Nephrotoxicity: a comparison in humans of gentamicin and gentamicin C1 administration

Dissociating antibacterial from ototoxic effects of gentamicin C-subtypes

Gentamicin is a potent broad-spectrum aminoglycoside antibiotic whose use is hampered by ototoxic side-effects. Hospital gentamicin is a mixture of five gentamicin C-subtypes and several impurities of various ranges of nonexact concentrations. We developed a purification strategy enabling assaying of individual C-subtypes and impurities for ototoxicity and antimicrobial activity. We found that C-subtypes displayed broad and potent in vitro antimicrobial activities comparable to the hospital gentamicin mixture. In contrast, they showed different degrees of ototoxicity in cochlear explants, with gentamicin C2b being the least and gentamicin C2 the most ototoxic. Structure-activity relationships identified sites in the C4'-C6' region on ring I that reduced ototoxicity while preserving antimicrobial activity, thus identifying targets for future drug design and mechanisms for hair cell toxicity. Structure-activity relationship data suggested and electrophysiological data showed that the C-subtypes both bind and permeate the hair cell mechanotransducer channel, with the stronger the binding the less ototoxic the compound. Finally, both individual and reformulated mixtures of C-subtypes demonstrated decreased ototoxicity while maintaining antimicrobial activity, thereby serving as a proof-of-concept of drug reformulation to minimizing ototoxicity of gentamicin in patients.

Comparisons of cochleotoxicity among three gentamicin compounds following intratympanic application

CONCLUSION

Among the three main gentamicin (GM) compounds following intratympanic application, the cochleotoxicity of C2 was the most severe, whereas that of C1a was the weakest. Understanding of the different cochleotoxicity characteristics of each compound may be of use in future custom-made intratympanic therapy for Ménière's disease.

OBJECTIVE

To investigate differences in cochleotoxicity among three major GM compounds following intratympanic application.

MATERIALS AND METHODS

Three GM compounds (C1, C2, and C1a) were isolated. Sprague-Dawley rats were treated every 2 days for 2 weeks with intratympanic application of saline, GM complex, C1, C2, and C1a. The cochleotoxicity of each compound was assessed by measuring auditory brainstem response (ABR) and through morphological analyses using scanning electron microscopy.

RESULTS

The ABR threshold of the C2 group was found to be more impaired than those of the other groups. The C1a group showed the mildest elevation of the ABR thresholds. Morphological analyses revealed that the proportion of remaining outer hair cells (OHCs) was the lowest in animals treated with C2. Morphologically, the C1 and C1a groups showed the least damage to OHCs.

Differing effects on the inner ear of three gentamicin compounds: GM-C1, -C2 and -C1a

OBJECTIVE

Gentamicin (GM) has been used to ablate the vestibular system function as a form of treatment for Ménière's disease. Generally, clinicians have not recognized that commercial GM comprises three major compounds--GM-C1, -C2 and -C1a--and the effects of the individual compounds on the inner ear have not been elucidated. In this study we investigated differences in cochleo- and vestibulotoxicity between the three compounds

MATERIAL AND METHODS

Hartley guinea pigs were treated for 2 weeks with daily subcutaneous injections (100 mg/kg) of the GM complex or the GM-C1, -C2 or -C1a compounds. After a further 2 weeks, we assessed the inner ear by measuring the auditory brainstem response (ABR) and performing a sinusoidal rotation test (SRT) and morphological analyses.

RESULTS

The ABR thresholds in the C2 group were found to be more impaired than those in the other groups In the SRT, the nystagmus counts in the C2 group were lower than those in the controls and in the C1 group. The counts in the C1a group were lower than those in the C1 group. Morphological analyses revealed that the proportion of remaining outer hair cells was less in the C2 group than in the other groups.

CONCLUSION

Our results suggest that characterization of differences in ototoxicity may be useful for treatment of Ménière's disease with GM.

Pharmacokinetics of gentamicin C1, C1a and C2 in horses after single intravenous dose

Gentamicin pharmacokinetics has not been studied in horses. Pharmacokinetics of gentamicin C1, C1a and C2 components following i.v. administration of total gentamicin at 6.6 mg/kg bwt to 6 healthy mature horses was determined. Significant differences in clearance, half-life (t 1/2), and mean residence time (MRT) between the gentamicin Cia and the 2 other components were found. The total body clearance (CL) of gentamicin C1a was 1.62 +/- 0.50 ml/min x kg and similar to the glomerular filtration rate (GFR) reported for horses. The CL of gentamicin C1 and C2 were 1.03 +/- 0.08 ml/min x kg and 1.10 +/- 0.15 ml/min x kg, respectively, and significantly slower than that of gentamicin C1a. The values of apparent volume of distribution at steady state were 0.22 +/- 0.05, 0.26 +/- 0.12 and 0.23 +/- 0.05 l/kg for gentamicin C1, C1a and C2, respectively. The MRT values were mean +/- s.d. 3.6 +/- 0.5, 2.7 +/- 0.3 and 3.5 +/- 0.4 h and the t 1/2 values were 3.1 (2.5-4.0), 2.4 (2.0-3.2) and 33 (2.4-4.3) h (harmonic mean and range) for gentamicin C1, C1a and C2, respectively. The MRT and t 1/2 values for gentamicin C1a were significantly shorter than those of gentamicin C1 and C2. It was concluded that the difference in pharmacokinetics between the gentamicin components has potential pharmacological and toxicological implications.

Pharmacokinetics of gentamicin C(1), C(1a), and C(2) in beagles after a single intravenous dose

The pharmacokinetics of gentamicin C(1), C(2), and C(1a) were studied in six beagles after administration of gentamicin at 4 mg/kg of body weight as a single intravenous bolus dose. Plasma concentrations of the gentamicin components were analyzed with a novel high-performance liquid chromatography method capable of identifying and quantifying each of the components. The pharmacokinetic analysis of the plasma concentration-versus-time data was performed using the noncompartmental approach. The results indicated significant differences in the pharmacokinetic characteristics between the gentamicin components C(1), C(1a), and C(2). The mean residence times of gentamicin C(1), C(1a), and C(2) were 81+/-13, 84+/-12, and 79+/-13 min (mean +/- standard deviation), respectively. The half-lives of the respective components were 64+/-12, 66+/-12 and 63+/-12 min. Clearance (CL) of gentamicin C(1), 4.62+/-0.71 ml min(-1) kg(-1), was significantly higher (P = 0.0156) than CL of gentamicin C(1a), 1.81+/-0.26 ml min(-1) kg(-1), and C(2), 1.82+/-0.25 ml min(-1) kg(-1). Similarly, the volume of distribution at steady state (V(ss)) of gentamicin C(1), 0.36+/-0.04 liter kg(-1), was significantly higher (P = 0.0156) than the V(ss) of gentamicin C(1a), 0.14+/-0.01 liter kg(-1), and C(2), 0.15+/-0.02 liter kg(-1). Tissue binding was considered the most likely cause for the difference. The difference may have clinical and toxicological significance.

Further studies of the response of kidney lysosomes to aminoglycosides and other cations

Rat renal cortical lysosomes were isolated in 0.3 M sucrose containing 1 mM EDTA by differential centrifugation. Lysosomes were incubated in isotonic sucrose or isotonic glycine with various concentrations of endogenous and exogenous compounds at 37 degrees for 1 hr. Lysosomes were resedimented, and the N-acetyl-beta-glucosaminidase (NAG) activity was measured in the supernatant fraction and the disrupted pellet and the percentage of total NAG released was calculated. Gentamicin and its C1 and C2 components had similar potencies for inhibiting NAG release from lysosomes at low concentrations. The release of alpha-galactosidase and beta-galactosidase from lysosomes was also inhibited by streptomycin and gentamicin. Mepacrine at low concentrations stabilized lysosomes and at high concentrations disrupted lysosomes. This drug also enhanced the effect of low concentrations of gentamicin on lysosomes. Inositol hexaphosphoric acid was a potent antagonist of the effect of low concentrations of gentamicin and mepacrine on lysosomes. Rats were treated with gentamicin at doses of 40, 80 and 160 mg/kg for 1 and 3 days. NAG excretion in gentamicin-treated groups as compared to saline controls was unchanged at day 1. Only the 160 mg/kg treatment group showed a tendency toward elevated renal cortical NAG at day 1 (P less than 0.06). All treatment groups had elevated renal cortical NAG at day 3, while the 160 mg/kg group also had elevated NAG excretion. Lysine, arginine, L-canavanine and polymyxin B all affected NAG release from lysosomes in vitro. Lysine enhanced the disruptive effect of high gentamicin concentrations on lysosomes. Ferric and ferrous ions, tested over widely varied concentrations, inhibited NAG release at low concentrations while enhancing NAG release at high concentrations. We therefore conclude that the nephrotoxicity of aminoglycoside and other endogenous and exogenous renally excreted cationic compounds may be produced by their effects on lysosomes in the proximal renal tubule.