The overall aim is to establish a 3D retinal cell model which is physiologically-competent and predictive of human physiology. The model should consist of all the major cell types of the retina and enable their interplay: Müller and microglia, RPE and neurons (including photoreceptors). The model needs to resemble key morphological and functional features which can easily be addressed with a panel of functional readouts.
Over 60 million people worldwide are blind (Zhang K. et al., 2012). One of the leading causes of blindness in the ageing population is Age Related Macular Degeneration. There is currently no cure for this debilitating condition.
The eye is a complex organ comprising three major structures: the cornea, the lens and the retina. For the cornea and to a minor extent for the lens, in vitro models are available that enable, for example, testing of compounds for their potential to induce corneal irritation.
For the retina, there are no adequate in vitro models mainly due to its complex structure which consists of multiple cells types, including glia, neurons and the retinal pigment epithelium cells (RPE). There are some simple preclinical models available - for example, retinal explant organotypic cultures from early postnatal rodents resemble the retinal morphology to a high degree and contain the relevant cell types including Müller cells and RPE. However, they do not fully mature when cultured in vitro and therefore their translation to the mature human retina is limited. Human tissue is not readily available (Caffe A.R. et al., 2000, Pinzón-Duarte G et al., 2002) for these studies. Human-based in vitro models using primary or cell lines such as the RPE cell line ARPE19 (Dunn K.C. et al.,1996; Seigel G.M. et al.,1999) and the Müller cell line (MIO-M1) (Limb G.A. et al., 2002) do provide useful tools but again, their relevance to the mature human retina is limited.
Complex human stem cell-derived models (Gonzalez-Cordero A. et al., 2013) are beginning to emerge which form retinal spheroids in vitro but these resemble embryonic retinal development whereas mature phenotypes for example a ganglion cell can take up to 30 days to reach maturity. The spheroids often lack Müller and RPE cells which are key to a fully predictive model. Throughput is another concern as the generation and maintenance of these spheroids is often not amenable to industry use.
In summary, the currently available in vitro models have a number of limitations:
- They are typically based on a single cell type in culture and do not reflect the architecture of the mature human retina and the complex interplay between multiple cell types (e.g. photoreceptors, glial cells, endothelial cells, RPE cells).
- Integrated and functional measurements such as electrophysiology and high content imaging other than single cell behavior cannot typically be performed.
- Organotypic cultures of retina explants are typically not stable beyond a maximum of two to three weeks.
As a result of the current limitations, most pharmacological and toxicological studies to assess effects of drug candidates on the retina need to be performed in vivo. The degree of visual acuity and structure of the retina differs between animals and humans, reflecting the distinctive role of visual function between species (e.g. rats have more rods for low light scotopic vision, whilst humans have more cones for bright light phototopic vision, Jacobs et al., 2001). For models more representative of human visual function, it would therefore be highly desirable to have access to a human-relevant retinal cell model to support compound testing in vitro.
Many other retinal diseases have a genetic background and are lacking suitable animal models. Utilising patient-derived iPS retinal cell models could integrate a disease background and phenotype for compound testing which would address these concerns and provide the opportunity for the development of novel safety testing strategies.
Recent advances in the field of tissue engineering, such as by bioprinting, now provide the capabiity to address the development of a 3D model comprising the three major retinal cell types: Müller and microglia, neurons and RPE. This is the aim of this CRACK IT Challenge.
Such a model would enable the in vitro assessment of drug candidate-induced alterations on the cellular phenotypes and functions which have relevance in vivo, and which can be used not only for pharmacological and early toxicological assessment but also for disease modelling. This would also provide the potential for a leaner early drug development programme for ophthalmology indications as, for example, optimisation related to mechanistic toxicity could be performed earlier in the cycle, shortening preclinical development and enabling faster time to the clinic.
Worldwide, there are currently more than 600 R&D projects in the field of ophthalmology and the market for therapies targeting retinal disorders is expected to grow to $14.8 billion by 2022 (Pharmaventures, 2015). The majority of studies for efficacy and safety testing in opthalmological drug development are performed in animals (mainly rodents and rabbits) that provide both functional and histological readouts. For example, in vivo studies investigating ocular safety of new compounds use at least 20 animals per compound.
The proposed new model set by the Retinal 3D Challenge has the potential to replace the use of animals in the discovery of new opthalmologic drugs as well as screen out a significant number of compounds that would fail later in the pipeline, thus avoiding unnecessary animal use. The in vivo studies that are still required will be designed with improved mechanistic toxicological and pharmacological knowledge to enable more relevant dosing and group numbers and potentially deliver more refined humane endpoints.
Phase 1 winners
- Professor Michael Cheetham, University College London, £99,999.
- Professor Stefan Liebau, University of Tübingen, £99,982.
- Professor Majlinda Lako, Newcastle University, £99,982.
Full Challenge information
The following Challenge Panel will consider applications submitted to this Challenge:
|Dr Malcolm Skingle (Chair)||GlaxoSmithKline|
|Dr Philip Hewitt (Sponsor)||Merck KGaA|
|Dr Michael Schmitt (Sponsor)||Merck KGaA|
|Dr Stefan Kustermann (Sponsor)||F. Hoffmann-La Roche Ltd|
|Dr Marianne Uteng (Sponsor)||Novartis|
|Professor Mark Hankins||University of Oxford|
|Dr Julie Sanderson||University of East Anglia|
|Professor Julie Daniels||University College London|
|Dr Francois Paquet-Durand||University of Tübingen|
|Professor Rachel Williams||University of LIverpool|
|Dr Martino Picardo||Stevenage Bioscience Catalyst|