Novel slow- and fast-type drug release round-window microimplants for local drug application to the cochlea: an experimental study in guinea pigs

A state-of-the-art analysis of pharmacological delivery and artificial intelligence techniques for inner ear disease treatment

Over the last several decades, both the academic and therapeutic fields have seen significant progress in the delivery of drugs to the inner ear due to recent delivery methods established for the systemic administration of drugs in inner ear treatment. Novel technologies such as nanoparticles and hydrogels are being investigated, in addition to the traditional treatment methods. Intracochlear devices, which utilize current developments in microsystems technology, are on the horizon of inner ear drug delivery methods and are designed to provide medicine directly into the inner ear. These devices are used for stem cell treatment, RNA interference, and the delivery of neurotrophic factors and steroids during cochlear implantation. An in-depth analysis of artificial neural networks (ANNs) in pharmaceutical research may be found in ANNs for Drug Delivery, Design, and Disposition. This prediction tool has a great deal of promise to assist researchers in more successfully designing, developing, and delivering successful medications because of its capacity to learn and self-correct in a very complicated environment. ANN achieved a high level of accuracy exceeding 0.90, along with a sensitivity of 95% and a specificity of 100%, in accurately distinguishing illness. Additionally, the ANN model provided nearly perfect measures of 0.99%. Nanoparticles exhibit potential as a viable therapeutic approach for bacterial infections that are challenging to manage, such as otitis media. The utilization of ANNs has the potential to enhance the effectiveness of nanoparticle therapy, particularly in the realm of automated identification of otitis media. Polymeric nanoparticles have demonstrated effectiveness in the treatment of prevalent bacterial infections in pediatric patients, suggesting significant potential for forthcoming therapeutic interventions. Finally, this study is based on a research of how inner ear diseases have been treated in the last ten years (2012-2022) using machine learning.

Drug Delivery across Barriers to the Middle and Inner Ear

The prevalence of ear disorders has spurred efforts to develop drug delivery systems to treat these conditions. Here, recent advances in drug delivery systems that access the ear through the tympanic membrane (TM) are reviewed. Such methods are either non-invasive (placed on the surface of the TM), or invasive (placed in the middle ear, ideally on the round window [RW]). The major hurdles to otic drug delivery are identified and highlighted the representative examples of drug delivery systems used for drug delivery across the TM to the middle and (crossing the RW also) inner ear.

[Topical drug administration to the inner ear. Modern state of the problem and development perspectives]

The work assessed modern methods of drug delivery through biological barriers to the lesion, in particular, through the most studied - skin. The main advantages and disadvantages of the existing methods for the topical administration of drugs into the inner ear - intra-imperial and intra-labyrinth delivery are analyzed. A brief review of medicinal substances for topical administration to the inner ear, both widely used (for example, aminoglycosides, steroid drugs) and undergoing clinical trials, is given. An assessment is made of the prospects for the use of transmembrane drug delivery to the inner ear using an electric field, which has a combined electro-creative and iontophoretic effect.

A Polymer-Based Extended Release System for Stable, Long-term Intracochlear Drug Delivery

OBJECTIVE

Investigate a new polymer-based drug coating suitability for safe intracochlear delivery and ability to maintain long-term physiologically active levels of the corticosteroid fluticasone propionate.

STUDY DESIGN

In vitro dissolution study to evaluate release profiles of polymer-coated drug particles and in vivo studies using a guinea pig model to measure perilymph drug concentrations at specific time points after implantation with polymer-coated drug particles and evaluate their effect on hearing function.

METHODS

Polymer-coated fluticasone propionate (FP) particles were surgically implanted in guinea pigs through the round window membrane into the cochlear scala tympani. In the pilot study, pre- and post-op hearing thresholds were conducted on days 7, 14, and 42. In a second study, post-op hearing thresholds were conducted on days 90, 120, and 180. Perilymph drug concentrations were measured on the same time points.

RESULTS

In 15 of 16 animals from day 7 through day 90, drug levels were within the targeted range, with no initial burst release detected. Drug was present in all animals on day 90 and was detected in some animals at 120 and 180 days. Hearing was tested and compared with non-implanted ears. Very good hearing preservation was observed in ears implanted with intracochlear particles when compared with contralateral ears.

CONCLUSIONS

The polymer-based extended release system is effective in providing long-term, stable drug delivery for at least 90 days with good hearing outcomes. The results of this study support the potential for achieving long-term drug delivery with a single intracochlear administration.

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.

Reduction of permanent hearing loss by local glucocorticoid application : Guinea pigs with acute acoustic trauma

BACKGROUND

High-intensity noise exposure from impulse and blast noise events often leads to acute hearing loss and may cause irreversible permanent hearing loss as a long-term consequence. Here, a treatment regime was developed to limit permanent damage based on a preclinical animal model of acute noise trauma.

AIM

To develop clinical trials for the treatment of acute noise traumas using approved drugs. The otoprotective potential of glucocorticoids applied locally to the inner ear was examined.

MATERIALS AND METHODS

A series of experiments with different impulse noise exposures were performed. Permanent hearing loss and hair cell density were assessed 14 days after exposure. Hearing and hair cell preservation were investigated as a function of the glucocorticoid dose.

RESULTS

After impulse noise exposure, local application to the round window of the cochlea of high-dose prednisolone (25 mg/ml) or methylprednisolone (12.5 mg/ml) resulted in a statistically significant reduction in hearing loss compared with the control group.

CONCLUSION

The local application of high doses of the drugs to the round window of the cochlea appears to be an effective treatment for acute noise trauma.

Intratympanic steroid delivery by an indwelling catheter in refractory severe sudden sensorineural hearing loss

OBJECTIVE

Many studies over the last decade showed favorable outcomes with intratympanic (IT) steroid treatment, alone as salvage treatment or in combination with conventional systemic therapy (ST). However, in severe to profound sensorineural hearing loss resistant to ST, the optimal infusion mode, the type and concentration of the solution, the preferable drug, its total amount, and the duration and fractionation of the treatment are still debated. Aim of the study was to investigate the feasibility and the outcomes of a direct and constant IT delivery of dexamethasone (DEX) by means of a new indwelling catheter.

METHODS

A prospective case-control study in a tertiary referral university hospital. Ninety-nine subjects treated with ST only and 28 with additional IT DEX have been included in the study. A 4 Fr catheter inserted in a sub-annular fashion with a minimal postero-inferior tympanotomy through and endocanalar approach under local anesthesia. DEX 4mg/ml delivered daily, up to 7 days. Daily bone and air-conducted pure tone and speech audiometry were performed with a follow-up at 1, 3, 6 months after treatment.

RESULTS

Twenty-one out of 28 patients (75%) refractory to ST gained on average 24.0dB±20.5dB HL after IT-DEX, compared to 35.4% (average 6.7dB±16.6dB HL) of those receiving only medical ST (p<0.001). No significant side effects were noted.

CONCLUSION

In severe to profound sudden deafness refractory to conventional ST, the daily perfusion of 4mg/ml DEX through an intratympanic catheter is an easy, well accepted procedure that enables patients to receive a drug in the middle ear in a repeatable or sustained form, with minimal discomfort and a partial rescue (67.86%) and a speech recognition gain of 39%.

Pharmacokinetic Properties of Adenosine Amine Congener in Cochlear Perilymph after Systemic Administration

Noise-induced hearing loss (NIHL) is a global health problem affecting over 5% of the population worldwide. We have shown previously that acute noise-induced cochlear injury can be ameliorated by administration of drugs acting on adenosine receptors in the inner ear, and a selective A₁ adenosine receptor agonist adenosine amine congener (ADAC) has emerged as a potentially effective treatment for cochlear injury and resulting hearing loss. This study investigated pharmacokinetic properties of ADAC in rat perilymph after systemic (intravenous) administration using a newly developed liquid chromatography-tandem mass spectrometry detection method. The method was developed and validated in accordance with the USA FDA guidelines including accuracy, precision, specificity, and linearity. Perilymph was sampled from the apical turn of the cochlea to prevent contamination with the cerebrospinal fluid. ADAC was detected in cochlear perilymph within two minutes following intravenous administration and remained in perilymph above its minimal effective concentration for at least two hours. The pharmacokinetic pattern of ADAC was significantly altered by exposure to noise, suggesting transient changes in permeability of the blood-labyrinth barrier and/or cochlear blood flow. This study supports ADAC development as a potential clinical otological treatment for acute sensorineural hearing loss caused by exposure to traumatic noise.

In vitro and in vivo pharmacokinetic study of a dexamethasone-releasing silicone for cochlear implants

Cochlear implants have been widely used for patients with profound hearing loss and partial deafness. Residual low-frequency hearing, however, may deteriorate due to insertion trauma and tissue response around the electrode array. The present study investigated in vitro and in vivo release of dexamethasone from silicone used for cochlear implant electrode carriers. The in vitro experiment involved an apparatus simulating the inner ear fluid environment in humans. Release from two sizes of silicone films (200 µm × 1 mm × 10 mm and 500 µm × 1 mm × 10 mm), each loaded with 2 % dexamethasone, and was measured for 24 weeks. In the in vivo experiment, silicone rods loaded with 2 or 10 % dexamethasone, respectively, were implanted into the scala tympani of guinea pigs. Perilymph concentrations were measured during the first week after implantation. The results showed that dexamethasone was released from the silicone in a sustained manner. After a burst release, perilymph concentration was similar for silicone incorporated with 2 and 10 % dexamethasone, respectively. The similar pharmacokinetic profile was found in the in vitro experiment. The period of sustained drug delivery was maintained for 20 weeks in vitro and for 1 week in vivo. The results of the present study suggest that drugs like dexamethasone are released in a controlled manner from silicon electrode carriers of cochlear implants. Further studies will identify optimal release profiles for the use with cochlear implants to improve their safety and long-term performance.

Therapeutic designed poly (lactic-co-glycolic acid) cylindrical oseltamivir phosphate-loaded implants impede tumor neovascularization, growth and metastasis in mouse model of human pancreatic carcinoma

Poly (lactic-co-glycolic acid) (PLGA) copolymers have been extensively used in cancer research. PLGA can be chemically engineered for conjugation or encapsulation of drugs in a particle formulation. We reported that oseltamivir phosphate (OP) treatment of human pancreatic tumor-bearing mice disrupted the tumor vasculature with daily injections. Here, the controlled release of OP from a biodegradable PLGA cylinder (PLGA-OP) implanted at tumor site was investigated for its role in limiting tumor neovascularization, growth, and metastasis. PLGA-OP cylinders over 30 days in vitro indicated 20%–25% release profiles within 48 hours followed by a continuous metronomic low dose release of 30%–50% OP for an additional 16 days. All OP was released by day 30. Surgically implanted PLGA-OP containing 20 mg OP and blank PLGA cylinders at the tumor site of heterotopic xenografts of human pancreatic PANC1 tumors in RAGxCγ double mutant mice impeded tumor neovascularization, growth rate, and spread to the liver and lungs compared with the untreated cohort. Xenograft tumors from PLGA and PLGA-OP-treated cohorts expressed significant higher levels of human E-cadherin with concomitant reduced N-cadherin and host CD31⁺ endothelial cells compared with the untreated cohort. These results clearly indicate that OP delivered from PLGA cylinders surgically implanted at the site of the solid tumor show promise as an effective treatment therapy for cancer.

Effects of a dexamethasone-releasing implant on cochleae: A functional, morphological and pharmacokinetic study

AIM

This study evaluated the impact of a dexamethasone-releasing silicone implant on hearing function preservation, cochlear morphology and perilymph pharmacokinetics after cochlear implantation.

METHODS

Guinea pigs were implanted unilaterally with silicone rods containing either 2% dexamethasone (DEXA group, n = 18) or no dexamethasone (control group, n = 17). Auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAEs) were measured preoperatively and over 6 months postoperatively. Cochlear histology using standard hematoxylin and eosin (H&E) staining and tumor necrosis factor (TNF)-alpha staining was performed 1 month postoperatively. Twenty-two guinea pigs were involved in the pharmacokinetic study, and real-time drug concentrations in perilymph were investigated using high-performance liquid chromatography (HPLC). The Mann-Whitney U test (1-tailed) was used for statistical analyses.

RESULTS

ABR and DPOAE testing demonstrated decreased hearing function immediately postoperatively followed by a progressive hearing loss within the first day postoperatively. There was almost no observable hearing improvement in the control group from 1 week to 6 months postoperatively, but hearing levels in the DEXA group improved gradually from 1 week to 12 weeks. Hearing loss in the DEXA and control group was 5.0 ± 3.4 dB and 21.7 ± 5.3 dB, respectively at a 16-kHz stimulus frequency 6 months postoperatively. The difference in threshold shifts was present throughout all measured frequencies, and it was significant at 4-24 kHz. The morphological study revealed new fibrosis formation in the scala tympani, which encapsulated the implanted electrode. TNF-alpha positive staining in the cochleae of the DEXA group was less evident than the control group. The pharmacokinetic study revealed a peak perilymph concentration 30 min postoperatively and sustained dexamethasone release at least 1 week postoperatively.

CONCLUSION

Cochlear implants that incorporate dexamethasone can release drug chronically in the inner ear and induce significant long-term recovery and preservation of auditory function after implantation.

A microfluidic reciprocating intracochlear drug delivery system with reservoir and active dose control

Reciprocating microfluidic drug delivery, as compared to steady or pulsed infusion, has unique features which may be advantageous in many therapeutic applications. We have previously described a device, designed for wearable use in small animal models, that periodically infuses and then withdraws a sub-microliter volume of drug solution to and from the endogenous fluid of the inner ear. This delivery approach results in zero net volume of liquid transfer while enabling mass transport of compounds to the cochlea by means of diffusion and mixing. We report here on an advanced wearable delivery system aimed at further miniaturization and complex dosing protocols. Enhancements to the system include the incorporation of a planar micropump to generate reciprocating flow and a novel drug reservoir that maintains zero net volume delivery and permits programmable modulation of the drug concentration in the infused bolus. The reciprocating pump is fabricated from laminated polymer films and employs a miniature electromagnetic actuator to meet the size and weight requirements of a head-mounted in vivo guinea pig testing system. The reservoir comprises a long microchannel in series with a micropump, connected in parallel with the reciprocating flow network. We characterized in vitro the response and repeatability of the planar pump and compared the results with a lumped element simulation. We also characterized the performance of the reservoir, including repeatability of dosing and range of dose modulation. Acute in vivo experiments were performed in which the reciprocating pump was used to deliver a test compound to the cochlea of anesthetized guinea pigs to evaluate short-term safety and efficacy of the system. These advances are key steps toward realization of an implantable device for long-term therapeutic applications in humans.

Noninvasive technique for monitoring drug transport through the murine cochlea using micro-computed tomography

Local delivery of drugs to the inner ear has the potential to treat inner ear disorders including permanent hearing loss or deafness. Current mathematical models describing the pharmacokinetics of drug delivery to the inner ear have been based on large rodent studies with invasive measurements of concentration at few locations within the cochlea. Hence, estimates of clearance and diffusion parameters are based on fitting measured data with limited spatial resolution to a model. To overcome these limitations, we developed a noninvasive imaging technique to monitor and characterize drug delivery inside the mouse cochlea using micro-computed tomography (μCT). To increase the measurement accuracy, we performed a subject-atlas image registration to exploit the information readily available in the atlas image of the mouse cochlea and pass segmentation or labeling information from the atlas to our μCT scans. The approach presented here has the potential to quantify concentrations at any point along fluid-filled scalae of the inner ear. This may permit determination of spatially dependent diffusion and clearance parameters for enhanced models.

A safety evaluation of dexamethasone-releasing cochlear implants: comparative study on the risk of otogenic meningitis after implantation

CONCLUSION

Dexamethasone released from a cochlear implant seems not to enhance the risk for postoperative infections.

OBJECTIVE

Dexamethasone has a positive impact on hearing preservation for electric acoustic stimulation (EAS). Due to their antiproliferative and immunosuppressive properties, steroids may enhance the risk of postoperative infections. A comparative study was performed to evaluate the risk of pneumococcal meningitis after implantation of dexamethasone-eluting cochlear implants.

METHODS

Thirty guinea pigs were implanted with non-eluting (n = 15) or dexamethasone-eluting (n = 15) cochlear implant electrode dummies. After 5 weeks, animals were exposed to a virulent strain of Streptococcus pneumoniae. The two groups were compared based on the meningitis rate. Animals were observed for 5 days for signs of meningitis. Meningitis was verified by clinical outcome as well as by pleocytosis and presence of bacteria in cerebrospinal fluid. Results were confirmed by histological examination of brains and cochleae, clinical findings and culture.

RESULTS

There was no significant difference in meningitis risk between the two groups. In the group with non-eluting implants, 3 of 15 animals developed meningitis, while in the group with dexamethasone-eluting implants 4 of 15 showed signs of meningitis. In this study dexamethasone-releasing implants did not significantly increase the risk of postoperative pneumococcal otogenic meningitis.

Dexamethasone levels and base-to-apex concentration gradients in the scala tympani perilymph after intracochlear delivery in the guinea pig

HYPOTHESIS

To determine whether intracochlearly applied dexamethasone will lead to better control of drug levels, higher peak concentrations, and lower base-to-apex concentration gradients in the scala tympani (ST) of the guinea pig than after intratympanic (round window [RW]) application.

BACKGROUND

Local application of drugs to the RW results in substantial variation of intracochlear drug levels and significant base-to-apex concentration gradients in ST.

METHODS

Two microliters of dexamethasone-phosphate (10 mg/ml) were injected into ST either through the RW membrane, which was covered with 1% sodium hyaluronate gel or through a cochleostomy with a fluid tight seal of the micropipette. Perilymph was sequentially sampled from the apex at a single time point for each animal, at 20, 80, or 200 min after the injection ended. Results were mathematically interpreted by means of an established computer model and compared with previous experiments performed by our group with the same experimental techniques but using intratympanic applications.

RESULTS

Single intracochlear injections of 20 minutes resulted in approximately 10 times higher peak concentrations (on average) than 2 to 3 hours of intratympanic application to the RW niche. Intracochlear drug levels were less variable and could be measured for over 220 minutes. Concentration gradients along the scala tympani were less pronounced. The remaining variability in intracochlear drug levels was attributable to perilymph and drug leak from the injection site.

CONCLUSION

With significantly higher, less variable drug levels and smaller base-to-apex concentration gradients, intracochlear applications have advantages to intratympanic injections. For further development of this technique, it is of importance to control leaks of perilymph and drug from the injection site and to evaluate its clinical feasibility and associated risks.

Development of a specially tailored local drug delivery system for the prevention of fibrosis after insertion of cochlear implants into the inner ear

A cochlear implant (CI)-associated local drug delivery system based on dexamethasone (DMS) was developed with the purpose to inhibit the growth of fibrotic tissue which influences the signal transmission from the CI to the neurons of the inner ear. For the realization of a targeted DMS delivery the following concepts were combined: modification of the silicone-based electrode carrier by incorporation of DMS and a DMS-containing polymeric coating chemically attached on the surface of the electrode carrier. It was demonstrated that the coated CI showed a high coating stability in a simulated implantation procedure. The in vitro drug release studies in a quasi-stationary model revealed a faster DMS release in the initial phase originating from the DMS-containing coatings and then a lower and sustained DMS release originating from the DMS-loaded silicone carrier. The performed in vitro biocompatibility study confirmed that the released DMS was non-toxic for cultured spiral ganglion cells.

Intracochlear drug delivery systems

INTRODUCTION

Advances in molecular biology and in the basic understanding of the mechanisms associated with sensorineural hearing loss and other diseases of the inner ear are paving the way towards new approaches for treatments for millions of patients. However, the cochlea is a particularly challenging target for drug therapy, and new technologies will be required to provide safe and efficacious delivery of these compounds. Emerging delivery systems based on microfluidic technologies are showing promise as a means for direct intracochlear delivery. Ultimately, these systems may serve as a means for extended delivery of regenerative compounds to restore hearing in patients suffering from a host of auditory diseases.

AREAS COVERED

Recent progress in the development of drug delivery systems capable of direct intracochlear delivery is reviewed, including passive systems such as osmotic pumps, active microfluidic devices and systems combined with currently available devices such as cochlear implants. The aim of this article is to provide a concise review of intracochlear drug delivery systems currently under development and ultimately capable of being combined with emerging therapeutic compounds for the treatment of inner ear diseases.

EXPERT OPINION

Safe and efficacious treatment of auditory diseases will require the development of microscale delivery devices, capable of extended operation and direct application to the inner ear. These advances will require miniaturization and integration of multiple functions, including drug storage, delivery, power management and sensing, ultimately enabling closed-loop control and timed-sequence delivery devices for treatment of these diseases.

Kinetics of reciprocating drug delivery to the inner ear

Reciprocating drug delivery is a means of delivering soluble drugs directly to closed fluid spaces in the body via a single cannula without an accompanying fluid volume change. It is ideally suited for drug delivery into small, sensitive and unique fluid spaces such as the cochlea. We characterized the pharmacokinetics of reciprocating drug delivery to the scala tympani within the cochlea by measuring the effects of changes in flow parameters on the distribution of drug throughout the length of the cochlea. Distribution was assessed by monitoring the effects of DNQX, a reversible glutamate receptor blocker, delivered directly to the inner ear of guinea pigs using reciprocating flow profiles. We then modeled the effects of those parameters on distribution using both an iterative curve-fitting approach and a computational fluid dynamic model. Our findings are consistent with the hypothesis that reciprocating delivery distributes the drug into a volume in the base of the cochlea, and suggest that the primary determinant of distribution throughout more distal regions of the cochlea is diffusion. Increases in flow rate distributed the drug into a larger volume that extended more apically. Over short time courses (less than 2h), the apical extension, though small, significantly enhanced apically directed delivery of drug. Over longer time courses (>5h) or greater distances (>3mm), maintenance of drug concentration in the basal scala tympani may prove more advantageous for extending apical delivery than increases in flow rate. These observations demonstrate that this reciprocating technology is capable of providing controlled delivery kinetics to the closed fluid space in the cochlea, and may be suitable for other applications such as localized brain and retinal delivery.

Round window perfusion dynamics: implications for intracochlear therapy

PURPOSE OF REVIEW

The treatments for inner ear diseases are evolving as the systemic administration of medication is replaced by novel intratympanic and intracochlear drug delivery. The current review explores the background and recent developments in this field.

RECENT FINDINGS

Although still in various stages of clinical development, novel drug delivery techniques such as the Silverstein MicroWick, the round window microcatheter, biodegradable hydrogels, biopolymers, nanoparticles, newly designed cochlear implant arrays, osmotic mini/micro pumps, and reciprocating perfusion systems hold significant promise. Animal data suggest that sustained delivery systems have more reliable inner ear pharmacokinetics than both systemic administration and intratympanic injections.

SUMMARY

As research scientists advance technologies for treating inner ear diseases, drug delivery techniques must keep pace. Viable treatment options for sensorineural hearing loss, tinnitus, and vestibular disorders are on the horizon and may usher in a new golden age for otology.

Distribution of dexamethasone and preservation of inner ear function following intratympanic delivery of a gel-based formulation

Intratympanic (IT) delivery of drugs to the ear is increasingly used for both clinical and research purposes. One limitation of IT delivery is that drugs are rapidly lost from the middle ear by a number of processes, so that prolonged delivery of drug is technically difficult. In the present study, the delivery characteristics of a poloxamer hydrogel formulation containing dexamethasone (dex) were evaluated. The gel is liquid at room temperature, allowing IT injection, but transitions to a gel at body temperature, providing a prolonged residence time in the middle ear. A 50-μl volume of control or dex-containing gel (dex-gel) was injected through the tympanic membrane of guinea pigs. Cochlear function was assessed with cochlear action potential and acoustic emission thresholds measured immediately, 6 or 24 h after IT gel injection. After 6- or 24-hour treatment with dex-gel, perilymph drug gradients along the cochlea were assessed by taking samples sequentially from the apex, and endolymph was sampled from the basal turn. Control gel injections caused small changes in sound field calibrations and functional measures for low-frequency stimuli, consistent with an induced conductive loss. Within 24 h, responses returned to normal. Twenty-four hours after dex-gel injection, low-frequency changes remained as the dex-gel was retained better in the middle ear, but there was no indication of high-frequency loss. While perilymph sample data showed that dex gradients were substantially lower than after single injections of dex solution, quantitative analysis of this result suggests that some dex may have entered the perilymph through the thin bone in the apical region of the cochlea. Endolymph levels of dex remained lower than those in the perilymph. This study confirms that a poloxamer hydrogel-based dex formulation provides an effective method for a prolonged delivery, providing a more uniform distribution of drug in the inner ear.

Visualization and analysis of superparamagnetic iron oxide nanoparticles in the inner ear by light microscopy and energy filtered TEM

Nanoparticles as potential carriers for local drug transfer are an alternative to systemic drug delivery into the inner ear. We report on the first in vitro tests of a new ferrogel consisting of superparamagnetic iron oxide nanoparticles (SPIONs) and a Pluronic(®) F127 (PF127) copolymer. Pluronic copolymers possess a unique viscosity-adjustable property that makes PF127 gels easy to handle compared to conventional cross-linked hydrogels. This ferrogel was successfully tested in cadaver human temporal bones as well as in organotypic explant cultures of mouse inner ears. SPIONs were identified by light microscopy and localized with different imaging modes in energy-filtered transmission electron microscopy. Our approach shows a promising possibility to use iron oxide nanoparticles, which are suitable for visualization and characterization at both the light- and electron-microscopic levels.

FROM THE CLINICAL EDITOR

The authors report the first in vitro tests of a new ferrogel consisting of superparamagnetic iron oxide nanoparticles (SPIONs) and a Pluronic® F127 (PF127) copolymer for drug delivery in the inner ear, demonstrasting a promising possibility to use iron oxide nanoparticles, which are suitable for visualization and characterization at both the light- and electron-microscopic levels.

Principles of local drug delivery to the inner ear

As more and more substances have been shown in preclinical studies to be capable of preventing damage to the inner ear from exposure to noise, ototoxic drugs, ischemia, infection, inflammation, mechanical trauma and other insults, it is becoming very important to develop feasible and safe methods for the targeted delivery of drugs to specific regions in the inner ear. Recently developed methods for sampling perilymph from the cochlea have overcome major technical problems that have distorted previous pharmacokinetic studies of the ear. These measurements show that drug distribution in perilymph is dominated by passive diffusion, resulting in large gradients along the cochlea when drugs are applied intratympanically. Therefore, in order to direct drugs to specific regions of the ear, a variety of delivery strategies are required. To target drugs to the basal cochlear turn and vestibular system while minimizing exposure of the apical cochlear turns, single one-shot intratympanic applications are effective. To increase the amount of drug reaching the apical cochlear turns, repeated intratympanic injections or controlled-release drug delivery systems, such as biodegradable biopolymers or catheters and pumps, are more effective. However, if the applied substance does not easily pass through the round window membrane, or if a more widespread distribution of drug in the ear is required, then intralabyrinthine injections of the substance may be required. Intralabyrinthine injection procedures, which are currently in development in animals, have not yet been proven safe enough for human use.

Randomized, double blind, placebo controlled trial on the safety and efficacy of continuous intratympanic dexamethasone delivered via a round window catheter for severe to profound sudden idiopathic sensorineural hearing loss after failure of systemic therapy

OBJECTIVES

To study the safety and efficacy of continuous intratympanic dexamethasone-phosphate (Dex-P) for severe to profound sudden idiopathic sensorineural hearing (ISSHL) or sudden idiopathic anacusis after failure of systemic therapy.

STUDY DESIGN

Randomized, double-blind, placebo controlled multicenter trial.

METHODS

Patients with ISSHL and insufficient recovery (mean 4PTA = 97 dB HL) after systemic high dose glucocorticoid therapy received either Dex-P (4 mg/ml) or placebo (NaCl 0.9%) continuously applied for 14 days into the round window niche via a temporarily implanted catheter. For ethical reasons, intratympanic treatment was continued with Dex-P in all patients for another 14 days after the placebo-controlled study period. According to a two-step adaptive study design an interim analysis was performed after inclusion of 23 patients.

RESULTS

Intention-to-treat analysis for the primary outcome criterion (4PTA: 0.5-3 kHz) during the placebo controlled study period (14 days) showed an average hearing improvement in the treatment group of 13.9 dB (SD: 21.3) and in the placebo group of 5.4 dB (SD: 10.4). This difference in hearing improvement between the two groups (mean: 8.4 dB, SD: 17.0, 95% CI: -7.1-24.1) was statistically not significant (p = .26). Of the secondary outcome parameters, the largest benefit of local salvage therapy was found for maximum speech discrimination with an improvement of 24.4% (SD: 32.0) in the treatment and 4.5% (SD: 7.6) in the placebo group (p = 0.07). After a 3 month follow-up period (i.e. after all patients received intratympanic Dex-P) hearing improvement in the two groups was very similar. No serious adverse events were observed. Sample size calculation after the interim analysis resulted in stopping of the trial.

CONCLUSIONS

The tendency toward better hearing improvement in the treatment group, the rather conservative inclusion criteria, the limited placebo-controlled observation period and the absence of serious adverse events supports further investigation local inner ear drug delivery as a first or second line treatment option for ISSHL.

Inner ear drug delivery for auditory applications

Many inner ear disorders cannot be adequately treated by systemic drug delivery. A blood-cochlear barrier exists, similar physiologically to the blood-brain barrier, which limits the concentration and size of molecules able to leave the circulation and gain access to the cells of the inner ear. However, research in novel therapeutics and delivery systems has led to significant progress in the development of local methods of drug delivery to the inner ear. Intratympanic approaches, which deliver therapeutics to the middle ear, rely on permeation through tissue for access to the structures of the inner ear, whereas intracochlear methods are able to directly insert drugs into the inner ear. Innovative drug delivery systems to treat various inner ear ailments such as ototoxicity, sudden sensorineural hearing loss, autoimmune inner ear disease, and for preserving neurons and regenerating sensory cells are being explored.

Glucocorticoid impact on cochlear function and systemic side effects in autoimmune C3.MRL-Faslpr and normal C3H/HeJ mice

Glucocorticoids are effective in reversing hearing loss, but their severe side effects limit long term management of many ear disorders. A clearer understanding of these side effects is critical for prolonged therapeutic control of hearing and vestibular dysfunction. Therefore, this study characterized the impact of the glucocorticoid prednisolone on cochlear dysfunction and systemic organ systems in C3.MRL-Fas(lpr) autoimmune mice and their normal C3H/HeJ parent strain. Following 3 months of treatment, autoimmune mice had better auditory thresholds and improved hematocrits, anti-nuclear antibodies, and immune complexes. Steroid treatment also lowered body and spleen weights, both of which rise with systemic autoimmune disease. Steroid treatment of the normal C3H/HeJ mice significantly elevated their blood hematocrits and lowered their body and spleen weights to abnormal levels. Thus, systemic autoimmune disease and its related hearing loss in C3.MRL-Fas(lpr) mice are steroid-responsive, but normal hemopoiesis and organ functions can be significantly compromised. This mouse model may be useful for studies of the detrimental side effects of steroid treatments for hearing loss.

Drug delivery systems for the treatment of sensorineural hearing loss

Sensorineural hearing loss is one of the most common disabilities in our society. Experimentally, many candidates for therapeutic molecules have been discovered. However, the lack of safe and effective methods for drug delivery to the cochlea has been a considerable obstacle to clinical application. Local application of therapeutic molecules into the cochlea has been used in clinic and in animal experiments. Advances in pharmacological technology provide various drug delivery systems via biomaterials, which can be utilized for local drug delivery to the cochlea. Recent studies in the field of otology have demonstrated the potential of synthetic and natural biomaterials for local drug delivery to the cochlea. Although problems still remain to be resolved for clinical application, introduction into clinical practice of these controlled-release systems may be reasonable because of their certain advantages over previous methods.

Simulation of application strategies for local drug delivery to the inner ear

Local, rather than systemic, drug delivery to the inner ear is becoming more widely used to treat inner ear disorders. While many substances are undergoing preclinical and clinical studies, it is equally important to develop appropriate drug delivery systems. Pharmacokinetic studies are technically demanding in animals and almost impossible in humans. Computer simulations have helped establish the basic principles of drug distribution in the inner ear. The distribution of methylprednisolone in the guinea pig cochlea has been simulated for different drug delivery systems based on kinetic parameters established in prior studies. Results were compared for different rates of drug clearance from the middle ear. Absolute and relative drug levels in the perilymph were highly dependent on how long the drug remained in the middle ear. For a brief (30 min) application, the basal to apical drug gradient was higher than for longer delivery times. These findings show that controlling middle ear drug clearance is of critical importance.

Therapeutic efficacy of intra-cochlear administration of methylprednisolone after acoustic trauma caused by gunshot noise in guinea pigs

The therapeutic efficacy of cochlear infusion of methylprednisolone (MP) after an impulse noise trauma (170dB SPL peak) was evaluated in guinea pigs. The compound action potential threshold shifts were measured over a 14 days recovery period after the gunshot exposure. For each animal, one of the cochlea was perfused directly into the scala tympani with MP during 7 days via a mini-osmotic pump, whereas the other cochlea was not pump-implanted. The functional study of hearing was supplemented by histological analysis. Forty eight hours after the trauma, significant differences between auditory threshold shifts in the implanted and non-implanted ears were observed for frequencies above 8kHz. At day 7, the difference was significant for only one frequency and no difference was observed after 14 days recovery. Cochleograms showed that the hair cell losses were significantly lower in the MP treated ears. This work indicates that direct infusion of MP into perilymphatic space accelerates hearing recovery, reduces hair cell losses after impulse noise trauma but does not limit permanent threshold shifts.

Cochlear Protection by Local Insulin-Like Growth Factor-1 Application Using Biodegradable Hydrogel

OBJECTIVE

The aim of this experimental study was to examine the potential of local recombinant human insulin-like growth factor-1 (rhIGF-1) application through a biodegradable hydrogel for the treatment of cochleae.

METHODS

A hydrogel immersed with rhIGF-1 was placed on the round window membrane of Sprague-Dawley rats while a hydrogel immersed with physiological saline was applied to control animals. On day 3 after drug application, the animals were exposed to white noise at 120 dB sound pressure level (SPL) for 2 hours. Cochlear function was monitored using measurements of auditory brain stem responses (ABRs) at frequencies of 8, 16, and 32 kHz. The temporal bones were collected 7 or 30 days after noise exposure and the loss of hair cells was quantitatively analyzed.

RESULTS

Local rhIGF-1 treatment significantly reduced the elevation of ABR thresholds on days 7 and 30 after noise exposure. Histologic analysis revealed that local rhIGF-1 treatment significantly prohibited the loss of outer hair cells.

CONCLUSIONS

These findings demonstrate that local IGF-1 application through the biodegradable hydrogel has the potential for protection of cochleae from noise trauma.

Perilymph sampling from the cochlear apex: a reliable method to obtain higher purity perilymph samples from scala tympani

Measurements of drug levels in the fluids of the inner ear are required to establish kinetic parameters and to determine the influence of specific local delivery protocols. For most substances, this requires cochlear fluids samples to be obtained for analysis. When auditory function is of primary interest, the drug level in the perilymph of scala tympani (ST) is most relevant, since drug in this scala has ready access to the auditory sensory cells. In many prior studies, ST perilymph samples have been obtained from the basal turn, either by aspiration through the round window membrane (RWM) or through an opening in the bony wall. A number of studies have demonstrated that such samples are likely to be contaminated with cerebrospinal fluid (CSF). CSF enters the basal turn of ST through the cochlear aqueduct when the bony capsule is perforated or when fluid is aspirated. The degree of sample contamination has, however, not been widely appreciated. Recent studies have shown that perilymph samples taken through the round window membrane are highly contaminated with CSF, with samples greater than 2microL in volume containing more CSF than perilymph. In spite of this knowledge, many groups continue to sample from the base of the cochlea, as it is a well-established method. We have developed an alternative, technically simple method to increase the proportion of ST perilymph in a fluid sample. The sample is taken from the apex of the cochlea, a site that is distant from the cochlear aqueduct. A previous problem with sampling through a perforation in the bone was that the native perilymph rapidly leaked out driven by CSF pressure and was lost to the middle ear space. We therefore developed a procedure to collect all the fluid that emerged from the perforated apex after perforation. We evaluated the method using a marker ion trimethylphenylammonium (TMPA). TMPA was applied to the perilymph of guinea pigs either by RW irrigation or by microinjection into the apical turn. The TMPA concentration of the fluid sample was compared with that measured in perilymph prior to taking the sample using a TMPA-selective microelectrode sealed into ST. Data were interpreted with a finite element model of the cochlear fluids that was used to simulate each aspect of the experiment. The correction of sample concentration back to the perilymph concentration prior to sampling can be performed based on the known ST volume (4.7microL in the guinea pig) and the sample volume. A more precise correction requires some knowledge of the profile of drug distribution along the cochlear prior to sampling. This method of sampling from the apex is technically simple and provides a larger sample volume with a greater proportion of perilymph compared to sampling through the RW.

Local inner-ear drug delivery and pharmacokinetics

Several drugs that are applied directly to the inner ear are in widespread clinical use for the treatment of inner-ear disorders. Many new substances and drug delivery systems specific to the inner ear are under development and in some cases are being evaluated in animal experiments and in clinical studies. However, the pharmacokinetics of drugs in the inner ear is not well defined and the field is plagued by technical problems in obtaining pure samples of the inner-ear fluids for analysis. Nevertheless, a basic understanding of the mechanisms of drug dispersal in the inner ear has emerged, which facilitates the design and interpretation of future pharmacokinetic studies.