In vitro models of the intestinal barrier. The report and recommendations of ECVAM Workshop 46. European Centre for the Validation of Alternative methods

Combining a microphysiological system of three organ equivalents and transcriptomics to assess toxicological endpoints for cosmetic ingredients

Animal testing for cosmetic ingredients and final products has been banned in Europe and is gaining legal force worldwide. However, the need for reliable testing methodologies remains for safety assessment of cosmetic ingredients. While new approach methodologies exist for many toxicological endpoints, some complex ones lack appropriate testing methods. Microphysiological systems (MPSs) have emerged as a promising tool to address this gap in pre-clinical testing, offering higher predictivity compared to animal models due to the phylogenetic distance between humans and animals. Moreover, they provide a more physiological approach than traditional in vitro testing by mimicking interconnections between different culture compartments as seen in complex organisms. This study presents a three-organ microfluidic MPS comprising skin, liver, and intestine equivalents. Combining this model with gene expression analysis, we evaluated toxicological endpoints of chemicals, demonstrating its potential for diverse applications. Our findings highlight the MPS model as a reliable and ethical method to be applied in an integrated approach for safety assessment in the cosmetic industry. It offers a promising strategy to evaluate toxicological endpoints for cosmetic ingredients and other chemicals, supporting the elimination of animal testing while ensuring consumer safety.

Development and Characterization of an In Vitro Intestinal Model Including Extracellular Matrix and Macrovascular Endothelium

In vitro intestinal models are used to study biological processes, drug and food absorption, or cytotoxicity, minimizing the use of animals in the laboratory. They usually consist of enterocytes and mucus-producing cells cultured for 3 weeks, e.g., on Transwells, to obtain a fully differentiated cell layer simulating the human epithelium. Other important components are the extracellular matrix (ECM) and strong vascularization. The former serves as structural support for cells and promotes cellular processes such as differentiation, migration, and growth. The latter includes endothelial cells, which coordinate vascularization and immune cell migration and facilitate the transport of ingested substances or drugs to the liver. In most cases, animal-derived hydrogels such as Matrigel or collagen are used as ECM in in vitro intestinal models, and endothelial cells are only partially considered, if at all. However, it is well-known that animal-derived products can lead to altered cell behavior and incorrect results. To circumvent these limitations, synthetic and modifiable hydrogels (Peptigel and Vitrogel) were studied here to mimic xenofree ECM, and the data were compared with Matrigel. Careful rheological characterization was performed, and the effect on cell proliferation was investigated. The results showed that Vitrogel exhibited shear-thinning behavior with an internal structure recovery of 78.9 ± 11.2%, providing the best properties among the gels investigated. Therefore, a coculture of Caco-2 and HT29-MTX cells (ratio 7:3) was grown on Vitrogel, while simultaneously endothelial cells were cultured on the basolateral side by inverse cultivation. The model was characterized in terms of cell proliferation, differentiation, and drug permeability. It was found that the cells cultured on Vitrogel induced a 1.7-fold increase in cell proliferation and facilitated the formation of microvilli and tight junctions after 2 weeks of cultivation. At the same time, the coculture showed full differentiation indicated by high alkaline phosphatase release of Caco-2 cells (95.0 ± 15.9%) and a mucus layer produced by HT29-MTX cells. Drug tests led to ex vivo comparable permeability coefficients (Papp) (i.e., Papp; antipyrine = (33.64 ± 5.13) × 10-6 cm/s, Papp; atenolol = (0.59 ± 0.16) × 10-6 cm/s). These results indicate that the newly developed intestinal model can be used for rapid and efficient assessment of drug permeability, excluding unexpected results due to animal-derived materials.

Human intestinal organoids as models to study enteric bacteria and viruses

Laboratory studies of host-microbe interactions have historically been carried out using transformed cell lines and animal models. Although much has been learned from these models, recent advances in the development of multicellular, physiologically active, human intestinal organoid (HIO) cultures are allowing unprecedented discoveries of host-microbe interactions. Here, we review recent literature using HIOs as models to investigate the pathogenesis of clinically important enteric bacteria and viruses and study commensal intestinal microbes. We also discuss limitations of current HIO culture systems and how technical advances and innovative engineering approaches are providing new directions to improve the model. The studies discussed here highlight the potential of HIOs for studying microbial pathogenesis, host-microbe interactions, and for preclinical development of therapeutics and vaccines.

Apple juice relieves loperamide-induced constipation in rats by downregulating the intestinal apical sodium-dependent bile acid transporter ASBT

Apples are known to exhibit various beneficial effects on human health. In the present study, we investigated the effect of continuous intake of apple juice (AJ) on constipation status. A single dose of loperamide in rats as the constipation model markedly decreased the weight and number of fecal pellets compared to saline-administered rats as a control. After the administration of AJ twice a day for seven days, recovery of defecation close to that of the control was observed in loperamide-treated rats. In addition, the total bile acid content in the feces increased from day 4 after the administration of AJ. Among hepatic and intestinal transporters and enzymes that regulate bile acids, the mRNA expression of the apical sodium-dependent bile acid transporter (Asbt, slc10a2) was decreased by AJ in rats. Furthermore, the Asbt-mediated bile acid transport activity in the rat ileum decreased after AJ administration. Moreover, in human colonic cancer-derived Caco-2 cells, AJ exposure for 24 and 48 h decreased the expressions of ASBT mRNA and protein, and the uptake activity of taurocholic acid in both 7- and 21-d cultures. Several components of AJ, such as procyanidins, decreased the expression of ASBT in Caco-2 cells. In conclusion, ASBT downregulation is a possible mechanism responsible for the constipation-relieving effect of apples, and procyanidins may play a role in downregulating ASBT, which leads to the beneficial effects of apples against constipation. Although it is generally agreed that the common dietary compositions play a role in constipation relief, the novel specific mechanism of apples found in this study would facilitate understanding food functions.

Comprehensive Study of Antiretroviral Drug Permeability at the Cervicovaginal Mucosa via an In Vitro Model

Modulation of drug transporter activity at mucosal sites of HIV-1 transmission may be exploited to optimize retention of therapeutic antiretroviral drug concentrations at target submucosal CD4+ T cells. Previously, we showed that darunavir was a substrate for the P-glycoprotein efflux drug transporter in colorectal mucosa. Equivalent studies in the cervicovaginal epithelium have not been reported. Here, we describe the development of a physiologically relevant model to investigate the permeability of antiretroviral drugs across the vaginal epithelium. Barrier properties of the HEC-1A human endometrial epithelial cell line were determined, in a dual chamber model, by measurement of transepithelial electrical resistance, immunofluorescent staining of tight junctions and bi-directional paracellular permeability of mannitol. We then applied this model to investigate the permeability of tenofovir, darunavir and dapivirine. Efflux ratios indicated that the permeability of each drug was transporter-independent in this model. Reduction of pH to physiological levels in the apical compartment increased absorptive transfer of darunavir, an effect that was reversed by inhibition of MRP efflux transport via MK571. Thus, low pH may increase the transfer of darunavir across the epithelial barrier via increased MRP transporter activity. In a previous in vivo study in the macaque model, we demonstrated increased MRP2 expression following intravaginal stimulation with darunavir which may further increase drug uptake. Stimulation with inflammatory modulators had no effect on drug permeability across HEC-1A barrier epithelium but, in the VK2/E6E7 vaginal cell line, increased expression of both efflux and uptake drug transporters which may influence darunavir disposition.

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.

Mimicking the Intestinal Host-Pathogen Interactions in a 3D In Vitro Model: The Role of the Mucus Layer

The intestinal mucus lines the luminal surface of the intestinal epithelium. This mucus is a dynamic semipermeable barrier and one of the first-line defense mechanisms against the outside environment, protecting the body against chemical, mechanical, or biological external insults. At the same time, the intestinal mucus accommodates the resident microbiota, providing nutrients and attachment sites, and therefore playing an essential role in the host-pathogen interactions and gut homeostasis. Underneath this mucus layer, the intestinal epithelium is organized into finger-like protrusions called villi and invaginations called crypts. This characteristic 3D architecture is known to influence the epithelial cell differentiation and function. However, when modelling in vitro the intestinal host-pathogen interactions, these two essential features, the intestinal mucus and the 3D topography are often not represented, thus limiting the relevance of the models. Here we present an in vitro model that mimics the small intestinal mucosa and its interactions with intestinal pathogens in a relevant manner, containing the secreted mucus layer and the epithelial barrier in a 3D villus-like hydrogel scaffold. This 3D architecture significantly enhanced the secretion of mucus. In infection with the pathogenic adherent invasive E. coli strain LF82, characteristic of Crohn's disease, we observed that this secreted mucus promoted the adhesion of the pathogen and at the same time had a protective effect upon its invasion. This pathogenic strain was able to survive inside the epithelial cells and trigger an inflammatory response that was milder when a thick mucus layer was present. Thus, we demonstrated that our model faithfully mimics the key features of the intestinal mucosa necessary to study the interactions with intestinal pathogens.

Does Bentonite Cause Cytotoxic and Whole-Transcriptomic Adverse Effects in Enterocytes When Used to Reduce Aflatoxin B1 Exposure?

Aflatoxin B1 (AFB1) is a major food safety concern, threatening the health of humans and animals. Bentonite (BEN) is an aluminosilicate clay used as a feed additive to reduce AFB1 presence in contaminated feedstuff. So far, few studies have characterized BEN toxicity and efficacy in vitro. In this study, cytotoxicity (WST-1 test), the effects on cell permeability (trans-epithelial electrical resistance and lucifer yellow dye incorporation), and transcriptional changes (RNA-seq) caused by BEN, AFB1 and their combination (AFB1 + BEN) were investigated in Caco-2 cells. Up to 0.1 mg/mL, BEN did not affect cell viability and permeability, but it reduced AFB1 cytotoxicity; however, at higher concentrations, BEN was cytotoxic. As to RNA-seq, 0.1 mg/mL BEN did not show effects on cell transcriptome, confirming that the interaction between BEN and AFB1 occurs in the medium. Data from AFB1 and AFB1 + BEN suggested AFB1 provoked most of the transcriptional changes, whereas BEN was preventive. The most interesting AFB1-targeted pathways for which BEN was effective were cell integrity, xenobiotic metabolism and transporters, basal metabolism, inflammation and immune response, p53 biological network, apoptosis and carcinogenesis. To our knowledge, this is the first study assessing the in vitro toxicity and whole-transcriptomic effects of BEN, alone or in the presence of AFB1.

Size effect and mucus role on the intestinal toxicity of the E551 food additive and engineered silica nanoparticles

The E551 food additive is composed of synthetic amorphous silica particles. The current regulation does not mention any specifications regarding their size and granulometric distribution, thus allowing the presence of silica nanoparticles despite their potential toxicity. The digestion process could modify their physicochemical properties and then influence their toxicological profile. After physicochemical characterization, subacute toxicity of engineered silica nanoparticles from 20 to 200 nm, native and digested E551 additives were evaluated from in vitro models of the intestinal barrier. Single cultures and a co-culture of enterocytes and mucus-secreting cells were established to investigate the mucus role. Toxicological endpoints including cytotoxicity, ROS production, intestinal permeability increase, and actin filament disruption were addressed after a 7-day exposure. The results showed a size-dependent effect of silica nanoparticles on cytotoxicity and intestinal permeability. A time-dependent disruption of actin filaments was observed in Caco-2 cells. The mucus layer spread on the HT29-MTX single culture acted as an efficient protective barrier while in the co-culture, small nanoparticles were able to cross it to reach the cells. From a hydrodynamic diameter of 70 nm, nanoparticles were not internalized in the intestinal cells, even in mucus-free models. Digestion did not affect the physicochemical properties of the additive. Due to a mean hydrodynamic diameter close to 200 nm, both native and digested E551 additives did not induce any toxic effect in intestinal barrier models. This study emphasized a cutoff size of 70 nm from which the interactions of the E551 additive with intestinal cells would be limited.

Quercetin Mitigates Endothelial Activation in a Novel Intestinal-Endothelial-Monocyte/Macrophage Coculture Setup

Atherosclerosis initiation is associated with a pro-inflammatory state of the endothelium. Quercetin is a flavonoid abundantly present in plant-based foods, with a possible impact on cardiovascular health. In this study, the effects of quercetin on lipopolysaccharide (LPS)-mediated endothelial inflammation and monocyte adhesion and migration, which are initial steps of the atherogenic process, are studied. Novel in vitro multicellular models simulating the intestinal-endothelial-monocytes/macrophages axis allowed to combine relevant intestinal flavonoid absorption, metabolism and efflux, and the consequent bioactivity towards peripheral endothelial cells. In this triple coculture, quercetin exposure decreased monocyte adhesion to and macrophage migration through an LPS-stressed endothelium, and this was associated with significantly lower levels of soluble vascular cell adhesion molecule-1 (sVCAM-1). Furthermore, quercetin decreased the pro-inflammatory cell environment upon LPS-induced endothelial activation, in terms of tumor necrosis factor- α (TNF-α), interleukin-6 (IL-6), interleukin-8 (IL-8), and sVCAM-1 expression. These findings highlight a mode-of-action by which quercetin may positively impact the initial states of atherosclerosis under more physiologically relevant conditions in terms of quercetin concentrations, metabolites, and intercellular crosstalk.

Recent advance of <i>in vitro</i> models in natural phytochemicals absorption and metabolism

Natural phytochemicals absorption and metabolic process are mainly in the human gut. Simulating the absorption and metabolism of natural phytochemicals in vitro to predict the rate and degree of absorption of natural phytochemicals provides convenience for many researchers. However, in this process, many physiological factors <i>in vitro</i> are affected, such as stomach and intestinal juice composition, pH, intestinal transmission rate and so on. In recent years, the research methods have gradually improved to make these models more suitable for the natural phytochemicals absorption process, <i>in vitro</i> simulation models have become an essential means to study natural phytochemicals absorption. Therefore, this paper introduces the advantages and disadvantages of commonly used <i>in vitro</i> simulation models of natural phytochemicals absorption and metabolism, as well as briefly introduces the working principle of each model. To provide a theoretical basis for simulating natural phytochemicals absorption <i>in vitro</i> and development and utilization of natural phytochemicals.

Gutsy science: In vitro systems of the human intestine to model oral drug disposition

The intestine has important gate-keeping functions that can profoundly affect the systemic blood exposure of orally administered drugs. Thus, characterizing a new molecular entity's (NME) disposition within the intestine is of utmost importance in drug development. While currently used in vitro systems, such as Ussing chamber, precision-cut intestinal slices, immortalized cell lines, and primary enterocytes provide substantial knowledge about drug absorption and the intestinal first-pass effect, they remain sub-optimal for quantitatively predicting this process and the oral bioavailability of many drugs. Use of novel in vitro systems such as intestinal organoids and intestinal microphysiological systems have provided substantial advances over the past decade, expanding our understanding of intestinal physiology, pathology, and development. However, application of these emerging in vitro systems in the pharmaceutical science is in its infancy. Preliminary work has demonstrated that these systems more accurately recapitulate the physiology and biochemistry of the intact intestine, as it relates to oral drug disposition, and thus they hold considerable promise as preclinical testing platforms of the future. Here we review currently used and emerging in vitro models of the human intestine employed in pharmaceutical science research. We also highlight aspects of these emerging tools that require further study.

Assessing Intestinal Health. In Vitro and Ex vivo Gut Barrier Models of Farm Animals: Benefits and Limitations

Animal performance is determined by the functionality and health of the gastrointestinal tract (GIT). Complex mechanisms and interactions are involved in the regulation of GIT functionality and health. The understanding of these relationships could be crucial for developing strategies to improve animal production yields. The concept of "gut health" is not well defined, but this concept has begun to play a very important role in the field of animal science. However, a clear definition of GIT health and the means by which to measure it are lacking. In vitro and ex vivo models can facilitate these studies, creating well-controlled and repeatable conditions to understand how to improve animal gut health. Over the years, several models have been developed and used to study the beneficial or pathogenic relationships between the GIT and the external environment. This review aims to describe the most commonly used animals' in vitro or ex vivo models and techniques that are useful for better understanding the intestinal health of production animals, elucidating their benefits and limitations.

S-layer protein 2 of vaginal Lactobacillus crispatus 2029 enhances growth, differentiation, VEGF production and barrier functions in intestinal epithelial cell line Caco-2

We have previously demonstrated the ability of the human vaginal strain Lactobacillus crispatus 2029 (LC2029) for strong adhesion to cervicovaginal epithelial cells, expression of the surface layer protein 2 (Slp2), and antagonistic activity against urogenital pathogens. Slp2 forms regular two-dimensional structure around the LC2029 cells,which is secreted into the medium and inhibits intestinal pathogen-induced activation of caspase-9 and caspase-3 in the human intestinal Caco-2 cells. Here, we elucidated the effects of soluble Slp2 on adhesion of proteobacteria pathogens inducing necrotizing enterocolitis (NEC), such as Escherichia coli ATCC E 2348/69, E. coli ATCC 31705, Salmonella Enteritidis ATCC 13076, Campylobacter jejuni ATCC 29428, and Pseudomonas aeruginosa ATCC 27853 to Caco-2 cells, as well as on growth promotion, differentiation, vascular endothelial growth factor (VEGF) production, and intestinal barrier function of Caco-2 cell monolayers. Slp2 acts as anti-adhesion agent for NEC-inducing proteobacteria, promotes growth of immature Caco-2 cells and their differentiation, and enhances expression and functional activity of sucrase, lactase, and alkaline phosphatase. Slp2 stimulates VEGF production, decreases paracellular permeability, and increases transepithelial electrical resistance, strengthening barrier function of Caco-2 cell monolayers. These data support the important role of Slp2 in the early postnatal development of the human small intestine enterocytes.

Sensing technologies and experimental platforms for the characterization of advanced oral drug delivery systems

Complex and miniaturized oral drug delivery systems are being developed rapidly for targeted, controlled drug release and improved bioavailability. Standard analytical techniques are widely used to characterize i) drug carrier and active pharmaceutical ingredients before loading into a delivery device (to ensure the solid form), and ii) the entire drug delivery system during the development process. However, in light of the complexity and the size of some of these systems, standard techniques as well as novel sensing technologies and experimental platforms need to be used in tandem. These technologies and platforms are discussed in this review, with a special focus on passive delivery systems in size range from a few 100 µm to a few mm. Challenges associated with characterizing these systems and evaluating their effect on oral drug delivery in the preclinical phase are also discussed.

Impaired Mucosal Integrity in Proximal Esophagus Is Involved in Development of Proton Pump Inhibitor-Refractory Nonerosive Reflux Disease

BACKGROUND AND OBJECTIVE

Weakly acidic reflux reaching to the proximal esophagus is closely related to the perception of gastroesophageal reflux in patients with nonerosive reflux disease despite treatment with a proton pump inhibitor (PPI). However, little is known about the involvement of the patients' mucosal integrity of the proximal esophagus.

METHODS

We recruited 15 symptomatic nonerosive gastroesophageal reflux disease (GERD) patients with a positive symptom index despite PPI treatment and 11 healthy asymptomatic volunteers as controls. The biopsy specimens obtained from the proximal and distal esophagus were applied to a mini-Ussing chamber system to measure transepithelial electrical resistance (TEER) against a pH 4 weak acid. The esophageal biopsy samples were subjected to quantitative real-time PCR and immunohistochemical analysis.

RESULTS

In the proximal esophagus, the weak acid exposure reduced the TEER in the PPI-refractory patients compared to that in the controls. The frequency of the reflux extending to the proximal esophagus had a significant correlation with the reduction in the proximal esophageal TEER in the patients. The reduced TEER in the proximal esophagus was accompanied by an increase in IL-8 and IL-1β mRNA and a decrease in occludin mRNA levels. The proximal esophageal mucosa in the patients presented infiltration of CD3-positive lymphocytes and an increased expression of solute carrier organic anion transporter family member 2A1 (SLCO2A1), a passage gate of reflux symptom-evoking molecules.

CONCLUSIONS

The reflux perception is related to an impairment of the proximal esophageal mucosal integrity in patients with nonerosive reflux disease despite PPI.

3D organoids derived from the small intestine: An emerging tool for drug transport research

Small intestine in vitro models play a crucial role in drug transport research. Although conventional 2D cell culture models, such as Caco-2 monolayer, possess many advantages, they should be interpreted with caution because they have relatively poor physiologically reproducible phenotypes and functions. With the development of 3D culture technology, pluripotent stem cells (PSCs) and adult somatic stem cells (ASCs) show remarkable self-organization characteristics, which leads to the development of intestinal organoids. Based on previous studies, this paper reviews the application of intestinal 3D organoids in drug transport mediated by P-glycoprotein (P-gp), breast cancer resistance protein (BCRP) and multidrug resistance protein 2 (MRP2). The advantages and limitations of this model are also discussed. Although there are still many challenges, intestinal 3D organoid model has the potential to be an excellent tool for drug transport research.

New Stable Cell Lines Derived from the Proximal and Distal Intestine of Rainbow Trout (Oncorhynchus mykiss) Retain Several Properties Observed In Vivo

We derived two novel cell lines from rainbow trout (RT) proximal (RTpi-MI) and distal intestine (RTdi-MI) and compared them with the previously established continuous cell line RTgutGC. Intestinal stem cells, differentiating and differentiated epithelial cells, and connective cells were found in all cell lines. The cell lines formed a polarized barrier, which was not permeable to large molecules and absorbed proline and glucose. High seeding density induced their differentiation into more mature phenotypes, as indicated by the downregulation of intestinal stem cell-related genes (i.e., sox9, hopx and lgr5), whereas alkaline phosphatase activity was upregulated. Other enterocyte markers (i.e., sglt1 and pept1), however, were not regulated as expected. In all cell lines, the presence of a mixed population of epithelial and stromal cells was characterized for the first time. The expression by the stromal component of lgr5, a stem cell niche regulatory molecule, may explain why these lines proliferate stably in vitro. Although most parameters were conserved among the three cell lines, some significant differences were observed, suggesting that characteristics typical of each tract are partly conserved in vitro as well.

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.

In vitro models replicating the human intestinal epithelium for absorption and metabolism studies: A systematic review

Absorption, distribution, metabolism and excretion (ADME) studies represent a fundamental step in the early stages of drug discovery. In particular, the absorption of orally administered drugs, which occurs at the intestinal level, has gained attention since poor oral bioavailability often led to failures for new drug approval. In this context, several in vitro preclinical models have been recently developed and optimized to better resemble human physiology in the lab and serve as an animal alternative to accomplish the 3Rs principles. However, numerous models are ineffective in recapitulating the key features of the human small intestine epithelium and lack of prediction potential for drug absorption and metabolism during the preclinical stage. In this review, we provide an overview of in vitro models aimed at mimicking the intestinal barrier for pharmaceutical screening. After briefly describing how the human small intestine works, we present i) conventional 2D synthetic and cell-based systems, ii) 3D models replicating the main features of the intestinal architecture, iii) micro-physiological systems (MPSs) reproducing the dynamic stimuli to which cells are exposed in the native microenvironment. In this review, we will highlight the benefits and drawbacks of the leading intestinal models used for drug absorption and metabolism studies.

Intestinal Excretion, Intestinal Recirculation, and Renal Tubule Reabsorption Are Underappreciated Mechanisms That Drive the Distribution and Pharmacokinetic Behavior of Small Molecule Drugs

Drug reabsorption following biliary excretion is well-known as enterohepatic recirculation (EHR). Renal tubular reabsorption (RTR) following renal excretion is also common but not easily assessed. Intestinal excretion (IE) and enteroenteric recirculation (EER) have not been recognized as common disposition mechanisms for metabolically stable and permeable drugs. IE and intestinal reabsorption (IR:EHR/EER), as well as RTR, are governed by dug concentration gradients, passive diffusion, active transport, and metabolism, and together they markedly impact disposition and pharmacokinetics (PK) of small molecule drugs. Disruption of IE, IR, or RTR through applications of active charcoal (AC), transporter knockout (KO), and transporter inhibitors can lead to changes in PK parameters. The impacts of intestinal and renal reabsorption on PK are under-appreciated. Although IE and EER/RTR can be an intrinsic drug property, there is no apparent strategy to optimize compounds based on this property. This review seeks to improve understanding and applications of IE, IR, and RTR mechanisms.

Influence of cellulose nanocrystals (CNC) on permeation through intestinal monolayer and mucus model in vitro

Cellulose nanocrystals (CNC) as a novel ingredient in foods and pharmaceuticals still lacks the safety and functionality information. We aimed to assess the absorption of CNC in small intestine and the effect on cell viability. In the second part, the impact of CNC on substance permeation through mucus layer, including the potential functionality in improving high blood cholesterol, was tested. No noticeable amount of CNC was found to penetrate through differentiated Caco-2 monolayer and in vitro mucus layer, and CNC had low toxicity on Caco-2 cell viability up to 10 mg/mL. CNC at 2 % (w/w) may affect the permeability of the mucus layer and larger molecules are more easily influenced. CNC may also alleviate hypercholesteremia by increasing viscosity of digesta, adsorbing cholesterol, and decreasing bile acids permeation. The results suggest CNC may not penetrate the small intestinal lining and may be used as a functional supplement.

Permeability of the Cyanotoxin Microcystin-RR across a Caco-2 Cells Monolayer

Microcystins (MCs) are toxins produced by several cyanobacterial species found worldwide. While MCs have a common structure, the variation of two amino acids in their structure affects their toxicity. As toxicodynamics are very similar between the MC variants, their differential toxicity could rather be explained by toxicokinetic parameters. Microcystin-RR (MC-RR) is the second most abundant congener and induces toxicity through oral exposure. As intestinal permeability is a key parameter of oral toxicokinetics, the apparent permeability of MC-RR across a differentiated intestinal Caco-2 cell monolayer was investigated. We observed a rapid and large decrease of MC-RR levels in the donor compartment. However, irrespective of the loaded concentration and exposure time, the permeabilities were very low from apical to basolateral compartments (from 4 to 15 × 10⁻⁸ cm·s⁻¹) and from basolateral to apical compartments (from 2 to 37 × 10⁻⁸ cm·s⁻¹). Our results suggested that MC-RR would be poorly absorbed orally. As similar low permeability was reported for the most abundant congener microcystin-LR, and this variant presented a greater acute oral toxicity than MC-RR, we concluded that the intestinal permeability was probably not involved in the differential toxicity between them, in contrast to the hepatic uptake and metabolism.

Resveratrol enhances trans-intestinal cholesterol excretion through selective activation of intestinal liver X receptor alpha

Resveratrol (RSV) is a dietary polyphenol with well-documented cardio-protective activity, but its effects on blood cholesterol levels remain to be established. Due to its poor bioavailability, tissue accumulation of RSV is extremely low except for that in the small intestine. In the present study, we aimed to investigate the dose-dependent effects of RSV on blood cholesterol levels and the involvement of small intestine in the cholesterol-lowering impacts of RSV. Mice were administrated with RSV at various doses with high-fat diet (HFD) or high-fat and high-cholesterol diet (HCD) for 12 weeks. The fecal neutral sterol contents were analyzed, and intestinal perfusion test was performed. An enteric barrier model using Caco-2 cells was established. We observed that RSV reduced blood cholesterol levels in a dose-dependent manner in mice fed with HFD or HCD. Further investigation revealed that RSV administration increased the bile acid pool size but did not affect cholesterol consumption or de novo cholesterol synthesis. Interestingly, RSV promoted trans-intestinal cholesterol excretion (TICE) by 2-fold in the intestinal perfusion test. In addition, RSV upregulated the expressions of ATP-binding cassette sub-family G member 5 or 8 (Abcg5/8) and ATP-binding cassette sub-family B member 1a or 1b (Abcb1a/b) by up to 8 times in the duodenum mucosa but not in the liver. RSV also significantly downregulated the expression of intestinal Niemann-Pick C1-Like 1 (Npc1l1). Knock-down of liver X receptor alpha (LXRα) but not Sirt1 by siRNA significantly blocked RSV-induced cholesterol excretion in Caco-2 cells. In conclusion, RSV could decrease circulating cholesterol levels through enhancing TICE and limiting cholesterol absorption via selective activation of intestinal LXRα.

Preparation of Biological Scaffolds and Primary Intestinal Epithelial Cells to Efficiently 3D Model the Fish Intestinal Mucosa

Tissue engineering is an elegant tool to create organs in vitro, that can help obviate the lack of organ donors in transplantation medicine and provide the opportunity of studying complex biological systems in vitro, thereby reducing the need for animal experiments. Artificial intestine models are at the core of Fish-AI, an EU FET-Open research project dedicated to the development of a 3D in vitro platform that is intended to enable the aquaculture feed industry to predict the nutritional and health value of alternative feed sources accurately and efficiently.At present, it is impossible to infer the health and nutrition value through the chemical characterization of any given feed. Therefore, each new feed must be tested through in vivo growth trials. The procedure is lengthy, expensive and requires the use of many animals. Furthermore, although this process allows for a precise evaluation of the final effect of each feed, it does not improve our basic knowledge of the cellular and molecular mechanisms determining such end-results. In turn, this lack of mechanistic knowledge severely limits the capacity to understand and predict the biological value of a single raw material and of their different combinations.The protocol described herein allows to develop the two main components essential to produce a functional platform for the efficient and reliable screening of feeds that the feed industry is currently developing for improving their health and nutritional value. It is here applied to the Rainbow Trout, but it can be fruitfully used to many other fish species.

Development of an in vitro coculture device for the investigation of host-microbe interactions via integrative multiomics approaches

The commensal gut bacterium Akkermansia muciniphila is well known as a promising probiotic candidate that improves host health and prevents diseases. However, the biological interaction of A. muciniphila with human gut epithelial cells has rarely been explored for use in biotherapeutics. Here, we developed an in vitro device that simulates the gut epithelium to elucidate the biological effects of living A. muciniphila via multiomics analysis: the Mimetic Intestinal Host-Microbe Interaction Coculture System (MIMICS). We demonstrated that both human intestinal epithelial cells (Caco-2) and the anaerobic bacterium A. muciniphila can remain viable for 12 h after coculture in the MIMICS. The transcriptomic and proteomic changes (cell-cell junctions, immune responses, and mucin secretion) in gut epithelial cells treated with A. muciniphila closely correspond with those reported in previous in vivo studies. In addition, our proteomic and metabolomic results revealed that A. muciniphila activates glucose and lipid metabolism in gut epithelial cells, leading to an increase in ATP production. This study suggests that A. muciniphila improves metabolism for ATP production in gut epithelial cells and that the MIMICS may be an effective general tool for evaluating the effects of anaerobic bacteria on gut epithelial cells.

Research in a time of enteroids and organoids: how the human gut model has transformed the study of enteric bacterial pathogens

Enteric bacterial pathogens cause significant morbidity and mortality globally. Studies in tissue culture and animal models shaped our initial understanding of these host-pathogen interactions. However, intrinsic shortcomings in these models limit their application, especially in translational applications like drug screening and vaccine development. Human intestinal enteroid and organoid models overcome some limitations of existing models and advance the study of enteric pathogens. In this review, we detail the use of human enteroids and organoids to investigate the pathogenesis of invasive bacteria Shigella, Listeria, and Salmonella, and noninvasive bacteria pathogenic Escherichia coli, Clostridium difficile, and Vibrio cholerae. We highlight how these studies confirm previously identified mechanisms and, importantly, reveal novel ones. We also discuss the challenges for model advancement, including platform engineering to integrate environmental conditions, innate immune cells and the resident microbiome, and the potential for pre-clinical testing of recently developed antimicrobial drugs and vaccines.

Artificial sweeteners affect the glucose transport rate in the Caco-2/NCI-H716 co-culture model

BACKGROUND

Artificial sweeteners have been used widely as substitutes for sugar for several decades. In recent years they have been reported to be harmful to human health - especially to glucose absorption. However, as conclusions from previous studies using a single Caco-2 cell model were not consistent, further studies with a more suitable cell model are needed.

RESULTS

We established a co-culture model with enterocyte Caco-2 and enteroendocrine NCI-H716 cell lines cultured in transwell inserts. The effects of artificial sweeteners, enhancing the glucose transport rate, lasted for 60 min and then began to diminish. Most importantly, different artificial sweeteners with the same sweetness intensity had similar effects on glucose transport. The sodium / glucose co-transporter member 1 (SGLT1) mRNA expression levels increased significantly with an initial glucose concentration of 20 mM, while glucose transporter 2 (GLUT2) mRNA expression significantly increased with initial glucose concentrations of 20 mM and 60 mM.

CONCLUSION

Based on the Caco-2/NCI-H716 co-culture model, SGLT1 and GLUT2 mediated the enhancing effects of artificial sweeteners on glucose transport, depending on the sweetness intensity and initial glucose concentration.

Bioavailability and bioaccessibility of food bioactive compounds; overview and assessment by in vitro methods

Oral bioavailability is the key to the bioefficiency of food bioactive ingredients; it evaluates the relationship between foods and their health benefits. The analysis of the main factors limiting the oral bioavailability (bioaccessibility, absorption, and transformation) has led to the proposal of classification systems for pharmaceuticals and nutraceuticals (Biopharmaceuticals Classification System and Nutraceutical Bioavailability Classification Scheme). Based on the relevant studies published in the last decade, this review presents the essential aspects regarding the factors limiting the oral bioavailability of the biocomponents and different in vitro methods used to investigate the mechanisms involved in the digestion, absorption, and metabolism of biocomponents, particularly encapsulated bioactive compounds. Oral bioavailability investigated by in vitro studies provides the food and drug manufacturers with information to formulate delivery systems more efficiently and to determine the dosage of biocomponents for increase the health benefits and avoid or reduce the risk of toxicity.

Tissue-on-a-Chip: Microphysiometry With Human 3D Models on Transwell Inserts

Microphysiometry has proved to be a useful tool for monitoring the energy metabolism of living cells and their interactions with other cells. The technique has mainly been used for monitoring two-dimensional (2D) monolayers of cells. Recently, our group showed that it is also possible to monitor the extracellular acidification rate and transepithelial electrical resistance (TEER) of 3D skin constructs in an automated assay maintaining an air–liquid interface (ALI) with a BioChip extended by 3D-printed encapsulation. In this work, we present an optimized multichannel intestine-on-a-chip for monitoring the TEER of the commercially available 3D small intestinal tissue model (EpiIntestinal^TM from MatTek). Experiments are performed for 1 day, during which a 60 min cycle is repeated periodically. Each cycle consists of three parts: (1) maintain ALI; (2) application of the measurement medium or test substance; and (3) the rinse cycle. A cytotoxic and barrier-disrupting benchmark chemical (0.2% sodium dodecyl sulfate) was applied after 8 h of initial equilibration. This caused time-dependent reduction of the TEER, which could not be observed with typical cytotoxicity measurement methods. This work represents a proof-of-principle of multichannel time-resolved TEER monitoring of a 3D intestine model using an automated ALI. Reconstructed human tissue combined with the Intelligent Mobile Lab for In vitro Diagnostic technology represents a promising research tool for use in toxicology, cellular metabolism studies, and drug absorption research.

Simulation of Human Small Intestinal Digestion of Starch Using an In Vitro System Based on a Dialysis Membrane Process

This work deepens our understanding of starch digestion and the consequent absorption of hydrolytic products generated in the human small intestine. Gelatinized starch dispersions were digested with α-amylase in an in vitro intestinal digestion system (i-IDS) based on a dialysis membrane process. This study innovates with respect to the existing literature, because it considers the impact of simultaneous digestion and absorption processes occurring during the intestinal digestion of starchy foods and adopts phenomenological models that deal in a more realistic manner with the behavior found in the small intestine. Operating the i-IDS at different flow/dialysate flow ratios resulted in distinct generation and transfer curves of reducing sugars mass. This indicates that the operating conditions affected the mass transfer by diffusion and convection. However, the transfer process was also affected by membrane fouling, a dynamic phenomenon that occurred in the i-IDS. The experimental results were extrapolated to the human small intestine, where the times reached to transfer the hydrolytic products ranged between 30 and 64 min, according to the flow ratio used. We consider that the i-IDS is a versatile system that can be used for assessing and/or comparing digestion and absorption behaviors of different starch-based food matrices as found in the human small intestine, but the formation and interpretation of membrane fouling requires further studies for a better understanding at physiological level. In addition, further studies with the i-IDS are required if food matrices based on fat, proteins or more complex carbohydrates are of interest for testing. Moreover, a next improvement step of the i-IDS must include the simulation of some physiological events (e.g., electrolytes addition, enzyme activities, bile, dilution and pH) occurring in the human small intestine, in order to improve the comparison with in vivo data.

Hydrophobic ion pair loaded self-emulsifying drug delivery system (SEDDS): A novel oral drug delivery approach of cromolyn sodium for management of bronchial asthma

The aim of the present study is to develop a self-emulsifying drug delivery system (SEDDS) for the hydrophobic ion pair (HIP) complex of cromolyn sodium (CS), in order to enhance its intestinal absorption and biological activity. Two ion pairing agents (IPAs) were investigated: hexadecyl pyridininum chloride (HPC) and myristyl trimethyl ammonium bromide (MTAB). The optimum binding efficiency for complexation between investigated IPAs and CS was observed at a molar ratio of 1.5:1, where CS binding efficiency was found to be 76.10 ± 2.12 and 91.37 ± 1.73% for MTAB and HPC, respectively. The two prepared complexes exhibited a significant increase in partition coefficient indicating increased lipophilicity. The optimized CS-HIP complex was incorporated into SEDDS formulations. SEDDS formulations F2 (40% oleic acid, 40% Brij^TM98, 20% propylene glycol) and F3 (25% oleic acid, 50% Brij^TM98, 25% propylene glycol) exhibited nanometric droplet diameters with monodisperse distribution and nearly neutral zeta potential values. Ex vivo intestinal permeation study, using the non-everted gut sac technique, revealed a significantly higher cumulative amount of permeated drug, after 2 h, for F2 and F3 (53.836 and 77.617 µg/cm², respectively) compared to 8.649 µg/cm² for plain CS solution. The in vivo evaluation of plain CS solution compared to F2 and F3 was conducted in an ovalbumin sensitization-induced bronchial asthma rat model. Lung function parameters (tidal volume and peak expiratory flow), biochemical parameters (interleukin-5, immunoglobulin-E, myeloperoxidase and airway remodelling parameters) were assessed in addition to histopathological examination. The results indicated the superiority of F3 followed by F2 compared to plain CS solution for prophylaxis of bronchial asthma in rats.

Interactions of graphene oxide and graphene nanoplatelets with the in vitro Caco-2/HT29 model of intestinal barrier

Carbon-based nanomaterials are being increasingly used, demanding strong information to support their safety in terms of human health. As ingestion is one of the most important exposure routes in humans, we have determined their potential risk by using an in vitro model simulating the human intestinal barrier and evaluated the effects of both graphene oxide (GO) and graphene nanoplatelets (GNPs). A coculture of differentiated Caco-2/HT29 cells presenting inherent intestinal epithelium characteristics (i.e. mucus secretion, brush border, tight junctions, etc.) were treated with GO or GNPs for 24 h. Different endpoints such as viability, membrane integrity, NPs localization, cytokines secretion, and genotoxic damage were evaluated to have a wide view of their potentially harmful effects. No cytotoxic effects were observed in the cells that constitute the barrier model. In the same way, no adverse effects were detected neither in the integrity of the barrier (TEER) nor in its permeability (LY). Nevertheless, a different bio-adhesion and biodistribution behavior was observed for GO and GNPs by confocal microscopy analysis, with a more relevant uptake of GNPs. No oxidative damage induction was detected, either by the DCFH-DA assay or the FPG enzyme in the comet assay. Conversely, both GO and GNPs were able to induce DNA breaks, as observed in the comet assay. Finally, low levels of anti-inflammatory cytokines were detected, suggesting a weak anti-inflammatory response. Our results show the moderate/severe risk posed by GO/GNPs exposures, given the observed genotoxic effects, suggesting that more extensive genotoxic evaluations must be done to properly assess the genotoxic hazard of these nanomaterials.

Hydrogel co-networks of gelatine methacrylate and poly(ethylene glycol) diacrylate sustain 3D functional in vitro models of intestinal mucosa

Mounting evidence supports the importance of the intestinal epithelial barrier and its permeability both in physiological and pathological conditions. Conventional in vitro models to evaluate intestinal permeability rely on the formation of tightly packed epithelial monolayers grown on hard substrates. These two-dimensional models lack the cellular and mechanical components of the non-epithelial compartment of the intestinal barrier, the stroma, which are key contributors to the barrier permeability in vivo. Thus, advanced in vitro models approaching the in vivo tissue composition are fundamental to improve precision in drug absorption predictions, to provide a better understanding of the intestinal biology, and to faithfully represent related diseases. Here, we generate photo-crosslinked gelatine methacrylate (GelMA)-poly(ethylene glycol) diacrylate (PEGDA) hydrogel co-networks that provide the required mechanical and biochemical features to mimic both the epithelial and stromal compartments of the intestinal mucosa, i.e. they are soft, cell adhesive and cell-loading friendly, and suitable for long-term culturing. We show that fibroblasts can be embedded in the GelMA-PEGDA hydrogels while epithelial cells can grow on top to form a mature epithelial monolayer that exhibits barrier properties which closely mimic those of the intestinal barrier in vivo, as shown by the physiologically relevant transepithelial electrical resistance (TEER) and permeability values. The presence of fibroblasts in the artificial stroma compartment accelerates the formation of the epithelial monolayer and boosts the recovery of the epithelial integrity upon temporary barrier disruption, demonstrating that our system is capable of successfully reproducing the interaction between different cellular compartments. As such, our hydrogel co-networks offer a technologically simple yet sophisticated approach to produce functional three-dimensional (3D) in vitro models of epithelial barriers with epithelial and stromal cells arranged in a spatially relevant manner and near-physiological functionality.

Evaluation of human primary intestinal monolayers for drug metabolizing capabilities

Background

The intestinal epithelium is a major site of drug metabolism in the human body, possessing enterocytes that house brush border enzymes and phase I and II drug metabolizing enzymes (DMEs). The enterocytes are supported by a porous extracellular matrix (ECM) that enables proper cell adhesion and function of brush border enzymes, such as alkaline phosphatase (ALP) and alanyl aminopeptidase (AAP), phase I DMEs that convert a parent drug to a more polar metabolite by introducing or unmasking a functional group, and phase II DMEs that form a covalent conjugate between a functional group on the parent compound or sequential metabolism of phase I metabolite. In our effort to develop an in vitro intestinal epithelium model, we investigate the impact of two previously described simple and customizable scaffolding systems, a gradient cross-linked scaffold and a conventional scaffold, on the ability of intestinal epithelial cells to produce drug metabolizing proteins as well as to metabolize exogenously added compounds. While the scaffolding systems possess a range of differences, they are most distinguished by their stiffness with the gradient cross-linked scaffold possessing a stiffness similar to that found in the in vivo intestine, while the conventional scaffold possesses a stiffness several orders of magnitude greater than that found in vivo.

Results

The monolayers on the gradient cross-linked scaffold expressed CYP3A4, UGTs 2B17, 1A1 and 1A10, and CES2 proteins at a level similar to that in fresh crypts/villi. The monolayers on the conventional scaffold expressed similar levels of CYP3A4 and UGTs 1A1 and 1A10 DMEs to that found in fresh crypts/villi but significantly decreased expression of UGT2B17 and CES2 proteins. The activity of CYP3A4 and UGTs 1A1 and 1A10 was inducible in cells on the gradient cross-linked scaffold when the cells were treated with known inducers, whereas the CYP3A4 and UGT activities were not inducible in cells grown on the conventional scaffold. Both monolayers demonstrate esterase activity but the activity measured in cells on the conventional scaffold could not be inhibited with a known CES2 inhibitor. Both monolayer culture systems displayed similar ALP and AAP brush border enzyme activity. When cells on the conventional scaffold were incubated with a yes-associated protein (YAP) inhibitor, CYP3A4 activity was greatly enhanced suggesting that mechano-transduction signaling can modulate drug metabolizing enzymes.

Conclusions

The use of a cross-linked hydrogel scaffold for expansion and differentiation of primary human intestinal stem cells dramatically impacts the induction of CYP3A4 and maintenance of UGT and CES drug metabolizing enzymes in vitro making this a superior substrate for enterocyte culture in DME studies. This work highlights the influence of mechanical properties of the culture substrate on protein expression and the activity of drug metabolizing enzymes as a critical factor in developing accurate assay protocols for pharmacokinetic studies using primary intestinal cells.

Graphical abstract

The Grouping and Assessment Strategy for Organic Pigments (GRAPE): Scientific evidence to facilitate regulatory decision-making

This article presents the Grouping and Assessment Strategy for Organic Pigments (GRAPE). GRAPE is driven by the hypotheses that low (bio)dissolution and low permeability indicate absence of systemic bioavailability and hence no systemic toxicity potential upon oral exposure, and, for inhalation exposure, that low (bio)dissolution (and absence of surface reactivity, dispersibility and in vitro effects) indicate that the organic pigment is a 'poorly soluble particle without intrinsic toxicity potential'. In GRAPE Tier 1, (bio)solubility and (bio)dissolution are assessed, and in Tier 2, in vitro Caco-2 permeability and in vitro alveolar macrophage activation. Thereafter, organic pigments are grouped by common properties (further considering structural similarity depending on the regulatory requirements). In Tier 3, absence of systemic bioavailability is verified by limited in vivo screening (rat 28-day oral and 5-day inhalation toxicity studies). If Tier 3 confirms no (or only very low) systemic bioavailability, all higher-tier endpoint-specific animal testing is scientifically not-relevant. Application of the GRAPE can serve to reduce animal testing needs for all but few representative organic pigments within a group. GRAPE stands in line with the EU REACH Regulation (Registration, Evaluation, Authorisation and Restriction of Chemicals). An ongoing research project aims at establishing a proof-of-concept of the GRAPE.

Hyperosmolar environment and intestinal epithelial cells: impact on mitochondrial oxygen consumption, proliferation, and barrier function in vitro

The aim of the present study was to elucidate the in vitro short-term (2-h) and longer-term (24-h) effects of hyperosmolar media (500 and 680 mOsm/L) on intestinal epithelial cells using the human colonocyte Caco-2 cell line model. We found that a hyperosmolar environment slowed down cell proliferation compared to normal osmolarity (336 mOsm/L) without inducing cell detachment or necrosis. This was associated with a transient reduction of cell mitochondrial oxygen consumption, increase in proton leak, and decrease in intracellular ATP content. The barrier function of Caco-2 monolayers was also transiently affected since increased paracellular apical-to-basal permeability and modified electrolyte permeability were measured, allowing partial equilibration of the trans-epithelial osmotic difference. In addition, hyperosmotic stress induced secretion of the pro-inflammatory cytokine IL-8. By measuring expression of genes involved in energy metabolism, tight junction forming, electrolyte permeability and intracellular signaling, different response patterns to hyperosmotic stress occurred depending on its intensity and duration. These data highlight the potential impact of increased luminal osmolarity on the intestinal epithelium renewal and barrier function and point out some cellular adaptive capacities towards luminal hyperosmolar environment.

Species-specific models in toxicology: in vitro epithelial barriers

Species-specific in vitro epithelial barriers represent interesting predictive tools for risk assessment evaluation in toxicological studies. Moreover, these models could be applied either as stand-alone methods for the study of absorption, bioavailability, excretion, transport, effects of xenobiotics, or through an Integrated Testing Strategy. The aim of this review is to give a comprehensive overview of in vitro species-specific epithelial barrier models from bovine, dog and swine. Bovine mammary epithelial barrier as a fundamental instrument for the evaluation of the toxicant excretion, the blood brain barrier as a useful first approach in toxicological and pharmacological studies, the porcine intestinal barrier, the canine skin barrier, and finally the pulmonary barrier from bovine and swine species are described in this review.

Exploration of biomedical dendrimer space based on in-vitro physicochemical parameters: key factor analysis (Part 1)

Dendrimers are highly branched, star-shaped macromolecules with nanometer-scale dimensions that can be readily modified with a range of functional groups, thus modifying their physicochemical and biological properties. In nanomedicine, dendrimers can be used as vectors for the targeted delivery strategy of a variety of biologically active agents or can be used as drug per se. In the future, it will be necessary to designate and develop 'safe' dendrimers, which is currently a crucial concern. Here, we analyze the key in vitro physicochemical parameters to be considered for preclinical evaluation of biomedical dendrimers.

In Silico Assessment of ADME Properties: Advances in Caco-2 Cell Monolayer Permeability Modeling

One of the main goals of in silico Caco-2 cell permeability models is to identify those drug substances with high intestinal absorption in human (HIA). For more than a decade, several in silico Caco-2 models have been made, applying a wide range of modeling techniques; nevertheless, their capacity for intestinal absorption extrapolation is still doubtful. There are three main problems related to the modest capacity of obtained models, including the existence of inter- and/or intra-laboratory variability of recollected data, the influence of the metabolism mechanism, and the inconsistent in vitro-in vivo correlation (IVIVC) of Caco-2 cell permeability. This review paper intends to sum up the recent advances and limitations of current modeling approaches, and revealed some possible solutions to improve the applicability of in silico Caco-2 permeability models for absorption property profiling, taking into account the above-mentioned issues.