Assessment of recovery from ocular irritancy using a human tissue equivalent model

The SKIN(2) ZK1200 tissue equivalent model has been used in an exploratory study of recovery from ocular irritation. Nine substances from the ECETOC eye irritation reference chemicals data bank were tested. The cellular viability of the tissue model was measured using the MTT assay immediately after chemical exposure and after incubation periods corresponding to observation times used in the Draize tests. The cellular viability of the tissue specimens exposed to moderate irritants, with modified maximum average score (MMAS) values between 15 and 40 (= 5), returned to control levels within 1-7 days. Tissue specimens exposed to 1 and 10% benzalkonium chloride showed significantly reduced MTT activities at day 14 and day 21. Mild ocular irritants, with MMAS values below 3 (n = 2), did not induce significant depressions of the MTT activities of the tissue specimens after 60 min exposure. A relatively good correlation (r = 0.73) was obtained between the exposure times used in vitro and Draize MMAS values. Moreover, the incubation periods needed for the exposed SKIN(2) ZK1200 to regain control MTT activities showed a good agreement (r > 0.90) with days-to-clear ocular irritancy in the Draize test.