Evaluation of two in vitro ciliated epithelial systems, dog trachea and frog palate, for potential as screens for acute inhalation toxicity

We have evaluated two in vitro ciliated epithelial systems, dog trachea and frog palate, for their potential as toxicological screens for inhaled chemicals. We examined the effects of a series of nonpolar narcotic agents (alcohols and ketones) and related observed toxicities (EC₅₀) to hydrophobicity (log K_o/w). The baseline QSAR for the dog trachea system is log 1/EC₅₀ = 0.80 (log K_o/w) - 2.99; n = 7, r² = 0.982, s = 0.18, F = 275.63, Pr > F = 0.0001. The frog palate baseline QSAR is log 1/EC₅₀ = 0.81 (log K_o/w) - 3.12; n = 7, r² = 0.991, s = 0.13, F = 565.66, Pr > F = 0.0001. The slopes of these QSARs are equivalent to the slopes of QSARs from established systems, including in vivo mouse inhalation, and differing intercepts have been explained on the basis of method of exposure. We also examined a set of bioreactive chemicals, whose toxicities were expected to elicit greater toxicity than would be predicted by their hydrophobicity alone, and reported computed excess toxicity (T_e) values greater than 2 for each bioreactive chemical. Mechanisms of toxicity represented by the set of bioreactive chemicals tested in our study include bimolecular nucleophilic substitution, Michael-type addition to a carbon-carbon double bond, Schiff base formation, uncoupling of oxidative phosphorylation, and reversible and irreversible inhibition of acetylcholinesterase. Ranking the bioreactive chemicals in increasing order of Te for the dog trachea results in: trans-2-octenal < benzyl chloride < carbaryl < malathion < heptaldehyde < 4,6-dinitro-o-cresol. In the frog palate, the increasing rank order of T_e is: trans-2-octenal < heptaldehyde < benzyl chloride < 4,6-dinitro-o-cresoI < carbaryl < malathion. The differences in rank orders between dog and frog indicate that more bioreactive chemicals need to be studied with these in vitro systems in order to determine if certain chemical classes are more or less appropriate for evaluation with one or the other system. Based on the positive preliminary data in this investigation, we feel that, as one component of a battery of tests, these ciliated in vitro systems would provide valuable, usable information about potential respiratory effects of inhaled chemicals. The additional advantages of lower cost and efficiency of data collection make these systems attractive as alternatives to the current whole animal test system (mouse inhalation chamber).