Prediction of phototoxic potential using human A431 cells and mouse 3T3 cells

An assay using established cell lines, human A431 epidermal cells and mouse 3T3 fibroblasts, has been developed to predict the phototoxic potential of compounds. The test determines the viability of the two cell lines in response to UV light in both the presence and absence of the test compound. The end point for cytotoxicity is determined from the mitochondrial dehydrogenase conversion of a tetrazolium salt (MTT) to a colored formazan product. The cytotoxicity of the test compound is established prior to UV exposure, and the highest no-effect concentration observed is then applied to cells that are subsequently exposed to different periods of UVA and UVA plus UVB light. A phototoxic effect is considered to have occurred when a biologically significant enhancement of toxicity is shown for the UV light with the compound present when compared to that of UV light alone. The test system has been validated with 30 compounds classified as strong, idiosyncratic, and negative based on the frequency of reported adverse reactions in humans. The in vitro phototoxicity assay was able to highlight the potential for phototoxicity in the strong category of phototoxic compounds and several of the idiosyncratic compounds. Only one of the negative compounds produced any activity in the assay in terms of enhancing UV toxicity. Some of the compounds were shown to protect the cells from the toxic effects of UV exposure.