Materials and Microenvironments for Engineering the Intestinal Epithelium

Challenges in Permeability Assessment for Oral Drug Product Development

Drug permeation across the intestinal epithelium is a prerequisite for successful oral drug delivery. The increased interest in oral administration of peptides, as well as poorly soluble and poorly permeable compounds such as drugs for targeted protein degradation, have made permeability a key parameter in oral drug product development. This review describes the various in vitro, in silico and in vivo methodologies that are applied to determine drug permeability in the human gastrointestinal tract and identifies how they are applied in the different stages of drug development. The various methods used to predict, estimate or measure permeability values, ranging from in silico and in vitro methods all the way to studies in animals and humans, are discussed with regard to their advantages, limitations and applications. A special focus is put on novel techniques such as computational approaches, gut-on-chip models and human tissue-based models, where significant progress has been made in the last few years. In addition, the impact of permeability estimations on PK predictions in PBPK modeling, the degree to which excipients can affect drug permeability in clinical studies and the requirements for colonic drug absorption are addressed.

Polymeric and biological membranes for organ-on-a-chip devices

Membranes are fundamental elements within organ-on-a-chip (OOC) platforms, as they provide adherent cells with support, allow nutrients (and other relevant molecules) to permeate/exchange through membrane pores, and enable the delivery of mechanical or chemical stimuli. Through OOC platforms, physiological processes can be studied in vitro, whereas OOC membranes broaden knowledge of how mechanical and chemical cues affect cells and organs. OOCs with membranes are in vitro microfluidic models that are used to replace animal testing for various applications, such as drug discovery and disease modeling. In this review, the relevance of OOCs with membranes is discussed as well as their scaffold and actuation roles, properties (physical and material), and fabrication methods in different organ models. The purpose was to aid readers with membrane selection for the development of OOCs with specific applications in the fields of mechanistic, pathological, and drug testing studies. Mechanical stimulation from liquid flow and cyclic strain, as well as their effects on the cell's increased physiological relevance (IPR), are described in the first section. The review also contains methods to fabricate synthetic and ECM (extracellular matrix) protein membranes, their characteristics (e.g., thickness and porosity, which can be adjusted depending on the application, as shown in the graphical abstract), and the biological materials used for their coatings. The discussion section joins and describes the roles of membranes for different research purposes and their advantages and challenges.

Harnessing conserved signaling and metabolic pathways to enhance the maturation of functional engineered tissues

The development of induced-pluripotent stem cell (iPSC)-derived cell types offers promise for basic science, drug testing, disease modeling, personalized medicine, and translatable cell therapies across many tissue types. However, in practice many iPSC-derived cells have presented as immature in physiological function, and despite efforts to recapitulate adult maturity, most have yet to meet the necessary benchmarks for the intended tissues. Here, we summarize the available state of knowledge surrounding the physiological mechanisms underlying cell maturation in several key tissues. Common signaling consolidators, as well as potential synergies between critical signaling pathways are explored. Finally, current practices in physiologically relevant tissue engineering and experimental design are critically examined, with the goal of integrating greater decision paradigms and frameworks towards achieving efficient maturation strategies, which in turn may produce higher-valued iPSC-derived tissues.

Changes of Intestinal Flora and Its Relationship with Nutritional Status for Patients with Cancer Pain

Objective. To study the changes in the intestinal flora and its relationship with nutritional status for patients with cancer pain. Methods. A prospective research method was adopted. One hundred twenty cancer patients with cancer pain were selected as the research objects, who were treated in our hospital from June 2019 to June 2020, and 120 cancer patients without cancer pain were selected as the control group, who were treated in the same period. The differences of the intestinal flora and nutritional status of patients with different severity between the observation group and the control group were compared to analyze the changes of intestinal flora in patients with cancer pain and its correlation with nutritional status. Results. Hemoglobin (HB) ( $t=17.141$ , $p\le 0.001$ ), albumin (ALB) ( $t=27.654$ , $p\le 0.001$ ), prealbumin (PAB) ( $t=96.192$ , $p\le 0.001$ ), and total protein (TP) ( $t=18.781$ , $p\le 0.001$ ) in the observation group were significantly lower than those in the control group. There were statistically significant differences in HB ( $f=13.569$ , $p\le 0.001$ ), ALB ( $f=22.229$ , $p\le 0.001$ ), PAB ( $f=19.521$ , $p\le 0.001$ ), and TP ( $f=21.451$ , $p\le 0.001$ ) among patients with cancer pain of different severity. Through these two comparisons, their nutritional indicators showed a significant downward trend with the increase in the severity for cancer pain patients; the levels of Lactobacillus ( $t=2.124$ , $p=0.035$ ), Bifidobacterium ( $t=4.823$ , $p\le 0.001$ ), Enterococcus ( $t=3.578$ , $p\le 0.001$ ), and Eubacterium ( $t=2.394$ , $p=0.017$ ) in the observation group were significantly lower than those in the control group. There were statistically significant differences in the levels of Lactobacillus ( $f=20.643$ , $p\le 0.001$ ), Bifidobacterium ( $f=19.129$ , $p\le 0.001$ ), Enterococcus ( $f=17.408$ , $p\le 0.001$ ), and Eubacterium ( $f=22.343$ , $p\le 0.001$ ) among patients with cancer pain of different severity. After pairwise comparison, their beneficial intestinal bacteria were significantly lower than those in the control group with the increase in pain in cancer pain patients. Nitric oxide (NO) ( $t=8.418$ , $p\le 0.001$ ), galectin-3 ( $t=14.043$ , $p\le 0.001$ ), occludin (OCLN) ( $t=47.308$ , $p\le 0.001$ ), galectin-1 ( $t=15.298$ , $p\le 0.001$ ), zonula occludens protein 1 (ZO-1) ( $t=23.093$ , $p\le 0.001$ ), and cingulin ( $t=340.198$ , $p\le 0.001$ ) in the observation group were significantly lower than those in the control group. There were statistically significant differences in NO, galectin-3, OCLN, galectin-1, ZO-1, and cingulin for patients with cancer pain of different severity. By comparison, the NO, galectin-3, OCLN, galectin-1, ZO-1, and cingulin of the patients showed a significant downward trend with the aggravation of cancer pain symptoms. Through correlation analysis, the nutritional indicators of patients were positively correlated with intestinal microorganisms and intestinal barrier function. Conclusion. There was a significant correlation between the changes in intestinal flora and nutritional status for patients with cancer pain, which could be used as an important basis for improving the treatment of cancer pain.

Advanced human developmental toxicity and teratogenicity assessment using human organoid models

Tremendous progress has been made in the field of toxicology leading to the advance of developmental toxicity assessment. Conventional animal models and in vitro two-dimensional models cannot accurately describe toxic effects and predict actual in vivo responses due to obvious inter-species differences between humans and animals, as well as the lack of a physiologically relevant tissue microenvironment. Human embryonic stem cell (hESC)- and induced pluripotent stem cell (iPSC)-derived three-dimensional organoids are ideal complex and multicellular organotypic models, which are indispensable in recapitulating morphogenesis, cellular interactions, and molecular processes of early human organ development. Recently, human organoids have been used for drug discovery, chemical toxicity and safety in vitro assessment. This review discusses the recent advances in the use of human organoid models, (i.e., brain, retinal, cardiac, liver, kidney, lung, and intestinal organoid models) for developmental toxicity and teratogenicity assessment of distinct tissues/organs following exposure to pharmaceutical compounds, heavy metals, persistent organic pollutants, nanomaterials, and ambient air pollutants. Combining next-generation organoid models with innovative engineering technologies generates novel and powerful tools for developmental toxicity and teratogenicity assessment, and the rapid progress in this field is expected to continue.

Multiscale Invasion Assay for Probing Macrophage Response to Gram-Negative Bacteria

The immune system is a complex network of various cellular components that must differentiate between pathogenic bacteria and the commensal bacteria of the human microbiome, where misrecognition is linked to inflammatory disorders. Fragments of bacterial cell wall peptidoglycan bind to pattern recognition receptors within macrophages, leading to immune activation. To study this complex process, a methodology to remodel and label the bacterial cell wall of two different species of bacteria was established using copper (I) catalyzed azide-alkyne cycloaddition (CuAAC) and strain-promoted azide-alkyne cycloaddition (SPAAC). Additionally, an approach for three-dimensional (3D) culture of human macrophages and their invasion with relevant bacteria in a well-defined hydrogel-based synthetic matrix inspired by the microenvironment of the gut was established. Workflows were developed for human monocyte encapsulation and differentiation into macrophages in 3D culture with high viability. Bacteria invaded into macrophages permitted in situ peptidoglycan labeling. Macrophages exhibited biologically-relevant cytokine release in response to bacteria. This molecularly engineered, multi-dimensional bacteria-macrophage co-culture system will prove useful in future studies to observe immunostimulatory, bacterial fragment production and localization in the cell at the carbohydrate level for insights into how the immune system properly senses bacteria.

From Patient Material to New Discoveries: a Methodological Review and Guide for Intestinal Stem Cell Researchers

Intestinal stem cells (ISC) are characterized by their ability to continuously self-renew and differentiate into various functionally distinct intestinal epithelial cell types. Impaired stem cell proliferation and differentiation can cause severe dysfunction of the gastrointestinal tract and lead to the development of several clinical disorders. Animal mouse models provide a valuable platform to study ISC function, disease mechanisms, and the intestinal epithelium's regenerative capacity upon tissue damage. However, advanced in vitro systems that are more relevant to human physiology are needed to understand better the diverse disease-triggering factors and the heterogeneity in clinical manifestations. Intestinal biopsies from patients might serve as potent starting material for such "gut-in-a-dish" approaches. While many promising tools for intestinal tissue processing, in vitro expansion, and downstream analysis have been developed in recent years, a comprehensive guide with recommendations to successfully launch or improve intestinal stem cell culture is missing. In this review, we present a selection of currently established methods, highlight recent publications and discuss the potential and limitations of those methodological approaches to facilitate and support the future design of novel and more personalized therapeutic options.

All layers matter: Innovative three-dimensional epithelium-stroma-endothelium intestinal model for reliable permeability outcomes

Drug development is an ever-growing field, increasingly requesting reliable in vitro tools to speed up early screening phases, reducing the need for animal experiments. In oral delivery, understanding the absorption pattern of a new drug in the small intestine is paramount. Classical two-dimensional (2D) in vitro models are generally too simplistic and do not accurately represent native tissues. The main goal of this work was to develop an advanced three-dimensional (3D) in vitro intestinal model to test absorption in a more reliable manner, by better mimicking the native environment. The 3D model is composed of a collagen-based stromal layer with embedded fibroblasts mimicking the intestinal lamina propria and providing support for the epithelium, composed of enterocytes and mucus-secreting cells. An endothelial layer, surrogating the absorptive capillary network, is also present. The cellular crosstalk between the different cells present in the model is unveiled, disclosing key players, namely those involved in the contraction of collagen by fibroblasts. The developed 3D model presents lower levels of P-glycoprotein (P-gp) and Multidrug Resistance Protein 2 (MRP2) efflux transporters, which are normally overexpressed in traditional Caco-2 models, and are paramount in the absorption of many compounds. This, allied with transepithelial electrical resistance (TEER) values closer to physiological ranges, leads to improved and more reliable permeability outcomes, which are observed when comparing our results with in vivo data.

Roadmap to functional characterization of the human intestinal microbiota in its interaction with the host

It is known for more than 100 years that the intestinal microbes are important for the host's health and the last decade this is being intensely studied with a focus on the mechanistic aspects. Among the fundamental functions of the intestinal microbiome are the priming of the immune system, the production of essential vitamins and the energy harvest from foods. By now, several dozens of diseases, both intestinal and non-intestinal related, have been associated with the intestinal microbiome. Initially, this was based on the description of the composition between groups of different health status or treatment arms based on phylogenetic approaches based on the 16S rRNA gene sequences. This way of analysis has mostly moved to the analysis of all the genes or transcripts of the microbiome i.e. metagenomics and meta-transcriptomics. Differences are regularly found but these have to be taken with caution as we still do not know what the majority of genes of the intestinal microbiome are capable of doing. To circumvent this caveat researchers are studying the proteins and the metabolites of the microbiome and the host via metaproteomics and metabolomics approaches. However, also here the complexity is high and only a fraction of signals obtained with high throughput instruments can be identified and assigned to a known protein or molecule. Therefore, modern microbiome research needs advancement of existing and development of new analytical techniques. The usage of model systems like intestinal organoids where samples can be taken and processed rapidly as well as microfluidics systems may help. This review aims to elucidate what we know about the functionality of the human intestinal microbiome, what technologies are advancing this knowledge, and what innovations are still required to further evolve this actively developing field.