Direct entry of gadolinium into the vestibule following intratympanic applications in Guinea pigs and the influence of cochlear implantation

Preliminary Evidence for the Effects of Gentamicin on Vertical Semicircular Canals

INTRODUCTION

Gentamicin is a vestibulotoxic antibiotic often used in patients with Ménière's disease for its vestibular ablative effects. Gentamicin's effect on the horizontal semicircular canal does not always correlate with the degree of vertigo control achieved by patients; its effect on the vertical semicircular canals remains unknown. We sought to examine the effect of intratympanic gentamicin on vertical semicircular canal function in patients with Ménière's disease using video head impulse testing.

METHODS

A retrospective case series was carried out at a tertiary academic center. Patients with Ménière's disease who received ≥1 intratympanic gentamicin injection from 2019-2022 and had video head impulse testing performed were included. Outcomes of interest were vertical semicircular canal function following intratympanic gentamicin, correlations between vertical semicircular canal function and horizontal semicircular canal function, and residual symptoms following injection.

RESULTS

Ten patients met inclusion criteria. Twenty percent had abnormal V-SCC function prior to any injection and 40% following the first injection. There was an association between abnormal vertical and horizontal semicircular canal function following the first intratympanic gentamicin injection, though the relationship did not reach statistical significance (p = 0.058). While patients with abnormal vertical semicircular canal function following the first injection were less likely to report ongoing vertigo attacks, the relationship was not statistically significant (p = 0.260).

CONCLUSIONS

Intratympanic gentamicin leads to changes in vertical semicircular canal function in at least a proportion of patients with Ménière's disease. Further study is required to better assess correlations between vertical semicircular canal function and symptom control following intratympanic gentamicin.

A Single Intratympanic Triamcinolone Acetonide Administration Elicits Long-Term Reduction in Impedances Following Cochlear Implantation

BACKGROUND

Intracochlear fibrosis and inflammation remain important limitations in cochlear implantation (CI). Glucocorticoids are routinely used to ameliorate the inflammatory response following CI. This study investigates the long-term effects of an intratympanically-applied triamcinolone-acetonide suspension on intracochlear impedance changes in CI recipients and investigates differences in drug concentrations and timepoints of injection.

METHODS

A total of 87 patients were included in the study, of whom 39 received an intratympanically-applied triamcinolone-acetonide suspension at either 10 or 40 mg/ml, 1 hour or 24 hours prior to cochlear implantation, while 48 patients served as an untreated control group. Electrode impedances were measured and compared over a period of 3 years following cochlear implantation.

RESULTS

The preoperative intratympanic application of a triamcinolone-acetonide suspension resulted in significantly lower mean impedances following cochlear implantation compared with an untreated control group at first fitting (4.66 ± 1.3 kΩ to 5.90 ± 1.4 kΩ, P = .0001), with sustained reduction observed over 3 months. A sustained reduction was observed after spatial grouping of the electrode contacts, with significant improvements in both the middle cochlear region over a 24 month period (from 3.97 ± 1.3 kΩ to 5.85 ± 1.3 kΩ, P = .049) and the basal region over a 6 month period (from 5.02 ± 1.3 kΩ to 5.85 ± 1.3 kΩ, P = .008). The injection of 10 mg/ml of triamcinolone-acetonide 1 hour prior to cochlear implantation resulted in higher impedances compared with 40 mg/ml and 24 hour time interval until surgery.

CONCLUSION

A single preoperative intratympanic injection of triamcinolone-acetonide significantly reduces electrode impedances across the entire cochlea. This effect is sustained for up to 2 years, after which impedances gradually equalize between the groups. A preoperative triamcinolone-acetonide injection could therefore be a favorable approach to attenuate the immediate tissue response following cochlear implantation.

Magnetic Targeting of Gadolinium Contrast to Enhance MRI of the Inner Ear in Endolymphatic Hydrops

OBJECTIVES

1. Determine the feasibility and efficiency of local magnetic targeting delivery of gadolinium (Gad) contrast to the inner ear in rodents. 2. Assess any potential ototoxicity of magnetic targeting delivery of Gad in the inner ear. 3. Study the utility of magnetic targeting delivery of Gad to visualize and quantify endolymphatic hydrops (EH) in a transgenic mouse model.

STUDY DESIGN

Controlled in vivo animal model study.

METHODS

Paramagnetic Gad was locally delivered to the inner ear using the magnetic targeting technique in both rat and mouse models. Efficiency of contrast delivery was assessed using magnetic resonance imaging (MRI). Ototoxicity of Gad was examined with histology of the cochlea and functional audiological tests. The Phex mouse model was used to study EH, hearing loss, and balance dysfunction. Magnetic targeting delivery of Gad contrast was used in the Phex mouse model to visualize the effects of EH using MRI.

RESULTS

Magnetic targeting improved the delivery of Gad to the inner ear and the technique was reproducible in both rat and mouse models. The delivery method did not result in microstructural damage or any significant hearing loss in a normal animal. Magnetic targeting of Gad in the Phex mouse model allowed detailed visualization and quantification of EH.

CONCLUSION

This study provided the first evidence of the effectiveness and efficiency of the local magnetic targeting delivery of gadolinium contrast to the inner ear and its application to the visualization and quantification of EH. Laryngoscope, 133:914-923, 2023.

Sonoporation of the Round Window Membrane on a Sheep Model: A Safety Study

Sonoporation using microbubble-assisted ultrasound increases the permeability of a biological barrier to therapeutic molecules. Application of this method to the round window membrane could improve the delivery of therapeutics to the inner ear. The aim of this study was to assess the safety of sonoporation of the round window membrane in a sheep model. To achieve this objective, we assessed auditory function and cochlear heating, and analysed the metabolomics profiles of perilymph collected after sonoporation, comparing them with those of the control ear in the same animal. Six normal-hearing ewes were studied, with one sonoporation ear and one control ear for each. A mastoidectomy was performed on both ears. On the sonoporation side, Vevo MicroMarker^® microbubbles (MBs; VisualSonics-Fujifilm, Amsterdam, The Netherlands) at a concentration of 2 × 10⁸ MB/mL were locally injected into the middle ear and exposed to 1.1 MHz sinusoidal ultrasonic waves at 0.3 MPa negative peak pressure with 40% duty cycle and 100 μs interpulse period for 1 min; this was repeated three times with 1 min between applications. The sonoporation protocol did not induce any hearing impairment or toxic overheating compared with the control condition. The metabolomic analysis did not reveal any significant metabolic difference between perilymph samples from the sonoporation and control ears. The results suggest that sonoporation of the round window membrane does not cause damage to the inner ear in a sheep model.

Intratympanic Substance Distribution After Injection of Liquid and Thermosensitive Drug Carriers: An Endoscopic Study

OBJECTIVE

In the treatment of inner ear conditions, intratympanic injection emerges as an important drug delivery method. Novel compounds designed for intratympanic injection are routinely loaded in viscous drug carriers. To date, it is unclear if they can freely distribute in the middle ear. The aims of this study were to investigate the middle ear distribution of different drug carriers during intratympanic injection and to determine an optimal injection method for thermosensitive hydrogels.

METHODS

Twenty-one human temporal bones were intratympanically injected with fluid drug carriers or poloxamer-407 hydrogels at different tympanic membrane injection sites (inferior, anterior-superior) using different needle types (Whitacre, Quincke). Fluid distribution was evaluated via an endoscopic view. Injection volume, duration, backflow, and overall safety were analyzed.

RESULTS

Liquid drug carriers distribute effortlessly in the middle ear, whereas an additional ventilation hole is advantageous when applying thermosensitive hydrogels. The round window is coated with required volumes between 150 and 200 μl, irrespective of the injection position. Required volumes to also coat the stapedial footplate ranged from 310 to 440 μl. Use of the Whitacre-type needle reduced backflow to the ear canal and enabled longer tympanic membrane visibility when no additional ventilation hole was placed.

CONCLUSION

Intratympanic injection is a safe and reliable method for the application of thermosensitive hydrogels. The round window niche is readily filled regardless of the injected formulation and injection position. Although fluid drug carriers distribute effortlessly in the middle ear, the placement of an additional ventilation hole might facilitate the application of viscous hydrogels.

Local Delivery of Therapeutics to the Cochlea Using Nanoparticles and Other Biomaterials

Hearing loss negatively impacts the well-being of millions of people worldwide. Systemic delivery of ototherapeutics has limited efficacy due to severe systemic side effects and the presence of the blood-labyrinth barrier that selectively limits or enables transfer of molecules between plasma and inner ear tissues and fluids. Local drug delivery into the middle and inner ear would be preferable for many newly emerging classes of drugs. Although the cochlea is a challenging target for drug delivery, recent technologies could provide a safe and efficacious delivery of ototherapeutics. Local drug delivery routes include topical delivery via the external auditory meatus, retroauricular, transtympanic, and intracochlear delivery. Many new drug delivery systems specifically for the inner ear are under development or undergoing clinical studies. Future studies into these systems may provide a means for extended delivery of drugs to preserve or restore hearing in patients with hearing disorders. This review outlines the anatomy of the (inner) ear, describes the various local delivery systems and routes, and various quantification methodologies to determine the pharmacokinetics of the drugs in the inner ear.

Intratympanic corticosteroids for sudden sensorineural hearing loss

BACKGROUND

Idiopathic sudden sensorineural hearing loss (ISSNHL) is common, and defined as a sudden decrease in sensorineural hearing sensitivity of unknown aetiology. Systemic corticosteroids are widely used, however their value remains unclear. Intratympanic injections of corticosteroids have become increasingly common in the treatment of ISSNHL.

OBJECTIVES

To assess the effects of intratympanic corticosteroids in people with ISSNHL.

SEARCH METHODS

The Cochrane ENT Information Specialist searched the Cochrane ENT Trials Register; CENTRAL (2021, Issue 9); PubMed; Ovid Embase; CINAHL; Web of Science; ClinicalTrials.gov; ICTRP and additional sources for published and unpublished trials (search date 23 September 2021).

SELECTION CRITERIA

We included randomised controlled trials (RCTs) involving people with ISSNHL and follow-up of over a week. Intratympanic corticosteroids were given as primary or secondary treatment (after failure of systemic therapy).

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methods, including GRADE to assess the certainty of the evidence. Our primary outcome was change in hearing threshold with pure tone audiometry. Secondary outcomes included the proportion of people whose hearing improved, final hearing threshold, speech audiometry, frequency-specific hearing changes and adverse effects.

MAIN RESULTS

We included 30 studies, comprising 2133 analysed participants. Some studies had more than two treatment arms and were therefore relevant to several comparisons. Studies investigated intratympanic corticosteroids as either primary (initial) therapy or secondary (rescue) therapy after failure of initial treatment. 1. Intratympanic corticosteroids versus systemic corticosteroids as primary therapy We identified 16 studies (1108 participants). Intratympanic therapy may result in little to no improvement in the change in hearing threshold (mean difference (MD) -5.93 dB better, 95% confidence interval (CI) -7.61 to -4.26; 10 studies; 701 participants; low-certainty). We found little to no difference in the proportion of participants whose hearing was improved (risk ratio (RR) 1.04, 95% CI 0.97 to 1.12; 14 studies; 972 participants; moderate-certainty). Intratympanic therapy may result in little to no difference in the final hearing threshold (MD -3.31 dB, 95% CI -6.16 to -0.47; 7 studies; 516 participants; low-certainty). Intratympanic therapy may increase the number of people who experience vertigo or dizziness (RR 2.53, 95% CI 1.41 to 4.54; 1 study; 250 participants; low-certainty) and probably increases the number of people with ear pain (RR 15.68, 95% CI 6.22 to 39.49; 2 studies; 289 participants; moderate-certainty). It also resulted in persistent tympanic membrane perforation (range 0% to 3.9%; 3 studies; 359 participants; very low-certainty), vertigo/dizziness at the time of injection (1% to 21%, 3 studies; 197 participants; very low-certainty) and ear pain at the time of injection (10.5% to 27.1%; 2 studies; 289 participants; low-certainty). 2. Intratympanic plus systemic corticosteroids (combined therapy) versus systemic corticosteroids alone as primary therapy We identified 10 studies (788 participants). Combined therapy may have a small effect on the change in hearing threshold (MD -8.55 dB better, 95% CI -12.48 to -4.61; 6 studies; 435 participants; low-certainty). The evidence is very uncertain as to whether combined therapy changes the proportion of participants whose hearing is improved (RR 1.27, 95% CI 1.15 to 1.41; 10 studies; 788 participants; very low-certainty). Combined therapy may result in slightly lower (more favourable) final hearing thresholds but the evidence is very uncertain, and it is not clear whether the change would be important to patients (MD -9.11 dB, 95% CI -16.56 to -1.67; 3 studies; 194 participants; very low-certainty). Some adverse effects only occurred in those who received combined therapy. These included persistent tympanic membrane perforation (range 0% to 5.5%; 5 studies; 474 participants; very low-certainty), vertigo or dizziness at the time of injection (range 0% to 8.1%; 4 studies; 341 participants; very low-certainty) and ear pain at the time of injection (13.5%; 1 study; 73 participants; very low-certainty).  3. Intratympanic corticosteroids versus no treatment or placebo as secondary therapy We identified seven studies (279 participants). Intratympanic therapy may have a small effect on the change in hearing threshold (MD -9.07 dB better, 95% CI -11.47 to -6.66; 7 studies; 280 participants; low-certainty). Intratympanic therapy may result in a much higher proportion of participants whose hearing is improved (RR 5.55, 95% CI 2.89 to 10.68; 6 studies; 232 participants; low-certainty). Intratympanic therapy may result in lower (more favourable) final hearing thresholds (MD -11.09 dB, 95% CI -17.46 to -4.72; 5 studies; 203 participants; low-certainty). Some adverse effects only occurred in those who received intratympanic injection. These included persistent tympanic membrane perforation (range 0% to 4.2%; 5 studies; 185 participants; very low-certainty), vertigo or dizziness at the time of injection (range 6.7% to 33%; 3 studies; 128 participants; very low-certainty) and ear pain at the time of injection (0%; 1 study; 44 participants; very low-certainty).  4. Intratympanic plus systemic corticosteroids (combined therapy) versus systemic corticosteroids alone as secondary therapy We identified one study with 76 participants. Change in hearing threshold was not reported. Combined therapy may result in a higher proportion with hearing improvement, but the evidence is very uncertain (RR 2.24, 95% CI 1.10 to 4.55; very low-certainty). Adverse effects were poorly reported with only data for persistent tympanic membrane perforation (rate 8.1%, very low-certainty).

AUTHORS' CONCLUSIONS

Most of the evidence in this review is low- or very low-certainty, therefore it is likely that further studies may change our conclusions.   For primary therapy, intratympanic corticosteroids may have little or no effect compared with systemic corticosteroids. There may be a slight benefit from combined treatment when compared with systemic treatment alone, but the evidence is uncertain. For secondary therapy, there is low-certainty evidence that intratympanic corticosteroids, when compared to no treatment or placebo, may result in a much higher proportion of participants whose hearing is improved, but may only have a small effect on the change in hearing threshold. It is very uncertain whether there is additional benefit from combined treatment over systemic steroids alone. Although adverse effects were poorly reported, the different risk profiles of intratympanic treatment (including tympanic membrane perforation, pain and dizziness/vertigo) and systemic treatment (for example, blood glucose problems) should be considered when selecting appropriate treatment.

Inner Ear Drug Delivery for Sensorineural Hearing Loss: Current Challenges and Opportunities

Most therapies for treating sensorineural hearing loss are challenged by the delivery across multiple tissue barriers to the hard-to-access anatomical location of the inner ear. In this review, we will provide a recent update on various pharmacotherapy, gene therapy, and cell therapy approaches used in clinical and preclinical studies for the treatment of sensorineural hearing loss and approaches taken to overcome the drug delivery barriers in the ear. Small-molecule drugs for pharmacotherapy can be delivered via systemic or local delivery, where the blood-labyrinth barrier hinders the former and tissue barriers including the tympanic membrane, the round window membrane, and/or the oval window hinder the latter. Meanwhile, gene and cell therapies often require targeted delivery to the cochlea, which is currently achieved via intra-cochlear or intra-labyrinthine injection. To improve the stability of the biomacromolecules during treatment, e.g., RNAs, DNAs, proteins, additional packing vehicles are often required. To address the diverse range of biological barriers involved in inner ear drug delivery, each class of therapy and the intended therapeutic cargoes will be discussed in this review, in the context of delivery routes commonly used, delivery vehicles if required (e.g., viral and non-viral nanocarriers), and other strategies to improve drug permeation and sustained release (e.g., hydrogel, nanocarriers, permeation enhancers, and microfluidic systems). Overall, this review aims to capture the important advancements and key steps in the development of inner ear therapies and delivery strategies over the past two decades for the treatment and prophylaxis of sensorineural hearing loss.

Application of New Materials in Auditory Disease Treatment

Auditory diseases are disabling public health problems that afflict a significant number of people worldwide, and they remain largely incurable until now. Driven by continuous innovation in the fields of chemistry, physics, and materials science, novel materials that can be applied to hearing diseases are constantly emerging. In contrast to conventional materials, new materials are easily accessible, inexpensive, non-invasive, with better acoustic therapy effects and weaker immune rejection after implantation. When new materials are used to treat auditory diseases, the wound healing, infection prevention, disease recurrence, hair cell regeneration, functional recovery, and other aspects have been significantly improved. Despite these advances, clinical success has been limited, largely due to issues regarding a lack of effectiveness and safety. With ever-developing scientific research, more novel materials will be facilitated into clinical use in the future.

Pharmacokinetics and biodistribution of supraparticle-delivered neurotrophin 3 in the guinea pig cochlea

Hearing loss is the most prevalent sensory disorder affecting nearly half a billion people worldwide. Aside from devices to assist hearing, such as hearing aids and cochlear implants, a drug treatment for hearing loss has yet to be developed. The neurotrophin family of growth factors has long been established as a potential therapy, however delivery of these factors into the inner ear at therapeutic levels over a sustained period of time has remained a challenge restricting clinical translation. We previously demonstrated that direct delivery of exogenous neurotrophin-3 (NT3) in the guinea pig cochleae via a bolus injection was rapidly cleared from the inner ear, with almost complete elimination 3 days post-treatment. Here, we explored the potential of suprapaticles (SPs) for NT3 delivery to the inner ear to achieve sustained delivery over time. SPs are porous spheroid structures comprised of smaller colloidal silica nanoparticles that provide a platform for long-term controlled release of therapeutics. This study aimed to assess the pharmacokinetics and biodistribution of SP-delivered NT3. We used a radioactive tracer (iodine 125: ¹²⁵I) to label the NT3 to determine the loading, retention and distribution of NT3 delivered via SPs. Gamma measurements taken from ¹²⁵I NT3 loaded SPs revealed high drug loading (an average of 5.3 μg of NT3 loaded per SP weighing 50 μg) and elution capacities in vitro (67% cumulative release over one month). Whole cochlear gamma measurements from SP-implanted cochleae harvested at various time points revealed detection of ¹²⁵I NT3 in the guinea pig cochlea after one month, with 3.6 and 10% of the loaded drug remaining in the intracochlear and round window-implanted cochleae respectively. Autoradiography analysis of cochlear micro-sections revealed widespread ¹²⁵I NT3 distribution after intracochlear SP delivery, but more restricted distribution with the round window delivery approach. Collectively, drug delivery into the inner ear using SPs support sustained, long-term availability and release of neurotrophins in the inner ear.

Efficiency of a dexamethasone nanosuspension as an intratympanic injection for acute hearing loss

Dexamethasone sodium phosphate (Dex-SP) is the most commonly used drug administered via intratympanic injection for the treatment of acute hearing loss, but its penetration efficiency into the inner ear is very low. To address this problem, we evaluated the possibility of administering dexamethasone nanosuspensions via intratympanic injection because hydrophobic drugs might be more effective in penetrating the inner ear. Three types of dexamethasone nanosuspensions were prepared; the dexamethasone nanoparticles in the three nanosuspensions were between approximately 250 and 350 nm in size. To compare the efficiency of Dex-SP and dexamethasone nanosuspension in delivering dexamethasone to the inner ear, the concentrations of dexamethasone in perilymph and cochlear tissues were compared by liquid chromatography–mass spectrometry. The dexamethasone nanosuspensions resulted in significantly higher drug concentrations in perilymph and cochlear tissues than Dex-SP at 6 h; interestingly, animals treated with nanosuspensions showed a 26-fold higher dexamethasone concentrations in their cochlear tissues than animals treated with Dex-SP. In addition, dexamethasone nanosuspension caused better glucocorticoid receptor phosphorylation than Dex-SP both in vitro and in vivo, and in the ototoxic animal model, the nanosuspension showed a significantly better hearing-protective effect against ototoxic drugs than Dex-SP. In the in vivo safety evaluation, the nanosuspension showed no toxicity at concentrations up to 20 mg/mL. In conclusion, a nanosuspension of dexamethasone was able to deliver dexamethasone to the cochlea very safely and efficiently and showed potential as a formula for intratympanic injection.

Drug distribution along the cochlea is strongly enhanced by low-frequency round window micro vibrations

The cochlea’s inaccessibility and complex nature provide significant challenges to delivering drugs and other agents uniformly, safely and efficiently, along the entire cochlear spiral. Large drug concentration gradients are formed along the cochlea when drugs are administered to the middle ear. This undermines the major goal of attaining therapeutic drug concentration windows along the whole cochlea. Here, utilizing a well-known physiological effect of salicylate, we demonstrate a proof of concept in which drug distribution along the entire cochlea is enhanced by applying round window membrane low-frequency micro vibrations with a probe that only partially covers the round window. We provide evidence of enhanced drug influx into the cochlea and cochlear apical drug distribution without breaching cochlear boundaries. It is further suggested that ossicular functionality is not required for the effective drug distribution we report. The novel method presented here of local drug delivery to the cochlea could be implemented when ossicular functionality is absent or impeded and can be incorporated in clinically approved auditory protheses for patients who suffer with conductive, sensorineural or mixed hearing loss.

[Development of novel drug carrier via round window membrane]

As isolated anatomical position, limited labyrinthine artery supply, and blood-labyrinth barrier hampers systemic drug delivery to the inner ear. The efficient concentration of drug treatment is unsatisfactory and there's possible side effects after systemic administration. Intratympanic injection of drug can bypass the blood-labyrinth and permeated to the hair cells or synaptic area via the round-or oval window of the cochlea. Efficacy and safety of pharmacotherapy has become increasingly relied on the inner ear delivery carrier system. The goal of this review focus on the anatomical barrier that need to be overcome in the intratympanic applications, the improvement of drug retention and specific targets, and the safety of novel drug carriers, these emerging strategies of local drug delivery promise novel and better guidance for the clinical application.

Consistent removal of hair cells in vestibular end organs by time-dependent transtympanic administration of gentamicin in guinea pigs

BACKGROUND

Vestibular hair cell loss and its role in balance disorders are not yet completely understood due largely to the lack of precise hair cell damage protocols.

NEW METHOD

Our damage protocol aims to selectively remove type I hair cells in a way that produces consistent and predictable lesions that can be used for reliable inter-animal and inter-group comparison in balance research. This objective is achieved by transtympanic injection of gentamicin on both the round window membrane and oval window over a fixed time period followed by thorough washing.

RESULTS

We achieved nearly total and consistent loss of type I hair cells at 94 % for the crista ampullaris of the lateral semicircular canal (LSC) and 86 % for the utricular macula with negligible loss of type II hair cells at 4% for the crista ampullaris of the LSC and 6% for the utricular macula. While the vestibular function was compromised in the relevant study group, this group had a zero mortality rate with no significant suppression of body weight gain.

COMPARISON WITH EXISTING METHODS

Gentamicin is typically administered via intraperitoneal systemic injection or, more recently, transtympanic injection. The intraperitoneal method is simple, but mortality rate is high. The transtympanic injection method produces ototoxic damage but with inconsistent lesion size. This inconsistency prevents reliable comparisons among animals.

CONCLUSIONS

This protocol employs a transtympanic injection method which selectively targets type I hair cells for removal in the vestibular epithelia in a time-dependent manner, uniformly damages vestibular function, and causes uniform hair cell loss.

A radiolabeled drug tracing method to study neurotrophin-3 retention and distribution in the cochlea after nano-based local delivery

Hearing loss is the most common sensory deficit worldwide with no approved therapeutics for treatment. Local neurotrophin delivery into the cochlea has shown great potential in protecting and repairing the sensory cells important for hearing. However, delivery of these factors into the inner ear at therapeutic levels over a sustained period of time has remained a challenge restricting clinical translation. We have developed a method to test the pharmacokinetics of neurotrophin released from porous silica particles called ‘supraparticles’ that can provide sustained release of neurotrophins to the inner ear.•

This report describes a radiolabeling method to examine neurotrophin retention and distribution in the cochlea. The neurotrophin was labeled with a radioactive tracer (iodine 125: ¹²⁵I) and delivered into the cochlea via the supraparticle system.

•

Gamma counts reveal drug levels and clearance in the intact cochlea, as well as accumulation in off-target organs (safety test). Autoradiography analyses using film and emulsion permit quantification and visualization of drug distribution at the cellular level. The method has a detection limit of 0.8 pg of radiolabeled neurotrophin-3 in cochlear sections exposed to film.

•

The tracer ¹²⁵I with a half-life of 59.4 days can be used to label other drugs/substances with a tyrosine residue and therefore be broadly applicable for long-term pharmacokinetic studies in other systems.

Development of nanoparticle drug-delivery systems for the inner ear

Hearing loss has become the most common sensory nerve disorder worldwide, with no effective treatment strategy. Low-permeability and limited blood supply to the blood-labyrinth barrier limit the effective delivery and efficacy of therapeutic drugs in the inner ear. Nanoparticle (NP)-based drugs have shown benefits of stable controlled release and functional surface modification, and NP-based delivery systems have become a research hotspot. In this review, we discuss the development of new targeted drug-delivery systems based on the biocompatibility and safety of different NPs in the cochlea, as well as the advantages and disadvantages of their prescription methods and approaches. We believe that targeted NP-based drug-delivery systems will be effective treatments for hearing loss.

Ultrasound-induced microbubble cavitation via a transcanal or transcranial approach facilitates inner ear drug delivery

Ultrasound-induced microbubble (USMB) cavitation is widely used to promote drug delivery. Our previous study investigated USMB targeting the round window membrane by applying the ultrasound transducer to the tympanic bulla. In the present study, we further extended the use of this technology to enhance drug delivery to the inner ear by introducing the ultrasound transducer into the external auditory canal (EAC) or applying it to the skull. Using a 3-dimensional-printed diffusion apparatus mimicking the pathway for ultrasound passing through and reaching the middle ear cavity in vitro, the models simulating the transcanal and transcranial approach demonstrated 4.8-fold- and 3.7-fold-higher delivery efficiencies, respectively. In an in vivo model of guinea pigs, by filling tympanic bulla with microbubbles and biotin-FITC, USMB applied transcanally and transcranially induced 2.8-fold and 1.5-fold increases in biotin-FITC delivery efficiencies, respectively. In addition, the gentamicin uptake by cochlear and vestibular hair cells and gentamicin-induced hair cell loss were significantly enhanced following transcanal application of USMB. On the 28th day after transcanal USMB, safety assessment showed no significant changes in the hearing thresholds and the integrity of cochlea. These are the first results to our knowledge to demonstrate the feasibility and support the potential clinical application of applying USMB via EAC to facilitate drug delivery into the inner ear.

Inner ear delivery: Challenges and opportunities

OBJECTIVES

The treatment of inner ear disorders remains challenging due to anatomic barriers intrinsic to the bony labyrinth. The purpose of this review is to highlight recent advances and strategies for overcoming these barriers and to discuss promising future avenues for investigation.

DATA SOURCES

The databases used were PubMed, EMBASE, and Web of Science.

RESULTS

Although some studies aimed to improve systemic delivery using nanoparticle systems, the majority enhanced local delivery using hydrogels, nanoparticles, and microneedles. Developments in direct intracochlear delivery include intracochlear injection and intracochlear implants.

CONCLUSIONS

In the absence of a systemic drug that targets only the inner ear, the best alternative is local delivery that harnesses a combination of new strategies to overcome anatomic barriers. The combination of microneedle technology with hydrogel and nanoparticle delivery is a promising area for future investigation.

LEVEL OF EVIDENCE

NA.

MRI detection of endolymphatic hydrops in Meniere's disease in 8 minutes using MIIRMR and a 20-channel coil after targeted gadolinium delivery

Background

Endolymphatic hydrops (EH) become visible in vertigo patients, particularly in those with Meniere's disease (MD), in vivo using gadolinium-enhanced MRI. However, the image quality is not satisfying after intravenous injection of gadolinium chelate (GdC), and occasional failure in GdC uptake has been noticed after traditional intratympanic injection. In the present report, targeted delivery of GdC and using a cost-effective MRI system to obtain high quality images of EH in only 8 min will be introduced.

Methods

39 MD patients were recruited in the study. First, 0.1 ml of 20-fold diluted gadolinium-diethylenetriamine acid (Gd-DTPA) was delivered onto the posterior upper part of the tympanic medial wall using a soft-tipped micro-irrigation catheter through an artificially perforated tympanic membrane. Inner ear MRI was performed 24 h after Gd-DTPA administration using a 3T MR machine and a 20-channel head/neck coil with an 8 min sequence of medium inversion time inversion recovery imaging with magnitude reconstruction (MIIRMR). The parameters were as follows: TR 16000 ms, TE 663 ms, inversion time 2700 ms, flip angle 180°, slices per slab 60.

Results

Efficient inner ear uptake of Gd-DTPA was detected 24 h after delivery and it created excellent contrast in the inner ear of all cases. High quality images demonstrating EH in the vestibule and cochlea were obtained.

Conclusion

Targeted delivery of minimum Gd-DTPA (0.1 ml, 20-fold dilution) onto the posterior upper portion of the tympanic medial wall and MRI with MIIRMR in a 3T machine and 20-channel head/neck coil are clinically practical to obtain high quality images displaying EH.

Developments in Bio-Inspired Nanomaterials for Therapeutic Delivery to Treat Hearing Loss

Sensorineural hearing loss affects millions of people worldwide and is a growing concern in the aging population. Treatment using aminoglycoside antibiotics for infection and exposure to loud sounds contribute to the degeneration of cochlear hair cells and spiral ganglion neurons. Cell loss impacts cochlear function and causes hearing loss in ∼ 15% of adult Americans (∼36 million). The number of individuals with hearing loss will likely grow with increasing lifespans. Current prosthesis such as hearing aids and cochlear implants can ameliorate hearing loss. However, hearing aids are ineffective if hair cells or spiral ganglion neurons are severely damaged, and cochlear implants are ineffective without properly functioning spiral ganglion neurons. As such, strategies that alleviate hearing loss by preventing degeneration or promoting cell replacement are urgently needed. Despite showing great promise from in vitro studies, the complexity and delicate nature of the inner ear poses a huge challenge for delivering therapeutics. To mitigate risks and complications associated with surgery, new technologies and methodologies have emerged for efficient delivery of therapeutics. We will focus on biomaterials that allow controlled and local drug delivery into the inner ear. The rapid development of microsurgical techniques in conjunction with novel bio- and nanomaterials for sustained drug delivery appears bright for hearing loss treatment.

Correlations Between the Degree of Endolymphatic Hydrops and Symptoms and Audiological Test Results in Patients With Menière's Disease: A Reevaluation

OBJECTIVE

This study was performed to reevaluate the diagnostic significance of clinically well-accepted audiological tests in indicating endolymphatic hydrops (EH) in Menière's disease (MD).

STUDY DESIGN

Retrospective case review.

SETTINGS

Hospital.

PATIENTS

Fifty patients (52 affected ears) diagnosed with MD were enrolled.

INTERVENTION

Diagnostic.

MAIN OUTCOME MEASURE

To analyze the correlations between endolymphatic hydrops and results of audiological test including the pure-tone audiometry threshold, suprathreshold function tests, electrocochleogram, and glycerol test.

RESULTS

Three-dimensional fluid-attenuated inversion recovery magnetic resonance imaging demonstrated EH in either the vestibule or cochlea to various degrees in all of the MD patients, 24 hours after intratympanic gadolinium chelate injection. Both vestibular and cochlear EH were significantly correlated with PTA threshold. However, EH was not associated with alternate binaural loudness balance or the tone decay test, although a correlation was observed with the short-increment sensitivity index. There was also a correlation between vestibular EH, but not cochlear EH, and the negative summating potential/action potential (-SP/AP) ratio. Neither vestibular EH nor cochlear EH was correlated with the glycerol test results. In addition, the frequency of vertigo attacks, the existence of tinnitus, and aural fullness did not correlate with EH.

CONCLUSIONS

Disrupted ionic homeostasis in the inner ear, but not the EH, may contribute to changes in the -SP/AP ratio. The relevance of glycerol test in identifying EH through detection of hearing changes needs further investigation in the future.

Gene Therapy for Human Sensorineural Hearing Loss

Hearing loss is the most common sensory impairment in humans and currently disables 466 million people across the world. Congenital deafness affects at least 1 in 500 newborns, and over 50% are hereditary in nature. To date, existing pharmacologic therapies for genetic and acquired etiologies of deafness are severely limited. With the advent of modern sequencing technologies, there is a vast compendium of growing genetic alterations that underlie human hearing loss, which can be targeted by therapeutics such as gene therapy. Recently, there has been tremendous progress in the development of gene therapy vectors to treat sensorineural hearing loss (SNHL) in animal models in vivo. Nevertheless, significant hurdles remain before such technologies can be translated toward clinical use. These include addressing the blood-labyrinth barrier, engineering more specific and effective delivery vehicles, improving surgical access, and validating novel targets. In this review, we both highlight recent progress and outline challenges associated with in vivo gene therapy for human SNHL.

Local Drug Delivery for Prevention of Hearing Loss

Systemic delivery of therapeutics for targeting the cochlea to prevent or treat hearing loss is challenging. Systemic drugs have to cross the blood-labyrinth barrier (BLB). BLB can significantly prevent effective penetration of drugs in appropriate concentrations to protect against hearing loss caused by inflammation, ototoxic drugs, or acoustic trauma. This obstacle may be obviated by local administration of protective agents. This route can deliver higher concentration of drug compared to systemic application and preclude systemic side effects. Protective agents have been administered by intra-tympanic injection in numerous preclinical studies. Drugs such as steroids, etanercept, D and L-methionine, pifithrin-alpha, adenosine agonists, melatonin, kenpaullone (a cyclin-dependent kinase 2 (CDK2) inhibitor) have been reported to show efficacy against cisplatin ototoxicity in animal models. Several siRNAs have been shown to ameliorate cisplatin ototoxicity when administered by intra-tympanic injection. The application of corticosteroids and a number of other drugs with adjuvants appears to enhance efficacy. Administration of siRNAs to knock down AMPK kinase, liver kinase B1 (LKB1) or G9a in the cochlea have been found to ameliorate noise-induced hearing loss. The local administration of these compounds appears to be effective in protecting the cochlea against damage from cisplatin or noise trauma. Furthermore the intra-tympanic route yields maximum protection in the basal turn of the cochlea which is most vulnerable to cisplatin ototoxicity and noise trauma. There appears to be very little transfer of these agents to the systemic circulation. This would avoid potential side effects including interference with anti-tumor efficacy of cisplatin. Nanotechnology offers strategies to effectively deliver protective agents to the cochlea. This review summarizes the pharmacology of local drug delivery by intra-tympanic injection to prevent hearing loss caused by cisplatin and noise exposure in animals. Future refinements in local protective agents provide exciting prospects for amelioration of hearing loss resulting from cisplatin or noise exposure.

Cochlear pharmacokinetics - Micro-computed tomography and learning-prediction modeling for transport parameter determination

Inner ear disorders such as sensorineural deafness and genetic diseases may one day be treated with local drug delivery to the inner ear. Current pharmacokinetic models have been based on invasive methods to measure drug concentrations, limiting them in spatial resolution, and restricting the research to larger rodents. We developed an intracochlear pharmacokinetic model based on an imaging, learning-prediction (LP) paradigm for learning transport parameters in the murine cochlea. This was achieved using noninvasive micro-computed tomography imaging of the cochlea during in vivo infusion of a contrast agent at the basal end of scala tympani through a cochleostomy. Each scan was registered in 3-D to a cochlear atlas to segment the cochlear regions with high accuracy, enabling concentrations to be extracted along the length of each scala. These spatio-temporal concentration profiles were used to learn a concentration dependent diffusion coefficient, and transport parameters between the major scalae and to clearance. The LP model results are comparable to the current state of the art model, and can simulate concentrations for cases involving different infusion molecules and different drug delivery protocols. Forward simulation results with pulsatile delivery suggest the pharmacokinetic model can be used to optimize drug delivery protocols to reduce total drug delivered and the potential for toxic side effects. While developed in the challenging murine cochlea, the processes are scalable to larger animals and different drug infusion paradigms.

Adjuvant agents enhance round window membrane permeability to dexamethasone and modulate basal to apical cochlear gradients

Glucocorticoids have direct anti-inflammatory, anti-oxidant and anti-apoptotic effects on cochlear hair cells. Cochlear glucocorticoid therapy has gained particular attention for its ability to enhance the protection of residual hearing following hearing preservation cochlear implantation. Local drug delivery methods achieve high drug concentrations within the inner ear fluids but are reliant upon diffusion across the round window membrane. Diffusion has been shown to demonstrate large individual variability. This study explores the role of "adjuvant agents", which when administered with glucocorticoids, enhance inner ear absorption and distribution. Guinea pig cochleae were administered either dexamethasone alone or in combination with hyaluronic acid, histamine, or combination histamine and hyaluronic acid, targeted at the round window membrane. Control subjects received saline. Perilymph was sampled from the cochlear apex, and basal to apical dexamethasone concentrations recorded with mass spectroscopy. Cochleae were harvested, and immunohistochemistry employed to explore dexamethasone tissue penetration and distribution. Basal to apical gradients were observed along the scala tympani, with higher dexamethasone concentrations observed at the cochlear base. Gradients were more pronounced and uniform when administered on a hyaluronic acid sponge, while histamine increased absolute concentrations reaching the inner ear. Tissue penetration correlated with perilymph concentration. Our results demonstrate that adjuvant agents can be employed to enhance dexamethasone absorption and distribution in the inner ear, thus proposing therapeutic strategies that may enhance steroid facilitated hearing protection.

Anatomical basis of drug delivery to the inner ear

The isolated anatomical position and blood-labyrinth barrier hampers systemic drug delivery to the mammalian inner ear. Intratympanic placement of drugs and permeation via the round- and oval window are established methods for local pharmaceutical treatment. Mechanisms of drug uptake and pathways for distribution within the inner ear are hard to predict. The complex microanatomy with fluid-filled spaces separated by tight- and leaky barriers compose various compartments that connect via active and passive transport mechanisms. Here we provide a review on the inner ear architecture at light- and electron microscopy level, relevant for drug delivery. Focus is laid on the human inner ear architecture. Some new data add information on the human inner ear fluid spaces generated with high resolution microcomputed tomography at 15 μm resolution. Perilymphatic spaces are connected with the central modiolus by active transport mechanisms of mesothelial cells that provide access to spiral ganglion neurons. Reports on leaky barriers between scala tympani and the so-called cortilymph compartment likely open the best path for hair cell targeting. The complex barrier system of tight junction proteins such as occludins, claudins and tricellulin isolates the endolymphatic space for most drugs. Comparison of relevant differences of barriers, target cells and cell types involved in drug spread between main animal models and humans shall provide some translational aspects for inner ear drug applications.

Communication pathways to and from the inner ear and their contributions to drug delivery

The environment of the inner ear is highly regulated in a manner that some solutes are permitted to enter while others are excluded or transported out. Drug therapies targeting the sensory and supporting cells of the auditory and vestibular systems require the agent to gain entry to the fluid spaces of the inner ear, perilymph or endolymph, which surround the sensory organs. Access to the inner ear fluids from the vasculature is limited by the blood-labyrinth barriers, which include the blood-perilymph and blood-strial barriers. Intratympanic applications provide an alternative approach in which drugs are applied locally. Drug from the applied solution enters perilymph through the round window membrane, through the stapes, and under some circumstances, through thin bone in the otic capsule. The amount of drug applied to the middle ear is always substantially more than the amount entering perilymph. As a result, significant amounts of the applied drug can pass to the digestive system, to the vasculature, and to the brain. Drugs in perilymph pass to the vasculature and to cerebrospinal fluid via the cochlear aqueduct. Conversely, drugs applied to cerebrospinal fluid, including those given intrathecally, can enter perilymph through the cochlear aqueduct. Other possible routes in or out of the ear include passage by neuronal pathways, passage via endolymph and the endolymphatic sac, and possibly via lymphatic pathways. A better understanding of the pathways for drug movements in and out of the ear will enable better intervention strategies.

Ultra-high-field (9.4 T) MRI Analysis of Contrast Agent Transport Across the Blood-Perilymph Barrier and Intrastrial Fluid-Blood Barrier in the Mouse Inner Ear

HYPOTHESIS

Effective paramagnetic contrast agent for the penetration of the perilymphatic spaces of the scala tympani, scala vestibuli, and scala media of the mouse inner ear can be determined using intravenous injection of various gadolinium (Gd) complexes and ultra-high-field magnetic resonance imaging (MRI) at 9.4 Tesla.

BACKGROUND

A number of contrast agents have been explored in experimental high-field MRI to determine the most effective Gd complex for ideal signal-to-noise ratio and maximal visualization of the in vivo mammalian inner ear in analyzing the temporal and spatial parameters involved in drug penetration of the blood-perilymph barrier and intrastrial fluid-blood barrier in the mouse model using MRI.

METHODS

Gadoteric acid (Dotarem), Gadobutrol (Gadovist), Gadodiamide (Omniscan), Gadopent acid (Magnevist), and Mangafodipir (Teslascan) were administered intravenously using the tail vein of 60 Balb/C mice. High-resolution T1 images of drug penetration were acquired with a horizontal 9.4 T Agilent magnet after intravenously injection. Signal intensity was used as a metric of temporal and spatial parameters of drug delivery and penetration of the perilymphatic and endolymphatic spaces.

RESULTS

ANOVA analysis of the area under the curve of intensity enhancement in perilymph revealed a significant difference (p < 0.05) in the scalae uptake using different contrast agents (F (3,25) = 3.54, p = 0.029). The Gadoteric acid complex Dotarem was found to be the most effective Gd compound in terms of rapid, morphological enhancement for analysis of the temporal, and spatial distribution in the perilymphatic space of the inner ear.

CONCLUSION

Gadoteric acid (Dotarem) demonstrated efficacy as a contrast agent for enhanced visualization of the perilymphatic spaces of the inner ear labyrinthine in the mouse, including the scala tympani and scala vestibuli of the cochlea, and the semicircular canals of the vestibular apparatus. These findings may inform the clinical application of Gd compounds in patients with inner ear fluid disorders and vertigo.

Pharmacokinetic principles in the inner ear: Influence of drug properties on intratympanic applications

Local drug delivery to the ear has gained wide clinical acceptance, with the choice of drug and application protocol in humans largely empirically-derived. Here, we review the pharmacokinetics underlying local therapy of the ear using the drugs commonly used in clinical practice as examples. Based on molecular properties and perilymph measurements interpreted through computer simulations we now better understand the principles underlying entry and distribution of these and other drugs in the ear. From our analysis, we have determined that dexamethasone-phosphate, a pro-drug widely-used clinically, has molecular and pharmacokinetic properties that make it ill-suited for use as a local therapy for hearing disorders. This polar form of dexamethasone, used as a more soluble agent in intravenous preparations, passes less readily through lipid membranes, such as those of the epithelia restricting entry at the round window membrane and stapes. Once within the inner ear, dexamethasone-phosphate is cleaved to the active form, dexamethasone, which is less polar, passes more readily through lipid membranes of the blood-perilymph barrier and is rapidly eliminated from perilymph without distributing to apical cochlear regions. Dexamethasone-phosphate therefore provides only a brief exposure of the basal regions of the cochlea to active drug. Other steroids, such as triamcinolone-acetonide, exhibit pharmacokinetic properties more appropriate to the ear and merit more detailed consideration.

Is oval window transport a royal gate for nanoparticle delivery to vestibule in the inner ear?

Drug delivery to the inner ear by nanomedicine strategies has emerged as an effective therapeutic approach for the management of inner ear diseases including hearing and balance disorders. It is well accepted that substance enters the perilymph from the middle ear through the round window membrane (RWM), but the passage through the oval window (OW) has long been neglected. Up to now, researchers still know little about the pathway via which nanoparticles (NPs) enter the inner ear or how they reach the inner ear following local applications. Herein, we engineered fluorescence traceable chitosan (CS) NPs, investigated the NP distribution within cochlear and vestibular organs, and assessed the availability of RWM and OW pathways to NP transport. Intriguingly, there were high levels of CS NPs in vestibular hair cells, dark cells and supporting cells, but negligible ones in cochlear hair cells and epithelial cells after intratympanic administration. However, the NPs were visualized in two cell models, L929 and HEI-OC1 cell lines, and in the hair cells of cochlear explants after co-incubation in vitro. These combined studies implied that CS NPs might enter the vestibule directly through the OW and then preferentially accumulated in the cells of vestibular organs. Thus, in vivo studies were carried out and clearly revealed that CS NPs entered the inner ear through both the RWM and OW, but the latter played a governing role in delivering NPs to the vestibule with vivid fluorescence signals in the thin bone of the stapes footplate. Overall, these findings firstly suggested that the OW, as a royal gate, afforded a convenient access to facilitate CS NPs transport into inner ear, casting a new light on future clinical applications of NPs in the effective treatment of vestibular disorders by minimizing the risk of hearing loss associated with cochlear hair cell pathology.

Hearing Changes After Intratympanically Applied Steroids for Primary Therapy of Sudden Hearing Loss: A Meta-analysis Using Mathematical Simulations of Drug Delivery Protocols

OBJECTIVE

Controlled and uncontrolled studies with primary intratympanic or combined intratympanic and systemic application of glucocorticosteroids for idiopathic sudden hearing loss were analyzed by means of a meta-analysis in an attempt to establish optimal local drug delivery protocols.

STUDY DESIGN

A total of 25 studies with 28 treatment groups between January 2000 and June 2014 were selected that adequately described drug delivery protocols. Cochlear drug levels were calculated by a validated computer model of drug dispersion in the inner ear fluids based on the concentration and volume of glucocorticoids applied, the time the drug remained in the middle ear, and the specific timing of injections. Various factors were compared with hearing outcome, including baseline data, individual parameters of the application protocols, calculated peak concentration (Cmax), and total dose (area under the curve).

RESULTS

There was no dependence of hearing outcome on individual parameters of the application protocol, Cmax, or area under the curve. Final hearing threshold was notably independent of delay of treatment.

CONCLUSION

During primary intratympanic or combined steroid therapy of idiopathic sudden hearing loss, the tendency toward early treatment having a positive effect on hearing improvement is thought to be a "sham effect," likely related to spontaneous recovery. Change in pure-tone average may not be an adequate outcome parameter to assess effectiveness of the intervention, as it depends on the degree of initial hearing loss. Final pure-tone average provides a better alternative.

Autophagy-related protein 12 associates with anti-apoptotic B cell lymphoma-2 to promote apoptosis in gentamicin-induced inner ear hair cell loss

The aim of the present study was to investigate the underlying mechanisms of autophagy in a gentamicin (GM)-induced ototoxic model, and to establish whether the blocking of autophagy significantly increases the survival of inner ear hair cells. Cochleae were carefully dissected from four day‑old C57BL/6J mice and randomly divided into three groups prior to explant culture: Control (culture medium), GM‑treated (culture medium + GM) and GM + 3-methyladenine (3-MA; culture medium + GM + 3‑MA). Transmission electron microscopy, immunofluorescence and western blotting were performed to observe the expression of the autophagy protein microtubule‑associated protein 1A/B‑light chain 3 in explant cultures treated with GM and the autophagy inhibitor 3‑MA. Administration of GM in in vitro mouse cochlear culture induced apoptosis and the formation of autophagic vesicles and autophagosomes in hair cells. Notably, combined treatment with GM and 3‑MA to block autophagy significantly increased the survival of inner ear hair cells. Furthermore, it was indicated that the simultaneous expression and interaction of Atg12 with Bcl‑2 following GM treatment co‑integrated autophagy with apoptosis in the cochlea. The results of the present study demonstrated that autophagy was involved in GM-induced ototoxicity. Additionally, Atg12 may serve a protective role by binding to Bcl‑2. Therefore, Atg12 may be a potential therapeutic target for the treatment of GM-induced cochlear hair loss.

Gentamicin Applied to the Oval Window Suppresses Vestibular Function in Guinea Pigs

Intratympanic gentamicin therapy is widely used clinically to treat the debilitating symptoms of Ménière's disease. Cochleotoxicity is an undesirable potential side effect of the treatment and the risk of hearing loss increases proportionately with gentamicin concentration in the cochlea. It has recently been shown that gentamicin is readily absorbed through the oval window in guinea pigs. The present study uses quantitative functional measures of vestibular and cochlea function to investigate the efficacy of treating the vestibule by applying a small volume of gentamicin onto the stapes footplate in guinea pigs. Vestibular and cochlea function were assessed by recording short latency vestibular evoked potentials in response to linear head acceleration and changes in hearing threshold, respectively, 1 and 2 weeks following treatment. Histopathology was analyzed in the crista ampullaris of the posterior semi-circular canal and utricular macula in the vestibule, and in the basal and second turns of the cochlea. In animals receiving gentamicin on the stapes footplate, vestibular responses were significantly suppressed by 72.7 % 2 weeks after treatment with no significant loss of hearing. This suggests that the vestibule can be treated directly by applying gentamicin onto the stapes footplate.

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.

Trans-Oval-Window Implants, A New Approach for Drug Delivery to the Inner Ear: Extended Dexamethasone Release From Silicone-based Implants

HYPOTHESIS

The purpose of this study was to develop a new strategy to deliver drugs to the inner ear from dexamethasone (DXM)-loaded silicone implants and to evaluate the distribution of the drug in the cochlea with confocal microscopy.

BACKGROUND

Systemic drug administration for the treatment of inner ear disorders is tricky because of the blood-cochlear barrier, a difficult anatomical access, the small size of the cochlea, and can cause significant adverse effects. An effective way to overcome these obstacles is to administer drugs locally.

METHODS

In vitro, the drug release from DXM-loaded silicone-based thin films and tiny implants into artificial perilymph was thoroughly analyzed by high-performance liquid chromatography. In vivo, a silicone implant loaded with 10% DXM and 5% polyethylene glycol 400 was implanted next to the stapes's footplate of gerbils. Delivery of DXM into the inner ear was proved by confocal microscopy imaging of the whole cochlea and the organ of Corti.

RESULTS

The study showed a continuous and prolonged release during 90 days in vitro. This was confirmed by confocal microscopy that allowed detection of DXM by fluorescence labeling in the cell body of the hair cells for at least 30 days. Interestingly, fluorescence was already observed after 20 minutes of implantation, reached a climax at day 7, and could still be detected 30 days after implantation.

CONCLUSIONS

Thus, we developed a new device for local corticosteroids delivery into the oval window with an extended drug release of DXM to the inner ear.

The correlation between symptoms of definite Meniere's disease and endolymphatic hydrops visualized by magnetic resonance imaging

OBJECTIVE/HYPOTHESIS

This study aimed at investigating the correlation between a battery of diagnostic symptoms of definite Meniere's disease (MD) and the degree of endolymphatic hydrops (EH) in the inner ear.

STUDY DESIGN

Prospective study.

METHODS

Fifty-four patients diagnosed with unilateral definite MD were enrolled in the study. The hearing levels of all patients at the low, middle, and high frequencies were evaluated. Three-dimensional fluid-attenuated inversion recovery (3D-FLAIR) and three-dimensional real inversion recovery (3D-real IR) magnetic resonance imaging (MRI) were performed 24 hours after bilateral intratympanic injection of gadolinium to assess the presence and grading of EH.

RESULTS

Various degrees of EH were observed in the vestibule and/or each turn of the cochlea in the affected ears of all patients. The duration of MD disease and low-tone and middle-tone hearing thresholds were proportional to the extent of EH in the vestibule and cochlear. However, no significant correlation was demonstrated between EH and other aspects of symptoms such as high-tone hearing loss, tinnitus, and aural fullness. Of all subjects, 16.7% exhibited bilateral EH on MRI exam who were diagnosed with unilateral MD based on diagnostic criteria.

CONCLUSIONS

EH in the inner ear of MD patients exhibits a progressive deteriorative trend over time. Low-tone and middle-tone hearing thresholds can indirectly reflect the severity of EH in the cochlea. EH may not be the primary cause of tinnitus and aural fullness in patients with MD. Patients diagnosed with unilateral MD should maintain long-term regular follow-ups for the possibility of developing bilateral EH.

LEVEL OF EVIDENCE

4.

Cisplatin-Induced Ototoxicity: Effects, Mechanisms and Protection Strategies

Cisplatin is a highly effective chemotherapeutic agent that is widely used to treat solid organ malignancies. However, serious side effects have been associated with its use, such as bilateral, progressive, irreversible, dose-dependent neurosensory hearing loss. Current evidence indicates that cisplatin triggers the production of reactive oxygen species in target tissues in the inner ear. A variety of agents that protect against cisplatin-induced ototoxicity have been successfully tested in cell culture and animal models. However, many of them interfere with the therapeutic effect of cisplatin, and therefore are not suitable for systemic administration in clinical practice. Consequently, local administration strategies, namely intratympanic administration, have been developed to achieve otoprotection, without reducing the antitumoral effect of cisplatin. While a considerable amount of pre-clinical information is available, clinical data on treatments to prevent cisplatin ototoxicity are only just beginning to appear. This review summarizes clinical and experimental studies of cisplatin ototoxicity, and focuses on understanding its toxicity mechanisms, clinical repercussions and prevention strategies.

Hearing function after intratympanic application of gadolinium-based contrast agent: A long-term evaluation

OBJECTIVES/HYPOTHESIS

The aim of this study was to evaluate the long-term influence of intratympanic gadolinium-based contrast agent on hearing function in patients with possible Ménière's disease and normal auditory thresholds who were undergoing locally enhanced magnetic resonance imaging scans.

STUDY DESIGN

Prospective observational cohort study in a tertiary referral university hospital ear, nose, and throat department.

METHODS

Between 2009 and 2012, 17 patients with possible or probable Ménière's disease and a four-tone pure-tone average baseline of <25 dB were recruited for our study. Before undergoing intratympanic injection of gadolinium-based contrast agent, all patients underwent a complete audiological evaluation. The study population was then invited back after at least 6 months postinjection for a follow-up auditory evaluation. This consisted of comprehensive clinical and audiological tests on both sides and were evaluated according to the ototoxicity guidelines.

RESULTS

A long-term evaluation of our study group revealed no significant difference in the air-conduction pure-tone average. Furthermore, no statistical difference at individual frequencies compared to baseline was found. There was no evidence of ototoxicity in the injected ear.

CONCLUSIONS

Long-term hearing function assessment after intratympanic application of gadolinium-based agent showed no evidence of ototoxicity. The use of intratympanic gadolinium-based agent in the diagnosis of Ménièrés disease is currently a helpful tool, and seems to be a safe method, especially with regard to auditory function.

LEVEL OF EVIDENCE

4.

Diverse patterns of perilymphatic space enhancement in the rat inner ear after intratympanic injection of two different types of gadolinium: a 9.4-tesla magnetic resonance study

OBJECTIVE

To compare the quality of perilymphatic enhancement in the rat inner ear after intratympanic injection of two types of gadolinium with a 9.4-tesla micro-MRI.

MATERIALS AND METHODS

Gadolinium was injected into the middle ear in 6 Sprague-Dawley rats via the transtympanic route. The left ear was injected with Gd-DO3A-butrol first, and then the right ear was injected with Gd-DOTA. MR images of the inner ear were acquired 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4 h after intratympanic (IT) injection using an Agilent MRI system 9.4T/160/AS. The normalized signal intensity was quantitatively analyzed at the scala vestibuli (SV), scala media, and scala tympani (ST) using a Marosis M-view system. Then the normalized signal intensities (SIs) were compared between the two contrast agents.

RESULTS

For Gd-DO3A-butrol, the SI was as low as 1.0-1.5 throughout 1-4 h at the SV and ST of the basal turn. The maximum SI was 1.5 ± 0.5 at the SV (2 h) and 1.3 ± 0.5 at the ST (2 h). For Gd-DOTA, the 1-hour postinjection SI at the basal turn was 2.5 ± 0.5 at the SV, 1.6 ± 0.3 at the ST, and 1.2 ± 0.3 at the scala media. In the apical turn, the maximum SI was reached after 2.5 h. The maximum SI in the apical turn was 1.8 ± 0.4 at the SV (3.5 h), 1.8 ± 0.4 at the ST (4 h), and 1.4 ± 0.3 at the scala media (4 h).

CONCLUSIONS

We were able to clearly visualize and separate the ST and SV using IT Gd and 9.4-tesla micro-MRI. We recommend using Gd-DO3A-butrol over Gd-DOTA and to perform the MRI 2.5 h after using IT Gd in the rat inner ear.

Magnetic resonance imaging of the middle and inner ear after intratympanic injection of a gadolinium-containing gel

OBJECTIVE

To investigate the distribution and elimination of a gadolinium containing high viscosity formulation of sodium hyaluronan (HYA gel) after injection to the middle ear.

MATERIALS AND METHODS

The T1 contrast agent gadolinium-diethylenetriamine pentaacetic acid-bis methylamine (Gd-DTPA-BMA) was added to HYA gel and delivered to the middle ear of 13 albino guinea pigs by 3 different ways of injection. Magnetic resonance imaging was performed with a 4.7 T MRI system using a T1-weighted 3-dimentional rapid acquisition with relaxation enhancement sequence.

RESULTS

An injection technique where the Gd-DTPA-BMA-containing HYA gel was delivered to the middle ear through a percutaneous injection through the auditory bulla after a small incision had been made in the tympanic membrane gave the best filling of the middle ear, covering the cochlea and the region of the round window niche for 24 hours in a majority of the ears studied. Ears injected without an incision in the tympanic membrane showed an immediate uptake of Gd-DTPA-BMA in the inner ear as a sign of rupture of the round window membrane.

CONCLUSION

A percutaneous injection of a HYA gel into the tympanic bulla is distributed in a predictable way and gives a good filling of the middle ear cavity. The HYA gel remains in close vicinity to the RWM for more than 24 hours. Injection should be performed after an incision of the tympanic membrane has been made to prevent rupture of the round window membrane.

The predominant vestibular uptake of gadolinium through the oval window pathway is compromised by endolymphatic hydrops in Ménière's disease

OBJECTIVE

Intratympanic administration of gentamicin is becoming a common therapy to control vertigo in Ménière's disease (MD). Drug entry through an oval window pathway was recently demonstrated in rats. The present study aimed to evaluate the permeability of the oval window to gadolinium in MD patients.

METHODS

Eight patients with MD and other inner ear diseases received a transtympanic injection of 5-fold diluted gadopentetate dimeglumine (Gd-DTPA). Three-dimensional fluid-attenuated inversion-recovery (3D-FLAIR) and 3D inversion recovery turbo spin echo with real reconstruction (3D-real IR) imaging was performed using a 3-Tesla MRI scanner at 3, 6, 12, and 24 h after injection. The extent of vestibular endolymphatic hydrops (EH) was determined, and the dynamics of the signal changes in the vestibule and the cochlear basal turn were evaluated.

RESULTS

Transtympanically injected Gd-DTPA entered the cochlea of all 8 of the patients and entered the vestibule of 7 of the 8 patients. EH was demonstrated in 2 patients with MD and the patient with idiopathic sensorineural hearing loss or idiopathic vertigo. In the patients lacking EH, the vestibules exhibited more efficient uptake of Gd-DTPA than did the cochleae. In the patients with vestibular EH, the Gd-DTPA signal in the vestibule was less intense than that in the cochlear basal turn.

CONCLUSION

In patients with inner ear diseases, the vestibular distribution of Gd-DTPA was compromised by vestibular EH, suggesting oval window passage of Gd-DTPA is reduced in the presence of EH.

Nanomedicine strategies for drug delivery to the ear

The highly compartmentalized anatomy of the ear aggravates drug delivery, which is used to combat hearing-related diseases. Novel nanosized drug vehicles are thought to overcome the limitations of classic approaches. In this article, we summarize the nanotechnology-based efforts involving nano-objects, such as liposomes, polymersomes, lipidic nanocapsules and poly(lactic-co-glycolic acid) nanoparticles, as well as nanocoatings of implants to provide an efficient means for drug transfer in the ear. Modern strategies do not only enhance drug delivery efficiency, in the inner ear these vector systems also aim for specific uptake into hair cells and spiral ganglion neurons. These novel peptide-mediated strategies for specific delivery are reviewed in this article. Finally, the biosafety of these vector systems is still an outstanding issue, since long-term application to the ear has not yet been assessed.

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.

Influence of cochleostomy and cochlear implant insertion on drug gradients following intratympanic application in Guinea pigs

Locally applied drugs can protect residual hearing following cochlear implantation. The influence of cochlear implantation on drug levels in the scala tympani (ST) after round window application was investigated in guinea pigs using the marker trimethylphenylammonium (TMPA) measured in real time with TMPA-selective microelectrodes. TMPA concentration in the upper basal turn of the ST rapidly increased during implantation and then declined due to cerebrospinal fluid entering the ST at the cochlear aqueduct and exiting at the cochleostomy. The TMPA increase was found to be caused by the cochleostomy drilling if the burr tip partially entered the ST. TMPA distribution in the second turn was less affected by implantation procedures. These findings show that basal turn drug levels may be changed during implantation and the changes may need to be considered in the interpretation of therapeutic effects of drugs in conjunction with implantation.

Gentamicin administration on the stapes footplate causes greater hearing loss and vestibulotoxicity than round window administration in guinea pigs

Clinically, gentamicin has been used extensively to treat the debilitating symptoms of Mèniére's disease and is well known for its vestibulotoxic properties. Until recently, it was widely accepted that the round window membrane (RWM) was the primary entry route into the inner ear following intratympanic drug administration. In the current study, gentamicin was delivered to either the RWM or the stapes footplate of guinea pigs (GPs) to assess the associated hearing loss and histopathology associated with each procedure. Vestibulotoxicity of the utricular macula, saccular macula, and crista ampullaris in the posterior semicircular canal were assessed quantitatively with density counts of hair cells, supporting cells, and stereocilia in histological sections. Cochleotoxicity was assessed quantitatively by changes in threshold of auditory brainstem responses (ABR), along with hair cell and spiral ganglion cell counts in the basal and second turns of the cochlea. Animals receiving gentamicin applied to the stapes footplate exhibited markedly higher levels of hearing loss between 8 and 32 kHz, a greater reduction of outer hair cells in the basal turn of the cochlea and fewer normal type I cells in the utricle in the vestibule than those receiving gentamicin on the RWM or saline controls. This suggests that gentamicin more readily enters the ear when applied to the stapes footplate compared with RWM application. These data provide a potential explanation for why gentamicin preferentially ablates vestibular function while preserving hearing following transtympanic administration in humans.

MRI evidence of endolymphatic impermeability to the gadolinium molecule in the in vivo mouse inner ear at 9.4 tesla

OBJECTIVE

Previous in vivo experimental magnetic resonance imaging (MRI) investigations of the mammalian inner ear at 4.7 Tesla have indicated that intravenously injected gadolinium (Gd) penetrates the perilymphatic labyrinth, but not the endolymphatic membranous labyrinth. In the present study, high field MRI at 9.4T was used to visualize the in vivo mouse vestibulo-cochlea system, and to determine whether the endolymphatic system is permeable to a Gd complex.

METHODS

A 9.4 T Varian magnet equipped with a 12 cm inner diameter gradient system with maximum gradient strength of 600 mT/m, a millipede coil (Varian design) and a Gd contrast agent were used for image acquisition in the normal C57 BL-6 mouse.

RESULTS

High-resolution 2D and 3D images of the mouse cochlea were acquired within 80 minutes following intravenous injection of Gd. Gd initially permeated the perilymphatic scala tympani and scala vestibuli, and permitted visualization of both cochlear turns from base to apex. The superior, inferior and lateral semicircular canals were subsequently visualized in 3 planes. The membranous endolymphatic labyrinth was impermeable to intravenously injected Gd, and thus showed no apparent uptake of Gd at 9.4T.

CONCLUSION

The 9.4T field strength MRI permitted acquisition of high resolution images of anatomical and physiological features of the normal, wild type mouse perilymphatic inner ear in vivo, and provided further evidence that the endolymphatic system is impermeable to intravenously injected Gd.