Substrate specificity and some properties of phenol sulfotransferase from human intestinal Caco-2 cells

Intestinal transport of HDND-7, a novel hesperetin derivative, in in vitro MDCK cell and in situ single-pass intestinal perfusion models

1. Hesperetin (HDND) possesses extensive bioactivities, however, its poor solubility and low bioavailability limit its application. HDND-7, a derivative of HDND, has better solubility and high bioavailability. In this study, we investigated the intestinal absorption mechanisms of HDND-7. 2. MDCK cells were used to examine the transport mechanisms of HDND-7 in vitro, and a rat in situ intestinal perfusion model was used to characterize the absorption of HDND-7. The concentration of HDND-7 was determined by HPLC. 3. In MDCK cells, HDND-7 was effectively absorbed in a concentration-dependent manner in both directions. Moreover, HDND-7 showed pH-dependent and TEER-independent transport in both directions. The transport of HDND-7 was significantly reduced at 4 °C or in the presence of NaN3. Furthermore, the efflux of HDND-7 was apparently reduced in the presence of MRP2 inhibitors MK-571 or probenecid. However, P-gp inhibitor verapamil had no effect on the transport of HDND-7. The in situ intestinal perfusion study indicated HDND-7 was well-absorbed in four intestinal segments. Furthermore, MRP2 inhibitors may slightly increase the absorption of HDND-7 in jejunum. 4. In summary, all results indicated that HDND-7 might be absorbed mainly by passive diffusion via transcellular pathway, MRP2 but P-gp may participate in the efflux of HDND-7.

Characterization of a gastrointestinal tract microscale cell culture analog used to predict drug toxicity

The lining of the gastrointestinal (GI) tract is the largest surface exposed to the external environment in the human body. One of the main functions of the small intestine is absorption, and intestinal absorption is a route used by essential nutrients, chemicals, and pharmaceuticals to enter the systemic circulation. Understanding the effects of digestion on a drug or chemical, how compounds interact with and are absorbed through the small intestinal epithelium, and how these compounds affect the rest of the body is critical for toxicological evaluation. Our goal is to create physiologically realistic in vitro models of the human GI tract that provide rapid, inexpensive, and accurate predictions of the body's response to orally delivered drugs and chemicals. Our group has developed an in vitro microscale cell culture analog (microCCA) of the GI tract that includes digestion, a mucus layer, and physiologically realistic cell populations. The GI tract microCCA, coupled with a multi-chamber silicon microCCA representing the systemic circulation, is described and challenged with acetaminophen. Proof of concept experiments showed that acetaminophen passes through and is metabolized by the in vitro intestinal epithelium and is further metabolized by liver cells, resulting in liver cell toxicity in a dose-dependent manner. The microCCA response is also consistent with in vivo measurements in mice. The system should be broadly useful for studies on orally delivered drugs or ingestion of chemicals with potential toxicity.

Novel bioassay system for evaluating anti-oxidative activities of food items: Use of basolateral media from differentiated Caco-2 cells

Reactive oxygen and nitrogen species, including superoxide and nitric oxide (NO), are known to be mediators of oxidative stress and play pivotal roles in the onset of numerous life style-related diseases. While a number of studies have shown that naturally occurring anti-oxidants may be applicable for prevention and therapy for those diseases, most in vitro anti-oxidation tests reported have not provided significant insight into the absorption efficiency or metabolism of dietary anti-oxidants in the gastrointestinal tract. In the present study, we established a novel assay system by focusing on the bioconversion of food constituents using differentiated Caco-2 cells as a model of human intestinal epithelial cells. Various fresh food preparations [ginger, garlic, shimeji (Hypsizigus marmoreus), onion, carrot] were added to the apical side of differentiated Caco-2 monolayers. After incubation, the medium was recovered and tested for its inhibitory effects on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced generation in differentiated HL-60 cells, and on combined lipopolysaccharide (LPS)- and interferon (IFN)-gamma -induced NO generation in RAW 264.7 macrophages. The garlic preparation (25% v/v) basolateral medium abolished generation without any cytotoxicity toward HL-60 cells, though it was cytotoxic to Caco-2 cells. In the NO generation tests, all of the food preparations showed notable inhibitory activity, while the garlic preparation (5% v/v) basolateral medium inhibited NO generation with substantial cytotoxicity toward RAW 264.7 cells. Interestingly, the carrot preparation (1% v/v) basolateral medium inhibited NO generation in both a concentration- and time-dependent manner without any cytotoxicity toward RAW 264.7 or Caco-2 cells, and its activities were higher than those of the carrot preparation alone (1% v/v). Our results indicate that the present assay system is appropriate and reliable for determination of the anti-oxidative efficacy of dietary phytochemicals in vivo.

Transport of high concentration of thiamin, riboflavin and pyridoxine across intestinal epithelial cells Caco-2

In this study, we examined the intestinal uptake of thiamin (vitamin B(1)), riboflavin (vitamin B(2)) and pyridoxine (vitamin B(6)) administered at high concentration using intestinal epithelial Caco-2 cells as an in vitro model of drugs and food absorption. The effect of vitamin concentration, culture age, transport direction and incubation temperature on vitamin transport was determined. The vitamin transport was expressed as an apparent permeability coefficient and changes in cumulative fraction transported across epithelial membrane in time. It was found that transepithelial transport of these vitamins is dependent on the experimental factors. At low concentrations an active transport mechanism was observed, whereas at high vitamin concentration a passive transport dominated. At high vitamin concentration the transepithelial flux of vitamins in both directions was similar, which proves the mechanism of passive transport.

The Caco-2 cell monolayer: usefulness and limitations

BACKGROUND

The Caco-2 monolayer has been used extensively for the high-throughput screening of drug permeability and identification of substrates, inhibitors, and inducers of intestinal transporters, especially P-glycoprotein (P-gp). Traditionally, the Caco-2 monolayer is viewed as a single barrier rather than a polarized cell monolayer consisting of metabolic enzymes that are sandwiched between two membrane barriers with distinctly different transporters.

OBJECTIVE

This review addressed the usefulness and limitations of the Caco-2 cell monolayer in drug discovery and mechanistic studies.

METHODS

This mini-review covered applications of the Caco-2 monolayer, clarified misconceptions, and critically addressed issues on data interpretation.

CONCLUSION

The catenary model extends the usefulness of Caco-2 monolayer and provides proper mechanistic insight and data interpretation.

Caco-2 cell permeability assays to measure drug absorption

Caco-2 cells are a human colon epithelial cancer cell line used as a model of human intestinal absorption of drugs and other compounds. When cultured as a monolayer, Caco-2 cells differentiate to form tight junctions between cells to serve as a model of paracellular movement of compounds across the monolayer. In addition, Caco-2 cells express transporter proteins, efflux proteins, and Phase II conjugation enzymes to model a variety of transcellular pathways as well as metabolic transformation of test substances. In many respects, the Caco-2 cell monolayer mimics the human intestinal epithelium. One of the functional differences between normal cells and Caco-2 cells is the lack of expression of the cytochrome P450 isozymes and in particular, CYP3A4, which is normally expressed at high levels in the intestine. However, Caco-2 cells may be induced to express higher levels of CYP3A4 by treatment with vitamin D3. Caco-2 cell monolayers are usually cultured on semipermeable plastic supports that may be fitted into the wells of multi-well culture plates. Test compounds are then added to either the apical or basolateral sides of the monolayer. After incubation for various lengths of time, aliquots of the buffer in opposite chambers are removed for the determination of the concentration of test compounds and the computation of the rates of permeability for each compound (called the apparent permeability coefficients). Although radiolabelled compounds were used in the original Caco-2 cells monolayer assays, radiolabelled compounds have been replaced in most laboratories by the use of liquid chromatography-mass spectrometry (LC-MS) and LC-tandem mass spectrometry (LC-MS-MS). Mass spectrometry not only eliminates the need for radiolabelled compounds, but permits the simultaneous measurement of multiple compounds. The measurement of multiple compounds per assay reduces the number of incubations that need to be carried out, thereby increasing the throughput of the experiments. Furthermore, LC-MS and LC-MS-MS add another dimension to Caco-2 assays by facilitating the investigation of the metabolism of compounds by Caco-2 cells.

An inter-laboratory study to evaluate the effects of medium composition on the differentiation and barrier function of Caco-2 cell lines

Differentiated human intestinal Caco-2 cells are frequently used in toxicology and pharmacology as in vitro models for studies on intestinal barrier functions. Since several discrepancies exist among the different lines and clones of Caco-2 cells, comparison of the results obtained and optimisation of models for use for regulatory purposes are particularly difficult, especially with respect to culture conditions and morphological and biochemical parameters. An inter-laboratory study has been performed on the parental cell line and on three clonal Caco-2 cell lines, with the aim of standardising the culture conditions and identifying the best cell line with respect to parameters relevant to barrier integrity, namely, trans-epithelial electrical resistance (TEER) and mannitol passage, and of epithelial differentiation (alkaline phosphatase activity). Comparison of the cell lines maintained in traditional serum-supplemented culture medium or in defined medium, containing insulin, transferrin, selenium and lipids, showed that parameter performance was better and more reproducible with the traditional medium. The maintenance of the cell lines for 15 days in culture was found to be sufficient for the development of barrier properties, but not for full epithelial differentiation. Caco-2/TC7 cells performed better than the other three cell lines, both in terms of reproducibility and performance, exhibiting low TEER and mannitol passage, and high alkaline phosphatase activity.

Specific antibody modulates absorptive transport and metabolic activation of benzo[a]pyrene across Caco-2 monolayers

It has been shown that oral anticarcinogen antibodies can decrease intestinal absorption of procarcinogens. So far, no attempts have been made to understand the potential modulatory effect of such antibodies on metabolic activation at mucosal surfaces. Moreover, the influence of naturally induced serosal-specific antibodies on absorptive transport of carcinogens remains unknown. In this study, the prototype food carcinogen benzo[a]pyrene (B[a]P) and a specific monoclonal antibody were used to address these questions in a bicompartmental model of polarized intestinal cells (Caco-2). Apical (i.e., luminal) administration of a 30-fold molar excess antibodies increased about 25-fold recovery of unmetabolized B[a]P, concomitantly with a decrease of 80% in both absorptive transport and formation of phenol metabolites. Interestingly, when metabolism was slowed down by antibodies, cross-reactive antibodies also increased at least 5-fold the extracellular levels of the 7,8-diol-B[a]P, interrupting subsequent activation steps. On the other hand, basolateral antibodies changed by 8-fold the rate of carcinogen appearance in the basolateral compartment, albeit without interfering with the apical cellular uptake or sequestration of either B[a]P or 7,8-diol-B[a]P by apical antibodies. Furthermore, basolateral antibodies reduced exposure of Caco-2 monolayers to B[a]P as demonstrated by a 50% decrease in apical efflux of 3-OH-B[a]P. These data show for the first time that both luminal and serosal antibodies may reduce the carcinogenic process by preventing high local concentrations, which would overload DNA repair mechanisms. This study also sheds light on the relevance of both naturally induced and immunoprophylactic antibodies against polycyclic aromatic hydrocarbon carcinogens.

Induction of gene expression of xenobiotic metabolism enzymes and ABC-transport proteins by PAH and a reconstituted PAH mixture in human Caco-2 cells

It was shown recently that in epithelial Caco-2 cells the food contaminant benzo[a]pyrene (B[a]P) is metabolized and B[a]P-sulfate metabolites were transported out of the cells. The aim of this study was to investigate whether B[a]P and other polycyclic aromatic hydrocarbons (PAH) such as chrysene, phenanthrene, benzo[k]fluoranthene (B[k]F), dibenzo[a,l]pyrene (DB[a,l]P), and pyrene alone or in a mixture in a ratio as they occur in tobacco smoke have effects on gene expression of intestinal cytochrome P450 enzymes (CYP), Phase II enzymes and ATP-binding cassette (ABC)-transport proteins in the human Caco-2 cells. B[a]P induced its own metabolism. Treatment of the Caco-2 cells with B[a]P, chrysene, B[k]F, or DB[a,l]P induced mRNA expression of CYP1A1 and CYP1B1 specifically as measured by RT-PCR. In contrast, the mRNA expression of the microsomal epoxide hydrolase (mEH) was not affected by PAH. The gene expression of the Phase II enzymes UDP-glucuronosyltransferase 1A6 (UGT1A6) and UGT1A7 was also induced by these PAH but treatment with them had no effect on gene expression of sulfotransferases (SULT) at all. Of the ABC-transport proteins, MDR1 mRNA expression was induced by treatment with carcinogenic PAH, whereas MRP2 mRNA expression was not changed. The mixture of PAH also induced CYP1A1, CYP1B1, UGT1A6, and UGT1A7 mRNA expression. We conclude that B[a]P, chrysene, B[k]F, and DB[a,l]P have specific effects on intestinal CYP1A1, CYP1B1, UGT1A6, and UDP1A7 mRNA expression but no effects on the expression of SULT.

Antioxidative flavonoid quercetin: implication of its intestinal absorption and metabolism

Quercetin is a typical flavonoid ubiquitously present in fruits and vegetables, and its antioxidant effect is implied to be helpful for human health. The bioavailability of quercetin glycosides should be clarified, because dietary quercetin is mostly present as its glycoside form. Although quercetin glycosides are subject to deglycosidation by enterobacteria for the absorption at large intestine, small intestine acts as an effective absorption site for glucose-bound glycosides (quercertin glucosides). This is because small intestinal cells possess a glucoside-hydrolyzing activity and their glucose transport system is capable of participating in the glucoside absorption. A study using a cultured cell model for intestinal absorption explains that the hydrolysis of the glucosides accelerates their absorption in the small intestine. Small intestine is also recognized as the site for metabolic conversion of quercetin and other flavonoids as it possesses enzymatic activity of glucuronidation and sulfation. Modulation of the intestinal absorption and metabolism may be beneficial for regulating the biological effects of dietary quercetin.

Tamoxifen induction of aryl sulfotransferase and hydroxysteroid sulfotransferase in male and female rat liver and intestine

The antiestrogenic drug tamoxifen (TAM) is widely used in the treatment of breast cancer. Species-specific mutagenic and carcinogenic potentialities have been reported and have raised concerns. Sulfotransferases (STs) are important phase II drug-metabolizing enzymes. STs are involved in the sulfation processes of some TAM metabolites (i.e., alpha-hydroxy tamoxifen and 4-hydroxy tamoxifen). Regulation of drug-metabolizing enzymes is important for the understanding of drug metabolism and detoxification. Studies on ST induction are limited. In the present investigation, protein and mRNA expression of aryl sulfotransferase (AST-IV) and hydroxysteroid sulfotransferase (STa) have been studied in liver and intestine of male and female Sprague-Dawley rats after TAM treatment with either 6.8 or 68 mg/kg/day for 1 or 2 weeks. Enzyme assay and Western blot methods were used for protein level determination; reverse transcription-polymerase chain reaction method was used for mRNA level determination. Here, for the first time, we have demonstrated that AST-IV and STa could be induced in intestine by tamoxifen. Furthermore, intestinal inductions were found to be much greater than the inductions found in the liver, suggesting a distinct potentiality of intestinal cells in TAM metabolism. The impact of induction and regulation of intestinal STs on TAM metabolism with respect to its toxicity has yet to be studied. The role of STs induction and relevant TAM metabolism is discussed in the context of organ- and species-specific variable carcinogenic manifestations.

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.

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.

Brown algae fucoxanthin is hydrolyzed to fucoxanthinol during absorption by Caco-2 human intestinal cells and mice

The metabolic fate in mammals of dietary fucoxanthin, a major carotenoid in brown algae, is not known. We investigated the absorption and metabolism of fucoxanthin in differentiated Caco-2 human intestinal cells, a useful model for studying the absorption of dietary compounds by intestinal cells. Fucoxanthin was taken up by Caco-2 cells incubated with micellar fucoxanthin composed of 1 micromol/L fucoxanthin, 2 mmol/L sodium taurocholate, 100 micromol/L monoacylglycerol, 33.3 micromol/L fatty acids and 50 micromol/L lysophosphatidylcholine. Fucoxanthinol, the deacetylated product of fucoxanthin, was also found in both medium and cells, with its level increasing significantly in a time-dependent manner. No conjugated forms of fucoxanthin and fucoxanthinol were found in either medium or cells. In the animal study, fucoxanthinol (10.4 +/- 5.3 nmol/L plasma, n = 4) was detected in plasma of mice 1 h after intubation of 40 nmol fucoxanthin. These results indicate that dietary fucoxanthin is incorporated as fucoxanthinol, the deacetylated form, from the digestive tract into the blood circulation system in mammals.

Epigallocatechin gallate (EGCG) inhibits the sulfation of 1-naphthol in a human colon carcinoma cell line, Caco-2

We have previously reported that epigallocatechin gallate (EGCG) strongly inhibits the in vitro phenol sulfotransferase (P-ST) activity of a human colon carcinoma cell line, Caco-2. In the present study, we examined the ability of EGCG to inhibit the sulfation of 1-naphthol in intact Caco-2 cells. Sulfation of 1-naphthol was detected in Caco-2 cells after 2 h of incubation, and was observed to continue for 24 h, resulting in an accumulation of sulfated 1-naphthol. Sulfation was strongly inhibited by the addition of EGCG to the culture medium. The IC50 of EGCG was calculated to be 20 microM; this value is similar to that obtained from in vitro assays (14 microM) [Ref. Tamura et al., Biol. Pharm. Bull., 23, 695, (2000)]. These results indicate that catechins are capable of inhibiting P-ST activity in intact cells as well as in vitro. We believe that the inhibitory activity of catechins might be the mechanism by which catechins (and green tea) exert anti-carcinogenic activity against procarcinogenic compounds that require P-ST activation in vivo.

Intestinal responses to xenobiotics

The gastrointestinal tract represents the first barrier met by the exogenous compounds of food or orally delivered drugs. To be transferred to the whole body, drugs and xenobiotics have first to pass through the intestinal epithelium, where detoxification systems have to minimize the potential of damage from toxic xenobiotics. However, most studies on xenobiotic-metabolizing enzymes have focused on liver enzymes. Such a situation may be explained by the fact that this organ is the site of toxification/detoxification for both endogenous and exogenous compounds, and also because adequate in vitro hepatocytes models have been available for a long time. By contrast, normal cellular models for the in vitro study of the intestinal processes of biotransformation still remain difficult to obtain. In the present report we will thus focus on the most commonly used models, which are Caco-2 cells and their derivative clones, and we will report recent procedures that allow the isolation of normal enterocytes which maintain their functions and integrity for several hours or even several days. Their respective performance and advantages for the study of the induction of the drug-metabolizing enzymes will be discussed.

Efficiency of absorption and metabolic conversion of quercetin and its glucosides in human intestinal cell line Caco-2

The efficiency of intestinal absorption and metabolic conversion of quercetin aglycone and its glucosides, quercetin-4'-O-beta-D-glucoside (Q4'G), quercetin-3-O-beta-D-glucoside (Q3G), and quercetin-3,4'-di-O-beta-D-glucoside (Q3,4'G), was estimated by using Caco-2 cell monolayers as an intestinal epithelial cell model. Aglycone was significantly lost from the apical side, resulting in the appearance of free and conjugated forms of quercetin and those of isorhamnetin in the cellular extracts. In the basolateral solution, the conjugated form of quercetin was predominant and increased with the elapse of incubation. As compared with quercetin aglycone, none of the quercetin glucosides were absorbed efficiently from apical side. However, Q4'G yielded conjugated quercetin and isorhamnetin in basolateral solution at higher amounts than Q3G or Q3,4'G. Lipophilicity of Q4'G was found to be higher than that of Q3G or Q3,4'G. This suggests that lipophilicity contributes to the relatively efficient absorption of Q4'G. It is likely that the occurrence of hydrolysis enhances the efficiency of intestinal absorption and metabolic conversion of dietary quercetin glucosides.

Induction of UDP-glucuronosyltransferase by the flavonoids chrysin and quercetin in Caco-2 cells

PURPOSE

Dietary flavonoids have been reported to be potent inhibitors of drug metabolizing enzymes. In the present study we examined the inducing effect of three of these compounds, chrysin, quercetin and genistein, on UDP-glucuronosyltransferase (UGT) in the human intestinal cell line Caco-2.

METHODS

The induction of UGT by flavonoid pretreatment was studied both in the intact cells and cell homogenates, measured as the glucuronidation of chrysin, and by immunoblot analysis of the UGT 1A protein.

RESULTS

Exposure of Caco-2 cells to 50 microM chrysin resulted in a 3.8-fold increase in chrysin glucuronidation in intact cells (p < 0.0001) with a 38% decrease in sulfation (p < 0.01). In the cell homogenate the induction was much larger, 14-fold. The induction was slow to develop with maximum induction after 3-4 days. Interestingly, the isoflavonoid genistein was without effect. Immunoblot analysis of Caco-2 cell microsomes with a UGT1A subfamily-selective antibody showed a markedly increased band at about 59 kDa, consistent with induction of one or more UGT1A isoforms. A 5-week exposure of Caco-2 cells to low concentrations (10 microM) of chrysin or quercetin also showed markedly increased glucuronidation activity.

CONCLUSIONS

Diet-mediated induction of intestinal UGT may be important for the bioavailability of carcinogens and other toxic chemicals as well as therapeutic drugs.

Comparison of CYP3A activities in a subclone of Caco-2 cells (TC7) and human intestine

PURPOSE

To compare the activity of the CYP3A enzyme expressed by TC7, a cell culture model of the intestinal epithelial cell, to the activity of human intestinal CYP3A4, using terfenadine as a substrate.

METHODS

The metabolism of terfenadine was investigated in intact cells and microsomal preparations from TC7, human intestine, and liver. The effect of two CYP3A inhibitors, ketoconazole and troleandomycin (TAO), on the metabolism of terfenadine was also examined.

RESULTS

Only hydroxy-terfenadine was detected in TC7 microsomal incubations. In contrast, azacyclonol and hydroxy-terfenadine were detected in human intestinal and hepatic microsomal incubations. The Km values for hydroxy-terfenadine formation in TC7 cells, intestine and liver microsomes were 1.91, 2.5, and 1.8, microM respectively. The corresponding Vmax values were 2.11, 61.0, and 370 pmol/min/mg protein. Km values for azacyclonol in intestinal and hepatic samples were 1.44 and 0.82 microM and the corresponding Vmax values were 14 and 60 pmol/min/mg protein. The formation of hydroxy-terfenadine was inhibited by ketoconazole and TAO in human intestine and TC7 cell microsomes. The Km and Vmax values for terfenadine metabolism in intact TC7 cells were similar to those from TC7 cell microsomes.

CONCLUSIONS

Our results indicate that TC7 cells are a potentially useful alternative model for studies of CYP3A mediated drug metabolism. The CYP3A expressed by TC7 cells is not CYP3A4, but probably CYP3A5, making this cell line suitable for studies of colonic drug transport and metabolism.

The human intestinal epithelial cell line Caco-2; pharmacological and pharmacokinetic applications

The gastrointestinal tract remains the most popular and acceptable route of administration for drugs. It offers the great advantage of convenience and many compounds are well absorbed and thereby provide acceptable plasma concentration-time profiles. Currently there is considerable interest from the pharmaceutical industry in development of cell culture systems that would mimic the intestinal mucosa in order to evaluate strategies for investigating and/or enhancing drug absorption. The intestinal epithelial cells of primary interest, from the standpoint of drug absorption and metabolism, are the villus cells, which are fully differentiated cells. An in vitro cell culture system consisting of a monolayer of viable, polarized and fully differentiated villus cells, similar to that found in the small intestine, would be a valuable tool in the study of drug and nutrient transport and metabolism. The Caco-2 cell line, which exhibits a well-differentiated brush border on the apical surface and tight junctions, and expresses typical small-intestinal microvillus hydrolases and nutrient transporters, has proven to be the most popular in vitro model (a) to rapidly assess the cellular permeability of potential drug candidates, (b) to elucidate pathways of drug transport (e.g., passive versus carrier mediated), (c) to assess formulation strategies designed to enhance membrane permeability, (d) to determine the optimal physicochemical characteristics for passive diffusion of drugs, and (e) to assess potential toxic effects of drug candidates or formulation components on this biological barrier. Since differentiated Caco-2 cells express various cytochrome P450 isoforms and phase II enzymes such as UDP-glucuronosyltransferases, sulfotransferases and glutathione-S-transferases, this model could also allow the study of presystemic drug metabolism.

Iron absorption by CaCo 2 cells cultivated in serum-free medium as in vitro model of the human intestinal epithelial barrier

A cell culture system consisting of confluent monolayer of human enterocyte-like CaCo 2 cells, cultivated in a serum-free nutritive medium, on microporous synthetic membranes has been used as an in vitro model of the intestinal epithelial barrier. The uptake of 55ferric citrate, as well as the transepithelial passage from the apical to the basolateral pole, have been studied. CaCo 2 cells accumulate iron in a time- and concentration-dependent process, largely specific from the apical pole. When 55ferric citrate is added at the apical pole, radioiron appears at the basal pole and the clearance rate is approximately four times higher than in the opposite direction; the amounts of 55Fe increase with the concentration in iron citrate and the duration of incubation. At least two concurrent mechanisms could be involved in iron absorption across monolayers of CaCo 2 cells. A first route would correspond to a paracellular passage of the metal from the apical to the basal pole. The second route would involve a selective intake of iron at the apical pole and could require a reduction of ferric iron, prior to the entry. Iron accumulated by the cells would, for a minor part, be stored within ferritin, whereas the major part would be excreted at the basolateral pole, either as low molecular weight material of undetermined chemical composition but from which iron is easily mobilized by apotransferrin or associated with neosynthesized apotransferrin. Vesicular transport and protein synthesis seem to be required.

Mechanism of intestinal absorption of ranitidine and ondansetron: transport across Caco-2 cell monolayers

We have investigated the transport of ranitidine and ondansetron across the Caco-2 cell monolayers. The apparent permeability co-efficients (Papp) were unchanged throughout the concentration range studied, indicating a passive diffusion pathway across intestinal mucosa. No metabolism was observed for ranitidine and ondansetron during the incubation with Caco-2 cell monolayers. Papp values for ranitidine and ondansetron (bioavailability of 50 and approximately 100% in humans, respectively) were 1.03 +/- 0.17 x 10(-7) and 1.83 +/- 0.055 x 10(-5) cm/sec, respectively. The Papp value for ranitidine was increased by 15- to 20-fold in a calcium-free medium or in the transport medium containing EDTA, whereas no significant change occurred with ondansetron, indicating that paracellular passive diffusion is not rate determining for ondansetron. Uptake of ondansetron by Caco-2 cell monolayers was 20- and 5-fold higher than that of ranitidine when the uptake study was carried out under sink conditions and at steady state. These results suggest that ranitidine and ondansetron are transported across Caco-2 cell monolayers predominantly via paracellular and transcellular pathways, respectively.

Importance of the paracellular pathway for the transport of a new bisphosphonate using the human CACO-2 monolayers model

The transport of a new bisphosphonate, Tiludronate, was investigated on the human adenocarcinoma cell line, CACO-2. Experiments were performed 7-16 days after cells achieved confluence, conditions under which they form well-differentiated monolayers joined by tight junctions. Tiludronate transport rate across CACO-2 monolayers was independent of the temperature (4 degrees versus 37 degrees), of the polarity of the cell membrane (apical-to-basolateral versus basolateral-to-apical), and of the presence of metabolic poisons (sodium azide). Its transport was enhanced by either the presence of EGTA in the incubation buffer, i.e. when extracellular Ca2+ concentration was reduced, or by the pretreatment of monolayers with EGTA, i.e. when the intercellular spaces and the tight junctions were widened. Based on these different observations, we could suggest that Tiludronate mainly used the paracellular pathway to cross the intestinal epithelium. An increase in the Tiludronate permeability coefficient was also observed following treatment of cells with high Tiludronate concentrations, as a consequence of the direct effect of this compound on the extracellular Ca2+ ions. Hence, for high drug concentrations, i.e. 20 mM, we observed a decrease in free extracellular Ca2+ concentration, an increase in the transepithelial electrical resistance and an increase in the transport of [14C]polyethyleneglycol ([14C]PEG400), a probe for the paracellular pathway. The results indicate that Tiludronate is transported across CACO-2 monolayers by the paracellular route. Moreover, it can affect its own transport by its concentration-dependent effect on tight junction widening.

Phase I and II biotransformations in living CaCo 2 cells cultivated under serum-free conditions. Selective apical excretion of reaction products

CaCo 2 cells, cultivated in a synthetic, serum-free nutritive medium on poly (ethylene terephthalate) membranes, form a confluent monolayer of differentiated cells, with the apical and basolateral poles exposed to the upper and lower compartments, respectively, of bicameral culture inserts (Halleux and Schneider, In Vitro Cell Dev Biol, 27A: 293-302, 1991). This cell culture system allows the passage of intact mannitol by the paracellular route and the transcellular diffusion of testosterone which appears mainly as a biotransformed unconjugated metabolite. When ethoxyresorufin is added to either the apical or basolateral poles of living CaCo 2 cells, resorufin is formed, and more than 80% is excreted at the apical pole. Under our experimental conditions, no detectable amounts of glucurono- or sulfconjugates are found. Methylcholanthrene and phenobarbital increase the biotransformation of ethoxyresorufin 50 and 3 times, respectively, and induce that of benzoxyresorufin, but not of pentoxyresorufin which remains absent under all conditions. These substances do not affect the proportion of resorufin recovered at the apical role. Verapamil inhibits by 25% the release of resorufin but does not affect its distribution. Chlorodinitrobenzene is conjugated with glutathione and at least two-thirds of the product is excreted at the apical pole; methylcholanthrene and phenobarbital do not increase this activity. These results demonstrate that differentiated CaCo 2 cells, under serum-free conditions, perform phase I and II reactions and that the biotransformation products are selectively excreted at the apical pole.

Hep G2 cell line as a human model for sulphate conjugation of drugs

1. The objective of this study was to examine the usefulness of the hepatoma cell line Hep G2 as a model for human sulphoconjugation of drugs, in particular stereoselective conjugation. 2. Using the substrates p-nitrophenol and dopamine, we found sulphation activities consistent with the presence of both the phenol (P) and the monoamine (M) form of the human phenolsulphotransferases in these cells. 3. The Kmapp was 3.0 microM for the sulphation of p-nitrophenol. This activity was inhibited selectively by 2,6-dichloro-4-nitrophenol, IC50 6 microM. The Kmapp was 39 microM for the sulphation of dopamine. This activity was selectively inhibited by elevated temperature. 4. The chiral adrenergic drugs (+/-)-terbutaline and (+/-)-4-hydroxypropranolol were both sulphated stereoselectively with Kmapp and Vmaxapp values for each enantiomer virtually identical to previous observations with human liver cytosol. 5. In a direct comparison, the estimated activity of the P form of phenolsulphotransferase in the Hep G2 cell line was 30% of that in human liver, whereas, surprisingly, the activity of the M form of phenolsulphotransferase was 4.5 times higher in the Hep G2 cells than in the liver.