Comparison of toxicity tests on human skin and epidermoid (A431) cells using free fatty acids as test substances

Several in-vivo methods can be used to determine the ability of chemical compounds to induce skin irritancy. In this study we estimated in vivo the capacity of several free fatty acids to induce skin irritancy and compared the results with those found in in vitro tests. Skin irritancy induced by free fatty acids (chain lengths: C6, C7, C9, C10, C11, C13 and C18) was evaluated in humans by means of laser-Doppler flowmetry (LDF) and visual scoring (VS). Both methods demonstrated that the toxic effect of free fatty acids determined by LDF and VS increased from C6 through C11 and decreased again for C13 and C18. The cytotoxic effect of these free fatty acids on cells was measured in vitro by incubation of human epidermoid cells (A431) with these compounds. It was determined by measuring: (a) the number of dead cells by inclusion of Trypan blue (TB); and (b) the number of living cells by mitochondrial metabolism of 3-[4,5-dimethylthiazole-2-yl]-2,5 diphenyltetrazolium bromide (MTT). The LD-50 concentrations decreased from C6 through C11 in both in-vitro assays. The results of the in-vitro assays for C13 and C18 both demonstrated a discrepancy. The cytotoxic effect of the free fatty acids expressed as LD-50 values, determined after 20 min with the TB assay, was seen at higher concentrations than after incubation for 18 h (MTT assay). From the results it was concluded that C13 in particular affected skin blood flow. We also determined correlation coefficients between the in-vivo and in-vitro methods. When C13 is excluded these coefficients ranged from -0.77 to -0.92.(ABSTRACT TRUNCATED AT 250 WORDS)