Limonene hydroperoxide analogues differ in allergenic activity

Background: The fragrance terpene R‐limonene is a very weak sensitizer but forms allergenic oxidation products upon contact with air. Oxidized (ox.) limonene is a frequent cause of contact allergy in clinical testing. Objectives: This study investigates the sensitizing potencies of ox. and non‐ox. limonene and of structurally closely related limonene hydroperoxides. The clinical importance of the difference in sensitizing potency of two hydroperoxides in autoxidized limonene was studied. Patients/Methods: Ox. and non‐ox. limonene were investigated in the murine local lymph node assay (LLNA). Limonene hydroperoxides were investigated using a modified LLNA involving non‐pooled lymph nodes and statistical calculations; patch testing of patients with known contact allergy to ox. limonene was performed. Results: A marked increase in the sensitizing potency of ox. limonene compared with that of pure limonene was observed in the LLNA. One analogue, limonene‐1‐hydroperoxide, was a significantly more potent sensitizer than the other hydroperoxides and gave more positive test reactions in the allergic patients. Conclusions: The results support that hydroperoxides have a specific reactivity indicating that oxygen‐centred radicals are important in hapten–protein complex formation of hydroperoxides. The primary oxidation products of ox. limonene, the hydroperoxides, have an important impact on the sensitizing capacity of the oxidation mixture.