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

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.

A bioprinted 3D gut model with crypt-villus structures to mimic the intestinal epithelial-stromal microenvironment

The intestine is a complex tissue with a characteristic three-dimensional (3D) crypt-villus architecture, which plays a key role in the intestinal function. This function is also regulated by the intestinal stroma that actively supports the intestinal epithelium, maintaining the homeostasis of the tissue. Efforts to account for the 3D complex structure of the intestinal tissue have been focused mainly in mimicking the epithelial barrier, while solutions to include the stromal compartment are scarce and unpractical to be used in routine experiments. Here we demonstrate that by employing an optimized bioink formulation and the suitable printing parameters it is possible to produce fibroblast-laden crypt-villus structures by means of digital light projection stereolithography (DLP-SLA). This process provides excellent cell viability, accurate spatial resolution, and high printing throughput, resulting in a robust biofabrication approach that yields functional gut mucosa tissues compatible with conventional testing techniques.

The shape of our gut: Dissecting its impact on drug absorption in a 3D bioprinted intestinal model

The small intestine is a complex organ with a characteristic architecture and a major site for drug and nutrient absorption. The three-dimensional (3D) topography organized in finger-like protrusions called villi increases surface area remarkably, granting a more efficient absorption process. The intestinal mucosa, where this process occurs, is a multilayered and multicell-type tissue barrier. In vitro intestinal models are routinely used to study different physiological and pathological processes in the gut, including compound absorption. Still, standard models are typically two-dimensional (2D) and represent only the epithelial barrier, lacking the cues offered by the 3D architecture and the stromal components present in vivo, often leading to inaccurate results. In this work, we studied the impact of the 3D architecture of the gut on drug transport using a bioprinted 3D model of the intestinal mucosa containing both the epithelial and the stromal compartments. Human intestinal fibroblasts were embedded in a previously optimized hydrogel bioink, and enterocytes and goblet cells were seeded on top to mimic the intestinal mucosa. The embedded fibroblasts thrived inside the hydrogel, remodeling the surrounding extracellular matrix. The epithelial cells fully covered the hydrogel scaffolds and formed a uniform cell layer with barrier properties close to in vivo. In particular, the villus-like model revealed overall increased permeability compared to a flat counterpart composed by the same hydrogel and cells. In addition, the efflux activity of the P-glycoprotein (P-gp) transporter was significantly reduced in the villus-like scaffold compared to a flat model, and the genetic expression of other drugs transporters was, in general, more relevant in the villus-like model. Globally, this study corroborates that the presence of the 3D architecture promotes a more physiological differentiation of the epithelial barrier, providing more accurate data on drug absorbance measurements.

Caco-2 Cell Sheet Partially Laminated with HT29-MTX Cells as a Novel In Vitro Model of Gut Epithelium Drug Permeability

The intestinal epithelial Caco-2 cell monolayer is a well-established in vitro model useful for predicting intestinal drug absorption in humans. Coculture models of Caco-2 and goblet-cell-like HT29-MTX cells have been developed to overcome the lack of a mucus layer; however, those models are much leakier compared to the intestinal epithelium. Here, we developed a partially laminated culture model where HT29-MTX cells were superimposed onto a Caco-2 monolayer to overcome this issue. A morphological study showed that the piled HT29-MTX cells were voluntarily incorporated into the Caco-2 monolayer, and mucus production was confirmed via periodic acid-Schiff and mucin protein 2 staining. Permeability was evaluated in terms of transepithelial electrical resistance (TEER) and the apparent permeability of paracellular markers with different molecular sizes. The partially laminated model maintained the high barrier function of the Caco-2 monolayer, whose permeability appeared adjustable according to the HT29-MTX/Caco-2 cell ratio. In contrast, the coculture models showed abnormally high permeability of those markers, correlated with low TEER. Thus, the partially laminated model enabled in vitro recapitulation of effective mucosal barrier function. Consequently, this novel model may be useful as an in vitro high-throughput evaluation system for enteral mucosal permeability and mucus-penetrating efficiency of drugs and nanocarriers.

Chitooligosaccharide reconstitutes intestinal mucus layer to improve oral absorption of water-soluble drugs

Intestinal mucus is a complex natural hydrogel barrier with unique physical properties that impede the absorption of various oral drugs. Both washout from the upper water layer and the physical resistance of the mucus layer particularly affect bioavailability of, especially, highly water-soluble molecules. One potential strategy for designing pharmaceutical formulations is to add absorption enhancers (AEs). However, there are few reports of AEs that work on mucus and their underlying mechanisms, leading to imprecise application. In this study, we investigated chitooligosaccharide (COS) as a safe, low-cost, and effective oral drug AE. We revealed the hydrodynamic law of interaction between COS and the intestinal mucus layer, which was associated with absorption benefiting mucus structural reconstruction. Based on this, we designed a translational strategy to improve the bioavailability of a group of soluble oral drugs by drinking COS solution before administration. Moreover, this research is expected to expand its application scenario by reducing drug dosage such as avoiding gastro-intestinal irritation and slowing veterinary antibiotic resistance.

Gut-on-a-chip for disease models

The intestinal tract is a vital organ responsible for digestion and absorption in the human body and plays an essential role in pathogen invasion. Compared with other traditional models, gut-on-a-chip has many unique advantages, and thereby, it can be considered as a novel model for studying intestinal functions and diseases. Based on the chip design, we can replicate the in vivo microenvironment of the intestine and study the effects of individual variables on the experiment. In recent years, it has been used to study several diseases. To better mimic the intestinal microenvironment, the structure and function of gut-on-a-chip are constantly optimised and improved. Owing to the complexity of the disease mechanism, gut-on-a-chip can be used in conjunction with other organ chips. In this review, we summarise the human intestinal structure and function as well as the development and improvement of gut-on-a-chip. Finally, we present and discuss gut-on-a-chip applications in inflammatory bowel disease (IBD), viral infections and phenylketonuria. Further improvement of the simulation and high throughput of gut-on-a-chip and realisation of personalised treatments are the problems that should be solved for gut-on-a-chip as a disease model.

Suspended Collagen Hydrogels to Replicate Human Colonic Epithelial Cell Interactions with Immune Cells

The human colon plays a critical role in fluid and salt absorption and harbors the largest immune compartment. There is a widespread need for in vitro models of human colon physiology with its innate immune system. A method is described to produce a cassette with a network of struts supporting a suspended, non-chemically cross-linked collagen hydrogel scaffold compatible with the co-culture of primary gastrointestinal epithelium and migratory inflammatory cells. The epithelial monolayer cultured on the suspended collagen possesses a population of polarized and differentiated cells similar to that present in vivo. This epithelial layer displays proper barrier function with a transepithelial electrical resistance (TEER) ≥ 1,500 Ω cm2 and an apparent permeability ≤10-5 cm2 s-1 . Immune cells plated on the basal face of the scaffold transmigrated over a period of 24 h to the epithelial layer in response to epithelial production of IL-8 induced by luminal stimulation of Clostridium difficile Toxin A. These studies demonstrate that this in vitro platform possesses a functional primary colonic epithelial layer with an immune cell compartment capable of recruitment in response to pro-inflammatory cues coming from the epithelium.

Oral delivery of camptothecin-loaded multifunctional chitosan-based micelles is effective in reduce colorectal cancer

Colorectal cancer (CRC) is a heterogeneous disease with high incidence and mortality worldwide. The efficacy of conventional CRC chemotherapy is hampered by poor drug solubility and bioavailability and suboptimal pharmacokinetic profiles. In this work, camptothecin (CPT), a potent anticancer drug, was loaded into an amphiphilic chitosan modified with PEG and oleic acid, to reduce CRC progression after oral administration. While CPT-loaded micelles presented anticancer activity against HCT116, Caco-2 and HT29 CRC cell lines in vitro, empty micelles demonstrated a safe profile when incubated with human blood cells and colorectal cancer cell lines. In a more complex 3D CRC multicellular spheroid model, CPT-loaded micelles also exhibited a significant effect on the spheroid's metabolic activity and size reduction. Remarkably, in vivo studies performed in a HCT116 xenograft model, showed a significant reduction on the tumor growth during and after treatment with CPT-loaded micelles. Moreover, in a more biological relevant in vivo model of chemically-induced CRC, orally administered CPT-loaded micelles demonstrated a significant reduction on tumor incidence and inflammation signs. The findings here reported indicate that CPT-loaded into chitosan-based micelles, by improving drug solubility, alongside its safety profile for normal tissues, may have a promising role CRC chemotherapy.

3D printed systems for colon-specific delivery of camptothecin-loaded chitosan micelles

The use of 3D printing technology in the manufacturing of drug delivery systems has expanded and benefit of a customized care. The ability to create tailor-made structures filled with drugs/delivery systems with suitable drug dosage is especially appealing in the field of nanomedicine. In this work, chitosan-based polymeric micelles loaded with camptothecin (CPT) were incorporated into 3D printing systems (printfills) sealed with an enteric layer, aiming to protect the nanosystems from the harsh environment of the gastrointestinal tract (GIT). Polymeric micelles and printfills were fully characterized and, a simulated digestion of the 3D systems upon an oral administration was performed. The printfills maintained intact at the simulated gastric pH of the stomach and, only released the micelles at the colonic pH. From there, the dissolution media was used to recreate the intestinal absorption and, chitosan micelles showed a significant increase of the CPT permeability compared to the free drug, reaching an apparent permeability coefficient (P_app) of around 9×10^-6 cm/s in a 3D intestinal cell-based model. The combination of 3D printing with nanotechnology appears to have great potential for the colon-specific release of polymeric micelles, thereby increasing intestinal absorption while protecting the system/drug from degradation throughout the GIT.