Formation of a toxic metabolite from gentamicin by a hepatic cytosolic fraction

Sex Difference in Gentamicin-induced Nephrotoxicity: Influence of L-arginine in Rat Model

Background:

L-arginine is an important precursor for the formation of nitric oxide (NO). According to previous studies, NO function is related to gender. Likewise, chronic renal diseases have lower prevalence in female. Gentamicin (GM) is an aminoglycoside antibiotic. According to some studies, males are more sensitive to GM renal nephrotoxicity. This study attempts to find protective effects of L-arginine on GM nephrotoxicity in male and female rats.

Methods:

Male and female rats were divided into eight groups: Rats were randomly assigned to 8 groups each including both male and female rats. The first and second groups received vehicle (saline), the third and fourth groups received gentamicin (80 mg/kg), the fifth and sixth groups received L-arginine (150 mg/kg), and finally, seventh and eighth groups received gentamicin+ L- arginine. Next, 9 days after administering drugs, blood samples were collected from the heart. After making sacrifices, the level of blood urea, creatinine (Cr), nitrite, and malondialdehyde (MDA) was measured in serums. Likewise, nitrite and MDA were measured in the homogenized kidney tissue.

Results:

GM significantly increased serum level of urea and Cr in male and female rats (P < 0.05). However, co-administration of GM + L-arginine significantly did not decrease urea and Cr level in male rats, whereas, in female rats, they significantly reduced (P < 0.05). In response to GM, renal MDA level increased in male and female rats (P < 0.05), and in the presence of GM + L-arginine, the level of MDA significantly decreased in both genders (P < 0.05).

Conclusions:

L-arginine demonstrated some protective effects in female rats but did not protect against GM nephrotoxicity in male rats for unknown reasons, probably related to the effects of sex hormones which needs further studies to be confirmed.

Hearing, reactive metabolite formation, and oxidative stress in cochleae after a single acute overdose of acetaminophen: an in vivo study

CONTEXT

Although the liver is the primary target organ in acetaminophen (APAP) toxicity, other organs are affected. Previous data suggested that chronic APAP abuse can be ototoxic and the mechanism involves APAP-induced oxidative stress and reactive metabolite (N-acetyl-p-benzoquinone imine, NAPQI)-induced endoplasmic reticulum stress. However, the effect of a single acute overdose on hearing has not been tested.

OBJECTIVES

To determine if a single acute APAP overdose causes hearing damage, and to explore possible mechanisms of APAP ototoxicity.

MATERIALS AND METHODS

Male C57BL/6 J mice were treated with a single human-relevant overdose of APAP (300 mg APAP per kg bodyweight). Blood, liver and cochleae were harvested at 0, 2, 6 and 24 h post-APAP. In some mice, auditory brainstem responses (ABRs) to a range of frequencies were measured at 24 h. The furosemide plus kanamycin (FS/K) model of drug ototoxicity was used as a positive control for hearing loss. NAPQI formation after APAP was assessed by measuring glutathione depletion and covalent protein binding, and oxidative stress was assessed by measuring glutathione disulfide.

RESULTS

There was no evidence of reactive metabolite formation or hearing loss after a single overdose of APAP at a clinically relevant dose. However, there was a transient increase in oxidative stress.

DISCUSSION

Although a single acute overdose was not ototoxic, there was evidence of oxidative stress which may support a role for oxidative stress in hearing loss due to chronic APAP abuse.

CONCLUSION

A single human-relevant acute overdose of APAP causes transient oxidative stress in cochleae but not hearing loss.

Modulation of melanogenesis and antioxidant defense system in melanocytes by amikacin

Amikacin is principally used to treat infections caused by microorganisms resistant to other aminoglycosides. Ototoxicity is one of the side effects of amikacin, but the causative mechanism of damage to the ear has not been fully established. Thus, the aim of this work was to examine the impact of amikacin on the melanogenesis and antioxidant defense system in cultured human normal melanocytes (HEMa-LP). Amikacin induced the concentration - dependent loss in melanocytes viability. The value of EC50 was determined to be ~7.5 mM. The analyzed antibiotic inhibited melanin biosynthesis in concentration-dependent manner. Increasing the amikacin concentration also resulted in a decrease in cellular tyrosinase activity. To study the antioxidant defense system in melanocytes, the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) in cells exposed to amikacin were determined. Significant changes in cellular antioxidant enzymes activities were observed. Modulation of melanogenesis and the antioxidant status of melanocytes resulting from the use of amikacin in vitro may explain a potential role of melanin and melanocytes in the mechanisms of aminoglycosides ototoxic effects in vivo.

Aminoglycoside ototoxicity

PURPOSE OF REVIEW

To summarize mechanisms of ototoxicity associated with aminoglycoside antibiotics and discuss possible protective strategies.

RECENT FINDINGS

Studies in the past 15 years have demonstrated that aminoglycoside ototoxicity is mediated by an apoptotic form of cell death which employs caspase-dependent pathways. Reactive oxygen species have been demonstrated in the sensory epithelium after aminoglycoside administration and methods of blocking reactive oxygen species in the cochlea have been attempted, but not found to be uniformly effective in protecting against cell loss or threshold shift. Aspirin has recently been studied in a human chemoprevention trial in China, and while data suggest the possibility of protection, there was a significant increase in gastrointestinal bleeds associated with aspirin use.

SUMMARY

There are currently no recommendations for pretreatment or posttreatment therapies to attenuate ototoxicity associated with aminoglycoside antibiotics. Our understanding of the mechanisms of ototoxicity has improved and apoptotic pathways are clearly responsible for hair cell demise. Further studies are necessary before significant improvement in hearing outcome can be expected after use of ototoxic antibiotics.

A direct HPLC method to estimate streptomycin and its putative ototoxic derivative, streptidine, in blood serum: Application to streptomycin-treated humans

Streptomycin is an aminoglycoside antibiotic with a well-known antituberculosis activity; it is commonly used in clinical practice because it is effective and cheap. However, streptomycin has severe ototoxic effects. The delayed and gradual onset may suggest that a metabolic derivative of the antibiotic could be a potential contributor to ototoxicity. As in a rat experimental model this compound was found to be streptidine, we investigated whether this ototoxic metabolite was also present in the blood of streptomycin-treated patients. To this end, we implemented and optimized a direct reverse-phase HPLC technique to identify and estimate streptomycin and streptidine in serum of streptomycin-treated patients. All criteria for validation of the method were implemented in standard curves in serum of healthy non-treated volunteers by addition of increasing concentration of both compounds and their determination in a trichloroacetic acid deproteinized extract. We found that recovery of streptomycin or streptidine was > or =91.5%. Linearity was r(2)> or =0.99. The intraday and interday precisions were < or =9.7 and < or =10.6%, respectively. The relative intraday and interday error ranged from -9.0 to 8.3% for both compounds in human serum. Studies in patients included five male individuals treated from 35 to 90 days with 1g/day of streptomycin, presenting inner ear malfunction from mild to severe, in whose serum streptidine was always present, and could be successfully separated from streptomycin. Therefore, the validated method used can be a valuable tool to measure and follow these compounds in serum of streptomycin-treated patients.

Effect of melanin on netilmicin-induced inhibition of collagen biosynthesis in human skin fibroblasts

It is known that various drugs form complexes with melanins and that melanins are abundant constituents of the inner ear. In this study, we determined whether the aminoglycoside antibiotic, netilmicin, interacts with melanin and how this process affects collagen biosynthesis in cultured human skin fibroblasts. The obtained results indicate that netilmicin forms stable complexes with melanin characterized by the association constants K(1) approximately 10(6)M(-1) and K(2) approximately 10(3)M(-1). We have suggested that prolidase, an enzyme involved in collagen metabolism, may be one of the targets for aminoglycoside-induced inhibition of collagen biosynthesis. We found that netilmicin strongly induced inhibition of prolidase activity (IC(50)<5microM) and collagen biosynthesis (IC(50) approximately 10microM). At 10microM concentration of netilmicin, prolidase activity in human skin fibroblasts was inhibited by about 80% and DNA biosynthesis-only by about 25%. Melanin at 100microg/mL produced about 30% inhibition of collagen biosynthesis and about 30% inhibition of prolidase activity in cultured fibroblasts. However, the addition of melanin (100microg/mL) to netilmicin-treated cells (10microM) restored the prolidase activity in fibroblasts to almost 100% of control values and partially reversed the inhibitory action of the drug on collagen and DNA biosynthesis. The data suggest that the ability of netilmicin to form stable complexes with melanin may prevent its toxicity on prolidase activity and collagen biosynthesis.

A PI3K pathway mediates hair cell survival and opposes gentamicin toxicity in neonatal rat organ of Corti

Gentamicin is well known to promote hair cell death in inner ear, but it also appears to activate opposing pathways that promote hair cell survival. In combination with others, our previous work has indicated that a K-Ras/Rac/JNK pathway is important for hair cell death and an H-Ras/Raf/MEK/Erk pathway is involved in promoting hair cell survival (Battaglia et al., Neuroscience 122(4):1025-1035, 2003). However, these data also suggested that a Ras-independent survival pathway for activation of MEK might be stimulated by gentamicin. To investigate alternatives to the Ras/Raf/MEK/Erk pathway in promoting hair cell survival, cochlear explants were exposed to gentamicin combined with several inhibitors of alternative pathways (LY294002, calphostin C, SH-6, U73122). When exposed to gentamicin with the PI3K inhibitor LY294002 (10, 50 microM), the protein kinase C (PKC) inhibitor calphostin C (50, 100 nM) or the PKB/Akt inhibitor SH-6 (5, 10 microM), hair cell damage was significantly increased compared to gentamicin alone. By Western blotting, strong PKB/Akt activation was observed in the organ of Corti following exposure to 50 microM gentamicin for 6 h. In addition, PKC activation by 12-O-tetradecanoylphorbol-13-acetate protected outer hair cells from gentamicin induced cell death. In contrast, the phospholipase C-gamma (PLCgamma) inhibitor U73122 (2, 5 microM) did not affect hair cell damage when combined with gentamicin. Also, phosphorylation of PLCgamma was not increased in the organ of Corti following gentamicin treatment, as evaluated by Western blot. The results indicate that PI3K promotes hair cell survival via its downstream targets, PKC and PKB/Akt. This suggests that both Ras-dependent and Ras-independent survival pathways are involved during gentamicin exposure. In contrast, PLCgamma activation of PKC does not appear to play a role.

Involvement of the mitochondrial permeability transition in gentamicin ototoxicity

Aminoglycosides may induce irreversible hearing loss in both animals and humans. In order to study the nature and mechanisms underlying gentamicin-induced cell death in the inner ear, the cochlear neurosensory epithelia were dissected from guinea pigs and incubated with 0.5-10 mM gentamicin. Concentration-dependent loss of cell viability was detected by the inability of damaged cells to exclude propidium iodide. Outer hair cells were most sensitive towards gentamicin toxicity, followed by inner hair cells whereas Deiters and Hensen cells were not affected by the gentamicin concentrations used. The iron chelators 2,2'-dipyridyl and deferoxamine provided partial protection against gentamicin-induced hair cell death while the calcium chelator Quin-2 AM had no effect. Gentamicin (0.5-1 mM) induced condensation of chromatin typical for apoptosis. Using the fluorescent dye tetramethyl-rhodamine methyl ester and laser scanning microscopy we could visualize a loss of the mitochondrial membrane potential in damaged outer hair cells about 1 h before cell death occurred. Cyclosporin A, an inhibitor of the mitochondrial permeability pore, provided partial protection against gentamicin toxicity. This strongly suggests an involvement of the mitochondrial permeability transition in gentamicin-induced apoptosis.

Melanin potentiates gentamicin-induced inhibition of collagen biosynthesis in human skin fibroblasts

One of the recognized side effects of gentamicin is ototoxicity. The mechanism underlying the organ specificity of this side effect of gentamicin has not been fully established. In view of the fact that a number of pharmacologic agents are known to form complexes with melanin and melanin is an abundant constituent of the inner ear tissues, we determined whether gentamicin interacts with melanin and how this process affects the biosynthesis of collagen in cultured human skin fibroblasts. Our results indicate that gentamicin forms complexes with melanin. The amount of gentamicin bound to melanin increases with increasing of initial drug concentration. The Scatchard plot analysis of drug binding to melanin showed that at least two classes of independent binding sites are implicated in gentamicin-melanin complex formation: one class with an association constant K(1) approximately 4 x 10(3) M(-1), and the second class with an association constant K(2) approximately 3 x 10(2) M(-1). The number of total binding sites (n(1)+n(2)) was calculated as about 1.36 micromol gentamicin per 1 mg melanin. We have suggested that prolidase, an enzyme involved in collagen metabolism, may be one of the targets for gentamicin-induced inhibition of collagen biosynthesis. We found that gentamicin-induced inhibition of prolidase activity (IC(50) approximately 100 microM) and collagen biosynthesis (IC(50) approximately 100 microM). At this concentration of gentamicin, DNA biosynthesis in human skin fibroblasts was inhibited only by about 30%. Melanin at 100 microg/ml produced about 25% inhibition of DNA synthesis and about 30% inhibition of prolidase activity, but it had no effect on collagen biosynthesis in cultured fibroblasts. However, the addition of melanin (100 microg/ml) to gentamicin-treated cells (100 microM) augmented the inhibitory action of gentamicin on collagen and DNA biosynthesis and partially reversed its inhibitory effect on prolidase activity. A melanin-induced augmentation of the inhibitory effects of gentamicin on collagen and DNA biosynthesis may explain the mechanism for the organ specificity of gentamicin-induced hearing loss in patients administered this drug.

Intra-cochlear administration of dexamethasone attenuates aminoglycoside ototoxicity in the guinea pig

This study demonstrates the attenuation of aminoglycoside ototoxicity by cochlear infusion of dexamethasone (Dex) using a microcannulation-osmotic pump delivery system. The results indicate that treating the cochlea with Dex both before and after kanamycin administration was more effective in preventing ototoxicity than Dex treatment only after kanamycin administration. A concentration of 1 ng/ml Dex showed the greatest protective effect on both kanamycin-induced threshold shift of the auditory brainstem response and outer hair cell survival. These results show that the Dex treatment attenuates both functional and structural damage of the inner ear from aminoglycoside toxicity.

Aminoglycoside antibiotics

In the 50 years since their discovery, the aminoglycoside antibiotics have seen unprecedented use. Discovered in the 1940s, they were the long-sought remedy for tuberculosis and other serious bacterial infections. The side effects of renal and auditory toxicity, however, led to a decline of their use in most countries in the 1970s and 1980s. Nevertheless, today the aminoglycosides are still the most commonly used antibiotics worldwide thanks to the combination of their high efficacy with low cost. This review first summarizes the history, chemistry, antibacterial actions and acute side effects of the drugs. It then details the pathophysiology of aminoglycoside ototoxicity including experimental and clinical observations, risk factors and incidence. Pharmacokinetics, cellular actions and our current understanding of the underlying molecular mechanisms of ototoxicity are discussed at length. The review concludes with recent advances towards therapeutic intervention to prevent aminoglycoside ototoxicity.

Capability of serum to convert streptomycin to cytotoxin in patients with aminoglycoside-induced hearing loss

Individual variations in sensitivity to the ototoxic effects of aminoglycoside antibiotics are well documented. Our research demonstrates that there is an apparent difference in serum from patients who are resistant or susceptible to aminoglycoside ototoxicity. In the first study, the cytotoxicity of sera from patients with and without hearing loss after various time periods following the discontinuation of aminoglycoside treatment was assayed using the isolated outer hair cell toxicity assay. The results indicate that sera from patients with hearing loss were significantly more toxic than sera from patients with normal hearing or minimal hearing loss. This toxicity may persist for up to 1 year after discontinuation of aminoglycoside therapy. In a second study, sera were obtained from patients who had received aminoglycoside therapy several years previously. None of these sera was toxic to isolated outer hair cells in vitro. Streptomycin was then incubated with the sera or a protein fraction isolated from sera, and the incubation mixtures were tested for toxicity. The percentage of damaged outer hair cells was significantly higher when streptomycin had been treated with sera or a serum protein fraction from patients with hearing loss (58+/-10% and 68+/-9%, respectively) than with sera or a serum protein fraction from a control group (10+/-5% and 17+/-4%, respectively). In addition, several incubation mixtures were analyzed using high performance liquid chromatography. A new chromatographic peak was only found in the incubations of streptomycin with serum protein from patients with hearing loss. The results suggest that sera from individuals sensitive to aminoglycoside antibiotics may metabolize these drugs to cytotoxins.

Pharmacokinetics and urinary excretion of amikacin in low-clearance unilamellar liposomes after a single or repeated intravenous administration in the rhesus monkey

Liposomal aminoglycosides have been shown to have activity against intracellular infections, such as those caused by Mycobacterium avium. Amikacin in small, low-clearance liposomes (MiKasome) also has curative and prophylactic efficacies against Pseudomonas aeruginosa and Klebsiella pneumoniae. To develop appropriate dosing regimens for low-clearance liposomal amikacin, we studied the pharmacokinetics of liposomal amikacin in plasma, the level of exposure of plasma to free amikacin, and urinary excretion of amikacin after the administration of single-dose (20 mg/kg of body weight) and repeated-dose (20 mg/kg eight times at 48-h intervals) regimens in rhesus monkeys. The clearance of liposomal amikacin (single-dose regimen, 0.023 +/- 0.003 ml min-1 kg-1; repeated-dose regimen, 0.014 +/- 0.001 ml min-1 kg-1) was over 100-fold lower than the creatinine clearance (an estimate of conventional amikacin clearance). Half-lives in plasma were longer than those reported for other amikacin formulations and declined during the elimination phase following administration of the last dose (from 81.7 +/- 27 to 30.5 +/- 5 h). Peak and trough (48 h) levels after repeated dosing reached 728 +/- 72 and 418 +/- 60 micrograms/ml, respectively. The levels in plasma remained > 180 micrograms/ml for 6 days after the administration of the last dose. The free amikacin concentration in plasma never exceeded 17.4 +/- 1 micrograms/ml and fell rapidly (half-life, 1.47 to 1.85 h) after the administration of each dose of liposomal amikacin. This and the low volume of distribution (45 ml/kg) indicate that the amikacin in plasma largely remained sequestered in long-circulating liposomes. Less than half the amikacin was recovered in the urine, suggesting that the level of renal exposure to filtered free amikacin was reduced, possibly as a result of intracellular uptake or the metabolism of liposomal amikacin. Thus, low-clearance liposomal amikacin could be administered at prolonged (2- to 7-day) intervals to achieve high levels of exposure to liposomal amikacin with minimal exposure to free amikacin.

Rabbits treated with chronic isepamicin are resistant to mivacurium and rocuronium

We compared the dose-response relationships and the neuromuscular blocking effects of mivacurium and rocuronium after chronic isepamicin therapy for 7 days in 56 anesthetized rabbits. Train-of-four stimuli were applied every 10 s to the common peroneal nerve, and the force of contraction of the tibialis anterior muscle was measured. Chronic isepamicin therapy is associated with a rightward shift of the mivacurium and rocuronium dose-response curves. The effective dose for 50% twitch depression of mivacurium and rocuronium increased significantly, from 16.9 +/- 4.8 and 56.5 +/- 5.3 microg/kg, respectively, with placebo to 30.6 +/- 5.3 and 75.6 +/- 4.7 microg/kg, respectively, during isepamicin therapy. The isepamicin rabbits receiving mivacurium 0.18 mg/kg or rocuronium 0.6 mg/kg had an accelerated recovery from neuromuscular blockade compared with those receiving placebo. The results of this study show that mivacurium and rocuronium have both a decreased effect and a shorter duration of action in rabbits when used during concurrent isepamicin therapy.

IMPLICATIONS

We studied the dose-response relationships and the neuromuscular blocking effects of mivacurium and rocuronium during chronic isepamicin therapy in rabbits. Mivacurium and rocuronium have both a decreased effect and a shorter duration of action during chronic aminoglycoside antibiotic therapy in rabbits.

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.

Iron chelators protect from aminoglycoside-induced cochleo- and vestibulo-toxicity

The attenuation of gentamicin-induced hearing loss by iron chelators and radical scavengers has recently been demonstrated in guinea pig in vivo. The present study investigated whether this protective treatment is effective against hearing loss and vestibular damage caused by other aminoglycosides. In a direct comparison, dihydroxybenzoate was chosen over deferoxamine because of its more effective action against gentamicin-induced hearing loss. Guinea pigs received daily injections of kanamycin (250 mg/kg/d) or streptomycin (300 mg/kg/d) for 23 d to induce severe cochlear or vestibular toxicity, respectively. Kanamycin injections resulted in a progressive threshold shift of 60 to 80 dB at 18 kHz, while streptomycin injections induced only a small threshold shift. In contrast, streptomycin abolished almost all vestibular responses. Coinjection of aminoglycosides with a mixture of dihydroxybenzoate (100 mg/kg/d) and mannitol (30 mg/kg/d) significantly attenuated kanamycin-induced hearing loss and protected against streptomycin-induced vestibulotoxicity. DHB/mannitol did not affect serum levels or the antibacterial efficacy of either aminoglycoside. This study supports the idea that iron and free radicals play a critical role in the toxic side effects of aminoglycoside antibiotics. Furthermore, the previously proposed therapeutic protection is not limited to gentamicin but applicable to other aminoglycosides as well.

Morphological evidence of ototoxicity of the iron chelator deferoxamine

Recent reports of the role of iron-catalyzed free radical formation in gentamicin ototoxicity and the successful attenuation of gentamicin ototoxicity by iron chelators led us to re-examine experimental material from a previously unpublished study of deferoxamine. Deferoxamine was injected i.m. into adult Japanese quail at either 300 or 750 mg/kg body weight for 30 days. Examination of sections from the basilar papilla at the light microscope level indicated that supporting cells were damaged after the lower drug dose, and that both supporting cells and hair cells were damaged after the higher drug dose. High, prolonged exposure to deferoxamine produced pathological changes similar to those seen in the basilar papilla after much lower, shorter doses of gentamicin. These results demonstrate that deferoxamine damages the quail inner ear and are consistent with the idea that the ototoxic actions of gentamicin may be mediated by iron chelation.

Transforming growth factor alpha treatment alters intracellular calcium levels in hair cells and protects them from ototoxic damage in vitro

To determine if transforming growth factor alpha (TGF alpha) pretreatment protects hair cells from aminoglycoside induced injury by modifying their intracellular calcium concentration, we assayed hair cell calcium levels in organ of Corti explants both before and after aminoglycoside (i.e. neomycin, 10(-3) M) exposure either with or without growth factor pretreatment. After TGF alpha (500 ng/ml) treatment, the intracellular calcium level of hair cells showed a five-fold increase as compared to the levels observed in the hair cells of control cultures. After ototoxin exposure, calcium levels in hair cells of control explants showed an increase relative to their baseline levels, while in the presence of growth factors pretreatment, hair cells showed a relative reduction in calcium levels. Pretreatment of organ of Corti explants afforded significant protection of hair cell stereocilia bundle morphology from ototoxic damage when compared to explants exposed to ototoxin alone. This study correlates a rise in hair cell calcium levels with the otoprotection of hair cells by TGF alpha in organ of Corti explants.

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.

Proteolytic cleavage and activation of pro-macrophage-stimulating protein by resident peritoneal macrophage membrane proteases

Macrophage stimulating protein (MSP), which is secreted as biologically inactive pro-MSP, is activated to MSP by cleavage at a single peptide bond. Our objectives were to determine the form of MSP in circulating blood and to study proteolytic activation of pro-MSP by its target cell. Western blot of immunoaffinity-purified serum MSP showed that all the protein was pro-MSP, without detectable MSP. The circulating form of the protein is therefore pro-MSP, and conversion to MSP does not occur when blood is shed. Incubation of radiolabeled pro-MSP with murine peritoneal macrophages caused proteolytic cleavage to predominantly inactive fragments. Among several protease inhibitors, soybean trypsin inhibitor was one of two that inhibited nonspecific cleavage and revealed a macrophage proteolysis of pro-MSP, and certain concentrations enhanced cleavage to mature MSP. Macrophage membranes had nonspecific and specific pro-MSP proteolytic activity, which was not present in macrophage culture fluids. The results suggest that control of MSP activity can occur at the level of the target cell by proteolytic cleavage of pro-MSP to mature MSP or to inactive fragments.

Formation of free radicals by gentamicin and iron and evidence for an iron/gentamicin complex

Participation of free radicals in the adverse renal and cochlear side effects of aminoglycoside antibiotics is controversial. We measured the production of free radicals by gentamicin in vitro through the oxidation of arachidonic acid. Gentamicin alone (0.05 to 10 mM) did not cause lipid peroxidation. However, it dramatically promoted radical formation in the presence of iron salts. Peroxidation was maximal at 1 mM gentamicin plus 0.1 mM Fe(II)/Fe(III) (0.05 mM FeSO4 and FeCl3 each). At these iron concentrations, peroxidation was not significant in the absence of gentamicin. Since chelators can enhance iron-catalyzed oxidations, this finding suggested that gentamicin-dependent radical formation was based upon iron chelation. This hypothesis was tested by measuring the influence of gentamicin on the oxidation of salicylate by Fe-EDTA complexes, a reaction that is inhibited by competing iron chelators. Gentamicin was a concentration-dependent inhibitor. In contrast, concentrations of gentamicin as high as 50 mM did not interfere with iron-independent salicylate oxidation. These results suggest that gentamicin acts as an iron chelator, and that the iron-gentamicin complex is a potent catalyst of free radical formation.

Gentamicin nephrotoxicity in humans and animals: some recent research

It would appear from the literature cited in this article, that interest in gentamicin nephrotoxicity is still thriving. Despite extensive studies, the mechanism(s) of the nephrotoxicity is uncertain. Several clinical and experimental strategies have been employed in order to ameliorate or abolish the signs of gentamicin nephrotoxicity. Most of these were unsuccessful, impractical or unsafe. Therefore there is still a need for further studies to elucidate the mechanism(s) of action of the drugs nephrotoxicity, and to discover safe, practical and effective agents to ameliorate the nephrotoxicity in patients at risk.

Glutathione protection against gentamicin ototoxicity depends on nutritional status

This study demonstrates that gentamicin ototoxicity depends on dietary factors and correlates with tissue glutathione levels. After 15 days of gentamicin injections (100 mg/kg/day s.c.) guinea pigs on a regular protein diet (18.5% protein) had an average hearing loss of 9 dB at 3 kHz, 31 dB at 8 kHz and 42 dB at 18 kHz. Guinea pigs on a 7% protein diet showed an increased hearing loss of 52 dB at 3 kHz, 63 dB at 8 kHz and 74 dB at 18 kHz. Supplementing the low protein diet with either essential or sulfur-containing amino acids did not protect against gentamicin ototoxicity. Glutathione levels in the cochlear sensory epithelium were decreased in animals on a low protein diet and could be restored to normal by oral administration of glutathione monoethyl ester (1.2 g/kg/day) in combination with vitamin C (100 mg/kg/day). Glutathione supplementation significantly reduced the magnitude of hearing loss in the low protein diet group at all frequencies (43 dB reduction at 3 kHz, 27 dB reduction at 8 kHz and 21 dB reduction at 18 kHz). In animals on a full protein diet, dietary glutathione neither increased cochlear glutathione levels nor attenuated hearing loss. Serum gentamicin levels did not differ between animals on the various diets with or without glutathione supplement. These results suggest that gentamicin toxicity and detoxifying mechanisms are affected by the metabolic state of the animal and the glutathione content of the tissue. Thus, compounds that could potentially protect against gentamicin ototoxicity may be more correctly assessed in animal models of deficient nutritional states in which endogenous detoxifying mechanisms are compromised. This animal model might also be more realistically related to the clinical situation of a critically ill patient receiving gentamicin treatment.

Tobramycin gender-related nephrotoxicity in Fischer but not Sprague-Dawley rats

Published reports suggest a gender-related difference in susceptibility to the nephrotoxicity of gentamicin which may also be strain-related in rats. However, certain ambiguities exist in the results obtained with the functional and morphologic determinants of nephrotoxicity used in these studies. Within the same experimental protocol we examined the potential gender-related differences in tobramycin nephrotoxicity in age-matched male and female rats of both the Sprague-Dawley (SD) and Fischer (F344) strains. Equal numbers of both genders were dosed at 30 mg/kg (F344) and 90 mg/kg (SD) twice daily for 9 days. Results of BUN, serum creatinine, whole body weight change and histologic score comparisons (P < 0.05) indicate that male F344 rats are more sensitive to tobramycin nephrotoxicity than F344 females but this gender-related susceptibility was not observed in SD rats.

Sulfhydryl compounds and antioxidants inhibit cytotoxicity to outer hair cells of a gentamicin metabolite in vitro

Aminoglycoside antibiotics such as gentamicin have long been known to destroy cochlear and vestibular hair cells in vivo. In the cochlea outer hair cells are preferentially affected. In contrast, gentamicin will not damage outer hair cells in vitro unless it has been enzymatically converted to a cytotoxic metabolite. Several potential inhibitors of this enzymatic reaction were tested in an in vitro assay against outer hair cells isolated from the guinea pig cochlea. Viability of hair cells (viable cells as per cent of total number of cells observed) averaged about 70% under control conditions. Addition of metabolized gentamicin significantly reduced viability to less than 50% in one hour. Sulfhydryl compounds (glutathione, dithioerythritol) and antioxidants (vitamin C, phenylene diamine, trolox) prevented the cytotoxic actions of the gentamicin metabolite. Inhibitors of amine oxidases and compounds reportedly protective against renal and acute lethal toxicity of aminoglycosides (poly-L-aspartate and pyridoxal phosphate, respectively) were ineffective as protectants. The results reinforce the hypothesis that gentamicin is enzymatically converted to a cytotoxin and imply the participation of sulfhydryl-sensitive groups or free radicals in this reaction. Alternatively or additionally, sulfhydryl compounds or antioxidants may participate in detoxification reactions.

Attenuation of gentamicin ototoxicity by glutathione in the guinea pig in vivo

The effect of glutathione co-therapy on the expression of gentamicin ototoxicity was tested in pigmented guinea pigs. The first group of animals was injected with gentamicin (100 mg/kg body weight/day) for two weeks followed by 10 weeks of rest. A second group received glutathione by gastric gavage immediately prior to each gentamicin injection. Two groups of controls were treated either with saline injections or glutathione gavage alone. Auditory brainstem responses, taken at 2-week intervals, revealed a progressive gentamicin-induced hearing loss reaching a 30 to 40 dB threshold shift at 2 kHz, approximately 60 dB at 8 kHz and 80 dB at 18 kHz. Glutathione co-therapy slowed the progression of hearing loss and significantly attenuated the final threshold shifts by 20 to 40 dB. Morphological evaluation confirmed hair cell loss after gentamicin treatment and protection by glutathione. Drug serum levels were assayed at 2 and 7 days of treatment. There were no differences between the gentamicin (mean = 183 micrograms/ml; range, 90 to 300) and the gentamicin/glutathione group (mean = 164 micrograms/ml; range, 80 to 320). Antimicrobial activity of gentamicin was tested against Staphylococcus aureus and Pseudomonas aeruginosa. A 30-fold molar excess of glutathione did not influence the efficacy of gentamicin. These studies suggest that glutathione protects against ototoxicity by interfering with the cytotoxic mechanism.

Acute gentamicin ototoxicity in cochlear outer hair cells of the guinea pig

The acute effects of the aminoglycoside antibiotics gentamicin on isolated cochlear outer hair cells (OHC) was investigated by whole-cell patch-clamp and measurements of the intracellular potassium level by means of the potassium-sensitive dye PBFI. In addition, the accompanying length changes of OHC are described. It could be shown that gentamicin at different concentrations reversibly induces a hyperpolarization by about 5-10 mV, potassium outflow from the cytoplasm (by about 22 mM) and a cellular elongation (by about 10%). It is suggested that these effects are the result of an interaction between gentamicin and the cochlear transduction channels in OHC as suggested earlier. These acute effects are distinctly different from the chronic gentamicin effects which are based on the metabolization of the antibiotics to cause the death of the OHC by interaction with the phosphoinositide signalling cascade.

Gentamicin ototoxicity in otoconia: quantitative electron probe X-ray microanalysis

Chronic gentamicin ototoxicity was evaluated in the otolithic membrane of adult OF1 mice at the otoconial layer of the saccule and utricle by quantitative electron probe X-ray microanalysis of Ca and K. The otolithic membranes were plunge-frozen and freeze-dried. The analysis was carried out with an energy dispersive detector using the peak-to-back-ground ratio method and different inorganic salts of Ca and K as standards to calibrate the microprobe. Ca and K in the otoconia are related via a linear function in both the saccule and the utricle. This association is not maintained after exposure to gentamicin, which suggests that this aminoglycoside antibiotic interferes with the Ca-K equilibrium in the otoconia. A dose of 200 mg/kg gentamicin twice a day for 5 days did not affect Ca in the mineral phase of the otoconia, but did increase K in both saccular (p < 0.05) and utricular (p < 0.01) otoconia. These increases in K may reflect a modification in the composition of the endolymph, resulting from cellular damage at the plasma membrane.

Effects of polyaspartic acid on pharmacokinetics of tobramycin in two strains of rat

To provide insight into polyaspartic acid nephroprotection and differences in aminoglycoside renal toxicity between two rat strains, the single-dose pharmacokinetics of tobramycin was examined in the presence and absence of polyaspartic acid. Following a single subcutaneous 6.5-mg/kg dose of tobramycin alone, higher aminoglycoside concentrations were measured in Sprague-Dawley rats than in Fischer rats (P < 0.05). Simultaneous administration of polyaspartic acid (50 mg/kg) and tobramycin did not alter the concentrations of tobramycin in serum. The amount of tobramycin in renal tissue and the amount recovered in urine over a 24-h period were greater in both rat strains when tobramycin and polyaspartic acid were given concomitantly. In summary, polyaspartic acid did not alter the concentrations in serum achieved after a single dose of tobramycin in two different rat strains but did result in higher renal concentrations and greater urinary excretion of tobramycin.

Sulphhydryl-modifying reagents alter ototoxin block of muscarinic receptor-linked phosphoinositide turnover in the cochlea

In the 12-day-old rat cochlea, the synthesis of inositol phosphates (IPs) can be activated via M3 cholinoceptors. This stimulation is blocked by ototoxins (mercury, ethacrynate, cisplatin, neomycin), drugs with side effects that lead to damage of hair cells and strial cells. As these toxic effects can be reversed in vivo by thiol molecules, we investigated whether modifications of thiol compounds could be involved in ototoxin-induced inhibition of the IP turnover in the cochlea. For this purpose, we assessed whether the sulphhydryl-modifying reagents N-ethylmaleimide and cadmium modify the carbachol-stimulated formation of IPs in the 12-day-old rat cochlea. Both molecules inhibit the carbachol effect on a dose-dependent way without altering the basal metabolism of IPs. As cadmium may block some calcium channels, the effect of verapamil, another calcium channel antagonist, was tested. Verapamil (1-50 microM) does not alter carbachol-evoked IP formation, suggesting that the inhibitory effect of cadmium is not due to a calcium influx block. Binding experiments with the muscarinic ligand quinuclidinyl benzylate (QNB) showed that the sulphhydryl-modifying reagents do not displace QNB from binding sites. Combining ototoxins and reagents shows that N-ethylmaleimide acts synergistically with all ototoxins but ethacrynate while cadmium does so only with mercury. Both N-ethylmaleimide and cadmium have additive effects with ethacrynate. As a supplement, disulphide bond-modifying agents do not alter the carbachol-enhanced metabolism of IPs. These results suggest that molecules having thiol-modifying properties inhibit the carbachol-induced turnover of IPs without acting at the muscarinic sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of in vivo evidence of a cytochrome P450 metabolite participating in aminoglycoside nephrotoxicity

Recent in vitro evidence has suggested that the cytotoxicity of aminoglycosides may be mediated by a metabolite generated by the hepatic cytochrome P450 drug-metabolizing system. This postulate has been tested by pretreating rats with cobalt protoporphyrin IX (CoP) to suppress hepatic P450 levels prior to administration of gentamicin. CoP pretreatment was observed to suppress antipyrine clearance markedly but not to alter gentamicin nephrotoxicity.