Chemical basis for the extreme skin sensitization potency of (E)-4-(ethoxymethylene)-2-phenyloxazol-5(4H)-one.

(E)-4-(ethoxymethylene)-2-phenyloxazol-5(4H)-one, commonly referred to as oxazolone, is the most potent skin sensitizer in published databases as determined with the murine local lymph node assay. It has been used very widely in immunological research to induce and elicit skin sensitization reactions in experimental animals. Nevertheless, no detailed study on the reactivity of oxazolone with proteins or peptides has been published, which would rationalize its unique sensitization potential from a chemical point of view. Peptide reactivity assays have been proposed as alternatives to animal tests to study the skin sensitization potential of test chemicals. Besides their application to reduce animal experimentation, peptide reactivity assays also allow one to gain mechanistic insights into the reactivity of test chemicals with proteins. In this case study, we applied different peptide reactivity assays to investigate and mechanistically rationalize the reactivity of oxazolone. Its sensitization potential could be linked to the following findings: (i) oxazolone reacts rapidly with the amine group in lysine with an addition-elimination reaction at the ethoxymethylene group to form stable products within minutes at physiological pH; (ii) sequentially different products with cysteine-peptides are formed, the most stable being an S-hippuryl-modification; and (iii) the S-hippuryl-modification can be shuttled to other nucleophilic sites; thus, also Lys residues can subsequently be modified with a hippuryl-moiety. This very rapid and diverse reactivity especially with lysine residues may explain why oxazolone forms sufficient stable novel epitopes on proteins to induce skin sensitization even at very low concentration.