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Caroverine, 1-(2-diethylaminoethyl)-3-(p-methoxy benzyl)-1,2-dihydro-2-quinoxalin-2-on-hydrochloride, is a class B calcium-channel-blocker and antiglutamatergic agent with significant effects on the brain function. Caroverine exhibits competitive AMPA antagonism, and at higher concentrations, noncompetitive NMDA antagonism. In clinical practice caroverine is used as a spasmolytic and otoneuroprotective agent. Since reactive oxygen species are supposed to be involved in the pathogenesis of inner ear diseases in which caroverine shows beneficial effects, the present study aimed to investigate the antioxidant properties of caroverine. Lipid peroxidation of liposomal membranes was suppressed in the presence of caroverine. In order to understand the mechanism of this antioxidant action of caroverine, we determined the rate constants both for a possible reaction with superoxide (O2•-) radicals from xanthine/xanthine oxidase and for a possible reaction with hydroxyl (•OH) radicals in Fenton system. Using a defined chemical reaction model O2•- scavenging was found to occur at a rather low rate constant only (3×102 M-1 s-1). Thus, a reaction of caroverine with O2•- radicals is of marginal significance. In contrast, the reaction of caroverine with •OH radicals occurs at an extremely high rate constant (k=1.9×1010 M-1 s-1). The strong antioxidant activity of caroverine is therefore based both on the partial prevention and highly active scavenging of hydroxyl radicals.