Quantitative assessment of primary skin irritants in vitro in a cytotoxicity model: Comparison with in vivo human irritation tests

Background: While great efforts have been made in recent years to develop in vitro methods for assessing skin irritation potential, there are relatively few data that correlate in vitro data with in vivo data. Objectives: To expand our previously reported investigations on in vitro vs. in vivo correlation of a series of homologous N-alkyl sulphates of different alkyl chain length to include primary skin irritants of different chemical classes. Methods: Anionic surfactants (three different sodium alkyl sulphonates and sodium lauryl sulphate), cationic surfactants (three alkyl trimethyl ammonium bromides), non-ionic surfactants (polyoxyethylene-20-cetyl ether and Tween 20), benzoic acid, dimethyl sulphoxide and phenol were chosen as model irritants. A spontaneously immortalized human keratinocyte line, HaCaT, was used as an in vitro model to predict the cutaneous irritation. The end-point used to assess toxicity was uptake of the vital dye neutral red (NR) 24 h after dosing. The cytotoxicity data from these assays were compared with the irritant responses (as evaluated by measurement of erythema and transepidermal water loss) obtained after 24-h application of the same compounds (100 μL of 20 mmol L^-1 aqueous solution) to the volar forearm of human volunteers. Results: All tested irritants had cytotoxic effects as demonstrated by a decreased NR uptake, which showed a clear dose-response relationship. Concentrations resulting in 50% inhibition of NR uptake (IC₅₀) ranged from 8 μmol L^-1 (hexadecyl trimethyl ammonium bromide) to 328 mmol L^-1 (dimethyl sulphoxide). We found a good overall correlation between in vitro cytotoxicity (NR uptake IC₅₀ values) and in vivo irritation potential in humans. Only the high molecular weight compounds Tween 20 and polyoxyethylene-20-cethyl ether were problematic, as their irritation potential was overestimated by the in vitro assay. This non-conformity of these high molecular weight (> 1000) compounds was expected, and can be largely attributed to the epidermal permeability barrier. The epidermal barrier, which greatly limits the percutaneous penetration of xenobiotics in vivo, does not exist in cell culture models. Conclusions: The in vitro cytotoxicity model is a useful screening tool, but data should be interpreted critically and require confirmation by appropriate in vivo studies.