Effect of changing intestinal flow rate on a measurement of intestinal permeability

In Vitro Modelling of a Typical Dietary Intake in Restrictive Anorexia Nervosa Results in Changes to Gut Microbial Community and Metabolites

Anorexia nervosa (AN) is a psychiatric illness with harmful physical consequences. Studies have observed differences in the faecal microbiota of patients with AN compared to healthy controls. Diet has an impact on the gut microbiota, facilitating an altered community, such changes could impact the gut–brain axis. In this study, a three-stage gut model system that mimics the luminal microbiology of the large intestine was conducted to identify relationships between diet and gut microbiota. A microbial medium was developed to provide nutrients more appropriate to restricting subtype AN (R-AN). The model was inoculated with faeces and samples were taken to compare differences in the microbiota and end products following the fermentation of healthy control medium (HC) compared to R-AN medium. Then, 16S amplicon sequencing along with flow cytometry–fluorescence in situ hybridisation were used to ascertain changes in the microbiota. Gas chromatography (GC) was used to assess changes in microbial metabolites. There were reduced levels of SCFA following the fermentation of R-AN medium. The fermentation of R-AN media led to fewer total bacteria numbers, along with less bifidobacteria and Rumincoccus proximally, but more Clostridium and Enterobacteriaceae. Nutrient-deficient medium resulted in reduced neurotransmitter-producing bacteria, reduced butyrate-producing bacteria, and increased protein-utilising bacteria, all of which could be maintaining factors in AN. The model system provides a novel tool for exploring how extreme dietary changes impact the microbiota and could therefore could be useful for assessing appropriate gut–brain targeted treatments.

Oral Nanomedicine: Challenges and Opportunities

Compared to injection administration, oral administration is free of discomfort, wound infection, and complications and has a higher compliance rate for patients with diverse diseases. However, oral administration reduces the bioavailability of medicines, especially biologics (e.g., peptides, proteins, and antibodies), due to harsh gastrointestinal biological barriers. In this context, the development and prosperity of nanotechnology have helped improve the bioactivity and oral availability of oral medicines. On this basis, first, the biological barriers to oral administration are discussed, and then oral nanomedicine based on organic and inorganic nanomaterials and their biomedical applications in diverse diseases are reviewed. Finally, the challenges and potential opportunities in the future development of oral nanomedicine, which may provide a vital reference for the eventual clinical transformation and standardized production of oral nanomedicine, are put forward.

Engineering Escherichia coli for diagnosis and management of hyperuricemia

Uric acid disequilibrium is implicated in chronic hyperuricemia-related diseases. Long-term monitoring and lowering of serum uric acid levels may be crucial for diagnosis and effective management of these conditions. However, current strategies are not sufficient for accurate diagnosis and successful long-term management of hyperuricemia. Moreover, drug-based therapeutics can cause side effects in patients. The intestinal tract plays an important role in maintaining healthy serum acid levels. Hence, we investigated the engineered human commensal Escherichia coli as a novel method for diagnosis and long-term management of hyperuricemia. To monitor changes in uric acid concentration in the intestinal lumen, we developed a bioreporter using the uric acid responsive synthetic promoter, pucpro, and uric acid binding Bacillus subtilis PucR protein. Results demonstrated that the bioreporter module in commensal E. coli can detect changes in uric acid concentration in a dose-dependent manner. To eliminate the excess uric acid, we designed a uric acid degradation module, which overexpresses an E. coli uric acid transporter and a B. subtilis urate oxidase. Strains engineered with this module degraded all the uric acid (250 µM) found in the environment within 24 h, which is significantly lower (p < 0.001) compared to wild type E. coli. Finally, we designed an in vitro model using human intestinal cell line, Caco-2, which provided a versatile tool to study the uric acid transport and degradation in an environment mimicking the human intestinal tract. Results showed that engineered commensal E. coli reduced (p < 0.01) the apical uric acid concentration by 40.35% compared to wild type E. coli. This study shows that reprogramming E. coli holds promise as a valid alternative synthetic biology therapy to monitor and maintain healthy serum uric acid levels.

Paracellular permeability and tight junction regulation in gut health and disease

Epithelial tight junctions define the paracellular permeability of the intestinal barrier. Molecules can cross the tight junctions via two distinct size-selective and charge-selective paracellular pathways: the pore pathway and the leak pathway. These can be distinguished by their selectivities and differential regulation by immune cells. However, permeability increases measured in most studies are secondary to epithelial damage, which allows non-selective flux via the unrestricted pathway. Restoration of increased unrestricted pathway permeability requires mucosal healing. By contrast, tight junction barrier loss can be reversed by targeted interventions. Specific approaches are needed to restore pore pathway or leak pathway permeability increases. Recent studies have used preclinical disease models to demonstrate the potential of pore pathway or leak pathway barrier restoration in disease. In this Review, we focus on the two paracellular flux pathways that are dependent on the tight junction. We discuss the latest evidence that highlights tight junction components, structures and regulatory mechanisms, their impact on gut health and disease, and opportunities for therapeutic intervention.

Comparisons of in Vitro Models to Evaluate the Membrane Permeability of Amorphous Drug Nanoparticles

The spontaneous formation of amorphous drug nanoparticles following the release of a drug from a supersaturating formulation is gaining increasing attention due to their potential contribution to increased oral bioavailability. The formation of nanosized drug particles also has considerable implications for the interpretation of in vitro and in vivo data. However, the membrane transport properties of these drug particles remain less well understood. Herein, the membrane permeation of nanosized amorphous drug particles of a model drug atazanavir was evaluated using different artificial membrane-based, cell-based, and animal tissue-based models. Results showed that flux enhancement by particles was different for the various systems used. Generally, good agreement was obtained among experiments performed using the same apparatus with different model membranes, with the exception of the Madin-Darby canine kidney cell monolayer and the Long-Evans rat intestine tissue, which showed lower flux enhancements. Franz cell-based models showed slightly higher flux enhancements by particles compared to Transwell and intestinal tissue sac models. Mass transport analysis suggested that the extent of flux enhancement by particles is dependent on the geometry of the apparatus as well as the properties of the membrane and buffer used, whereas the flux plateau concentration is dependent on the unstirred water later (UWL) asymmetry. These results highlight the complexity in characterizing the permeability advantage of these nonmembrane permeable drug particles and suggest that caution should be used in selecting the appropriate in vitro model to evaluate the overall permeability of colloidal drug particles.

Wireless soft millirobots for climbing three-dimensional surfaces in confined spaces

Wireless soft-bodied robots at the millimeter scale allow traversing very confined unstructured terrains with minimal invasion and safely interacting with the surrounding environment. However, existing untethered soft millirobots still lack the ability of climbing, reversible controlled surface adhesion, and long-term retention on unstructured three-dimensional (3D) surfaces, limiting their use in biomedical and environmental applications. Here, we report a fundamental peeling-and-loading mechanism to allow untethered soft-bodied robots to climb 3D surfaces by using both the soft-body deformation and whole-body motion of the robot under external magnetic fields. This generic mechanism is implemented with different adhesive robot footpad designs, allowing vertical and inverted surface climbing on diverse 3D surfaces with complex geometries and different surface properties. With the unique robot footpad designs that integrate microstructured adhesives and tough bioadhesives, the soft climbing robot could achieve controllable adhesion and friction to climb 3D soft and wet surfaces including porcine tissues, which paves the way for future environmental inspection and minimally invasive medicine applications.

Engineering in vitro immune-competent tissue models for testing and evaluation of therapeutics

Advances in 3D cell culture, microscale fluidic control, and cellular analysis have enabled the development of more physiologically-relevant engineered models of human organs with precise control of the cellular microenvironment. Engineered models have been used successfully to answer fundamental biological questions and to screen therapeutics, but these often neglect key elements of the immune system. There are immune elements in every tissue that contribute to healthy and diseased states. Including immune function will be essential for effective preclinical testing of therapeutics for inflammatory and immune-modulated diseases. In this review, we first discuss the key components to consider in designing engineered immune-competent models in terms of physical, chemical, and biological cues. Next, we review recent applications of models of immunity for screening therapeutics for cancer, preclinical evaluation of engineered T cells, modeling autoimmunity, and screening vaccine efficacy. Future work is needed to further recapitulate immune responses in engineered models for the most informative therapeutic screening and evaluation.

Formulation strategies to improve the efficacy of intestinal permeation enhancers. Adv Drug Deliv Rev 2021;177:113925. [PMID: 34418495 DOI: 10.1016/j.addr.2021.113925]

The use of chemical permeation enhancers (PEs) is the most widely tested approach to improve oral absorption of low permeability active agents, as represented by peptides. Several hundred PEs increase intestinal permeability in preclinical bioassays, yet few have progressed to clinical testing and, of those, only incremental increases in oral bioavailability (BA) have been observed. Still, average BA values of ~1% were sufficient for two recent FDA approvals of semaglutide and octreotide oral formulations. PEs are typically screened in static in vitro and ex-vivo models where co-presentation of active agent and PE in high concentrations allows the PE to alter barrier integrity with sufficient contact time to promote flux across the intestinal epithelium. The capacity to maintain high concentrations of co-presented agents at the epithelium is not reached by standard oral dosage forms in the upper GI tract in vivo due to dilution, interference from luminal components, fast intestinal transit, and possible absorption of the PE per se. The PE-based formulations that have been assessed in clinical trials in either immediate-release or enteric-coated solid dosage forms produce low and variable oral BA due to these uncontrollable physiological factors. For PEs to appreciably increase intestinal permeability from oral dosage forms in vivo, strategies must facilitate co-presentation of PE and active agent at the epithelium for a sustained period at the required concentrations. Focusing on peptides as examples of a macromolecule class, we review physiological impediments to optimal luminal presentation, discuss the efficacy of current PE-based oral dosage forms, and suggest strategies that might be used to improve them.

Soybean oil and coconut oil enhance the absorption of chlorogenic acid in humans

The effect of a mainly polyunsaturated oil (soybean oil) and a mainly medium chain triglyceride oil (coconut oil) on the absorption of the phenolic antioxidant chlorogenic acid (5-caffeoylquinic acid) was investigated using 90 healthy volunteers. Serum concentrations and the absorbed percentages of chlorogenic acid of volunteers who received chlorogenic acid without oils (0.006 ± 0.001 mg/ml, 5.7 ± 0.2%), chlorogenic acid with soybean oil (0.012 ± 0.001 mg/ml, 11.8 ± 1.3%), and chlorogenic acid with coconut oil (0.067 ± 0.014 mg/ml, 65.6 ± 18.1%) were significantly different from each other (p < .05). There was a strong positive correlation between the increase in serum and plasma antioxidant capacity and the absorption of chlorogenic acid. The major fatty acid of each of soybean oil and coconut oil also improved the permeability of chlorogenic acid in Caco-2 cell monolayers. The results suggest that the tested edible oils may improve the nutritional value of chlorogenic acid-containing foods by improving the absorption of chlorogenic acid. PRACTICAL APPLICATIONS: Small polar antioxidants such as phenolic acids and flavonoids are poorly absorbed through the intestinal epithelium. Chlorogenic acid was used in the present study as a model for small polar phenolic antioxidants. According to the present study, soybean oil with mainly polyunsaturated fatty acids and coconut oil with mainly medium chain fatty acids improve the absorption of these antioxidants. These findings suggest that proper planning of diets or food supplements containing phenolic antioxidants with medium chain or polyunsaturated fatty acid-rich edible oils may enhance the nutritional benefits expected from phenolic antioxidants.

Go with the flow: modeling unique biological flows in engineered in vitro platforms

Interest in recapitulating in vivo phenomena in vitro using organ-on-a-chip technology has grown rapidly and with it, attention to the types of fluid flow experienced in the body has followed suit. These platforms offer distinct advantages over in vivo models with regards to human relevance, cost, and control of inputs (e.g., controlled manipulation of biomechanical cues from fluid perfusion). Given the critical role biophysical forces play in several tissues and organs, it is therefore imperative that engineered in vitro platforms capture the complex, unique flow profiles experienced in the body that are intimately tied with organ function. In this review, we outline the complex and unique flow regimes experienced by three different organ systems: blood vasculature, lymphatic vasculature, and the intestinal system. We highlight current state-of-the-art platforms that strive to replicate physiological flows within engineered tissues while introducing potential limitations in current approaches.

An Openable Artificial Intestinal Tract System Enables the Evaluation of Drug Absorption in Caco-2 Cells through the Reduction in Thickness of the Unstirred Water Layer

In drug absorption and permeability experiments, an unstirred water layer (UWL) is known to cause differences in the estimated permeability of drugs between in vitro and in vivo experiments. Therefore, it is necessary to develop a new method to allow for accurate measurements of in vitro drug absorption through the reduction of the UWL effect. Previously, we have developed an artificial intestinal tract that mimics the tubular structure of the human intestine and enables study of drug absorption under flow conditions. In order to determine whether our artificial intestinal tract has the potential to reduce the effect of a UWL on drug absorption, the present study evaluated drug absorption in Caco-2 cells using this artificial system. The viability and tight junction structure of Caco-2 cells on the artificial intestinal tract were intact during perfusion. The cumulative amount of the highly lipophilic drugs imipramine and chlorpromazine accumulated in Caco-2 cells cultured on the cell culture plate was 1.5 times higher under mechanical agitation, whereas that of cells on the artificial intestinal tract was 6.5 times higher when internal flow was applied. In addition, the cumulative amounts of 5-aminosalicylic acid and clonidine, drugs with low lipophilicity, accumulated in Caco-2 cells on the artificial intestinal tract were unchanged by internal flow. These results indicate that the artificial intestinal tract enables effective reduction of the UWL thickness at the Caco-2 cell-surface, and allows evaluation of in vitro drug absorption under conditions similar to those found in vivo.

Integrative physiology of transcellular and paracellular intestinal absorption

Glucose absorption by the small intestine has been studied for nearly a century. Despite extensive knowledge about the identity, functioning and regulation of the relevant transporters, there has been and there remains controversy about how these transporters work in concert to determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) over a wide range of dietary and/or luminal concentrations. Our broader, integrative understanding of intestinal absorption requires more than the reductionist dissection of all the components and their elaboration at molecular and genetic levels. This Commentary emphasizes the integration of discrete molecular players and processes (including paracellular absorption) that, in combination, determine the overall epithelial absorption of key nutrients (e.g. sugars, amino acids) and putative anti-nutrients (water-soluble toxins), and the integration of that absorption with other downstream processes related to metabolic demands. It identifies historic key advances, controversies and future research ideas, as well as important perspectives that arise through comparative as well as biomedical physiological research.

Microfluidic Gut-liver chip for reproducing the first pass metabolism

After oral intake of drugs, drugs go through the first pass metabolism in the gut and the liver, which greatly affects the final outcome of the drugs' efficacy and side effects. The first pass metabolism is a complex process involving the gut and the liver tissue, with transport and reaction occurring simultaneously at various locations, which makes it difficult to be reproduced in vitro with conventional cell culture systems. In an effort to tackle this challenge, here we have developed a microfluidic gut-liver chip that can reproduce the dynamics of the first pass metabolism. The microfluidic chip consists of two separate layers for gut epithelial cells (Caco-2) and the liver cells (HepG2), and is designed so that drugs go through a sequential absorption in the gut chamber and metabolic reaction in the liver chamber. We fabricated the chip and showed that the two different cell lines can be successfully co-cultured on chip. When the two cells are cultured on chip, changes in the physiological function of Caco-2 and HepG2 cells were noted. The cytochrome P450 metabolic activity of both cells were significantly enhanced, and the absorptive property of Caco-2 cells on chip also changed in response to the presence of flow. Finally, first pass metabolism of a flavonoid, apigenin, was evaluated as a model compound, and co-culture of gut and liver cells on chip resulted in a metabolic profile that is closer to the reported profile than a monoculture of gut cells. This microfluidic gut-liver chip can potentially be a useful platform to study the complex first pass metabolism of drugs in vitro.

Re-assembled casein micelles improve in vitro bioavailability of vitamin D in a Caco-2 cell model

The pandemic of vitamin D (VD) deficiency, and the global rise in obesity stimulate a need for staple low-fat foods and beverages enriched with VD. In light of consumer demand for a clean label, the use of natural endogenous food ingredients as delivery vehicles is of great interest. To this end, re-assembled casein micelles (rCM) have been shown to help retain VD during processing and shelf life and provide high bioavailability in low-fat milk and non-fat yoghurt. This follow-up study focused on the performance of VD-loaded rCM after drying and reconstitution, considering VD retention during simulated digestion, and the subsequent in vitro bioavailability of the vitamin. rCM conferred great protection to VD₃ during simulated digestion with a significant increase in vitamin retention for 1 h under gastric conditions. This observation is believed to arise from the vitamin-casein binding and the system's natural gelation (curd formation) near the casein isoelectric point that seclude the vitamin from environmental stressors and couple its release with digestive proteolysis of the rCM matrix. Vitamin absorption by Caco-2 cells from digested rCM was not significantly different from the absorption of the digested free VD. However, thanks to the highly protective effect of the rCM, against VD gastric degradation, the overall effect of the rCM was a 4-fold higher bioavailability, compared to the free VD.

DFT study of the physicochemical properties of A- and B-type procyanidin oligomers

The objective of this study was to determine the physicochemical properties of the oligomers of procyanidins (PCs) including PA1, PA2, PB1, PB2, PC1 and a B-type tetramer, taking into account of their conformations related to the interflavan links using the density functional theory (DFT). This information may provide useful insight into the potential effect of physicochemical properties on the absorption of PCs. The results indicate that A-type and B-type PCs in all of their conformations tend to be more stable in water than in octanol, showing a hydrophilic character due to their negative log [Formula: see text] values, which increase with the degree of polymerization (DP). The studied PCs, including the B-type tetramer, can achieve an appropriate molecular size (i.e. width and length) that can allow them to pass through the pores in the paracellular route in the human small intestine. The factor that could limit the absorption of the PC oligomers with increases in size is the higher number of hydroxyl groups exposed to the outside of the molecule due to their potential to interact with other molecules, which is based on electrostatic potential maps.

Supramolecular Metallo-Bioadhesive for Minimally Invasive Use

A novel metallo-bioadhesive to be used as tissue sealant in minimally invasive procedures is reported. Metal complexation can be used to render gelatin derivatives adhesive, which occurs in minutes, is efficient, and fully biodegradable within weeks.

Resilience of bacterial quorum sensing against fluid flow

Quorum sensing (QS) is a population-density dependent chemical process that enables bacteria to communicate based on the production, secretion and sensing of small inducer molecules. While recombinant constructs have been widely used to decipher the molecular details of QS, how those findings translate to natural QS systems has remained an open question. Here, we compare the activation of natural and synthetic Pseudomonas aeruginosa LasI/R QS systems in bacteria exposed to quiescent conditions and controlled flows. Quantification of QS-dependent GFP expression in suspended cultures and in surface-attached microcolonies revealed that QS onset in both systems was similar under quiescent conditions but markedly differed under flow. Moderate flow (Pe > 25) was sufficient to suppress LasI/R QS recombinantly expressed in Escherichia coli, whereas only high flow (Pe > 102) suppressed QS in wild-type P. aeruginosa. We suggest that this difference stems from the differential production of extracellular matrix and that the matrix confers resilience against moderate flow to QS in wild-type organisms. These results suggest that the expression of a biofilm matrix extends the environmental conditions under which QS-based cell-cell communication is effective and that findings from synthetic QS circuits cannot be directly translated to natural systems.

Local and global consequences of flow on bacterial quorum sensing

Bacteria use a chemical communication process called quorum sensing (QS) to control collective behaviours such as pathogenesis and biofilm formation(1,2). QS relies on the production, release and group-wide detection of signal molecules called autoinducers. To date, studies of bacterial pathogenesis in well-mixed cultures have revealed virulence factors and the regulatory circuits controlling them, including the overarching role of QS(3). Although flow is ubiquitous to nearly all living systems(4), much less explored is how QS influences pathogenic traits in scenarios that mimic host environments, for example, under fluid flow and in complex geometries. Previous studies(5-7) have shown that sufficiently strong flow represses QS. Nonetheless, it is not known how QS functions under constant or intermittent flow, how it varies within biofilms or as a function of position along a confined flow, or how surface topography (grooves, crevices, pores) influence QS-mediated communication. We explore these questions using two common pathogens, Staphylococcus aureus and Vibrio cholerae. We identify conditions where flow represses QS and other conditions where QS is activated despite flow, including characterizing geometric and topographic features that influence the QS response. Our studies highlight that, under flow, genetically identical cells do not exhibit phenotypic uniformity with respect to QS in space and time, leading to complex patterns of pathogenesis and colonization. Understanding the ramifications of spatially and temporally non-uniform QS responses in realistic environments will be crucial for successful deployment of synthetic pro- and anti-QS strategies.

Beyond Ussing's chambers: contemporary thoughts on integration of transepithelial transport

In the mid-20th century, Hans Ussing developed a chamber that allowed for the simultaneous measurement of current and labeled probe flux across epithelia. Using frog skin as a model, Ussing used his results to propose mechanisms of transcellular Na(+) and K(+) transport across apical (exterior/luminal) and basolateral (interior) membranes that is essentially unchanged today. Others took advantage of Ussing's chambers to study mucosal tissues, including bladder and intestines. It quickly became clear that, in some tissues, passive paracellular flux, i.e., across the tight junction, was an important component of overall transepithelial transport. Subsequent work demonstrated that activation of the apical Na(+)-glucose cotransporter SGLT1 regulated paracellular permeability such that intestinal paracellular transport could coordinate with and amplify transcellular transport. Intermediates in this process include activation of p38 MAPK, the apical Na(+)/H(+) exchanger NHE3, and myosin light chain kinase (MLCK). Investigators then focused on these processes in disease. They found that TNF induces barrier dysfunction via MLCK activation and downstream caveolin-1-dependent endocytosis of the tight junction protein occludin. TNF also inhibited NHE3, and both barrier loss and PKCα-dependent NHE3 inhibition were required for TNF-induced acute diarrhea, emphasizing the interplay between transcellular and paracellular transport. Finally, studies using immune-mediated inflammatory bowel disease models showed that mice lacking epithelial MLCK were initially protected, but became ill as epithelial damage progressed and provided a tight junction-independent means of barrier loss. None of these advances would have been possible without the insights provided by Ussing and others using Ussing's ingenious, and still useful, chambers.

The oral delivery of peptides and proteins: established versus recently patented approaches

Over the past 30 years there has been significant research into technologies that promote the delivery of high molecular weight, poor membrane-permeable compounds across the gut. Most work has concentrated on the delivery of peptides and proteins. However, technologies have also been applied to compounds such as poorly membrane-permeable small molecules, heparin and oligonucleotides. Much of this research has been characterized by early promise with many systems showing positive results in animal studies. Success in man has proven more elusive. In 2011, however, the oral delivery of peptides took one step closer to commercial reality when Tarsa Therapeutics announced that it had achieved positive Phase III data for oral recombinant salmon calcitonin. This article reviews the current development status of oral delivery systems for peptides and proteins and examines recent patent activity in this field based mainly on US patents issued in the last 2–3 years.

Oral delivery of macromolecules: rationale underpinning Gastrointestinal Permeation Enhancement Technology (GIPET)

Oral delivery of macromolecular drugs, particularly peptides and proteins, is the focus of many academic and industrial laboratories. Armed with an increased understanding of the structure and regulation of intestinal epithelial junctional complexes of the paracellular barrier, the development of permeation enhancement technology initially focused on the specific and reversible opening of tight junctions in order to enable oral delivery. Despite intense research, none of these specific tight junction-opening technologies has yet been approved in an oral drug product, likely because of poor efficacy. Less specific enhancer technologies with a long history of safe use in man have additional surfactant-like effects on the transcellular pathway that lead to improved efficacy. These are likely to be the first to market for selected poorly permeable peptides. This review presents a summary of some approaches taken to intestinal permeation enhancement and explores in detail the oral delivery system developed by Merrion Pharmaceuticals, Gastrointestinal Permeation Enhancement Technology (GIPET).

The significance of altered gastrointestinal permeability in cancer patients

PURPOSE OF REVIEW

The diagnosis and assessment of severity of intestinal mucosal damage in cancer patients treated by anticancer therapy still rely mostly on anamnestic data. We review here studies reporting on the use of intestinal permeability measurements in cancer patients before and during treatment.

RECENT FINDINGS

The concept of intestinal permeability is based on differential permeability of intestinal mucosa to molecular markers, including monosaccharides and disaccharides, along the crypt-villus axis. Cytotoxic drugs and/or radiation impair replacement of intestinal epithelia and induce flattening of the villi, leading to increased exposure of luminal contents to crypts and increased disaccharide absorption. Increased disaccharide/monosaccharide ratio and decreased xylose absorption have been described in patients treated by radiotherapy as well as different cytotoxic or targeted agents across a spectrum of malignant disorders. Intestinal permeability changes correlated with clinical manifestations, including diarrhea, mucositis, neutropenic enterocolitis and systemic infections. The measurement of intestinal permeability has also been used as a surrogate end-point in interventional studies.

SUMMARY

Intestinal permeability testing using nonmetabolized sugars may represent a tool for noninvasive objective assessment of intestinal toxicity of anticancer therapy.

How well can the Caco-2/Madin-Darby canine kidney models predict effective human jejunal permeability?

The study aimed to predict effective human jejunal permeability (P(eff)) using a biophysical model based on parametrized paracellular, aqueous boundary layer, and transcellular permeabilities, and the villus-fold surface area expansion factor (k(VF)). Published human jejunal data (119 P(eff), 53 compounds) were analyzed by a regression procedure incorporating a dual-pore size paracellular model. Transcellular permeability, scaled by k(VF), was equated to that of Caco-2 at pH 6.5. The biophysical model predicted human jejunal permeability data within the experimental uncertainty. This investigation revealed several surprising predictions: (i) many molecules permeate predominantly (but not exclusively) by the paracellular route, (ii) the aqueous boundary layer thickness in the intestinal perfusion experiments is larger than expected, (iii) the mucosal surface area in awake humans is apparently nearly entirely accessible to drug absorption, and (iv) the relative "leakiness" of the human jejunum is not so different from that observed in a number of published Caco-2 studies.

The Caco-2 cell monolayer: usefulness and limitations

BACKGROUND

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

OBJECTIVE

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

METHODS

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

CONCLUSION

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

Mechanistic bases for differences in passive absorption

Increasing evidence indicates that small birds have more extensive non-mediated, paracellular intestinal absorption of hydrosoluble compounds than do mammals, although studies have not employed uniform methodologies or demonstrated differences at the tissue level. The mechanistic bases behind apparent species differences are poorly understood. We show using uniform methodology at the whole-animal level that intact, unanesthetized pigeons had significantly higher absorption of l-arabinose and l-rhamnose, two water-soluble compounds used to measure paracellular absorption, than similarly sized laboratory rats. The species differences were also evident using perfused isolated loops of duodenum,showing that the difference in paracellular absorption occurred at the tissue level, even when d-glucose absorption rates(transcellular+paracellular) were similar between the two species. The greater absorption of these probes in pigeons could not be explained by mediated uptake of the putative paracellular probes, or by increased nominal surface area, increased villus area or increased number of tight junctions. Rats and pigeons had comparable absorption of larger probes, which is consistent with similar effective pore size of the tight junction between enterocytes. The elimination of these mechanistic explanations might suggest that pigeon intestine has relatively higher paracellular solvent drag, but pigeon duodenal segments did not have higher net water absorption than rat duodenal segments. Whatever the exact mechanism(s), the paracellular pathway of both species limits substantial (&gt;5%) fractional absorption to molecules smaller than about 4.8 Å (Mr ca. 350), and permeability to smaller molecules at the tissue level is higher in pigeons than in rats.

Ethanol enhanced in vivo gene delivery with non-ionic polymeric micelles inhalation

Modifications of both carriers and host barriers have been investigated for efficient inhalation gene delivery to lung. Here we used a biocompatible, non-ionic poly(ethyleneoxide)-poly(propyleneoxide)-poly(ethyleneoxide) (PEO-PPO-PEO) polymeric micelles (PM) as a carrier and combined it with ethanol to enhance membrane penetration of delivered DNA. The inhalation delivery with six 100 microg doses of pCMV-Lac Z with PM co-formulated with 10%-40% ethanol to nude mice in 2 days at 8 h interval was performed. The beta-galatosidase (beta-Gal) activity was assessed using chlorophenol red-beta-d galactopyranoside (CPRG) and X-gal staining for quantitative and qualitative analysis in tissues. The results showed that beta-Gal activity was significantly increased by 38% in lung around bronchioles when inhalation with PM and 10% ethanol was given. The 10% ethanol also increased the intracellular apparent permeability by 42% in stomach and by 141% in intestine at 48 h after the first dosage of delivery. Also delivery of DNA encoding a functional human cystic fibrosis transmembrane protein (CFTR) using the same inhalation delivery method co-formulated with 10% ethanol, an increased expression of CFTR in lung was detected by immunostaining. We concluded that 10% ethanol co-formulated with the PM system could enhance inhaled gene delivery to airway and gastrointestinal (GI) tract.

Abnormal passive chloride absorption in cystic fibrosis jejunum functionally opposes the classic chloride secretory defect

Due to genetic defects in apical membrane chloride channels, the cystic fibrosis (CF) intestine does not secrete chloride normally. Depressed chloride secretion leaves CF intestinal absorptive processes unopposed, which results in net fluid hyperabsorption, dehydration of intestinal contents, and a propensity to inspissated intestinal obstruction. This theory is based primarily on in vitro studies of jejunal mucosa. To determine if CF patients actually hyperabsorb fluid in vivo, we measured electrolyte and water absorption during steady-state perfusion of the jejunum. As expected, chloride secretion was abnormally low in CF, but surprisingly, there was no net hyperabsorption of sodium or water during perfusion of a balanced electrolyte solution. This suggested that fluid absorption processes are reduced in CF jejunum, and further studies revealed that this was due to a marked depression of passive chloride absorption. Although Na+-glucose cotransport was normal in the CF jejunum, absence of passive chloride absorption completely blocked glucose-stimulated net sodium absorption and reduced glucose-stimulated water absorption 66%. This chloride absorptive abnormality acts in physiological opposition to the classic chloride secretory defect in the CF intestine. By increasing the fluidity of intraluminal contents, absence of passive chloride absorption may reduce the incidence and severity of intestinal disease in patients with CF.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Use of an isolated intestinal circuit to evaluate the effect of ischemia and reperfusion on mucosal permeability of the equine jejunum

OBJECTIVES

To evaluate the efficacy of an isolated perfusion circuit and the effect of ischemia-reperfusion on mucosal permeability of the jejunum.

STUDY DESIGN

In vitro study of intestinal mucosal permeability.

ANIMALS

Twelve healthy adult horses.

METHODS

A control segment of jejunum was placed in an isolated perfusion circuit for 240 minutes and mucosal permeability was measured. After detecting no deleterious effects of the isolated system on the control intestine, low flow ischemia was created in experimental segments for 20, 40, 60 and 90 minutes followed by 60 minutes of reperfusion and mucosal permeability was evaluated. At the completion of the studies, histologic evaluation was used to determine mucosal grades, surface area, and volume.

RESULTS

Control tissue was maintained in the isolated circuit for 240 minutes without effect on mucosal grade, surface area, or volume relative to intact tissue. After ischemia-reperfusion, mucosal grade increased, and volume and surface area decreased progressively with longer periods of ischemia. Mucosal clearance of albumin remained constant during 240 minutes of perfusion in control tissue and was elevated after ischemia-reperfusion.

CONCLUSIONS

No deleterious changes were noted in jejunum perfused with this isolated circuit, whereas alterations in mucosal permeability were present after ischemia-reperfusion.

CLINICAL RELEVANCE

The isolated perfusion circuit successfully maintained an isolated segment of jejunum within physiologic limits, and can be used to evaluate the effects of injury and the efficacy of pharmaceuticals to attenuate these changes.

Permeability of the rat small intestinal epithelium along the villus-crypt axis: effects of glucose transport

BACKGROUND & AIMS

The aim of this study was to elucidate the permeability characteristics of the epithelium along the villus-crypt axis and investigate the effect of glucose transport on these characteristics along this axis.

METHODS

The disappearance rates of (14)C-mannitol and (51)Cr-EDTA or (3)H-inulin were determined as clearance (Cl(x)) from a recirculating perfusion system of the jejunal lumen in anesthetized rats. Net fluid transport was varied over a large range by exchanging mannitol with glucose in the perfusate solution and by inhibition of nervously mediated secretory processes with hexamethonium. The perfusion rate was 0.5 or 0.2 mL/min.

RESULTS

Cl(Man) enhanced significantly with increasing net fluid transport (secretion 8.50+/-1.88, to absorption 16.72+/-1.75 microL x min(-1) x g(-1)) and with glucose perfusates. Cl(Cr-EDTA) was constant irrespective of net fluid transport and was reduced to insignificant values at a perfusion rate of 0.2 mL/min. Cl(In) was not different from zero.

CONCLUSIONS

The absorbing apical part of the villus contains small pores (radius, <6 A) allowing passive transport via solvent drag of, e.g., monosaccharides, whereas the pores in the crypts are large (50-60 A) and inaccessible to the luminal content. The basal part of the villus contains medium-sized pores (10-15 A) through which no solvent drag occurs. Active glucose transport in the rat mainly increases the number of small pores accessible for passive transport, whereas the size of these pores seems to stay constant.

Modeling intermittent digesta flow to calculate glucose uptake capacity of the bovine small intestine

To test the hypothesis that the uptake capacity of the bovine small intestine for glucose is upregulated to match or slightly exceed glucose delivery, glucose was continuously infused into the proximal duodenum of four cannulated holstein heifers. Every 3 days, infusion rates were increased by an average of 34 mmol/h. A model of glucose disappearance from multiple boluses of intestinal digesta was used to estimate the transporter maximum velocity and functional maximum uptake capacity for the entire small intestine from average ileal glucose flows during the third day of each period. Because of its intermittency, digesta flow remained independent of simulated transit time. For each unit increase in glucose infusion rate, uptake capacity increased by only 0.55 units. Excess capacity for glucose uptake was approximately twofold in forage-fed cattle and declined to below delivery at infusions of >208 mmol/h added glucose, approximately three times the normal load. Calculations for cattle, sheep, and rats indicate that the glucose transport capacity of the small intestine is typically underutilized because of a fraction of time that transporters are not in contact with digesta.

Altered intestinal permeability to mannitol in diabetes mellitus type I

BACKGROUND

Intestinal permeability has seldom been investigated in diabetes mellitus, even though patients frequently report gastrointestinal symptoms, and it has recently been shown that the prevalence of celiac disease associated with diabetes mellitus is higher than expected.

METHODS

Intestinal permeability to cellobiose and mannitol was investigated in 31 patients affected by type I uncomplicated diabetes mellitus. Values were compared with those obtained in 32 normal subjects.

RESULTS

The percentage of mannitol recovery was far higher than normal in two thirds of the investigated patients and correlated with the length of disease, even though the probes' ratio (cellobiose/mannitol) was in the normal range.

CONCLUSIONS

A not previously reported increase of intestinal permeability to mannitol, clear-cut and not associated with that of the larger probe, is found in type I uncomplicated diabetes mellitus. These results may describe a primary feature of type I diabetes mellitus and the initial steps of evolution to celiac disease.

Influence of morphometric factors on quantitation of paracellular permeability of intestinal epithelia in vitro

PURPOSE

The relative contribution of the small and large intestine to paracellular absorption is a subject of some controversy. Direct comparison of paracellular permeability in different epithelia is complicated by variations in junctional density and/or the absorptive surface area.

METHODS

This study used a combination of morphometric analyses and in vitro absorption studies to define permeability characteristics in relation to the amount of paracellular pathway present in rat ileum, colon and the model epithelium, Caco-2.

RESULTS

Mucosal to serosal amplification was higher in ileum (3.9) than colon (1.9) or Caco-2 (1). Tight junctional density (lp) of ileal crypts was approximately 3 fold greater (91 m/cm2) than that measured in ileal villi, colonic surface and crypt cells or Caco-2 monolayers (34-37 m/cm2). However, when the relative contributions of the crypts and villi was taken into account there was no significant difference in the mean lp per mucosal area for the three epithelia studied. Using these data to correct for morphometric differences the permeabilities of a range of small hydrophilic molecules (atenolol, D-PheAsp and PEG oligomers MW 282-634) was measured. Permeability of rat ileum and colon were virtually identical for all compounds studied. In contrast, Caco-2 monolayers showed a significantly lower permeability than intestinal tissues with the difference increasing markedly with molecular size.

CONCLUSIONS

These studies suggest the importance of accounting for morphological variation when comparing the permeability characteristics of different epithelial systems.

Effect of restricted conformational flexibility on the permeation of model hexapeptides across Caco-2 cell monolayers

PURPOSE

To determine how restricted conformational flexibility of hexapeptides influences their cellular permeation characteristics.

METHODS

Linear (Ac-Trp-Ala-Gly-Gly-X-Ala-NH2; X = Asp, Asn, Lys) and cyclic (cyclo[Trp-Ala-Gly-Gly-X-Ala]; X = Asp, Asn, Lys) hexapeptides were synthesized, and their transport characteristics were assessed using the Caco-2 cell culture model. The lipophilicities of the hexapeptides were determined using an immobilized artificial membrane. Diffusion coefficients used to calculate molecular radii were determined by NMR. Two-dimensional NMR spectroscopy, circular dichroism, and molecular dynamic simulations were used to elucidate the most favorable solution structure of the cyclic Asp-containing peptide.

RESULTS

The cyclic hexapeptides used in this study were 2-3 times more able to permeate (e.g., Papp = 9.3 +/- 0.3 x 10(-8) cm/sec, X = Asp) the Caco-2 cell monolayer than were their linear analogs (e.g., Papp = 3.2 +/- 0.3 x 10(-8) cm/sec, X = Asp). In contrast to the linear hexapeptides, the flux of the cyclic hexapeptides was independent of charge. The cyclic hexapeptides were shown to be more lipophilic than the linear hexapeptides as determined by their retention times on an immobilized phospholipid column. Determination of molecular radii by two different techniques suggests little or no difference in size between the linear and cyclic hexapeptides. Spectroscopic data indicate that the Asp-containing linear hexapeptide exists in a dynamic equilibrium between random coil and beta-turn structures while the cyclic Asp-containing hexapeptide exists in a well-defined compact amphophilic structure containing two beta-turns.

CONCLUSIONS

Cyclization of the linear hexapeptides increased their lipophilicities. The increased permeation characteristics of the cyclic hexapeptides as compared to their linear analogs appears to be due to an increase in their flux via the transcellular route because of these increased lipophilicities. Structural analyses of the cyclic Asp-containing hexapeptide suggest that its well-defined solution structure and, specifically the existence of two beta-turns, explain its greater lipophilicity.

Esterase-sensitive cyclic prodrugs of peptides: evaluation of a phenylpropionic acid promoiety in a model hexapeptide

PURPOSE

To evaluate a cyclic phenylpropionic acid prodrug of a model hexapeptide (H-Trp-Ala-Gly-Gly-Asp-Ala-OH) as a novel approach to enhance the membrane permeation of a peptide and stabilize it to metabolism.

METHODS

Conversion to the linear hexapeptide was studied at 37 degrees C in HBSS, pH 7.4, and in various biological milieus having measurable esterase activities. Transport and metabolism characteristics were assessed using the Caco-2 cell culture model.

RESULTS

In aqueous buffered solution, pH 7.4, the cyclic prodrug degraded quantitatively (t1/2 = 1795 +/- 289 min) to the linear hexapeptide and the lactone. Substantially faster degradation of the cyclic prodrug was observed in 90% human plasma (t1/2 = 508 +/- 24 min), and in homogenates of Caco-2 cells (t1/2 = 940 +/- 13 min), the rat intestinal mucosa (t1/2 = 1286 +/- 32 min), and rat liver (t1/2 = 840 +/- 42 min). Pretreatment of these biological media with paraoxon significantly decreased the degradation rate of the prodrug. When applied to the apical side of Caco-2 cell monolayers, the cyclic prodrug was significantly more stable than the hexapeptide and at least 71-fold more able to permeate (P(app) = 1.21 +/- 0.12 X 10(-7) cm/s) than was the parent peptide (P(app) < or = 0.17 x 10(-8) cm/s). In the presence of 0.1 mM palmitoyl-DL-carnitine, the transport rate of the cyclic prodrug (P(app) = 2.19 X 10(-6) cm/s) was 1250-fold greater than that of the linear hexapeptide.

CONCLUSIONS

Preparation of a cyclic peptide using a phenylpropionic acid promoiety reduced the lability of the peptide to peptidase metabolism and substantially increased its permeation through biological membranes. In various biological media the parent peptide was released from the prodrug by an apparent esterase-catalyzed reaction, sensitive to paraoxon inhibition.