Predictive ability of reconstructed human epidermis equivalents for the assessment of skin irritation of cosmetics.

The aim of this study was to examine the concordance between human in vivo and in vitro skin irritation classifications of cosmetic products and to evaluate the correlations between the different parameters. For that purpose, 22 formulations from product development test series, covering the full range of in vivo scores and representing different cosmetic product classes, were tested in vivo (modified Frosch-Kligman Soap Chamber Patch Test with repetitive occlusive application) and in vitro using two epidermis equivalents commercially available as kits (EpiDerm and EPISKIN) and one in-house model (Cosmital). In vivo, skin reactions (erythema, dryness and fissures) were visually evaluated and, in addition, skin redness and transepidermal water loss (TEWL) were measured by means of technical instruments. The parameters measured in vitro were the percent cell viability in the MTT reduction assay, with ET(50) determination, and the extracellular release of the pro-inflammatory mediator IL-1alpha and of the cytosolic enzyme lactate dehydrogenase (LDH), into the culture medium collected after topical application of the products for different exposure times (time-course assay). In general, good Spearman rank correlations could be observed between the different in vivo parameters (with the exception of TEWL and dryness at day 2). Furthermore, high correlation coefficients were obtained by comparing the different in vitro parameters (except for LDH release) and different models, which allowed to conclude that the results obtained with the different reconstructed epidermis models were very similar. A comparison between in vivo and in vitro parameters resulted in the best rank correlation for ET(50), then in decreasing order, for the percent MTT viability at 16 h, the IL-1alpha release and finally, for LDH release, where the correlation was generally low. A direct comparison of the mean total scores (sum of erythema, dryness and fissures at day 5) of the 22 products with the best predictor, ET(50) obtained with the three reconstructed epidermis models, using simple linear regression analysis resulted in a coefficient of correlation R=0.94 for EpiDerm, R=0.90 for Cosmital and R=0.84 for EPISKIN. Multivariate descriptive statistics showed that the in vitro parameters, MTT viability evaluated after the 16-h exposure and ET(50), as well as the in vivo parameters, sum of visual scores at day 5 and chromameter value, were the best endpoints to discriminate between irritant and non-irritant products. Using the in vivo mean total scores at day 5 with a cut-off value at 2 and the in vitro percent MTT viability after the 16-h exposure with a cut-off value at 50% to classify the products, the same two-by-two contingency table was obtained for all the three reconstructed epidermis models with sensitivity=92%, specificity=100% and observed concordance=95% (kappa=0.91; 95% confidence interval 0.74-1.08). This classification system was a satisfactory and relevant approach to discriminate the "irritant" from the "non-irritant" cosmetic products in this study. In conclusion, this study demonstrated the usefulness of reconstructed human epidermis equivalents for the in vitro assessment of the irritation potential of a series of cosmetic products. These models allow the measurement of quantifiable and objective endpoints relevant to in vivo irritative phenomena.