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

Abstract

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

Keywords: cell therapy; drug delivery; gene therapy; inner ear; sensorineural hearing loss; small molecule.

FIGURE 1

(A) A schema of the structure of an ear; sensorineural hearing loss (SNHL) is caused by lesions to the inner ear or neurons along the vestibular auditory nerve from the cochlea to the brain. (B) A cross-sectional schema of the cochlea showing the three scalae and associated anatomical structures. (C) A schema of a sensory hair cell. (B,C) Reprinted from Willems and Epstein (2000) with permission.

FIGURE 2

An illustration showing possible sites of genetic defects in the cochlea and a subset of genes involved in SNHL at each location.

FIGURE 3

(A) Possible anatomical routes for therapeutic delivery into the inner ear; adapted from Delmaghani and El-Amraoui (2020), available under Creative Commons license (CC BY 4.0). (B) Distribution of the blood labyrinthine barrier (BLB), proposed in the literature based on existing experimental evidence; reprinted from Nyberg et al. (2019) with permission. (C) A schema of the structures and cell types in the tympanic membrane. (D) Structure of the round window membrane (RWM); adapted from Pyykkö et al. (2013), available under Creative Commons license (CC BY-NC 4.0).

FIGURE 4

A selection of SNHL gene therapies studied in vivo. (A) Dual AAV-based packaging of the gene encoding Otoferlin, bridged by inverted terminal repeats (ITR). (B) The mid-to-apical turn of injected mouse cochlea showed strong expression of otoferlin (green) in inner hair cells (IHCs) but not in outer hair cells (OHCs), nuclear backstained in blue. (C) Auditory brainstem response (ABR) for dual-AAV injected mice (green) was similar to wild-type (black), but single-AAV injected mice (orange) and untreated mice (blue) had no measurable ABR threshold. The recombinant AAV-Otof NT and AAV-Otof CT vectors contain the 5′ and 3′ parts of the otoferlin cDNA, respectively. (A–C) Reproduced from Akil et al. (2019), available under Creative Commons license (CC BY-NC-ND 4.0). (D) A cytosine base editor composed of a nickase Cas9 (nCas9) fused to a deaminase can convert C:G base pair to T:A along with bystander edits or unwanted edits. Reproduced from Antoniou et al. (2021), available under Creative Commons license (CC BY 4.0). (E) Confocal images of mid-turn cochlea excised from base editing-treated Baringo mouse showing uptake of FM1-43 (green) in IHCs and OHCs, indicating restored mechanotransduction (scale bar = 50 μm). (F) Scanning electron microscopy (SEM) images of apical OHCs and IHCs of [left] untreated Baringo mouse and [right] base editing-treated Baringo mice (scale bar = 10 μm). (E,F) Reproduced from Yeh et al. (2020) with permission. (G) Sham-treated (scRNA) noise-deafened guinea pig [left] mid-turn hair cells immunostained with anti-myosin VIIa (green), stereocilia with phalloidin (yellow), nuclei (blue) (scale bar = 50 μm) and [right] SEM showing complete ablation of basal OHCs (scale bar = 10 μm). (H) Noise-deafened guinea pig treated with Hes1 silencing RNA (siRNA) for Notch inhibition [left] immunohistochemical staining showing supernumery IHCs; arrowheads indicate ectopic IHCs with stereocilia, arrows indicated those without (scale bar = 50 μm) and [right] SEM showing regenerated basal OHCs, some with abnormal stereocilia lacking the canonical stair-step organization (scale bars = 10 μm and 1 μm). (G,H) Reproduced from Du et al. (2018), available under (CC BY-NC-ND 4.0).

FIGURE 5

Differentiation of otic progenitor cells in vitro. (A) (Left) hair cell-like cells derived from otic epithelial progenitors (OEPs) expressed hair cell markers BRN3C (green), MYO7A (red), and ATOH1 (red) (scale bars = 20 μm). (Right) SEM showing apical projections outside hair cell-like cells which are reminiscent of stereocilia (scale bar = 1 μm). (B) SGN-like cells derived from otic neural progenitors (ONPs) expressed markers NF200 (green), TUJ1 (green), and BRN3A (red) and formed dendrite-like protrusions (scale bars = 20 μm). (A,B) Adapted from Chen et al. (2018), available under Creative Commons license (CC BY 4.0).