As part of the ongoing effort to reduce animal testing, the OECD recently released final test guidelines for two in vitro assays used to assess eye irritation or serious eye damage potential.
OECD Test Guideline 491 (adopted 28 July 2015) defines the Short Time Exposure (STE) test for identifying chemicals that cause serious eye damage. This test distinguishes whether a chemical is classified as UN GHS Category 1 or UN GHS No Category. According to the Test Guideline, a confluent monolayer of Statens Seruminstitut Rabbit Cornea (SIRC) cells are cultured and exposed to test articles diluted to 5% and 0.05%. After the exposure period, cell viability is measured using the MTT assay. If cell viability is less than or equal to 70% at both concentrations, the chemical is classified as GHS Category 1. If cell viability is above 70% at both concentrations tested, the chemical is classified as GHS No Category. Cell viability less than or equal to 70% at 5% and greater than 70% at 0.05% is not considered classifiable, and would require further testing.
OECD Test Guideline 492 (adopted 28 July 2015) defines the reconstructed human cornea-like epithelium (RhCE) test method of which the EpiOcular™ Eye Irritation Test (EIT) is the only currently available in vitro test method covered by this Test Guideline. The test is intended to evaluate chemicals that either do not require UN GHS labelling and classification (No category) or chemicals that potentially do require labelling (Category 1 and Category 2). Under Test Guideline 492, the EpiOcularTM tissue is exposed to a test substance, and cytotoxicity is measured by the MTT assay. A mean tissue viability greater than 60% categorises a material as UN GHS No Category whilst a result of less than or equal to 60% identifies a material as potentially requiring classification and labelling according to UN GHS (Category 2 or Category 1). In the latter situation, further testing with other test methods is required to determine the categorisation.
Employing these two assays in combination will determine if a compound is UN GHS No Category, GHS Category 1 or GHS Category 2. If it is determined that a compound is positive in the EIT, and unable to classify in the STE test then a Category 2 classification is probable, and further testing may be required to confirm this category and subsequent sub-categorisation. The flowchart below illustrates the steps involved in this approach.
Learn more about the Short Time Exposure (STE) and EpiOcular™ Eye Irritation assay.