A component of the EPA Administrator’s 2019 directive to staff to reduce the requests for and funding of animal studies by 2035 was the request to develop a work plan that addresses the subject of “validation to ensure that NAMs [new approach methods] are equivalent to or better than the animal tests replaced” [107]. This work plan was released in June 2020 [108]. Similar language is present in the amended Toxic Substances Control Act (TSCA) that requires that NAMs provide “information of equivalent or better scientific quality and relevance” than traditional animal tests. A key consideration in the evaluation of what it means to be “equal to or better” is the explicit consideration of the variability, human relevance, and predictivity of the animal data. This paper is designed to consider these factors for eye irritation.
The rabbit eye test is widely used to meet regulatory requirements for agrochemical formulations, and has been a requirement for pesticide registration for decades [109]. However, the rabbit test involves subjective examination of ocular lesions, uses an exaggerated exposure duration, provides limited mechanistic information, has never been validated for its relevance to humans, uses live animals, and is associated with considerable variability (see Table 1 for details). Furthermore, interspecies differences in structure, anatomy, and physiology exist between rabbit and human eyes, for example, rabbits have a nictitating membrane, higher pH of the eye, a larger conjunctival cul-de-sac, and are not as efficient in tear production. Thus, the rabbit test is not appropriate for use as a standard for evaluating new methods.
The in vitro and ex vivo models described herein are more human relevant than the rabbit test because they include one or more of the following properties: (a) they allow for more precise control of the application and termination of dosing, (b) they model corneal tissue barrier functions and penetration kinetics, (c) they include relevant cell types within each of the tissue layers, (d) they provide quantitative results, (e) they have been shown to be reproducible and repeatable, (f) they do not directly use live animals for testing, and/or (g) they discriminate a range of cytotoxic responses within each layer. It is not necessary for a test system to include all of these traits to be useful and relevant; model with any one of these characteristics could be useful to address specific events in the assessment of eye irritation.
Considering all available information, the in vitro/ex vivo methods presented in this paper are equivalent or scientifically superior to the use of the rabbit test. Where discordant results exist between NAMs and the rabbit test, findings from the in vitro and ex vivo systems described herein should carry more weight than the rabbit data. The scientific validity of an in vitro/ex vivo method should be assessed by understanding the assay’s relevance to human biology and mechanisms of eye irritation. Ultimately, a replacement method that provides a model grounded in human biology will be as good as or better than the currently used rabbit test at protecting human health.
For more information read the paper at:
CUTANEOUS AND OCULAR TOXICOLOGY
https://doi.org/10.1080/15569527.2021.1910291
