Experimental estimation of the effective unstirred water layer thickness in the human jejunum, and its importance in oral drug absorption

Prediction of human pharmacokinetics--gastrointestinal absorption

Permeability (P(e)) and solubility/dissolution are two major determinants of gastrointestinal (GI) drug absorption. Good prediction of these is crucial for predicting doses, exposures and potential interactions, and for selecting appropriate candidate drugs. The main objective was to evaluate screening methods for prediction of GI P(e), solubility/dissolution and fraction absorbed (f(a)) in humans. The most accurate P(e) models for prediction of f(a) of passively transported and highly soluble compounds appear to be the 2/4/A1 rat small intestinal cell model (in-vitro and in-silico), a newly developed artificial-membrane method, and a semi-empirical approach based on in-vitro membrane affinity to immobilized lipid bilayers, effective molecular weight and physiological GI variables. The predictability of in-vitro Caco-2, in-situ perfusion and other artificial membrane methods seems comparably low. The P(e) and f(a) in humans for compounds that undergo mainly active transport were predicted poorly by all models investigated. However, the rat in-situ perfusion model appears useful for prediction of active uptake potential (complete active uptake is generally well predicted), and Caco-2 cells are useful for studying bidirectional active transport, respectively. Human intestinal in-vitro P(e), which correlates well with f(a) for passively transported compounds, could possibly also have potential to improve/enable predictions of f(a) for actively transported substances. Molecular descriptor data could give an indication of the passive absorption potential. The 'maximum absorbable dose' and 'dose number' approaches, and solubility/dissolution data obtained in aqueous media, appear to underestimate in-vivo dissolution to a considerable extent. Predictions of in-vivo dissolution should preferably be done from in-vitro dissolution data obtained using either real or validated simulated GI fluids.

The role of permeability in drug ADME/PK, interactions and toxicity--presentation of a permeability-based classification system (PCS) for prediction of ADME/PK in humans

PURPOSE

The objective was to establish in vitro passive permeability (Pe) vs in vivo fraction absorbed (fa)-relationships for each passage through the human intestine, liver, renal tubuli and brain, and develop a Pe-based ADME/PK classification system (PCS).

MATERIALS AND METHODS

Pe- and intestinal fa-data were taken from an available data set. Hepatic fa was calculated based on extraction ratios of the unbound fraction of drugs (with support from animal in vivo uptake data). Renal fa (reabsorption) was estimated using renal pharmacokinetic data, and brain fa was predicted using animal in vitro and in vivo brain Pe-data. Hepatic and intestinal fa-data were used to predict bile excretion potential.

RESULTS

Relationships were established, including predicted curves for bile excretion potential and minimum oral bioavailability, and a 4-Class PCS was developed: I (very high Pe; elimination mainly by metabolism); II (high Pe) and III (intermediate Pe and incomplete fa); IV (low Pe and fa). The system enables assessment of potential drug-drug transport interactions, and drug and metabolite organ trapping.

CONCLUSIONS

The PCS and high quality Pe-data (with and without active transport) are believed to be useful for predictions and understanding of ADME/PK, elimination routes, and potential interactions and organ trapping/toxicity in humans.

Oral Delivery of Peptide Drugs

A wide variety of peptide drugs are now produced on a commercial scale as a result of advances in the biotechnology field. Most of these therapeutic peptides are still administered by the parenteral route because of insufficient absorption from the gastrointestinal tract. Peptide drugs are usually indicated for chronic conditions, and the use of injections on a daily basis during long-term treatment has obvious drawbacks. In contrast to this inconvenient and potentially problematic method of drug administration, the oral route offers the advantages of self-administration with a high degree of patient acceptability and compliance. The main reasons for the low oral bioavailability of peptide drugs are pre-systemic enzymatic degradation and poor penetration of the intestinal mucosa. A considerable amount of research has focused on overcoming the challenges presented by these intestinal absorption barriers to provide effective oral delivery of peptide and protein drugs. Attempts to improve the oral bioavailability of peptide drugs have ranged from changing the physicochemical properties of peptide molecules to the inclusion of functional excipients in specially adapted drug delivery systems. However, the progress in developing an effective peptide delivery system has been hampered by factors such as the inherent toxicities of absorption-enhancing excipients, variation in absorption between individuals, and potentially high manufacturing costs. This review focuses on the intestinal barriers that compromise the systemic absorption of intact peptide and protein molecules and on the advanced technologies that have been developed to overcome the barriers to peptide drug absorption.

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.

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

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

Theoretical predictions of drug absorption in drug discovery and development

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

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.

Gastrointestinal absorption of drugs: methods and studies

Physico-chemical descriptors of drug molecules are often not adequate in predicting their oral bioavailability. In vitro methods can be useful in evaluating some of the different factors contributing to bioavailability. While physical parameters such as drug solubility may effect oral bioavailability, in most cases, the major determining factors are likely to be metabolism, and absorption at the intestinal level. Metabolism may be preabsorptive, as with peptides, or during absorption, particularly as a result of the activity of the intracellular enzyme CYP3A4. Absorption may be transcellular (membrane diffusion, carrier-mediated, endocytosis) or paracellular, while p-glycoprotein activity in the apical cell membrane may limit bioavailability by expelling drugs from the mucosal cells. Knowledge of the absorption mechanism is important in determining formulation strategies. The different in vitro techniques used to study absorption have advantages and disadvantages. Ussing chambers can be useful to measure bidirectional transport, but most studies use simple salt media, and full tissue viability is doubtful. Caco-2 cell monolayers are human cells, but the system is static, and gives very low rates of transport, and exagerated enhancement of the paracellular route compared with small intestine. The rat everted gut sac incubated in tissue culture medium maintains tissue viability and gives reliable data, although it is a closed system. In situ perfusion gives no information on events at the cellular level, and absorption may be reduced by anaesthesia and surgical manipulation. In vivo perfusion in man, with multichannel tubes, gives valuable data, but is not practical for screening. Pharmacokinetic modelling can also give useful data such as the existence of different absorption sites. Permeability values from the literature show that for small hydrophilic molecules, which pass by the paracellular route, the improved everted sac gives values close to those for humans, while values with Caco-2 cells are orders of magnitude lower.

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.

Correlation of human jejunal permeability (in vivo) of drugs with experimentally and theoretically derived parameters. A multivariate data analysis approach

The effective permeability (Peff) in the human jejunum (in vivo) of 22 structurally diverse compounds was correlated with both experimentally determined lipophilicity values and calculated molecular descriptors. The permeability data were previously obtained by using a regional in vivo perfusion system in the proximal jejunum in humans as part of constructing a biopharmaceutical classification system for oral immediate-release products. pKa, log P, and, where relevant, log Pion values were determined using the pH-metric technique. On the basis of these experiments, log D values were calculated at pH 5.5, 6.5, and 7.4. Multivariate data analysis was used to derive models that correlate passive intestinal permeability to physicochemical descriptors. The best model obtained, based on 13 passively transcellularly absorbed compounds, used the variables HBD (number of hydrogen bond donors), PSA (polar surface area), and either log D5.5 or log D6.5 (octanol/water distribution coefficient at pH 5.5 and 6.5, respectively). Statistically good models for prediciting human in vivo Peff values were also obtained by using only HBD and PSA or HBD, PSA, and CLOGP. These models can be used to predict passive intestinal membrane diffusion in humans for compounds that fit within the defined property space. We used one of the models obtained above to predict the log Peff values for an external validation set consisting of 34 compounds. A good correlation with the absorption data of these compounds was found.

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.

A residence-time distribution analysis of the hydrodynamics within the intestine in man during a regional single-pass perfusion with Loc-I-Gut: in-vivo permeability estimation

The goal of this study was to determine the most appropriate hydrodynamic model for the Loc-I-Gut in-vivo perfusion system. The general mixing-tank-in-series model, which can approximate single mixing tank and laminar and plug-flow hydrodynamics, was fitted to the observed experimental residence-time distribution curves for the non-absorbable marker [14C]PEG 4000. The residence-time distribution analysis shows that the hydrodynamics of the perfusion solution within the jejunal segment in man is well approximately by a model containing on average between 1-2 mixing tanks in series. The solution is well mixed when using perfusion rates of 2.0, 3.0 and 6.0 mL min-1. The average mean residence time estimates from the fitted residence-time distribution were 12 +/- 7.6, 15 +/- 4.2 and 7.7 +/- 4.6 min, respectively, at these three perfusion rates. The mean volumes of the segment (Vs) were 25 +/- 15, 45 +/- 12 and 46 +/- 27 mL, respectively. There were no statistical differences between 2.0, 3.0 and 6.0 mL min-1 in respect of the number of mixing tanks (n) and mean residence times. This residence-time distribution analysis indicates that the luminal fluid in the Loc-I-Gut perfusion system is well-mixed, and that permeability calculations based on the well-mixed assumption most closely approximate the actual local (average) membrane permeability within the perfused segment.

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.

The biopharmaceutic drug classification and drugs administered in extended release (ER) formulations

A biopharmaceutic drug classification scheme for correlating the in-vitro drug product dissolution and in-vivo bioavailability for IR products was proposed by Amidon et al (1995). The classification arose from drug dissolution and absorption models which identified the key parameters controlling drug absorption as the dimensionless numbers; the Absorption number (A(n)), the Dissolution number (Dn) and the Dose number (D(o)). This led to a biopharmaceutic classification of drugs into four groups, the establishment of a basis for determining the conditions under which in-vitro-in-vivo (IVIV) correlation's are expected and the use of the classification to set drug bioavailability standards for IR products. These developments raise the issue of whether the biopharmaceutic classification has relevance to ER products. In contrast to IR products, drugs selected for ER products should have good gastrointestinal (GI) permeability and an extended site of absorption. However their permeability(Papp) may change depending on the site. Solubility(Cs), effective fluid volume and hence D(o) may also vary with site. Of particular relevance to both permeability and solubility is the degree of ionization of the drug. Residence time at each site, pH changes and the potential for drug degradation at different sites, the latter resulting in a restricted absorption window, will influence the time frame over which an IVIV relationship is possible. Of the drugs available in ER dosage forms approximately 63% are bases, 15% acids and the remainder either unionizable or small inorganic ions. Acidic drugs will tend to have lower solubility's high up in the gastrointestinal tract (GIT), with solubility increasing down the GIT. In contrast with increased ionization permeability should fall. Thus with acids, as the dosage form moves to a more alkaline environment down the GIT, absorption may change from dissolution control to membrane control depending on the pK.a of the drug. In contrast bases will loose solubility with transit down the GIT, but become more permeable; absorption becoming more dissolution/release controlled or in extreme cases solubility controlled in the latter stages of the absorption phase. In the light of the above considerations a modified biopharmaceutic classification is proposed for ER products.

Peritrophic matrix structure and function

Formed of proteins, glycoproteins, and chitin microfibrils in a proteoglycan matrix, the peritrophic matrix (PM) separates the food from the midgut epithelium in most but not all insects. A PM occurs in two forms. A type I PM is delaminated from the entire midgut epithelium and, in some cases, may only be formed in response to feeding and the type of meal ingested. A type II PM is produced by a specialized region of the anterior midgut called the cardia and forms a continuous sleeve (or sleeves) that is always present. As it is positioned between food and midgut epithelium, the PM plays key roles in the intestinal biology of the insect. The PM may protect the midgut epithelium from mechanical damage and insult from pathogens and toxins; it must act as a semipermeable membrane regulating passage of molecules between the different midgut compartments; and it may separate the midgut lumen into different, physiologically significant compartments.