A drug absorption model based on the mucus layer producing human intestinal goblet cell line HT29-H

Effect of experimental pH on the in vitro permeability in intact rabbit intestines and Caco-2 monolayer

The effect of experimental (apical) pH on absorptive permeability (Pe) was investigated in animal intestinal tissues and Caco-2 cell monolayers to examine whether the introduction of physiological pH such as 6.5 relates to the better prediction of animal intestinal Pe. Transport studies were conducted in a 24-well transwell for Caco-2 and diffusion chambers for rabbit intestinal permeability. Twenty-four test compounds were chosen (seven acidic, seven basic, eight neutral, and two zwitterionic) and Pe was measured at a 100microM donor concentration with two apical pHs, Krebs Bicarbonate Ringer's buffer (pH 7.4) and 4-morpholineethanesulfonic acid (MES) buffer (pH 6.5). Samples were collected over a 90-min interval and analyzed by LC/UV, LC/MS, or LSC. Upon the apical pH change from 7.4 to 6.5, Caco-2 Pe of acidic and basic compounds changed significantly, whereas rabbit intestinal Pe did not change possibly by the presence of mucous layer. When the intestinal Pe was correlated with pH 6.5 or 7.4 Caco-2 Pe, the correlation of pH 6.5 duodenum and jejunum Pe with pH 6.5 Caco-2 Pe was very poor. However, pH 7.4 Caco-2 Pe correlated relatively well with pH 6.5 duodenum and jejunum Pe and pH 7.4 ileum and colon Pe. The results suggested that pH 7.4 Caco-2 Pe is a good qualitative predictor for physiological intestinal permeability from duodenum to colon.

Effect of hydroxypropyl beta cyclodextrin complexation on aqueous solubility, stability, and corneal permeation of acyl ester prodrugs of ganciclovir

The purpose of the study was to investigate the effect of hydroxypropyl beta cyclodextrin (HPbetaCD) on aqueous solubility, stability, and in vitro corneal permeation of acyl ester prodrugs of ganciclovir (GCV). Aqueous solubility and stability of acyl ester prodrugs of Ganciclovir (GCV) were evaluated in pH 7.4 isotonic phosphate buffer solution (IPBS) in the presence and absence of HPbetaCD. Butyryl cholinesterase-mediated enzymatic hydrolysis of the GCV prodrugs was studied using various percentage w/v HPbetaCD. In vitro corneal permeation of GCV and its prodrugs (with and without 5% HPbetaCD) across isolated rabbit cornea was studied using side-by-side diffusion cells. HPbetaCD-prodrug complexation was of the A(L) type with values for complexation constants ranging between 12 and 108 M(-1). Considerable improvement in chemical and enzymatic stability of the GCV prodrugs was observed in the presence of HPbetaCD. The stabilizing effect of HPbetaCD was found to depend on the degree of complexation and the degradation rate of prodrug within the complex. Five percent w/v HPbetaCD was found to enhance the corneal permeation of only the most lipophilic prodrug GCV dibutyrate (2.5-fold compared with 0% HPbetaCD). All other prodrugs showed little or no difference in transport in the presence of 5% w/v HPbetaCD. Agitation in the donor chamber largely influenced the transport kinetics of GCV dibutyrate across cornea. Results indicate the presence of an unstirred aqueous diffusion layer at the corneal surface that restricts the transport of the highly lipophilic GCV dibutyrate prodrug. HPbetaCD improves corneal permeation by solubilizing the hydrophobic prodrug and delivering it across the mucin layer at the corneal surface.

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.

Mechanistic studies on nonviral gene delivery to the intestine using in vitro differentiated cell culture models and an in vivo rat intestinal loop

PURPOSE

To identify factors influencing nonviral vector transfection in differentiated CaCo-2 and mucus-secreting coculture, CaCo-2: Ht29GlucH, cell culture models and to compare these in vitro results with in vivo transfection efficiency in rat intestine.

METHODS

A range of nonviral vectors including DOTAP, Lipofectin, Superfect, PEI, and polylysine were investigated. CaCo-2 and a mucus-secreting coculture were used at 21 days. Transfection efficiency was assessed using pCMVluc (firefly luciferase) plasmid, and radio-labeled plasmid was used to determine the binding and internalization of plasmid DNA. The in vivo model used was a ligated rat intestinal loop.

RESULTS

Transfection levels decreased by over 1000-fold in differentiated models relative to nondifferentiated COS-7 cells and were related to reductions in luciferase production by individual cells. Active internalization of DNA by the differentiated cells decreased. Removal of mucus by the mucolytic agent N-acetylcysteine, from the coculture system significantly reduced (p < 0.05) transfection efficiency. In vivo the transfection efficiency of PEI proved superior to DOTAP.

CONCLUSIONS

Nonviral gene delivery to the hostile environment of the intestine is possible. Mechanistic studies using differentiated intestinal cell models aid identification of the rate-limiting steps to transfection and represent a more physiologically relevant approach to predict gene delivery to the intestine.

Comparative uptake studies of bioadhesive and non-bioadhesive nanoparticles in human intestinal cell lines and rats: the effect of mucus on particle adsorption and transport

PURPOSE

The interaction of nanoparticles (NP), consisting of hydrophobic polystyrene, bioadhesive chitosan, and stealth PLA-PEG with two human intestinal cell lines, the enterocyte-like Caco-2 and mucus-secreting MTX-E12, was investigated and compared to the in vivo NP uptake in rats.

METHODS

The extent and mechanism of cellular association of different NP with Caco-2 and MTX-E12 was investigated using confocal laser scanning microscopy (CLSM) and a cellular association assay. In vitro results were compared to gastrointestinal distribution of chitosan NP in rats after intra-duodenal injection.

RESULTS

Cellular association of NP with Caco-2 cell monolayers showed the following rank order: polystyrene > chitosan >> PLA-PEG. Mucus (MTX-E12) significantly decreased the association of hydrophobic polystyrene NP. While no mucus binding was observed for PLA-PEG, association of chitosan NP with mucus strongly increased. Intra-duodenal administration of chitosan NP in rats confirmed these in vitro results, demonstrating that NP could be detected in both epithelial cells and Peyer's patches. Chitosan NP internalization was saturable, as well as energy- and temperature-dependent. It could be inhibited by an excess of protamine and by removal of anionic sites of the apical membrane. By contrast, polystyrene NP uptake was found to be largely independent of these factors, except for a temperature-dependency.

CONCLUSIONS

In contrast to Caco-2 cells, the presence of mucus presented a major barrier for the uptake of hydrophobic polystyrene NP and showed an even more profound effect upon the uptake of chitosan NP. A correlation between the uptake in cell culture models and in vivo rat epithelial cells was confirmed for chitosan NP. Moreover, chitosan NP seemed to be taken up and transported by adsorptive transcytosis, while polystyrene NP uptake was probably mediated by non-adsorptive transcytosis.

Comparative uptake studies of bioadhesive and non-bioadhesive nanoparticles in human intestinal cell lines and rats: the effect of mucus on particle adsorption and transport

PURPOSE

The interaction of nanoparticles (NP), consisting of hydrophobic polystyrene, bioadhesive chitosan, and stealth PLA-PEG with two human intestinal cell lines, the enterocyte-like Caco-2 and mucus-secreting MTX-E12, was investigated and compared to the in vivo NP uptake in rats.

METHODS

The extent and mechanism of cellular association of different NP with Caco-2 and MTX-E12 was investigated using confocal laser scanning microscopy (CLSM) and a cellular association assay. In vitro results were compared to gastrointestinal distribution of chitosan NP in rats after intra-duodenal injection.

RESULTS

Cellular association of NP with Caco-2 cell monolayers showed the following rank order: polystyrene > chitosan >> PLA-PEG. Mucus (MTX-E12) significantly decreased the association of hydrophobic polystyrene NP. While no mucus binding was observed for PLA-PEG, association of chitosan NP with mucus strongly increased. Intra-duodenal administration of chitosan NP in rats confirmed these in vitro results, demonstrating that NP could be detected in both epithelial cells and Peyer's patches. Chitosan NP internalization was saturable, as well as energy- and temperature-dependent. It could be inhibited by an excess of protamine and by removal of anionic sites of the apical membrane. By contrast, polystyrene NP uptake was found to be largely independent of these factors, except for a temperature-dependency.

CONCLUSIONS

In contrast to Caco-2 cells, the presence of mucus presented a major barrier for the uptake of hydrophobic polystyrene NP and showed an even more profound effect upon the uptake of chitosan NP. A correlation between the uptake in cell culture models and in vivo rat epithelial cells was confirmed for chitosan NP. Moreover, chitosan NP seemed to be taken up and transported by adsorptive transcytosis, while polystyrene NP uptake was probably mediated by non-adsorptive transcytosis.

A systematic examination of the in vitro Ussing chamber and the in situ single-pass perfusion model systems in rat ileum permeation of model solutes

In situ and in vitro intestinal absorption in the rat ileum was systematically studied and mechanistically quantified in terms of permeability coefficients (P) of a series of [(3)H]steroids as model transcellular permeants, [(3)H]taurocholate utilizing the active membrane transport systems to define the aqueous boundary layer (ABL), and [(14)C]urea and [(14)C]mannitol as pore-hindered paracellular diffusants. In situ single-pass perfusion experiments were performed in isolated ileal segments and blood samples were collected from the cannulated mesenteric vein. For the in vitro experiments, an excised, serosal and muscular layer-removed, ileal tissue was mounted in the Ussing chamber diffusion cells. In situ and in vitro P values versus logarithm of the partition coefficient in n-octanol/water (log K) of the steroids were characterized by a sigmoidal-shaped curve in which plateau values were attained for the highly lipophilic steroids with log K greater, similar 2.5. The in situ and in vitro transport barriers in series were viewed as ABL/mucosal epithelium and ABL/mucosal epithelium/submucosal tissue, respectively. Within this framework and the use of experimental strategies and theoretical reasoning, the transport barriers of the steroids were quantitatively delineated and the rate-determining barriers identified. In the plateau region, the analyses indicate that the in situ absorption of the lipophilic steroids was essentially ABL controlled, whereas the in vitro absorption was about equally controlled by diffusion across the ABL and submucosal tissue. The in situ and in vitro pore radii of the paracellular route were 7.2 and 9.2 A, respectively, and the difference was likely the result of perturbation of the tight junctions during the in vitro preparation of the ileal tissue.

HT29-MTX and Caco-2/TC7 monolayers as predictive models for human intestinal absorption: role of the mucus layer

The permeability of 19 compounds in both the Caco-2/TC7 and HT29-MTX models was determined, and the ability of each model to predict intestinal absorption in humans was compared. Similar apparent permeability values (log P(app)) were obtained in both models for the majority of compounds tested, and plots of log P(app) versus fraction absorbed in humans gave comparable sigmoidal curves. A linear correlation was also observed between the log P(app) values derived from these two models, which suggests that HT29-MTX is an alternative model for absorption prediction in humans. The similarity of both the diffusion coefficients and permeability values obtained for a range of hydrophilic and lipophilic compounds in the two models indicates that the mucus layer secreted by the human adenocarcinoma HT29-MTX goblet cells does not constitute a diffusion barrier to such compounds. The lack of P-glycoprotein (P-gp) in the HT29-MTX cell line may explain the higher permeability values obtained for cimetidine and sumatriptan in this model compared with those derived from the Caco-2/TC7 monolayers. The results suggest that the HT29-MTX model can be used to rank order the passive permeability of compounds, irrespective of their potential interaction with P-gp, which may facilitate optimization of the physicochemical features of compounds within a chemical series.

Transport of lipophilic drug molecules in a new mucus-secreting cell culture model based on HT29-MTX cells

PURPOSE

A new mucus-secreting in vitro drug absorption model based on monolayers of goblet-cell like sub-clones of the human colon carcinoma cell line HT29 obtained by methotrexate (MTX) treatment was investigated.

METHODS

Twelve sub-clones were isolated and characterized by light microscopy (LM), transelectron microscopy (TEM), confocal laser scanning microscopy (CLSM), transepithelial electrical resistance (TEER) and the transport of a paracellular marker FITC-Dextran (Mw 4400) (FD-4).

RESULTS

Significant differences of microscopical appearance, TEER-values and permeability of FD-4 between the sub-clones were evident. However, two of them, namely MTX-D1 and MTX-E12. formed tight confluent monolayers with a thick mucus-layer on the apical surface. They were used to compare the apparent permeability coefficient (Papp) of a series of lipophilic drugs, which should be affected by the mucus-layer, namely barbiturates (barbituric acid, barbital, phenobarbital, methylphenobarbital and heptabarbital) and testosterone, as a reference, to mucus-free Caco-2 cells. The permeability of drugs with a partition coefficient (log P) > 1 was decreased in the mucus-producing cell lines. Testosterone, the most lipophilic compound, showed a decrease of up to 43%.

CONCLUSIONS

We demonstrated that the mucus layer is a significant barrier to drug absorption for lipophilic drugs. In conclusion, our model may serve as a suitable in-vitro cell culture model to study the influence of the mucus layer on drug diffusion.

Caco-2 monolayers in experimental and theoretical predictions of drug transport

This review examines the use of Caco-2 monolayers in the prediction of intestinal drug absorption. First, the different routes of drug transport in Caco-2 monolayers are compared with those seen in vivo. Second, the prediction of drug absorption in vivo from transport experiments in cell monolayers is discussed for different classes of drugs. Finally, the use of Caco-2 monolayers as a reference model in physico-chemical and theoretical predictions of drug absorption is discussed. We conclude that Caco-2 monolayers can be used to identify drugs with potential absorption problems, and possibly also to select drugs with optimal passive absorption characteristics from series of pharmacologically active molecules generated in drug discovery programs.

Interaction of bile salt and phospholipids with bovine submaxillary mucin

PURPOSE

The purpose of this study was to determine the distribution and diffusion of sodium taurocholate-phospholipid micelles with mucin in order to provide the foundation for understanding the transport of ingested fat and poorly water-soluble drugs through the intestinal mucous layer.

METHODS

Sodium taurocholate (NaTC) was dispersed with egg phosphatidylcholines (PC) to yield mixed micelles of a specific size and concentration. A preliminary study was conducted to determine the time required for equilibration of PC/TC micellar solutions with mucin. PC/TC micellar solutions were dialyzed against fixed and variable concentrations of bovine submaxillary mucin after which the concentration of PC and NaTC was measured by an assay for total phosphorus and by HPLC, respectively. In addition, a quantitative assay of TC and PC by NMR was developed and used to estimate the mobile fraction of lipids in the samples. Finally, pulsed-field gradient spin echo NMR self-diffusion measurements were made of the water, TC, and PC in the samples obtained from dialysis.

RESULTS

TC/PC micellar solutions achieved equilibrium with mucin in 7 days. Mucin did not affect the equilibrium concentration of PC or TC, except at high concentrations of mucin (5%), and then the effect was small. NMR quantitation was valid for PC and TC systems containing small micelles, but deviated significantly with systems containing large micelles. Mucin decreased the diffusivity of water and the phospholipids, but the effect was relatively small. Mucin dramatically affected the mobility of TC, which prevented a straightforward interpretation of the calculated diffusion coefficients.

CONCLUSIONS

Mucin has a minor effect on the equilibrium distribution of phospholipids and bile salts. However, lipids are readily accommodated by mucus, which can significantly increase the permeability of the mucous layer, particularly for poorly water-soluble drugs.

Enhancing paracellular permeability by modulating epithelial tight junctions

The intestinal epithelium is a major barrier to the absorption of hydrophilic drugs. The presence of intercellular junctional complexes, particularly the tight junctions (zona occludens), renders the epithelium impervious to hydrophilic drugs, which cannot diffuse across the cells through the lipid bilayer of the cell membranes. There have been significant advances in understanding the structure and cellular regulation of tight junctions over the past decade. This article reviews current knowledge regarding the physiological regulation of tight junctions and paracellular permeability, and recent progress towards the rational design of agents that can effectively and safely increase paracellular permeability via modulation of tight junctions.

Caco-2 versus Caco-2/HT29-MTX co-cultured cell lines: permeabilities via diffusion, inside- and outside-directed carrier-mediated transport

PURPOSE

The objective of this study was a systematic characterization and evaluation of cell culture models based on mixtures of Caco-2/HT29-MTX co-cultures for their use in screening for drug absorption and intestinal permeability in comparison to the properties of the respective mono-cultures.

METHODS

Co-cultures of Caco-2 cells (absorptive-type) and HT29-MTX cells (goblet-type) were set up. Three different co-cultures (initial seeding ratios Caco-2/HT29-MTX: 90/10, 70/30, and 50/50) were grown on permeable filter supports, and monolayers were used for permeability studies with model compounds for paracellular absorption (atenolol, furosemide, H334/75, mannitol, terbutaline), transcellular absorption (antipyrine, ketoprofen, metoprolol, piroxicam), carrier-mediated absorption (D-glucose, Gly-Pro, and L-phenylalanine) as well as substrates for carrier-mediated secretion via P-glycoprotein (cimetidine and talinolol). Electrophysiological and microscopic controls were performed to characterize the cell cultures.

RESULTS

For compounds undergoing passive intestinal absorption permeabilities were generally higher in co-cultures than in Caco-2 monolayers, yielding highest values in pure HT29-MTX monolayers. This difference was most obvious for compounds transported via the paracellular pathway, where HT29-MTX cells may be up to 30 times more permeable than Caco-2 cells, whereas for lipophilic and highly permeable compounds, the difference in permeability values was less obvious. For drugs undergoing intestinal secretion mediated by P-glycoprotein, co-cultivation of Caco-2 cells with HT29-MTX cells led to increased apical to basolateral permeability which was decreased in the opposite direction, consistent with the fact that HT29-MTX cells do not express P-glycoprotein. When a carrier-mediated absorption mechanism is involved, the permeabilities observed were lower than the values reported for human small intestine and co-cultivation of HT29-MTX cells with Caco-2 cells resulted in even lower values as compared to the plain Caco-2 cultures.

CONCLUSIONS

Co-cultures of HT29-MTX and Caco-2 cells offer the opportunity of modifying the permeability barrier of the cell monolayers both with respect to paracellular resistance and secretory transport via P-gp. Thus, in special cases, they allow more flexibility in adapting the in vitro system to the in vivo situation as compared to the monocultures. Another advantage is the obvious robustness of the method with respect to the reproducibility of the results. A problem remaining, however, is the quantitative expression of carriers involved in intestinal uptake of many nutrients and drugs.

Chitosans as absorption enhancers of poorly absorbable drugs. 3: Influence of mucus on absorption enhancement

Chitosans are potent nontoxic absorption enhancers after nasal administration but their effects on the intestinal epithelium in vivo has not been studied in detail. In this study, the effects of chitosans with varying molecular weights and degrees of acetylation on the absorption of a poorly absorbed model drug (atenolol) were studied in intestinal epithelial cell layers with or without a mucus layer and in an in situ perfusion model of rat ileum. The effects of the chitosans on epithelial morphology and release of lactate dehydrogenase (LDH) into the perfusate were investigated in the in situ model. The chitosans had pronounced effects on the permeability of mucus-free Caco-2 layers and enhanced the permeation of atenolol 10- to 15-fold, with different absorption kinetics for different chitosans, in accordance with previous results. In contrast, enhancement of atenolol absorption through rat ileum was modest. LDH release from the tissues perfused with chitosans did not increase, indicating that the chitosans were used at nontoxic concentrations. Morphological examination of the perfused ileal tissues revealed more mucus discharge from the tissues exposed to chitosans than from controls, which suggested that the discharged mucus may inhibit the binding of chitosan to the epithelial surface and hence decrease the absorption-enhancing effect. This hypothesis was supported by studies with intestinal epithelial HT29-H goblet cells covered with a mucus layer. The binding of chitosan to the epithelial cell surface and subsequent absorption-enhancing effects were significantly reduced in mucus-covered HT29-H cultures. When the mucus layer was removed prior to the addition of chitosan, the cell surface binding and absorption-enhancing effects of the chitosans were increased. We conclude that the modest absorption-enhancing effects of unformulated chitosan solutions in the perfused rat ileum are a result of the mucus barrier in this tissue. This effect may be overcome by increasing the local concentrations of both chitosan and drug, i.e,. through formulation of the chitosan into a particulate dosage form.

Mucus as a barrier to the permeability of hydrophilic and lipophilic compounds in the absence and presence of sodium taurocholate micellar systems using cell culture models

A mucus secreting CaCo-2-Ht29GlucH cell co-culture model was characterised and used to examine the influence of mucus as a barrier to the transport of hydrophilic and lipophilic compounds in the absence and presence of sodium taurocholate micellar (NaTC) systems. TEER measurements and permeability studies using the hydrophilic markers (mannitol, polyethylene glycols (PEGS) 900 and 4000) indicated that the paracellular permeability of the co-culture model was greater than that of the CaCo-2 model. At pH 7.4, no difference in the transport of a model lipophilic drug, dextropropoxyphene, was observed between the two models. However, at pH 4.5, when the drug was highly ionised the transport was significantly lower across the co-culture monolayers. NaTC micellar systems appeared to affect the different cell culture models in the order CaCo-2>CaCo-2-Ht29GlucH>Ht29GlucH. Following removal of the mucus layer by incubation with the mucolytic agent, N-acetyl-l-cysteine, the absorption enhancing potential of NaTC micellar systems was increased in the co-culture model.

The influence of intestinal mucus components on the diffusion of drugs

PURPOSE

Mucus, a potential diffusional barrier to drug absorption, is a complex mixture of mucin and other components. The objective of this study was to investigate the composition of native pig intestinal mucus (PIM) and the influence of identified mucus components on drug diffusion.

METHODS

The mucus components were separated by CsCl-density gradient centrifugation and further analyzed. The self-diffusion coefficients of mannitol, metoprolol, propranolol, hydrocortisone, and testosterone, ranging in lipophilicity from logK = -3.1 to logK = 3.3, were determined, using a small scale tracer technique. The diffusion of drugs in PIM, in solutions or dispersions of individual mucus components, and in an artificial mucus model (MLPD) reconstituted from the major mucus components mucin, lipids, protein, and DNA was compared.

RESULTS

The dry weight of pig intestinal mucus contained (%, w/w); mucin (5%), lipids (37%), proteins (39%), DNA (6%), and unidentified materials. The most commonly occurring lipids were free fatty acids, cholesterol, and phospholipids while the most common protein was serum albumin. In PIM, but not in the purified pig gastric mucin (PPGM) solution, the diffusion of the lipophilic drugs metoprolol, propranolol, hydrocortisone, and testosterone was reduced compared to that of the hydrophilic drug mannitol. The diffusion of the lipophilic drugs was also significantly reduced in a dispersion of identified mucus lipids compared to that of mannitol. The diffusion in MLPD was similar to that in PIM for mannitol, propranolol, hydrocortisone, and testosterone, but somewhat lower for metoprolol.

CONCLUSIONS

Lipids, rather than mucin glycoproteins, are a major component which contributes to reduced diffusion of drugs in native intestinal mucus. The results suggest that reconstituted artificial mucus models are interesting alternatives to native mucus models.

First-pass effect: significance of the intestine for absorption and metabolism

The occurrence of low systemic availability due to significant metabolism or poor absorption of orally administered drugs has been well recognized. Three rate controlling factors affecting the oral absorption: unstirred water layer, membrane limitation, or flow limitation, have been identified. These are much affected by the physicochemical properties of the drug: pKA, water/lipid solubility, structural mimicry to endogenous substrates for transport proteins, and the physiology of the GI tract. Drug metabolizing enzymes are found to be present in the intestine, albeit the content is lower than that found in liver. The presence of pre-absorptive versus post-absorptive intestinal metabolism is presently discussed in experimental sets of data with luminal and systemic administration of the drugs in the vascularly perfused rat small intestine preparation. The effect of the anterior anatomical placement of the intestine and its contribution to metabolism, in relation to that for the liver, has been examined in our laboratory by the perfused intestine-liver preparation. The effect of concentration and flow have been studied and general principles governing drug absorption and metabolism in the intestine and the subsequent effects on the liver have been discussed.

Diffusion of drugs in native and purified gastrointestinal mucus

The mucus layer covering the surface of the gastrointestinal tract may act as a barrier to drug absorption. The aim of this investigation was to study the self-diffusion coefficients of model drugs with different physicochemical properties in gastrointestinal mucus. An in vitro method was used to determine the self-diffusion coefficients of radiolabeled model drugs in different diffusion media. Glucosamine, mannitol, glucuronic acid, glucose, metoprotol, antipyrine, propranolol, hydrocortisone, and testosterone, which display large differences in charge and octanol/water distribution ratios (K), were used as model drugs. The diffusion coefficients of model drugs were compared in phosphate buffer (PB), native pig intestinal mucus (PIM), and purified pig gastric much (PPGM). PIM was not purified and therefore contained all the original components of native mucus, whereas PPGM contained only high molecular weight mucin molecules. Charge had only minor effects on the diffusion coefficients of the model drugs. Lipophilicity, however, had a much larger effect, the largest decrease in diffusion coefficient, 58%, was observed for testosterone in PIM. A negative relationship between the diffusion coefficient and log K was observed in PIM, but no relationship was observed in PPGM and PB. In contrast, the diffusion coefficients for two larger molecules of comparable size, the lipophilic peptide cyclosporin and the hydrophilic peptide D-arginine vasopressin, were markedly reduced in PIM. In conclusion, the most important physicochemical characteristic influencing the diffusion coefficient of most drugs in gastrointestinal mucus appears to be lipophilicity, whereas molecular size appears to have more influence for larger peptide drugs.

Current concepts in intestinal peptide absorption

Today there is considerable interest in oral peptide delivery. However, oral administration of peptides is limited by a low bioavailability and a high variability in plasma levels. A review is given of the literature describing the major barriers in peptide absorption, the basic mechanisms of intestinal peptide transport, the experimental models and the pharmaceutical approaches currently used in the investigation of peptide and protein absorption processes.

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.

Effect of 'unstirred' water layer in the intestine on the rate and extent of absorption after oral administration

The presence of an aqueous diffusion layer or 'unstirred' water layer adjacent to the intestinal membrane has long been regarded as a potential barrier for intestinal absorption of compounds. Theoretical analyses were performed in the present study to quantitatively determine the effect of this layer on the rate and extent of absorption of passively absorbed compounds with different membrane absorption half-lives (10 to 300 min) in humans, dogs, rabbits, rats and mice. Diffusion half-lives across this (40 microns thick) layer were estimated to be 5.8, 2.5, 1.1, 0.65 and 0.32 min, respectively, in the distended intestine of the above five species. These half-lives are reduced by about 5-fold when the intestine is about 80% 'flat' in fasting state. The results of extensive analysis indicate that the presence of such a layer is generally expected to have a relatively mild or insignificant effect on the rate of absorption and an insignificant effect on the extent of absorption. The study also indicates that an aqueous layer of 708 microns has practically no effect on the extent of absorption of progesterone, a highly lipophilic compound, in rats. For prediction of or correlation with the fraction of oral dose absorbed after oral administration, the present study indicates that use of apparent or effective permeability rather than unbiased or true wall (membrane) permeability, as advocated earlier by others, should generally suffice.

Quantitative approaches to delineate paracellular diffusion in cultured epithelial cell monolayers

When using cultured cell monolayers to determine the mechanism of transcellular diffusion of molecules, it may be important to identify the fraction that moves through the paracellular route or passively diffuses through tight junctions. We characterized the apparent diameter of the junctional pore in a variety of epithelial cell monolayers (Caco-2, MDCK, alveolar). Using hydrophilic extracellular permeants varying in molecular radii and charge (neutral, anionic, cationic, zwitterionic), rate-determining steps and factors of the paracellular route were quantitatively delineated by the model for molecular size-restricted diffusion within a negative electrostatic field of force. Protonated amines permeated the pores faster than their neutral images while organic anions were slower. With increasing molecular size the influence of charge diminished. This approach was used to quantify the relationship between permeant radius and transepithelial electrical resistance and to analyze changes in junctional pore size as a function of pharmacological perturbation, such as in the use of absorption promoters or adjuvants.