In vitro alternatives and phototoxicity testing. I. Evaluation of in vitro phototoxicity assays

Three in vitro phototoxicity assays -- the photohaemolysis assay, the haemoglobin (Hb) photo-oxidation assay, and the 3T3 cell neutral red uptake phototoxicity (3T3 NRU PT) assay -- were evaluated for use as screening methods in predicting the phototoxicity of test substances. Twenty-seven test substances, including cosmetic ingredients and drugs, were assessed in this study. The phototoxicity predicted in each assay was compared with that in guinea-pigs. In total, nine phototoxic substances used in this study could be detected by some of the in vitro phototoxicity assays. Eleven test substances regarded as non-phototoxic in vivo were predicted as non-phototoxic in the in vitro assays. Of the eight false positives revealed in some of the in vitro assays, five are believed to be photo-allergens. This suggests that in vitro phototoxicity assays might not be able to discriminate clearly between in vivo phototoxicity and in vivo photo-allergy, because the mechanisms in both processes are based on photodynamic reactions. The predictability of the three in vitro assays was comparatively good, but positive predictive values were low. The equivalence values of the photohaemolysis assay, the Hb photo-oxidation assay and the 3T3 NRU PT assay were 81%, 70% and 70%, respectively. From these results, we suggest that the three in vitro phototoxicity assays could be used as screens in predicting phototoxicity.