Inner ear concentrations and ototoxicity of different antibiotics in local and systemic application

Can Gentamicin-collagen Sponges Prevent Seroma Formation Following Mastectomy?

BACKGROUND

The aim of this study was to investigate whether gentamicin-collagen (GC) sponges can lower the incidence of seroma and surgical site infections following breast cancer surgery.

PATIENTS AND METHODS

A retrospective cohort study was performed. Two consecutive cohorts of patients who underwent a mastectomy with or without an axillary lymph node dissection were compared. The first cohort was treated conventionally (n = 38), the second cohort received GC sponges (n = 39). Endpoints were the incidence of clinical significant seroma (CSS) and surgical site infections (SSI), the mean number of aspirations, and the mean aspirated volume.

RESULTS

GC sponges lowered the CSS incidence from 73.7% to 38.5% (P = .002). The mean number of aspirations and the mean aspirated volume were not affected. SSI incidence was 15.8% in the conventional cohort compared with 7.7% in the GC cohort (P = .23).

CONCLUSION

Application of GC sponges significantly lowered the incidence of CSS. The incidence of SSI was halved, although this was not significant.

Complications associated with antibiotic administration: neurological adverse events and interference with antiepileptic drugs

Antibiotic use is associated with toxic effects involving the peripheral and central nervous systems and it may interfere with antiepileptic drugs, causing significant variations in their serum levels and activity. Prompt identification of neurological complications during antibiotic therapy is important in order to make appropriate modifications to medication. Characteristics of the drug and the patient, including age and underlying diseases, may favour these complications. The main aim of this study was to review the neurological adverse events that may follow antibiotic administration, the mechanisms that cause them, and the possibility of prevention and treatment. Moreover, the interference of antibiotics with serum levels and the activity of antiepileptic drugs are discussed. The results demonstrate that antibiotic-associated adverse events involving the nervous system are relatively uncommon and are only rarely severe and irreversible, although neurotoxicity has been reported for several antibiotics. Moreover, for patients receiving antiepileptic drugs, monitoring of drug serum levels to avoid the risk of toxicity or inadequate therapy is mandatory during antibiotic treatment. Areas for future research include the effects of combined antibiotic therapies as well as multiple antiepileptic drugs in study populations with an adequate sample size, including neonates and infants, patients with pharmacoresistant epilepsy and elderly patients.

III. Comparative Quantitative Analysis

Hypotheses to explain the selective ototoxicity of aminoglycoside antibiotics may be grouped into two categories: 1) a partition effect and 2) innate hair cell selectivity. The results of this study give evidence that, in fact, both mechanisms may interplay. This conclusion is induced from the observations that following the transtympanic administration of streptomycin, dihydrostreptomycin and neomycin: 1) All three drugs elicited nearly the same topographic pattern and degree of damage in the vestibular compartment. 2) All three drugs elicited nearly the same topographic pattern but widely different degrees of damage in the cochlear compartment (neomycin > > streptomycin > dihydrostreptomycin). 3) Neomycin caused the same devastating cochlear hair cell damage as reported for its systemic administration. 4) Dihydrostreptomycin was significantly more vestibulotoxic than cochleotoxic, and streptomycin was both significantly more cochleotoxic than vestibulotoxic and significantly more cochleotoxic than was dihydrostreptomycin. The latter observations are discussed with regard to contrasts with the literature reporting the drug effects following systemic administration.

IV. Influence of Vestibular Stimulation

Guinea pigs were given transtympanic dosages of neomycin followed by three hours of either vestibular stimulation or immobilization to test the hypothesis that inner ear hair cell membrane permeability is increased for drugs as a consequence of the transduction process. It was found, however, that neither the topographic pattern nor the degree of hair cell damage differed between the two groups. It is concluded that if the transduction process is accompanied by cell membrane permeability changes, the effect is not an increased susceptibility to neomycin toxicity. A measurable effect of stimulation was the significant decrease in the standard deviation of the mean dosage-damage response at each dosage level. This increase in reliability probably resulted from mixing of the inner ear fluids (especially in the vestibular compartment) which encouraged consistent dosages to the hair cells with each repeated trial. Finally, the impact of these findings on hypotheses related to the mechanism of selective ototoxicity is discussed briefly.

Erythromycin-Induced Ototoxicity: Case Report

Objective:

To report a case of erythromycin-induced ototoxicity and to discuss the occurrence of this event.

Case Summary:

A 26-year-old woman was admitted to the medical intensive care unit with a two-day history of progressive shortness of breath, high-grade fever, cough, and pleuritic chest pain. Arterial blood gases on room air showed severe hypoxemia, and a chest X-ray revealed right lower-lobe infiltrates. Provisional diagnosis was atypical pneumonia, for which erythromycin lactobionate 1 g q6h iv was administered. All other chronic medications were maintained at the same dosage and frequency. All laboratory work remained stable. After 36 hours, the patient developed sensorineural hearing loss. Erythromycin was stopped immediately. After 24 hours, there was subjective improvement of hearing, with complete return to pretreatment levels in 72 hours.

Discussion:

A review of the literature showed only 40 reported cases of reversible ototoxicity, mainly with high dosages of erythromycin (4 g/d).

Conclusions:

High-dose erythromycin therapy can cause reversible sensorineural hearing impairment. Treatment with erythromycin 4 g/d should be reserved for immunosuppressed patients with Legionnaires' disease and patients with Legionella endocarditis. Patients should have a baseline audiogram and regular monitoring for subjective evidence of sensorineural hearing loss, and the drug should be discontinued if ototoxicity is suspected.

A Sox10(rtTA/+) Mouse Line Allows for Inducible Gene Expression in the Auditory and Balance Organs of the Inner Ear

Genetic mouse models provide invaluable tools for discerning gene function in vivo. Tetracycline-inducible systems (Tet-On/Off) provide temporal and cell-type specific control of gene expression, offering an alternative or even complementary approach to existing Cre/LoxP systems. Here we characterized a Sox10(rtTA/+) knock-in mouse line which demonstrates inducible reverse tetracycline trans-activator (rtTA) activity and Tet-On transgene expression in the inner ear following induction with the tetracycline derivative doxycycline (Dox). These Sox10(rtTA/+) mice do not exhibit any readily observable developmental or hearing phenotypes, and actively drive Tet-On transgene expression in Sox10 expressing cells in the inner ear. Sox10(rtTA/+) activity was revealed by multiple Tet-On reporters to be nearly ubiquitous throughout the membranous labyrinth of the developing inner ear, and notably absent from hair cells, tympanic border cells, and ganglion neurons following postnatal Dox inductions. Interestingly, Dox-induced Sox10(rtTA/+) activity declined with induction age, where Tet-On reporters became uninducible in adult cochlear epithelium. Co-administration of the loop diuretic furosemide was able to rescue Dox-induced reporter expression, though this method also caused significant cochlear hair cell loss. Surprisingly, Sox10(rtTA/+) driven reporter expression in the cochlea persists for at least 54 days after cessation of neonatal induction, presumably due to the persistence of Dox within inner ear tissues. These findings highlight the utility of the Sox10(rtTA/+) mouse line as a powerful tool for functional genetic studies of the auditory and balance organs in vivo, but also reveal some important considerations that must be adequately controlled for in future studies that rely upon Tet-On/Off systems.

Minocycline protection of neomycin induced hearing loss in gerbils

This animal study was designed to determine if minocycline ameliorates cochlear damage is caused by intratympanic injection of the ototoxic aminoglycoside antibiotic neomycin. Baseline auditory-evoked brainstem responses were measured in gerbils that received 40 mM intratympanic neomycin either with 0, 1.2, or 1.5 mg/kg intraperitoneal minocycline. Four weeks later auditory-evoked brainstem responses were measured and compared to the baseline measurements. Minocycline treatments of 1.2 mg/kg and 1.5 mg/kg resulted in significantly lower threshold increases compared to 0 mg/kg, indicating protection of hearing loss between 6 kHz and 19 kHz. Cochleae were processed for histology and sectioned to allow quantification of the spiral ganglion neurons and histological evaluation of organ of Corti. Significant reduction of spiral ganglion neuron density was demonstrated in animals that did not receive minocycline, indicating that those receiving minocycline demonstrated enhanced survival of spiral ganglion neurons, enhanced survival of sensory hairs cells and spiral ganglion neurons, and reduced hearing threshold elevation correlates with minocycline treatment demonstrating that neomycin induced hearing loss can be reduced by the simultaneous application of minocycline.

Generation of Atoh1-rtTA transgenic mice: a tool for inducible gene expression in hair cells of the inner ear

Atoh1 is a basic helix-loop-helix transcription factor that controls differentiation of hair cells (HCs) in the inner ear and its enhancer region has been used to create several HC-specific mouse lines. We generated a transgenic tetracycline-inducible mouse line (called Atoh1-rtTA) using the Atoh1 enhancer to drive expression of the reverse tetracycline transactivator (rtTA) protein and human placental alkaline phosphatase. Presence of the transgene was confirmed by alkaline phosphatase staining and rtTA activity was measured using two tetracycline operator (TetO) reporter alleles with doxycycline administered between postnatal days 0–3. This characterization of five founder lines demonstrated that Atoh1-rtTA is expressed in the majority of cochlear and utricular HCs. Although the tetracycline-inducible system is thought to produce transient changes in gene expression, reporter positive HCs were still observed at 6 weeks of age. To confirm that Atoh1-rtTA activity was specific to Atoh1-expressing cells, we also analyzed the cerebellum and found rtTA-driven reporter expression in cerebellar granule neuron precursor cells. The Atoh1-rtTA mouse line provides a powerful tool for the field and can be used in combination with other existing Cre recombinase mouse lines to manipulate expression of multiple genes at different times in the same animal.

In vitro and in vivo models of drug ototoxicity: studying the mechanisms of a clinical problem

INTRODUCTION

Drug ototoxicity represents one of the main preventable causes of deafness. Ototoxicity is a trait shared by aminoglycoside and macrolide antibiotics, antimalarial medications, loop diuretics, platinum-based chemotherapeutic agents, some NSAIDs and most recently described, acetaminophen when abused with narcotic medication. These medications are prescribed despite their side effects, which includes inner ear toxicity, because they are life-saving drugs or there is a lack of better treatment.

AREAS COVERED

This review will discuss in vitro and in vivo models of ototoxicity highlighting recently published ototoxicity research. The reader will learn the strengths and limitations of different ototoxicity models and what molecular insights have been gained from their application. A better understanding of the cellular mechanisms of these ototoxins will help in the discovery of ways to prevent and treat hearing loss associated with ototoxic medications.

EXPERT OPINION

There are benefits to both in vitro and in vivo models of ototoxicity. Research of a particular medication and its ototoxic mechanisms should draw from several models, enabling a better answer to the clinical question of prevention and treatment of inner ear drug toxicity.

Understanding drug ototoxicity: molecular insights for prevention and clinical management

Ototoxicity is a trait shared by aminoglycoside and macrolide antibiotics, loop diuretics, platinum-based chemotherapeutic agents, some NSAIDs and antimalarial medications. Because their benefits in combating certain life-threatening diseases often outweigh the risks, the use of these ototoxic drugs cannot simply be avoided. In this review, the authors discuss some of the most frequently used ototoxic drugs and what is currently known about the cell and molecular mechanisms underlying their noxious effects. The authors also provide suggestions for the clinical management of ototoxic medications, including ototoxic detection and drug monitoring. Understanding the mechanisms of drug ototoxicity may lead to new strategies for preventing and curing drug-induced hearing loss, as well as developing new pharmacological drugs with less toxic side effects.

The biology of intratympanic drug administration and pharmacodynamics of round window drug absorption

The application of therapeutic agents to the round window holds great promise in the treatment of inner ear disease. The primary benefit of this route of administration is the ability to achieve high inner ear concentrations of drugs without systemic side effects. Recent research has elucidated the anatomy and physiology of the round window and provided important information on the inner ear pharmacokinetics and the pharmacodynamics of drugs administered intratympanically. Although amino-glycosides and steroids have been most thoroughly studied, many other classes of pharmaceuticals, including otoprotective agents, other antibiotics,and topical anesthetics, have therapeutic potential in the inner ear and will probably be the subject of future studies. The authors believe that direct delivery approaches other than through the round window, perhaps with slow or sustained release formulations, may hold the key to the future treatment of inner ear disorders.

Mechanisms of antibiotic neurotoxicity in renal failure

Neurological complications of antibiotics are relatively common in renal failure. Central nervous system neurotoxicity due to penicillin and beta-lactam antibiotics is best documented with fewer accounts of ototoxicity, peripheral nerve toxicity and neuromuscular blockade. In the context of risk stratification, the goal of this review is to explore the mosaic of factors in renal impairment that may contribute to susceptibility to antibiotic neurotoxicity. Improved knowledge of the pathogenesis of these formidable adverse events among the renal failure subjects should help prevent antibiotic neurotoxicity in the future.

Round window gentamicin absorption: an in vivo human model

OBJECTIVE/HYPOTHESIS

Using a novel human labyrinthine sampling model, in vivo gentamicin absorption through the round window can be measured.

STUDY DESIGN

A prospective study.

METHODS

Gentamicin was delivered either transtympanically (preoperative) or through a facial recess approach (intraoperative). The lateral semicircular canal and vestibule were opened, and by means of a microsyringe, labyrinthine fluid was aspirated. A sample of serum was also drawn. In all patients cerebrospinal fluid was also drawn. The samples were analyzed using a standard chemistry analyzer.

RESULTS

Intratympanic gentamicin diffused through the round window membrane and achieved concentrations in the labyrinthine fluid ranging from 0 to 16 mg/L. Intratympanic gentamicin was absorbed into the systemic circulation in 4 of 11 patients with serum levels ranging from 0.3 to 0.4 mg/L. No gentamicin was detected in the cerebrospinal fluid.

CONCLUSIONS

Intratympanic gentamicin diffuses rapidly through the round window membrane and achieves significant levels in the inner ear. Thus, this new model can be used to assess round window permeability to clinically relevant medications such as steroids and ototopical antibiotics.

Normal and altered cytoarchitecture of the inner ear

Scanning electron microscope gave new insights into real morphology of the inner ear and has enhanced basic research. In fact, during the last twenty years, the cellular system that composes the organ of Corti and the vestibular apparatus has been clearly described by many authors. We now have a better understanding of each cell type and that they have a physiological functioning as ear and vestibular system. In addition, new insights into the damage caused by many drugs, such as aminoglucosides, can be investigated by studying the morphological changes and the reparative process following the injury. Animals such as guinea pigs can be used as ideal models to investigate the alterations in the sensory epithelium of the cochlear and vestibular receptors following ototoxic poisoning.

Intravenous azithromycin-induced ototoxicity

Intravenous azithromycin is increasingly administered for treatment of hospitalized patients with community-acquired pneumonia. Macrolide antibiotics cause ototoxicity, which occurs most frequently when high serum concentrations are achieved. Current dosing guidelines for intravenous azithromycin can result in much higher serum concentrations than is seen with oral administration. We describe a 47-year-old woman who developed complete deafness after receiving 8 days of intravenous azithromycin.

Treatment of chronic suppurative otitis media with topical ciprofloxacin

To date, only ofloxacin has been approved by the US Food and Drug Administration for treatment of ears with a nonintact tympanic membrane. The purpose of this study was to determine the safety and efficacy of topical ciprofloxacin hydrochloride in the treatment of experimental chronic suppurative otitis media caused by Pseudomonas aeruginosa infection in cynomolgus monkeys. Forty adult cynomolgus monkeys were divided into 4 equal groups, and their ears were challenged with P aeruginosa, drained for 3 weeks, then treated twice daily for 4 weeks with 1 of 4 randomly assigned agents: 1) ciprofloxacin, 2) saline, 3) Cortisporin, or 4) vehicle. The animals were followed up with auditory brain stem response testing, culture, otoscopy, and histopathology. Both ciprofloxacin and Cortisporin treatment resulted in a significantly more rapid rate of clearance of P aeruginosa as compared to treatment with saline (100% versus 20%). Eradication was not associated with resolution of otorrhea after a 4-week period of treatment. There were no significant changes in auditory brain stem response wave latencies for any of the treatment groups. Histopathologic data revealed that there was no statistically significant difference in the amount of outer hair cell loss for the ciprofloxacin group as compared to the control ear and other treatment groups. We conclude, therefore, that topical ciprofloxacin is not ototoxic and is effective in sterilizing the otorrhea, but does not promote resolution of the drainage, in this animal model.

Topical gentamicin and ethacrynic acid: effects on cochlear function

OBJECTIVE

To determine whether concurrent intravenous administration of the loop diuretic ethacrynic acid potentiates the toxicity of the aminoglycoside antibiotic gentamicin applied topically on the round window.

STUDY DESIGN

The authors studied the effects on cochlear sensitivity of co-administered intracardiac ethacrynic acid (40 mg/kg) and high-dose topical gentamicin solution (100%) applied to the round window. Comparisons were made with animals receiving ethacrynic acid plus systemic gentamicin (100 mg/kg); topical gentamicin alone; systemic gentamicin alone; and intravenous ethacrynic acid alone.

METHODS

Experiments were carried out on pigmented guinea pigs weighing 400 to 500 g. Changes in cochlear function were characterized by monitoring shifts in compound action potential (CAP) thresholds by use of chronic indwelling electrodes implanted at the round window, vertex, and contralateral mastoid.

RESULTS

After 20 days animals receiving ethacrynic acid in combination with topical gentamicin to the round window failed to demonstrate a significant deterioration in cochlear sensitivity, whereas all animals receiving systemic gentamicin plus ethacrynic acid experienced profound increases in CAP thresholds.

CONCLUSIONS

This study supports the contention that ethacrynic acid potentiates aminoglycoside ototoxicity by facilitating the entry of the antibiotics from the systemic circulation into the endolymph. In addition, this study answers important clinical concerns regarding the safety of the use of topical aminoglycoside agents in combination with loop diuretics.

Ototoxic effect of erythromycin on cochlear potentials in the guinea pig

The mechanism of hearing loss due to the administration of intravenous erythromycin was investigated in the albino guinea pig, and it was found for the first time that this drug causes cochlear dysfunction. The endocochlear potential (EP) and the cochlear microphonics (CM) recorded at the first cochlear turn transiently decreased when erythromycin was administered intravenously at dosages of 100 and 150 mg/kg. The averaged maximum decrease in EP was 16 mV (n = 5) and 33 mV (n = 5) for 100 and 150 mg/kg, respectively. The maximum decrease in the CM was about 25% when the EP reached its lowest value with the injection of 150 mg/kg. A complete recovery of the EP and CM ensured within 20 minutes after each erythromycin dose. The perilymphatic perfusion of 3 mmol/L of erythromycin decreased the EP and CM; however, in contrast to the intravenous administration, the decrease of the CM was nearly complete and both the EP and CM were irreversible. Hearing loss due to intravenously administered erythromycin could likely be attributle to the transient dysfunction of the stria vascularis, although concomitant dysfunction of the central auditory pathway cannot be excluded.

Prospective evaluation of neomycin serum concentrations after direct corpora cavernosa irrigation during penile prosthesis placement

OBJECTIVES

To determine if toxic serum levels of neomycin are generated after direct corpora cavernosa irrigation during penile prosthesis placement.

METHODS

We have used an infection prophylaxis technique that involves directly irrigating the corpora cavernosa tissue (through the corporotomy) with 0.5% neomycin solution. Serum neomycin concentrations were measured at 1 hour and 4 hours after irrigation in 13 patients undergoing penile prosthesis placement. A subset of patients who had preimplant and postimplant serum creatinine concentrations was evaluated for changes in renal function.

RESULTS

The mean 1-hour postirrigation serum neomycin level was 1.2 micrograms/mL and the mean 4-hour postirrigation level was 1.2 micrograms/mL. These serum neomycin concentrations are lower than those thought to be necessary to produce nephrotoxicity or ototoxicity. Renal function was not significantly affected by the neomycin irrigation.

CONCLUSIONS

Although aminoglycosides are ototoxic and neomycin has the highest nephrotoxic potential of the aminoglycosides, we conclude that direct irrigation of the corpora cavernosa with 0.5% neomycin solution does not produce significant systemic exposure to result in nephrotoxicity or ototoxicity. One-time prophylactic neomycin irrigation remains an effective, safe, and economic adjunct to penile prosthesis placement.

Erythromycin ototoxicity in liver transplant patients

We report on three liver transplant patients who developed erythromycin-related ototoxicity. This complication has been described in renal transplant patients and in patients with liver dysfunction, but to our knowledge it has not yet been reported in liver transplant patients. The influence of hepatic dysfunction, common renal failure, and the interaction between cyclosporin and erythromycin in the development of erythromycin ototoxicity are discussed.

Erythromycin-induced hypoacusis: 11 new cases and literature review

OBJECTIVE

To report 11 cases of possible erythromycin-induced hearing loss and to review all cases reported in the literature.

CASE SUMMARY

In the 11 cases reported, the following are reviewed: age, gender, time to onset of and recovery from hypoacusis in relation to erythromycin administration, presence or absence of renal or hepatic disease, underlying disorders, and concurrent administration of other drugs. Hypoacusis appeared with dosages equal to or higher than 4 g/d in patients with a mean age of 52.5 +/- 19 years, a high percentage of whom (45 percent) presented with renal impairment. The hearing loss was reversible in all cases, and subsided a few days (median = 3) after dosage reduction or drug discontinuation.

DISCUSSION

Our patients' characteristics are similar to those of patients reported in the literature. Most data indicate that erythromycin-induced hypoacusis is a dose-dependent effect; however, its occurrence in patients otherwise free from disposing factors suggests that it is idiosyncratic.

CONCLUSIONS

Erythromycin administered for appropriate indications and dosage adjustments in patients with impaired renal and/or liver function may prevent or reduce the incidence of erythromycin-induced hypoacusis.

Cochlear permeability of neomycin and gentamicin: an immunohistochemical study

Aminoglycoside antibiotics (AGAs) target specifically the cochleo-vestibular hair cells, but with varied ototoxicity. Differences in their penetration and clearance rates into the membranous labyrinth may play a role. This in turn may be related to a difference in the number of amine groups, the cationic nature, as well as the molecular weight and size of the AGA molecule. Immunohistochemical labeling techniques were used to study the pathways of gentamicin and neomycin from the perilymph into cochlear tissues and target cells. The more cochleotoxic AGA, neomycin, penetrated into cochlear tissues faster than the less cochleotoxic AGA, gentamicin.

Aminoglycoside induced ototoxicity in patients with cystic fibrosis

The improved survival of cystic fibrosis (CF) patients is partly due to intensive treatment for their chronic infections. Treatment usually includes intravenous and nebulised aminoglycoside antibiotics and they receive a large cumulative dose of these antibiotics over their lifetime. There is little information in the literature on the prevalence of ototoxicity due to aminoglycoside in these patients. We performed pure tone audiometry on 43 CF patients aged 14-42 years. Seven (16%) had bilateral sensorineural hearing loss (SNHL) for high frequency sounds, consistent with aminoglycoside induced ototoxicity. However, only 2 of these patients had documented toxic serum levels in the past. The identification of bilateral SNHL in one in six adult CF patients is a cause for concern. It may be that the high cumulative dose of aminoglycosides received by these patients may be causing inner ear injury in the absence of specific episodes of toxic serum levels.

Evidence that amikacin ototoxicity is related to total perilymph area under the concentration-time curve regardless of concentration

Previous studies have failed to fully establish whether ototoxicity is related in any way to the levels of an aminoglycoside antibiotic in the perilymph. To study this we exposed guinea pigs to continuously infused amikacin at four different dosing rates under conditions parallel to those used in our previous study which related ototoxicity to total plasma area under the concentration-time curve regardless of the level in plasma. It was found that at all dosing rates, levels in the perilymph and ratios of levels in perilymph/plasma remained constant as the dosing duration increased from nonototoxic to strongly ototoxic. Plasma and perilymph amikacin levels were found to be linear functions of the dosing rate even at ototoxic dosing exposures, and ratios of levels in perilymph/plasma did not differ between dosing rates. The total perilymph area under the concentration-time curve was not different between dosing rates either for a total dose associated with threshold ototoxicity or for one associated with severe ototoxicity. The results suggest that amikacin ototoxicity is related to the integral of the concentration in the perilymph over the total time of amikacin exposure regardless of the level in the perilymph.

Drug-induced ototoxicity. Pathogenesis and prevention

Ototoxicity is a disabling adverse effect of several widely used classes of drugs, such as diuretics, anti-inflammatory agents, antineoplastic agents and aminoglycoside antibiotics. High-dose therapy with either diuretics or anti-inflammatory agents is primarily associated with acute and transient impairment of hearing or tinnitus. In contrast, long term treatment with antineoplastic agents or aminoglycoside antibiotics is typically associated with delayed and irreversible loss of hearing; lesion in the organ of Corti include the destruction of auditory sensory cells. Vestibular function can also be compromised by ototoxic drugs. Occasional cases of ototoxicity have been reported for a variety of other therapeutic compounds and environmental toxins. In addition, the simultaneous administration of multiple agents which are potentially ototoxic can lead to synergistic loss of hearing. Exposure to loud noise may also potentiate the hearing loss due to cochleotoxic drugs. Ototoxic agents can impair the sensory processing of sound at many cellular or subcellular sites. However, the molecular mechanisms of ototoxicity have not been established for most of these drugs, and structure-toxicity relationships have not been determined. It has therefore been difficult to predict the ototoxic potential of new drugs, and rational approaches to the prevention of ototoxicity are still lacking. The clinical and experimental features of ototoxicity are reviewed for several classes of drugs, with an emphasis on current knowledge of the mechanism and the possibilities for the prevention of ototoxicity for each.

The use of streptomycin to induce unilateral ablation of vestibular function in the rat: a preliminary report

The goal of our research is to develop a method of applying streptomycin to the middle ear so as to destroy vestibular function unilaterally while avoiding damage to auditory function. In this preliminary experiment, a total of 42 mg of streptomycin was infused continuously over a seven-day period into one middle ear cavity of seven rats, with the opposite ear serving as a control. Histologic study revealed a moderate to severe degree of vestibular neuroepithelial damage in all of the infused ears. However, significant damage also occurred in the basal turn of the cochlea in a majority of cases.

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.

The reproducibility of caloric tests of vestibular function in young and old subjects

The test-retest repeatability of the hot caloric response alone and of the hot and cold caloric responses combined, was evaluated in 29 subjects, over a 6-month period. The subjects were from two different age groups, 20-30 years (15 subjects) and 65-75 years (14 subjects). Intersubject variability was statistically significant in both age groups, though of greater magnitude in the older subjects. Once a vestibular caloric response baseline was established, for any given subject, there was reasonably reliable test-retest repeatability over time, in both age groups, with coefficients of reliability greater than 0.90.

Furosemide-induced alteration of drug pathway to cochlea

The ototoxicity of aminoglycosides is known to be enhanced by post-treatment with loop diuretics. The present experimental study was undertaken to elucidate the mechanism of this enhancement immunohistologically. Intravenously administered kanamycin (KM) reached the organ of Corti rapidly (within 10 min) via the capillaries of the basilar membrane and spiral limbus. Besides this direct pathway, KM penetrated through the perilymphatic space pathway. This penetration, however, took a much longer time (90 min). In contrast, in the case when KM was followed by intravenous furosemide (FM), KM reached the perilymphatic region within a short time (1-5 min). The KM content of the organ of Corti ascertained 1-5 min after FM injection seemed almost equivalent to that determined 90 min after injection of KM alone. This FM-induced change in the time-course penetration pattern of KM might have a bearing on the enhancement of its otoxicity.

Pharmacokinetics of aminoglycoside antibiotics in blood, inner-ear fluids and tissues and their relationship to ototoxicity

This review critically evaluates the literature on aminoglycoside pharmacokinetics in order to answer the question how fluid and tissue levels of the drugs relate to the development of ototoxic and nephrotoxic side effects. We will summarize the evidence that: (1) aminoglycosides do not accumulate in inner-ear fluids; (2) aminoglycoside levels in fluids do not correlate with the ototoxic potential of a drug, and (3) selective toxicity cannot be explained by selective tissue penetration of the drugs. We suggest that studies of drug disposition at the cellular level after chronic aminoglycoside treatment be conducted to establish whether a cell-specific uptake contributes to the selective toxicity of the aminoglycoside antibiotics. A sequence of biochemical events that may lead to the development of toxicity at the molecular level is briefly described.

Intratympanic ototoxicity: influence of post-injection survival period

Many studies have been reported on the intratympanic ototoxicity of different drugs in animal models. The recovery periods of the animals following intratympanic drug applications varied among these studies. The present study compares the cochlear damage caused by intratympanic kanamycin following short (4 days) and long (30 days) post-injection survival periods, using the guinea pig as the animal model. The degree of cochlear damage 4 days after kanamycin injection was consistent among the tested animals. The degeneration was mainly confined to the outer hair cells and almost all inner hair were spared. The change 30 days after kanamycin injection was more variable among the animals and both inner and outer hair cells were damaged. This shows that, although the damage to the cochlea after intratympanic aminoglycoside injection is progressive, a short post-injection recovery period is suitable for comparative intratympanic ototoxicity studies.

A histopathological study of the inner ear after the administration of hyaluronan into the middle ear of the guinea pig

Hyaluronan, 19 mg/ml, was administered into the middle ear cavity of guinea pigs. A control group was subjected to a sham operation. Fourteen days after administration the animals were killed and a macroscopic examination of all middle ears was performed. Surface preparations of the organ of Corti were made and examined by light microscopy. Hair cell loss along the length of the sensory epithelium was quantified. No toxic effect of hyaluronan could be detected either in the middle ear or in the sensory cells in the cochlea. Thus hyaluronan is potentially useful for facilitating middle ear surgery.

Comparative uptake of gentamicin, netilmicin, and amikacin in the guinea pig cochlea and vestibule

The kinetics of the entry of three aminoglycosides into inner-ear tissues of the guinea pig after acute and chronic administration were compared: gentamicin toxic to the cochlea and the vestibule, amikacin preferentially cochleotoxic, and netilmicin of low ototoxic liability. During constant intravenous infusion, levels of the three drugs in plasma tended to reach a plateau after 1 h, while levels in perilymph did not reach a plateau within 6 h. The drug concentrations in both vestibular and cochlear tissues quickly reached saturation. Amikacin and gentamicin concentrations were similar in vestibular and cochlear tissues, while netilmicin values were somewhat lower. After 1 week of chronic treatment (100 mg of drug per kg of body weight daily subcutaneously), levels of gentamicin and amikacin in tissue were similar to each other and were not significantly different between cochlear and vestibular tissues. Netilmicin concentrations again were somewhat lower in the tissues, but identical to those of the other drugs in the perilymph. After 3 weeks of treatment, all of the drugs were equally distributed in the inner-ear tissues. Release of the drug from the tissues after the 3-week treatment was faster for amikacin (83% decrease after 20 days) than for netilmicin and gentamicin (approximately 50% decrease). There was no correlation, under any of the experimental conditions, between the drug concentrations and their degrees of toxicity. These results demonstrate that selective aminoglycoside ototoxicity cannot be explained by a preferential uptake or accumulation of drugs in the afflicted tissues or in the perilymph.

Ototoxicity of neomycin and its penetration through the round window membrane into the perilymph

The ototoxicity of neomycin and its concentration in the perilymph after direct application on the round window membrane were studied. After placing 5 mg of neomycin on the round window membrane of guinea pigs for various time intervals, concentration of the drug in the perilymph was determined by high performance liquid chromatography, and the cochlea was examined by light microscopy. Neomycin penetrated the round window membrane quite easily, and its concentration in the perilymph became extremely high in a short time and then decreased gradually. This indicates that high concentration of neomycin in the perilymph can be attained by application of a small amount of the drug on the round window membrane. Ototoxicity of neomycin was observed after application for 4 hours. Cochlear damage increased as neomycin application time became longer, but no consistent relationship was noted between the concentration of neomycin and the amount of damage. This result is discussed from the point of concentration and persistence of the drug in the inner ear fluids.

Kinetics of gentamicin uptake and release in the rat. Comparison of inner ear tissues and fluids with other organs

The kinetics of entry and release of gentamicin was investigated in fluids and tissues of the inner ear of the rat, as well as in renal cortex, and in organs that do not share susceptibility to the toxic effects of aminoglycosides. Various modes of administration were used to achieve different patterns of drug plasma concentrations. Electrophysiological and histological examinations were performed to correlate pharmacokinetics and ototoxicity. Results show that: the uptake of the drug by the inner ear tissues is dose dependent and manifests a rapid saturation kinetics with a concentration plateau of about 1 micrograms/mg of protein. The low ratio of the perilymph and endolymph to plasma concentrations argues against the concept of an accumulation of the drug in the inner ear over drug levels in plasma, which has been considered as the basic mechanism of ototoxicity. In renal cortex, the kinetics appears similar to that of the inner ear but the concentrations achieved are 10-fold higher than in cochlear tissues. In other organs (liver, heart, lung, and spleen), no saturation could be demonstrated within the duration of the experiment. Ototoxicity seems to be related to the penetration of the drug into compartment(s) from which the half-life of disappearance is extremely slow. Rapid uptake, early saturation, and long exposure of the tissues to the drug may account for the development of toxicity in inner ear and kidney.

Effects of noise and ototoxic drugs at the cellular level in the cochlea: a review

Currently available information concerning the cellular mechanisms involved in acoustic trauma and aminoglycoside ototoxicity is reviewed to shed some new light on the cellular events that may be related to functional impairment of the auditory organ. Based on the available data, the following postulations can be made concerning the cellular mechanisms involved. 1) The macromolecular disruption of the stereocilia and cuticular plates is the initial cellular event in acoustic trauma. This disruption would affect the micromechanics of the transduction process, leading to temporary threshold shift. Further cellular impairment would involve basic cellular functions such as the protein, lipid, and glucose synthesis needed for cell repair and survival, and such impairment would result in permanent cell injury or cell death, leading to permanent threshold shift. 2) It can be postulated that the cellular mechanisms involved in aminoglycoside ototoxicity include two events. The early event is the reversible blockage of the transduction channels from the endolymph side of the hair cells. The later event is the interference in such cellular functions as protein and/or phospholipid synthesis because of binding of aminoglycoside to the phospholipids and/or protein, leading to cell death. The latter event may be facilitated by penetration or membrane-mediated internalization of the aminoglycoside from the perilymph side of the hair cell.

Gentamycin for prophylaxis of bacterial endocarditis: a review for the dentist

New guidelines have recommended that gentamicin in combination with ampicillin be used for prophylaxis of bacterial endocarditis in patients with prosthetic heart valves. This article reviews some of the important and practical considerations for its use by the dentist. Gentamicin is an aminoglycoside antibiotic most exclusively reserved for treatment of serious infections caused by gram-negative bacteria in which less toxic antibacterials are ineffective. It has also been shown to be impressive in combination with penicillin in treating high-risk endocarditis patients. All strains of enterococci that are resistant to penicillin plus streptomycin are almost always sensitive to penicillin plus gentamicin. There is minimal absorption into the bloodstream from the gastrointestinal tract after oral administration but rapid absorption after intramuscular injection. Peak serum concentrations appear 30 to 90 minutes after intramuscular injection. The T1/2 is 2 hours, and in normal kidneys 85% to 95% of the drug is excreted within 24 hours by glomerular filtration. Ototoxicity and nephrotoxicity are the most serious toxic effects resulting from gentamicin therapy. The incidence of ototoxicity is about 2%, with affected patients experiencing vestibular effects rather than hearing loss. Nephrotoxicity is usually not seen before the patient has had 5 to 7 days of frequent dosing for treatment of systemic infections; the incidence is 2% to 4%. There are no data to suggest that ototoxicity or nephrotoxicity will occur in the patient given a single intramuscular injection of gentamicin for the prophylaxis of bacterial endocarditis. A single intramuscular or intravenous injection each of ampicillin and gentamicin should provide adequate blood levels for protection in the endocarditis patient for at least 4 to 5 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Progressive ototoxicity of neomycin monitored using derived brainstem response audiometry

Progressive hearing loss following the systemic administration of neomycin was investigated using derived brainstem response audiometry. Cats were given three to five times the maximum recommended clinical dose of neomycin over a period of 10 days. Their hearing was monitored prior to and during the administration of the drug, and periodically following its completion. The results of this study showed that the induced hearing loss generally proceeded from high to low frequencies as an advancing lesion, with regions apical to the lesion functioning normally. Although considerable variability in response to the drug existed among animals, the evoked responses from both ears of each animal showed close bilateral symmetry during the deafening process. Furthermore, the present results highlight the long-term ongoing ototoxicity associated with neomycin, and the importance of monitoring high frequencies for initial signs of an aminoglycoside induced hearing loss.

Effect of pentobarbital anesthesia on amikacin concentrations in plasma and perilymph and evaluation of multiple sampling in perilymph of guinea pigs

The purpose of this study was to determine whether a multiple-sampling procedure could be used in guinea pigs to study the kinetics of amikacin in perilymph. Amikacin was infused intravenously for 6 h into conscious anesthetized guinea pigs, and the concentrations of the drug in plasma and perilymph were measured. From each anesthetized guinea pig, five to six perilymph samples were collected from one ear, and one sample was collected from the other ear at 6 h. The concentrations of amikacin in perilymph were dose proportional and increased slowly during the 6-h infusion. However, after 6 h of intravenous infusion, the concentrations of amikacin in perilymph of the multiply sampled ears were significantly higher than those of the singly sampled ears, indicating that the multiple-sampling procedure should not be used as is to study the kinetics of amikacin in perilymph. Amikacin concentrations in perilymph were linearly related to amikacin concentrations in plasma in pentobarbital-anesthetized animals, as had previously been observed for conscious guinea pigs. However, the slope of the regression line was only 0.09 for anesthetized animals compared with 0.24 for conscious animals. Drug concentrations in plasma were found to be threefold higher in anesthetized animals, whereas drug levels in perilymph were the same in both groups at similar dosing rates. These results indicate that the amikacin concentration in perilymph is not solely dependent upon its concentration in plasma and that other factor(s) can affect the entry of amikacin into the inner ear.