Antioxidants attenuate gentamicin-induced free radical formation in vitro and ototoxicity in vivo: D-methionine is a potential protectant.

We have recently suggested antioxidant therapy against aminoglycoside-induced hearing loss based on the hypothesis of a redox-active aminoglycoside-iron complex causing ototoxicity. The present study compares seven antioxidants and iron chelators for their ability to attenuate gentamicin-induced free radical generation in vitro and ototoxicity in guinea pig in vivo. Free radical formation by gentamicin was measured by chemiluminescence detection both in a non-enzymatic system in vitro and in cell culture. Deferoxamine, 2,3-dihydroxybenzoate, or salicylic acid suppressed gentamicin-induced luminescence in both tests. This indicated the usefulness of the assay as a screen for potential protectants since these agents had previously been shown to attenuate gentamicin-induced ototoxicity in vivo. Histidine and D-methionine, amino acids with chelating and antioxidant properties, also suppressed gentamicin-mediated luminosity both in vitro and in cell culture. In contrast, the metal chelators succimer (2, 3-dimercaptosuccinic acid (DMSA)) and trientine (N, N'-bis[2-aminoethyl]-1,2 ethanediamine) promoted free radical formation and were excluded from further studies. Histidine and D-methionine were then administered to guinea pigs receiving concurrent treatment with gentamicin (120 mg/kgx19 days). Threshold shifts induced by gentamicin were significantly attenuated by twice-daily injections of D-methionine. Once-daily injections of histidine or D-methionine were less effective, pointing to the importance of pharmacokinetics in antioxidant protection in vivo. The study presents a simple screening system for agents with the potential to attenuate gentamicin-induced hearing loss. It also supports the hypothesis of free radical formation as an underlying cause of gentamicin ototoxicity.