Anti-oxidant actions of oxymethazoline and xylomethazoline

Anti-oxidant actions of oxymethazoline and xylomethazoline were investigated by measuring inhibition io microsomal lipid peroxidation and hydroxyl radical scavenging activity. Oxymethazoline was shown to be potent inhibitir of lipid peroxidation (IC₅₀ = 4.9 μM at t = 15 min, IC₅₀ = 81. μM at t = 30 min), in contrast to xylomethazoline. Both compounds were excellent hydroxyl radical scavengers. Their rate constants (k_S = 1.1 × 10¹² M⁻¹s⁻¹ for oxymethozoline and k_S = 4.7 × 10¹⁰ M⁻¹s⁻¹ for xylomethazoline) exceeded the rate constant of a known powerful scavenger cimetidine (k_S = 1.8 × 10¹⁰ M⁻¹S⁻¹). The difference in inhibiting lipid peroxidation might be explained by the fact that only oxymethazoline has a hydroxy group which can donate a hydrogen atom and terminate the chain reaction of lipid peroxidation. The mechanism of hydroxyl radical scavenging activity is still unclear. Moreover oxymethazoline seems to have a different mode of action in scavenging hydroxyl radicals than oxylomethazoline and cimetidine which results in an extremely high rate constant. Because oxidants play a role in tissue damage in inflammation, it was hypothesized that especially oxymethazoline and to a lesser extent xylomethazoline may have an additional beneficial effect, due to their anti-oxidant properties, in the topical treatment of nasal inflammation.