Evaluation of mechanistic in vitro tests for the discrimination of photoallergic and photoirritant potential.

Photochemical tests were used to discriminate photoallergens and photoirritants. UV absorption spectrometry was employed to identify chemicals which absorbed sunlight wavelengths and which required further testing. Photoallergic potential was assessed by studying photobinding of the test chemicals to human serum albumin. Photobinding was determined by increased UV absorbance of the protein fraction after gel filtration chromatography. Photooxidation of histidine was used to screen for a mechanism of photoirritancy. Efficient photooxidisers may be considered photoirritant rather than photoallergic. The substances selected for the EU/COLIPA phototoxicity project were tested. There were 14 photoirritants (three tested as both free acid/base and salts, i.e., 17 samples in total), four photoallergens, three which were photoirritant and photoallergenic (i.e., 17 photoirritants and seven photoallergens) and six "negatives" (four clearly non-phototoxic and two unclear). UV spectrometry showed that 28 of the 30 substances absorbed sunlight significantly and had the potential for adverse photoreaction. Six of seven photoallergens were identified as such by the photobinding assay. Most photoirritants also caused photomodification of protein, but eleven of these photooxidised histidine efficiently and so were classified as photoirritants. Four photoirritants remained falsely predicted as photoallergens. Two photoirritants were negative for both photomodification of protein and for histidine photooxidation. Four chemicals negative in vivo were negative in vitro. The remaining two chemicals could not be classified because of unclear data both in vivo and in vitro. The in vitro test battery, therefore, was useful for the discrimination of photoallergic and photoirritant potential.