There are three primary categories into which ocular irritants can be classified under the United Nations Globally Harmonised System (GHS). The most serious irreversible effects are typically caused by ocular corrosion and are classified as Category 1. Chemicals that cause ocular irritation but not lasting damage are designated Category 2, and non-irritating compounds are labelled as No Category. In vitro assays that can assess either ocular irritation or serious eye damage have been validated and accepted by the OECD. Despite this, there is currently no single in vitro method that can fully replace in vivo testing unless combined with data from other in vitro or ex vivo tests.
We recently performed a study to explore the possibility of a tiered testing cascade to accurately assign test articles to their appropriate categories. In the study, 15 compounds with known and varying categorisation levels were initially screened using the MatTek Eye Irritation Test (EIT) – an assay that is capable of distinguishing between a substance that is No Category and one that is either Category 1 or 2. Within the study, performance of the EIT was excellent with 14 out of 15 of the test compounds being correctly classified. The Short Time Exposure (STE) assay was used to then assess the same set of substances tested in the EIT assay. Although the STE assay was capable of correctly identifying No Category (100%), it failed to identify many of the Category 2/1 substances, especially if solid compound or insoluble compound was being assessed. This is unsurprising as the STE assay is a 2D cell-based assay. Additionally, some of the compounds were MTT auto-inducers or interfered with the MTT assay which further impacted on the STE assay performance.
The data from this study indicate the EIT has high sensitivity and should remain an integral part of the tiered testing strategy. The STE assay has limitations in terms of its predictivity which to a certain extent is linked to the fact it is a 2D monolayer model and so is not appropriate for testing solid preparations. Pre-assessment of the test articles for solubility and potential interference with the MTT assay may be one approach to improve the predictivity.
This research was presented as SOT 2016 and was supported by the CON4EI consortium.