Validation of in vitro methods to single out photoirritants using mechanistically based tests

In the recent OECD draft proposal for a new guideline on acute dermal photoirritation testing, in vitro screening tests have been included as part of the sequential test strategy. These screening tests were placed directly prior to animal tests proposed. In Europe some in vitro techniques - cell culture and mechanistic tests - are under validation in a joint project of the European Center for the Validation of Alternative Methods (ECVAM) and the European Cosmetic, Toiletry, and Perfumery Association (COLIPA). Two promising cellular in vitro tests are presented and discussed as tool for the screening of photoirritancy. The first one as a general core test performed in each participating laboratory is the 3T3 mouse fibroblast Neutral Red Uptake Phototoxicity test determining the cell viability by uptake of Neutral Red as end point, whereas the second performed only by three participating laboratories was the the Red Blood Cell phototoxicity test comprising a combination of two end points, the photohaemolysis and the oxyhaemoglobin oxidation. Besides this, other mechanistic tests can be used as additional support. Identification of photoirritation is generally considered to be one area for the successful research and validation of in vitro techniques.