In vitro cytotoxicity test for estimating non-ocular irritation dose of ophthalmic solutions

The in vitro cytotoxicity test for estimating the non-ocular irritation dose of ophthalmic solutions was investigated. In the in vitro test, normal human epidermal keratinocytes (NHEK) in a confluent monolayer were incubated for 48 hr in a medium with test compounds. The concentration of a test compound which causes a 50% reduction in NHEK viability was determined as IC50 by MTT colorimetric assay. For comparison, the in vivo rabbit ocular irritation tests were carried out by the standard Draize method. The maximum concentration, which did not show any ocular irritation, was determined as DS0. The results showed the correlation coefficient between the IC50 values and the DS0 values for 19 test compounds to be 0.82. However, the correlation coefficients for 10 compounds, which have IC50 values of less than 300 micrograms/ml, and for 7 alcohols were 0.99. The IC50-DS0 correlation curves obtained could be utilized as the critical concentrations for ocular irritation. These results suggest that our in vitro/in vivo test can estimate non-ocular irritation dose of the ophthalmic preparations in advance of the in vivo tests.