CRISPR-Cas9: A New Addition to the Drug Metabolism and Disposition Tool Box

Clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR associated protein 9 (Cas9), i.e., CRISPR-Cas9, has been extensively used as a gene-editing technology during recent years. Unlike earlier technologies for gene editing or gene knockdown, such as zinc finger nucleases and RNA interference, CRISPR-Cas9 is comparably easy to use, affordable, and versatile. Recently, CRISPR-Cas9 has been applied in studies of drug absorption, distribution, metabolism, and excretion (ADME) and for ADME model generation. To date, about 50 papers have been published describing in vitro or in vivo CRISPR-Cas9 gene editing of ADME and ADME-related genes. Twenty of these papers describe gene editing of clinically relevant genes, such as ATP-binding cassette drug transporters and cytochrome P450 drug-metabolizing enzymes. With CRISPR-Cas9, the ADME tool box has been substantially expanded. This new technology allows us to develop better and more predictive in vitro and in vivo ADME models and map previously underexplored ADME genes and gene families. In this mini-review, we give an overview of the CRISPR-Cas9 technology and summarize recent applications of CRISPR-Cas9 within the ADME field. We also speculate about future applications of CRISPR-Cas9 in ADME research.

Oral absorption of peptides and nanoparticles across the human intestine: Opportunities, limitations and studies in human tissues

In this contribution, we review the molecular and physiological barriers to oral delivery of peptides and nanoparticles. We discuss the opportunities and predictivity of various in vitro systems with special emphasis on human intestine in Ussing chambers. First, the molecular constraints to peptide absorption are discussed. Then the physiological barriers to peptide delivery are examined. These include the gastric and intestinal environment, the mucus barrier, tight junctions between epithelial cells, the enterocytes of the intestinal epithelium, and the subepithelial tissue. Recent data from human proteome studies are used to provide information about the protein expression profiles of the different physiological barriers to peptide and nanoparticle absorption. Strategies that have been employed to increase peptide absorption across each of the barriers are discussed. Special consideration is given to attempts at utilizing endogenous transcytotic pathways. To reliably translate in vitro data on peptide or nanoparticle permeability to the in vivo situation in a human subject, the in vitro experimental system needs to realistically capture the central aspects of the mentioned barriers. Therefore, characteristics of common in vitro cell culture systems are discussed and compared to those of human intestinal tissues. Attempts to use the cell and tissue models for in vitro-in vivo extrapolation are reviewed.

Validation and development of MTH1 inhibitors for treatment of cancer

BACKGROUND

Previously, we showed cancer cells rely on the MTH1 protein to prevent incorporation of otherwise deadly oxidised nucleotides into DNA and we developed MTH1 inhibitors which selectively kill cancer cells. Recently, several new and potent inhibitors of MTH1 were demonstrated to be non-toxic to cancer cells, challenging the utility of MTH1 inhibition as a target for cancer treatment.

MATERIAL AND METHODS

Human cancer cell lines were exposed in vitro to MTH1 inhibitors or depleted of MTH1 by siRNA or shRNA. 8-oxodG was measured by immunostaining and modified comet assay. Thermal Proteome profiling, proteomics, cellular thermal shift assays, kinase and CEREP panel were used for target engagement, mode of action and selectivity investigations of MTH1 inhibitors. Effect of MTH1 inhibition on tumour growth was explored in BRAF V600E-mutated malignant melanoma patient derived xenograft and human colon cancer SW480 and HCT116 xenograft models.

RESULTS

Here, we demonstrate that recently described MTH1 inhibitors, which fail to kill cancer cells, also fail to introduce the toxic oxidized nucleotides into DNA. We also describe a new MTH1 inhibitor TH1579, (Karonudib), an analogue of TH588, which is a potent, selective MTH1 inhibitor with good oral availability and demonstrates excellent pharmacokinetic and anti-cancer properties in vivo.

CONCLUSION

We demonstrate that in order to kill cancer cells MTH1 inhibitors must also introduce oxidized nucleotides into DNA. Furthermore, we describe TH1579 as a best-in-class MTH1 inhibitor, which we expect to be useful in order to further validate the MTH1 inhibitor concept.

Genotype-dependent effects of inhibitors of the organic cation transporter, OCT1: predictions of metformin interactions

Common genetic variants of the liver-specific human organic cation transporter 1 (OCT1; SLC22A1) have reduced transport capacity for substrates such as the antidiabetic drug metformin. The effect of the reduced OCT1 function on drug interactions associated with OCT1 has not been investigated and was, therefore, the focus of the study presented here. HEK293 cells expressing human OCT1-reference or the variants R61C, V408M, M420del and G465R were first used to study the kinetics and inhibition pattern of the OCT1 substrate 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)). In the second part OCT1-mediated (14)C-metformin uptake was studied in the presence of drugs administered concomitantly with metformin. Transport studies using ASP(+) showed that the function of the variants decreased in the following order: OCT1-reference=V408M=M420del >R61C >>G465R. Variants M420del and R61C were more sensitive to drug inhibition, with IC(50) values up to 23 times lower than those of the OCT1-reference. Uptake studies using (14)C-metformin were in qualitative agreement with those using ASP(+), with the exception that a larger reduction in transport capacity was observed for M420del. Concomitantly administered drugs, such as verapamil and amitriptyline, revealed potential drug-drug interactions at clinical plasma concentrations of metformin for OCT1-M420del.

Ketobemidone is a substrate for cytochrome P4502C9 and 3A4, but not for P-glycoprotein

The role of the major drug-metabolizing cytochrome P450 (CYP) enzymes as well as P-glycoprotein (PGP) was investigated in the disposition of ketobemidone in vitro. Formation of norketobemidone from ketobemidone was studied and compared with the activities of 11 major CYP enzymes in human liver microsomes. The formation of norketobemidone from ketobemidone (1 microM) correlated best with CYP2C9 activity, measured as losartan oxidation (rs = 0.82, n = 19, p < 0.001), but there was also a strong correlation with CYP3A4 activity. Additionally, a good correlation was observed with CYP2C19, CYP2C8 and CYP2B6 at a ketobemidone concentration of 50 microM. Inhibition studies confirmed the involvement of CYP2C9 and CTP3A4 in the formation of norketobemidone. The formation rate of norketobemidone was three times higher in the CYP2C9*1*1 genotype group compared with the CYP2C9*1*2, CYP2C9*1*3 and CYP2C9*3*3 genotypes (p < 0.01). Treatment with verapamil as a PGP inhibitor did not affect the transport of ketobemidone in Caco-2 cells, indicating that PGP is not involved. The data suggest that CYP2C9 and CYP3A4 play a major role in the formation of norketobemidone at clinically relevant concentrations.

Improved chitosan-mediated gene delivery based on easily dissociated chitosan polyplexes of highly defined chitosan oligomers

Nonviral gene delivery systems based on conventional high-molecular-weight chitosans are efficient after lung administration in vivo, but have poor physical properties such as aggregated shapes, low solubility at neutral pH, high viscosity at concentrations used for in vivo delivery and a slow dissociation and release of plasmid DNA, resulting in a slow onset of action. We therefore developed highly effective nonviral gene delivery systems with improved physical properties from a series of chitosan oligomers, ranging in molecular weight from 1.2 to 10 kDa. First, we established structure-property relationships with regard to polyplex formation and in vivo efficiency after lung administration to mice. In a second step, we isolated chitosan oligomers from a preferred oligomer fraction to obtain fractions, ranging from 10 to 50-mers, of more homogeneous size distributions with polydispersities ranging from 1.01 to 1.09. Polyplexes based on chitosan oligomers dissociated more easily than those of a high-molecular-weight ultrapure chitosan (UPC, approximately a 1000-mer), and released pDNA in the presence of anionic heparin. The more easily dissociated polyplexes mediated a faster onset of action and gave a higher gene expression both in 293 cells in vitro and after lung administration in vivo as compared to the more stable UPC polyplexes. Already 24 h after intratracheal administration, a 120- to 260-fold higher luciferase gene expression was observed compared to UPC in the mouse lung in vivo. The gene expression in the lung was comparable to that of PEI (respective AUCs of 2756+/-710 and 3320+/-871 pg luciferase x days/mg of total lung protein). In conclusion, a major improvement of chitosan-mediated nonviral gene delivery to the lung was obtained by using polyplexes of well-defined chitosan oligomers. Polyplexes of oligomer fractions also had superior physicochemical properties to commonly used high-molecular-weight UPC.

PEI - a potent, but not harmless, mucosal immuno-stimulator of mixed T-helper cell response and FasL-mediated cell death in mice

Polyethyleneimine (PEI) is one of the most effective gene delivery systems available today. However, very little is known about its ability to stimulate a systemic immune response and the molecular mechanisms thereof. However, this information is vital for the future development of new gene delivery systems. Here we address this issue by studying gene expression profiles from spleen lymphocytes after in vivo immunization of mice with PEI formulated with a reporter plasmid (PEI+) or the formulation alone (PEI-). PEI- was found to provoke the activation of genes with important immunostimulatory functions, but without the necessary costimulatory signals. PEI+ resulted in: a mixed Th1/Th2 response; activation of both CD8(+) and CD4(+) T cells, with a larger effect on CD4(+); and FasL-mediated antigen-induced cell death. A comparison of the immune responses of PEI+ with that of the clinically used tetanus toxoid-aluminum phosphate vaccine showed that the DNA vaccine provoked a stronger immune response as compared to the protein vaccine. However, many genes involved in other cellular responses such as apoptosis, stress responses and oncogenesis were activated in PEI+, supporting the theory of immunostimulation by danger genes, but also pointing toward possible adverse reactions such as Alzheimer's disease.

Relationship between the physical shape and the efficiency of oligomeric chitosan as a gene delivery system in vitro and in vivo

BACKGROUND

Chitosans of high molecular weights have emerged as efficient nonviral gene delivery systems, but the properties and efficiency of well-defined low molecular weight chitosans (<5 kDa) have not been studied. We therefore characterized DNA complexes of such low molecular weight chitosans and related their physical shape and stability to their efficiency as gene delivery systems in vitro and in vivo.

METHODS

Individual complexes between six different chitosan oligomers (6-, 8-, 10-, 12-, 14- and 24-mers) and fluorescence-labeled T4 DNA were visualized and classified into six physical shapes using video-enhanced fluorescence microscopy. The effects of chitosan chain length, charge ratio (+/-) and solvent properties (pH and ionic strength) on the stability and structure of the complexes were studied. Gene expression in vitro and in vivo were studied using a luciferase reporter gene.

RESULTS

Free DNA appeared as extended coils. Chitosan complexes had a variety of physical shapes depending on the experimental conditions. In general, the fraction of complexes that had nonaggregated, globular structures increased with increasing chain length of the chitosan oligomer, increasing charge ratio and reduction of pH (from 6.5 to 3.5). A further increase in charge ratio for globular complexes or a further reduction in pH (to 2.5) increased the fraction of aggregates, indicating a window where pharmaceutically desirable globules are obtained. Gene transfection efficiencies in vitro and in vivo were related to the physical shape and stability of the complexes. Only the 24-mer formed stable complexes that gave a high level of gene expression comparable to that of high molecular weight ultrapure chitosan (UPC) in vitro and in vivo.

CONCLUSIONS

Chitosan oligomers form complexes with DNA in a structure-dependent manner. We conclude that the 24-mer, which has more desirable physical properties than UPC, is more attractive as a gene delivery system than the conventional high molecular weight chitosans.

Tetanus antigen modulates the gene expression profile of aluminum phosphate adjuvant in spleen lymphocytes in vivo

Adjuvants play an important role in stimulation of the immune response to antigens. Very little is known about the molecular mechanisms of this stimulation. Here we address this issue by studying gene expression profiles from spleen lymphocytes after in vivo immunization of mice with a clinically relevant vaccine, tetanus toxoid formulated with aluminum phosphate as adjuvant (TT(ADJ)), or the adjuvant alone (ADJ). The Th1/Th2 response to TT(ADJ) was obtained from a combination of up- and downstream markers to conventional cytokines, which were in good agreement with cytokine protein levels. A clustering algorithm revealed that ADJ elicited expression of 47 genes active in cytotoxic lymphocytes, inflammation, oncogenesis, stress, toxicity and cell cycle regulation. In TT(ADJ) these adjuvant-elicited genes were expressed at lower levels and a compensatory onset of protective and inhibitory genes was observed. We conclude that the antigen, to a larger extent than previously recognized, modulates the molecular mechanism of the aluminum phosphate adjuvant and that the identified genes may serve as predictive biomarkers in the development of new adjuvants and vaccines.

CYP3A4, CYP3A5, and MDR1 in human small and large intestinal cell lines suitable for drug transport studies

The aim of this study was to find a cell culture model of the intestinal epithelium for use in studies of CYP3A4-mediated first-pass metabolism of drugs and also for studies of the interplay between CYP3A4 metabolism and P-glycoprotein efflux. For this purpose, the expression of CYP3A4, CYP3A5, and MDR1 mRNA was studied in three cell lines of the normal human intestinal epithelium and three transformed cell lines of colonic (Caco-2) origin. Surprisingly, only transformed cell lines were induced by 1alpha,25-dihydroxy vitamin D3 (D3) to express high amounts of CYP3A4. In contrast to the original findings for this model, the monolayer integrity was maintained during D3 treatment. Levels of CYP3A mRNA expression in Caco-2 and TC7 cells differed dramatically. The highest levels of CYP3A4 and lowest levels of CYP3A5 mRNA expression were observed in D3 treated Caco-2 cells of high passage numbers, resulting in a CYP3A4/3A5 expression ratio greater than fourfold higher than that seen in TC7 cells. Functional studies, using the CYP3A probe testosterone, showed that CYP3A activity was completely absent only in uninduced Caco-2 cells. After D3 induction, high levels of the metabolite were produced in both cell lines (149.4 +/- 12.3 and 86.5 +/- 3.8 pmol 6beta-OH testosterone/min/mg cellular protein from 75 microM testosterone in Caco-2 and TC7 cells, respectively). The maximum velocity (Vmax) and the apparent Michaelis constant (Km) for the 6beta-hydroxylation of testosterone by CYP3A4 in intact Caco-2 monolayers were similar to those obtained from human intestinal microsomes. Only minor changes in P-glycoprotein activity were observed when the metabolically stable P-glycoprotein substrate celiprolol was used. In conclusion, these results show that the features of the generally available Caco-2 cell line from American Type Culture Collection make it suitable for studies of CYP3A4-mediated first-pass metabolism and also for studies of the interplay between CYP3A4 and drug efflux mechanisms.

Correlation of gene expression of ten drug efflux proteins of the ATP-binding cassette transporter family in normal human jejunum and in human intestinal epithelial Caco-2 cell monolayers

This investigation describes the expression and interindividual variability in transcript levels of multiple drug efflux systems in the human jejunum and compares the expression profiles in these cells with that of the commonly used Caco-2 cell drug absorption model. Transcript levels of ten-drug efflux proteins of the ATP-binding cassette (ABC) transporter family [MDR1, MDR3, ABCB5, MRP1-6, and breast cancer resistance protein (BCRP)], lung resistance-related protein (LRP), and CYP3A4 were determined using quantitative polymerase chain reaction in jejunal biopsies from 13 healthy human subjects and in Caco-2 cells. All genes except ABCB5 were expressed, and transcript levels varied between individuals only by a factor of 2 to 3. Surprisingly, BCRP and MRP2 transcripts were more abundant in jejunum than MDR1 transcripts. Jejunal transcript levels of the different ABC transporters spanned a range of three log units with the rank order: BCRP approximately MRP2 > MDR1 approximately MRP3 approximately MRP6 approximately MRP5 approximately MRP1 > MRP4 > MDR3. Furthermore, transcript levels of 9 of 10 ABC transporters correlated well between jejunum and Caco-2 cells (r2 = 0.90). However, BCRP exhibited a 100-fold lower transcript level in Caco-2 cells compared with jejunum. Thus, the expression of a number of efflux protein transcripts in jejunum are equal to, or even higher than, that of MDR1, suggesting that the roles of these proteins (in particular BCRP and MRP2) in intestinal drug efflux have been underestimated. Also, we tentatively conclude that the Caco-2 cell line is a useful model of jejunal drug efflux, if the low expression of BCRP is taken into account.

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.

Chitosan as a nonviral gene delivery system. Structure-property relationships and characteristics compared with polyethylenimine in vitro and after lung administration in vivo

Chitosan is a natural cationic linear polymer that has recently emerged as an alternative nonviral gene delivery system. We have established the relationships between the structure and the properties of chitosan-pDNA polyplexes in vitro. Further, we have compared polyplexes of ultrapure chitosan (UPC) of preferred molecular structure with those of optimised polyethylenimine (PEI) polyplexes in vitro and after intratracheal administration to mice in vivo. Chitosans in which over two out of three monomer units carried a primary amino group formed stable colloidal polyplexes with pDNA. Optimized UPC and PEI polyplexes protected the pDNA from serum degradation to approximately the same degree, and they gave a comparable maximal transgene expression in 293 cells. In contrast to PEI, UPC was non toxic at escalating doses. After intratracheal administration, both polyplexes distributed to the mid-airways, where transgene expression was observed in virtually every epithelial cell, using a sensitive pLacZ reporter containing a translational enhancer element. However, the kinetics of gene expression differed - PEI polyplexes induced a more rapid onset of gene expression than UPC. This was attributed to a more rapid endosomal escape of the PEI polyplexes. Although this resulted in a more efficient gene expression with PEI polyplexes, UPC had an efficiency comparable to that of commonly used cationic lipids. In conclusion, this study provides insights into the use of chitosan as a gene delivery system. It emphasises that chitosan is a nontoxic alternative to other cationic polymers and it forms a platform for further studies of chitosan-based gene delivery systems.

Experimental and computational screening models for the prediction of intestinal drug absorption

The aim of this study was to devise experimental protocols and computational models for the prediction of intestinal drug permeability. Both the required experimental and computational effort and the accuracy and quality of the resulting predictions were considered. In vitro intestinal Caco-2 cell monolayer permeabilities were determined both in a highly accurate experimental setting (Pc) and in a faster, but less accurate, mode (Papp). Computational models were built using four different principles for generation of molecular descriptors (atom counts, molecular mechanics calculations, fragmental, and quantum mechanics approaches) and were evaluated for their ability to predict intestinal membrane permeability. A theoretical deconvolution of the polar molecular surface area (PSA) was also performed to facilitate the interpretation of this composite descriptor and allow the calculation of PSA in a simplified and fast mode. The results indicate that it is possible to predict intestinal drug permeability from rather simple models with little or no loss of accuracy. A new, fast computational model, based on partitioned molecular surface areas, that predicts intestinal drug permeability with an accuracy comparable to that of time-consuming quantum mechanics calculations is presented.

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.

Expression of specific markers and particle transport in a new human intestinal M-cell model

The aim of this work was to establish a new, simplified in vitro model of the human M-cell. Cocultures of physically separated human intestinal epithelial Caco-2 cells and B-cell lymphoma Raji cells were established. The cocultures were characterized under the criteria of morphology, integrity, expression of M-cell markers and cell adhesion molecules (CAMs), and altered particle transport. Using this construct, the epithelial cells were transformed to cells with an M-cell-like morphology and had altered expression of potential human M-cell markers (alkaline phosphatase down-regulation and Sialyl Lewis A antigen up-regulation). The expression of intercellular adhesion molecule-1 and vascular cell adhesion molecule was altered and there was an increased binding of lectins wheat germ agglutinin and peanut agglutinin with a 40-fold increase in microparticle transport. The particle transport was size-dependent and could be inhibited at 4 degrees C or by replacing the Raji B-cells with Jurkat T-cells. This new coculture model will enable controlled studies of M-cell development and function in vitro.

A new method for preparing biodegradable microparticles and entrapment of hydrocortisone in DL-PLG microparticles using supercritical fluids

An improved process for the production of polymeric microparticles, based on solution-enhanced dispersion by supercritical fluids (SEDS) using a combination of supercritical N(2) and CO(2), was evaluated. The biodegradable polymers, poly(D,L-lactide-co-glycolide): copolymer composition 50:50 (DL-PLG), poly(L-lactide) (L-PLA), poly(D,L-lactide) (DL-PLA) and polycaprolactone, were used for preparation of microparticles by a modified SEDS process. Solutions of the polymers in organic solvents were dispersed and the solvent was extracted with supercritical CO(2) and N(2). The morphology, the size distributions and degree of hydrocortisone entrapment were determined. The combination of supercritical N(2) and CO(2) led to a more efficient dispersion of the polymer solutions than CO(2) alone. This resulted in a reduction of particle size of the microparticles produced from all of the amorphous polymers. Discrete spherical microparticles with a mean volumetric diameter of less than 10 microm were produced from DL-PLG, DL-PLA and L-PLA. Hydrocortisone was successfully entrapped within the DL-PLG microparticles. The modified SEDS process improved the dispersion of amorphous polymer solutions resulting in formation of small spherical microparticles. The SEDS process can be used for incorporation of drugs into the DL-PLG microparticles.

Virtual screening of intestinal drug permeability

Lead compounds generated in high throughput drug discovery programmes often have unfavorable biopharmaceutical properties, resulting in a low success rate of such drug candidates in clinical development. Drug companies and researchers would thus like to have methods of predicting biopharmaceutical properties accurately. The intestinal permeability to a lead compound is one such property which is particularly important. Therefore, access to methods to accurately predict biopharmaceutical properties, such as the intestinal permeability of a large series of compounds, is of particular importance. This review deals with new theoretical methods used to predict intestinal drug permeability. There are several possible transport routes across the intestine, but theoretical methods generally deal with only one of them, the passive transcellular route. Therefore, this review will also discuss the relative importance of passive and active drug transport and efflux routes using recent data generated in cell cultures, animal models and human subjects.

Effect of molecular charge on intestinal epithelial drug transport: pH-dependent transport of cationic drugs

The aim of this study was to investigate the effect of ionization on drug transport across the intestinal epithelium in order to include this effect in structure-absorption relationships. The pH-dependent permeation of one rapidly (alfentanil) and one slowly (cimetidine) transported basic model drug across Caco-2 cell monolayers was investigated. Both drugs had pK(a)values in the physiological pH range. The permeability coefficients (P(c)) of the model drugs were obtained at varying apical buffer pHs, thus varying the degree of drug ionization (from 5 to 95%). The relationship between P(c) and the fraction of the drug in un-ionized form (f(u)) was analyzed to delineate the permeability coefficients of the un-ionized (P(c,u)) and ionized (P(c,i)) forms of the drugs. Theoretical estimates of the pK(a) values were also calculated from ionization energies for each model compound. For both drugs, a linear increase in P(c) was observed with increasing f(u). Transport of the un-ionized form was 150- and 30-fold more rapid than transport of the ionized form for alfentanil and cimetidine, respectively. However, when f(u) <0.1, the contribution of the ionized form was significant. Because f(u) is <0.1 over the entire physiological pH range for a large number of drugs, these results will have implications on predictions of in vivo intestinal drug absorption both from in vitro studies in cell cultures and from computed structural properties of drug molecules.

Prediction of the intestinal absorption of endothelin receptor antagonists using three theoretical methods of increasing complexity

PURPOSE

Three new computational strategies have been evaluated for their ability to predict intestinal membrane permeability to a series of endothelin receptor antagonists.

METHODS

The three methods were evaluated using a set of ten nonpeptide endothelin receptor antagonists. The simplest method, "the rule of five", is based on 2D parameters such as the number of potential hydrogen bonds, molecular weight and calculated lipophilicity. A method based on molecular mechanics calculations is used to calculate 3D parameters such as polar and non-polar parts of the molecular surface area. The third method uses quantum mechanics to calculate molecular properties related to the valence region.

RESULTS

Descriptors derived by the latter two methods correlated well with permeability coefficients of the endothelin receptor antagonists. On the other hand, the rule of five failed to discriminate between drugs with high and low permeability.

CONCLUSIONS

Molecular surface descriptors and descriptors derived from quantum mechanics are potentially useful for the virtual screening of the permeability of the intestinal membrane to endothelin receptor antagonists.

Paracellular drug transport across intestinal epithelia: influence of charge and induced water flux

The influence of drug charge and transepithelial water flux on passive paracellular drug transport was investigated in Caco-2 cell monolayers and rat ileal mucosa in vitro. Three small hydrophilic compounds with different net charges (creatinine, erythritol and foscarnet) were used as model drugs. A hypotonic glucose-rich solution was applied apically to induce epithelial absorption of water. In the Caco-2 monolayers, permeability to creatinine (positively charged) was 25-fold greater than to foscarnet (negatively charged), indicating a pronounced cation selective paracellular permeability. During apical exposure to the hypotonic glucose-rich solution, transport of all model drugs increased in both the absorptive and secretory directions. This enhanced transport coincided with a decrease in transepithelial resistance. Further, fluorescence and transmission electron microscopy indicated dilatations of the paracellular spaces but no damage to the cell membranes. These findings suggested that the enhancement in drug transport was attributable to increased paracellular tight junction permeability rather than to "solvent drag". In the ileal segments, mucosal exposure to the hypotonic glucose-rich solution had no effect on transepithelial resistance and only a marginal increase in drug transport was observed. Taken together, the modest absorption enhancement demonstrated in the in vitro models agrees with results obtained in vivo, supporting the conclusion that a more pronounced disruption of the tight junction barrier than that obtained through stimulation of epithelial absorption of water is required for efficient enhancement of paracellular intestinal drug absorption.

Novel lipoamino acid- and liposaccharide-based system for peptide delivery: application for oral administration of tumor-selective somatostatin analogues

Lipoamino acid and liposaccharide conjugates of somatostatin analogue TT-232 were synthesized to modify the physicochemical properties of the parent peptide. The relative position, the number, and the nature of the lipid and/or saccharide moieties were varied. Experiments in vitro clearly showed that many compounds modified at the N- and/or C-terminus with lipid or sugar moieties retained the biological activity of the parent compound. An interesting construct was synthesized containing lipid and sugar units at opposite ends of the somatostatin analogue, so that the entire molecule could be considered as an amphipathic surfactant.

A conditionally immortalized epithelial cell line for studies of intestinal drug transport

A new cell culture model that better mimics the permeability of the human small intestine was developed for studies of passive drug transport. The intestinal epithelial cell line, 2/4/A1, conditionally immortalized with a temperature-sensitive mutant of the growth-promoting oncogene simian virus 40 (SV40) large T, was grown on permeable supports. The cells grew at 33 degrees C, where the oncogene is fully active, but stopped growing and entered a differentiation program at 39 degrees C, where the oncogene is inactive. Significant cell death was observed at 39 degrees C and, therefore, growth conditions under which 2/4/A1 cells survive during the differentiation process were developed. Cells grown on extracellular matrices which contained laminin at an intermediate temperature of 37 degrees C formed viable differentiated monolayers with tight junctions, an increased expression of brush border enzymes, and a paracellular permeability that was comparable to that of the human small intestine. The permeability of 17 structurally diverse drugs gave a sigmoidal relationship with the absorbed fraction of the drugs after oral administration to humans. The relationship was compared with those obtained with the well established Caco-2 model and after in vivo perfusion of the human jejunum. The transport of drugs with low permeability in 2/4/A1 monolayers was comparable to that in the human jejunum, and up to 300 times faster than that in Caco-2 monolayers. The transport of drugs with high permeability was comparable in all models. These results indicate that 2/4/A1 monolayers are promising alternatives to Caco-2 monolayers for studies of passive drug transport.

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.

Encapsulation of rotavirus into poly(lactide-co-glycolide) microspheres

Two small-scale double emulsion techniques for incorporation of formaldehyde-inactivated rotavirus particles (FRRV) into poly(lactide-co-glycolide) (PLG) microspheres were developed and optimised. The effects of high-speed homogenisation versus vortex mixing on the double emulsion stability, microsphere size, entrapment efficiency and in vitro release of FRRV in the second emulsification step were studied. A stable double emulsion was verified only when using vortex mixing in this step. Slow removal of the organic phase allowed measurement of the size of the emulsion droplets and subsequent prediction of the size of the resulting microspheres. Microspheres in the size range of 1-10 microm were prepared using both techniques. The homogenisation technique was sensitive to changes in the operating time, the emulsification energy and the volume of the outer aqueous phase, while the vortex technique was more robust. Rotavirus was released in vitro in a triphasic manner with both techniques. The more robust vortex technique was selected for preparation of PLG microspheres containing rotavirus for in vivo studies. After immunisation of mice with a single intramuscular injection, the PLG-FRRV microspheres elicited an IgG antibody response in serum detected by ELISA equally high as that elicited with FRRV alone. These results indicate that the antigenicity of FFRV was retained after incorporation into PLG microspheres using the vortex technique.

Preparation of biodegradable microparticles using solution-enhanced dispersion by supercritical fluids (SEDS)

PURPOSE

We have evaluated a new process, involving solution-enhanced dispersion by supercritical fluids (SEDS), for the production of polymeric microparticles.

METHODS

The biodegradable polymers, Poly (DL-lactide-co-glycolide): copolymer composition 50:50 (DL-PLG), Poly (L-lactide) (L-PLA), Poly (DL-lactide) (DL-PLA) and Polycaprolactone (PCL), were used for preparation of microparticles using SEDS. Solutions of the polymers in organic solvents were dispersed and sprayed with supercritical CO2. Extraction of the organic solvents resulted in the formation of solid microparticles. The amounts of highly toxic solvents such as dichloromethane (MC) were reduced in the process.

RESULTS

Microparticles were obtained from all polymers. The mean particle size and shape varied with the polymer used. The morphology of the particles was strongly affected by the choice of polymer solvent. Discrete spherical microparticles of DL-PLG were produced with a mean volumetric diameter of 130 microm. The microparticles of the L-PLA were almost spherical, and their size increased from 0.5 to 5 microm as the density of supercritical CO2 decreased. PCL formed microparticles with diameters of 30-210 microm and showed a strong tendency to form films at high pressure.

CONCLUSIONS

The SEDS process appears a promising method for production of microparticles from biodegradable polymers without the use of toxic solvents.

Interplay between CYP3A-mediated metabolism and polarized efflux of terfenadine and its metabolites in intestinal epithelial Caco-2 (TC7) cell monolayers

PURPOSE

To further characterize cytochrome P450 (CYP) and P-glycoprotein (Pgp) expression in monolayers of the Caco-2 cell clone TC7, a cell culture model of the human intestinal epithelium. To study the interplay between CYP3A and Pgp as barriers to intestinal drug absorption in TC7 cells using terfenadine and its metabolites as substrates.

METHODS

mRNA expression of eight CYPs and Pgp was investigated in TC7 and parental Caco-2 (Caco-2p) cell monolayers using RT-PCR. The CYP3A kinetics was determined in microsomes from both cell lines. The transport, metabolism and efflux of terfenadine and its metabolites were investigated in TC7 monolayers.

RESULTS

Both TC7 and Caco-2p cells expressed mRNA for Pgp and several important CYPs. However, mRNA for CYP3A4 was detectable anly from TC7 cells. The relative affinity of CYP3A for terfenadine metabolism in the two cell lines was comparable, but the maximum reaction rate in the TC7 cells was 8-fold higher. The rate of transport of terfenadine and its metabolites hydroxy-terfenadine (HO-T) and azacyclonol across TC7 monolayers was 7.1-, 3.5- and 2.1-fold higher, respectively, in the basolateral to apical direction than it was in the apical to basolateral (AP-BL) direction. Inhibition studies indicated that the efflux was mediated by Pgp. Ketoconazole increased the AP-BL transport terfenadine dramatically by inhibiting both terfenadine metabolism and Pgp efflux.

CONCLUSIONS

Cell culture models such as TC7 provide qualitative information on drug interactions involving intestinal CYP3A and Pgp.

Theoretical calculation and prediction of intestinal absorption of drugs in humans using MolSurf parametrization and PLS statistics

A method for modeling and prediction of the intestinal absorption of drugs in humans using theoretically computed molecular descriptors and multivariate statistics has been investigated using 20 diverse drug-like compounds. The program MolSurf was used to compute theoretical molecular descriptors related to physicochemical properties such as lipophilicity, polarity, polarizability and hydrogen bonding. The multivariate Partial Least Squares Projections to Latent Structures (PLS) method was used to delineate the relationship between the intestinal absorption of drugs in humans and the theoretically computed molecular descriptors.Good statistical models were derived. Properties associated with hydrogen bonding had the largest impact on absorption and should be kept to a minimum to promote high absorption. High charge-transfer properties and the presence of surface electrons, i.e. valence electrons, which are not tightly bonded to the molecule, were also found to promote high absorption.

Prediction of membrane permeability to peptides from calculated dynamic molecular surface properties

PURPOSE

To develop a theoretical method for prediction of transcellular permeability to peptides.

METHODS

The dynamic molecular surface properties of 19 oligopeptide derivatives, divided into three homologous series were calculated. The dynamic molecular surface properties were compared with commonly used experimental predictors of membrane permeability such as partition coefficients. Relationships between the dynamic molecular surface properties and intestinal epithelial permeability, as determined in Caco-2 cell monolayers, were used to develop a model for prediction of the transmembrane permeability to the oligopeptide derivatives.

RESULTS

A theoretical model was derived which takes both the polar and non-polar part of the dynamic molecular surface area of the investigated molecule into consideration. The model provided a strong relationship with transepithelial permeability for the oligopeptide derivatives. The predictability of transepithelial permeability from this model was comparable to that from the best experimental descriptor.

CONCLUSIONS

To our knowledge, this is the first example of a theoretical model that gives a satisfactory relationship between calculated molecular properties and epithelial permeability to peptides by accounting for both the hydrogen bonding capacity and the hydrophobicity of the investigated molecule. This model may be used to differentiate poorly absorbed oligopeptide drugs at an early stage of the drug discovery process.

Evaluation of dynamic polar molecular surface area as predictor of drug absorption: comparison with other computational and experimental predictors

The relationship between various molecular descriptors and transport of drugs across the intestinal epithelium was evaluated. The monolayer permeability (Pc) of human intestinal Caco-2 cells to a series of nine beta-receptor-blocking agents was investigated in vitro. The dynamic polar molecular surface area (PSAd) of the compounds was calculated from all low-energy conformations identified in molecular mechanics calculations in vacuum and in simulated chloroform and water environments. For most of the investigated drugs, the effects of the different environments on PSAd were small. The exception was H 216/44, which is a large flexible compound containing several functional groups capable of hydrogen bonding (PSAd,chloroform = 70.8 A2 and PSAd,water = 116.6 A2). The relationship between Pc and PSAd was stronger than those between Pc and the calculated octanol/water distribution coefficients (log Dcalc) or the experimentally determined immobilized liposome chromatography (ILC) retention. Pc values for two new practolol analogues and H 216/44 were predicted from the structure-permeability relationships of a subset of the nine compounds and compared with experimental values. The Pc values of the two practolol analogues were predicted well from both PSAd calculations and ILC retention studies. The Pc value of H 216/44 was reasonably well-predicted only from the PSAd of conformations preferred in vacuum and in water. The other descriptors overestimated the Pc of H 216/44 100-500-fold.

Absorption enhancement in intestinal epithelial Caco-2 monolayers by sodium caprate: assessment of molecular weight dependence and demonstration of transport routes

Sodium caprate (C10), a medium chain fatty acid, is used clinically to enhance rectal absorption of the low molecular weight (MW) drug ampicillin. The main aim of this study was to investigate whether C10 also enhances the permeability of high MW model drugs in a model of the intestinal epithelium. The second aim was to present visual evidence of the route of enhanced transport across the epithelial cell layer. The studies were performed in Caco-2 monolayers cultured on permeable supports. The effects of non-toxic concentrations (< or = 13 mM) of C10 on drug transport across the monolayers was studied using monodisperse 14C-polyethylene glycols (MW 238-502; 14C-PEGs), 125I-Arg5-vasopressin (MW 1,208), 125I-insulin (MW 6,000) and FITC-labelled dextrans (MW 4,400 and 19,600; FD4 and FD20 respectively) as model drugs. Electron and confocal laser scanning microscopy were used to demonstrate transport routes across the epithelium. 10 mM C10 increased the permeability of all 14C-PEGs to approximately the same extent. 13 mM C10 increased the permeability of 125I-Arg8-vasopressin 10-fold. Only small increases in FD4 and FD20 permeabilities were observed. After C10 exposure, both tight junctions with normal morphology and those with dilatations showed an increased permeability to ruthenium red, indicating that C10 enhanced the paracellular transport of molecules with a MW < 1,000. Confocal microscopy showed that C10 increased the transport of FD4 and FD20 by the paracellular route. In conclusion, non-toxic concentrations of C10 can be used to enhance the permeability of drugs of MW up to approximately 1,200. Enhancement of the absorption of molecules larger than 4,000 is quantitatively insignificant. The enhanced permeability occurred via the paracellular pathway.

Integrity and metabolism of human ileal mucosa in vitro in the Ussing chamber

The Ussing chamber is increasingly being used for in vitro studies of human intestinal mucosa, but little attention has been paid to the viability of specimens over time. Ninety-one mucosal specimens from the ileum in 19 patients operated on for colonic cancer were studied in regard to intestinal barrier function, metabolism, electrophysiology and histology during 360 min of incubation in Ussing chambers. Steady-state permeability to 51Cr-EDTA was maintained for 120 min. Mucosal ATP and lactate levels were stable for 180 min and transmucosal glucose flux for 240 min. Lactate dehydrogenase leakage was limited within 120 min. Transepithelial potential difference was 9.0 +/- 3.0 mV at the start, and declined slowly throughout 360 min. Light microscopy revealed epithelial lifting from the basal lamina at 90 min. Transmission electron microscopy demonstrated preserved ultrastructure for 120 min. Specimens with a transepithelial potential difference below 6 mV at the start were associated with increased 51Cr-EDTA permeability and lactate dehydrogenase leakage and more pronounced light microscopy changes. All studied parameters pointed to preserved viability if experiments were kept within a period of 90 min after equilibration. The few specimens with early viability derangement were identified by a transepithelial potential difference below 6 mV at the start. The Ussing chamber provides a tool for in vitro studies of human intestinal epithelium, including permeability. To minimize viability problems, experiments should be limited in time and monitored by measurements of transepithelial potential difference.

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.

Absorption enhancement through intracellular regulation of tight junction permeability by medium chain fatty acids in Caco-2 cells

Medium chain fatty acids (MCFAs) are used to enhance the permeability of mucosal tissues to hydrophilic drugs, but their mechanism of action is largely unknown. In this study, the absorption-enhancing effects of the sodium salts of two MCFAs, capric acid (C10) and lauric acid (C12), were studied in monolayers of human intestinal epithelial Caco-2 cells. Both MCFAs induced a rapid increase in epithelial permeability to the hydrophilic marker molecule sodium fluorescein. Inhibition of phospholipase C and inhibition or activation of various kinases and buffering of intracellular calcium indicated that the effects on epithelial permeability were mediated through phospholipase C-dependent inositol triphosphate/diacylglycerol pathways. Surprisingly, the inositol triphosphate and diacylglycerol pathways were found to have opposing effects on paracellular permeability. Exposure to the MCFAs also resulted in a concentration dependent reduction of cellular dehydrogenase activity and ATP levels. C10, but not C12, induced redistribution of the tight junction proteins ZO-1 and occludin. These results indicate that the two MCFAs have partially different and more complex mechanisms than previously recognized, which has important implications for their use in vivo.

Theoretical calculation and prediction of Caco-2 cell permeability using MolSurf parametrization and PLS statistics

PURPOSE

To statistically model the permeability across Caco-2 cell monolayers using theoretically computed molecular descriptors and multivariate statistics.

METHODS

Seventeen structurally diverse compounds were investigated. The program MolSurf was used to compute theoretical molecular descriptors related to physico-chemical properties such as lipophilicity, polarity, polarizability and hydrogen bonding. The multivariate Partial Least Squares Projections to Latent Structures (PLS) method was used to delineate the relationship between the permeability across Caco-2 cell monolayers and the theoretically computed molecular descriptors.

RESULTS

Excellent statistical models were derived. Properties associated with hydrogen bonding had the largest impact on diffusion through the monolayers and should be kept at a minimum to promote high permeability. High lipophilicity and the presence of surface electrons, i.e. valence electrons, which are not tightly bonded to the molecule, were also found to have a favorable influence to achieve high permeability.

CONCLUSIONS

The results indicate that theoretically computed molecular MolSurf descriptors in conjunction with multivariate statistics of PLS type can be used to successfully model permeability across Caco-2 cell monolayers and, thus, differentiate drugs with poor permeability from those with acceptable permeability at an early stage of the preclinical drug discovery process.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Chitosans as absorption enhancers for poorly absorbable drugs 2: mechanism of absorption enhancement

PURPOSE

It has recently been shown that the absorption enhancing and toxic effects of chitosans are dependent on their chemical composition. In this study, the mechanisms underlying these effects were investigated at the cellular level.

METHODS

The effects on epithelial cells of chitosans with different chemical composition, absorption enhancing properties and toxicities were studied in Caco-2 monolayers. Chitosan C( 1:31) has a low degree of acetylation (DA) (1%) and a low m.w. (31 kD), and displays dose-dependent absorption enhancement and cytotoxicity; chitosan C(35:170) has a higher DA (35%) and a higher m.w. (170 kD), is less dose-dependent in absorption enhancement, and is not cytotoxic. A third non-toxic chitosan C(49:22) with a high DA (49%), a low m.w. (22 kD), and no influence on epithelial permeability was used as control.

RESULTS

C(1:31) and C(35:170) bound tightly to the epithelium. Cellular uptake of the chitosans was not observed. Both chitosans increased apical but not basolateral cell membrane permeability and induced a redistribution of cytoskeletal F-actin and the tight junction protein ZO-1. This resulted in increased paracellular permeability of hydrophilic marker molecules of different molecular weights. Addition of negatively charged heparin inhibited the cellular and the absorption enhancing effects of the chitosans, indicating that these effects are mediated via their positive charges. The onset of the effects of C(35:170) on apical membrane permeability and tight junction structure was much faster than that of C(1:31). C(49:22) did not influence any of the properties of the Caco-2 cell monolayers studied.

CONCLUSIONS

The binding and absorption enhancing effects of chitosans on epithelial cells are mediated through their positive charges. The interaction of chitosans with the cell membrane results in a structural reorganisation of tight junction-associated proteins which is followed by enhanced transport through the paracellular pathway.

Mechanism of absorption enhancement in humans after rectal administration of ampicillin in suppositories containing sodium caprate

PURPOSE

The medium chain fatty acid sodium caprate (C10) is approved as an absorption enhancer but its mechanism of action has not been studied in humans. The aim of this study was to investigate the mechanism of action of C10 in human subjects after rectal administration.

METHODS

Twelve healthy human subjects were randomised to receive ampicillin suppositories with (AM-C10) or without (AM) C10. Serum and urine samples were collected and analysed for ampicillin by HPLC. Rectal biopsies were taken before and 25 min (approximate time of maximum serum concentration, Cmax, for ampicillin) and 185 min (during the final part of the elimination phase) after rectal administration of the suppositories. The osmolality of the rectal fluid was also measured.

RESULTS

AM-C10 administration increased Cmax, area under the serum concentration-time curve (AUC) and urinary recovery of ampicillin 2.6-, 2.3- and 1.8-fold, respectively, compared to AM. Histological examination of the biopsies showed that AM-C10 exposure resulted in reversible mucosal damage that occurred at the same time as the Cmax for ampicillin while AM prolonged mucosal damage. A reversible increase in rectal fluid osmolality was observed with both treatments.

CONCLUSIONS

AM-C10-enhanced absorption of ampicillin coincides with non-specific damage to the rectal mucosa. C10 itself as well as the suppository base and the hyperosmolality of the rectal fluid contributed to this effect. However, the histological damage was reversible with AM-C10, suggesting that C10 also has a protective effect on the rectal mucosa.

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.

Polar molecular surface properties predict the intestinal absorption of drugs in humans

PURPOSE

A theoretical method has been devised for prediction of drug absorption after oral administration to humans.

METHODS

Twenty structurally diverse model drugs, ranging from 0.3 to 100% absorbed, were investigated. The compounds also displayed diversity in physicochemical properties such as lipophilicity, hydrogen bonding potential and molecular size. The dynamic molecular surface properties of the compounds were calculated, taking into account their three-dimensional shape and flexibility.

RESULTS

An excellent sigmoidal relationship was established between the absorbed fraction after oral administration to humans (FA) and the dynamic polar molecular surface area (PSAd) (r2 = 0.94). The relationship was stronger than those obtained for more established predictors of drug absorption. Drugs that are completely absorbed (FA > 90%) had a PSAd < or = 60 A2 while drugs that are < 10% absorbed had a PSAd > or = 140 A2.

CONCLUSIONS

The results indicate that PSAd can be used to differentiate poorly absorbed drugs at an early stage of the drug discovery process.

Chitosans as absorption enhancers for poorly absorbable drugs. 1: Influence of molecular weight and degree of acetylation on drug transport across human intestinal epithelial (Caco-2) cells

PURPOSE

Chitosan has recently been demonstrated to effectively enhance the absorption of hydrophilic drugs such as peptides and proteins across nasal and intestinal epithelia (1-3). In this study, the effect of the chemical composition and molecular weight of chitosans on epithelial permeability and toxicity was investigated using monolayers of human intestinal epithelial Caco-2 cells as a model epithelium.

METHODS

Eight chitosans varying in degree of acetylation (DA) and molecular weight were studied. The incompletely absorbed hydrophilic marker molecule 14C-mannitol was used as a model drug to assess absorption enhancement. Changes in intracellular dehydrogenase activity and cellular morphology were used to assess toxicity.

RESULTS

Chitosans with a low DA (1 and 15%) were active as absorption enhancers at low and high molecular weights. However, these chitosans displayed a clear dose-dependent toxicity. Chitosans with DAs of 35 and 49% enhanced the transport of 14C-mannitol at high molecular weights only, with low toxicity. One chitosan (DA = 35%; MW = 170 kD) was found to have especially advantageous properties such as an early onset of action, very low toxicity, and a flat dose-absorption enhancement response relationship.

CONCLUSIONS

The structural features of chitosans determining absorption enhancement are not correlated with those determining toxicity, which makes it possible to select chitosans with maximal effect on absorption and minimal toxicity.

Transport of an influenza virus vaccine formulation (iscom) in Caco-2 cells

The influenza virus envelope glycoproteins hemagglutinin and neuraminidase were administered to the apical or basolateral sides of Caco-2 monolayers either as native protein micelles (mic-ag) or after incorporation into the orally active adjuvant formulation, immune stimulating complexes (iscoms) (isc-ag). Biotin-conjugated isc-ag were localized in intracellular vesicles as early as 2 min after administration to the apical side at 37 degrees C. Ten minutes after administration, both intracellular vesicles and intercellular spaces were labeled, and extracellular labeling was observed on the basolateral side of the cells, indicating that isc-ag were transported across the epithelium within 10 min of exposure. Transport of 125I-labeled isc-ag and mic-ag in the apical-to-basolateral and basolateral-to-apical directions across Caco-2 monolayers was comparable at 37 degrees C. Gel chromatography analysis revealed that only 0.55-3.1% of transported isc-ag and mic-ag had a molecular weight of > 5,000, while 21.0-42.3% was eluted at a position corresponding to peptides of approximately 10 amino acids. Although isc-ag and mic-ag were transported and degraded by Caco-2 monolayers in comparable amounts, only transported isc-ag induced a dose-dependent proliferative response in vitro of T cells primed with influenza virus antigen. High-performance gel chromatography and reverse-phase high-performance liquid chromatography indicated that transported antigenic isc-ag consisted of hydrophobic peptides with a molecular weight of < or = 3,000. These results indicate that antigens incorporated into the orally active adjuvant formulation iscom are degraded to antigenic peptides during transport across the intestinal epithelium.

Correlation of drug absorption with molecular surface properties

The correlation between dynamic surface properties of drug molecules and drug absorption in two common in vitro models of the intestinal wall (Caco-2 monolayers and rat intestinal segments) has been investigated. A homologous series of beta-adrenoreceptor antagonists were used as model compounds. Dynamic molecular surface properties, considering all low-energy conformations, of the compounds were calculated. The flexibility of the molecules was studied by molecular mechanics calculations (MM2) and the van der Waals' (vdW), and water accessible surface areas were calculated and averaged according to a Boltzmann distribution. Excellent correlations were obtained between the dynamic polar vdW surface areas and cell permeabilities in Caco-2 cells and rat ileum (r2 = 0.99 and 0.92, respectively). These correlations were stronger than those between calculated octanol/buffer partition coefficients (log Doct,7.4) and permeability (r2 = 0.80 and 0.73, respectively). Moreover, the calculated log Doct,7.4 values failed to rank the permeability coefficients through Caco-2 monolayers and rat ileum in the correct order. The results indicate that dynamic polar surface area is a promising alternative model for the prediction of oral drug absorption.