“Open Source”–Based Engineered Human Tissue Models: A New Gold Standard for Nonanimal Testing Through Openness, Transparency, and Collaboration, Promoted by the ALEXANDRA Association

Skin Equivalent Models: Protocols for In Vitro Reconstruction for Dermal Toxicity Evaluation

This chapter presents the protocols for developing of skin equivalents (SE) and reconstructed human epidermis (RHE) models for dermal toxicity evaluation as an alternative method to animal use in research. It provides a detailed protocol for the in vitro reconstruction of human skin from primary keratinocytes, melanocytes, and fibroblasts obtained from foreskin biopsies, including the procedures for reconstruction of a stratified epidermis on a polyester membrane. SE and RHE developed through these methods have been proven suitable not only for dermal toxicity studies, but also for investigating of pathological conditions in the skin, such as diabetes and invasion of melanoma.

In vitro models of dermatophyte infection to investigate epidermal barrier alterations

Fungal infections of the skin, known as dermatophytoses, are initiated at the epidermal barrier and lead to dysfunctions of the stratum corneum and cornified skin appendages. Dermatophytosis affects a significant part of the human population and, despite the availability of effective treatments, its prevalence is still increasing. Numerous dermatophyte species are able to induce lesions in both animals and humans, with different clinical pictures and host inflammatory responses. The understanding of the infectious process and of tissue responses has been impeded by discrepancies between observations in vivo or in research models. Indeed, cells cultured as monolayers do not undergo the keratinization process required to study the adherence and invasion of dermatophytes. Animal models lack relevance to study human dermatophytosis because of species-specific differences in the development of lesions and inflammatory responses. This review focuses on the recent development of cultured human skin equivalents, which partly overcomes those limitations and allows improved understanding of the pathogenesis of dermatophytosis in human being, especially the impacts of infection on epidermal barrier integrity.

Skin-on-a-Chip: Transepithelial Electrical Resistance and Extracellular Acidification Measurements through an Automated Air-Liquid Interface

Skin is a critical organ that plays a crucial role in defending the internal organs of the body. For this reason, extensive work has gone into creating artificial models of the epidermis for in vitro skin toxicity tests. These tissue models, called reconstructed human epidermis (RhE), are used by researchers in the pharmaceutical, cosmetic, and environmental arenas to evaluate skin toxicity upon exposure to xenobiotics. Here, we present a label-free solution that leverages the use of the intelligent mobile lab for in vitro diagnostics (IMOLA-IVD), a noninvasive, sensor-based platform, to monitor the transepithelial electrical resistance (TEER) of RhE models and adherent cells cultured on porous membrane inserts. Murine fibroblasts cultured on polycarbonate membranes were first used as a test model to optimize procedures using a custom BioChip encapsulation design, as well as dual fluidic configurations, for continuous and automated perfusion of membrane-bound cultures. Extracellular acidification rate (EAR) and TEER of membrane-bound L929 cells were monitored. The developed protocol was then used to monitor the TEER of MatTek EpiDerm^TM RhE models over a period of 48 h. TEER and EAR measurements demonstrated that the designed system is capable of maintaining stable cultures on the chip, monitoring metabolic parameters, and revealing tissue breakdown over time.

A new reconstructed human epidermis for in vitro skin irritation testing

Different models of reconstructed human epidermis (RHE) are currently validated to assess skin irritation in vitro and ultimately to the animal replacement of the Draize test. The development of a new RHE model is a challenge for many laboratories, representing a potential gain of autonomy and improvement of technological knowledge. The Organization for Economic Co-operation and Development (OECD) encourages the development of new models and, for this purpose, offers a thorough guideline on quality control parameters (OECD TG 439 performance standards). This work aimed to develop an RHE model (i.e. USP-RHE) for in vitro skin irritation assays, following the OECD TG 439. The developed model presents a well-differentiated epidermis similar to the Validated Reference Methods (VRM) and to native human epidermis. Quality parameters, i.e. optical density of negative control, tissue integrity and barrier function, were similar to VRM and in accordance with OECD TG 439. Moreover, the USP-RHE model was shown to have 85,7% of specificity (6/7), 100% of sensitivity (6/6) and 92,3% of accuracy (12/13) when compared to in vivo UN GHS classification. The within-laboratory reproducibility was 92.3% (12/13). Thus, we demonstrated that USP-RHE model attends to all OECD TG 439 performance standards and is ready to be used by private and public laboratories and companies for future validation studies.

In vitro models for evaluating safety and efficacy of novel technologies for skin drug delivery

For preclinical testing of novel therapeutics, predictive in vitro models of the human skin are required to assess efficacy, absorption and safety. Simple as well as more sophisticated three-dimensional organotypic models of the human skin emerged as versatile and powerful tools simulating healthy as well as diseased skin states. Besides addressing the demands of research and industry, such models serve as valid alternative to animal testing. Recently, the acceptance of several models by regulatory authorities corroborates their role as important building block for preclinical development. However, valid assessment of readout parameters derived from these models requires suitable analytical techniques. Standard analytical methods are mostly destructive and limited regarding in-depth investigation on molecular level. The combination of adequate in vitro models with modern non-invasive analytical modalities bears a great potential to address important skin drug delivery related questions. Topics of interest are for instance the assessment of repeated dosing effects and xenobiotic biotransformation, which cannot be analyzed by destructive techniques. This review provides a comprehensive overview of current in vitro skin models differing in functional complexity and mimicking healthy as well as diseased skin states. Further, benefits and limitations regarding analytical evaluation of efficacy, absorption and safety of novel drug carrier systems applied to such models are discussed along with a prospective view of anticipated future directions. In addition, emerging non-invasive imaging modalities are introduced and their significance and potential to advance current knowledge in the field of skin drug delivery is explored.

Animal-Free Chemical Safety Assessment

The exponential growth of the Internet of Things and the global popularity and remarkable decline in cost of the mobile phone is driving the digital transformation of medical practice. The rapidly maturing digital, non-medical world of mobile (wireless) devices, cloud computing and social networking is coalescing with the emerging digital medical world of omics data, biosensors and advanced imaging which offers the increasingly realistic prospect of personalized medicine. Described as a potential “seismic” shift from the current “healthcare” model to a “wellness” paradigm that is predictive, preventative, personalized and participatory, this change is based on the development of increasingly sophisticated biosensors which can track and measure key biochemical variables in people. Additional key drivers in this shift are metabolomic and proteomic signatures, which are increasingly being reported as pre-symptomatic, diagnostic and prognostic of toxicity and disease. These advancements also have profound implications for toxicological evaluation and safety assessment of pharmaceuticals and environmental chemicals. An approach based primarily on human in vivo and high-throughput in vitro human cell-line data is a distinct possibility. This would transform current chemical safety assessment practice which operates in a human “data poor” to a human “data rich” environment. This could also lead to a seismic shift from the current animal-based to an animal-free chemical safety assessment paradigm.

Catch-up validation study of an in vitro skin irritation test method based on an open source reconstructed epidermis (phase II)

To replace the Draize skin irritation assay (OECD guideline 404) several test methods based on reconstructed human epidermis (RHE) have been developed and were adopted in the OECD test guideline 439. However, all validated test methods in the guideline are linked to RHE provided by only three companies. Thus, the availability of these test models is dependent on the commercial interest of the producer. To overcome this limitation and thus to increase the accessibility of in vitro skin irritation testing, an open source reconstructed epidermis (OS-REp) was introduced. To demonstrate the capacity of the OS-REp in regulatory risk assessment, a catch-up validation study was performed. The participating laboratories used in-house generated OS-REp to assess the set of 20 reference substances according to the performance standards amending the OECD test guideline 439. Testing was performed under blinded conditions. The within-laboratory reproducibility of 87% and the inter-laboratory reproducibility of 85% prove a high reliability of irritancy testing using the OS-REp protocol. In addition, the prediction capacity was with an accuracy of 80% comparable to previous published RHE based test protocols. Taken together the results indicate that the OS-REp test method can be used as a standalone alternative skin irritation test replacing the OECD test guideline 404.