An evaluation of mechanistic in vitro tests for the discrimination of photoallergic and photoirritant potential

Photochemical tests were used to discriminate between photoallergens and ph otoirritants. UV absorption spectrometry was employed to identify chemicals that absorbed sunlight wavelengths and which required further testing. Pho toallergic potential was assessed by studying photobinding of the test chem icals to human serum albumin. Photobinding was determined by increased UV a bsorbance of the protein fraction after gel filtration chromatography. Phot ooxidation of histidine was used to screen for a mechanism of photoirritanc y. Efficient photooxidisers can be considered to be photoirritant rather th an photoallergic. The substances selected for the EU/COLIPA phototoxicity p roject were tested. There were 14 photoirritants (three tested as both free acid/base and salts, i.e. a total of 17 samples), four photoallergens, thr ee of which were photoirritant and photoallergenic (i.e. 17 photoirritants and seven pho toallergens) and six "negatives" (four clearly non-phototoxic and two unclear). UV spectrometry showed that 28 of the 30 substances abso rbed sunlight significantly and had the potential for adverse photoreaction . Six of seven photoallergens were identified as such by the photobinding a ssay. Most photoirritants also caused photomodification of protein, but ele ven of these photooxidised histidine efficiently and so were classified as photoirritants. Four photoirritants remained falsely predicted as photoalle rgens. Two photoirritants were negative far both photomodification of prote in and histidine photooxidation. Four chemicals negative in vivo were negat ive in. vitro. The remaining two chemicals could not be classified, because of unclear data bath in vivo and in vitro. Therefore, the in vitro test ba ttery was useful for the discrimination of pho toallergic and photoirritant potential.