Characterization of the regional intestinal kinetics of drug efflux in rat and human intestine and in Caco-2 cells

Development and characterization of a recombinant Madin-Darby canine kidney cell line that expresses rat multidrug resistance-associated protein 1 (rMRP1)

Multidrug resistance-associated protein 1 (MRP1) is one of the major proteins shown to mediate efflux transport of a broad range of antitumor drugs, glucuronide conjugates, and glutathione, in addition to endogenous substrates. Significant differences in substrate selectivity were reported for murine and human MRP1. As preclinical drug disposition and pharmacokinetics studies are often conducted in rats, we have recently cloned the rat MRP1 (rMRP1) and demonstrated that rMRP1 expressed in transfected cells effluxes calcein, a commonly used fluorescence substrate for human MRP1. To further characterize the rat ortholog of MRP1, we isolated a cell line stably expressing recombinant rMRP1. These cells were tested for their ability to transport calcein and a range of chemotherapeutic drugs. Our results showed that cells expressing rMRP1 consistently efflux calcein at a rate 5-fold greater than control cells. The rMRP1 transfected cells, like their human ortholog, can confer drug resistance to vinca alkaloid (vinblastine and vincristine) and anthracycline drugs (daunorubcin and doxorubicin), and the resistance conferred by the MRP1 can be partially abolished by the MRP-specific inhibitors. The transepithelial permeability due to rMRP1 expression in differentiated Madin-Darby canine kidney cells (MDCK) cells was also investigated. The MRP1 transport activity is directional, as demonstrated by directional vinblastine transport. Collectively, our results demonstrate that the cellular expression of rMRP1, like its human ortholog, could confer resistance to anticancer drugs.

Characterization of jejunal absorption and apical efflux of ropivacaine, lidocaine and bupivacaine in the rat using in situ and in vitro absorption models

The purpose of this study was to characterize the rat jejunal passive transport and the possible active efflux of three local anaesthetics, ropivacaine, lidocaine and bupivacaine using two different absorption models, the in situ single-pass intestinal perfusion and the in vitro Ussing chamber model, as well as P-glycoprotein (Pgp)-mediated calcein transport inhibition in Caco-2 cells. Concentration and pH dependence, efflux inhibition by verapamil and digoxin and bi-directional permeability studies were performed to investigate the potential involvement of efflux carriers in the intestinal absorption of the local anaesthetics. In the jejunal perfusion the permeability of these agents appeared to be high, predicting complete intestinal absorption (>90%). There was no effect of the Pgp inhibitors on net absorption for any of the local anaesthetics in the two absorption models. However, in the Ussing chamber at an equal pH of 7.4 at mucosal and serosal sides, the observed jejunal permeability ratios (S-M)/(M-S), of 2.3, 1.8 and 3.0 for ropivacaine, lidocaine and bupivacaine, respectively, indicated at least some involvement of carrier-mediated intestinal secretion. This idea was supported in the calcein AM Pgp transport assay in which two of the tested local anaesthetic agents affected cellular calcein retention. As anticipated for these agents, the mucosal pH conditions were shown to largely affect the gut permeability. The jejunal permeabilities of the local anaesthetics as measured in the two absorption models fitted well in a model comparison that incorporated the permeabilities of six other structurally unrelated drugs. In conclusion, the jejunal permeability of ropivacaine, lidocaine and bupivacaine was high and although evidence was obtained for carrier-mediated intestinal efflux this process appeared not to have a significant influence on the rate and extent of in vivo intestinal absorption. Rather, passive diffusion of these agents seems to be the major mechanism for the intestinal absorption.

Protein expression pattern of P-glycoprotein along the gastrointestinal tract of the yucatan micropig

The purpose of this study is to characterize the distribution pattern of P-gp protein levels along the entire GI tract in the Yucatan micropig, which is being developed as a model for human drug bioavailability. Small and large intestines were freshly obtained and divided into about 37 segments and 10 segments, respectively (ca., 1 foot/segment). Epithelial cells from the small intestine were obtained by an elution method; whereas, a scraping method was applied to the large intestine. Total cellular protein was isolated from the epithelial cells. Western blot analysis using P-gp antibody showed that the amount of P-gp protein increased distally from the duodenum to the ileum over approximately a 10-fold range. P-gp protein in the large intestine was present at a higher level in the central portion, but the absolute amount was much less than what was found in the small intestine.

The ABCs of drug transport in intestine and liver: efflux proteins limiting drug absorption and bioavailability

Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (Pgp; MDR1) and MRP2, may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into blood. Drugs may also be modified by intracellular phase I and phase II metabolising enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for Pgp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subject to further metabolism and biliary excretion, often by a similar system of ATP-binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolising enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. The effectiveness of this system is optimised through dynamic regulation of transporter and enzyme expression; tissues have a remarkable capacity to regulate the amounts of protein both at transcriptional and post-transcriptional levels in order to maintain homeostasis. This review addresses the progress to date on what is known about the role and regulation of drug efflux mechanisms in the intestine and liver.

Limited Influence of P‐Glycoprotein on Small‐Intestinal Absorption of Cilostazol, a High Absorptive Permeability Drug

Intestinal transport of the type III phosphodiesterase inhibitor cilostazol was characterized to evaluate the influence of secretory transporter. Intestinal absorption of cilostazol measured by the in situ closed loop method, showed regional differences, with high permeability in the upper part of the small intestine. Intestinal secretory transport of cilostazol at the ileum was tended to be decreased by the increase of tested concentration of cilostazol from 10 to 20 microM when evaluated by means of a Ussing-type chamber method with mounted rat intestinal tissues. Transcellular transport of cilostazol in the basolateral-to-apical direction in LLC-GA5-COL150 cells, which overexpress P-glycoprotein, was higher than that in parental LLC-PK1 cells. In addition, cilostazol reduced the basolateral-to-apical transport and increased the accumulation of [(3)H]daunomycin in LLC-GA5-COL150 cells. Accordingly, cilostazol was demonstrated to be transported by P-glycoprotein, while cilostazol is not likely to cause induction of the expression level of P-glycoprotein by the same manner with rifampin. Apical-to-basolateral transport of cilostazol in Caco-2 cells was increased in a low concentration range, followed by a decrease with further increase of the concentration, while the permeability coefficient of cilostazol was above 1 x 10(-6) cm/s at any concentration. Initial uptake of [(14)C]cilostazol by Caco-2 cells was temperature dependent and was reduced in the presence of unlabeled cilostazol, suggesting that apical uptake is also mediated by a transporter(s). In conclusion, intestinal absorption of cilostazol, which has a high absorptive permeability, may not be significantly hampered by efflux transporters, such as P-glycoprotein.

Expression of genes encoding for drug metabolising cytochrome P450 enzymes and P-glycoprotein in the rat small intestine; comparison to the liver

The level of expression of genes encoding for nine major xenobiotic metabolising Cytochrome P450s (CYPs) and the P-glycoprotein (Pgp) was determined in three different regions of the small intestine of male and female Sprague Dawley rats and the expression was compared with that in the liver. A semi-quantitative RT-PCR method, using the total RNA from the tissues, was established for the determination of the level of gene expression. Four of the CYP genes: the CYP2B1, CYP2C6, CYP2C11 and CYP2D1 and the Pgp were expressed at as high levels in the small intestine as in the liver. The expression of the other CYP genes was remarkably different in the two organs. The CYP1A2, CYP2A3, CYP2E1 and CYP3A1 showed a strong expression in the liver but only a comparatively weak or no expression in the small intestine. The CYP1A1 on the other hand exhibited a stronger expression in the small intestine than in the liver. With the exception of the CYP2A3, none of the genes showed a clear regional distribution in their small intestinal expression. Furthermore, no obvious sex difference in the expression of the CYP and Pgp genes could be observed. Our results indicate that several of the enzymes, central for drug metabolism are differently expressed in the liver and in the small intestine of the rat which should be taken into account when using rat as a model for the bioavailability and organ specific toxicity studies of orally administered xenobiotics. The apparently strong small intestinal expression of the CYP2C genes suggests that these enzymes could play a key role in the intestinal drug metabolism in rats and therefore affect the bioavailability of those orally used drugs which are substrates of the CYP2Cs. This possibility should be investigated in more detail both in rats and humans.

Application of compartmental modeling to an examination of in vitro intestinal permeability data: assessing the impact of tissue uptake, P-glycoprotein, and CYP3A

P-glycoprotein (P-gp)-mediated drug efflux and cytochrome p450 3A (CYP3A) metabolism within the enterocyte have been implicated as potential biochemical barriers to intestinal drug permeability. The current studies examined the in vitro intestinal permeability of verapamil, a common P-gp and CYP3A substrate, using both disappearance and appearance measurements, and investigated the possible impact of P-gp efflux on the intestinal extraction of verapamil. Bidirectional permeability and metabolism studies were conducted across rat jejunal tissue in side-by-side diffusion chambers and data were modeled using compartmental kinetics. Substantial tissue uptake of verapamil was evident in the in vitro model and resulted in a disappearance permeability coefficient that was approximately 10-fold greater than that determined from verapamil appearance in the receptor chamber. Polarization of the bidirectional transport of verapamil was evident due to P-gp efflux (efflux ratio of 2.5), and significant intestinal extraction of verapamil on passage across the tissue was observed (mucosal to serosal extraction ratio of 0.31 +/- 0.04). Surprisingly, the selective P-gp inhibitor, valspodar (PSC833), had an insignificant impact on P-gp-mediated efflux of verapamil; however, selective CYP3A inhibition (afforded by midazolam) increased mucosal to serosal verapamil transport 1.6-fold, presumably through a reduction in intestinal metabolism. Using a four-compartment model, simulations of the impact of P-gp on the intestinal extraction ratio of verapamil demonstrated that for efflux to increase intestinal extraction, a nonlinear relationship must exist between the extent of drug metabolism and the extent of drug transport; the origin of this "nonlinearity" may include saturable drug metabolism, accumulation, and/or distribution.

Biological, pharmaceutical, and analytical considerations with respect to the transport media used in the absorption screening system, Caco-2

During the evaluation and selection of drug candidates, the Caco-2 cell culture system is commonly used for the determination of intestinal transport characteristics and to anticipate permeability limited drug absorption. Although classic HBSS-like buffered salt solutions are commonly used to perform Caco-2 transport experiments, different shortcomings (e.g., adsorption and low solubility) have been associated with the use of plain aqueous buffers. As transport experiments performed with unoptimized conditions may compromize the value of the Caco-2 model as a permeation screening tool, many efforts have been made to optimize the experimental conditions of Caco-2 transport assays. In this minireview, the hurdles associated with the use of saline aqueous buffers in Caco-2 transport experiments are summarized and the different options, which have been proposed to overcome these issues, are reviewed and discussed. Biologically, pharmaceutically, as well as analytically relevant media affecting the outcome of the transport experiments are described. Unfortunately, up to now, no systematic studies comparing the different experimental conditions have been performed, jeopardizing the possibility to define a (single) optimal solution to overcome the different issues associated with the use of saline aqueous buffers. Based on the reported options it can be proposed to use DMSO (<or=1%) in standard screening procedures for the ranking of compounds based on their apical to basolateral transport. If compounds are not soluble in DMSO 1%, dimethylacetamide (3%) or N-1-methyl-pyrrolidone (2.5%) are good alternatives. However, these options do not imitate the in vivo situation. If one wants to take into account the physiological relevance of the media, the use of a biologically relevant apical medium (e.g., FASSIF) in combination with an analytically friendly, sink condition creating basolateral solvent (e.g., containing a micelle forming agent) can be suggested.

The site-specific transport and metabolism of tacrolimus in rat small intestine

The objective of this study was to evaluate the absorption of tacrolimus by means of simultaneous perfusion of intestinal lumen and blood vessels in rats. In our previous report, the permeability of tacrolimus was found to be higher in the jejunum than in the ileum or colon, suggesting the site-dependent absorption after oral administration. However, in this article, simultaneous perfusion technique revealed that the extent of absorption into blood vessels was similar in the jejunum and the ileum regardless of the site difference in permeability as the absorption rate. In addition to the multidrug resistance-associated protein-mediated efflux, cytochrome P450 (P450)-mediated metabolism could be a possible mechanism to explain the inconsistencies in the site dependence of tacrolimus absorption. Two enzyme inhibitors, ketoconazole and midazolam, were coperfused in rat intestinal lumen with tacrolimus to specify the effect of P-gp and P450. In the jejunum, both inhibitors significantly enhanced the absorbed amount of tacrolimus, whereas the permeability was not affected. It was suggested that both inhibitors mainly suppress P450-mediated metabolism in the upper region of the intestine. In contrast, in the ileum, ketoconazole significantly enhanced both the absorbed amount and the permeability of tacrolimus. However, midazolam failed to enhance the absorption of tacrolimus, indicating the dominant role of P-glycoprotein (P-gp)-mediated efflux in the lower region. From these findings, it is concluded that the site-dependent differences in P-gp and/or P450 activity could be the prime cause of large intra- and interindividual variability in the oral absorption of tacrolimus.

Regional transport and metabolism of ropivacaine and its CYP3A4 metabolite PPX in human intestine

The major aim of this study was to investigate the CYP3A4 metabolism and polarized transport of ropivacaine and its metabolite 2',6'-pipecoloxylidide (PPX) in tissue specimens from the human small and large intestine. Ropivacaine has been shown to be effective in the treatment of ulcerative colitis in human colon. This study was conducted using a modified Ussing-chamber technique with specimens from jejunum, ileum and colon collected from 11 patients. The local kinetics of ropivacaine and PPX were assessed from their concentration-time profiles in mucosal and serosal compartments. The permeability (P(app)) in the absorptive direction for both ropivacaine and PPX increased regionally in the order jejunum < ileum < colon. Ropivacaine was not found to be subjected to any carrier-mediated intestinal efflux. However, the CYP3A4 metabolite left the human enterocyte in a polarized manner and both the extent of CYP3A4 metabolism of ropivacaine and the extrusion of its metabolite to the mucosal chamber were more efficient in jejunum than in ileum. P-glycoprotein was probably not involved in the metabolite extrusion. No other metabolite than PPX was found. This in-vitro study with human intestinal tissues provides new mechanistic insights into regional transport and metabolism of drugs.

Relationships between the hydrophilic-lipophilic balance values of pharmaceutical excipients and their multidrug resistance modulating effect in Caco-2 cells and rat intestines

The effects of a series of pharmaceutical excipients, including Span 80, Brij 30, Tween 20, Tween 80, Myrj 52, and sodium lauryl sulfate (with increasing hydrophilic-lipophilic balance (HLB) values) on the intracellular accumulation, transport kinetics, and intestinal absorption of epirubicin were investigated in both the human colon adenocarcinoma (Caco-2) cell line and the everted gut sacs of rat jejunum and ileum. The possible use of these excipients as multidrug resistance (MDR) reversing agents also was examined. Epirubicin uptake experiments using a flow cytometer showed that these selected excipients markedly enhanced the intracellular accumulation of epirubicin in Caco-2 cells in a dose-dependent manner. The optimal effect on the epirubicin uptake was characteristic of excipients with intermediate HLB values ranging from 10 to 17. Moreover, the optimal net efficacy was observed for excipients with polyoxyethylene chains and intermediate chain length of fatty acid and fatty alcohol (monolaurate for Tween 20, monooleate for Tween 80, monostearate for Myrj 52, and lauryl alcohol for Brij 30). These excipients significantly increased apical to basolateral absorption and substantially reduced basolateral to apical efflux of epirubicin across Caco-2 monolayers. Furthermore, the addition of Tween 20, Tween 80, Myrj 52, and Brij 30 markedly enhanced mucosal to serosal absorption of epirubicin in the rat jejunum and ileum. This study suggests that inhibition of intestinal P-glycoprotein (P-gp), multidrug resistance associated protein family (MRPs), or other transporter proteins by pharmaceutical excipients may improve oral absorption of drugs in MDR spectrum. The optimal HLB values of surfactant systems with suitable hydrocarbon chains and polar groups are an important factor in designing promising epirubicin formulations for reversing MDR. In conclusion, therapeutic efficacy of epirubicin may be enhanced by the use of such low toxicity excipients as absorption enhancers and MDR modulators in formulations. This provides a potential strategy for improving bioavailability in the optimization of formulations for drugs performing intestinal absorption and secretion.

Arbekacin is actively secreted in the rat intestine via a different efflux system from P-glycoprotein

This study was undertaken to examine the secretory transport of arbekacin, an aminoglycoside antibiotic, in the rat small intestine and to compare it with those in Caco-2 and LLC-PK1 cells. In vitro permeation of arbekacin was examined using an Ussing chamber technique. Serosal-to-mucosal (secretory)/mucosal-to-serosal (absorptive) permeation ratios of 0.5 mM arbekacin were 2.8 in the jejunum and 7.0 in the ileum, respectively, indicating that arbekacin permeation was highly secretory-oriented. In the ileum, the ratios became smaller with increase in arbekacin concentration applied. When D-glucose was replaced with 3-o-methyl-D-glucose in the experimental medium, the directionality of the arbekacin permeation disappeared almost completely. Absorptive permeation of arbekacin was not significantly influenced by verapamil, cyclosporin A, or probenecid. On the other hand, when gentamicin sulfate was added to the serosal medium, secretory transport of arbekacin was significantly inhibited. The results of this study strongly suggest that a specialized efflux system other than P-glycoprotein and multidrug resistance proteins was involved in the secretory transport of arbekacin in the rat intestine. There was no directionality in arbekacin permeation across Caco-2 cell monolayers, suggesting the absence or very slight expression of the secretory system for arbekacin in this cell line.

Enantioselective transport and CYP3A4-mediated metabolism of R/S-verapamil in Caco-2 cell monolayers

We have evaluated the passive and carrier-mediated intestinal transport and CYP3A4-mediated metabolism of R/S-verapamil with respect to dose dependency and enantioselectivity in modified Caco-2 cells. The present in vitro results were compared to published data from human in vivo and rat in situ jejunal perfusions with R/S-verapamil. Caco-2 cell permeability to enantiomers of verapamil and norverapamil was weakly concentration dependent (2.5-100 microM). While Caco-2 permeability to verapamil was 2.6- to 3.7-fold lower than in the human jejunum, it was 1.4- to 2.3-fold higher than in rats. However, all three models classified R- and S-verapamil as high permeability compounds according to the biopharmaceutical classification system. In accordance with human and rat data, R/S-verapamil was transported to a minor extent by carrier-mediated mechanisms in Caco-2 cells. Neither the passive nor the carrier-mediated permeability was enantioselective in any of the three models. CYP3A4-mediated demethylation to R/S-norverapamil was enantioselective in Caco-2 cells. Apparent V(max) and K(m) values for the conversion of R-verapamil were 3.2 pmol/min/insert and 0.7 microM, respectively, and for S-verapamil, 5.4 pmol/min/insert and 0.6 microM, respectively. The enantioselectivity in the CYP3A4-metabolism observed in Caco-2 cells was in agreement with human data, but not with rat data, indicating that Caco-2 cells better reflect the human small intestine in this regard. However, all three models suggested that intestinal permeability to verapamil is unaffected by CYP3A4-activity. In summary, modified Caco-2 cells and human jejunum were qualitatively related with respect to R-and S-verapamil transport and CYP3A4-metabolism.

Role of P-glycoprotein in pharmacokinetics: clinical implications

P-glycoprotein, the most extensively studied ATP-binding cassette (ABC) transporter, functions as a biological barrier by extruding toxins and xenobiotics out of cells. In vitro and in vivo studies have demonstrated that P-glycoprotein plays a significant role in drug absorption and disposition. Because of its localisation, P-glycoprotein appears to have a greater impact on limiting cellular uptake of drugs from blood circulation into brain and from intestinal lumen into epithelial cells than on enhancing the excretion of drugs out of hepatocytes and renal tubules into the adjacent luminal space. However, the relative contribution of intestinal P-glycoprotein to overall drug absorption is unlikely to be quantitatively important unless a very small oral dose is given, or the dissolution and diffusion rates of the drug are very slow. This is because P-glycoprotein transport activity becomes saturated by high concentrations of drug in the intestinal lumen. Because of its importance in pharmacokinetics, P-glycoprotein transport screening has been incorporated into the drug discovery process, aided by the availability of transgenic mdr knockout mice and in vitro cell systems. When applying in vitro and in vivo screening models to study P-glycoprotein function, there are two fundamental questions: (i) can in vitro data be accurately extrapolated to the in vivo situation; and (ii) can animal data be directly scaled up to humans? Current information from our laboratory suggests that in vivo P-glycoprotein activity for a given drug can be extrapolated reasonably well from in vitro data. On the other hand, there are significant species differences in P-glycoprotein transport activity between humans and animals, and the species differences appear to be substrate-dependent. Inhibition and induction of P-glycoprotein have been reported as the causes of drug-drug interactions. The potential risk of P-glycoprotein-mediated drug interactions may be greatly underestimated if only plasma concentration is monitored. From animal studies, it is clear that P-glycoprotein inhibition always has a much greater impact on tissue distribution, particularly with regard to the brain, than on plasma concentrations. Therefore, the potential risk of P-glycoprotein-mediated drug interactions should be assessed carefully. Because of overlapping substrate specificity between cytochrome P450 (CYP) 3A4 and P-glycoprotein, and because of similarities in P-glycoprotein and CYP3A4 inhibitors and inducers, many drug interactions involve both P-glycoprotein and CYP3A4. Unless the relative contribution of P-glycoprotein and CYP3A4 to drug interactions can be quantitatively estimated, care should be taken when exploring the underlying mechanism of such interactions.

Interaction of ochratoxin A with human intestinal Caco-2 cells: possible implication of a multidrug resistance-associated protein (MRP2)

Ochratoxin A (OTA), a nephrotoxic mycotoxin, is absorbed from small intestine and, in plasma, binds to serum albumin. Prolonged half-live results from reabsorption by proximal tubules and enterohepatic circulation. The mechanism whereby OTA crosses intestine was investigated by means of a cell culture system consisting of Caco-2 cells, as in vitro model of human intestinal epithelium. Cytotoxicity assays on proliferating Caco-2 cells showed that 0.4 microM OTA inhibits MTT reduction by 50%. Transepithelial transport and intracellular accumulation of OTA were studied in Caco-2 cells, differentiated in bicameral inserts. At pH 7.4, OTA is transported preferentially in basolateral (BL) to apical (AP) direction, suggesting a net secretion. Conditions closer to in vivo situation in duodenum (AP pH 6.0, BL pH 7.4) increase intracellular accumulation and transepithelial transport. AP to BL transport becomes higher than BL to AP transport, suggesting OTA absorption. Addition of serum albumin in BL compartment further increases OTA absorption across Caco-2 cells and suggests that in vivo OTA transport from serosal to luminal side of enterocytes is prevented, due to its binding to plasma proteins. Competition experiments showed that carrier systems for large neutral amino acids, H(+)/dipeptides cotransporter, organic anion (p-aminohippurate) carrier and organic anion transporter (oatp) are not implicated in OTA transport across Caco-2 cells, in contrast to what was reported in kidney and liver. AP and BL transport and intracellular accumulation of OTA are increased in the presence of non specific inhibitors of MRPs (indomethacin, genistein and probenecid) and of 1-chloro-2,4-dinitrobenzene (biotransformed into 2,4-dinitrophenyl-gluthatione, a specific inhibitor of MRPs), but are affected by verapamil, an inhibitor of P-gp. This suggests that the multidrug resistance-associated protein (MRP2) could be implicated in transepithelial transport. Therefore, absorption of OTA across the intestinal mucosa would be limited thanks to its excretion through MRP2 at the apical pole of enterocytes.

Drug-drug interaction mediated by inhibition and induction of P-glycoprotein

P-glycoprotein (P-gp), the most extensively studied ATP-binding cassette transporter, functions as a biological barrier by extruding toxic substances and xenobiotics out of cells. In vitro and in vivo studies have demonstrated that P-gp plays a significant role in drug absorption and disposition. Like cytochrome P450 enzymes, inhibition and induction of P-gp have been reported as the causes of drug-drug interactions. Because many prototypic inhibitors and inducers affect both CYP3A4 and P-gp, many drug interactions caused by these inhibitors and inducers involve these two systems. Clinically, it is very difficult to quantitatively differentiate P-gp-mediated drug interactions versus CYP3A4-mediated drug interactions, unless their relative contributions can be accurately estimated. Therefore, care should be exercised when interpreting drug interaction data and exploring the underlying mechanisms of drug interactions.

In vitro and in vivo models for assessing drug efflux transporter activity

Determining the activity of drug efflux transport proteins has important implications in the identification of substrates and/or inhibitors of the various transport systems, as well as mechanistic determination of localization, and functional role of the transporters in absorption, distribution and elimination of compounds in the body. This review examines both in vitro and in vivo approaches used to determine drug efflux transporter activity, their applications, and advantages and potential limitations.

Human intestinal Caco-2 cells display active transport of benzo[a]pyrene metabolites

Epithelial cells of the gastrointestinal tract are challenged by exposure to many potentially toxic agents including the well-known food contaminant benzo[a]pyrene (B[a]P). They are equipped with a variety of Phase 1- and Phase 2-enzymes that are able to metabolize B[a]P. Furthermore, transmembranous ABC-transport proteins are expressed at the apical pole of these cells. The aim of this study was to investigate whether [14C]B[a]P or products of the metabolism are transported by intestinal cells back into the gut lumen. The intestinal Caco-2 cell line was used as a metabolism and transport model. Experiments with Caco-2 monolayers in the Transwell-system revealed that radiolabeled substance is transported towards the apical (luminal) region. This transport was characterized as active and increased after induction of cytochromes P450 1A1 and 1B1 by beta-naphthoflavone. On the other hand, transport was decreased with the concomitant inhibition of Phase 1-metabolism. TLC-analysis revealed that the primary metabolites of B[a]P found in the supernatant were very polar; other metabolites of less polarity could only be detected in trace amounts. These results indicate that B[a]P is metabolized by Caco-2 cells to highly polar metabolites resulting from biphasic metabolism and that these polar metabolites are subject to an apically directed transport. Chemical inhibition studies showed that P-glycoprotein and MRP1 or 2 were not involved in this polarized B[a]P-metabolite secretion.

Region-dependent modulation of intestinal permeability by drug efflux transporters: in vitro studies in mdr1a(-/-) mouse intestine

Information on the extent to which xenobiotics interact with P-glycoprotein (PGP) during transit through the intestine is crucial in determining the influence of PGP on oral drug absorption. We have recently described a novel use of isolated ileum from PGP-deficient mdr1a(-/-) mice to resolve PGP- and non-PGP-dependent drug efflux and provide a definitive measure of intrinsic drug permeability without recourse to inhibitors. The present study uses this approach to investigate the impact of PGP on intestinal permeability of paclitaxel and digoxin in different regions of the mouse intestine (jejunum, ileum, and proximal and distal colon). Absorption of paclitaxel and digoxin in tissues from wild-type mice was low and showed little regional variation. In contrast, absorption of both drugs was markedly higher in mdr1a(-/-) intestine, although the increase was highly region-dependent, with the ileum and distal colon showing the greatest effect and much smaller changes in the jejunum and proximal colon. These effects were accompanied by the abolition of paclitaxel and digoxin secretion in mdr1a(-/-) mice, suggesting that regional variations in intestinal permeability are masked by differential PGP expression, confirmed by immunoblotting studies. Propranolol permeability, which is not influenced by PGP, showed similar regional variation in both wild-type and mdr1a(-/-) tissues, suggesting that differences are at the level of transcellular permeability. These data suggest that the ileum and the distal colon are regions of relatively high transcellular permeability for xenobiotics that are compensated by enhanced expression of PGP.

Kinetic characterization of secretory transport of a new ciprofloxacin derivative (CNV97100) across Caco-2 cell monolayers

The kinetics of transport of a new fluoroquinolone antibiotic (CNV97100) and its analogs were characterized using the Caco-2 cell culture model. Unidirectional permeabilities of these analogs were greater (p < 0.05) than that of ciprofloxacin. The absorptive permeabilities (P(AB)) of 4'-N-substituted analogs (CNV97101-104) were 400-600% greater, whereas the secretory permeability (P(BA)) was 25-80% greater than unsubstituted analogs because CNV97101-104 were poor substrates for efflux transporters (efflux ratio approximately 1). The transport of compounds without 4'-N-substitution (i.e., ciprofloxacin and CNV97100) favored secretion (efflux ratio approximately 4). Further characterization of CNV97100 transport revealed that it was concentration dependent (apparent K(m) = 0.484 mM, and apparent V(max) = 17.5 nmol x cm(-2) x h(-1)), and temperature dependent (E(a) = 20.57 for P(AB) and 31.45 kcal/mol for P(BA), respectively). p-Glycoprotein (p-gp)inhibitors, such as verapamil (100 microM) and cyclosporin A (CsA, 20 microM) significantly (p < 0.05) inhibited P(BA) but significantly (p < 0.05) enhanced P(AB). Multidrug resistance related protein (MRP) inhibitor leukotriene C(4) only decreased (p < 0.05) P(BA) of ciprofloxacin but not that of CNV97100. In the presence of increasing concentrations of verapamil, the P(BA) of CNV97100 decreased significantly (p < 0.05), with an IC(90) value of 96.5 microM. Taken together, these results suggested that 4'-N-alkylation of fluoroquinolones improves their absorptive permeability. Secretion of CNV97100 is dominated by p-gp, whereas the secretion of ciprofloxacin is via a combination of efflux transporters.

Fexofenadine transport in Caco-2 cells: inhibition with verapamil and ritonavir

This study investigated fexofenadine (FXD) transport and the inhibition of FXD transport in Caco-2 cell monolayer transwells, using rhodamine 123 (RH123) transport as a positive control. FXD transport from the basolateral (B) to apical (A) compartment was fivefold higher than A to B transport. FXD transport was linear with respect to time (up to 270 min) and concentration (up to 300 microm). Similar results were seen with the positive control RH123. Ritonavir (100 PM) and verapamil (100 microm) reduced transport of FXD and RH123 by more than 80%, whereas transport was not inhibited by 100 m indomethacin or 2 mM probenecid. This suggests predominantly P-glycoprotein (P-gp)-mediated transport as opposed to transport by multidrug resistance protein. In concentration-response experiments, FXD transport was inhibited by verapamil and ritonavir with IC50 values of 6.5 microm and 5.4 microm, respectively. Results from this in vitro study demonstrate differential transport of FXD across Caco-2 cell monolayers and inhibition of FXD transport by established P-gp inhibitors. Thefindings support the use of FXD as an index or probe compound to reflect P-gp activity in vivo.

Theoretical predictions of drug absorption in drug discovery and development

The clinical development of new drugs is often terminated because of unfavourable pharmacokinetic properties such as poor intestinal absorption after oral administration. Intestinal permeability and solubility are two of the most important factors that determine the absorption properties of a compound. Efficient and reliable computational models that predict these properties as early as possible in drug discovery and development are therefore desirable. In this review, we first discuss the implementation of predictive models of intestinal drug permeability and solubility in drug discovery and development. Secondly, we discuss the mechanisms of intestinal drug permeability and computational methods that can be used to predict it. We then discuss factors influencing drug solubility and models for predicting it. We finally speculate that once these and other predictive computational models are implemented in drug discovery and development, these processes will become much more effective. Further, an increased fraction of drug candidates that are less likely to fail during clinical development will be selected.

Interaction between metabolism and transport of benzo[a]pyrene and its metabolites in enterocytes

Epithelial cells of the small intestine are responsible for the resorption of different food components as well as potentially toxic agents such as benzo[a]pyrene (B[a]P), a particular contaminant of charcoal-grilled meat. This study was undertaken to investigate any functional relationship between the metabolism of B[a]P and the unidirectional transport of metabolites back into the intestinal lumen mediated by ATP-binding cassette (ABC) transport proteins. The human intestinal Caco-2 cell line was used. In addition, mdr1- and mrp2-transfected MDCK cells were employed to characterize the possible role of these ABC transport proteins in the polarized transport. After incubations of Caco-2 cells with B[a]P, HPLC analysis revealed that the primary metabolites of B[a]P were B[a]P-1-sulfate and B[a]P-3-sulfate. Other metabolites, such as B[a]P-3-glucuronide, B[a]P-9,10-diol, or B[a]P-3,6-quinone, could be detected only in small amounts. The transport experiments using Transwell chambers clearly showed that B[a]P-1- and B[a]P-3-sulfate were actively transported toward the apical (luminal) region. This transport increased after induction of CYP1A1/CYP1B1 (Phase 1)-metabolism, although a decrease was observed during concomitant inhibition. Inhibition studies using chemical inhibitors of P-glycoprotein, MRPs, showed no effects. A comparison between the transport of B[a]P-1- and B[a]P-3-sulfate in wild-type and mrp2-transfected MDCKII cells revealed no differences at all. The results indicate that B[a]P is metabolized by Caco-2 cells mainly to B[a]P-1- and B[a]P-3-sulfate, which are subject to an apically directed transport. Furthermore ABC transport proteins P-glycoprotein, MRP1, and MRP2 are not involved in this polarized B[a]P-sulfate secretion.

Novel choline transport characteristics in Caco-2 cells

Choline transport is characterized by sodium-dependent high-affinity, sodium-independent low-affinity, and sodium-independent blood-brain barrier transport mechanisms. Each defined mechanism has specific characteristics with regard to affinity for choline, transport capacity, and inhibition by hemicholinium. The purpose of this study is to determine the characteristics of choline transport across Caco-2 monolayers.

METHODS

Choline transport across Caco-2 cell monolayers was determined in both the apical to basal direction and the opposite direction. Further, the determination of calcium dependence and specific inhibitors was made. Determination of the apparent permeability of choline was calculated by established methods.

RESULTS

The apical to basal Caco-2 permeability coefficient is 11.11 +/- 0.33 x 10(-6) cm/sec with 21.3% of the choline associating with the cells. Meanwhile the basal to apical value is approximately 50% less (5.55 +/- 0.14 x 10(-6) cm/sec), suggesting an active apical to basal transport mechanism. Choline transport in this system was inhibited by nifedipine (82%), verapamil (80%), EGTA (36%), and cyclosporin (15%).

CONCLUSIONS

Choline transport across Caco-2 cells is demonstrated to be active and both pH- and Ca(2+)-dependent. Furthermore, choline transport across Caco-2 monolayers has unique characteristics when compared to traditional choline transport models.

Differential modulation of P-glycoprotein expression and activity by non-nucleoside HIV-1 reverse transcriptase inhibitors in cell culture

PURPOSE

This study investigated the effects of the non-nucleoside HIV-1 reverse transcriptase inhibitors (NNRTI) nevirapine (NVR), efavirenz (EFV), and delavirdine (DLV) on P-glycoprotein (P-gp) activity and expression to anticipate P-gp related drug-drug interactions associated with combination therapy.

METHODS

NNRTIs were evaluated as P-gp substrates by measuring differential transport across Caco-2 cell monolayers. Inhibition of P-gp mediated rhodaminel23 (Rh123) transport in Caco-2 cells was used to assess P-gp inhibition by NNRTIs. Induction of P-gp expression and activity in LS180V cells following 3-day exposure to NNRTIs was measured by western blot analysis and cellular Rh123 uptake, respectively.

RESULTS

The NNRTIs showed no differential transport between the basolateral to apical and apical to basolateral direction. NNRTI transport in either direction was not affected by the P-gp inhibitor verapamil. DLV inhibited Rh123 transport, causing a reduction to 15% of control at 100 microM (IC50 = 30 microM). NVR caused a concentration-dependent induction of P-gp expression in LS180V cells resulting in a 3.5-fold increase in immunoreactive P-gp at 100 microM NVR. Induction attributable to EFV and DLV was quantitatively smaller. NVR significantly reduced cellular uptake of Rh123 into LS180V cells, indicating increased drug efflux due to induced P-gp activity; effects of EFV and DLV were smaller. Acute DLV treatment of LS180V cells previously induced with NVR or ritonavir did not reverse the decreased Rh123 cell accumulation.

CONCLUSIONS

NNRTIs show differential effects on P-gp activity and expression in vitro. Clinical studies are required to elucidate the clinical importance of potential drug interactions.

The mucosa of the small intestine: how clinically relevant as an organ of drug metabolism?

The intestinal mucosa is capable of metabolising drugs via phase I and II reactions. Increasingly, as a result of in vitro and in vivo (animal and human) data, the intestinal mucosa is being implicated as a major metabolic organ for some drugs. This has been supported by clinical studies of orally administered drugs (well-known examples include cyclosporin, midazolam, nifedipine and tacrolimus) where intestinal drug metabolism has significantly reduced oral bioavailability. This review discusses the intestinal properties and processes that contribute to drug metabolism. An understanding of the interplay between the processes controlling absorption, metabolism and P-glycoprotein-mediated efflux from the intestinal mucosa into the intestinal lumen facilitates determination of the extent of the intestinal contribution to first-pass metabolism. The clinical relevance of intestinal metabolism, however, depends on the relative importance of the metabolic pathway involved, the therapeutic index of the drug and the inherent inter- and intra-individual variability. This variability can stem from genetic (metabolising enzyme polymorphisms) and/or non-genetic (including concomitant drug and food intake, route of administration) sources. An overwhelming proportion of clinically relevant drug interactions where the intestine has been implicated as a major contributor to first-pass metabolism involve drugs that undergo cytochrome P450 (CYP) 3A4-mediated biotransformation and are substrates for the efflux transporter P-glycoprotein. Much work is yet to be done in characterising the clinical impact of other enzyme systems on drug therapy. In order to achieve this, the first-pass contributions of the intestine and liver must be successfully decoupled.

Intestinal secretion is a major route for parent ivermectin elimination in the rat

The transepithelial intestinal elimination of ivermectin was studied using the intestinal closed-loop model in the rat. The common bile duct was cannulated, and duodenum, jejunum, and ileum were isolated in situ with their intact blood supplies. Following administration of 100, 200, or 400 microg/kg b.wt. ivermectin via the carotid artery, the elimination of parent ivermectin into the small intestinal lumen over 90 min was approximately 5-fold higher than in bile. The major amount of secreted ivermectin was recovered in the jejunum, but the duodenum showed a higher intestinal elimination capacity than the other intestinal segments with respect to the intestinal length. Systemic coadministration of the P-glycoprotein blocker verapamil significantly reduced the elimination capacity of jejunum by 50%, which resulted in a 30% decrease of ivermectin overall elimination by the small intestine. In contrast, verapamil did not significantly affect ivermectin secretion in duodenum, ileum, or bile in the same animals. Ivermectin small intestinal and biliary clearances were estimated to account for 27 and 5.5% of the total drug clearance, which was evaluated from a parallel in vivo experiment in which rats were given 200 microg/kg b.wt. ivermectin intra-arterially. In conclusion, intestinal secretion plays a greater role than biliary secretion in the overall elimination of ivermectin in the rat, providing major amounts of active drug to the intestinal lumen and to feces. This is discussed in terms of therapeutic efficacy against intestinal parasites in humans and animals and of ecotoxicity resulting from the contamination of livestock dung with parent drug.

A mechanistic approach to understanding the factors affecting drug absorption: a review of fundamentals

This article provides an overview of the patient-specific and drug-specific variables that can affect drug absorption following oral product administration. The oral absorption of any chemical entity reflects a complex spectrum of events. Factors influencing product bioavailability include drug solubility, permeability, and the rate of in vivo dissolution. In this regard, the Biopharmaceutics Classification System has proven to be an important tool for predicting compounds likely to be associated with bioavailability problems. It also helps in identifying those factors that may alter the rate and extent of drug absorption. Product bioavailability can also be markedly influenced by patient attributes such as the integrity of the gastrointestinal tract, physiological status, site of drug absorption, membrane transporters, presystemic drug metabolism (intrinsic variables), and extrinsic variables such as the effect of food or concomitant medication. Through an awareness of a drug's physicochemical properties and the physiological processes affecting drug absorption, the skilled pharmaceutical scientist can develop formulations that will maximize product availability. By appreciating the potential impact of patient physiological status, phenotype, age, gender, and lifestyle, dosing regimens can be tailored to better meet the needs of the individual patient.

Effects of grapefruit juice and orange juice on the intestinal efflux of P-glycoprotein substrates

PURPOSE

The aim of this study is to investigate the effects of 50% ethyl acetate extracts of grapefruit juice (GFJ) and orange juice (OJ) on the transport activity of P-glycoprotein (P-gp) in the rat small intestine.

METHODS

The efflux of P-gp substrates from rat everted sac in the absence or presence of verapamil, GFJ, OJ or erythromycin was measured. Rhodamine123, fexofenadine and saquinavir were used as P-gp substrates. P-gp expression levels in the rat jejunum and ileum were determined by Western blot analysis.

RESULTS

The efflux of rhodamine123 from the everted sac increased from the apex of the jejunum to the low ileum and the expression of P-gp in the ileum was 2.31-fold higher than that in the jejunum. Verapamil and the 50% GFJ and OJ extracts inhibited the efflux from the intestine of all three drugs tested. Erythromycin decreased the efflux of rhodaminel23 and fexofenadine, but did not affect the efflux of saquinavir in the intestine.

CONCLUSIONS

GFJ and OJ extracts inhibited the efflux of P-gp substrates from the small intestine. Therefore, they may enhance the oral bioavailability of P-gp substrates by increasing absorption in the small intestine.

Intestinal metabolism promotes regional differences in apical uptake of indinavir: coupled effect of P-glycoprotein and cytochrome P450 3A on indinavir membrane permeability in rat

The purpose of this study was to investigate transport and metabolism contributions to low indinavir permeability in rat ileum and enhanced drug permeability in the jejunum. Permeability models utilized included single pass in situ rat intestinal perfusion and rat intestinal tissue mounted in Ussing chambers. Intestinal metabolism was measured by fractional appearance of metabolite (F(met)), determined as the percentage of the predominant metabolite M6 over luminal loss of indinavir in the perfusion model. Among the results, indinavir exhibited bidirectional transport across rat ileum. Verapamil and cyclosporin A inhibited net flux by 37 and 38%, respectively. Intestinal metabolism of indinavir was most significant in upper jejunum (F(met) = 65.78 +/- 19.02%), decreasing in midjejunum (F(met) = 31.58 +/- 5.63%). M6 was not detectable in ileum or colon. Western blot analysis of rat intestinal mucosal tissue samples confirmed that the axial expression of CYP3A was consistent with the regional pattern of formation of M6. Intestinal metabolism was saturable and could be inhibited by the CYP3A inhibitor, ketoconazole. A low luminal concentration of indinavir (1 microM) was associated with high F(met) (87.90 +/- 14.30%), whereas a high luminal concentration of indinavir (50 microM) was associated with low F(met) (35.84 +/- 11.59%). In the presence of ketoconazole, both F(met) and permeability of indinavir were reduced in the jejunum. These results suggest that 1) intracellular metabolism of indinavir enhances apical uptake of indinavir in the rat jejunum as a function of the increased concentration gradient generated across the epithelial cell membrane and 2) the efflux transporter P-glycoprotein limits apical uptake of indinavir in the ileum, resulting in low apparent permeability.

The influence of P-glycoprotein on morphine transport in Caco-2 cells. Comparison with paclitaxel

In vitro monolayer studies using Caco-2 cells were employed here to explore P-glycoprotein mediated transport of morphine. Bi-directional transport studies of 10-75 microM morphine showed efflux to be twofold higher than influx (4 x 10(-6) compared to 2 x 10(-6) cm/s) and cellular accumulation in the efflux direction was eightfold higher. The cyclosporin analogue (PSC-833) equilibrated morphine transport in both directions. Depletion of intracellular glutathione had a greater effect on increasing cellular morphine accumulation than P-glycoprotein inhibitors, suggesting a role for glutathione in morphine transport. P-glycoprotein had a substantially greater effect on paclitaxel accumulation, efflux and bi-directional transport than for morphine. Paclitaxel transport was below detection (<0.1 x 10(-6) cm/s) in the influx direction, yet efflux was very high (18.4 x 10(-6) cm/s) and P-glycoprotein inhibition increased accumulation >100-fold. These results reinforce the substantial role P-glycoprotein has in paclitaxel transport. Conversely, P-glycoprotein regulated morphine transport is weak. Nevertheless, morphine transport rates could be doubled when administered with P-glycoprotein substrates. Therefore, increased analgesia through P-glycoprotein inhibition should be possible.

Delineating the contribution of secretory transporters in the efflux of etoposide using Madin-Darby canine kidney (MDCK) cells overexpressing P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP1), and canalicular multispecific organic anion transporter (cMOAT)

Multidrug resistance conferred to cancer cells is often mediated by the expression of efflux transporter "pumps". It is also believed that many of the same transporters are involved in drug efflux from numerous normal endothelial and epithelial cell types in the intestine, brain, kidney, and liver. Etoposide transport kinetics were characterized in Caco-2 cells and in well established Madin-Darby canine kidney (MDCKII) cell lines that were stably-transfected with a human cDNA encoding P-glycoprotein (Pgp), human multidrug resistance protein (MRP1), or the canalicular multispecific organic anion (cMOAT) transporters to determine the roles of these transporters in etoposide efflux. Etoposide transport kinetics were concentration-dependent in the MDCKII-MDR1 and MDCKII-cMOAT cells. The apparent secretory Michaelis constant (Km) and carrier-mediated permeability (Pc) values for Pgp and cMOAT were 254.96 +/- 94.39 microM and 5.96 +/- 0.41 x 10(-6) cm/s and 616.54 +/- 163.15 microM and 1.87 +/- 0.10 x 10(-5) cm/s, respectively. The secretory permeability of etoposide decreased significantly in the basal to apical (B to A) (i.e., efflux) direction, whereas the permeability increased 2.3-fold in the apical to basal (A to B) direction in MDCKII-MDR1 cells in the presence of elacridar (GF120918). Moderate inhibition of etoposide efflux by leukotriene C4 (LTC4) was observed in MDCKII-cMOAT cells. Furthermore, etoposide inhibited LTC4 efflux, confirming the involvement of cMOAT. The flux of etoposide in MDCKII-MRP1 cells was similar to that in MDCKII/wt control cells. The current results demonstrate that the secretory transport mechanism of etoposide involves multiple transporters, including Pgp and cMOAT but not MRP1. These results demonstrate that Pgp and cMOAT are involved in the intestinal secretory transport of etoposide. Since the intestinal secretion of etoposide was previously reported in the literature, it also suggests that they may be involved in the in vivo intestinal secretion of etoposide; however, mechanistic in vivo studies are required to confirm this.

Resolution of P-glycoprotein and non-P-glycoprotein effects on drug permeability using intestinal tissues from mdr1a (-/-) mice

1. Intestinal xenobiotic transporters are a significant barrier to the absorption of many orally administered drugs. P-glycoprotein (PGP) is the best known, but several others, including members of the multidrug resistance-associated protein (MRP) family, are also expressed. Definitive information on their precise effect on intestinal drug permeability is scarce due to a lack of specific inhibitors and the difficulty of studying non-PGP activity in the presence of high PGP expression. 2. We have investigated the in vitro use of intestinal tissues from PGP knockout (mdr1a (-/-)) mice as a tool for dissecting the mechanisms of intestinal drug efflux. The permeability characteristics of digoxin (DIG), paclitaxel (TAX) and etoposide (ETOP) were measured in ileum from mdr1a (-/-) and wild-type (FVB) mice mounted in Ussing chambers. 3. DIG and TAX exhibited marked efflux across FVB tissues (B-A : A-B apparent permeability (P(app)) ratio 10 and 17 respectively) which was absent in mdr1a (-/-) tissues, confirming that PGP is the sole route of intestinal efflux for these compounds. The A-B P(app) of both compounds was 3 - 5 fold higher in mdr1a (-/-) than in FVB. 4. Polarized transport of ETOP in FVB tissues was reduced but not abolished in mdr1a (-/-) tissues. Residual ETOP efflux in mdr1a (-/-) tissues was abolished by the MRP inhibitor MK571, indicating involvement of both PGP and MRP. 5. MK571 abolished calcein efflux in mdr1a (-/-) tissues, while quinidine had no parallel effect in FVB tissues, suggesting involvement of MRP but not PGP. 6. Tissues from mdr1a (-/-) mice provide a novel approach for investigating the influence of PGP ablation on intestinal permeability and for resolving PGP and non-PGP mechanisms that modulate drug permeability.

Tacrolimus is a class II low-solubility high-permeability drug: the effect of P-glycoprotein efflux on regional permeability of tacrolimus in rats

The objective of this study is to investigate the role of P-glycoprotein (P-gp), a membrane efflux pump associated with multidrug resistance (MDR) and a known substrate for tacrolimus, in determining the regional intestinal permeability of tacrolimus in rats. Thus, isolated segments of rat jejunum, ileum, or colon were perfused with tacrolimus solutions containing polyethoxylated hydrogenated castor oil 60 surfactant, and with or without verapamil, a P-gp substrate used to reverse the MDR phenotype. The results indicated that the intrinsic permeability of tacrolimus in the jejunum, calculated on the basis of the concentration of non-micellized free tacrolimus, was quite high ( approximately 1.4 x 10(-4) cm/s). The apparent permeability (P(app)) in the jejunum was unaffected by the presence of verapamil; however, the P(app) in the ileum and the colon increased significantly in the presence of verapamil and were similar to the values observed in the jejunum. The results suggest that systemic absorption of tacrolimus from the gastrointestinal tract could be significantly affected by P-gp efflux mechanisms. It is also possible that differences in P-gp function at various intestinal sites in a subject or at a given intestinal site in various subjects could lead to large intra- and interindividual variability in bioavailability of tacrolimus following oral administration.

No evidence for the involvement of the multidrug resistance-associated protein and/or the monocarboxylic acid transporter in the intestinal transport of fluvastatin in the rat

Fluvastatin, an amphiphilic anion, shows a nonlinear increase in effective intestinal permeability (P(eff)) with increasing lumenal concentrations in rats. The main objective of this study was to investigate whether or not this observation could be attributed to an efflux-mediated transport by the multidrug resistance-associated protein (MRP). In parallel, we investigated the possible involvement of the monocarboxylic acid transporter (MCT) in the rapid intestinal absorption of fluvastatin. Single-pass perfusions were performed in the ileum and colon of the rat, with and without the presence of well-established inhibitors/substrates for the MRP (probenecid) and the MCT (nicotinic acid). The results suggest that neither the MRP nor the MCT are involved to any significant extent in the absorption process of fluvastatin in the rat intestine. Thus, the previously reported concentration-dependent P(eff) of fluvastatin in these intestinal regions of the rat is probably not attributable to saturation of any efflux mediated by MRP.

Relevance of multidrug resistance proteins for intestinal drug absorption in vitro and in vivo

Multidrug resistance proteins (p-glycoprotein and mrps) are becoming increasingly important to explain the pharmacokinetics and action of drugs. Located in epithelial and endothelial cells of the gastrointestinal tract, liver, kidney, blood brain barrier, choroid plexus and other organs, they are critical determinants for the movement of a large number of commonly prescribed drugs across cellular barriers. Here we provide a brief overview of the role of multidrug resistance proteins in drug absorption from the gastrointestinal tract. We address the different types of multidrug resistance proteins involved, describe experimental models to study the influence of these proteins on transcellular transport and discuss the impact of multidrug resistance proteins on overall drug bioavailability in vivo.

Differentiation of gut and hepatic first-pass effect of drugs: 1. Studies of verapamil in ported dogs

PURPOSE

To investigate the relative contributions of the gut and liver to the first-pass loss of verapamil (VL) using an in vivo intestinal-vascular access port (IVAP) dog model.

METHODS

Basic pharmacokinetics of VL were determined after intravenous (IV: 0.5 mg/kg), portal venous (PV: 2 mg/kg), and duodenal (ID: 2 mg/kg) administration in IVAP dogs. Serial blood samples were collected for 8 h after dosing, and plasma was analyzed for unchanged drug by a high-performance liquid chromatography-fluorescence method. Extraction ratios in the liver and intestinal tract were determined from the area under the concentration-time curves for ID, PV, and IV administration. The functional role of CYP450 or secretory transporters such as P-gp on the gut and liver first-pass loss of VL was further studied using ritonavir, a known substrate or inhibitor of these processes.

RESULTS

The liver had a high intrinsic capacity for clearing VL because the absolute bioavailability (BA) of VL was 21.7% after PV administration. The BA of VL after ID administration was 23.5%; therefore, intestinal absorption was complete and intestinal extraction was negligible (ER(GI) approximately 0). The BA of VL increased from 23.5% to 66.2% in the presence of ritonavir primarily due to a reduction in hepatic extraction.

CONCLUSIONS

Although the liver had a high intrinsic capacity for extracting VL, the contribution of gut to the first-pass loss of VL was negligible. Because of the additive effects of intestinal CYP3A-mediated metabolism and secretory transport, a significant gut first-pass effect was expected, but not observed in dogs. These studies demonstrate the utility of the in vivo IVAP dog model for evaluating the relative contribution of the gut and liver to the first-pass loss of drugs and for characterizing the functional role that CYP450 metabolism and/or secretory transporters play in drug-drug interactions and reduced oral bioavailability.

The impact of P-glycoprotein efflux on enterocyte residence time and enterocyte-based metabolism of verapamil

P-glycoprotein (P-gp) can limit the intestinal permeability of a number of compounds and may therefore influence their exposure to metabolizing enzymes within the enterocyte (e.g. cytochrome P450 3A, CYP 3A). In this study, the intestinal metabolic profile of verapamil, the influence of P-gp anti-transport on the cellular residence time of verapamil, and the impact of this change in residence time on the extent of enterocyte-based metabolism have been investigated in-vitro, utilizing segments of rat jejunum and side-by-side diffusion chambers. Verapamil exhibited concentration-dependent P-gp efflux and CYP 3A metabolism. The P-gp efflux of verapamil (1 microM) increased the cellular residence time across the intestinal membrane (approximately 3-fold) in the mucosal to serosal (m to s) direction relative to serosal to mucosal (s to m), yielding significantly greater metabolism (approximately 2-fold), presumably as a result of the prolonged exposure to CYP 3A. Intestinal metabolism of verapamil generated not only norverapamil, but resulted also in the formation of an N-dealkylated product (D-617). Norverapamil and D-617 accumulated significantly in mucosal chambers, relative to serosal chambers, over the time course of the experiment. Based on these in-vitro data, it was apparent that P-gp efflux prolonged the cellular residence time of verapamil (m to s) and therefore increased the extent of intestinal metabolism, and also played a role in metabolite secretion from within the enterocyte.

Predicting the impact of physiological and biochemical processes on oral drug bioavailability

Recent advances in computational methods applied to the fields of drug delivery and biopharmaceutics will be reviewed with a focus on prediction of the impact of physiological and biochemical factors on simulation of gastrointestinal absorption and bioavailability. Our application of a gastrointestinal simulation for the prediction of oral drug absorption and bioavailability will be described. First, we collected literature data or we estimated biopharmaceutical properties by application of statistical methods to a set of 2D and 3D molecular descriptors. Second, we integrated the differential equations for an advanced compartmental absorption and transit (ACAT) model in order to determine the rate, extent, and approximate gastrointestinal location of drug liberation (for controlled release), dissolution, passive and carrier-mediated absorption, and saturable metabolism and efflux. We predict fraction absorbed, bioavailability, and C(p) vs. time profiles for common drugs and compare those estimates to literature data. We illustrate the simulated impact of physiological and biochemical processes on oral drug bioavailability.

Characterizing the expression of CYP3A4 and efflux transporters (P-gp, MRP1, and MRP2) in CYP3A4-transfected Caco-2 cells after induction with sodium butyrate and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate

PURPOSE

To examine the changes in expression levels of CYP3A4 and efflux transporters in CYP3A4-transfected Caco-2 (colon carcinoma) cells in the presence of the inducers sodium butyrate (NaB) and 12-O-tetradecanoylphorbol-13-acetate (TPA). To characterize the transport of [3H]-digoxin and the metabolism of midazolam in the cells under different inducing conditions.

METHODS

CYP3A4-Caco-2 cells were seeded onto cell culture inserts and were grown for 13-14 days. Transport and metabolism studies were performed on cells induced with NaB and/or TPA for 24 h. The expression and localization of P-gp, MRP1, MRP2, and CYP3A4 were examined by Western blot and confocal microscopy.

RESULTS

In the presence of both inducers, CYP3A4 protein levels were increased 40-fold over uninduced cells, MRP2 expression was decreased by 90%, and P-gp and MRP1 expression were unchanged. Midazolam 1-OH formation exhibited a rank order correlation with increased CYP3A4 protein, whereas [3H]-digoxin transport (a measure of P-gp activity) was unchanged with induction. P-gp and MRP2 were found on the apical membrane, whereas MRP1 was found perinuclear within the cell. CYP3A4 displayed a punctate pattern of expression consistent with endoplasmic reticulum localization and exhibited preferential polarization towards the apical side of the cell.

CONCLUSIONS

The present study characterized CYP3A4-Caco-2 cell monolayers when induced for 24 h in the presence of both NaB and TPA. These conditions provide intact cells with significant CYP3A4 and P-gp expression suitable for the concurrent study of transport and metabolism.

Evaluation of a single-pass intestinal-perfusion method in rat for the prediction of absorption in man

Prediction of the fraction of dose absorbed from the intestine (Fa) in man is essential in the early drug discovery stage. In-vitro assays in Caco-2 and MDCK cells are routinely used for that purpose, and their predictive value has been reported. However, in-situ techniques might provide a more accurate estimation of Fa. In this study, we evaluated a single-pass intestinal-perfusion (SPIP) method in the rat for its use in the prediction of absorption in man and compared it with a previous report using cell-based assays. Effective permeability coefficients (Peff) were determined in rats for 14 compounds, and ranged from 0.043x 10(-4) cm s(-1) to 1.67 x 10(-4) cm s(-1). These values strongly correlated (r2 = 0.88) with reported Peff values for man. In addition, the Spearman rank correlation coefficient calculated for in-situ-derived Peff and absorption in man was 0.92 while for the previously tested in-vitro Caco-2 and MDCK systems vs absorption in man, the correlation coefficients were 0.61 and 0.59, respectively. SPIP provided a better prediction of human absorption than the cell-based assays. This method, although time consuming, could be used as a secondary test for studying the mechanisms governing the absorption of new compounds, and for predicting more accurately the fraction absorbed in man.

P-glycoprotein interactions of nefazodone and trazodone in cell culture

This study investigated the effects of nefazodone (NFZ) and trazodone (TZD) on P-glycoprotein (P-gp) activity and expression in cell culture. NFZ and TZD showed no differential transport between the basolateral to apical and apical to basolateral direction across Caco-2 cell monolayers. Transport in either direction was not affected by verapamil. NFZ was a potent inhibitor (IC50 = 4.7 microM) of rhodamine123 (Rh123) B to A transport across Caco-2 cell monolayers, while TZD had minimal effect. Following 72-hour exposure of LS180V cells to NFZ and TZD (10 microM), a twofold increase in immunoreactive P-gp was observed. Rh123 accumulation into these cells was reduced to 65% and 74% of control by NFZ and TZD (10 microM), respectively. It was concluded that differential rates of transport of NFZ and TZD in Caco-2 cells were not evident. However, NFZ is an inhibitor of P-gp activity at clinically relevant in vivo concentrations and may have the potential to increase bioavailability of coadministered compounds that are substrates for transport. Concentrations of NFZ and TZD achieved in the intestine after chronic oral dosing may induce P-gp expression and reduce absorption of coadministered drugs.

P-glycoprotein efflux pump expression and activity in Calu-3 cells

The purpose of this work was to determine if the sub-bronchial epithelial cell model, Calu-3, expresses the functionally active P-glycoprotein (Pgp) efflux pump. Calu-3 cells express lower levels of Pgp than both Caco-2 and A549 cells as determined by Western Blot analysis. In Calu-3 cells, accumulation of the Pgp substrates rhodamine 123 (Rh123) and calcein acetoxymethyl ester (calcein-AM) was increased in the presence of the specific Pgp inhibitors cyclosporin A (CsA), vinblastine, and taxol. Significant inhibition of Pgp activity was not observed until after 2 h in both cell lines. The organic anion/multidrug resistance associated protein-1 (MRP1) inhibitors, probenecid and indomethacin, did not affect Rh123 accumulation, whereas an increase in calcein accumulation was observed by both agents. The metabolic inhibitor sodium azide decreased the efflux of Rh123 out of Calu-3 cells to the same degree as CsA, supporting inhibition of an active, efflux pathway. The basolateral-to-apical transport of Rh123 was significantly higher than that in the reverse direction, indicating a secretory pathway of efflux that was inhibited 25-fold by CsA. Basolateral-to-apical transport of Rh123 was inhibited slightly with both MRP1 inhibitors; however, no significant effect of Rh123 net secretion was observed. Mixed inhibitor studies demonstrated that Rh123 efflux was mainly Pgp mediated. These results support an energy-dependent Pgp efflux pump pathway that is sensitive to inhibition with CsA in Calu-3 cells.

The use of transgenic mice in pharmacokinetic and pharmacodynamic studies

Transgenic technology has made it possible to alter the genetic make-up of a laboratory mouse through the removal or insertion of selected genes. The resulting transgenic mouse provides a means for determining the developmental and functional contributions of selected genes and the proteins they encode. The current article reviews examples of the use of transgenic mice in pharmacokinetic and pharmacodynamic studies. In addition to examining current applications of transgenic technology in the areas of pharmacokinetics and pharmacodynamics, the potential for future advancements as well as limitations of the technology are discussed.

A functional assay for quantitation of the apparent affinities of ligands of P-glycoprotein in Caco-2 cells

PURPOSE

To develop a facile functional assay for quantitative determination of the apparent affinities of compounds that interact with the taxol binding site of P-glcoprotein (P-gp) in Caco-2 cell monolayers.

METHODS

A transport inhibition approach was taken to determine the inhibitory effects of compounds on the active transport of [3H]-taxol, a known substrate of P-gp. The apparent affinities (K(I) values) of the compounds were quantitatively determined based on the inhibitory effects of the compounds on the active transport of [3H]-taxol. Intact Caco-2 cell monolayers were utilized for transport inhibition studies. Samples were analyzed by liquid scintillation counting.

RESULTS

[3H]-Taxol (0.04 microM) showed polarized transport with the basolateral (BL) to apical (AP) flux rate being about 10-20 times faster than the flux rate in the AP-to-BL direction. This difference in [3H]-taxol flux could be totally abolished by inclusion of (+/-)-verapamil (0.2 mM), a known inhibitor of P-gp, in the incubation medium. However, inclusion of probenecid (1.0 mM), a known inhibitor for the multidrug resistance associated protein (MRP), did not significantly affect the transport of [3H]-taxol under the same conditions. These results suggest that P-gp, not MRP, was involved in taxol transport. Quinidine, daunorubicin, verapamil, taxol, doxorubicin, vinblastine, etoposide, and celiprolol were examined as inhibitors of the BL-to-AP transport of [3H]-taxol with resulting K(I) values of 1.5+/-0.8, 2.5+/-1.0, 3.0+/-0.3, 7.3+/-0.7, 8.5+/-2.8, 36.5+/-1.5, 276+/-69, and 313+/-112 microM, respectively. With the exception of that of quinidine, these K(I) values were comparable with literature values.

CONCLUSIONS

This assay allows a facile quantitation of the apparent affinities of compounds to the taxol-binding site in P-gp, however, this assay does not permit the differentiation of substrates and inhibitors. The potential of drug-drug interactions involving the taxol binding site of P-gp can be conveniently estimated using the protocol described in this paper.

Secretory transport of p-aminohippuric acid across intestinal epithelial cells in Caco-2 cells and isolated intestinal tissue

The intestinal transport of an organic anion, p-aminohippuric acid (PAH), was studied in Caco-2 cell monolayers and rat intestinal tissue mounted in Ussing chambers. In both experimental methods, PAH exhibited vectorial transport with significantly greater permeability in the secretory direction than the absorptive direction, indicating net secretion. This secretory transport required metabolic energy, but protons or hydroxyl ions were not involved as the driving force. In Caco-2 monolayers, secretory transport of [3H]PAH was decreased, and the intracellular accumulation of PAH was increased with increasing concentration of unlabelled PAH at the basolateral side. Addition of probenecid and genistein at the basolateral side decreased the secretory transport of [3H]PAH; the accumulation was not changed by probenecid, but was increased by genistein. In addition, the initial uptake rate of [3H]PAH from the basolateral side was decreased by both PAH and probenecid, but not by genistein. Therefore, it is suggested that the transport of PAH in Caco-2 cells is regulated by several transporters: a genistein-sensitive transporter on the apical membrane and probenecid-sensitive transporters on both the basolateral and apical membranes. In rat intestinal tissues, the transport rate of PAH showed regional variation (ileum > jejunum > duodenum), suggesting that secretory transporters with high activity exist predominantly in the lower region of the small intestine. The results suggest that PAH transport in both Caco-2 cells and rat intestinal tissues is regulated by multiple transporters on the apical and basolateral membranes, and these transporters have different characteristics.

Role of metabolic enzymes and efflux transporters in the absorption of drugs from the small intestine

It has been established that the absorption of many drugs from the small intestine is hindered by the detoxification systems which are present in this epithelial tissue. In this article, we will summarize the significant role of small intestine in reducing the oral bioavailability of drugs, particularly focusing on the role of metabolic enzymes and efflux transporters. Since the role of cytochrome P450 3A (CYP3A) and MDR1 P-glycoprotein (P-gp) in intestinal drug disposition has been highlighted, the disposition of CYP3A substrates, P-gp substrates and CYP3A/P-gp bisubstrates are summarized. Moreover, it is plausible that conjugative enzymes and/or carboxyesterases act synergistically with efflux transporters of organic anions, affecting the intestinal availability, i.e. many xenobiotics and ester-type prodrugs are metabolized to the corresponding glucuronide and sulfate conjugates and carboxylates (active drugs), respectively, followed by cellular extrusion. The characteristics of the efflux transporters of organic anions across the apical and basal membrane of enterocytes and Caco-2 cells are also summarized from this point of view.