited by the P-gp inhibitor LY335979 (zosuquidar trihydrochloride). Along with CYP3A4-mediated metabolism and P-gp-mediated efflux, a novel elimination pathway was identified that may also contribute to the first-pass extraction, and hence low oral bioavailability, of sirolimus. This new insight into the intestinal elimination of sirolimus, which was not identified using traditional drug metabolism/transport screening methods, may represent another source for the limited absorption of sirolimus.

Lack of strain-related differences in drug metabolism and efflux transporter characteristics between CD-1 and athymic nude mice

CD-1 mice are commonly used in oncology metabolism and toxicity to support drug discovery and development and to examine drug metabolism and toxicity properties of new chemical entities. On the other hand, athymic nude mice are the preferred animals to investigate tumor growth inhibition. Therefore, a frequently asked question is: are the metabolic and pharmacokinetic characteristics of xenobiotics in these two mouse strains comparable or not? To address this issue, we characterized drug metabolism and efflux transporter properties in both strains and in different organs. The metabolic stability of a set of 20 compounds and metabolite formation of cytochrome P450 (CYP) marker substrates (testosterone, ethoxyresorufin and pentoxyresorufin) were measured in liver microsomes. Drug conjugation was studied by following the disappearance of 7-hydroxycoumarin and the formation of its glucuronide and sulfate conjugates in freshly prepared liver slices. In addition, mRNA expression levels of the main cyp genes and drug efflux transporters were investigated by real-time RT-PCR in the liver, kidney, intestine and adrenal glands. No significant differences in enzymatic activities and metabolite formation were observed between the two strains. Also mRNA expression profiles of cyp and drug transporter genes were similar between CD-1 and nude mice.

Rapid induction of P-glycoprotein expression by high permeability compounds in colonic cells in vitro: a possible source of transporter mediated drug interactions?

P-glycoprotein (PGP) substrates with high membrane permeability, such as propranolol and verapamil, are considered to be essentially "transparent" to PGP since the transporter does not significantly limit their absorption or elimination. However, the question of whether such compounds can modulate PGP expression in epithelial cells following short-term exposure, with potential consequences for drug interactions, has not been addressed. LS180 colonic epithelial cells were exposed to propranolol or verapamil at concentrations (50-300 microM) consistent with those likely to be present in the gut lumen during oral dosing. Both compounds stimulated four to six-fold increases in MDR1 mRNA and PGP protein expression measured by quantitative real-time PCR and immunoblotting, respectively. These changes were accompanied by an induction in transporter activity measured by rhodamine 123 efflux. In contrast, metoprolol, a compound with similar permeability but no affinity for PGP had no effect on PGP expression. The induction of PGP by propranolol and verapamil was rapid with significant increases occurring within 3h with maximal stimulation after 6h exposure. Rifampicin, shown to cause clinical drug interactions via a PXR-mediated increase in PGP expression, exhibited a very similar time-course and extent of induction. In conclusion, verapamil and propranolol, whose trans-epithelial permeability are unaffected by PGP, appear to be effective inducers of PGP expression in gut epithelial cells in vitro. While the in vivo significance of these observations is unknown, this questions whether high permeability, "PGP-transparent" compounds, currently favoured in drug selection strategies, should be evaluated in terms of their potential for transporter-mediated drug interactions.

Interaction of triazolam and ketoconazole in P-glycoprotein-deficient mice

The role of P-glycoprotein (P-gp) on the distribution of the benzodiazepine triazolam (TRZ) and the azole antifungal agent ketoconazole (KET), and on the TRZ-KET interaction, was studied using mdr1a(-) or mdr1a/b(-/-) mice (P-gp-deficient mice) and matched controls. TRZ and KET also were studied in Caco-2 cells in Transwell culture. After single i.p. injections of TRZ or KET in separate groups of control mice, brain concentrations of TRZ exceeded those in serum [brain/serum area under the concentration curve (AUC) ratio, 5.0], whereas brain/serum AUC ratios for KET were approximately 0.5. On the basis of single time points, brain concentrations of TRZ, or brain/serum ratios, were similar in P-gp-deficient animals compared with controls, whereas P-gp-deficient animals had significantly higher KET brain concentrations and brain/serum ratios. Coadministration of KET with TRZ increased TRZ concentrations in serum, liver, and brain, both in controls and in P-gp-deficient animals, probably attributable to impairment by KET of CYP3A-mediated clearance of TRZ. However, KET did not increase brain/serum ratios of TRZ in either group. In Caco-2 cells, basal-to-apical flux of TRZ was higher than apical-to-basal flux. However, verapamil (100 microM) did not alter flux in either direction. KET inhibited basal-to-apical transport of rho-damine-123, with a 50% inhibitory concentration of 2.7 microM. Thus, TRZ does not appear to undergo measurable blood-brain barrier efflux transport by P-gp in this animal model. KET impairs clearance of TRZ but does not increase tissue uptake. However, KET itself may be a substrate for efflux transport at the blood-brain barrier.

P-glycoprotein increases from proximal to distal regions of human small intestine

PURPOSE

The contribution of the efflux transporter P-glycoprotein (P-gp) as a barrier to drug absorption may depend on its level of expression at the site of absorption. Accordingly, the distribution of P-gp was examined along the entire length of the human small intestine.

METHODS

Homogenates prepared from mucosal scrapings from every other 30-cm segment of four unrelated human donor small intestines were analyzed for P-gp and the control protein villin by Western blot.

RESULTS

In each donor intestine, relative P-gp expression (P-gp/villin integrated optical density ratio) progressively increased from proximal to distal regions. Among individuals, relative P-gp levels varied 2.1-fold in the duodenal/proximal jejunal region, 1.5- to 2.0-fold in the middle/distal jejunal region, and 1.2- to 1.9-fold in the ileal region. Within-donor variation was somewhat greater, from 1.5- to 3.0-fold.

CONCLUSIONS

These results provide further evidence that the site of absorption can represent another source for the interindividual variation in the oral bioavailability of drugs.

Classification analysis of P-glycoprotein substrate specificity

Prediction of P-glycoprotein substrate specificity (S(PGP)) can be viewed as a constituent part of a compound's "pharmaceutical profiling" in drug design. This task is difficult to achieve due to several factors that raised many contradictory opinions: (i) the disparity between the S(PGP) values obtained in different assays, (ii) the confusion between Pgp substrates and inhibitors, (iii) the confusion between lipophilicity and amphiphilicity of Pgp substrates, and (iv) the dilemma of describing class-specific relationships when Pgp has no binding sites of high ligand specificity. In this work, we compiled S(PGP) data for 1000 compounds. All data were represented in a binary format, assigning S(PGP) = 1 for substrates and S(PGP) = 0 for non-substrates. Each value was ranked according to the reliability of experimental assay. Two data sets were considered. Set 1 included 220 compounds with S(PGP) from polarized transport across MDR1 transfected cell monolayers. Set 2 included the entire list of 1000 compounds, with S(PGP) values of generally lower reliability. Both sets were analysed using a stepwise classification structure-activity relationship (C-SAR) method, leading to derivation of simple rules for crude estimation of S(PGP) values. The obtained rules are based on the following factors: (i) compound's size expressed through molar weight or volume, (ii) H-accepting given by the Abraham's beta (that can be crudely approximated by the sum of O and N atoms), and (iii) ionization given by the acid and base pKa values. Very roughly, S(PGP) can be estimated by the "rule of fours". Compounds with (N + O) > or = 8, MW > 400 and acid pKa > 4 are likely to be Pgp substrates, whereas compounds with (N + O) < or = 4, MW < 400 and base pKa < 8 are likely to be non-substrates. The obtained results support the view that Pgp functioning can be compared to a complex "mini-pharmacokinetic" system with fuzzy specificity. This system can be described by a probabilistic version of Abraham's solvation equation, suggesting a certain similarity between Pgp transport and chromatographic retention. The chromatographic model does not work in the case of "marginal" compounds with properties close to the "global" physicochemical cut-offs. In the latter case various class-specific rules must be considered. These can be associated with the "amphiphilicity" and "biological similarity" of compounds. The definition of class-specific effects entails construction of the knowledge base that can be very useful in ADME profiling of new drugs.

The systemic exposure of an N-methyl-D-aspartate receptor antagonist is limited in mice by the P-glycoprotein and breast cancer resistance protein efflux transporters

GV196771 [E-4,6-dichloro-3-(2-oxo-1-phenyl-pyrrolidin-3-glydenemethyl)-1H-indole-2 carboxylic acid] is a potent antagonist of the modulatory glycine site of the N-methyl-d-aspartate receptor. GV196771 has low oral bioavailability (<10%) and plasma clearance ( approximately 2 ml/min/kg) in rats. P-Glycoprotein (Pgp) and breast cancer resistance protein (Bcrp) are ATP-binding cassette (ABC) transporters that limit the oral absorption of drugs and dietary constituents. The objective of this work was to assess the involvement of Pgp and/or Bcrp on the systemic exposure of GV196771 in mice. In vitro, GV196771 was a Bcrp substrate [basolateral-to-apical/apical-to-basolateral (B-->A/A-->B) ratio = 5.1] with high passive membrane permeability (P(app) = 64-170 nm/s); however, GV196771 was not an in vitro Mdr1a substrate (B-->A/A-->B ratio = 1.9; no effect of GF120918 on efflux ratio). The role of Pgp and Bcrp on the systemic exposure of GV196771 was assessed by pretreatment of wild-type and Pgp-deficient mdr1a/1b(-/-) mice with a single oral dose of GF120918 (50 mg/kg; a dual Pgp and Bcrp inhibitor) or vehicle (0.5% hydroxypropylmethylcellulose and 1% Tween 80) 2 h before administration of a single oral dose of GV196771 (2 mg/kg). Compared with wild-type animals, the GV196771 area under the plasma concentration-time curve [AUC((0-->6 h))] increased 6.2-fold in Pgp-deficient mice, 10.3-fold in GF120918-pretreated wild-type mice, and 16.4-fold in GF120918-pretreated Pgp-deficient mice. C(max) values changed in parallel with the AUC((0-->6 h)) values; however, t(max) remained relatively unchanged. This study supports a role for Pgp and Bcrp in attenuating the systemic exposure of GV196771 in mice and demonstrates that two ABC efflux transporters can have nonredundant roles in attenuating the disposition of a compound.

Ritonavir Decreases the Nonrenal Clearance of Digoxin in Healthy Volunteers with Known MDR1 Genotypes

Our objective was to examine the influence of ritonavir on P-glycoprotein (P-gp) activity in humans by characterizing the effect of ritonavir on the pharmacokinetics of the P-gp substrate digoxin in individuals with known MDR1 genotypes. Healthy volunteers received a single dose of digoxin 0.4 mg orally before and after 14 days of ritonavir 200 mg twice daily. After each digoxin dose blood and urine were collected over 72 hours and analyzed for digoxin. Digoxin pharmacokinetic parameter values were determined using noncompartmental methods. MDR1 genotypes at positions 3435 and 2677 in exons 26 and 21, respectively, were determined using PCR-RFLP analysis. Ritonavir increased the digoxin AUC(0-72) from 26.20 +/- 8.67 to 31.96 +/- 11.24 ng x h/mL (P = 0.03) and the AUC(0-8) from 6.25 +/- 1.8 to 8.04 +/- 2.22 ng x h/mL (P = 0.02) in 12 subjects. Digoxin oral clearance decreased from 149 +/- 101 mL/h x kg to 105 +/- 57 mL/h x kg (P = 0.04). Other digoxin pharmacokinetic parameter values, including renal clearance, were unaffected by ritonavir. Overall, 75% (9/12) of subjects had higher concentrations of digoxin after ritonavir administration. The majority of subjects were heterozygous at position 3435 (C/T) (6 subjects) and position 2677 (G/T,A) (7 subjects); although data are limited, the effect of ritonavir on digoxin pharmacokinetics appears to occur across all tested MDR1 genotypes. Concomitant low-dose ritonavir reduced the nonrenal clearance of digoxin, thereby increasing its systemic availability. The most likely mechanism for this interaction is ritonavir-associated inhibition of P-gp. Thus, ritonavir can alter the pharmacokinetics of coadministered medications that are P-gp substrates.

SolEmuls-novel technology for the formulation of i.v. emulsions with poorly soluble drugs

Intravenously injectable o/w emulsions of drugs being poorly soluble in water and simultaneously in oils need to be produced by locating the drug in the interfacial lecithin layer, e.g. amphotericin B. For achieving this, up to now organic solvents were required. The objective was to develop a solvent-free production method for such emulsions. Drug and the pre-formed parenteral emulsion Lipofundin were mixed and subjected to high pressure homogenisation. Drug powder and emulsions were characterised regarding size and physical stability by photon correlation spectroscopy (PCS), laser diffractometry (LD) and zeta potential measurements. Drug incorporation was studied using light microscopy, electron microscopy (EM) and a centrifugation test to separate non-dissolved drug. Amphotericin B and carbamazepine were used as model drugs. The high streaming velocities lead to accelerated drug dissolution and partitioning into the interfacial layer (so-called "solubilisation by emulsification", SolEmuls Technology). The interfacial layer could incorporate (solubilise) a certain amount of drug, revealed by EM pictures. Exceeding this concentration, hybrid dispersions were formed consisting of drug-loaded oil droplets and drug nanocrystals of similar size (approximately 200 nm). Both dispersion types are i.v. injectable opening the opportunity to deliver the drug in a concentrated form at desired low injection volume, e.g. 10 mg/ml.

Polyphenols: food sources and bioavailability

Polyphenols are abundant micronutrients in our diet, and evidence for their role in the prevention of degenerative diseases such as cancer and cardiovascular diseases is emerging. The health effects of polyphenols depend on the amount consumed and on their bioavailability. In this article, the nature and contents of the various polyphenols present in food sources and the influence of agricultural practices and industrial processes are reviewed. Estimates of dietary intakes are given for each class of polyphenols. The bioavailability of polyphenols is also reviewed, with particular focus on intestinal absorption and the influence of chemical structure (eg, glycosylation, esterification, and polymerization), food matrix, and excretion back into the intestinal lumen. Information on the role of microflora in the catabolism of polyphenols and the production of some active metabolites is presented. Mechanisms of intestinal and hepatic conjugation (methylation, glucuronidation, sulfation), plasma transport, and elimination in bile and urine are also described. Pharmacokinetic data for the various polyphenols are compared. Studies on the identification of circulating metabolites, cellular uptake, intracellular metabolism with possible deconjugation, biological properties of the conjugated metabolites, and specific accumulation in some target tissues are discussed. Finally, bioavailability appears to differ greatly between the various polyphenols, and the most abundant polyphenols in our diet are not necessarily those that have the best bioavailability profile. A thorough knowledge of the bioavailability of the hundreds of dietary polyphenols will help us to identify those that are most likely to exert protective health effects.

The effect of age on P-glycoprotein expression and function in the Fischer-344 rat

We investigated the effect of age on P-glycoprotein (P-gp) expression and function in rat liver, intestine, kidney, and endothelial cells of the blood-brain barrier (BBB) and lymphocytes. Flow cytometric analysis was used to examine P-gp expression in lymphocytes from male Fischer-344 rats from three age groups (young at 3-4 months, intermediate at 13-14 months, and old at 25-26 months). In addition, P-gp function in lymphocytes was assessed by measuring the ability of the P-gp inhibitor verapamil to limit the efflux of the fluorescent P-gp substrate rhodamine 123. P-gp expression was evaluated in the remaining four tissues by Western blot analysis. The effect of age on P-gp expression was tissue-specific. Although lymphocytic and hepatic P-gp expression increased with age, renal P-gp content was lower in the old kidneys. No statistical difference was observed in P-gp expression in intestinal microsomes or in BBB cell lysates among the three age groups. P-gp function was also increased by 6- to 8-fold in lymphocytes from the old rats. When P-gp expression was compared with CYP3A expression in these rats (reported elsewhere in this journal), we found that P-gp expression increased with age, whereas CYP3A expression and activity declined in the old livers. The converse pattern was observed in the kidney. Thus, age-related changes in P-gp expression and function are likely to be tissue-specific, and these changes may be inversely related to differences in CYP3A expression.

Antihypertensive therapy: special focus on drug interactions

Today, the lifetime risk of patients aged 55-65 years to receive antihypertensive drugs approaches 60%. Yet, recent trials suggest that hypertension is not adequately controlled in the majority of patients. The prevalence of hypertension increases with advancing age, as does the prevalence of comorbid conditions and the total number of medications taken. Multi-drug therapy, advancing age and comorbid conditions are also key risk factors for adverse drug reactions and drug interactions. In this review, the authors evaluate the most frequently used antihypertensive drugs (diuretics, beta-adrenergic blockers, angiotensin-converting enzyme inhibitors, calcium channel blockers, angiotensin II receptor Type 1 blockers and alpha-adrenergic blockers) with special reference to pharmacodynamic and pharmacokinetic drug interactions. As the spectrum of drugs prescribed is constantly changing, safety yesterday does not imply safety today and safety today does not imply safety tomorrow. Furthermore, therapeutic efficacy should not be neglected over concerns regarding drug interactions. Many patients are at risk of clinically relevant drug interactions involving antihypertensive drugs but, presently, even more patients may be at risk of suffering from the consequences of their inadequately treated hypertension. In this respect, the authors discuss controversial viewpoints on the overall clinical relevance of drug interactions occurring at the level of cytochrome P450 metabolism.

Molecular cloning and functional characterization of the bovine (Bos taurus) organic anion transporting polypeptide Oatp1a2 (Slco1a2)

We describe the cloning, functional characterization and tissue localization of a novel membrane transporter of the OATP/Oatp-gene family obtained from liver and kidney of cattle (Bos taurus). The carrier protein exhibits highest sequence identity to the human OATP1A2 (previously called OATP-A) and is, therefore, named bovine Oatp1a2. Bovine Oatp1a2 received the gene symbol Slco1a2 that is identical to the SLC classification of human OATP1A2 (SLCO1A2, previously called SLC21A3) and is likely an orthologue of the human gene. Two different full-length bOatp1a2 cDNAs of 2316-bp and 3504-bp were obtained and encoded for a 666 amino acid membrane protein, which contains twelve putative transmembrane spanning domains. Bovine Oatp1a2 expression was detected in liver, kidney, brain and adrenal gland. Uptake studies in cRNA-injected oocytes demonstrated that bOatp1a2 transports estrone-3-sulfate and taurocholate, with K(m) values of 9.6 microM and 51 microM, respectively, and estradiol-17beta-glucuronide. However, the structurally-related heart glycosides ouabain (1 microM) and digoxin (1 microM) are neither transported by bovine Oatp1a2 nor by human OATP1A2. We conclude that based on the tested substrates bovine Oatp1a2 shows functional homology to human OATP1A2.

Development of an in vitro blood–brain barrier model based on immortalized porcine brain microvascular endothelial cells

Immortalized porcine brain microvessel endothelial cells (PBMEC/C1-2) were used to develop a model for measurement of blood-brain barrier permeation of central nervous system active drugs. Previous studies showed that a system using C6 astrocyte glioma conditioned medium leads to cell layers with transendothelial electrical resistance values up to 300 Omega cm(2) and a permeability coefficient P(e) of 3.24 +/- 0.14 x 10(-4) cm/min for U-[(14)C]sucrose, which is in good agreement to published values and thus indicates the formation of tight junctions in vitro. However, commercially available inserts for the Transwell system were not permeable for highly lipophilic compounds, such as diazepam. Systematic studies with different insert showed, that inserts with a pore width of 1 microm proved to be optimal for permeation studies of lipophilic compounds. Permeability studies with a set of three benzodiazepines further supported this finding.

Nonlinear oral pharmacokinetics of the alpha-antagonist 4-amino-5-(4-fluorophenyl)-6,7-dimethoxy-2-[4-(morpholinocarbonyl)-perhydro-1,4-diazepin-1-yl]quinoline in humans: use of preclinical data to rationalize clinical observations

4-amino-5-(4-fluorophenyl)-6,7-dimethoxy-2-[4-(morpholinocarbonyl)-perhydro-1,4-diazepin-1-yl]quinoline (UK-294,315) is an antagonist of the human alpha1-adrenoceptor and exhibits nonlinear oral pharmacokinetics in humans. Superproportional increases in Cmax occur (220-fold, over a 1- to 50-mg dose range), area under the curve increases linearly, but time to maximum concentration decreases with dose, suggesting variation in rate but not extent of absorption. Oral absorption in humans is extensive, with only 14% of an orally administered (20 mg) radiolabeled dose excreted unchanged in the feces. In rats and dogs, UK-294,315 is partially eliminated as unchanged drug in feces (29 and 14% of an intravenous dose, respectively). Oral bioavailability is low in rats (11%) and high in dogs (71%), in keeping with systemic clearance. Fecal elimination of unchanged drug was 60% after oral administration to rats, indicating incomplete absorption in this species, whereas absorption in dogs is complete. UK-294,315 is a P-glycoprotein (P-gp) substrate (Km, 15 microM) exhibiting polarized flux in Caco-2 cell monolayers, saturable across a concentration range of 5 to 200 microM. Furthermore, the observations in vitro occurred at similar concentrations to those estimated in the gut lumen in clinical trials (dose range, 1-100 mg). It is considered that P-gp acts as a saturable absorption barrier to UK-294,315, slowing the rate of absorption at low doses, and is responsible for the observed nonlinearity in oral disposition in humans. Rat and dog pharmacokinetic studies offered limited insight into the process(es) driving nonlinear pharmacokinetics in humans. Our current understanding of the functional effects of P-gp in the human intestine, in combination with in vitro studies at clinically relevant concentrations, has helped rationalize the clinical data for UK-294,315.

The genetic polymorphism of drug transporters: functional analysis approaches

Evidence is accumulating to strongly suggest that drug transporters are one of the determining factors governing the pharmacokinetic profile of drugs. To date, a variety of drug transporters have been cloned and classified as solute carriers and ATP-binding cassette transporters. Such drug transporters are expressed in various tissues such as the intestine, brain, liver, and kidney, and play critical roles in the absorption, distribution and excretion of drugs. However, at the present time, information is limited regarding the genetic polymorphism of drug transporters and its impact on their function. In this context, we have undertaken the functional analyses of the polymorphisms identified in drug transporter genes. This article aims to provide an overview on the functional aspects of the non-synonymous polymorphisms of drug transporters and to present standard methods for the evaluation of the effect of polymorphisms on their function.

Functional Evaluation of ABCB1 (P-Glycoprotein) Polymorphisms: High-Speed Screening and Structure-Activity Relationship Analyses

Evidence is accumulating to strongly suggest that drug transporters are one of the determinant factors governing the pharmacokinetic profile of drugs. Effort has been made to identify genetic variation in drug transporter genes. In particular, genetic variations of the human ABCB1 (MDR1) gene have been most extensively studied. Hitherto more than fifty single nucleotide polymorphisms (SNPs) and insertion/deletion polymorphisms in the ABCB1 gene have been reported. However, at the present time, information is still limited with respect to the actual effect of those genetic polymorphisms on the function of ABCB1. In this context, we have undertaken functional analyses of ABCB1 polymorphisms. To quantify the impact of genetic polymorphisms on the substrate specificity of ABCB1, we have developed a high-speed screening system and a new structure-activity relationship (SAR) analysis method. This review addresses functional aspects of the genetic polymorphism of ABCB1 and provides the standard method to evaluate the effect of polymorphisms on the function.