Distribution of gentamicin in the guinea pig inner ear after local or systemic application

Aminoglycoside-induced sensorineural hearing loss in pediatric cystic fibrosis patients: A retrospective cohort study

Background

Pulmonary infections by gram-negative organisms are important in cystic fibrosis (CF). Aminoglycosides (AG) are often part of the treatment regimen. However, they are a well-known cause of ototoxicity. Even minimal hearing impairment in children could have a future impact on functional well-being.We aimed to investigate the progression of sensorineural hearing loss (SNHL) over several years in pediatric CF patients, and to identify risk factors, such as the use of AG, including both intravenous (IV) and inhaled AG.

Methods

Retrospective analyses of patient records from children and adolescents followed up at the CF clinic of the Antwerp University Hospital, Belgium, were performed. We collected data on age, sex, pure-tone audiometry, and the use of AG. Descriptive and binary logistic regression analyses, and if indicated generalized estimating equations (GEE) analyses were performed.

Results

Forty pediatric patients were enrolled in the study taking part from 2013 to 2020. Pure-tone audiometry revealed an important rate of SNHL over several years, with a prevalence of 29 % for high-frequency SNHL (i.e. 8 kHz). Increasing age was identified as a significant risk factor for the development of SNHL at 8 kHz if 5 or more IV AG courses (p = 0.01) were reported or when IV AG were combined with inhaled AG (p = 0.002).

Conclusions

Age combined with the use of IV AG (≥5 courses or in combination with inhaled AG) are predictive for developing high-frequency SNHL (i.e. 8 kHz). We suggest routine annual hearing screening (incl. high-frequency thresholds) in CF patients, starting from childhood.

Mechanisms and Impact of Aminoglycoside-Induced Vestibular Deficits

PURPOSE

Acquired vestibulotoxicity from hospital-prescribed medications such as aminoglycoside antibiotics affects as many as 40,000 people each year in North America. However, there are no current federally approved drugs to prevent or treat the debilitating and permanent loss of vestibular function caused by bactericidal aminoglycoside antibiotics. This review will cover our current understanding of the impact of, and mechanisms underlying, aminoglycoside-induced vestibulotoxicity and highlight the gaps in our knowledge that remain.

CONCLUSIONS

Aminoglycoside-induced vestibular deficits have long-term impacts on patients across the lifespan. Additionally, the prevalence of aminoglycoside-induced vestibulotoxicity appears to be greater than cochleotoxicity. Thus, monitoring for vestibulotoxicity should be independent of auditory monitoring and encompass patients of all ages from young children to older adults before, during, and after aminoglycoside therapy.

Macrophage depletion attenuates degeneration of spiral ganglion neurons in kanamycin-induced unilateral hearing loss model

Pathological conditions in cochlea, such as ototoxicity, acoustic trauma, and age-related cochlear degeneration, induce cell death in the organ of Corti and degeneration of the spiral ganglion neurons (SGNs). Although macrophages play an essential role after cochlear injury, its role in the SGNs is limitedly understood. We analyzed the status of macrophage activation and neuronal damage in the spiral ganglion after kanamycin-induced unilateral hearing loss in mice. The number of ionized calcium-binding adapter molecule 1 (Iba1)-positive macrophages increased 3 days after unilateral kanamycin injection. Macrophages showed larger cell bodies, suggesting activation status. Interestingly, the number of activating transcription factor 3 (ATF3)-positive-neurons, an indicator of early neuronal damage, also increased at the same timing. In the later stages, the number of macrophages decreased, and the cell bodies became smaller, although the number of neuronal deaths increased. To understand their role in neuronal damage, macrophages were depleted via intraperitoneal injection of clodronate liposome 24 h after kanamycin injection. Macrophage depletion decreased the number of ATF3-positive neurons at day 3 and neuronal death at day 28 in the spiral ganglion following kanamycin injection. Our results suggest that suppression of inflammation by clodronate at early timing can protect spiral ganglion damage following cochlear insult.

Off target toxicities and links with physicochemical properties of medicinal products, including antibiotics, oligonucleotides, lipid nanoparticles (with cationic and/or anionic charges). Data review suggests an emerging pattern

Toxicology is an essential part of any drug development plan. Circumnavigating the risk of failure because of a toxicity issue can be a challenge, and failure in late development is extremely costly. To identify potential risks, it requires more than just understanding the biological target. The toxicologist needs to consider a compound's structure, it's physicochemical properties (including the impact of the overall formulation), as well as the biological target (e.g., receptor interactions). Understanding the impact of the physicochemical properties can be used to predict potential toxicities in advance by incorporating key endpoints in early screening strategies and/or used to compare toxicity profiles across lead candidates. This review discussed the risks of off-target and/or non-specific toxicities that may be associated with the physicochemical properties of compounds, especially those carrying dominant positive or negative charges, including amphiphilic small molecules, peptides, oligonucleotides and lipids/liposomes/lipid nanoparticles. The latter of which are being seen more and more in drug development, including the recent Covid pandemic, where mRNA and lipid nanoparticle technology is playing more of a role in vaccine development. The translation between non-clinical and clinical data is also considered, questioning how a physicochemical driven toxicity may be more universal across species, which means that such toxicity may be reassuringly translatable between species and as such, this information may also be considered as a support to the 3 R's, particularly in the early screening stages of a drug development plan.

Scientific and Regulatory Policy Committee Technical Review: Biology and Pathology of Ganglia in Animal Species Used for Nonclinical Safety Testing

Dorsal root ganglia (DRG), trigeminal ganglia (TG), other sensory ganglia, and autonomic ganglia may be injured by some test article classes, including anti-neoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, nerve growth factor inhibitors, and aminoglycoside antibiotics. This article reviews ganglion anatomy, cytology, and pathology (emphasizing sensory ganglia) among common nonclinical species used in assessing product safety for such test articles (TAs). Principal histopathologic findings associated with sensory ganglion injury include neuron degeneration, necrosis, and/or loss; increased satellite glial cell and/or Schwann cell numbers; and leukocyte infiltration and/or inflammation. Secondary nerve fiber degeneration and/or glial reactions may occur in nerves, dorsal spinal nerve roots, spinal cord (dorsal and occasionally lateral funiculi), and sometimes the brainstem. Ganglion findings related to TA administration may result from TA exposure and/or trauma related to direct TA delivery into the central nervous system or ganglia. In some cases, TA-related effects may need to be differentiated from a spectrum of artifactual and/or spontaneous background changes.

The vestibular calyceal junction is dismantled following subchronic streptomycin in rats and sensory epithelium stress in humans

Hair cell (HC) loss by epithelial extrusion has been described to occur in the rodent vestibular system during chronic 3,3'-iminodipropionitrile (IDPN) ototoxicity. This is preceded by dismantlement of the calyceal junction in the contact between type I HC (HCI) and calyx afferent terminals. Here, we evaluated whether these phenomena have wider significance. First, we studied rats receiving seven different doses of streptomycin, ranging from 100 to 800 mg/kg/day, for 3-8 weeks. Streptomycin caused loss of vestibular function associated with partial loss of HCI and decreased expression of contactin-associated protein (CASPR1), denoting calyceal junction dismantlement, in the calyces encasing the surviving HCI. Additional molecular and ultrastructural data supported the conclusion that HC-calyx detachment precede HCI loss by extrusion. Animals allowed to survive after the treatment showed functional recuperation and rebuilding of the calyceal junction. Second, we evaluated human sensory epithelia obtained during therapeutic labyrinthectomies and trans-labyrinthine tumour excisions. Some samples showed abnormal CASPR1 label strongly suggestive of calyceal junction dismantlement. Therefore, reversible dismantlement of the vestibular calyceal junction may be a common response triggered by chronic stress, including ototoxic stress, before HCI loss. This may partly explain clinical observations of reversion in function loss after aminoglycoside exposure.

A chemo-mechanical cochleostomy preserves hearing for the in vivo functional imaging of cochlear cells

In vivo and real-time multicellular imaging enables the decoding of sensory circuits and the tracking of systemic drug uptake. However, in vivo imaging of the auditory periphery remains technically challenging owing to the deep location, mechanosensitivity and fluid-filled, bone-encased nature of the cochlear structure. Existing methods that expose the cochlea invariably cause irreversible damage to auditory function, severely limiting the experimental measurements possible in living animals. Here we present an in vivo surgical protocol that permits the imaging of cochlear cells in hearing mice. Our protocol describes a ventro-lateral approach for preserving external and middle ear structures while performing surgery, the correct mouse positioning for imaging cochlear cells with effective sound transmission into the ear, the chemo-mechanical cochleostomy for creating the imaging window in the otic capsule bone that prevents intracochlear fluid leakage by maintaining an intact endosteum, and the release of intracochlear pressure that separates the endosteum from the otic capsule bone while creating an imaging window. The procedure thus preserves hearing thresholds. Individual inner and outer hair cells, supporting cells and nerve fibers can be visualized in vivo while hearing function is preserved. This approach may enable future original investigations, such as the real-time tracking of ototoxic drug transport into the cochleae. The technique may be applied to the monitoring of sound-evoked functional activity in multiple cochlear cells, in combination with optogenetic tools, and may help to improve cochlear implantation in humans. The cochleostomy takes ~1 h and requires experience in surgery.

An in vivo Biomarker to Characterize Ototoxic Compounds and Novel Protective Therapeutics

There are no approved therapeutics for the prevention of hearing loss and vestibular dysfunction from drugs like aminoglycoside antibiotics. While the mechanisms underlying aminoglycoside ototoxicity remain unresolved, there is considerable evidence that aminoglycosides enter inner ear mechanosensory hair cells through the mechanoelectrical transduction (MET) channel. Inhibition of MET-dependent uptake with small molecules or modified aminoglycosides is a promising otoprotective strategy. To better characterize mammalian ototoxicity and aid in the translation of emerging therapeutics, a biomarker is needed. In the present study we propose that neonatal mice systemically injected with the aminoglycosides G418 conjugated to Texas Red (G418-TR) can be used as a histologic biomarker to characterize in vivo aminoglycoside toxicity. We demonstrate that postnatal day 5 mice, like older mice with functional hearing, show uptake and retention of G418-TR in cochlear hair cells following systemic injection. When we compare G418-TR uptake in other tissues, we find that kidney proximal tubule cells show similar retention. Using ORC-13661, an investigational hearing protection drug, we demonstrate in vivo inhibition of aminoglycoside uptake in mammalian hair cells. This work establishes how systemically administered fluorescently labeled ototoxins in the neonatal mouse can reveal important details about ototoxic drugs and protective therapeutics.

Identifying targets to prevent aminoglycoside ototoxicity

Aminoglycosides are potent antibiotics that are commonly prescribed worldwide. Their use carries significant risks of ototoxicity by directly causing inner ear hair cell degeneration. Despite their ototoxic side effects, there are currently no approved antidotes. Here we review recent advances in our understanding of aminoglycoside ototoxicity, mechanisms of drug transport, and promising sites for intervention to prevent ototoxicity.

Hearing Function, Degeneration, and Disease: Spotlight on the Stria Vascularis

The stria vascularis (SV) is a highly vascularized tissue lining the lateral wall of the cochlea. The SV maintains cochlear fluid homeostasis, generating the endocochlear potential that is required for sound transduction. In addition, the SV acts as an important blood-labyrinth barrier, tightly regulating the passage of molecules from the blood into the cochlea. A healthy SV is therefore vital for hearing function. Degeneration of the SV is a leading cause of age-related hearing loss, and has been associated with several hearing disorders, including Norrie disease, Meniere's disease, Alport syndrome, Waardenburg syndrome, and Cytomegalovirus-induced hearing loss. Despite the SV's important role in hearing, there is still much that remains to be discovered, including cell-specific function within the SV, mechanisms of SV degeneration, and potential protective or regenerative therapies. In this review, we discuss recent discoveries elucidating the molecular regulatory networks of SV function, mechanisms underlying degeneration of the SV, and otoprotective strategies for preventing drug-induced SV damage. We also highlight recent clinical developments for treating SV-related hearing loss and discuss future research trajectories in the field.

In vivo real-time imaging reveals megalin as the aminoglycoside gentamicin transporter into cochlea whose inhibition is otoprotective

Aminoglycosides (AGs) are commonly used antibiotics that cause deafness through the irreversible loss of cochlear sensory hair cells (HCs). How AGs enter the cochlea and then target HCs remains unresolved. Here, we performed time-lapse multicellular imaging of cochlea in live adult hearing mice via a chemo-mechanical cochleostomy. The in vivo tracking revealed that systemically administered Texas Red-labeled gentamicin (GTTR) enters the cochlea via the stria vascularis and then HCs selectively. GTTR uptake into HCs was completely abolished in transmembrane channel-like protein 1 (TMC1) knockout mice, indicating mechanotransducer channel-dependent AG uptake. Blockage of megalin, the candidate AG transporter in the stria vascularis, by binding competitor cilastatin prevented GTTR accumulation in HCs. Furthermore, cilastatin treatment markedly reduced AG-induced HC degeneration and hearing loss in vivo. Together, our in vivo real-time tracking of megalin-dependent AG transport across the blood-labyrinth barrier identifies new therapeutic targets for preventing AG-induced ototoxicity.

Sex differences in the auditory functions of rodents

BACKGROUND

In humans, it is well known that females have better hearing than males. The mechanism of this influence of sex on auditory function in humans is not well understood. Testing the hypothesis of underlying mechanisms often relies on preclinical research, a field in which sex bias still exists unconsciously. Rodents are popular research models in hearing, thus it is crucial to understand the sex differences in these rodent models when studying health and disease in humans.

OBJECTIVES

This review aims to summarize the existing sex differences in the auditory functions of rodent species including mouse, rat, Guinea pig, Mongolian gerbil, and chinchilla. In addition, a concise summary of the hearing characteristics and the advantages and the drawbacks of conducting auditory experiments in each rodent species is provided.

DESIGNS

Manuscripts were identified in PubMed and Ovid Medline for the queries "Rodent", "Sex Characteristics", and "Hearing or Auditory Function". Manuscripts were included if they were original research, written in English, and use rodents. The content of each manuscript was screened for the sex of the rodents and the discussion of sex-based results.

CONCLUSIONS

The sex differences in auditory function of rodents are prevalent and influenced by multiple factors including physiological mechanisms, sex-based anatomical variations, and stimuli from the external environment. Such differences may play a role in understanding and explaining sex differences in hearing of humans and need to be taken into consideration for developing clinical therapies aim to improve auditory performances.

Intratympanic Lipopolysaccharide Elevates Systemic Fluorescent Gentamicin Uptake in the Cochlea

Objectives/Hypothesis

Lipopolysaccharide (LPS), a key component of bacterial endotoxins, activates macrophages and triggers the release of inflammatory cytokines in mammalian tissues. Recent studies have shown that intratympanic injection of LPS simulates acute otitis media (AOM) and results in morphological and functional changes in the inner ear. Here we established an AOM mouse model with LPS to investigate the uptake of ototoxic gentamicin in the inner ear, and elucidated the underlying mechanism by focusing on cochlear inflammation as a result of AOM.

Study Design

Preclinical rodent animal model.

Methods

Fluorescently tagged gentamicin (GTTR) was systemically administered to mice with AOM. Iba1‐positive macrophage morphology and inner ear cytokine profile were evaluated by immunofluorescence technique and a mouse cytokine array kit, respectively.

Results

We observed characteristic symptoms of AOM in the LPS‐treated ears with elevated hearing thresholds indicating a conductive hearing loss. More importantly, the LPS‐induced AOM activated cochlear inflammatory responses, manifested by macrophage infiltration, particularly in the organ of Corti and the spiral ligament, in addition to the up‐regulation of proinflammatory cytokines. Meanwhile, GTTR uptake in the stria vascularis and sensory hair cells from all the LPS‐treated ears was significantly enhanced at 24, 48, and 72‐hour post‐treatment, as the most prominent enhancement was observed in the 48‐hour group.

Conclusion

In summary, this study suggests that the pathological cochlea is more susceptible to ototoxic drugs, including aminoglycosides, and justified the clinical concern of aminoglycoside ototoxicity in the AOM treatment. Laryngoscope, 131:E2573–E2582, 2021

Non-Absorbable Oral Gentamicin Sulphate: Biopharmaceutical and Dosage Form Evaluation

Gentamicin sulphate is an antibiotic belonging to the aminoglycosides and to class III of the Biopharmaceutical Classification System (BCS). Gentamicin sulphate is highly water soluble, but has very low intestinal permeability. The wide use is because of its broad spectrum of activity. In the current study, the suitability of administering gentamicin sulphate orally for local action against susceptible gastrointestinal tract (GIT) bacteria was investigated. The possibility of the drug escaping into the systemic circulation even in the presence of some permeation enhancers was ascertained. Representatives of potential GIT bacteria pathogens were evaluated for their susceptibility to the drug at concentrations obtainable in the GIT using standard microbiological methods. Dose levels that will inhibit these potential bacteria pathogens were also established, as well as the frequency of their administration. Different batches of oral capsules of 250 mg gentamicin sulphate were formulated and their release profiles ascertained using standard methods. The results showed that the selected representatives of the GIT potential pathogenic bacteria were all susceptible to gentamicin sulphate. The drug at its plausible dosage levels of 14.28 mg/kg (1,000 mg/70 kg), 10.71 mg/kg (750 mg/70 kg) and 7.14 mg/kg (500 mg/70 kg) did not cross the mucosal barrier into the systemic circulation even in the presence of some permeation enhancers. The drug's frequency of administration were found to be on 8-hourly bases. Gentamicin sulphate (250 mg) granules formulated with polyethylene glycol (PEG 4000) as granulating aid were quick drying because the granules were not hygroscopic. The formulated gentamicin sulphate capsule batch released enough concentration of the drug that inhibits the test organism within 2 min of dissolution. The above stated doses are acceptable in the dosage form design; it is possible to formulate non-absorbable oral gentamicin sulphate dosage form for local activity in the GIT using existing conventional solid dosage formulating equipment.

Hair cell uptake of gentamicin in the developing mouse utricle

Intratympanic injection of gentamicin has proven to be an effective therapy for intractable vestibular dysfunction. However, most studies to date have focused on the cochlea, so little is known about the distribution and uptake of gentamicin by the counterpart of the auditory system, specifically vestibular hair cells (HCs). Here, with a combination of in vivo and in vitro approaches, we used a gentamicin-Texas Red (GTTR) conjugate to investigate the mechanisms of gentamicin vestibulotoxicity in the developing mammalian utricular HCs. In vivo, GTTR fluorescence was concentrated in the apical cytoplasm and the cellular membrane of neonatal utricular HCs, but scarce in the nucleus of HCs and supporting cells. Quantitative analysis showed the GTTR uptake by striolar HCs was significantly higher than that in the extrastriola. In addition, the GTTR fluorescence intensity in the striola was increased gradually from 1 to 8 days, peaking at 8-9 days postnatally. In vitro, utricle explants were incubated with GTTR and candidate uptake conduits, including mechanotransduction (MET) channels and endocytosis in the HC, were inhibited separately. GTTR uptake by HCs could be inhibited by quinine, a blocker of MET channels, under both normal and stressed conditions. Meanwhile, endocytic inhibition only reduced GTTR uptake in the CoCl₂ hypoxia model. In sum, the maturation of MET channels mediated uptake of GTTR into vestibular HCs. Under stressed conditions, MET channels play a pronounced role, manifested by channel-dependent stress enhanced GTTR permeation, while endocytosis participates in GTTR entry in a more selective manner.

Contralateral spreading of substances following intratympanic nanoparticle-conjugated gentamicin injection in a rat model

This study was performed to investigate the Eustachian tube as a potential route for contralateral spreading following intratympanic nanoparticle (NP)-conjugated gentamicin injection in a rat model. Sprague–Dawley rats were divided into three groups and substances were injected in the right ear: group 1 (fluorescent magnetic nanoparticles [F-MNPs], n = 4), group 2 (F-MNP-conjugated gentamicin [F-MNP@GM], n = 2), and control group (no injections, n = 2). T2-weighted sequences corresponding to the regions of interest at 1, 2, and 3 h after intratympanic injection were evaluated, along with immunostaining fluorescence of both side cochlea. The heterogeneous signal intensity of F-MNPs and F-MNP@GM on T2-weighted images, observed in the ipsilateral tympanum, was also detected in the contralateral tympanum in 4 out of 6 rats, recapitulating fluorescent nanoparticles in the contralateral cochlear hair cells. Computational simulations demonstrate the contralateral spreading of particles by gravity force following intratympanic injection in a rat model. The diffusion rate of the contralateral spreading relies on the sizes and surface charges of particles. Collectively, the Eustachian tube could be a route for contralateral spreading following intratympanic injection. Caution should be taken when using the contralateral ear as a control study investigating inner-ear drug delivery through the transtympanic approach.

In vitro and in silico characterization of the inhibition of Kir4.1 channels by aminoglycoside antibiotics

BACKGROUND AND PURPOSE

Aminoglycoside antibiotics are positively charged molecules that are known to inhibit several ion channels. In this study, we have shown that aminoglycosides also inhibit the activity of Kir4.1 channels. Aminoglycosides inhibit Kir4.1 channels by a pore-blocking mechanism, plugging the central vestibule of the channel.

EXPERIMENTAL APPROACH

Patch-clamp recordings were made in HEK-293 cells transiently expressing Kir4.1 channels to analyse the effects of gentamicin, neomycin and kanamycin. In silico modelling followed by mutagenesis were realized to identify the residues critical for aminoglycosides binding to Kir4.1.

KEY RESULTS

Aminoglycoside antibiotics block Kir4.1 channels in a concentration- and voltage-dependent manner, getting access to the protein from the intracellular side of the plasma membrane. Aminoglycosides block Ki4.1 with a rank order of potency as follows: gentamicin ˃ neomycin ˃ kanamycin. The residues T128 and principally E158, facing the central cavity of Kir4.1, are important structural determinants for aminoglycosides binding to the channel, as determined by our in silico modelling and confirmed by mutagenesis experiments.

CONCLUSION AND IMPLICATIONS

Kir4.1 channels are also target of aminoglycoside antibiotics, which could affect potassium transport in several tissues.

Prospective cohort study of ototoxicity in persons with cystic fibrosis following a single course of intravenous tobramycin

INTRODUCTION

Aminoglycoside (AG) antibiotics, such as tobramycin, are known to be ototoxic but important clinically due to their bactericidal efficacy. Persons with cystic fibrosis (CF) are at risk for AG-induced ototoxicity due to the repeated use of intravenous (IV) tobramycin for the treatment of pulmonary exacerbations. While it is well-established that ototoxic hearing loss is highly prevalent in this clinical population, the progression of hearing loss over time remains unclear. Cumulative IV-AG dosing has been associated with a higher risk of ototoxic hearing loss, yet some individuals lose substantial hearing after a single IV-AG treatment, while others never seem to lose hearing.

METHODS

31 persons with CF (18 on IV tobramycin, 13 controls) were enrolled in an observational study. Pure-tone hearing thresholds (0.25-16 kHz) were measured at baseline (pre-treatment) and at follow-up for each subject. A hearing shift was determined using various metrics, and outcomes were compared to characterize changes in hearing bilaterally for both study groups.

RESULTS

Comparison of pure-tone threshold shifts between baseline and follow-up audiograms following either a course of IV tobramycin (n = 18) or no intervening therapy (n = 13) demonstrated significant (p < 0.05) threshold shifts in all continuous metrics tested.

CONCLUSION

A single course of IV tobramycin causes ototoxic hearing loss in some people with CF, which supports the need for routine ototoxicity monitoring and management in this clinical population. These findings also suggest that people with CF are a suitable population for clinical trials examining ototherapeutics in single IV-tobramycin treatment episodes.

Local Drug Delivery to the Entire Cochlea without Breaching Its Boundaries

The mammalian cochlea is one of the least accessible organs for drug delivery. Systemic administration of many drugs is severely limited by the blood-labyrinth barrier. Local intratympanic administration into the middle ear would be a preferable option in this case, and the only option for many newly emerging classes of drugs, but it leads to the formation of drug concentration gradients along the extensive, narrow cochlea. The gradients are orders of magnitude and well outside the therapeutic windows. Here we present an efficient, quick, and simple method of cochlear pumping, through large-amplitude, low-frequency reciprocal oscillations of the stapes and round window, which can consistently and uniformly deliver drugs along the entire length of the intact cochlea within minutes without disrupting the cochlear boundaries. The method should facilitate novel ways of approaching the treatment of inner ear disorders because it overcomes the challenge of delivering therapeutics along the entire cochlear length.

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Systemic delivery of drugs to the inner ear is limited by the blood-labyrinth barrier

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Middle ear administration results in pronounced drug gradients along the cochlea

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Cochlear pumping distributes drugs evenly along the entire cochlea within minutes

Delivery of therapeutics to the inner ear: The challenge of the blood-labyrinth barrier

Permanent hearing loss affects more than 5% of the world's population, yet there are no nondevice therapies that can protect or restore hearing. Delivery of therapeutics to the cochlea and vestibular system of the inner ear is complicated by their inaccessible location. Drug delivery to the inner ear via the vasculature is an attractive noninvasive strategy, yet the blood-labyrinth barrier at the luminal surface of inner ear capillaries restricts entry of most blood-borne compounds into inner ear tissues. Here, we compare the blood-labyrinth barrier to the blood-brain barrier, discuss invasive intratympanic and intracochlear drug delivery methods, and evaluate noninvasive strategies for drug delivery to the inner ear.

Intravenously delivered aminoglycoside antibiotics, tobramycin and amikacin, are not ototoxic in mice

Many drugs on the World Health Organization's list of critical medicines are ototoxic, destroying sensory hair cells within the ear. These drugs preserve life, but patients can experience side effects including permanent hearing loss and vestibular dysfunction. Aminoglycoside ototoxicity was first recognised 80 years ago. However, no preventative treatments have been developed. In order to develop such treatments, we must identify the factors driving hair cell death. In vivo, studies of cell death are typically conducted using mouse models. However, a robust model of aminoglycoside ototoxicity does not exist. Previous studies testing aminoglycoside delivery via intraperitoneal or subcutaneous injection have produced variable ototoxic effects in the mouse. As a result, surgical drug delivery to the rodent ear is often used to achieve ototoxicity. However, this technique does not accurately model clinical practice. In the clinic, aminoglycosides are administered to humans intravenously (i.v.). However, repeated i.v. delivery has not been reported in the mouse. This study evaluated whether repeated i.v. administration of amikacin or tobramycin would induce hearing loss. Daily i.v. injections over a two-week period were well tolerated and transient low frequency hearing loss was observed in the aminoglycoside treatment groups. However, the hearing changes observed did not mimic the high frequency patterns of hearing loss observed in humans. Our results indicate that the i.v. delivery of tobramycin or amikacin is not an effective technique for inducing ototoxicity in mice. This result is consistent with previously published reports indicating that the mouse cochlea is resistant to systemically delivered aminoglycoside ototoxicity.

Protection of Hair Cells from Ototoxic Drug-Induced Hearing Loss

Hair cells are specialized sensory epithelia cells that receive mechanical sound waves and convert them into neural signals for hearing, and these cells can be killed or damaged by ototoxic drugs, including many aminoglycoside antibiotics, platinum-based anticancer agents, and loop diuretics, leading to drug-induced hearing loss. Studies of therapeutic approaches to drug-induced hearing loss have been hampered by the limited understanding of the biological mechanisms that protect and regenerate hair cells. This review briefly discusses some of the most common ototoxic drugs and describes recent research concerning the mechanisms of ototoxic drug-induced hearing loss. It also highlights current developments in potential therapies and explores current clinical treatments for patients with hearing impairments.

Local Drug Delivery for the Treatment of Neurotology Disorders

Neurotology disorders such as vertigo, tinnitus, and hearing loss affect a significant proportion of the population (estimated 39 million in the United States with moderate to severe symptoms). Yet no pharmacological treatments have been developed, in part due to limitations in effective drug delivery to the anatomically protected inner ear compartment. Intratympanic delivery, a minimally invasive injection performed in the office setting, offers a potential direct route of administration. Currently, off-label use of therapeutics approved to treat disorders via systemic administration are being injected intratympanically, mostly in the form of aqueous solutions, but provide variable levels of drug exposure for a limited time requiring repeated injections. Hence, current drug delivery approaches for neurotology disorders are sub-optimal. This review, following a description of pharmacokinetic considerations of the inner ear, explores the merits of novel delivery approaches toward the treatment of neurotology disorders. Methodologies employing local delivery to the inner ear are described, including direct intracochlear delivery as well as intratympanic methods of infusion and injection. Intratympanic injection delivery formulation strategies including hydrogels, polymers and nanoparticulate systems are explored. These approaches represent progress toward more effective delivery options for the clinical treatment of a variety of neurotology disorders.

Drug Diffusion Along an Intact Mammalian Cochlea

Intratympanic drug administration depends on the ability of drugs to pass through the round window membrane (RW) at the base of the cochlea and diffuse from this location to the apex. While the RW permeability for many different drugs can be promoted, passive diffusion along the narrowing spiral of the cochlea is limited. Earlier measurements of the distribution of marker ions, corticosteroids, and antibiotics demonstrated that the concentration of substances applied to the RW was two to three orders of magnitude higher in the base compared to the apex. The measurements, however, involved perforating the cochlear bony wall and, in some cases, sampling perilymph. These manipulations can change the flow rate of perilymph and lead to intake of perilymph through the cochlear aqueduct, thereby disguising concentration gradients of the delivered substances. In this study, the suppressive effect of salicylate on cochlear amplification via block of the outer hair cell (OHC) somatic motility was utilized to assess salicylate diffusion along an intact guinea pig cochlea in vivo. Salicylate solution was applied to the RW and threshold elevation of auditory nerve responses was measured at different times and frequencies after application. Resultant concentrations of salicylate along the cochlea were calculated by fitting the experimental data using a mathematical model of the diffusion and clearing of salicylate in a tube of variable diameter combined with a model describing salicylate action on cochlear amplification. Concentrations reach a steady-state at different times for different cochlear locations and it takes longer to reach the steady-state at more apical locations. Even at the steady-state, the predicted concentration at the apex is negligible. Model predictions for the geometry of the longer human cochlea show even higher differences in the steady-state concentrations of the drugs between cochlear base and apex. Our findings confirm conclusions that achieving therapeutic drug concentrations throughout the entire cochlear duct is hardly possible when the drugs are applied to the RW and are distributed via passive diffusion. Assisted methods of drug delivery are needed to reach a more uniform distribution of drugs along the cochlea.

Adverse outcome pathway for aminoglycoside ototoxicity in drug-resistant tuberculosis treatment

Individuals treated for multidrug-resistant tuberculosis (MDR-TB) with aminoglycosides (AGs) in resource-limited settings often experience permanent hearing loss. However, AG ototoxicity has never been conceptually integrated or causally linked to MDR-TB patients' pre-treatment health condition. We sought to develop a framework that examines the relationships between pre-treatment conditions and AG-induced hearing loss among MDR-TB-infected individuals in sub-Saharan Africa. The adverse outcome pathway (AOP) approach was used to develop a framework linking key events (KEs) within a biological pathway that results in adverse outcomes (AO), which are associated with chemical perturbation of a molecular initiating event (MIE). This AOP describes pathways initiating from AG accumulation in hair cells, sound transducers of the inner ear immediately after AG administration. After administration, the drug catalyzes cellular oxidative stress due to overproduction of reactive oxygen species. Since oxidative stress inhibits mitochondrial protein synthesis, hair cells undergo apoptotic cell death, resulting in irreversible hearing loss (AO). We identified the following pre-treatment conditions that worsen the causal linkage between MIE and AO: HIV, malnutrition, aging, noise, smoking, and alcohol use. The KEs are: (1) nephrotoxicity, pre-existing hearing loss, and hypoalbuminemia that catalyzes AG accumulation; (2) immunodeficiency and antioxidant deficiency that trigger oxidative stress pathways; and (3) co-administration of mitochondrial toxic drugs that hinder mitochondrial protein synthesis, causing apoptosis. This AOP clearly warrants the development of personalized interventions for patients undergoing MDR-TB treatment. Such interventions (i.e., choosing less ototoxic drugs, scheduling frequent monitoring, modifying nutritional status, avoiding poly-pharmacy) will be required to limit the burden of AG ototoxicity.

Pasireotide protects mammalian cochlear hair cells from gentamicin ototoxicity by activating the PI3K-Akt pathway

Gentamicin is a widely used antibiotic for the treatment of gram-negative bacterial infections; however, its use often results in significant and permanent hearing loss. Hearing loss resulting from hair cell (HC) degeneration affects millions of people worldwide, and one major cause is the loss of sensory HCs in the inner ear due to aminoglycoside exposure. Strategies to overcome the apparently irreversible loss of HCs in mammals are crucial for hearing protection. Here, we report that the somatostatin analog pasireotide protects mouse cochlear HCs from gentamicin damage using a well-established in vitro gentamicin-induced HC loss model and that the otoprotective effects of pasireotide are due to Akt up-regulation via the PI3K–Akt signal pathway activation. We demonstrate active caspase signal in organ of Corti (OC) explants exposed to gentamicin and show that pasireotide treatment activates survival genes, reduces caspase signal, and increases HC survival. The neuropeptide somatostatin and its selective analogs have provided neuroprotection by activating five somatostatin receptor (SSTR1–SSTR5) subtypes. Pasireotide has a high affinity for SSTR2 and SSTR5, and the addition of SSTR2- and SSTR5-specific antagonists leads to a loss of protection. The otoprotective effects of pasireotide were also observed in a gentamicin-injured animal model. In vivo studies have shown that 13 days of subcutaneous pasireotide application prevents gentamicin-induced HC death and permanent hearing loss in mice. Auditory brainstem response analysis confirmed the protective effect of pasireotide, and we found a significant threshold shift at all measured frequencies (4, 8, 16, 24, and 32 kHz). Together, these findings indicate that pasireotide is a novel otoprotective peptide acting via the PI3K–Akt pathway and may be of therapeutic value for HC protection from ototoxic insults.

Evaluation of Mitoquinone for Protecting Against Amikacin-Induced Ototoxicity in Guinea Pigs

HYPOTHESIS

Mitoquinone (MitoQ) attenuates amikacin ototoxicity in guinea pigs.

BACKGROUND

MitoQ, a mitochondria-targeted derivative of the antioxidant ubiquinone, has improved bioavailability and demonstrated safety in humans. Thus, MitoQ is a promising therapeutic approach for protecting against amikacin-induced ototoxicity.

METHODS

Both oral and subcutaneous administrations of MitoQ were tested. Amikacin-treated guinea pigs (n = 12-18 per group) received water alone (control) or MitoQ 30 mg/l-supplemented drinking water; or injected subcutaneously with 3 to 5 mg/kg MitoQ or saline (control). Auditory brainstem responses and distortion product otoacoustic emissions were measured before MitoQ or control solution administration and after amikacin injections. Cochlear hair cell damage was assessed using scanning electron microscopy and Western blotting.

RESULTS

With oral administration, animals that received 30 mg/l MitoQ had better hearing than controls at only 24 kHz at 3-week (p = 0.017) and 6-week (p = 0.027) post-amikacin. With subcutaneous administration, MitoQ-injected guinea pigs had better hearing than controls at only 24 kHz, 2-week post-amikacin (p = 0.013). Distortion product otoacoustic emission (DPOAE) amplitudes were decreased after amikacin injections, but were not different between treatments (p > 0.05). Electron microscopy showed minor difference in outer hair cell loss between treatments. Western blotting demonstrated limited attenuation of oxidative stress in the cochlea of MitoQ-supplemented guinea pigs.

CONCLUSIONS

Oral or subcutaneous MitoQ provided limited protection against amikacin-induced hearing loss and cochlear damage in guinea pigs. Other strategies for attenuating aminoglycoside-induced ototoxicity should be explored.

In vitro gentamicin exposure alters caveolae protein profile in cochlear spiral ligament pericytes

Background

The aminoglycoside antibiotic gentamicin is an ototoxic drug and has been used experimentally to investigate cochlear damage induced by noise.We have investigated the changes in the protein profile associated with caveolae in gentamicin treated and untreated spiral ligament (SL) pericytes, specialized cells in the blood labyrinth barrier of the inner ear microvasculature. Pericytes from various microvascular beds express caveolae, protein and cholesterol rich microdomains, which can undergo endocytosis and transcytosis to transport small molecules in and out the cells. A different protein profile in transport-specialized caveolae may induce pathological changes affecting the integrity of the blood labyrinth barrier and ultimately contributing to hearing loss.

Method

Caveolae isolation from treated and untreated cells is achieved through ultracentrifugation of the lysates in discontinuous gradients. Mass spectrometry (LC-MS/MS) analysis identifies the proteins in the two groups. Proteins segregating with caveolae isolated from untreated SL pericytes are then compared to caveolae isolated from SL pericytes treated with the gentamicin for 24 h. Data are analyzed using bioinformatic tools.

Results

The caveolae proteome in gentamicin treated cells shows that 40% of total proteins are uniquely associated with caveolae during the treatment, and 15% of the proteins normally associated with caveolae in untreated cell are suppressed. Bioinformatic analysis of the data shows a decreased expression of proteins involved in genetic information processing, and an increase in proteins involved in metabolism, vesicular transport and signal transduction in gentamicin treated cells. Several Rab GTPases proteins, ubiquitous transporters, uniquely segregate with caveolae and are significantly enriched in gentamicin treated cells.

Conclusion

We report that gentamicin exposure modifies protein profile of caveolae from SL pericytes. We identified a pool of proteins which are uniquely segregating with caveolae during the treatment, mainly participating in metabolic and biosynthetic pathways, in transport pathways and in genetic information processing. Finally, we show for the first time proteins associated with caveolae SL pericytes linked to nonsyndromic hearing loss.

Current practice of ototoxicity management across the United Kingdom (UK)

OBJECTIVE

Effective management of patients diagnosed with ototoxicity is needed to reduce hearing and balance damage which affects communication and life quality. Despite widespread recommendations to monitor and manage ototoxicity in an early and effective manner, there is limited evidence to support the actual implementation of these recommendations for affected patient groups in healthcare services across the UK with limited publications available. In this study, an online questionnaire analysed the current practice of ototoxicity management and patient pathways across the UK once the diagnosis of ototoxicity was confirmed, targeting Audiologists, ENTs/AVPs and GPs.

DESIGN

Qualitative Survey Study.

STUDY SAMPLE

A randomised sample of hearing services in the UK, including audiology departments; GP practices and local health settings were targeted with a total of 134 completed surveys.

RESULTS

About 72% reported the absence of ototoxicity management protocols within their centre. Results depicted great inconsistency and variation across the UK in ototoxicity management services provided, treatment modification, monitoring and referral pathways.

CONCLUSION

Developing and advocating national guidelines are intended not only to inform clinical decision making but to provide minimum standards of care in ototoxicity management and offer greater awareness and education to improve patients' quality of life.

Kanamycin Damages Early Postnatal, but Not Adult Spiral Ganglion Neurons

Although aminoglycoside antibiotics such as kanamycin are widely used clinically to treat life-threatening bacterial infections, ototoxicity remains a significant dose-limiting side effect. The prevailing view is that the hair cells are the primary ototoxic target of aminoglycosides and that spiral ganglion neurons begin to degenerate weeks or months after the hair cells have died due to lack of neurotrophic support. To test the early developmental aspects of this issue, we compared kanamycin-induced hair cell and spiral ganglion pathology in rat postnatal day 3 cochlear organotypic cultures with adult whole cochlear explants. In both adult and postnatal day 3 cultures, hair cell damage began at the base of the cochleae and progressed toward the apex in a dose-dependent manner. In postnatal day 3 cultures, spiral ganglion neurons were rapidly destroyed by kanamycin prior to hair cell loss. In contrast, adult spiral ganglion neurons were resistant to kanamycin damage even at the highest concentration, consistent with in vivo models of delayed SGN degeneration. In postnatal day 3 cultures, kanamycin preferentially damaged type I spiral ganglion neurons, whereas type II neurons were resistant. Spiral ganglion degeneration of postnatal day 3 neurons was associated with upregulation of the superoxide radical and caspase-3-mediated cell death. These results show for the first time that kanamycin is toxic to postnatal day 3 spiral ganglion neurons, but not adult neurons.

Partial Aminoglycoside Lesions in Vestibular Epithelia Reveal Broad Sensory Dysfunction Associated with Modest Hair Cell Loss and Afferent Calyx Retraction

Although the effects of aminoglycoside antibiotics on hair cells have been investigated for decades, their influences on the dendrites of primary afferent neurons have not been widely studied. This is undoubtedly due to the difficulty in disassociating pathology to dendritic processes from that resulting from loss of the presynaptic hair cell. This was overcome in the present investigation through development of a preparation using Chinchilla laniger that enabled direct perilymphatic infusion. Through this strategy we unmasked gentamicin's potential effects on afferent calyces. The pathophysiology of the vestibular neuroepithelia after post-administration durations of 0.5 through 6 months was assessed using single-neuron electrophysiology, immunohistochemistry, and confocal microscopy. Hair cell densities within cristae central zones (0.5-, 1-, 2-, and 6-months) and utricle peri- and extrastriola (6-months) regions were determined, and damage to calretinin-immunoreactive calyces was quantified. Gentamicin-induced hair cell loss exhibited a profile that reflected elimination of a most-sensitive group by 0.5-months post-administration (18.2%), followed by loss of a second group (20.6%) over the subsequent 5.5 months. The total hair cell loss with this gentamicin dose (approximately 38.8%) was less than the estimated fraction of type I hair cells in the chinchilla's crista central zone (approximately 60%), indicating that viable type I hair cells remained. Extensive lesions to afferent calyces were observed at 0.5-months, though stimulus-evoked modulation was intact at this post-administration time. Widespread compromise to calyx morphology and severe attenuation of stimulus-evoked afferent discharge modulation was found at 1 month post-administration, a condition that persisted in preparations examined through the 6-month post-administration interval. Spontaneous discharge was robust at all post-administration intervals. All calretinin-positive calyces had retracted at 2 and 6 months post-administration. We found no evidence of morphologic or physiologic recovery. These results indicate that gentamicin-induced partial lesions to vestibular epithelia include hair cell loss (ostensibly reflecting an apoptotic effect) that is far less extensive than the compromise to stimulus-evoked afferent discharge modulation and retraction of afferent calyces (reflecting non-apoptotic effects). Additionally, calyx retraction cannot be completely accounted for by loss of type I hair cells, supporting the possibility for direct action of gentamicin on the afferent dendrite.

Aminoglycoside-Induced Cochleotoxicity: A Review

Aminoglycoside antibiotics are used as prophylaxis, or urgent treatment, for many life-threatening bacterial infections, including tuberculosis, sepsis, respiratory infections in cystic fibrosis, complex urinary tract infections and endocarditis. Although aminoglycosides are clinically-essential antibiotics, the mechanisms underlying their selective toxicity to the kidney and inner ear continue to be unraveled despite more than 70 years of investigation. The following mechanisms each contribute to aminoglycoside-induced toxicity after systemic administration: (1) drug trafficking across endothelial and epithelial barrier layers; (2) sensory cell uptake of these drugs; and (3) disruption of intracellular physiological pathways. Specific factors can increase the risk of drug-induced toxicity, including sustained exposure to higher levels of ambient sound, and selected therapeutic agents such as loop diuretics and glycopeptides. Serious bacterial infections (requiring life-saving aminoglycoside treatment) induce systemic inflammatory responses that also potentiate the degree of ototoxicity and permanent hearing loss. We discuss prospective clinical strategies to protect auditory and vestibular function from aminoglycoside ototoxicity, including reduced cochlear or sensory cell uptake of aminoglycosides, and otoprotection by ameliorating intracellular cytotoxicity.

Structural and Ultrastructural Changes to Type I Spiral Ganglion Neurons and Schwann Cells in the Deafened Guinea Pig Cochlea

Sensorineural hearing loss is commonly caused by damage to cochlear sensory hair cells. Coinciding with hair cell degeneration, the peripheral fibres of type I spiral ganglion neurons (SGNs) that normally form synaptic connections with the inner hair cell gradually degenerate. We examined the time course of these degenerative changes in type I SGNs and their satellite Schwann cells at the ultrastructural level in guinea pigs at 2, 6, and 12 weeks following aminoglycoside-induced hearing loss. Degeneration of the peripheral fibres occurred prior to the degeneration of the type I SGN soma and was characterised by shrinkage of the fibre followed by retraction of the axoplasm, often leaving a normal myelin lumen devoid of axoplasmic content. A statistically significant reduction in the cross-sectional area of peripheral fibres was evident as early as 2 weeks following deafening (p < 0.001, ANOVA). This was followed by a decrease in type I SGN density within Rosenthal's canal that was statistically significant 6 weeks following deafening (p < 0.001, ANOVA). At any time point examined, few type I SGN soma were observed undergoing degeneration, implying that once initiated, soma degeneration was rapid. While there was a significant reduction in soma area as well as changes to the morphology of the soma, the ultrastructure of surviving type I SGN soma appeared relatively normal over the 12-week period following deafening. Satellite Schwann cells exhibited greater survival traits than their type I SGN; however, on loss of neural contact, they reverted to a non-myelinating phenotype, exhibiting an astrocyte-like morphology with the formation of processes that appeared to be searching for new neural targets. In 6- and 12-week deafened cochlea, we observed cellular interaction between Schwann cell processes and residual SGNs that distorted the morphology of the SGN soma. Understanding the response of SGNs, Schwann cells, and the complex relationship between them following aminoglycoside deafening is important if we are to develop effective therapeutic techniques designed to rescue SGNs.

Clinical predictors of aminoglycoside-induced ototoxicity in drug-resistant Tuberculosis patients on intensive therapy

OBJECTIVE

The study objectives were to determine the incidence of aminoglycoside-induced ototoxicity in institutionalized patients on intensive phase of therapy for drug-resistant Tuberculosis (DR Tb) and also to assess clinical factors which could predict the ototoxicity.

METHODS

The study was a prospective analytical study among consecutive DR Tb patients who were admitted for intensive phase of therapy (of 4 months) at the DR-Tb center over a 12-month period. Patients were diagnosed as DR Tb using the Gene Xpert machine to confirm Rifampicin resistance. All eligible 70 out of 87 consenting patients were consecutively recruited into the study. Patients had baseline (admission) and serial pure tone audiometries (PTAs) performed at 4 weekly intervals until discharge after 4 months of admission. Audiometric confirmation of aminoglycoside-induced ototoxicity was done by comparing serial with baseline PTA.

RESULTS

Among the 70 patients the male:female ratio was 1.7:1. Nine patients (12.9%) were retroviral-positive, and 16 patients (22.9%) were confirmed to have ototoxicity by audiometric criteria. The duration of treatment when ototoxicity was detected in the patients ranged 4-17 (Mean±SD; 9.4±3.4) weeks. Ototoxicity was detected in the audiometric low frequency ranges in 7 (43.8%) and at the high frequencies in 4 (25.0%) of the patients. Univariate analyses of clinical parameters found that age, underlying diabetes mellitus, deranged baseline PTAv >25dB HL, BMI on admission and retroviral status were significantly associated, while sex and previous drug regimen failure were not associated with ototoxicity. Multivariate adjusted logistic regression analyses, controlling for sex, revealed age (OR=1.068, p=0.018), BMI on admission (OR=0.673, p=0.012) and retroviral positivity (OR=8.822, p=0.014) of patients could significantly predict aminoglycoside-induced ototoxicity.

CONCLUSION

Incidence of aminoglycoside-induced ototoxicity in DR Tb patients was 22.9%. The clinical predictors for ototoxicity were age, BMI on admission, and co-existing retroviral infection in the patients. Clinicians should consider these factors in making choices of aminoglycosides to be used during intensive phase of treatment with second line anti-Tuberculous therapy.

Perilymph pharmacokinetics of locally-applied gentamicin in the guinea pig

Intratympanic gentamicin therapy is widely used clinically to suppress the vestibular symptoms of Meniere's disease. Dosing in humans was empirically established and we still know remarkably little about where gentamicin enters the inner ear, where it reaches in the inner ear and what time course it follows after local applications. In this study, gentamicin was applied to the round window niche as a 20 μL bolus of 40 mg/ml solution. Ten 2 μL samples of perilymph were collected sequentially from the lateral semi-circular canal (LSCC) at times from 1 to 4 h after application. Gentamicin concentration was typically highest in samples originating from the vestibule and was lower in samples originating from scala tympani. To interpret these results, perilymph elimination kinetics for gentamicin was quantified by loading the entire perilymph space by injection at the LSCC with a 500 μg/ml gentamicin solution followed by sequential perilymph sampling from the LSCC after different delay times. This allowed concentration decline in perilymph to be followed with time. Gentamicin was retained well in scala vestibuli and the vestibule but declined rapidly at the base of scala tympani, dominated by interactions of perilymph with CSF, as reported for other substances. Quantitative analysis, taking into account perilymph kinetics for gentamicin, showed that more gentamicin entered at the round window membrane (57%) than at the stapes (35%) but the lower concentrations found in scala tympani were due to greater losses there. The gentamicin levels found in perilymph of the vestibule, which are higher than would be expected from round window entry alone, undoubtedly contribute to the vestibulotoxic effects of the drug. Furthermore, calculations of gentamicin distribution following targeted applications to the RW or stapes are more consistent with cochleotoxicity depending on the gentamicin concentration in scala vestibuli rather than that in scala tympani.

Effects of cochlear implant surgical technique on post-operative electrode impedance

CONCLUSIONS

The mCI surgical technique led to reduced impedance and minimized disturbance of the microenvironment inside the cochlea. Atraumatic surgical techniques and inflammation-reduction strategies may preserve the cochlear architecture and prevent fibrotic development.

OBJECTIVES

To assess the advantages of a modified minimal access technique in cochlear implantation as well as to investigate the effects of intra-operative application of inflammation reduction strategies on the intra-cochlear impedance.

METHODS

One hundred patients received a 31.5 mm long Med-El standard electrode array fully inserted into the cochlea and did not have surgical complications post-operation. Patients were divided into two groups according to the surgical technique that was used for implantation: 50 were in the modified minimal access cochlear implant (mCI) surgery group and 50 were in the traditional CI surgery group. Intra-cochlear impedance values were measured at initial activation (4 weeks post-operatively). Electrode impedance values were compared between the surgery groups.

RESULTS

Electrode impedance values were significantly lower in the mCI group than in the CI surgery group at initial activation (5.01 kOhm vs 6.10 kOhm, respectively, F = 13.761, p = 0.000). The differences between the two groups were most prominent for the electrodes located at the basal region of the cochlea.

Activation of PI3K signaling prevents aminoglycoside-induced hair cell death in the murine cochlea

Loss of sensory hair cells of the inner ear due to aminoglycoside exposure is a major cause of hearing loss. Using an immortalized multipotent otic progenitor (iMOP) cell line, specific signaling pathways that promote otic cell survival were identified. Of the signaling pathways identified, the PI3K pathway emerged as a strong candidate for promoting hair cell survival. In aging animals, components for active PI3K signaling are present but decrease in hair cells. In this study, we determined whether activated PI3K signaling in hair cells promotes survival. To activate PI3K signaling in hair cells, we used a small molecule inhibitor of PTEN or genetically ablated PTEN using a conditional knockout animal. Hair cell survival was challenged by addition of gentamicin to cochlear cultures. Hair cells with activated PI3K signaling were more resistant to aminoglycoside-induced hair cell death. These results indicate that increased PI3K signaling in hair cells promote survival and the PI3K signaling pathway is a target for preventing aminoglycoside-induced hearing loss.

Endotoxemia-mediated inflammation potentiates aminoglycoside-induced ototoxicity

The ototoxic aminoglycoside antibiotics are essential to treat severe bacterial infections, particularly in neonatal intensive care units. Using a bacterial lipopolysaccharide (LPS) experimental model of sepsis, we tested whether LPS-mediated inflammation potentiates cochlear uptake of aminoglycosides and permanent hearing loss in mice. Using confocal microscopy and enzyme-linked immunosorbent assays, we found that low-dose LPS (endotoxemia) greatly increased cochlear concentrations of aminoglycosides and resulted in vasodilation of cochlear capillaries without inducing paracellular flux across the blood-labyrinth barrier (BLB) or elevating serum concentrations of the drug. Additionally, endotoxemia increased expression of both serum and cochlear inflammatory markers. These LPS-induced changes, classically mediated by Toll-like receptor 4 (TLR4), were attenuated in TLR4-hyporesponsive mice. Multiday dosing with aminoglycosides during chronic endotoxemia induced greater hearing threshold shifts and sensory cell loss compared to mice without endotoxemia. Thus, endotoxemia-mediated inflammation enhanced aminoglycoside trafficking across the BLB and potentiated aminoglycoside-induced ototoxicity. These data indicate that patients with severe infections are at greater risk of aminoglycoside-induced hearing loss than previously recognized.

c-Jun N-Terminal Phosphorylation: Biomarker for Cellular Stress Rather than Cell Death in the Injured Cochlea

Prevention of auditory hair cell death offers therapeutic potential to rescue hearing. Pharmacological blockade of JNK/c-Jun signaling attenuates injury-induced hair cell loss, but with unsolved mechanisms. We have characterized the c-Jun stress response in the mouse cochlea challenged with acoustic overstimulation and ototoxins, by studying the dynamics of c-Jun N-terminal phosphorylation. It occurred acutely in glial-like supporting cells, inner hair cells, and the cells of the cochlear ion trafficking route, and was rapidly downregulated after exposures. Notably, death-prone outer hair cells lacked c-Jun phosphorylation. As phosphorylation was triggered also by nontraumatic noise levels and none of the cells showing this activation were lost, c-Jun phosphorylation is a biomarker for cochlear stress rather than an indicator of a death-prone fate of hair cells. Preconditioning with a mild noise exposure before a stronger traumatizing noise exposure attenuated the cochlear c-Jun stress response, suggesting that the known protective effect of sound preconditioning on hearing is linked to suppression of c-Jun activation. Finally, mice with mutations in the c-Jun N-terminal phosphoacceptor sites showed partial, but significant, hair cell protection. These data identify the c-Jun stress response as a paracrine mechanism that mediates outer hair cell death.

Cell-specific accumulation patterns of gentamicin in the guinea pig cochlea

Intratympanic gentamicin therapy has become a popular treatment modality for Ménière's disease (MD) through controlled elimination of vertigo spells caused by the balance organ. However, the known ototoxic properties of aminoglycosides lead to cochlear damage. In order to gain more information about cellular preferences for aminoglycoside accumulation within the cochlea, gentamicin was immuno histochemically localized by light microscopy in male guinea pigs 1 and 7 days after intratympanic application (n = 8 ears/incubation time). Differences in the gentamicin-specific cellular storage capacities were quantified by determination of the local immuno staining intensities. Gentamicin was detected in every cochlear cell type, but with spatiotemporal variability. One day after application, an intense staining reaction was found in all cell types except the spiral ganglion cells and the stria vascularis. Six days later, gentamicin staining intensities were additionally reduced in the nerve fibers and the spiral ligament. Statistic analysis revealed strong cellular associations in respect to aminoglycoside accumulation. Furthermore, associations with recorded hearing losses were identified comparing the cellular gentamicin content in the organ of Corti, in the stria vascularis, in the spiral ganglion cells and in fibrocytes of the Limbus. In the lateral wall, clear differences in cellular gentamicin accumulation were found between type I fibrocytes of the spiral ligament compared with basal and intermediate cells of the stria vascularis. This finding was unexpected as these three cell types belong to a well-developed gap-junction system which normally enables unhampered cell communication. Cellular differences in local gentamicin storage capacities, transport processes and inherent diffusion barriers are discussed.

Uptake of fluorescent gentamicin by peripheral vestibular cells after systemic administration

Objective

In addition to cochleotoxicity, systemic aminoglycoside pharmacotherapy causes vestibulotoxicity resulting in imbalance and visual dysfunction. The underlying trafficking routes of systemically-administered aminoglycosides from the vasculature to the vestibular sensory hair cells are largely unknown. We investigated the trafficking of systemically-administered gentamicin into the peripheral vestibular system in C56Bl/6 mice using fluorescence-tagged gentamicin (gentamicin-Texas-Red, GTTR) imaged by scanning laser confocal microscopy to determine the cellular distribution and intensity of GTTR fluorescence in the three semicircular canal cristae, utricular, and saccular maculae at 5 time points over 4 hours.

Results

Low intensity GTTR fluorescence was detected at 0.5 hours as both discrete puncta and diffuse cytoplasmic fluorescence. The intensity of cytoplasmic fluorescence peaked at 3 hours, while punctate fluorescence was plateaued after 3 hours. At 0.5 and 1 hour, higher levels of diffuse GTTR fluorescence were present in transitional cells compared to hair cells and supporting cells. Sensory hair cells typically exhibited only diffuse cytoplasmic fluorescence at all time-points up to 4 hours in this study. In contrast, non-sensory cells rapidly exhibited both intense fluorescent puncta and weaker, diffuse fluorescence throughout the cytosol. The numbers and size of fluorescent puncta in dark cells and transitional cells increased over time. There is no preferential GTTR uptake by the five peripheral vestibular organs’ sensory cells. Control vestibular tissues exposed to Dulbecco’s phosphate-buffered saline or hydrolyzed Texas Red had negligible fluorescence.

Conclusions

All peripheral vestibular cells rapidly take up systemically-administered GTTR, reaching peak intensity 3 hours after injection. Sensory hair cells exhibited only diffuse fluorescence, while non-sensory cells displayed both diffuse and punctate fluorescence. Transitional cells may act as a primary pathway for trafficking of systemic GTTR from the vasculature to endolymph prior to entering hair cells.

Gentamicin concentration gradients in scala tympani perilymph following systemic applications

It has been shown in prior studies that round window membrane (RWM) application of gentamicin produced a robust basal-apical concentration gradient in the perilymph of scala tympani (ST) with peak concentrations in the basal turn of ST. These gradients potentially contribute to the clinical efficacy and safety of intratympanic gentamicin applications for the treatment of Ménière's disease. The present study aimed to establish the distribution of gentamicin along ST perilymph after systemic applications. Gentamicin sulfate was applied intravenously in the amounts of 100, 300 and 600 mg/kg body weight (BW) over a period of 3 h or as a 300 mg/kg BW subcutaneous bolus injection. At 3 and 5 h after the start of the application perilymph of ST was aspirated from the cochlea apex of the right and left cochlea, respectively, and 10 sequential 1-µl perilymph samples from the apex of each cochlea were quantitatively analyzed using a fluorescence polarization immunoassay. In contrast to local RWM delivery, systemic application of gentamicin resulted in the highest perilymph levels in the apex of the cochlea with decreasing concentrations towards the basal regions of ST. The absolute gentamicin concentrations increased with the amount of drug applied and time before sampling. While it is likely that the basal-apical gradient measured after local drug applications to the round window niche is the result of the direct uptake of drugs into the perilymph of the ST, distribution by diffusion and a very low perilymph flow towards the cochlear apex, computer simulations suggested that the apical-basal gradient observed with these systemic applications can be explained by higher entry rates of gentamicin in the apex compared to the basal turns of the cochlea. It is also possible that gentamicin enters perilymph indirectly from the blood via the endolymph. In this case the faster kinetics in apical turns could be due to the smaller cross-sectional area of ST relative to endolymph in the apical turns.

Expression and dexamethasone-induced nuclear translocation of glucocorticoid and mineralocorticoid receptors in guinea pig cochlear cells

Glucocorticoids (GC) are powerful anti-inflammatory agents frequently used to protect the auditory organ against damage associated with a variety of conditions, including noise exposure and ototoxic drugs as well as bacterial and viral infections. In addition to glucocorticoid receptors (GC-R), natural and synthetic GC are known to bind mineralocorticoid receptors (MC-R) with great affinity. We used light and laser scanning confocal microscopy to investigate the expression of GC-R and MC-R in different cell populations of the guinea pig cochlea, and their translocation to different cell compartments after treatment with the synthetic GC dexamethasone. We found expression of both types of receptors in the cytoplasm and nucleus of sensory inner and outer hair cells as well as pillar, Hensen and Deiters cells in the organ of Corti, inner and outer sulcus cells, spiral ganglion neurons and several types of spiral ligament and spiral limbus cells; stria vascularis cells expressed mostly MC-R whereas fibrocytes type IV were positive for GC-R only. GC-R and MC-R were also localized at or near the plasma membrane of pillar cells and outer hair cells, whereas GC-R were found at or near the plasma membrane of Hensen cells only. We investigated the relative levels of receptor expression in the cytoplasm and the nucleus of Hensen cells treated with dexamethasone, and found they varied in a way suggestive of dose-induced translocation. These results suggest that the oto-protective effects of GC could be associated with the concerted activation of genomic and non-genomic, GC-R and MC-R mediated signaling pathways in different regions of the cochlea.

Different uptake of gentamicin through TRPV1 and TRPV4 channels determines cochlear hair cell vulnerability

Hair cells at the base of the cochlea appear to be more susceptible to damage by the aminoglycoside gentamicin than those at the apex. However, the mechanism of base-to-apex gradient ototoxicity by gentamicin remains to be elucidated. We report here that gentamicin caused rodent cochlear hair cell damages in a time- and dose-dependent manner. Hair cells at the basal turn were more vulnerable to gentamicin than those at the apical turn. Gentamicin-conjugated Texas Red (GTTR) uptake was predominant in basal turn hair cells in neonatal rats. Transient receptor potential vanilloid 1 (TRPV1) and 4 (TRPV4) expression was confirmed in the cuticular plate, stereocilia and hair cell body of inner hair cells and outer hair cells. The involvement of TRPV1 and TRPV4 in gentamicin trafficking of hair cells was confirmed by exogenous calcium treatment and TRPV inhibitors, including gadolinium and ruthenium red, which resulted in markedly inhibited GTTR uptake and gentamicin-induced hair cell damage in rodent and zebrafish ototoxic model systems. These results indicate that the cytotoxic vulnerability of cochlear hair cells in the basal turn to gentamicin may depend on effective uptake of the drug, which was, in part, mediated by the TRPV1 and TRPV4 proteins.

Changes in the adult vertebrate auditory sensory epithelium after trauma

Auditory hair cells transduce sound vibrations into membrane potential changes, ultimately leading to changes in neuronal firing and sound perception. This review provides an overview of the characteristics and repair capabilities of traumatized auditory sensory epithelium in the adult vertebrate ear. Injured mammalian auditory epithelium repairs itself by forming permanent scars but is unable to regenerate replacement hair cells. In contrast, injured non-mammalian vertebrate ear generates replacement hair cells to restore hearing functions. Non-sensory support cells within the auditory epithelium play key roles in the repair processes.

"In-bone" utricle cultures--a simplified, atraumatic technique for in situ cultures of the adult mouse (Mus musculus) utricle

HYPOTHESIS

The "in-bone" method of culturing utricles described here is a reliable and atraumatic technique for culturing mature mouse hair cells and studying hair cell death and protection.

BACKGROUND

The current in vitro technique for studying hair cells of the mature mouse utricle involves removal from the temporal bone and free floating culture in media. This technique can be problematic because of variability in the preservation of the sensory epithelium and a steep learning curve that results in injury of the sensory epithelium in less experienced hands. We present a new atraumatic technique of culturing the utricle in situ within the temporal bone.

METHODS

Leaving the temporal bone largely intact, a window is opened in the bony vestibule overlying the mouse utricle. The entire temporal bone is then placed into culture media. Utricles were cultured in situ for several days with minimal damage to the epithelium. The utricles are then fixed in situ, removed from the temporal bone, and processed. A standardized aminoglycoside-induced hair cell damage protocol was developed.

RESULTS

Mature mouse utricles maintained hair cell numbers for 3 days in culture. Exposure to neomycin resulted in significant dose-dependent hair cell toxicity (p < 0.0001, 1-way analysis of variance). Exposure to the protective drug tacrine resulted in significant protection against neomycin (p < 0.05, 3-way analysis of variance).

CONCLUSION

The "in-bone" technique is a reliable and atraumatic method for culturing mature mouse utricles and studying hair cell death and protection. It is easily mastered and can make in vitro study of hair cells accessible to more research groups.

Temporal and spatial distribution of gentamicin in the peripheral vestibular system after transtympanic administration in guinea pigs

BACKGROUND AND OBJECTIVE

Transtympanic administration of gentamicin is effective for treating patients with intractable vertigo. This study explored the spatial and temporal distribution of gentamicin in vestibular end-organs after transtympanic administration.

METHODS

Thirty guinea pigs were transtympanically injected with gentamicin conjugated to Texas Red (GTTR) and their vestibular end-organs examined after various survival periods. Another 9 guinea pigs received GTTR at different doses. Nine animals received Texas Red only and served as controls. We used confocal microscopy to determine the cellular distribution of GTTR in semicircular canal cristae, as well as the utricular and saccular maculae.

RESULTS

The most intense GTTR labeling was present in the saccule compared to other vestibular end-organs. GTTR fluorescence was detected predominantly in type I hair cells, type II hair cells and transitional cells after a single transtympanic dose of GTTR (0.1 mg/ml, 0.05 ml), while only weak fluorescence was observed in non-sensory cells such as supporting cells, dark cells and lumenal epithelial cells. Transitional cells displayed intense GTTR fluorescence in the supra-nuclear regions 24 h after transtympanic injection that was retained for at least 4 weeks. A decreasing spatial gradient of GTTR fluorescence was observed sensory epithelial regions containing central type I to peripheral type I and then type II hair cells in the crista ampullaris, and from striolar to extra-striolar hair cells within the vestibular macula. GTTR fluorescence extended from being restricted to the apical cytoplasm at lower doses to the entire cell body of type I hair cells with increasing dose. GTTR fluorescence reached peak intensities for individual regions of interest within the cristae and maculae between 3 and 7 days after transtympanic injection.

CONCLUSION

The saccular uptake of GTTR is greater than other vestibular end-organs after transtympanic injection in the semicircular canals.

A regulated delivery system for inner ear drug application

OBJECTIVE

We have recently developed a novel inner ear drug delivery system using chitosan glycerophosphate (CGP) hydrogel loaded with drugs commonly used for treatment of inner ear diseases, significantly improving the drugs' sustained delivery. The goal of this study is to evaluate the effectiveness of chitosanase as a "switch off" mechanism for this drug delivery system when side effects and potential ototoxicities appear during treatment. To evaluate this effect, we tested gentamicin (GENT) in the inner ear following CGP delivery with/without regulation.

METHODS

Purified chitosanase was obtained and used for regulating the CGP delivery system. In vitro studies were performed to evaluate the effect of the interaction between chitosanase and CGP-hydrogel loaded with GENT or Texas Red-labeled GENT (GTTR). In vivo studies were performed using our mouse model to investigate the regulatory effect of chitosanase application on the delivery of GENT to the inner ear. To assess the potential drug rerouting regulatory effect of chitosanase the GTTR fluorescence intensity was evaluated at the round window niche (RWN) and the Eustachian tube (ET). To further characterize this regulatory effect, GENT concentration in the perilymph of the inner ear was analyzed by chromatographic tandem mass spectrometry (LC-MS/MS), and the uptake in the inner ear cells was measured using fluorescence microscopy following CGP delivery with/without chitosanase application.

RESULTS

The chitosanase effectively digested the CGP-hydrogel, quickly releasing GENT and GTTR from the system in vitro. When reacted with GENT alone chitosanase did not produce any reducing sugars and did not affect GENT's antimicrobial activity. In vivo GTTR was effectively rerouted from the RWN to the ET, limiting its uptake in inner ear hair cells. Concurrent with these findings, GENT concentration in the inner ear perilymph was significantly decreased after chitosanase application.

CONCLUSION

Our study findings suggest that, for the first time, sustained and controlled inner ear drug delivery can be successfully regulated enhancing its translation potential for clinical application. The use of chitosanase to digest the CGP-hydrogel results in the rerouting of the loaded drug away from the RWN, effectively downregulating its delivery to the inner ear. This important modification to our drug delivery system has the ability to deliver therapy to the inner ear until desired effect is achieved and to stop this process when side effects or treatment-related ototoxicities start to occur, providing a novel and salient approach for safe and effective delivery to the inner ear.