Tissue binding kinetics of tobramycin. An experimental study in the mouse inner ear in vitro

Loss of spiral ganglion cells as primary manifestation of aminoglycoside ototoxicity

Although pulmonary infections caused by Pseudomonas aeruginosa can hardly be eradicated in patients with cystic fibrosis (CF, the most common genetic disease among Caucasians), these patients are mainly treated with intravenous and nebulized tobramycin. Long-term treatment with tobramycin, however, may induce ototoxic effects. We assessed the clinical histories and postmortem temporal bones of six patients with CF for signs of this ototoxicity. Four bones showed typical manifestations of ototoxicity induced by aminoglycosides (AGs): loss of hair cells in the lower turns, and degeneration of ganglion cells. Six bones revealed no loss or scattered loss of hair cells, however, degeneration of the spiral ganglion cells was observed. This suggests that degeneration of the spiral ganglion may occur as a primary manifestation in some cases of ototoxicity due to aminoglycosides. Recent reports have shown that trophic factors (neurotrophins and acidic fibroblast growth factor) interacting with hair cells and the spiral ganglion protect the inner ear from damage. It may be that disturbances in supply of such trophic factors caused degeneration of ganglion cells without loss of hair cells in the cases we studied.

Potentiation of cochlear hair cell loss by acoustic stimulus and gentamicin in the guinea pig

We explored the possibility of synergism between a pure-tone stimulus and gentamicin in causing cochlear injury by analyzing hair cell loss. Guinea pigs receiving daily injections of gentamicin (200 mg/kg body wt.) for 1 week were exposed to a 2 kHz tone (95 dB SPL, 2 hours daily). Surface preparations of the spiral organ were studied by phase contrast microscopy, and the extent of hair cell loss in the entire organ of Corti was recorded in cytocochleograms. Gentamicin by itself was slightly ototoxic, damaging the innermost row of outer hair cells, whereas exposure to sound alone caused no hair cell loss. Combined antibiotic and acoustic exposure produced extensive cochlear damage. A few animals showed massive hair cell degeneration and collapse of the organ of Corti, except in the apical turn. The site of damage was possibly determined by the frequency of the sound stimulus. Thus, an intermittent tonal stimulus such as that used in the present experiment can be harmless by itself, but causes injury to cochlear hair cells in guinea pigs when administered in combination with gentamicin.

Pharmacokinetics of amikacin in serum and in tissue contiguous with pressure sores in humans with spinal cord injury

Pressure sores are a common occurrence in immobilized patients. They increase morbidity and mortality and impede rehabilitation. Antibiotics are routinely used to assist in effecting a cure when infection is present. Nevertheless, for patients with spinal cord injuries (SCI), strategies for effective therapy with antibiotics based on measurement of concentrations in tissue and pharmacokinetic behavior in extravascular spaces do not exist. By analyzing the concentration-time profile and protein binding of amikacin in the interstitial fluid (IF) in contact with pressure sores, we found that the disposition of amikacin in the tissue contiguous with pressure sores appears to be governed by simultaneous first-order and capacity-limited pharmacokinetic behavior. Amikacin disposition in IF proceeded without a simple relationship to amikacin concentrations in serum, and the time course in IF was not accurately simulated by linear models of amikacin pharmacokinetic behavior. Total amikacin clearance estimated from a pharmacokinetic model using simultaneous first-order and nonlinear intercompartmental transfer of amikacin was not significantly different from clearance calculated by us in a prior study of amikacin pharmacokinetic behavior in patients with SCI. In patients with SCI, optimal use of amikacin in the treatment of infected pressure sores is contingent upon accurate characterization of the pharmacokinetic behavior of this aminoglycoside in serum and in the IF in contact with these lesions. Only methods which quantitate amikacin concentration and protein binding in IF and incorporate a model that can simultaneously simulate nonlinear and linear disposition processes should be relied upon to influence therapeutic decision making.

The round window as access route for agents injurious to the inner ear

The ototoxicity of 50% propylene glycol, 70% isopropyl alcohol, 2% acetic acid, Otic Domeboro solution (Miles Pharmaceuticals, West Haven, CT), 1% Gentian violet (Purepac Pharmaceutical, Elizabeth, NJ), Vosol Otic solution (Wallace Laboratories, Cranbury, NJ), Genoptic Ophthalmic solution (Allergan Pharmaceuticals, Irvine, CA), Cortisporin Otic suspension (Burroughs Wellcome, Greenville, NC), Coly-Mycin S Otic (Parke-Davis, Rochester, MI), and Pyocidin Otic (Berlex Laboratories, Cedar Knolls, NJ) was studied in rats according to their effect on the latencies of the second peak of the auditory brainstem responses (ABR) at different sound pressure levels. The substances were instilled into the round window (RW) niches of rats, and the ABR to 1-kHz and 6-kHz tonebursts were obtained, 30 minutes, two hours, and 1 week after exposure. For all substances except isopropyl alcohol and propylene glycol, which evidently quickly penetrated the RW, approximately two hours of exposure were required before inner ear function was affected. The ototoxic effect of the antibiotic drugs was to some degree reversed with time, whereas the recorded potentials for antiseptics such as 1% Gentian violet, 2% acetic acid, and Otic Domeboro indicate that they caused severe damage to inner ear function. Some increases in latencies were also noted after exposure to propylene glycol and isopropyl alcohol, except when the latter was quickly withdrawn from the RW niche.