Quantification of Eye Irritation Based upon In Vitro Changes of Corneal Thickness

Abstract

Measurement of corneal swelling is an objective assessment of irritation in in vivo tests. The same measurement may be made in in vitro tests using the isolated eye test (IET), which has been recommended by the European Community as an alternative for in vivo eye irritation tests. In order to compare the corneal swelling in vivo and in vitro, tests were performed with 34 substances. The in vitro test results were assessed for their ability to predict eye-irritant potential.

The corneal swelling data in vivo after 4, 24, 48 and 72 hours were compared with the in vitro data obtained after 2 and 4 hours. Slight linear correlation was found between the corneal swelling in vivo after 4 hours and the corneal swelling in vitro after 4 hours (r=0.77).

The substances tested can be divided into two groups, according to their ability to create opacity in either the epithelial layer or in the stroma. When the substances causing epithelial opacity were omitted from the comparison, a much better linear correlation was obtained between the mean corneal swelling 121 vitro over 2 and 4 hours and the mean corneal swelling calculated in vivo for all animals and over three observation times (24, 48 and 72 hours; r=0.91), the latter being the observation times prescribed by EC legislation (1). A comparison of the mean corneal opacity scores observed in vivo and the mean percentage corneal swelling in vitro gave a satisfactory linear correlation (r=0.89). From this study it can be deduced that a mean corneal swelling of 55%, obtained in isolated eyes over 2 and 4 hours, corresponds with the limit for classification as irritant, to the eye. When this criterion was applied to all substances causing no epithelial opacity (n=28), only one false positive and no false negatives were found.

Keywords

corneal swelling eye irritation pachometer opacity in vitro

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