The influence of net water absorption on the permeability of antipyrine and levodopa in the human jejunum

Quantitative analysis of gastrointestinal fluid absorption and secretion to estimate luminal fluid dynamics in rats

The drug absorption profile is dependent on the luminal drug concentration, which in turn is influenced by the gastrointestinal (GI) fluid dynamics. In the present study, therefore, we aimed to examine the luminal fluid dynamics by kinetically analyzing fluid absorption and secretion along the GI tract in rats using the in situ closed-loop technique with non-absorbable fluorescein isothiocyanate-dextran 4000 (FD-4) and tritium water labeling ([3H]water) under different osmotic conditions. We found that the luminal fluid volume in the jejunum and ileum, but not the colon, gradually decreased and reached a steady state. In contrast, [3H]water almost completely disappeared in all intestinal regions. Kinetic analysis revealed the following rank order for the rate constant of fluid secretion: jejunum > ileum > colon, whereas a negligible regional difference was observed in the rate constant of fluid absorption. Fluid secretion under an isosmotic condition (300 mOsm/kg) was higher than that at 0 mOsm/kg in all intestinal regions, though no significant changes in fluid absorption were observed. Thus, the fluid secretion process appears to be the major determinant of the regional differences in GI fluid dynamics. Our findings indicate that the luminal fluid volume is altered as a result of water ingestion, absorption, and secretion, and finally reaches an apparent steady state, which is regulated mainly by the process of fluid secretion.

Best practices in current models mimicking drug permeability in the gastrointestinal tract - an UNGAP review

The absorption of orally administered drug products is a complex, dynamic process, dependent on a range of biopharmaceutical properties; notably the aqueous solubility of a molecule, stability within the gastrointestinal tract (GIT) and permeability. From a regulatory perspective, the concept of high intestinal permeability is intrinsically linked to the fraction of the oral dose absorbed. The relationship between permeability and the extent of absorption means that experimental models of permeability have regularly been used as a surrogate measure to estimate the fraction absorbed. Accurate assessment of a molecule's intestinal permeability is of critical importance during the pharmaceutical development process of oral drug products, and the current review provides a critique of in vivo, in vitro and ex vivo approaches. The usefulness of in silico models to predict drug permeability is also discussed and an overview of solvent systems used in permeability assessments is provided. Studies of drug absorption in humans are an indirect indicator of intestinal permeability, but in vitro and ex vivo tools provide initial screening approaches are important tools for direct assessment of permeability in drug development. Continued refinement of the accuracy of in silico approaches and their validation with human in vivo data will facilitate more efficient characterisation of permeability earlier in the drug development process and will provide useful inputs for integrated, end-to-end absorption modelling.

Are intact peptides absorbed from the healthy gut in the adult human?

For over 100 years it was believed that dietary protein must be completely hydrolysed before its constituent amino acids could be absorbed via specific amino acid transport systems. It is now known that the uptake of di- and tripeptides into the enterocyte is considerable, being transported across the intestinal endothelium by the PepT1 H+/peptide co-transporter. There is also evidence that some di- and tripeptides may survive cytosolic hydrolysis and be transported intact across the basolateral membrane. However, other than antigen sampling, the transport of larger intact macromolecules across the intestinal endothelium of the healthy adult human remains a controversial issue as there is little unequivocal in vivo evidence to support this postulation. The aim of the present review was to critically evaluate the scientific evidence that peptides/proteins are absorbed by healthy intestinal epithelia and pass intact into the hepatic portal system. The question of the absorption of oliogopeptides is paramount to the emerging science of food-derived bioactive peptides, their mode of action and physiological effects. Overall, we conclude that there is little unequivocal evidence that dietary bioactive peptides, other than di- and tripeptides, can cross the gut wall intact and enter the hepatic portal system in physiologically relevant concentrations.

Direct In Vivo Human Intestinal Permeability (Peff ) Determined with Different Clinical Perfusion and Intubation Methods

Regional in vivo human intestinal effective permeability (Peff ) is calculated by measuring the disappearance rate of substances during intestinal perfusion. Peff is the most relevant parameter in the prediction of rate and extent of drug absorption from all parts of the intestine. Today, human intestinal perfusions are not performed on a routine basis in drug development. Therefore, it would be beneficial to increase the accuracy of the in vitro and in silico tools used to evaluate the intestinal Peff of novel drugs. This review compiles historical Peff data from 273 individual measurements of 80 substances from 61 studies performed in all parts of the human intestinal tract. These substances include: drugs, monosaccharaides, amino acids, dipeptides, vitamins, steroids, bile acids, ions, fatty acids, and water. The review also discusses the determination and prediction of Peff using in vitro and in silico methods such as quantitative structure-activity relationship, Caco-2, Ussing chamber, animal intestinal perfusion, and physiologically based pharmacokinetic (PBPK) modeling. Finally, we briefly outline how to acquire accurate human intestinal Peff data by deconvolution of plasma concentration-time profiles following regional intestinal bolus dosing.

Human in vivo regional intestinal permeability: importance for pharmaceutical drug development

Both the development and regulation of pharmaceutical dosage forms have undergone significant improvements and development over the past 25 years, due primarily to the extensive application of the biopharmaceutical classification system (BCS). The Biopharmaceutics Drug Disposition Classification System, which was published in 2005, has also been a useful resource for predicting the influence of transporters in several pharmacokinetic processes. However, there remains a need for the pharmaceutical industry to develop reliable in vitro/in vivo correlations and in silico methods for predicting the rate and extent of complex gastrointestinal (GI) absorption, the bioavailability, and the plasma concentration-time curves for orally administered drug products. Accordingly, a more rational approach is required, one in which high quality in vitro or in silico characterizations of active pharmaceutical ingredients and formulations are integrated into physiologically based in silico biopharmaceutics models to capture the full complexity of GI drug absorption. The need for better understanding of the in vivo GI process has recently become evident after an unsuccessful attempt to predict the GI absorption of BCS class II and IV drugs. Reliable data on the in vivo permeability of the human intestine (Peff) from various intestinal regions is recognized as one of the key biopharmaceutical requirements when developing in silico GI biopharmaceutics models with improved predictive accuracy. The Peff values for human jejunum and ileum, based on historical open, single-pass, perfusion studies are presented in this review. The main objective of this review is to summarize and discuss the relevance and current status of these human in vivo regional intestinal permeability values.

Current Perspectives of Solubilization: Potential for Improved Bioavailability

This review focuses on the recent techniques of solubilization for the attainment of effective absorption and improved bioavailability. Solubilization may be affected due to cosolvent water interaction or altered crystal structure by cosolvent addition. Micellar solubilization could be affected by both ionic strength and pH. Addition of cosolvents to the surfactant solutions offers only a small advantage because of the decrease in the solubilization capacity of the micelles. Polymorphism is known to influence dissolution and bioavailability of the drugs. Molecular modeling study of cyclodextrin inclusion complexations can predict the inclusion modes, stoichiometry of the complex, and the relative complexing efficiency of cyclodextrins with various drug molecules.

The use of BDDCS in classifying the permeability of marketed drugs

We recommend that regulatory agencies add the extent of drug metabolism (i.e., >or=90% metabolized) as an alternate method in defining Class 1 marketed drugs suitable for a waiver of in vivo studies of bioequivalence. That is, >or=90% metabolized is an additional methodology that may be substituted for >or=90% absorbed. We propose that the following criteria be used to define>or=90% metabolized for marketed drugs: Following a single oral dose to humans, administered at the highest dose strength, mass balance of the Phase 1 oxidative and Phase 2 conjugative drug metabolites in the urine and feces, measured either as unlabeled, radioactive labeled or nonradioactive labeled substances, account for >or=90% of the drug dosed. This is the strictest definition for a waiver based on metabolism. For an orally administered drug to be >or=90% metabolized by Phase 1 oxidative and Phase 2 conjugative processes, it is obvious that the drug must be absorbed. This proposal, which strictly conforms to the present>or=90% criteria, is a suggested modification to facilitate a number of marketed drugs being appropriately assigned to Class 1.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

Why is it Challenging to Predict Intestinal Drug Absorption and Oral Bioavailability in Human Using Rat Model

PURPOSE

To study the correlation of intestinal absorption for drugs with various absorption routes between human and rat, and to explore the underlying molecular mechanisms for the similarity in drug intestinal absorption and the differences in oral bioavailability between human and rat.

MATERIALS AND METHODS

The intestinal permeabilities of 14 drugs and three drug-like compounds with different absorption mechanisms in rat and human jejunum were determined by in situ intestinal perfusion. A total of 48 drugs were selected for oral bioavailability comparison. Expression profiles of transporters and metabolizing enzymes in both rat and human intestines (duodenum and colon) were measured using GeneChip analysis.

RESULTS

No correlation (r(2) = 0.29) was found in oral drug bioavailability between rat and human, while a correlation (r(2) = 0.8) was observed for drug intestinal permeability with both carrier-mediated absorption and passive diffusion mechanisms between human and rat small intestine. Moderate correlation (with r(2) > 0.56) was also found for the expression levels of transporters in the duodenum of human and rat, which provides the molecular mechanisms for the similarity and correlation of drug absorption between two species. In contrast, no correlation was found for the expressions of metabolizing enzymes between rat and human intestine, which indicates the difference in drug metabolism and oral bioavailability in two species. Detailed analysis indicates that many transporters (such as PepT1, SGLT-1, GLUT5, MRP2, NT2, and high affinity glutamate transporter) share similar expression levels in both human and rat with regional dependent expression patterns, which have high expression in the small intestine and low expression in the colon. However, discrepancy was also observed for several other transporters (such as MDR1, MRP3, GLUT1, and GLUT3) in both the duodenum and colon of human and rat. In addition, the expressions of metabolizing enzymes (CYP3A4/CYP3A9 and UDPG) showed 12 to 193-fold difference between human and rat intestine with distinct regional dependent expression patterns.

CONCLUSIONS

The data indicate that rat and human show similar drug intestinal absorption profiles and similar transporter expression patterns in the small intestine, while the two species exhibit distinct expression levels and patterns for metabolizing enzymes in the intestine. Therefore, a rat model can be used to predict oral drug absorption in the small intestine of human, but not to predict drug metabolism or oral bioavailability in human.

Effect of intestinal fluid flux on ibuprofen absorption in the rat intestine

Previously the apparent permeability coefficient (P(app)) of ibuprofen was observed to vary depending on the perfusion medium employed. The present work explores the possible contributions to these differences. Studies were undertaken using an in situ single pass rat gut technique. Lumenal drug concentrations and plasma drug levels were assayed by HPLC. Absorption rate constants (k(0)) were determined from fractions of drug unabsorbed from the intestineat steady state. Plasma data were fitted to a two compartment open model with zero-order input. Significant differences in net fluid flux were observed between the various buffered perfusion media, with fluxes varying from -0.044+/-0.006 ml min(-1) to +0.057+/-0.013 ml min(-1), the lower and negative values occurring for lower pH media and the larger positive values tending to occur with media of higher pH. A linear relationship was found between the P(app) of ibuprofen and net water flux (y=1.13+11.3x; r(2)=0.80). Apparent zero-order rate constants for ibuprofen appearance in plasma correlated well with absorption rate constants estimated from steady state lumenal drug concentration [k(0(gut))]. From the linear relationship between P(app) and fluid flux a normalized P(app) for ibuprofen (i.e. the P(app) in the absence of net fluid flux) of 1.1 x 10(-4)cms(-1) was determined Net lumenal fluid flux is dependent on perfusion medium composition and significantly alters ibuprofen absorption. The differences observed for P(app) were reflected in systemic drug absorption concentrations. The findings of these studies underline the importance of standardizing the osmolarity of experimental media used for the determination of intestinal permeability data.

Hydrosoluble fiber (Plantago ovata husk) and levodopa I: experimental study of the pharmacokinetic interaction

Fiber therapy could be used in patients with Parkinson disease to reduce the symptoms of gastrointestinal disorders; however, it could interact with levodopa reducing its effectiveness. In this experimental study we have investigated whether the presence of Plantago ovata husk (water-soluble fiber) modifies in rabbits the bioavailability and other pharmacokinetic parameters of levodopa (20 mg/kg) when administered by the oral route at the same time. We have also studied whether pharmacokinetic modifications are fiber-dose dependent (100 and 400 mg/kg). The extent of levodopa absorbed when administering 100 mg/kg of fiber (AUC=43.4 mug min ml(-1)) is approximately the same as when levodopa is administered alone (AUC=47.1 microg min ml(-1)); however, Cmax is lower (1.04 versus 1.43 microg ml(-1)). Results obtained indicate that fiber at the higher dose increases the extent of levodopa absorbed (AUC=62.2 microg min ml(-1)), being the value of Cmax similar (1.46 microg ml(-1)). The value of tmax increases from 10 min when levodopa is administered alone to 20 min when the animals receive fiber. On the other hand, since certain time on, levodopa concentrations are always higher in the groups that receive fiber: 60 min with 100 mg/kg fiber and 20 min with 400 mg/kg fiber. Fiber also increases the mean residence time (MRT). P. ovata husk administration with levodopa could be beneficial, not only in patients with constipation, due to: lower adverse reactions (lower values of Cmax) and longer and more stable effects (higher final concentrations and more time in the body).

In vitro Nimesulide Absorption from Different Formulations

In light of improving the bioavailability of poorly water-soluble drugs, this work focused on the comparison among different nimesulide formulations resorting to in vitro absorption experiments through everted rat intestine. The performance of a nimesulide ethanol-triacetin solution, an activated system made up by cogrinding nimesulide/polyvinylpyrrolidone and simple solid nimesulide were compared with that of a reference nimesulide solution. Although ethanol-triacetin solution showed a better performance than the solid nimesulide because wettability problems connected with nimesulide were completely zeroed, the activated system showed a better performance than the reference solution one. This was due to the fact that the activated system allowed to overcome both the wettability and solubility problems connected with nimesulide. Moreover, as proved by intestinal pictures taken before and after permeation experiments, we observed the adhesion of polymeric particles to intestinal villi, this giving origin to a thin layer, surrounding the intestine, characterized by a nimesulide concentration higher than that in the release environment bulk. A proper mathematical model, based on Fick's second law, was developed to model drug absorption in the case of solution and activated system. In this manner, we could calculate nimesulide permeability through the intestinal wall, and we could better define the nature of the above-mentioned thin layer surrounding the intestine. Finally, the mathematical model was used to verify the theoretical correctness of the widely employed technique consisting in data correction for dilution when sample withdrawal and replacement were needed to measure drug concentration in the receiver environment.

Prediction of the oral absorption of low-permeability drugs using small intestine-like 2/4/A1 cell monolayers

PURPOSE

To characterize the paracellular route of 2/4/A1 monolayers and to compare the permeabilities of incompletely absorbed oral drugs in 2/4/A1 with those in Caco-2 monolayers.

METHODS

The cells were cultivated on permeable supports. The 2/4/ A1 expression of genes associated with tight junctions was compared with that in the small intestine using RT-PCR. The aqueous pore radii were determined using paracellular marker molecules. The permeabilities of a series of incompletely absorbed drugs (defined as having a fraction absorbed 0 to 80%) after oral administration to humans were studied.

RESULTS

Occludin and claudin 1 and 3 were expressed in 2/4/A1. The pore radius of 2/4/A1 was 9.0 +/- 0.2 A. which is similar to that in the human small intestine, although the pore radius was smaller (3.7 +/- 0.1 A) in Caco-2. The relationship between permeability and fraction absorbed of 13 drugs was stronger in 2/4/A1 than in Caco-2. The relationships were used to predict the intestinal absorption of another seven drugs. The prediction was more accurate in 2/4/A1 (RMSE = 15.6%) than in Caco-2 (RMSE = 21.1%). Further, Spearman's rank coefficient between FA and permeability was higher in 2/4/A1.

CONCLUSION

The improved 2/4/A1 cell culture model has a more in vivo-like permeability and predicted the oral absorption of incompletely absorbed drugs better than Caco-2 cells.

Theoretical considerations on the in vivo intestinal permeability determination by means of the single pass and recirculating techniques

This paper deals with the development of proper mathematical models for the calculation of the in vivo rat intestinal drug permeability resorting to two different kinds of experimental methods: the single pass and the recirculating perfusion techniques. In particular, in the single pass case, attention is focused on the effect of water exchange between the flowing solution and the intestinal wall, as this can sensibly affect the permeability determination. In both the single pass and the recirculating perfusion method, a complete radial mixing of the flowing solution is supposed to hold, so that drug concentration and solution velocity are radius independent. Nevertheless, they depend on the intestinal axial position. Accordingly, two distinct models are built up by resorting to microscopic mass balances. The reasonably good data fitting performed by the recirculating perfusion model ensures that the most important factors affecting the passive drug (Antipyrine) diffusion through a rat intestinal wall are properly accounted for. Moreover, the reliability of the developed models and the experimental tests is proved by the fact that the drug (Antipyrine) permeability determined by means of the two methods is statistically equal.

Setting bioequivalence requirements for drug development based on preclinical data: optimizing oral drug delivery systems

The recently proposed Biopharmaceutics Classification System can be used to classify drugs and set standards for scale-up and post-approval changes as well as standards for in vitro/in vivo correlation for immediate and controlled release products. This classification scheme is based on determining the underlying process that is controlling the drug absorption rate and extent, namely, drug solubility and intestinal membrane permeability. Theoretical analysis and experimental results suggest that a permeability/solubility classification scheme can be used to set more rationale drug standards. In particular, high solubility/high permeability, rapidly dissolving drugs may be regulated on the basis of a single point rapid dissolution test while low solubility dissolution rate limited drugs can be regulated based on an in vitro dissolution test that reflects the in vivo dissolution process. This dissolution test may include multiple time points, media change, as well as surfactants in order to reflect the in vivo dissolution process and would be used by the manufacturer for requesting a waiver from a bioequivalence (BE) trial. For controlled release products, the regulation of bioequivalence standards is more complex due to the potential differences in position-dependent permeability/solubility and metabolism of drugs along the gastrointestinal tract. These differences may result in drug absorption rates that are highly transit time dependent. This paper will present the current status of the biopharmaceutic drug classification scheme, the underlying developed data base and its application to optimizing IR and CR products.

Jejunal permeability in humans in vivo and rats in situ: investigation of molecular size selectivity and solvent drag

The mechanisms controlling rates and routes for intestinal absorption of nutrients and small compounds are still not fully clarified. In the present study we aimed to investigate the effect of solvent drag on intestinal permeability of compounds with different molecular sizes in humans and rats. The effective intestinal permeabilities (Peff) of hydrophilic compounds (MW 60-4000) were determined in the single-pass perfused jejunum in humans in vivo and rats in situ under iso- and hypotonic conditions. The transport mechanism(s) of water and the importance of the solvent drag effect were investigated by the use of D2O. This is the first report in humans establishing the relation between in vivo measured jejunal Peff and molecular size for hydrophilic compounds. In addition, in rats we also found a molecular-size selectivity for hydrophilic compounds similar to man. The jejunal Peff of water and urea (MW 60) in both species were several times higher than predicted from their physicochemical properties. In humans, the jejunal absorption of urea and creatinine (MW 113) was increased by solvent drag, while no effect was found for the other investigated compounds. In rats, Peff for urea and creatinine were unaffected. In conclusion, it is still unclear if solvent drag occurs mainly through the para- or transcellular route, although, results from this study further add to our earlier reports suggesting that the transcellular route is most important from a quantitative point of view regardless of physicochemical properties of the transported compounds.

Human intestinal permeability

This review focuses on permeability measurements in humans, briefly discussing different perfusion techniques, the relevance of human Peff values, and various aspects of in vivo transport mechanisms. In addition, human Peff values are compared with corresponding data from three preclinical transport models. The regional human jejunal perfusion technique has been validated in several important ways. One of the most important findings is that there is a good correlation between the measured human effective permeability values and the extent of absorption of drugs in humans determined by pharmacokinetic studies. Estimations of the absorption half-lives from the measured Peff agree very well with the time to maximal amount of the dose absorbed achieved after an oral dose in humans. We have also shown that it is possible to determine the Peff for carrier-mediated transported compounds and to classify them according to the proposed biopharmaceutical classification system (BCS). Furthermore, human in vivo permeabilities can be predicted using preclinical permeability models, such as in situ perfusion of rat jejunum, the Caco-2 model, and excised intestinal segments in the Ussing chamber. The permeability of passively transported compounds can be predicted with a particularly high degree of accuracy. However, special care must be taken for drugs with a carrier-mediated transport mechanism, and a scaling factor has to be used. Finally, the data obtained in vivo in humans emphasize the need for more clinical studies investigating the effect of physiological in vivo factors and molecular mechanisms influencing the transport of drugs across the intestinal and as well as other membrane barriers. It will also be important to study the effect of antitransport mechanisms (multidrug resistance, MDR), such as efflux by P-glycoprotein(s) and gut wall metabolism, for example CYP 3A4, on bioavailability.

Human jejunal effective permeability and its correlation with preclinical drug absorption models

This review focuses on intestinal permeability measurements in humans and various aspects of in-vivo transport mechanisms. In addition, comparisons of human data with preclinical models and the blood-brain barrier is discussed. The regional human jejunal perfusion technique has been validated by several crucial points. One of the most important findings is that there is a good correlation between the measured human effective permeability values and the extent of absorption of drugs in humans determined by pharmacokinetic studies. We have also shown that it is possible to determine the effective permeability (Peff) for carrier-mediated transported compounds, and to classify them according to the proposed Biopharmaceutical Classification System (BCS). Furthermore, it is possible to predict human in-vivo permeability using preclinical permeability models, such as in-situ perfusion of rat jejunum, the Caco-2 model and excized intestinal segments in the Ussing chamber. The permeability of passively transported compounds can be predicted with a particularly high degree of accuracy. However, special care must be taken for drugs with a carrier-mediated transport mechanism, and a scaling factor has to be used. It is also suggested that it is possible to roughly estimate the permeability of the blood-brain barrier using measurements of intestinal permeability, even if the quantitative role of efflux of P-glycoprotein(s) in-vivo still remains to be clarified. Finally, the data obtained in-vivo in humans emphasize the need for more clinical studies investigating the effect of physiological in-vivo factors and molecular mechanisms influencing the transport of drugs across the intestinal and as well as other membrane barriers. It is also important to study the effect of anti-transport mechanisms, such as efflux by P-glycoprotein(s), and gut wall metabolism, for example CYP 3A4, on the bioavailability.

Comparison between permeability coefficients in rat and human jejunum

PURPOSE

Our main aim is to determine the effective intestinal permeability (Peff) in the rat jejunum in situ for 10 compounds with different absorption mechanisms and a broad range of physico chemical properties, and then compare them with corresponding historical human in vivo Peff values.

METHODS

The rat Peff coefficients are determined using an in situ perfusion model in anaesthetized animals. The perfusion flow rate used is 0.2 ml/min, which is 10 times lower than that used in humans. The viability of the method is assessed by testing the physiological function of the rat intestine during perfusions.

RESULTS

The Peff for passively absorbed compounds is on average 3.6 times higher in humans compared to rats (Peff, man = 3.6 x Peff.rat+ 0.03.10(-4); R]2 = 1.00). Solutes with carrier-mediated absorption deviate from this relationship, which indicates that an absolute scaling of active processes from animal to man is difficult, and therefore needs further investigation. The fraction absorbed of drugs after oral administration in humans (fa) can be estimated from 1-e-(-2.Peff,man t rex/r.2.8).

CONCLUSIONS

Rat and human jejunum Peff-estimates of passively absorbed solutes correlate highly, and both can be used with precision to predict in vivo oral absorption in man. The carrier-mediated transport requires scaling between the models, since the transport maximum and/or substrate specificity might differ. Finally, we emphasize the absolute necessity of including marker compounds for continuous monitoring of intestinal viability.

Does fluid flow across the intestinal mucosa affect quantitative oral drug absorption? Is it time for a reevaluation?

PURPOSE

Hydrophilic and charged solutes have a lower membrane permeability which is due to a lower partition into the lipid membrane (low solubility in the membrane phase) and/or a slower transcellular diffusion coefficient. They are therefore anticipated to be absorbed through the paracellular route, which is a consequence of diffusion and a convective volume flow through the water-filled intercellular space.

METHODS

Two approaches have been used to investigate the mechanisms underlying the paracellular drug transport across the intestinal mucosa: (a) including water transport by exposing the apical side of the epithelium with a hypotonic solution, and (b) stimulated paracellular transport by widening of tight junction and increased water absorption as a consequence of the sodium-coupled transport of nutrients.

RESULTS

Among the first studies that recognized this fluid flux dependent transmucosal transport of drugs, was one published by Oschenfahrt & Winne in 1973 and the one by Kitazawa et al. in 1975. During the last two decades the importance of this paracellular route for drug delivery have been explored in vitro and in situ.

CONCLUSIONS

The limits concerning molecular weight, shape, ionization and the effect of physiological stimulants, such as luminal concentrations of nutrients, osmolality and motility, are currently under investigation. However, recently published in vivo human data by ourselves and others indicate that the promising results obtained in vitro and in situ for various hydrophilic compounds might not be valid in quantitative aspects in humans, especially not for drugs with a molecular weight over 200.

The lack of effect of induced net fluid absorption on the in vivo permeability of terbutaline in the human jejunum

The absorption mechanism(s) of terbutaline in the human jejunum was studied by using the intestinal perfusion instrument, Loc-I-Gut. The present study was divided into three parts. In Part I the absorption of 10 and 20 microM of both (+) and (-)-terbutaline enantiomers was studied. The influence of D-glucose (80 mM) on the net fluid transport across the intestinal wall and the effective intestinal permeability (Peff) of both (+/-)-terbutaline (10 microM) and antipyrine (0.5 mM) was investigated in Part II. The experimental design of Part III was similar to that in Part II, with the exception that the perfusion solution was hypotonic and had a D-glucose concentration of 80 mM. No statistical differences were found in the Peff between terbutaline enantiomers or their concentrations. D-glucose (80 mM) did neither affect net fluid transport nor the Peff of (+/-)-terbutaline and antipyrine in the human jejunum. In contrast, hypotonic D-glucose (80 mM) solution induced a net fluid absorption (p < 0.01). In parallel with this observation, the Peff -value of (+/-)-terbutaline was unchanged, whereas the absorption of antipyrine was found to be significantly increased (p < 0.05). The increased permeability of antipyrine during the net fluid absorption phase might be due to convective paracellular flow, but more likely is it a consequence of a higher concentration gradient of the drug close to the intestinal wall, and thereby increased transcellular absorption. Based on these findings we propose that the major route for the oral absorption of terbutaline and antipyrine might be passive transcellular diffusion.