Oxygen free radicals are not the main factor in experimental gentamicin nephrotoxicity

Formation of reactive oxygen species following bioactivation of gentamicin

The present study investigated the ability of gentamicin to catalyze free radical reactions and probed the underlying mechanisms by hydroethidine imaging, oxygen consumption, and reduction of cytochrome c. In Epstein-Barr virus-transformed lymphoblastoid cells, a respiratory burst was induced by phorbol ester and detected by hydroethidine, a fluorescent indicator of superoxide radical. The addition of gentamicin increased the fluorescence two-fold while gentamicin did not produce fluorescence in the absence of phorbol ester. In membrane preparations, gentamicin did not enhance NADPH consumption ruling out a direct activation of NADPH oxidase. The formation of reactive oxygen species by gentamicin was additionally supported by experiments that showed gentamicin increased oxygen consumption two-fold in intact cells and a cell-free system. In addition, generation of superoxide was indicated by the gentamicin-stimulated reduction of cytochrome c. The stimulation by gentamicin depended upon the presence of iron (FeII/FeIII) and of arachidonic acid as an electron donor. These results support the hypothesis that an iron-gentamicin complex can increase reactive oxygen species in nonenzymatic and in biological systems. The requirement for a reductive activation in intact cells (e.g., by a respiratory burst) is interpreted as the conversion of an inactive FeIII-gentamicin to a redox-active FeII-gentamicin complex.

Stimulation of free radical formation by aminoglycoside antibiotics

We have previously shown gentamicin to form a redox-active iron chelate. This study investigates whether other aminoglycosides can likewise stimulate the generation of reactive oxygen species (free radicals). Kanamycin, neomycin and streptomycin were compared to gentamicin in intact cells and in cell-free in vitro assays using luminescence detection with lucigenin or luminol. Neutrophils and Epstein-Barr virus-transformed lymphoblastoid cells served as cell models in which a respiratory burst of superoxide was induced by phorbol ester. The addition of millimolar amounts of any of the aminoglycosides increased the luminescence significantly. The drugs also increased the formation of free radicals in an enzymatic (hypoxanthine-xanthine oxidase) and a non-enzymatic (phenazine methosulfate-NADH) superoxide-generating system. Half-maximal stimulation was reached with (0.4 mM gentamicin, and there was an absolute requirement for an electron donor, arachidonic acid. In both intact cells and cell-free systems, gentamicin-enhanced luminosity was suppressed by iron chelators. These results demonstrate that different aminoglycoside antibiotics can stimulate the formation of free radicals in biological and in cell-free systems. Luminescence detection is a convenient assay method to investigate the redox properties of these drugs.

Variable efficacy of radical scavengers and iron chelators to attenuate gentamicin ototoxicity in guinea pig in vivo

Recent studies from our laboratory have suggested that the ototoxic side effects of gentamicin are caused by a metabolized or 'activated' from the drug. Furthermore, we have postulated that the activation proceeds via the formation of an iron-gentamicin complex and that this complex produces free radicals. The present study assessed the protection effects of free radical scavengers and iron chelators on gentamicin-induced ototoxicity in guinea pigs in vivo. Gentamicin (120 mg/kg per day for 19 days) caused progressive threshold shifts reaching 50-65 dB at 18 kHz. Co-therapy with different radical scavengers yielded results ranging from no protection (with allopurinol, dimethyl sulfoxide, benzoate, lazaroid U74389G) to a moderate attenuation of hearing loss (with mannitol, 4-methylthiobenzoate, WR-2721). This finding agrees well with previous reports of inconsistent effects of scavengers on aminoglycoside-induced ototoxicity although it should be cautioned that only a single dose and route of application was tested. Two iron chelators, deferoxamine and 2,3-dihydroxybenzoate, significantly reduced the gentamicin-induced threshold shifts to about 10 dB or less. Iron chelators markedly decreased total serum iron levels while gentamicin treatment alone had no influence. There were no differences in serum gentamicin levels among all treated groups. This study confirms that iron plays a critical role in gentamicin ototoxicity and suggests that iron chelators, which are well-established drugs in clinical therapy, may be promising therapeutic agents to reduce aminoglycoside ototoxicity.