Corneal epithelium models for safety assessment in drug development: Present and future directions

The human corneal epithelial barrier plays a crucial role in drug testing studies, including drug absorption, distribution, metabolism, and excretion (ADME), as well as toxicity testing during the preclinical stages of drug development. However, despite the valuable insights gained from animal and current in vitro models, there remains a significant discrepancy between preclinical drug predictions and actual clinical outcomes. Additionally, there is a growing emphasis on adhering to the 3R principles (refine, reduce, replace) to minimize the use of animals in testing. To tackle these challenges, there is a rising demand for alternative in vitro models that closely mimic the human corneal epithelium. Recently, remarkable advancements have been made in two key areas: microphysiological systems (MPS) or organs-on-chips (OoCs), and stem cell-derived organoids. These cutting-edge platforms integrate four major disciplines: stem cells, microfluidics, bioprinting, and biosensing technologies. This integration holds great promise in developing powerful and biomimetic models of the human cornea.

Research and Development of Microphysiological Systems in Japan Supported by the AMED-MPS Project

Microphysiological systems (MPS) have been actively developed as a new technology for in vitro toxicity testing platforms in recent years. MPS are culture techniques for the reconstruction of the specific functions of human organs or tissues in a limited space to create miniaturized human test systems. MPS have great promise as next-generation in vitro toxicity assessment systems. Here, I will review the current status of MPS and discuss the requirements that must be met in order for MPS to be implemented in the field of drug discovery, presenting the example of an in vitro cell assay system for drug-induced liver injury, which is the research subject in our laboratory. Projects aimed at the development of MPS were implemented early in Europe and the United States, and the AMED-MPS project was launched in Japan in 2017. The AMED-MPS project involves industry, government, and academia. Researchers in the field of drug discovery in the pharmaceutical industry also participate in the project. Based on the discussions made in the project, I will introduce the requirements that need to be met by liver-MPS as in vitro toxicity test platforms.

HepG2-NIAS cells, a new subline of HepG2 cells that can enhance not only CYP3A4 activity but also expression of drug transporters and form bile canaliculus-like networks by the oxygenation culture via a collagen vitrigel membrane

We reported the enhanced liver-specific function and structure of HepG2 cells by the oxygenation culture via a collagen vitrigel membrane (CVM). The cells were conditioned in our laboratory for a long period, so their characteristics may change from the original HepG2 cells registered in RIKEN cell bank (RCB) with the number of 1648 (HepG2-RCB1648 cells). We named the conditioned HepG2-RCB1648 cells in our laboratory as HepG2-NIAS cells. Here, we clarified the features of HepG2 cells with three different culture histories by analyzing their morphology and viability, CYP3A4 activity, the potential to form bile canaliculus-like structures, and the expression of drug transporters. On plastic, HepG2-NIAS cells grew as a monolayer without the formation of large aggregates involving dead cells that were observed in HepG2-RCB1648 cells and HepG2-RCB1886 cells. In the oxygenation culture via a CVM, the CYP3A4 activity of HepG2-NIAS cells increased to almost half level in direct comparison to that of differentiated HepaRG cells cultured on a collagen-coated plate; however, that of HepG2-RCB1648 cells and HepG2-RCB1886 cells was almost not detected. HepG2-NIAS cells formed bile canaliculus-like networks in which fluorescein was accumulated after the exposure of fluorescein diacetate, although HepG2-RCB1648 cells and HepG2-RCB1886 cells did not possess the potential. Also, immunohistological observations revealed that HepG2-NIAS cells remarkably enhanced the expression of drug transporters, NTCP, OATP1B1, OATP1B3, BSEP, MDR1, MRP2, and BCRP. These results suggest that HepG2-NIAS cells are a new subline of HepG2 cells useful for drug development studies. HepG2-NIAS cells were registered in RCB with the number of 4679.

Collagen Vitrigel Membrane-Coated Nylon Line Prevents Stenosis After Conization of the Cervix Uteri

Cervical stenosis is a postoperative complication of conization for uterine cervical malignancy, but a standard method of preventing this complication has yet to be established. Collagen vitrigel is a collagen-based biomaterial that has antifibrotic and epithelization promoting actions. We evaluated the antistenotic effect of an indwelling collagen vitrigel membrane-coated nylon line (CVNL) after cervical conization in rabbits. In one group of rabbits, a CVNL was placed in the cervical canal after conization. In another group, a nylon line without a collagen coating was placed in the cervical canal after conization. The control group underwent cervical conization without placement of a device. The control (conization alone) and nylon (conization plus indwelling nylon line) groups exhibited cervical swelling. Rabbits in the CVNL group (cervical conization plus indwelling CVNL in the xerogel state) had a normal cervical surface. The cervical canal in the control group was enlarged and showed cystic changes attributed to cervical stenosis. The nylon group exhibited a trend toward cervical canal dilatation. In the CVNL group, the cervical canal was normal and did not show cystic dilatation. Fibrosis occurred to a lesser degree in the nylon group than in the control group, and the CVNL group exhibited minimal interstitial fibrosis. The control and nylon groups showed increased numbers of myofibroblasts in the regenerated cervix, but few myofibroblasts were observed in the CVNL group. Abundant collagen type III was observed in regenerated cervical tissue in the control and nylon groups but not in the CVNL group. The number of proliferative mesenchymal cells in the regenerated cervix was lowest in the CVNL group. The expressions of connective tissue growth factor (CTGF, a regulator of fibroblast growth and extracellular matrix secretion), extracellular signal-regulated protein kinases 1 and 2, and c-Jun N-terminal kinase (which are involved in the induction of CTGF by transforming growth factor-β) were lower in the CVNL group than in the control or nylon groups. This study describes an indwelling CVNL that prevents cervical stenosis and cystic changes after conization. These effects were likely mediated by inhibition of fibrosis, myofibroblast emergence, CTGF expression, and collagen type III deposition in regenerating cervix. Impact statement Collagen vitrigel is a high-density collagen material that promotes epithelization, inhibits fibrosis, and suppresses inflammation in regenerating tissue. We evaluated whether a collagen vitrigel membrane-coated nylon line would prevent cervical stenosis after conization in the rabbit. We found that an indwelling collagen vitrigel membrane-coated nylon line prevented cervical canal stenosis and cystic changes after cervical conization by inhibiting fibrosis, myofibroblast emergence, connective tissue growth factor expression, and collagen type III deposition in the regenerating cervix. Our device has potential as a new method of preventing cervical canal fibrosis and stenosis after conization for cervical cancer.

In vitro reconstructed 3D corneal tissue models for ocular toxicology and ophthalmic drug development

Testing of all manufactured products and their ingredients for eye irritation is a regulatory requirement. In the last two decades, the development of alternatives to the in vivo Draize eye irritation test method has substantially advanced due to the improvements in primary cell isolation, cell culture techniques, and media, which have led to improved in vitro corneal tissue models and test methods. Most in vitro models for ocular toxicology attempt to reproduce the corneal epithelial tissue which consists of 4-5 layers of non-keratinized corneal epithelial cells that form tight junctions, thereby limiting the penetration of chemicals, xenobiotics, and pharmaceuticals. Also, significant efforts have been directed toward the development of more complex three-dimensional (3D) equivalents to study wound healing, drug permeation, and bioavailability. This review focuses on in vitro reconstructed 3D corneal tissue models and their utilization in ocular toxicology as well as their application to pharmacology and ophthalmic research. Current human 3D corneal epithelial cell culture models have replaced in vivo animal eye irritation tests for many applications, and substantial validation efforts are in progress to verify and approve alternative eye irritation tests for widespread use. The validation of drug absorption models and further development of models and test methods for many ophthalmic and ocular disease applications is required.

An alternative predictor of eye irritation that utilizes potential parameters of the human corneal epithelium model calculated based on Hansen solubility parameters

Eye irritation predictions are very important in the development of cosmetics and pharmaceuticals. For animal protection, alternative tests are being developed to replace the Draize test, which involves the use of rabbits to test eye irritation. The Vitrigel-eye irritancy test (Vitrigel-EIT), is one such alternative. As a preliminary study, we evaluated if Hansen solubility parameter (HSP) values can be used to predict Vitrigel-EIT results. An Hansen sphere was created based on the HSP values and Vitrigel-EIT results from 61 substances. Substances inside and outside of the sphere were designated as dangerous and safe substances, respectively. The safety of each test substance was predicted by comparing the center point (R_o) of the sphere with the relative energy difference, i.e., the ratio of each test substance (R_a). The accuracy, false negativity, and false positivity of the "irritant" and "nonirritant" designations, as determined by the Vitrigel-EIT results and Hansen sphere, were 91.8% (56/61), 2.3% (1/43), and 22.2% (4/18), respectively. These results indicated that HSP values can be used to predict Vitrigel-EIT results with high reproducibility, and thus are useful for evaluating the safety of substances.

Multi-laboratory Validation Study of the Vitrigel-Eye Irritancy Test Method as an Alternative to In Vivo Eye Irritation Testing

Collagen vitrigel membranes (CVMs) comprising high-density collagen fibrils equivalent to in vivo connective tissues have been widely used in cell culture applications. A human corneal epithelium (hCE) model was previously developed by the Takezawa group, by culturing HCE-T cells (derived from hCE cells) on a CVM scaffold in a chamber that provided an air–liquid interface culture system. This hCE model was used to establish a new test method, known as the Vitrigel-Eye Irritancy Test (Vitrigel-EIT) method, which can be used to estimate the ocular irritation potential of test chemicals by analysing relative changes in transepithelial electrical resistance (TEER) over time. The current study was conducted in order to assess the reliability and relevance of the Vitrigel-EIT method at three participating laboratories by determining the method’s within-laboratory reproducibility and between-laboratory reproducibility, as well as its capacity for distinguishing non-irritants from irritants in a bottom-up approach. The initial test sample size was found to be too low to evaluate the predictive capacity of the test method, and so it was evaluated with additional in-house data for a total of 93 test chemicals. The results showed 80–100% within-laboratory reproducibility and an excellent between-laboratory reproducibility that met the acceptance criteria of 80%. However, the method’s predictive capacity for distinguishing non-irritants (test chemicals not requiring classification and labelling for eye irritation or serious eye damage, i.e. United Nations Globally Harmonised System of Classification and Labelling of Chemicals (GHS) No Category) from irritants (GHS Categories 1 and 2) in a bottom-up approach was unacceptable because of false negative rates as high as 16.7%. After considerable review of the data with a view to using the method for regulatory purposes, it was determined that a more defined applicability domain, excluding test chemical solutions with a pH of 5 or less and solid test chemicals, improved the false negative rate to 4.2%. These results suggested that, within this carefully defined applicability domain, the Vitrigel-EIT method could be a useful alternative for distinguishing test chemicals that are ocular non-irritants from those that are irritants as part of a bottom-up approach.

Collagen vitrigel promotes hepatocytic differentiation of induced pluripotent stem cells into functional hepatocyte-like cells

Differentiation of stem cells to hepatocytes provides an unlimited supply of human hepatocytes and therefore has been vigorously studied. However, to date, the stem cell-derived hepatocytes were suggested to be of immature features. To obtain matured hepatocytes from stem cells, we tested the effect of culturing human-induced pluripotent stem (hiPS) cell-derived endoderm cells on collagen vitrigel membrane and compared with our previous reported nanofiber matrix. We cultured hiPS cell-derived endoderm cells on a collagen vitrigel membrane and examined the expression profiles, and tested the activity of metabolic enzymes. Gene expression profile analysis of hepatocytic differentiation markers revealed that upon culture on collagen vitrigel membrane, immature markers of AFP decreased, with a concomitant increase in the expression of mature hepatocyte transcription factors and mature hepatocyte markers such as ALB, ASGR1. Mature markers involved in liver functions, such as transporters, cytochrome P450 enzymes and phase II metabolic enzymes were also upregulated. We observed the upregulation of the liver markers for at least 2 weeks. Gene array profiling analysis revealed that hiPS cell-derived hepatocyte-like cells (hiPS-hep) resemble those of the primary hepatocytes. Functions of the CYP enzyme activities were tested in multi-institution and all revealed high CYP1A, CYP2C19, CYP2D6, CYP3A activity, which could be maintained for at least 2 weeks in culture. Taken together, the present approach identified that collagen vitrigel membrane provides a suitable environment for the generation of hepatocytes from hiPS cells that resemble many characteristics of primary human hepatocytes.

Summary: We found that collagen vitrigel membrane used as scaffold potentiates differentiation of human induced pluripotent stem cells to differentiate into mature hepatocyte-like cells that exhibit mature functions of the hepatocytes.

Fabrication of a Corneal Model Composed of Corneal Epithelial and Endothelial Cells via a Collagen Vitrigel Membrane Functioned as an Acellular Stroma and Its Application to the Corneal Permeability Test of Chemicals

A collagen vitrigel membrane (CVM) we developed can function as both a scaffold for cells and a pathway for chemicals. To extrapolate the corneal permeability of chemicals in vivo, we proposed six corneal models using the CVM. Thin and thick CVMs were used as models for Bowman's membrane (BM) and an acellular stroma (AS), respectively. Models for a corneal epithelium (CEpi), a CEpi-AS, a CEpi-endothelium (Endo), and a CEpi-AS-Endo were fabricated by culturing corneal epithelial cells and/or corneal endothelial cells on the surface of CVMs. Subsequently, the permeability coefficient (P_app) value of each model was calculated using five chemicals with different molecular radii; cyanocobalamin and four fluorescein isothiocyanate-dextrans (FD) (FD-4, FD-10, FD-20, and FD-40). The slopes of P_app versus molecular radii of those chemicals in the both BM and AS models were almost similar to data using an excised rabbit corneal stroma. The ratios of P_app values in models for BM, CEpi, and CEpi-Endo against those in data using an excised rabbit cornea were calculated as 75.4-fold, 6.4-fold, and 4.5-fold for FD-4, and 38.7-fold, 10.0-fold, and 4.2-fold for FD-10, respectively. Similarly, those in models for AS, CEpi-AS, and CEpi-AS-Endo were calculated as 26.1-fold, 2.5-fold, and 0.6-fold for FD-4, and 26.1-fold, 1.5-fold, and 0.6-fold for FD-10, respectively. These results suggest that the CEpi-AS-Endo model with both the barrier function of corneal cell layers and the diffusion capacity of chemicals in thick CVM is most appropriate for extrapolating the corneal permeability of chemicals in vivo.

Increase of β2-integrin on adhesion of THP-1 cells to collagen vitrigel membrane

When human monocyte-derived leukemia (THP-1) cells, which are floating cells, are stimulated with lipid peroxides, or Streptococcus suis, these cells adhere to a plastic plate or endothelial cells. However, it is unclear whether or not non-stimulated THP-1 cells adhere to collagen vitrigel membrane (CVM). In this study, firstly, we investigated the rate of adhesion of THP-1 cells to CVM. When THP-1 cells were not stimulated, the rate of adhesion to CVM was high. Then, to identify adhesion molecules involved in adhesion of THP-1 cells to CVM, expressions of various cell adhesion molecules on the surface of THP-1 cells adhering to CVM were measured. β-actin, β-catenin, and β1-integrin expressions did not change in non-stimulated THP-1 cells cultured on CVM compared with those in cells cultured in a flask, but β2-integrin expression markedly increased.

Development of an oxygenation culture method for activating the liver-specific functions of HepG2 cells utilizing a collagen vitrigel membrane chamber

We recently developed a collagen vitrigel membrane (CVM) chamber possessing a scaffold composed of high-density collagen fibrils. In this study, we first confirmed that the advantage of CVM chamber in comparison to the traditional culture chamber with porous polyethylene terephthalate membrane is to preserve a culture medium poured in its inside even though the under side is not a liquid phase but solid and gas phases. Subsequently, we designed three different culture systems to grow HepG2 cells in a culture medium (liquid phase) on the CVM which the under side is a culture medium, a plastic surface (solid phase) or 5 % CO₂ in air (gas phase) and aimed to develop a brief culture method useful for activating the liver-specific functions and analyzing the pharmacokinetics of fluorescein diacetate. HepG2 cells cultured for 2 days on the liquid–solid interface and subsequently for 1 day on the liquid–gas interface represented excellent cell viability and morphology in comparison to the others, and remarkably improved albumin secretion and urea synthesis to almost the same level of freshly isolated human hepatocytes and CYP3A4 activity to about half the level of differentiated HepaRG cells. Also, the cells rapidly absorbed fluorescein diacetate, distributed it in cytosol, metabolized it into fluorescein, and speedily excreted fluorescein into both bile canaliculus-like networks and extracellular solution. These data suggest that hepatic structure and functions of monolayered HepG2 cells can be induced within a day after the oxygenation from beneath the CVM.

Predictive performance of the Vitrigel-eye irritancy test method using 118 chemicals

We recently developed a novel Vitrigel‐eye irritancy test (EIT) method. The Vitrigel‐EIT method is composed of two parts, i.e., the construction of a human corneal epithelium (HCE) model in a collagen vitrigel membrane chamber and the prediction of eye irritancy by analyzing the time‐dependent profile of transepithelial electrical resistance values for 3 min after exposing a chemical to the HCE model. In this study, we estimated the predictive performance of Vitrigel‐EIT method by testing a total of 118 chemicals. The category determined by the Vitrigel‐EIT method in comparison to the globally harmonized system classification revealed that the sensitivity, specificity and accuracy were 90.1%, 65.9% and 80.5%, respectively. Here, five of seven false‐negative chemicals were acidic chemicals inducing the irregular rising of transepithelial electrical resistance values. In case of eliminating the test chemical solutions showing pH 5 or lower, the sensitivity, specificity and accuracy were improved to 96.8%, 67.4% and 84.4%, respectively. Meanwhile, nine of 16 false‐positive chemicals were classified irritant by the US Environmental Protection Agency. In addition, the disappearance of ZO‐1, a tight junction‐associated protein and MUC1, a cell membrane‐spanning mucin was immunohistologically confirmed in the HCE models after exposing not only eye irritant chemicals but also false‐positive chemicals, suggesting that such false‐positive chemicals have an eye irritant potential. These data demonstrated that the Vitrigel‐EIT method could provide excellent predictive performance to judge the widespread eye irritancy, including very mild irritant chemicals. We hope that the Vitrigel‐EIT method contributes to the development of safe commodity chemicals. Copyright © 2015 The Authors. Journal of Applied Toxicology published by John Wiley & Sons Ltd.

The sensitivity, specificity and accuracy of Vitrigel‐EIT method in comparison to GHS were 90.1%, 65.9% and 80.5%, respectively. In case of eliminating nine chemicals showing pH 5 or lower, those were improved to 96.8%, 67.4% and 84.4%, respectively. Meanwhile, nine of 16 false‐positive chemicals were classified irritant by EPA and immunohistologically confirmed to have an eye irritant potential. These data demonstrated that the Vitrigel‐EIT method could provide excellent predictive performance to judge the widespread eye irritancy, including mild irritant chemicals.

Fluid dwell impact induces peritoneal fibrosis in the peritoneal cavity reconstructed in vitro

Peritoneal fluid dwell impacts the peritoneum by creating an abnormal physiological microenvironment. Little is known about the precise effects of fluid dwell on the peritoneum, and no adequate in vitro models to analyze the impact of fluid dwell have been established. In this study, we developed a peritoneal fluid dwell model combined with an artificial peritoneal cavity and fluid stirring generation system to clarify the effects of different dwelling solutions on the peritoneum over time. To replicate the peritoneal cavity, we devised a reconstructed peritoneal cavity utilizing a mesothelial layer, endothelial layer, and collagen membrane chamber. The reconstructed peritoneal cavity was infused with Dulbecco's modified Eagle's medium, saline, lactated Ringer's solution or peritoneal dialysis solution with repeated 4-h dwells for 10 or 20 consecutive days. The above-described solutions induced epithelial-mesenchymal transition (EMT) and hyperplasia of mesothelial cells. All solution types modulated nitric oxide synthase activities in mesothelial and endothelial cells and nitric oxide concentrations in dwelling solutions. Inhibition of nitric oxide synthase activity acted synergistically on mesothelial EMT and hyperplasia. The present findings suggest that solutions infused into the peritoneal cavity are likely to affect nitric oxide production in the peritoneum and promote peritoneal fibrosis. Our newly devised peritoneal cavity model should be a promising tool for understanding peritoneal cellular kinetics and homeostasis.

Critical appraisal of alternative irritation models: three decades of testing ophthalmic pharmaceuticals

BACKGROUND

Testing ocular tolerability of ocular pharmaceuticals is an essential regulatory requirement. The current approved reference model (gold standard) for ocular irritation testing is the Draize test. However this method is subjective and involves using live animals, hence the need to develop alternative in vitro and ex vivo testing strategies.

SOURCE OF DATA

Pubmed, Science Direct, Scopus, Google Scholar, Medline, Current Content, Web of Science and validation reports from international regulatory bodies; The Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM) and European Centre for the Validation of Alternative Methods (ECVAM) were searched for in vitro alternatives.

AREA OF AGREEMENT

Whilst no single in vitro test can effectively replace the Draize eye irritation test, regulatory bodies and cosmetic/pharmaceutical industries agree that there is a need for in vitro alternatives with validated endpoints to evaluate pharmaceutical ingredients and finished eye products.

AREA OF CONTROVERSY

There is no single in vitro test / assay that can predict the ocular irritation potential of mild to moderate test substances.

AREA TIMELY FOR DEVELOPING RESEARCH

This review provides a critical appraisal of the selected in vitro and ex vivo ocular toxicity models recommended by international regulatory bodies. These include cytotoxicity methods, biochemical systems and ex vivo assays. The latter are approved by ECVAM as in vitro alternatives for the well-known Draize test. Hen's egg test-chorioallantoic membrane and the isolated rabbit eye test are also accepted by regulatory agencies in France, Germany, the Netherlands and the UK. A combination of ex vivo assays along with histological examination of excised bovine cornea can predict the conjunctival and corneal tolerability and cover a wider range of ocular pharmaceutical substances.

Epithelial-to-mesenchymal transition and slit function of mesothelial cells are regulated by the cross talk between mesothelial cells and endothelial cells

Peritoneal dysfunction is a major factor leading to treatment failure of peritoneal dialysis (PD). However, the precise mechanism of the peritoneal diffusion changes related to PD remains to be elucidated. To this end, we have established a novel peritoneal diffusion model in vitro, which consists of a three-dimensional culture system using a collagen vitrigel membrane chamber and a fluid-stream generation system. This artificial peritoneal model revealed that high-glucose culture medium and fluid flow stress promoted the epithelial-mesenchymal transition (EMT) process of mesothelial cells and that endothelial cells inhibited this mesothelial EMT process. Mesothelial cells in the EMT state showed high expression of connective tissue growth factor and low expression of bone morphogenic protein-7, while non-EMT mesothelial cells showed the opposite expression pattern of these two proteins. In addition, these protein expressions were dependent on the presence of endothelial cells in the model. Our model revealed that the endothelial slit function was predominantly dependent on the covering surface area, while the mesothelial layer possessed a specific barrier function for small solutes independently of the surface area. Notably, a synergic barrier effect of mesothelial cells and endothelial cells was present with low-glucose pretreatment, but high-glucose pretreatment abolished this synergic effect. These findings suggest that the mesothelial slit function is not only regulated by the high-glucose-induced EMT process but is also affected by an endothelial paracrine effect. This peritoneal diffusion model could be a promising tool for the development of PD.