A critical evaluation of predicting ocular irritancy potential from an in vitro cytotoxicity assay

Numerous in vitro cytotoxicity assays have been proposed as potential alternatives to the Draize eye irritancy test. The results reported, based upon the rank correlation of ocular irritancy with cytotoxicity, have been encouraging. However, direct calibration of in vivo to in vitro data utilizing several categories of chemicals has not been reported. This study evaluated the use of in vitro cytotoxicity data for predicting the ocular irritancy potential of 24 chemicals (six surfactants, seven alcohols, four ketones, four acetates, and three aromatics). BALB/c 3T3 cells were grown overnight, then exposed for 30 min to at least four different concentrations of each chemical (expressed as volume percentage). Linear regression analysis of the log concentration versus percentage of control growth was used to calculate the concentration of toxicant that inhibited the normal growth rate by 50% (GI50). The rank ordering of cytotoxicity based upon the GI50s was surfactants greater than aromatics greater than alcohols greater than ketones or acetates. The larger molecular weight representative of each series (i.e., 2-ethyl-1-hexanol for alcohols) had lower GI50 values than those of the lower molecular weight substances. The GI50 values were then directly calibrated against in vivo ocular irritancy quantitated as percentage corneal swelling following exposure of rabbits to the same test chemicals. A significant linear correlation between cytotoxicity and ocular irritancy was established only for surfactants and alcohols. For acetates, ketones, and aromatics there was little correlation. The overall poor correlation between cytotoxicity and ocular irritancy was attributed to differences in mechanisms of irritancy. The lack of correlation illustrates that in vitro cytotoxicity data cannot be used to predict the ocular irritancy potential of a broad spectrum of chemicals.