Comparison of in vitro and human in vivo dermal irritancy data for four primary irritants

The value of the Skintex dermal assay system as a means of predicting the irritancy of four primary irritants at various concentrations was investigated. The four compounds tested were: benzalkonium chloride at 0.1%, 0.5%, 1.0% and 2.0%; hydrochloric acid at 2.0%, 5.0%, 10% and 20%; phenol at 3.0%, 7.5%, 10%, 12%, 15% and 20%; trichloracetic acid at 7.5%, 10%, 20% and 30%. The results from the in vitro system were compared with published human in vivo data, which was obtained using 100 volunteers, and which graded the irritant reactions using a visual scale. A 2 x 2 contingency table was constructed and the following parameters of the assay were determined: sensitivity, 82%; specificity, 71%; and positive predictive value, 82%. The in vivo dose-response curves for each of the four substances were compared with the in vitro dose-response curves, and correlation coefficients were calculated. The in vitro dose-response curves for benzalkonium chloride (r(2) = 0.987) and phenol (r(2) = 0.994) were strikingly similar to those generated in vivo, possibly indicating that the mechanisms of action in vivo and in vitro are similar for these two compounds. These studies should be extended to a broader variety of chemicals before unknowns can be characterized with a reasonable degree of certainty using this in vitro method.