The Short Time Exposure (STE) assay, developed by Kao Corporation (Japan), is an in vitro assay used to assess acute eye irritation potential as an alternative to the traditional in vivo Draize test. The test method evaluates the cytotoxicity induced by a series of test chemical dilutions in a monolayer of rabbit corneal fibroblasts (Statens Seruminstitut Rabbit Cornea – SIRC) after a single five-minute exposure.
Two prediction models were initially developed for the STE assay – one categorizes the test material as either an ocular irritant or non-irritant; the second provides a rank classification of ocular irritation (mild, moderate or severe). The prediction models are based on the mean % viability of a 5% and 0.05% dilution of the test material relative to solvent controls.
A test material is categorized as Non-Irritant if the mean % viability of the 5% dilution was > 70%; and categorized as Irritant if the mean % viability was ≤ 70%.
A rank irritancy classification of a test material may be further determined based upon the evaluation of cytotoxicity at both dilutions. For the 5% dilution, a score of 0 is assigned if the cell viability is > 70%, and a score of 1 is assigned if the cell viability is ≤ 70%. For the 0.05% dilution, a score of 1 is assigned if the cell viability is > 70%, and a score of 2 is assigned if the cell viability is ≤ 70%. The sum of the scores, ranging from 1 to 3, determines the eye irritation rank classification as a minimal, moderate, or severe eye irritant.
These two approaches may be taken to support test material screening or product development and characterization goals.
For regulatory classification and labeling, the assay may be used to identify test substances and mixtures that induce serious eye damage as well as those that do not require classification for either serious eye damage or eye irritation, as defined by the United Nations (UN) Globally Harmonized System of Classification and Labeling of Chemicals (GHS), as described in OECD Test Guideline 491.
As stated in OECD TG 491: “The STE test method cannot be used for the identification of test chemicals as UN GHS Category 2, Category 2A (eye irritation) or UN GHS Category 2B (mild eye irritation), due to the considerable number of UN GHS Category 1 chemicals under-predicted as UN GHS Category 2, 2A, or 2B and UN GHS No Category chemicals over-predicted as UN GHS Category 2, 2A, or 2B. For this purpose, further testing with another suitable method may be required.”