Skin sensitization to eugenol and isoeugenol in mice: Possible metabolic pathways involving ortho-quinone and quinone methide intermediates

With the aim of providing further mechanistic insights into the mode of action of eugenol (4-allyl-2-methoxyphenol) and isoeugenol (4-propenyl-2-methoxyphenol), we have synthesized two series of modified compounds which were tested in the mouse local lymph node assay for their skin sensitizing potential. The replacement of the methoxy group by an isopropoxy group led to a complete loss of sensitization for the eugenol derivative 6a, while no significant effect was observed for the isoeugenol derivative 6b. In the eugenol series, when methyl groups were present in the 3-, 5-, or 6-position a significant reduction in sensitization potential was observed while in the isoeugenol series only methyl substitution in the 3- and 5-position had a discernable effect. Introduction of three methyl groups on the aromatic ring of eugenol (3,5,6-trimethyl-4-allyl-2-methoxyphenol, 7) and of a tert-butyl substituent at the gamma-position of the alkyl chain of isoeugenol (4-[3',3',3'-trimethylpropenyl]-2-methoxyphenol, 8) led to a strong decrease of the sensitizing capacity. Our findings indicate that, at least in the mouse, eugenol could sensitize via a demethylation pathway followed by oxidation to the o-quinone which could act directly as a hapten even if we cannot exclude a reaction via its tautomeric p-quinone methide. Isoeugenol, on the other hand, could act via a mechanism not involving demethylation and for which the evidence is consistent with a direct oxidation to the p-quinone methide.