Rhesus Cochlear and Vestibular Functions Are Preserved After Inner Ear Injection of Saline Volume Sufficient for Gene Therapy Delivery

Surgical procedure of intratympanic injection and inner ear pharmacokinetics simulation in domestic pigs

Introduction: One major obstacle in validating drugs for the treatment or prevention of hearing loss is the limited data available on the distribution and concentration of drugs in the human inner ear. Although small animal models offer some insights into inner ear pharmacokinetics, their smaller organ size and different barrier (round window membrane) permeabilities compared to humans can complicate study interpretation. Therefore, developing a reliable large animal model for inner ear drug delivery is crucial. The inner and middle ear anatomy of domestic pigs closely resembles that of humans, making them promising candidates for studying inner ear pharmacokinetics. However, unlike humans, the anatomical orientation and tortuosity of the porcine external ear canal frustrates local drug delivery to the inner ear. Methods: In this study, we developed a surgical technique to access the tympanic membrane of pigs. To assess hearing pre- and post-surgery, auditory brainstem responses to click and pure tones were measured. Additionally, we performed 3D segmentation of the porcine inner ear images and used this data to simulate the diffusion of dexamethasone within the inner ear through fluid simulation software (FluidSim). Results: We have successfully delivered dexamethasone and dexamethasone sodium phosphate to the porcine inner ear via the intratympanic injection. The recorded auditory brainstem measurements revealed no adverse effects on hearing thresholds attributable to the surgery. We have also simulated the diffusion rates for dexamethasone and dexamethasone sodium phosphate into the porcine inner ear and confirmed the accuracy of the simulations using in-vivo data. Discussion: We have developed and characterized a method for conducting pharmacokinetic studies of the inner ear using pigs. This animal model closely mirrors the size of the human cochlea and the thickness of its barriers. The diffusion time and drug concentrations we reported align closely with the limited data available from human studies. Therefore, we have demonstrated the potential of using pigs as a large animal model for studying inner ear pharmacokinetics.

Intrinsic mechanism and pharmacologic treatments of noise-induced hearing loss

Noise accounts for one-third of hearing loss worldwide. Regretfully, noise-induced hearing loss (NIHL) is deemed to be irreversible due to the elusive pathogenic mechanisms that have not been fully elucidated. The complex interaction between genetic and environmental factors, which influences numerous downstream molecular and cellular events, contributes to the NIHL. In clinical settings, there are no effective therapeutic drugs other than steroids, which are the only treatment option for patients with NIHL. Therefore, the need for treatment of NIHL that is currently unmet, along with recent progress in our understanding of the underlying regulatory mechanisms, has led to a lot of new literatures focusing on this therapeutic field. The emergence of novel technologies that modify local drug delivery to the inner ear has led to the development of promising therapeutic approaches, which are currently under clinical investigation. In this comprehensive review, we focus on outlining and analyzing the basics and potential therapeutics of NIHL, as well as the application of biomaterials and nanomedicines in inner ear drug delivery. The objective of this review is to provide an incentive for NIHL's fundamental research and future clinical translation.

Advances in gene therapy hold promise for treating hereditary hearing loss

Gene therapy focuses on genetic modification to produce therapeutic effects or treat diseases by repairing or reconstructing genetic material, thus being expected to be the most promising therapeutic strategy for genetic disorders. Due to the growing attention to hearing impairment, an increasing amount of research is attempting to utilize gene therapy for hereditary hearing loss (HHL), an important monogenic disease and the most common type of congenital deafness. Several gene therapy clinical trials for HHL have recently been approved, and, additionally, CRISPR-Cas tools have been attempted for HHL treatment. Therefore, in order to further advance the development of inner ear gene therapy and promote its broad application in other forms of genetic disease, it is imperative to review the progress of gene therapy for HHL. Herein, we address three main gene therapy strategies (gene replacement, gene suppression, and gene editing), summarizing the strategy that is most appropriate for particular monogenic diseases based on different pathogenic mechanisms, and then focusing on their successful applications for HHL in preclinical trials. Finally, we elaborate on the challenges and outlooks of gene therapy for HHL.

Patient Acceptance of Novel Therapeutic Options for Sensorineural Hearing Loss-A Pilot Study

OBJECTIVES

Numerous preclinical experiments over the past years have shown the potential of novel therapeutic approaches for sensorineural hearing loss (SNHL) that are now awaiting clinical translation. In this pilot study, we aimed to evaluate the patient acceptance of these future innovative therapies in individuals with SNHL.

STUDY DESIGN

Cross-sectional exploratory pilot study.

SETTING

Tertiary care academic hospital.

PATIENTS

In total, 72 individuals (43 female and 29 male, 59 affected subjects and 13 parents) with different types of SNHL were surveyed between May 2020 and November 2020.

INTERVENTION

The interest/willingness to consider new therapeutic options (viral vectors, stem cells, CRISPR/Cas) for themselves or their children was assessed with the help of a questionnaire, and the answers were matched with a quality-of-life score and sociodemographic as well as clinical characteristics.

MAIN OUTCOME MEASURE

Acceptance of new therapeutic strategies for SNHL in a representative population.

RESULTS

Even with the currently associated treatment uncertainties, 48 patients (66.7%) suffering from SNHL stated that new therapies could be a potential future option for them. Half of these (24 individuals; 33.3%) expressed high acceptance toward the novel strategies. Subjects with a positive attitude toward new therapies in general and viral vectors specifically were significantly older.

CONCLUSION

With two-thirds of patients affected by SNHL expressing acceptance toward novel therapies, this pilot study highlights the importance of investigating such attitudes and motivates further translational research to offer additional treatment strategies to this patient population.

Investigation of inner ear drug delivery with a cochlear catheter in piglets as a representative model for human cochlear pharmacokinetics

Hearing impairment is the most common sensory disorder in humans, and yet hardly any medications are licensed for the treatment of inner ear pathologies. Intricate pharmacokinetic examinations to better understand drug distribution within this complex organ could facilitate the development of novel therapeutics. For such translational research projects, animal models are indispensable, but differences in inner ear dimensions and other anatomical features complicate the transfer of experimental results to the clinic. The gap between rodents and humans may be bridged using larger animal models such as non-human primates. However, their use is challenging and impeded by administrative, regulatory, and financial hurdles. Other large animal models with more human-like inner ear dimensions are scarce. In this study, we analyzed the inner ears of piglets as a potential representative model for the human inner ear and established a surgical approach for intracochlear drug application and subsequent apical sampling. Further, controlled intracochlear delivery of fluorescein isothiocyanate-dextran (FITC-d) was carried out after the insertion of a novel, clinically applicable CE-marked cochlear catheter through the round window membrane. Two, six, and 24 hours after a single injection with this device, the intracochlear FITC-d distribution was determined in sequential perilymph samples. The fluorometrically assessed concentrations two hours after injection were compared to the FITC-d content in control groups, which either had been injected with a simple needle puncture through the round window membrane or the cochlear catheter in combination with a stapes vent hole. Our findings demonstrate not only significantly increased apical FITC-d concentrations when using the cochlear catheter but also higher total concentrations in all perilymph samples. Additionally, the concentration decreased after six and 24 hours and showed a more homogenous distribution compared to shorter observation times.

Towards the Clinical Application of Gene Therapy for Genetic Inner Ear Diseases

Hearing loss, the most common human sensory defect worldwide, is a major public health problem. About 70% of congenital forms and 25% of adult-onset forms of deafness are of genetic origin. In total, 136 deafness genes have already been identified and there are thought to be several hundred more awaiting identification. However, there is currently no cure for sensorineural deafness. In recent years, translational research studies have shown gene therapy to be effective against inherited inner ear diseases, and the application of this technology to humans is now within reach. We provide here a comprehensive and practical overview of current advances in gene therapy for inherited deafness, with and without an associated vestibular defect. We focus on the different gene therapy approaches, considering their prospects, including the viral vector used, and the delivery route. We also discuss the clinical application of the various strategies, their strengths, weaknesses, and the challenges to be overcome.

Altering gene expression using antisense oligonucleotide therapy for hearing loss

Hearing loss affects more than 430 million people, worldwide, and is the third most common chronic physical condition in the United States and Europe (GBD Hearing Loss Collaborators, 2021; NIOSH, 2021; WHO, 2021). The loss of hearing significantly impacts motor and cognitive development, communication, education, employment, and overall quality of life. The inner ear houses the sensory organs for both hearing and balance and provides an accessible target for therapeutic delivery. Antisense oligonucleotides (ASOs) use various mechanisms to manipulate gene expression and can be tailor-made to treat disorders with defined genetic targets. In this review, we discuss the preclinical advancements within the field of the highly promising ASO-based therapies for hereditary hearing loss disorders. Particular focus is on ASO mechanisms of action, preclinical studies on ASO treatments of hearing loss, timing of therapeutic intervention, and delivery routes to the inner ear.

Genetic insights, disease mechanisms, and biological therapeutics for Waardenburg syndrome

Waardenburg syndrome (WS), also known as auditory-pigmentary syndrome, is the most common cause of syndromic hearing loss (HL), which accounts for approximately 2-5% of all patients with congenital hearing loss. WS is classified into four subtypes depending on the clinical phenotypes. Currently, pathogenic mutations of PAX3, MITF, SOX10, EDN3, EDNRB or SNAI2 are associated with different subtypes of WS. Although supportive techniques like hearing aids, cochlear implants, or other assistive listening devices can alleviate the HL symptom, there is no cure for WS to date. Recently major progress has been achieved in preclinical studies of genetic HL in animal models, including gene delivery and stem cell replacement therapies. This review focuses on the current understandings of pathogenic mechanisms and potential biological therapeutic approaches for HL in WS, providing strategies and directions for implementing WS biological therapies, as well as possible problems to be faced, in the future.

Toward Personalized Diagnosis and Therapy for Hearing Loss: Insights From Cochlear Implants

ABSTRACT

Sensorineural hearing loss (SNHL) is the most common sensory deficit, disabling nearly half a billion people worldwide. The cochlear implant (CI) has transformed the treatment of patients with SNHL, having restored hearing to more than 800,000 people. The success of CIs has inspired multidisciplinary efforts to address the unmet need for personalized, cellular-level diagnosis, and treatment of patients with SNHL. Current limitations include an inability to safely and accurately image at high resolution and biopsy the inner ear, precluding the use of key structural and molecular information during diagnostic and treatment decisions. Furthermore, there remains a lack of pharmacological therapies for hearing loss, which can partially be attributed to challenges associated with new drug development. We highlight advances in diagnostic and therapeutic strategies for SNHL that will help accelerate the push toward precision medicine. In addition, we discuss technological improvements for the CI that will further enhance its functionality for future patients. This report highlights work that was originally presented by Dr. Stankovic as part of the Dr. John Niparko Memorial Lecture during the 2021 American Cochlear Implant Alliance annual meeting.

Emerging therapies for human hearing loss

INTRODUCTION

More than 5% of the world's population have a disabling hearing loss which can be managed by hearing aids or implanted electrical devices. However, outcomes are highly variable, and the sound perceived by recipients is far from perfect. Sparked by the discovery of progenitor cells in the cochlea and rapid progress in drug delivery to the cochlea, biological and pharmaceutical therapies are currently in development to improve the function of the cochlear implant or eliminate the need for it altogether.

AREAS COVERED

This review highlights progress in emerging regenerative strategies to restore hearing and adjunct therapies to augment the cochlear implant. Novel approaches include the reprogramming of progenitor cells to restore the sensory hair cell population in the cochlea, gene therapy and gene editing to treat hereditary and acquired hearing loss. A detailed review of optogenetics is also presented as a technique that could enable optical stimulation of the spiral ganglion neurons, replacing or complementing electrical stimulation.

EXPERT OPINION

Increasing evidence of substantial reversal of hearing loss in animal models, alongside rapid advances in delivery strategies to the cochlea and learnings from clinical trials will amalgamate into a biological or pharmaceutical therapy to replace or complement the cochlear implant.

Choice of vector and surgical approach enables efficient cochlear gene transfer in nonhuman primate

Inner ear gene therapy using adeno-associated viral vectors (AAV) promises to alleviate hearing and balance disorders. We previously established the benefits of Anc80L65 in targeting inner and outer hair cells in newborn mice. To accelerate translation to humans, we now report the feasibility and efficiency of the surgical approach and vector delivery in a nonhuman primate model. Five rhesus macaques were injected with AAV1 or Anc80L65 expressing eGFP using a transmastoid posterior tympanotomy approach to access the round window membrane after making a small fenestra in the oval window. The procedure was well tolerated. All but one animal showed cochlear eGFP expression 7–14 days following injection. Anc80L65 in 2 animals transduced up to 90% of apical inner hair cells; AAV1 was markedly less efficient at equal dose. Transduction for both vectors declined from apex to base. These data motivate future translational studies to evaluate gene therapy for human hearing disorders.

Gene therapy using Adeno-associated viral vectors (AAV) rescues hearing and balance deficits in mouse models of human disorders. Here, the authors show that AAVAnc80L65 allows efficient cochlear gene transfer in nonhuman primates, and motivate future studies to evaluate gene therapy for hearing and balance disorders.

Efficient delivery of adeno-associated virus (AAV) into inner ear in vivo via trans-stapes route in adult guinea pig

Adeno-associated virus (AAV) are potent vectors to achieve treatment against hearing loss resulting from genetic defects. However, the effects of delivery routes and the corresponding transduction efficiencies for clinical applications remain elusive. Here, we screened AAV vectors of three serotypes (AAV 8, 9 and Anc80L65) into the inner ears of adult normal guinea pigs through trans-stapes (oval window) and trans-round window delivery routes in vivo. Trans-stapes route is akin to stape surgeries in humans. Then, auditory brainstem response (ABR) measurements were conducted to evaluate postoperative hearing, and inner ear tissues were harvested for transduction efficiency analysis. Results showed that AAV8 targeted partial inner hair cells (IHCs) in cochlear basal turn; AAV9 targeted IHCs in cochlear basal and second turn, also a part of vestibular hair cells (VHCs). In contrast, Anc80L65 contributed to GFP signals of 80%-95% IHCs and 67%-91% outer hair cells (OHCs), as well as 69% VHCs via the trans-round window route, with 15-20 dB ABR thresholds shifts. And, via trans-stapes (oval window) route, there were 71%-90% IHCs and 42%-81% OHCs, along with 64% VHCs demonstrating GFP positive, and the ABR thresholds shifts were within 10 dB. This study revealed AAV could be efficiently delivered into mammalian inner ear cells in vivo via trans-stapes (oval window) route with postoperative hearing preservation, and both delivery routes showed promise of virus-based clinical translation of hearing impairment treatment.

AAV-S: A versatile capsid variant for transduction of mouse and primate inner ear

Gene therapy strategies using adeno-associated virus (AAV) vectors to treat hereditary deafnesses have shown remarkable efficacy in some mouse models of hearing loss. Even so, there are few AAV capsids that transduce both inner and outer hair cells-the cells that express most deafness genes-and fewer still shown to transduce hair cells efficiently in primates. AAV capsids with robust transduction of inner and outer hair cells in primate cochlea will be needed for most clinical trials. Here, we test a capsid that we previously isolated from a random capsid library, AAV-S, for transduction in mouse and non-human primate inner ear. In both mice and cynomolgus macaques, AAV-S mediates highly efficient reporter gene expression in a variety of cochlear cells, including inner and outer hair cells, fibrocytes, and supporting cells. In a mouse model of Usher syndrome type 3A, AAV-S encoding CLRN1 robustly and durably rescues hearing. Overall, our data indicate that AAV-S is a promising candidate for therapeutic gene delivery to the human inner ear.

Stem Cells and Gene Therapy in Progressive Hearing Loss: the State of the Art

Progressive non-syndromic sensorineural hearing loss (PNSHL) is the most common cause of sensory impairment, affecting more than a third of individuals over the age of 65. PNSHL includes noise-induced hearing loss (NIHL) and inherited forms of deafness, among which is delayed-onset autosomal dominant hearing loss (AD PNSHL). PNSHL is a prime candidate for genetic therapies due to the fact that PNSHL has been studied extensively, and there is a potentially wide window between identification of the disorder and the onset of hearing loss. Several gene therapy strategies exist that show potential for targeting PNSHL, including viral and non-viral approaches, and gene editing versus gene-modulating approaches. To fully explore the potential of these therapy strategies, a faithful in vitro model of the human inner ear is needed. Such models may come from induced pluripotent stem cells (iPSCs). The development of new treatment modalities by combining iPSC modeling with novel and innovative gene therapy approaches will pave the way for future applications leading to improved quality of life for many affected individuals and their families.

Gene Therapy to the Retina and the Cochlea

Vision and hearing disorders comprise the most common sensory disorders found in people. Many forms of vision and hearing loss are inherited and current treatments only provide patients with temporary or partial relief. As a result, developing genetic therapies for any of the several hundred known causative genes underlying inherited retinal and cochlear disorders has been of great interest. Recent exciting advances in gene therapy have shown promise for the clinical treatment of inherited retinal diseases, and while clinical gene therapies for cochlear disease are not yet available, research in the last several years has resulted in significant advancement in preclinical development for gene delivery to the cochlea. Furthermore, the development of somatic targeted genome editing using CRISPR/Cas9 has brought new possibilities for the treatment of dominant or gain-of-function disease. Here we discuss the current state of gene therapy for inherited diseases of the retina and cochlea with an eye toward areas that still need additional development.

The DNA methylation inhibitor RG108 protects against noise-induced hearing loss

Background

Noise-induced hearing loss represents a commonly diagnosed type of hearing disability, severely impacting the quality of life of individuals. The current work is aimed at assessing the effects of DNA methylation on noise-induced hearing loss.

Methods

Blocking DNA methyltransferase 1 (DNMT1) activity with a selective inhibitor RG108 or silencing DNMT1 with siRNA was used in this study. Auditory brainstem responses were measured at baseline and 2 days after trauma in mice to assess auditory functions. Whole-mount immunofluorescent staining and confocal microcopy of mouse inner ear specimens were performed to analyze noise-induced damage in cochleae and the auditory nerve at 2 days after noise exposure.

Results

The results showed that noise exposure caused threshold elevation of auditory brainstem responses and cochlear hair cell loss. Whole-mount cochlea staining revealed a reduction in the density of auditory ribbon synapses between inner hair cells and spiral ganglion neurons. Inhibition of DNA methyltransferase activity via a non-nucleoside specific pharmacological inhibitor, RG108, or silencing of DNA methyltransferase-1 with siRNA significantly attenuated ABR threshold elevation, hair cell damage, and the loss of auditory synapses.

Conclusions

This study suggests that inhibition of DNMT1 ameliorates noise-induced hearing loss and indicates that DNMT1 may be a promising therapeutic target.

Graphical abstract

Steady streaming as a method for drug delivery to the inner ear

The inner ear, or cochlea, is a fluid-filled organ housing the mechanosensitive hair cells. Sound stimulation is relayed to the hair cells through waves that propagate on the elastic basilar membrane. Sensorineural hearing loss occurs from damage to the hair cells and cannot currently be cured. Although drugs have been proposed to prevent damage or restore functionality to hair cells, a difficulty with such treatments is ensuring adequate drug delivery to the cells. Because the cochlea is encased in the temporal bone, it can only be accessed from its basal end. However, the hair cells that are responsible for detecting speech-frequency sounds reside at the opposite, apical end. In this paper we show that steady streaming can be used to transport drugs along the cochlea. Steady streaming is a nonlinear process that accompanies many fluctuating fluid motions, including the sound-evoked waves in the inner ear. We combine an analytical approximation for the waves in the cochlea with computational fluid dynamic simulations to demonstrate that the combined steady streaming effects of several different frequencies can transport drugs from the base of the cochlea further towards the apex. Our results therefore show that multi-frequency sound stimulation can serve as a non-invasive method to transport drugs efficiently along the cochlea.

Breaking the sound barrier: Towards next-generation AAV vectors for gene therapy of hearing disorders

Owing to the advances in transgenic animal technology and the advent of the next-generation sequencing era, over 120 genes causing hereditary hearing loss have been identified by now. In parallel, the field of human gene therapy continues to make exciting and rapid progress, culminating in the recent approval of several ex vivo and in vivo applications. Despite these encouraging developments and the growing interest in causative treatments for hearing disorders, gene therapeutic interventions in the inner ear remain in their infancy and await clinical translation. This review focuses on the adeno-associated virus (AAV), which nowadays represents one of the safest and most promising vectors in gene therapy. We first provide an overview of AAV biology and outline the principles of therapeutic gene transfer with recombinant AAV vectors, before pointing out major challenges and solutions for clinical translation including vector manufacturing and species translatability. Finally, we highlight seminal technologies for engineering and selection of next-generation "designer" AAV capsids, and illustrate their power and potential with recent examples of their application for inner ear gene transfer in animals.

Adeno-associated virus gene replacement for recessive inner ear dysfunction: Progress and challenges

Approximately 3 in 1000 children in the US under 4 years of age are affected by hearing loss. Currently, cochlear implants represent the only line of treatment for patients with severe to profound hearing loss, and there are no targeted drug or biological based therapies available. Gene replacement is a promising therapeutic approach for hereditary hearing loss, where viral vectors are used to deliver functional cDNA to "replace" defective genes in dysfunctional cells in the inner ear. Proof-of-concept studies have successfully used this approach to improve auditory function in mouse models of hereditary hearing loss, and human clinical trials are on the immediate horizon. The success of this method is ultimately determined by the underlying biology of the defective gene and design of the treatment strategy, relying on intervention before degeneration of the sensory structures occurs. A challenge will be the delivery of a corrective gene to the proper target within the therapeutic window of opportunity, which may be unique for each specific defective gene. Although rescue of pre-lingual forms of recessive deafness have been explored in animal models thus far, future identification of genes with post-lingual onset that are amenable to gene replacement holds even greater promise for treatment, since the therapeutic window is likely open for a much longer period of time. This review summarizes the current state of adeno-associated virus (AAV) gene replacement therapy for recessive hereditary hearing loss and discusses potential challenges and opportunities for translating inner ear gene replacement therapy for patients with hereditary hearing loss.

Inner Ear Gene Delivery: Vectors and Routes

Objectives

Current treatments for hearing loss offer some functional improvements in hearing, but do not restore normal hearing. The aim of this review is to highlight recent advances in viral and non-viral vectors for gene therapy and to discuss approaches for overcoming barriers inherent to inner ear delivery of gene products.

Data Sources

The databases used were Medline, EMBASE, Web of Science, and Google Scholar. Search terms were [("cochlea*" or "inner ear" or "transtympanic" or "intratympanic" or "intracochlear" or "hair cells" or "spiral ganglia" or "Organ of Corti") and ("gene therapy" or "gene delivery")]. The references section of resulting articles was also used to identify relevant studies.

Results

Both viral and non-viral vectors play important roles in advancing gene delivery to the inner ear. The round window membrane is one significant barrier to gene delivery that intratympanic delivery methods attempt to overcome through diffusion and intracochlear delivery methods bypass completely.

Conclusions

Gene therapy for hearing loss is a promising treatment for restoring hearing function by addressing innate defects. Recent technological advances in inner ear drug delivery techniques pose exciting opportunities for progress in gene therapy.

Diagnostic and therapeutic applications of genomic medicine in progressive, late-onset, nonsyndromic sensorineural hearing loss

The progressive, late-onset, nonsyndromic, sensorineural hearing loss (PNSHL) is the most common cause of sensory impairment globally, with presbycusis affecting greater than a third of individuals over the age of 65. The etiology underlying PNSHL include presbycusis, noise-induced hearing loss, drug ototoxicity, and delayed-onset autosomal dominant hearing loss (AD PNSHL). The objective of this article is to discuss the potential diagnostic and therapeutic applications of genomic medicine in PNSHL. Genomic factors contribute greatly to PNSHL. The heritability of presbycusis ranges from 25 to 75%. Current therapies for PNSHL range from sound amplification to cochlear implantation (CI). PNSHL is an excellent candidate for genomic medicine approaches as it is common, has well-described pathophysiology, has a wide time window for treatment, and is amenable to local gene therapy by currently utilized procedural approaches. AD PNSHL is especially suited to genomic medicine approaches that can disrupt the expression of an aberrant protein product. Gene therapy is emerging as a potential therapeutic strategy for the treatment of PNSHL. Viral gene delivery approaches have demonstrated promising results in human clinical trials for two inherited causes of blindness and are being used for PNSHL in animal models and a human trial. Non-viral gene therapy approaches are useful in situations where a transient biologic effect is needed or for delivery of genome editing reagents (such as CRISPR/Cas9) into the inner ear. Many gene therapy modalities that have proven efficacious in animal trials have potential to delay or prevent PNSHL in humans. The development of new treatment modalities for PNSHL will lead to improved quality of life of many affected individuals and their families.

Preclinical testing of AAV9-PHP.B for transgene expression in the non-human primate cochlea

In a number of mouse models of hereditary deafness, therapeutic transgene delivery to the cochlea and vestibular organs using adeno-associated viral vectors (AAVs) has shown striking rescue of hearing and balance. However, only a subset of AAV capsids have shown efficacy in transducing both inner hair cells and outer hair cells, and it is also not clear which of these can be translated to treatment of human inner ear. We recently reported efficient transgene expression of a GFP reporter in a non-human primate cochlea, in both inner and outer hair cells, following injection of the AAV9 capsid variant PHP.B via the round window membrane (RWM). However efficiency was poor at a lower dose. To further define the transduction potential of AAV9-PHP.B, we have performed a dosing study in the cynomolgus monkey and assessed vector-encoded GFP expression. Three animals were injected in both ears and four doses were tested. We describe a transmastoid surgical approach needed to access the RWM of this common primate model. We found that AAV9-PHP.B transduced nearly 100% of both IHCs and OHCs, from base to apex, at the higher doses (3.5 × 10¹¹ and 7 × 10¹¹ vector genomes). However, at lower doses there was a steep reduction in viral transduction. Thus, AAV9-PHP.B efficiently transduces the IHCs and OHCs of nonhuman primates, and should be considered as an AAV capsid for inner ear gene therapy in humans.

Viral vectors for gene delivery to the inner ear

Gene therapy using virus vectors to treat hereditary diseases has made remarkable progress in the past decade. There are FDA-approved products for ex-vivo gene therapy for diseases such as immunodeficiencies (e.g., SCID), and in vivo gene therapy for a rare blindness and neuro-muscular disease. Gene therapy for hereditary hearing loss has picked up pace in the past five years due to progress in understanding disease gene function as well as the development of better technologies such as adeno-associated virus (AAV) vectors, to deliver nucleic acid to target cells in the inner ear. This review has two major goals. One is to review the state of the art for investigators already working in preclinical cochlear gene therapy. The other is to present the language of vectorology and important considerations for designing and using AAV vectors to inner ear neurobiologists who might use AAV vectors in the cochlea for either therapeutic or basic biological applications.

rAAV-Mediated Cochlear Gene Therapy: Prospects and Challenges for Clinical Application

Over the last decade, pioneering molecular gene therapy for inner-ear disorders have achieved experimental hearing improvements after a single local or systemic injection of adeno-associated, virus-derived vectors (rAAV for recombinant AAV) encoding an extra copy of a normal gene, or ribozymes used to modify a genome. These results hold promise for treating congenital or later-onset hearing loss resulting from monogenic disorders with gene therapy approaches in patients. In this review, we summarize the current state of rAAV-mediated inner-ear gene therapies including the choice of vectors and delivery routes, and discuss the prospects and obstacles for the future development of efficient clinical rAAV-mediated cochlear gene medicine therapy.

The use of nonhuman primates in studies of noise injury and treatment

Exposure to prolonged or high intensity noise increases the risk for permanent hearing impairment. Over several decades, researchers characterized the nature of harmful noise exposures and worked to establish guidelines for effective protection. Recent laboratory studies, primarily conducted in rodent models, indicate that the auditory system may be more vulnerable to noise-induced hearing loss (NIHL) than previously thought, driving renewed inquiries into the harmful effects of noise in humans. To bridge the translational gaps between rodents and humans, nonhuman primates (NHPs) may serve as key animal models. The phylogenetic proximity of NHPs to humans underlies tremendous similarity in many features of the auditory system (genomic, anatomical, physiological, behavioral), all of which are important considerations in the assessment and treatment of NIHL. This review summarizes the literature pertaining to NHPs as models of hearing and noise-induced hearing loss, discusses factors relevant to the translation of diagnostics and therapeutics from animals to humans, and concludes with some of the practical considerations involved in conducting NHP research.

Dye Tracking Following Posterior Semicircular Canal or Round Window Membrane Injections Suggests a Role for the Cochlea Aqueduct in Modulating Distribution

The inner ear houses the sensory epithelium responsible for vestibular and auditory function. The sensory epithelia are driven by pressure and vibration of the fluid filled structures in which they are embedded so that understanding the homeostatic mechanisms regulating fluid dynamics within these structures is critical to understanding function at the systems level. Additionally, there is a growing need for drug delivery to the inner ear for preventive and restorative treatments to the pathologies associated with hearing and balance dysfunction. We compare drug delivery to neonatal and adult inner ear by injection into the posterior semicircular canal (PSCC) or through the round window membrane (RWM). PSCC injections produced higher levels of dye delivery within the cochlea than did RWM injections. Neonatal PSCC injections produced a gradient in dye distribution; however, adult distributions were relatively uniform. RWM injections resulted in an early base to apex gradient that became more uniform over time, post injection. RWM injections lead to higher levels of dye distributions in the brain, likely demonstrating that injections can traverse the cochlea aqueduct. We hypothesize the relative position of the cochlear aqueduct between injection site and cochlea is instrumental in dictating dye distribution within the cochlea. Dye distribution is further compounded by the ability of some chemicals to cross inner ear membranes accessing the blood supply as demonstrated by the rapid distribution of gentamicin-conjugated Texas red (GTTR) throughout the body. These data allow for a direct evaluation of injection mode and age to compare strengths and weaknesses of the two approaches.

Toward the Optical Cochlear Implant

When hearing fails, cochlear implants (CIs) provide open speech perception to most of the currently half a million CI users. CIs bypass the defective sensory organ and stimulate the auditory nerve electrically. The major bottleneck of current CIs is the poor coding of spectral information, which results from wide current spread from each electrode contact. As light can be more conveniently confined, optical stimulation of the auditory nerve presents a promising perspective for a fundamental advance of CIs. Moreover, given the improved frequency resolution of optical excitation and its versatility for arbitrary stimulation patterns the approach also bears potential for auditory research. Here, we review the current state of the art focusing on the emerging concept of optogenetic stimulation of the auditory pathway. Developing optogenetic stimulation for auditory research and future CIs requires efforts toward viral gene transfer to the neurons, design and characterization of appropriate optogenetic actuators, as well as engineering of multichannel optical implants.

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.

Nonhuman primate vestibuloocular reflex responses to prosthetic vestibular stimulation are robust to pulse timing errors caused by temporal discretization

Electrical stimulation of vestibular afferent neurons to partially restore semicircular canal sensation of head rotation and the stabilizing reflexes that sensation supports has potential to effectively treat individuals disabled by bilateral vestibular hypofunction. Ideally, a vestibular implant system using this approach would be integrated with a cochlear implant, which would provide clinicians with a means to simultaneously treat loss of both vestibular and auditory sensation. Despite obvious similarities, merging these technologies poses several challenges, including stimulus pulse timing errors that arise when a system must implement a pulse frequency modulation-encoding scheme (as is used in vestibular implants to mimic normal vestibular nerve encoding of head movement) within fixed-rate continuous interleaved sampling (CIS) strategies used in cochlear implants. Pulse timing errors caused by temporal discretization inherent to CIS create stair step discontinuities of the vestibular implant's smooth mapping of head velocity to stimulus pulse frequency. In this study, we assayed electrically evoked vestibuloocular reflex responses in two rhesus macaques using both a smooth pulse frequency modulation map and a discretized map corrupted by temporal errors typical of those arising in a combined cochlear-vestibular implant. Responses were measured using three-dimensional scleral coil oculography for prosthetic electrical stimuli representing sinusoidal head velocity waveforms that varied over 50-400°/s and 0.1-5 Hz. Pulse timing errors produced negligible effects on responses across all canals in both animals, indicating that temporal discretization inherent to implementing a pulse frequency modulation-coding scheme within a cochlear implant's CIS fixed pulse timing framework need not sacrifice performance of the combined system's vestibular implant portion. NEW & NOTEWORTHY Merging a vestibular implant system with existing cochlear implant technology can provide clinicians with a means to restore both vestibular and auditory sensation. Pulse timing errors inherent to integration of pulse frequency modulation vestibular stimulation with fixed-rate, continuous interleaved sampling cochlear implant stimulation would discretize the smooth head velocity encoding of a combined device. In this study, we show these pulse timing errors produce negligible effects on electrically evoked vestibulo-ocular reflex responses in two rhesus macaques.

Gene Transfer with AAV9-PHP.B Rescues Hearing in a Mouse Model of Usher Syndrome 3A and Transduces Hair Cells in a Non-human Primate

Hereditary hearing loss often results from mutation of genes expressed by cochlear hair cells. Gene addition using AAV vectors has shown some efficacy in mouse models, but clinical application requires two additional advances. First, new AAV capsids must mediate efficient transgene expression in both inner and outer hair cells of the cochlea. Second, to have the best chance of clinical translation, these new vectors must also transduce hair cells in non-human primates. Here, we show that an AAV9 capsid variant, PHP.B, produces efficient transgene expression of a GFP reporter in both inner and outer hair cells of neonatal mice. We show also that AAV9-PHP.B mediates almost complete transduction of inner and outer HCs in a non-human primate. In a mouse model of Usher syndrome type 3A deafness (gene CLRN1), we use AAV9-PHP.B encoding Clrn1 to partially rescue hearing. Thus, we have identified a vector with promise for clinical treatment of hereditary hearing disorders, and we demonstrate, for the first time, viral transduction of the inner ear of a primate with an AAV vector.

Sound abnormally stimulates the vestibular system in canal dehiscence syndrome by generating pathological fluid-mechanical waves

Individuals suffering from Tullio phenomena experience dizziness, vertigo, and reflexive eye movements (nystagmus) when exposed to seemingly benign acoustic stimuli. The most common cause is a defect in the bone enclosing the vestibular semicircular canals of the inner ear. Surgical repair often corrects the problem, but the precise mechanisms underlying Tullio phenomenon are not known. In the present work we quantified the phenomenon in an animal model of the condition by recording fluid motion in the semicircular canals and neural activity evoked by auditory-frequency stimulation. Results demonstrate short-latency phase-locked afferent neural responses, slowly developing sustained changes in neural discharge rate, and nonlinear fluid pumping in the affected semicircular canal. Experimental data compare favorably to predictions of a nonlinear computational model. Results identify the biophysical origin of Tullio phenomenon in pathological sound-evoked fluid-mechanical waves in the inner ear. Sound energy entering the inner ear at the oval window excites fluid motion at the location of the defect, giving rise to traveling waves that subsequently excite mechano-electrical transduction in the vestibular sensory organs by vibration and nonlinear fluid pumping.

Controlled drug release to the inner ear: Concepts, materials, mechanisms, and performance

Progress in drug delivery to the ear has been achieved over the last few years. This review illustrates the main mechanisms of controlled drug release and the resulting geometry- and size-dependent release kinetics. The potency, physicochemical properties, and stability of the drug molecules are key parameters for designing the most suitable drug delivery system. The most important drug delivery systems for the inner ear include solid foams, hydrogels, and different nanoscale drug delivery systems (e.g., nanoparticles, liposomes, lipid nanocapsules, polymersomes). Their main characteristics (i.e., general structure and materials) are discussed, with special attention given to underlining the link between the physicochemical properties (e.g., surface areas, glass transition temperature, microviscosity, size, and shape) and release kinetics. An appropriate characterization of the drug, the excipients used, and the formulated drug delivery systems is necessary to achieve a deeper understanding of the release process and decrease variability originating from the drug delivery system. This task cannot be solved by otologists alone. The interdisciplinary cooperation between otology/neurotology, pharmaceutics, physics, and other disciplines will result in improved drug delivery systems for the inner ear.

Hearing Loss in the Elderly

Presbycusis, or age-related hearing loss (ARHL), is the result of physiologic and pathologic changes associated with advancing age. ARHL presents typically with a high-frequency hearing loss, which contributes to greater trouble hearing consonants within words. Consonants convey the bulk of meaning within a word, and this loss of linguistic information results in complaints associated with ARHL. Hearing aids and cochlear implants significantly improve the lives of older adults with hearing loss, in particular, those with depression and dementia. Successful current research in gene therapy, pharmacotherapy, and stems cells holds the promise of being able to restore native cochlear function.

A Protocol for Decellularizing Mouse Cochleae for Inner Ear Tissue Engineering

In mammals, mechanosensory hair cells that facilitate hearing lack the ability to regenerate, which has limited treatments for hearing loss. Current regenerative medicine strategies have focused on transplanting stem cells or genetic manipulation of surrounding support cells in the inner ear to encourage replacement of damaged stem cells to correct hearing loss. Yet, the extracellular matrix (ECM) may play a vital role in inducing and maintaining function of hair cells, and has not been well investigated. Using the cochlear ECM as a scaffold to grow adult stem cells may provide unique insights into how the composition and architecture of the extracellular environment aids cells in sustaining hearing function. Here we present a method for isolating and decellularizing cochleae from mice to use as scaffolds accepting perfused adult stem cells. In the current protocol, cochleae are isolated from euthanized mice, decellularized, and decalcified. Afterward, human Wharton's jelly cells (hWJCs) that were isolated from the umbilical cord were carefully perfused into each cochlea. The cochleae were used as bioreactors, and cells were cultured for 30 days before undergoing processing for analysis. Decellularized cochleae retained identifiable extracellular structures, but did not reveal the presence of cells or noticeable fragments of DNA. Cells perfused into the cochlea invaded most of the interior and exterior of the cochlea and grew without incident over a duration of 30 days. Thus, the current method can be used to study how cochlear ECM affects cell development and behavior.