A conjugated diene identified as a prohapten:: Contact allergenic activity and chemical reactivity of proposed epoxide metabolites

A hapten causing allergic contact dermatitis binds covalently to macromolecules via nucleophilic-electrophilic reactions or radical couplings. A prohapten can be seen as a chemically inert compound without electrophilic or radical forming properties. To exert its activity, the prohapten is activated, for example, metabolically, to the hapten. We have investigated the contact allergenic properties of a diene, (5R)-5-isopropenyl-2-methyl-1-methylene-2-cyclohexene (1), as a potential probapten, and we found it to be a sensitizer in animal studies. The activity is likely to be exerted via epoxide metabolites. Thus, two potential metabolites of the investigated diene, (4S)-1,2-epoxy-4-isopropenyl-1-methyl-6-methylene-cyclohexane (3) and (7R)-7-isopropenyl-4-methyl-1-oxa-spiro[2.5]oct-4-ene (4), were synthesized and subjected to animal tests. Both epoxides were sensitizers. They also elicited significant reactions when tested in animals induced with 1, which indicates that they are formed from the diene in the skin. Furthermore, incubation of 1 with human liver microsomes produced both epoxides. The chemical reactivity of 1, 3, and 4 was investigated in relation to a hexapeptide, H-Pro-His-Cys-Lys-Arg-Met-OH. No adducts were obtained from reactions between the peptide and 1. However, epoxide 3 bound covalently to the cysteine residue and epoxide 4 to both the cysteine and proline residues. Since it is possible to relate the sensitizing capacity of a compound to its key physicochemical properties, knowledge-based expert systems have been developed to predict the toxicity of novel compounds by comparing the structure with activity data stored in the computer database. A diene related to 1 found in the knowledge-based expert system DEREK was considered as a nonsensitizer by this system. Our study indicates that conjugated dienes can be metabolized to contact allergens in the skin. Thus, when constructing predictive test methods based on SARs, it is important to analyze not only the virtual chemical structure of a compound but also its ability to act as a prohapten.