A spontaneously immortalized human keratinocyte line, HaCaT, was used as an in vitro model to predict the cutaneous irritation of anionic surfactants. For this purpose, a number of sodium salts of N-alkyl sulphates with hydrocarbon chain lengths varying between C-8 and C-16 were studied for possible cytotoxic effects. The endpoints used to assess toxicity were uptake of the vital dye neutral red (NR) and cell morphology criteria 24 h after dosing. A linear proportionality between keratinocyte number and NR uptake was established. All tested surfactants 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 (IC-50) ranged from 0.15 mmol (sodium lauryl sulphate, C-12) to 1.23 mmol (sodium octyl sulphate, C-8) The in vitro cytotoxicity data were highly reproducible when the test was repeated after several weeks. 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 (300 mu l of 20 mmol aqueous solution) to the volar forearm of human volunteers. There were significant linear correlations between the IC-50 values and both barrier damage (transepidermal water loss) and erythema (as evaluated by skin colour reflectance measurements). For the test substances, however, the sensitivity of the in vitro system was between 10 and 100 times higher than that observed in human skin in vivo. This quantitative difference can largely be ascribed to the effective permeability barrier of normal human skin in vivo, which protects living keratinocytes from the cytotoxic effects of surfactant molecules. The results indicate that normal human keratinocytes in culture are a promising screening method for predicting the irritation potential of anionic surfactants. Confirmation, however, has still to be obtained by appropriate in vivo testing in human volunteers.
