Introduction

Among the five senses, hearing is one of the most important in the interaction between humans and the external environment. Humans sense surrounding airborne sounds based on a complex anatomical structure called the ear. The ear provides the process for converting sound waves into electrical signals that the brain can process. The organ responsible for hearing is the cochlea, which is embedded in the innermost part of the ear. The well-maintained structural integrity and cellular organization of the cochlea play a pivotal role in converting mechanical stimuli into electrical signals (Ekdale 2016; Fettiplace 2017).

SNHL is considered a global problem and a significant handicap that greatly affects quality of life. In 1985, the worldwide disease prevalence of SNHL was first reported by the World Health Organization (WHO) with approximately 1% of the world’s population (42 million people) estimated to have moderate to severe hearing disorders or impaired hearing (Davis and Hoffman 2019). From 1990 to 2019, the number of people suffering from moderate to complete hearing loss increased from 225.3 million to 403.3 million (Haile et al. 2021). According to WHO statistics, the most recent estimate shows that more than 1.5 billion people (nearly 20% of the world’s population) live with hearing loss, of whom 430 million have hearing loss (over 5% of the world’s population) (WHO 2021a). This estimate predicts that nearly 2.5 billion people (1 in 4 people) will live with hearing loss, and at least 700 million people (1 in 10 people) worldwide will develop hearing loss by 2050 (WHO 2021b). Hearing impairment can negatively affect an individual’s social and emotional development as well as communication and education. People with hearing impairments tend to avoid social situations and experience with increased rates of depression, anxiety, and feelings of inadequacy because they have difficulty acquiring surrounding information.

With the increasing prevalence of hearing loss worldwide, it is necessary to explore and develop treatments for hearing loss due to the negative impact of this condition on quality of life. SNHL can result from several diseases and external stimuli that damage the sensorineural structures of the cochlea (Cohen et al. 2014; Yang and Chung 2016; Vona and Haaf 2016; Cardin 2016; Rizk, et al. 2020; Gacek 2021). The treatment of such inner ear disorders has been a challenge for physicians, as there are few drugs on the market for cochlear indications and it is difficult to achieve effective drug concentrations in the cochlea (Nyberg et al. 2019). In the last two decades, IT administration, a safe procedure that can overcome the pharmacokinetic limitations of systemic administration for the treatment of inner ear diseases, has attracted considerable attention as a potential method for drug delivery to the cochlea. Many conventional dosage forms of medication on the market (e.g., dexamethasone solution 4 mg/mL or 5 mg/mL; prednisolone solution 0.25 mg/mL; methylprednisolone solution 62.5 mg/mL; triamcinolone acetonide suspension 10 mg/mL or 40 mg/mL) are currently used in clinical practice by IT injection instead of systemic application (Lechner et al. 2019; Roßberg et al. 2020). However, although IT administration helps drugs reach the inner ear at a greater level than that achieved through the systemic route, it still has several limitations that make it very difficult to deliver drugs to the cochleae by conventional formulations (including short residence time in the middle ear and low passage of drug through the RWM) (Szeto et al. 2020). These limitations indicate the need for the development of new strategies to improve the efficiency of IT drug delivery systems. A variety of innovative systems have been researched and evaluated in in vitro studies, in vivo studies and clinical trials (Pyykkö et al. 2011; Pararas et al. 2012; El Kechai et al. 2015; Rathnam et al. 2019; Freitas et al. 2021).

The purpose of this review is to provide an overview of drug delivery systems for IT administration. First, we describe the most critical features of the anatomy and physiology of the inner ear that should be considered during the design of drug delivery systems for IT administration. Next, promising systems for future clinical applications and current obstacles to overcome for efficient inner ear treatment are discussed.

Structure of the inner ear and auditory transduction mechanism

Anatomically, the ear is divided into three parts: the external ear, the middle ear, and the inner ear (Fig. 1A). The external ear consists of the pinna, the auditory canal, and the tympanic membrane (eardrum). The middle ear consists of the tympanic cavity including three auditory ossicles (the malleus, the incus, and the stapes), the round window (RW), the oval window (OW), and the Eustachian tube. The external and middle ears are responsible for guiding and conducting sound waves to the inner ear. The inner ear is an anatomically isolated structure comprising the vestibular system responsible for balance and the spiral cochlea responsible for auditory perception. The cochlea consists of three constitutively fluid-filled cavities. The middle cavity filled with endolymph is called the scala media (SM) and is separated from the superior cavity (scala vestibuli—SV) and inferior cavity (scala tympani – ST) by the vestibular membrane and basilar membrane, respectively. The SV communicates with the middle ear via an OW, whereas the ST communicates with the middle ear via an RW; and both of these cavities are filled with perilymph. The scala media contains the organ of Corti (OC), an essential organ considered to be an auditory receptor that converts sound vibrations into nerve signals (Fig. 1B). The OC is composed of sensory hair cells (inner hair cells (IHC), which are the actual sensory cells, and outer hair cells (OHC), which are cochlear signal amplifiers), supporting cells, basilar membrane, tectorial membrane as well as nerve fibers connecting the organ to the nearby spiral ganglion (SG) (Fig. 1C) (Ekdale 2016). Each auditory hair cell has the potential to detect a distinctive range of sound frequencies varying tonotopically along the cochlear axis. In addition, the scala media includes another vital structure called the stria vascularis that is responsible for producing and maintaining the ionic composition of the endolymph. For sounds to be detected, sound waves must first travel across the external ear canal and middle ear and through the inner ear fluids to stimulate the mechanosensory hair cells housed in the OC to release the neurotransmitters, which are then transferred to the spiral ganglion neurons (SGNs) and further propagated to the auditory cortex of the brainstem, where they are processed to cause auditory sensation (Fettiplace 2017).

Fig. 1
figure 1

The anatomic structure of ear and organ of corti. A The anatomic structure of mammalian ear composed of external ear, middle ear and inner ear, B the cross section of cochlea including 3 compartments [scala vestibuli (SV), scala media (SM) and scala tympani (ST)], and C the anatomic of organ of corti (OC) indicates the relation of cochlear hair cells, tectorial membrane, nerve fibers

Full size image

Cochlear diseases, molecular mechanism, and therapeutic treatment

Any damage to sensory cells or auditory nerves can cause SNHL, which is the most frequent inner ear impairment in humans. This sensory deficit can originate from external stimuli (such as excessive noise (Yang and Chung 2016), infections (Cohen et al. 2014), and ototoxic drugs (Rizk, et al. 2020)) or intrinsic causes such as genetic mutations (Vona and Haaf 2016), aging (Cardin 2016) and Meniere’s disease (Gacek 2021). Overexposure to ototoxic compounds (for example, aminoglycoside antibiotics (gentamicin, streptomycin, etc.), chemotherapy agents (cisplatin, etc.), and acoustic injury as well as aging affect the distinct cellular signaling pathways to activate apoptosis leading to the degeneration of hair cells and SGNs. Moreover, both infections and Meniere’s disease can cause endolymphatic hydrops, inducing SNHL.

A more comprehensive understanding of the molecular mechanisms related to the degeneration and regeneration of sensory cells and the auditory nerve is of paramount importance in identifying the therapeutic candidates to prevent and treat inner ear diseases (Fig. 2). Most causes of SNHL induce cell death in response to the overproduction of reactive oxygen species (ROS) in both hair cells and SGNs. The high level of ROS activates the MAPK-JNK pathway, triggering the production of pro-apoptotic mediators and pro-inflammatory cytokines, leading to apoptosis or necroptosis (Kurabi et al. 2017). Antioxidants and apoptosis inhibitors as well as anti-inflammatory agents have been investigated to improve the defense capacity of hair cells and SGNs against damage to hinder cell death. On the other hand, the excessive generation of ROS mediates the activation of AMPK to inhibit mTOR, the central regulator of cell growth and cell proliferation, which plays a crucial role not only in cell survival but also in cell regeneration (Zhao et al. 2020; Cortada et al. 2021). Therefore, the stimulation of cell survival which can be achieved by using growth factors is another strategy to prevent hair cells and spiral ganglion nerves from damaging elements. A further approach is the application of molecular candidates (such as ATOH1 and neutrophins) or stem cells for the enhancement of cell regeneration to rescue injured hair cells and SGNs. In addition, gene therapy is a prospective means for treating hearing loss related to the functional deficits caused by genetic mutations (Table 1).

Fig. 2
figure 2

Cellular mechanism of hearing loss. AMPK, AMP-activated kinase; AP-1, activator-protein 1; CytC, cytochrome c; GF, growth factor; GFR, growth factor receptor; JNK, a-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; PI3K, phosphoinositide 3-kinase; ROS, reactive oxygen species; TNF-α, tumor necrosis factor-alpha; m-TOR, mammalian target of rapamycin

Full size image
Table 1 Therapeutic candidates for treatment of inner ear disorders
Full size table

Intratympanic administration

There are two main approaches to the treatment of inner ear disorders: systemic drug delivery and local drug delivery. The presence of the blood labyrinth barrier (BLB), which inhibits the exchange of blood and inner ear fluids, is a major factor in the difficulty delivering drugs into the cochlea with sufficient concentration via systemic administration (Nyberg et al. 2019). The necessity of a more effective delivery route that can maximize the drug concentration in the inner ear and minimize the undesired side effects caused by systemic drug delivery has increased interest in local drug deliveries (including IT and intracochlear administration (Salt and Plontke 2009) from many researchers. Intracochlear drug delivery is rarely applied in clinical practice, as it requires surgical access to the cochlea for the sole purpose of local drug delivery. Therefore, IT administration, which is minimally invasive, opens new horizons for drug delivery to the inner ear for the treatment of SNHL.

In IT administration, the drug is transported directly into the middle ear cavity to diffuse the drug through the RWM into the cochlea. The discovery and development of IT administration over time is presented in Fig. 3. In 1956, this route was introduced by Schuknecht to treat Meniere’s disease with gentamicin (Schuknecht 1956). However, it was not until 1996 that IT injections were used in patients with sudden hearing loss (Silverstein et al. 1996), and this drug delivery route has received much attention in the treatment of various inner ear diseases since then. Over the past two decades, much evidence regarding the efficacy of this approach has been reported for the clinical application of steroids, gentamicin, and several other therapeutic candidates in animal studies. More details are described in the section “Drug delivery system for IT administration”.

Fig. 3
figure 3

The diagram presents the discovery and development of IT administration route over time

Full size image

Pharmacokinetics and challenges in intratympanic drug delivery

Similar to the pharmacokinetic processes that occur in the human body after systemic administration, the pharmacokinetics of drugs after IT delivery to the inner ear also include liberation, absorption, distribution, metabolism, and elimination (Fig. 4). Liberation refers to the release of a drug from an administered dosage form; absorption is the entry of drug into the internal fluid from the administration site through an RWM and an OW; distribution is the process by which drugs spread within the inner ear fluid (perilymph and endolymph) and are delivered to cochlear tissues; metabolism is a chemical alteration of drugs catalyzed by the enzymes in the inner ear fluid; and elimination includes removal of the drug and its metabolites from the inner ear and clearance through the Eustachian tube from the middle ear cavity (Salt and Plontke 2009, 2018). The bioavailability of a drug is determined primarily by the permeability of the drug through the RWM, the residence time of the drug in the middle ear, and the basal to peak concentration gradient of the drug within the cochlea, which are the leading challenges for drug delivery into the cochlea via the IT route.

Fig. 4
figure 4

Pharmacokinetic processes from application position to therapeutic targets including absorption, distribution and clearance of drug after IT administration by direct injection into middle ear cavity. Red arrows indicate the path of the drug into the ear from the middle ear to the cochlea and Eustachian tube. TM, tympanic membrane; RWM, round window membrane

Full size image

Round window membrane

The RWM that allows drugs in the middle ear to penetrate into the perilymph can be considered the main pathway for drug delivery to the cochlea; however, it is also the fundamental obstacle to this administration route. The RWM, which consists of 3 layers, acts as a semipermeable membrane that allows the passage of low-molecular-weight molecules (such as antibiotics, local anesthetics, and corticosteroids). The permeability of the RWM is affected not only by the characteristics of drugs (lipophilicity and charges) but also by their properties (morphologic strength, thickness, and pathological condition) (Goycoolea and Lundman 1997).

Eustachian tube clearance

The Eustachian tube, which connects the middle ear cavity with the nasopharynx to equalize the middle ear pressure (Smith et al. 2016), also removes drugs administered to the middle ear. This drug loss reduces the residence time of the drugs in the middle ear and consequently decreases the amount of drug that diffuses through the RWM into the inner ear (Plontke et al. 2008b).

Base-to-apex concentration gradient in the cochlea

The base-to-apex concentration gradient occurs due to diffusion of the drug in the spiral structure of the cochlea after IT administration. This phenomenon poses a challenge to the treatment of disorders in the apex due to the difficulty of drug transport into the apical turn as a therapeutic concentration (Plontke et al. 2007, 2008a).

Strategies for overcoming the challenges of intratympanic administration

To overcome the above hurdles and effectively transport drugs to the treatment site, two parameters affecting drug concentration in the perilymph after IT injection should be considered: (1) residence time in the middle ear and (2) passage of drug through the RWM. Various strategies for the development of devices and drug delivery systems have been studied to increase the residence time of a drug in the middle ear cavity and to enhance drug delivery through the RWM. The former goal can be achieved using either several devices (in particular, microwicks, microcatheters, gelfoam, and Seprapack) (Pararas et al. 2012) or gelling systems (Rathnam et al. 2019). To achieve the latter, there are two main strategies: increasing the permeability of the three-layer barrier by a microneedle or penetrating facilitators and improving cellular uptake based on nanoparticle systems (Pyykkö et al. 2011). Although drug delivery devices are not mentioned in this review, we focus on the advanced technologies applied in preparation for intratympanically administered drug delivery systems.

Drug delivery system for intratympanic administration

Conventional drug delivery systems are still being used to treat inner ear diseases through the IT route, but the development of innovative drug delivery systems that overcome the limitations of these conventional dosage forms has been an attractive field for many researchers. A drug delivery system suitable for this route is a system that provides a prolonged residence time and/or sustained release to achieve a therapeutic concentration of drug in the inner ear with minimal invasiveness. In addition, it should be easily injected, biodegradable, and biocompatible. Various systems including hydrogels, nanoparticles, and combinations thereof, have been designed in recent years.

Conventional delivery systems

Conventional delivery systems, including solutions and suspensions, are the first dosage forms applied intratympanically to deliver streptomycin and dexamethasone into the middle ear cavity for the treatment of Meniere’s disease and sudden hearing loss, respectively (Schuknecht 1956; Silverstein et al. 1996). Although these conventional formulations have better therapeutic efficacy than systemic administration, frequent repeated injections (usually 2–5 times per week (Alles et al. 2006)) cause the typical undesired side effects of local injections such as pain, brief caloric vertigo, otitis media or lingering tympanic membrane perforation (Rauch 2011). These limitations are due to the difficulty of reaching therapeutic concentrations in the inner ear fluid due to the rapid drainage of intratympanically injected fluids via the Eustachian tube and the low transportability of the drug through the RWM. In this context, Eustachian tube clearance is the most important route that significantly shortens drug residence time at the absorption site, as demonstrated in the research of Plontke et al. on the IT use of methylprednisolone in humans (Plontke et al. 2008b). Another limitation of treatment with conventional dosage forms is the morphological change of the RWM after some therapeutic agents (e.g., hydrocortisone) come into direct contact with middle ear structures (Spandow et al. 1990; Nordang et al. 2003). Despite these drawbacks, transtympanic injection of conventional solutions or suspensions is widely used to manage common inner ear diseases. Aqueous solutions or suspensions of some therapeutic molecules, such as gentamicin sulfate (Liu et al. 2015a), dexamethasone (Liu et al. 2015a), methylprednisolone (Özel et al. 2016), N-acetyl cysteine (Somdaş et al. 2020), triamcinolone acetonide (Salt et al. 2019), and ciprofloxacin (Mair et al. 2016), have been investigated for the treatment of inner ear impairments through IT application.

Hydrogels

To prevent unintentional leakage via the Eustachian tube described above, viscous hydrogels seem to be a possible strategy to prevent rapid flow through the Eustachian tube, thereby increasing the residence time of the drug in the middle ear cavity and consequently improving the biodistribution of the drug in the cochlea. However, the delivery of formulations to the middle ear is conducted using a long and fine needle (smaller in diameter than G22) (El Kechai et al. 2015). Therefore, the residence time and injectability are two key parameters to consider when developing a hydrogel delivery system for IT administration. Injectability is the ability of a formulation to be injected into the tympanic cavity through a needle with an acceptable force, and residence time is the period of time at the application site calculated from the time the formulation is injected into the middle ear until it is washed out from the middle ear. In general, either injectability or residence time is affected by the viscosity of the hydrogel. The higher the viscosity, the longer the residence time and the harder it is to pass through the needle.

Conventional hydrogels

Several conventional hydrogel-based delivery systems investigated for IT drug delivery have been formulated from a variety of natural polymers including collagen, gelatin, chitosan, and hyaluronic acid (shown in Table 2). Collagen and gelatin are biodegradable polymers that can increase the viscosity of liquid dosage forms. However, since collagen or gelatin alone has limitations in thermal and mechanical stability and aqueous solubility, it cannot be accommodated in the site of administration for a long period of time. For this reason, glutaraldehyde is used as a crosslinking agent for collagen and gelatin, which reduces the degradation rate of collagen and gelatin, enabling long-term applications (Graziola et al. 2016; Peng et al. 2017). Hydrogels generated by glutaraldehyde crosslinking of gelatin or porcine type I collagen have proven effective for the delivery of some proteins, including brain-derived neurotrophic factor (BDNF) (Endo et al. 2005), insulin-like growth factor 1 (IGF-1) (Iwai et al. 2006; Lee et al. 2007; Fujiwara et al. 2008), and hepatocyte growth factor (HGF) (Inaoka et al. 2009). Chitosan is a nontoxic biodegradable cationic polymer that is prepared by alkaline n-deacetylation of chitin obtained from the exoskeleton of crustaceans (Sreenivas and Pai 2008). The positive charge of chitosan prolongs the residence time in the middle ear due to its electrostatic interaction with the negatively charged mucosal surface. In a study by Saber and coworkers, three types of chitosan were investigated to deliver neomycin to the inner ear. In the middle ear, low/medium-molecular-weight chitosan (Ch1 and Ch2) with lower solubility could be observed 7 days after application, while oligomeric chitosan (Ch3) with higher solubility was not observed at the same time. All formulations successfully delivered neomycin to the cochlear and chitosans have been demonstrated to be resistant to cochlea tissues (Saber et al. 2010). Hyaluronic acid is an anionic polymer present in the extracellular matrix of many tissues in the body and has mucoadhesive properties, biodegradability, and biocompatibility. The effect of drug delivery into the inner ear by the hyaluronic acid hydrogel was evidenced by the loss of hair cells after placing the neomycin-loaded hydrogel into the RWM, while there was no significant difference between the results obtained with neomycin solution (Saber et al. 2009). Hyaluronic acid can form highly viscous aqueous hydrogels that can extend residence time up to 72 h, as demonstrated in the study by Borden and colleagues (Borden et al. 2011). However, the high viscosity of the hyaluronic acid hydrogel does not affect the syringeability due to the shear-thinning and self-healing property of this polymeric system (Kechai et al. 2016). In addition to the achievement of higher drug concentrations in the perilymph based on prolonged contact with RWM, hyaluronic acid has been reported to facilitate drug diffusion to the inner ear based on its osmotic effect on the perilymph (Bagger-Sjöbäck et al. 1993) and RWM permeability modulating potential (Selivanova et al. 2003). Most conventional hydrogels mentioned above are in liquid form both before and after injection. It is difficult to deliver hydrogels with very high viscosity to the middle ear via the RWM with the aim of a very long residence time. To overcome these limitations, an in-situ gelling system was investigated.

Table 2 Conventional hydrogels for IT administration
Full size table

In situ hydrogels

An in-situ hydrogel is a system that remains in a liquid state before entering the body for easy injection and then undergoes sol–gel transition process to form a semisolid gel at the site of administration after a stimulus or a period of time. The unique viscosity-adjustable property makes in situ hydrogels easy to handle compared to conventional hydrogels. The in-situ gelling hydrogel systems studied for IT administration include thermosensitive hydrogels and time-dependent hydrogels (shown in Table 3).

Table 3 In situ hydrogels for IT administration
Full size table

In-situ thermosensitive hydrogels that retain fluidity at room temperature and gel at body temperature are very attractive for inner ear drug delivery via transtympanic injection. Such thermoreversible systems can be generated from synthetic copolymers such as poloxamer 407 (P407) and PLGA-PEG-PLGA. Both P407 and PLGA-PEG-PLGA copolymers are triblock copolymers composed of a hydrophobic central chain and two hydrophilic side chains. Because of these amphiphilic properties, these copolymers tend to form micelles in water with a hydrophobic core and a hydrophilic shell above the critical micelle concentration (CMC). Intermicellar crosslinking between different micelles at concentrations higher than the critical gel concentration (CGC) leads to the creation of a gel matrix. Heating reduces the CMC and CGC, causing gelation of the thermosensitive copolymer solution (Gong et al. 2012; Fakhari et al. 2017; Russo and Villa 2019). The hydrogels made of either P407 or PLGA-PEG-PLGA are biodegradable, biocompatible, stable, and suitable for controlled release. The concentration of P407 used in hydrogels is typically in the range of 16–20% (w/v). The obtained hydrogel has a gelation temperature of 37℃ (physiological temperature) or less. P407 hydrogel has been extensively investigated as a drug delivery system for the IT administration of several therapeutic agents, such as dexamethasone, methylprednisolone, triamcinolone, and n-acetyl cysteine (Wang et al. 2009, 2011; Piu et al. 2011; Honeder et al. 2014; Engleder et al. 2014; Gausterer et al. 2020). The PLGA-PEG-PLGA solution dissolves at temperatures of 2 to 15℃ and turns into a gel at body temperature. The CGC of PLGA-PEG-PLGA solution can be controlled by changing the length of the PLGA or PEG block (Gong et al. 2012; Wang et al. 2017). The PLGA-PEG-PLGA system was reported to be effective in controlling the delivery of dexamethasone acetate, cidofovir, and levothyroxine (Gao et al. 2010; Feng et al. 2014; Liu et al. 2014; Kamali et al. 2020).

Chitosan glycerophosphate (CGP), a chitosan derivative, also has the ability to form a temperature-sensitive gelling matrix. However, unlike the abovementioned copolymers, the mechanism of gel formation in CGP solutions is driven by hydrophobic association of the chitosan molecules with increasing temperature. At low temperatures, the CGP system remains in a liquid state based on a hydrated protective layer of glycerophosphate around the chitosan chains created through weak hydrogen bonds. As the temperature increases, this layer breaks down, allowing the chitosan molecules to interact through stronger hydrophobic bonds and form hydrogels at body temperature. The gelation temperature and gelation rate of the CGP solution are affected by the deacetylation degree of chitosan, the concentration of chitosan, and the concentration of glycerophosphate (Gong et al. 2012; Liu et al. 2016; Rahmanian-Devin et al. 2021). Moreover, CGP hydrogels have a high affinity for mucosa due to their positively charged nature and have the potential to provide a sustained release delivery system due to their slow degradation over time by lysozyme available in the middle ear. Studies by Paulson et al. and Luo et al. demonstrated that the CGP hydrogel system can deliver dexamethasone and gentamicin to the cochlear fluid of guinea pigs with a steady release profile after IT injection (Paulson et al. 2008; Luo and Xu 2012).

In addition, a time-sensitive silk-polyethylene glycol (PEG) hydrogel that remains in the sol state for sufficient time for injection into the middle ear before the sol–gel transition, has also been investigated for the introduction of dexamethasone into the cochlea (Yu et al. 2016; Chen et al. 2019). Gelation occurs due to the structural transition of silk fibroin from the random coil-dominant structure of the silk solution to the β-sheet structure of the silk hydrogel when the silk solution is incorporated with a polymer (such as PEG). The gelation time depends on the concentration of the silk solution and the type and concentration of PEG (Wang et al. 2015).

In general, due to the advantages of improving residence time and providing sustained release in the RWM, a single IT injection of hydrogel resulted in higher drug levels in the inner ear fluid over an extended period of time that the injection of drug solution. This hydrogel has been proven to be safe and well tolerated. In most studies, only mild conductive transient hearing loss was observed due to interference of the viscous hydrogel with ossicular chain mobility, and no histological damage to the RWM and cochlea after IT administration was observed. However, drug permeability through the RWM is still highly dependent on the drug properties as they do not alter the physicochemical properties of the drug loaded into the gel system. Furthermore, hydrogels cannot stabilize drugs (especially biotherapeutic agents) from degradable reactions or provide targeted delivery. These limitations can be overcome by the application of nanotechnology.

Nanoparticles

Nanoparticle (NP)-based systems have been extensively explored for the IT drug delivery of various therapeutics for the treatment of inner ear disorders due to their potential to stabilize loaded materials, improve diffusion kinetics through RWM, and enhance cellular uptake and specific targeting. NPs encapsulate the drug into a structure, providing a barrier to protect the drug from degradative reactions and providing a controlled and sustained release profile. Likewise, due to the encapsulation of the drug into NPs, the drug properties are modulated by the NPs and the diffusion kinetics of the drug depend on the particle size and surface modification of the NPs. Their small size (typically in the range of tens to hundreds of nanometers in diameter) and surface modifications (including charged, hydrophobic, and tethered cell-penetrating peptides) have been shown to improve the transport of NPs across the row window barrier and increase the uptake and accumulation of drugs in certain cell types of the cochlea. In addition, NP labeling with ligands that interact with specific cellular receptors including tropomyocin receptor kinase B (TrkB) receptor (Bitsche et al. 2011), trisialoganglioside clostridial toxin (GT1b) receptors (Santi et al. 1994), and prestin (Zheng et al. 2000; Liberman et al. 2002), provides targeted delivery to specific cell types such as auditory hair cells and spiral ganglion cells. The advantages of NP-mediated drug delivery mentioned above reduce the dose required and cause unwanted side effects. Many types of NP-based systems for transtympanic administration that can be classified into four main groups: polymer-based, lipid-based, inorganic, and hybrid (Fig. 5).

Fig. 5
figure 5

Structure of various nanoparticle systems used in IT administration. HAT, hydroxyapatite; LNCs, lipid nanocapsules; MNPs, magnetic nanoparticles; NPs, nanoparticles; SLNs, solid lipid nanoparticles

Full size image

Polymer-based nanoparticles

Polymer-based NPs are biocompatible, degradable, and maintainable systems prepared from natural and synthetic materials (including preformed polymers and monomers) by a variety of techniques (e.g., emulsification, precipitation, ionic gelation, and microfluidics) for application in the drug delivery of numerous hydrophilic and hydrophobic compounds. The loading efficiency and release kinetics of these drugs can be controlled by modulating formulation parameters such as composition, stability, and surface charge. Nanosystems of this category have been used in many studies to deliver drugs to the inner ear via IT administration, and according to their structure, they can be further divided into four subsets: polymersomes, polymeric NPs, dendrimers, and micelles.

Polymersomes

Polymersomes are a subset of polymer-based NPs with a core–shell structure with a central aqueous core surrounded by an outer bilayer shell composed of a hydrophobic membrane and hydrophilic corona formed by the self-assembly of an amphiphilic copolymer in an aqueous solution. Polymersomes have the ability to encapsulate hydrophilic and hydrophobic drugs with an aqueous core and hydrophobic membrane, respectively, to facilitate codelivery. They are resistant to the immune system due to their biomimetic structure (Lee and Feijen 2012). Many polymersomes studied for transtympanic drug delivery have been manipulated from the di-block copolymer poly(ethylene glycol)-b-poly(ε-caprolactone) (PEG-b-PCL), which showed the ability to penetrate into the inner ear and distribute in various cochlear tissues (Buckiová et al. 2012; Roy et al. 2012). After conjugation to peptides that specifically bind to the surface receptors in selected cell lines (including nerve growth factor-derived peptide (hNgf_EE) and prestin-binding peptides (665 and A666), functionalized PEG-b-PCL polymersomes can specifically target SGNs and outer hair cells, respectively (Roy et al. 2010; Surovtseva et al. 2012). Polymersomes composed of poly(2-hydroxyethyl aspartamide) (PHEA) with or without ligand binding have also been examined to evaluate safety and permeability through RWM (Kim et al. 2015);(Yoon et al. 2015, 2016) (shown in Table 4).

Table 4 Polymersomes for IT administration
Full size table
Polymeric nanoparticles

Polymeric NPs are the most commonly used subset of polymer-based NPs with solid matrix structures created from various types of natural and synthetic polymers (shown in Table 5). One of the most extensively investigated polymers is poly(lactic-co-glycolic acid) (PLGA) because of its biodegradability and biocompatibility, approval by the Food and Drug Administration (FDA) and European Medicines Agency (EMA), surface modification ability, and ability to carry either hydrophilic or hydrophobic therapeutics (Tabatabaei Mirakabad et al. 2014). After it was demonstrated in 2005 that PLGA NPs containing rhodamine can cross the RWM after local injection into the middle ear (Tamura et al. 2005), much focus has been placed on PLGA NP systems for drug delivery into the inner ear. PLGA NPs can be used for the delivery of a single drug such as dexamethasone (Sun et al. 2016), or multiple therapeutic candidates such as the combination of salvianolic acid B, tanshinone IIA, and total panax notoginsenoside (Cai et al. 2014) for the purpose of synergistic actions. The accumulation of PLGA NP-based systems can be increased using surface-modified NPs alone or in combination with cell penetrating peptides (CPPs) (Cai et al. 2017; Wen et al. 2016).

Table 5 Polymeric nanoparticles for IT administration
Full size table

Other polymers of interest for IT drug delivery have also been studied, including bovine serum albumin (BSA) and chitosan. BSA is a biodegradable and nontoxic protein capable of forming spherical NPs that can remain in the middle ear cavity for long periods and can penetrate through the RWM and internalize in the cochlea (Yu et al. 2014). Chitosan NPs can be prepared from chitosan by a different technique from chitosan hydrogels (Agnihotri et al. 2004). The application of chitosan NPs has the potential to successfully introduce drugs into the inner ear fluid in a controlled manner, mainly via the oval window (Ding et al. 2019).

Dendrimers

Dendrimers are synthetic polymers with a hyperbranched architecture whose size and shape van be controlled by the creation of repeating units. Dendrimers consist of a hydrophobic core surrounded by hydrophilic branches, and functional groups in hydrophobic core allow it to trap hydrophobic drugs and interact with functional groups to modify the surface (Chauhan 2018). The dendrimers used in otology for IT delivery are commonly formed by polyamidoamine (PAMAM) and hyperbranched poly-l-lysine (HBPL) (shown in Table 6) (Zhang et al. 2011a; Roy et al. 2012; Wu et al. 2013). Activation by heat coupling for surface functionalization with sodium-carboxymethyl-β-cyclodextrin (CM-β-CD) gives dendrimer NPs a higher transfection efficiency for DNA delivery (Wu et al. 2013).

Table 6 Dendrimers and micelles for IT administration
Full size table
Micelles

Another polymer-based NP with core–shell structure is polymeric micelles prepared by the self-association of amphiphilic block copolymers in an aqueous solution, which have many advantages for drug delivery including biocompatibility, low toxicity, and relatively high stability (Ahmad et al. 2014). NPs of a PEG-PLA micellar structure containing Dex prolonged the circulation time of Dex in the perilymph compared to Dex solution (≥ 48 h vs. < 12 h) and were more resistant to cisplatin-induced damage, as evaluated by the ABR test and hair cell counting test (Yu et al. 2015).

Lipid-based nanoparticles

In addition to polymer-based nanoparticles, lipid-based nanoparticle systems have been widely studied to deliver drugs from the middle ear to the inner ear via RWM, including liposomes, lipid nanoparticles, and cubosomes. As a delivery system, lipid-based NPs have many benefits including biocompatibility, large payload, and the ability to regulate their surface properties.

Liposomes

Liposomes are a subcategory of lipid-based NPs with a core–shell structure typically composed of phospholipids, a major component of the phospholipid bilayer membrane surrounding an aqueous central core. Similar to polymersomes, liposomes can deliver either hydrophilic therapeutics in the aqueous core or hydrophobic therapeutics in the phospholipid bilayer. Because of their cell membrane-like structure, liposomes are highly resistant. The stability of liposomes is controlled by the particle size, surface charge, lipid composition, number of lamellae, and spatial effect of surface modification (Alavi et al. 2017). Liposomes labeled with traceable agents were observed to have the ability to pass through the middle-inner ear barrier and distribute in the cochlea (Zou et al. 2010a, 2012; Buckiová et al. 2012). Permeation through the RWM of liposomes is size dependent, and smaller NPs have higher transport efficiency (Zou et al. 2012). Many in vivo studies have shown that liposomes can deliver genes for expression in various cochlear organs (Wareing et al. 1999; Jero et al. 2001; Maeda et al. 2007). Surface functionalization with A371 (a TrkB ligand) revealed greater targeted accumulation of therapeutic agents in SGNs and inner hair cells than was achieved by unmodified liposomes (Zou et al. 2009) (shown in Table 7).

Table 7 Liposomes for IT administration
Full size table
Lipid nanoparticles

Many other lipid-based NPs lacking an aqueous core-bilayer shell structure, such as liposomes are considered attractive delivery systems for carrying hydrophobic drugs to the inner ear, including lipid nanocapsules (LNCs), solid lipid nanoparticles (SLNs), and cubosomes (shown in Table 8).

Table 8 Lipid nanoparticles for IT administration
Full size table

LNCs consist of a liquid lipid core composed of triglycerides and mineral oil and an amphiphilic shell composed of nonionic surfactants. It was reported that LNCs could penetrate through the RWM and distribute throughout the cochlear cell populations of human and Sprague–Dawley rat ears without damaging the inner ear structure and function (Zou et al. 2008; Zhang et al. 2011b; Roy et al. 2012). The uptake of LNCs can be altered by the hydrophilicity of the surface. Cationic-PEGylated NPs had the highest cellular uptake in both HEI-OC1 cells and organotypic cultures, which resulted in significantly better otoprotective effects (Yang et al. 2018).

SLNs are solid-core lipid nanocarriers composed mainly of fatty acids or mono-, di- or tri-glycerides, which maintain the solid state of the systems at normal body temperature. SLNs have various advantages over liposomes and LNCs: biocompatibility, biodegradability, high stability, extended drug release, and better target delivery ability (Paliwal et al. 2020). SLNs have been reported to be effective in delivering some glucocorticoids (dexamethasone, dexamethasone acetate, hydrocortisone) and antioxidants (edaravone, clozapine) to the inner ear and provide significant protection against ototoxicity induced by noise or cisplatin overexposure (Chen et al. 2008; Gao et al. 2015; Yang et al. 2018; Cervantes et al. 2019; Wang et al. 2020).

Cubosomes are another type of lipid NP that are bicontinuous cubic phase liquid crystalline systems. Due to the honeycomb structure with bicontinuous domains of water and lipid, cubosomes can load hydrophilic, hydrophobic, and amphiphilic drugs and provide sustained release of the entrapped drug (Naveentaj and Muzib 2020). The tested cubosomes composed of amphiphilic lipids (such as phytantriol and glyceromonooleate) exhibit low toxicity and prospective transport ability across the RWM into the inner ear (Liu et al. 2013b, a, 2015b).

Inorganic NPs

Inorganic NPs are also attractive for application in drug delivery because of their unique physical, electrical, magnetic, and optical properties. Inorganic NPs can be prepared from metals, silica, and hydroxyapatite (Table 9). However, since metallic NPs alone cannot encapsulate drugs, metallic NPs are used in combination with another type of NP for drug delivery.

Table 9 Inorganic nanoparticles for IT administration
Full size table

Silica-based NPs are biocompatible, biodegradable, porous, and spherical NPs. Silica NPs are the most studied inorganic NPs for drug delivery, enabling a high payload of drug within the pores due to their unique porous structure. In addition, silica NPs release drugs loaded by the pore gating strategy and can be easily surface modified (Şen Karaman and Kettiger 2018). Praetorius et al. reported that silica NPs can cross the RWM and distribute in SGNs and inner hair cells without any harmful impacts (Praetorius et al. 2007), and Wise and colleagues demonstrated an effective strategy for delivering BDNF to the inner ear to improve spiral ganglion neuron survival (Wise et al. 2016). In addition, hollow mesoporous silica NPs have been tested as a system carrying gentamycin for the treatment of Meniere’s disease (Xu et al. 2018).

Hydroxyapatite NPs (HAT-NPs) are another widely studied inorganic NP for drug delivery. Hydroxyapatite (a formula formulation of Ca10(PO4)6(OH)2) is the principal constituent of bones and hard tissues, and exhibits excellent biocompatibility, biodegradability, bioresorbability, osteogenesis, osteoconductivity, and osteoinductivity. The proprietary properties of large surface area, high loading capacity, and intracellular transportation capacity make HAT-NPs preferred for use in drug delivery systems (Syamchand and Sony 2015; Murata et al. 2018; Ghiasi et al. 2019). Many studies have shown that HAT-NP can mediate NT3 gene transfection into SGNs in both in vitro and in vivo models (Jiang et al. 2007; Sun et al. 2008; Wu et al. 2012).

Hybrid NPs

Hybrid nanosystems are aimed integrating the advantages of these components into nanoscale systems that can combine materials with different properties, including organic-organic, organic–inorganic, and inorganic-inorganic blends. Compared with nonhybrid NPs, hybrid NPs have several advantages, such as improved circulation time, high stability, and outstanding improvements in drug targeting. In addition, the combination of various available ingredients can avoid the costly and time-consuming synthesis of new molecules (Hadinoto et al. 2013; Tahir et al. 2020; Ferreira Soares et al. 2020). Inorganic magnetic NPs combined with another inorganic constituent such as silica (Dormer et al. 2005; Kopke et al. 2006) or polymers, such as PLGA and poloxamer 407 (Kopke et al. 2006; Barnes et al. 2007; Ge et al. 2007; Zou et al. 2010b; Du et al. 2013a) have also been tested for otologic treatment by IT application (shown in Table 10).

Table 10 Hybrid nanoparticles for IT administration
Full size table

Despite these advantages, similar to conventional delivery systems, the use of nanoparticles alone has some limitations with respect to rapid clearance from the middle ear cavity through the Eustachian tube, reducing the contact of nanoparticles with the RWM for permeation.

Hydrogels combined with nanoparticles

As described above, hydrogels and nanoparticles are beneficial for drug delivery to the inner ear via IT injection. However, there are still individual limitations to the single use of each system. That is, hydrogels are hampered by RWM permeability, whereas nanoparticles are limited by leakage through the Eustachian tube. Therefore, synergistically increasing the control of drug delivery by incorporating hydrogel and nanoparticles in a single formulation is a highly promising strategy to utilize both hydrogel and nanoparticle systems and overcome the drawbacks of each system used alone. Many studies have been conducted to evaluate the effectiveness of the combination of NPs and hydrogels for inner ear drug delivery (shown in Table 11). SPIONs in hydrogel form can pass through the RWM and can be prominently located in the perilymphatic fluid space and over the cochlear tissues in either in vitro or ex vivo model studies without the use of magnetic forces (Thaler et al. 2011). The combination of liposomes and hyaluronic acid hydrogels significantly prolonged the residence time (30 days) compared to liposomes alone (15 days), which facilitated dexamethasone transport into the inner ear (El Kechai et al. 2016). Liposomes could also be entrapped into CGP hydrogels and exhibit higher fluorescence intensity colocalized to inner ear cells than NPs alone (Lajud et al. 2015). The integration of PLGA NPs and CGP hydrogels has the advantage of a longer mean residence time in the perilymph than is achieved by either NPs or hydrogels alone (Dai et al. 2018). In addition, enhancement of the residence time of particle-based carriers in the middle ear cavity is also achieved by using microsphere carriers in combination with film forming agents that can create a dry film to retain drug-loaded particles in the RWM (Dormer et al. 2019). A recent study of our research group on thermosensitive poloxamer hydrogels containing dexamethasone-loaded PLGA NPs also demonstrated the superiority of the combination between PLGA NPs and hydrogel versus PLGA NPs alone for delivering dexamethasone into the inner ear. In vivo studies in BALB/c mice showed that NP-gel remained in the application position for more than 2 days, which led to significantly greater absorption of dexamethasone in the cochleae (7.20 ± 1.76 ng versus 1.83 ± 0.14 ng and 2.71 ± 0.53 ng of dexamethasone solution and dexamethasone-NPs, respectively) (Kim et al. 2021).

Table 11 Nanoparticle-loaded hydrogels for IT administration
Full size table

Current status, development, and future perspectives of the application of the intratympanic route

The most commonly used therapeutic agents currently administered through the IT route in the treatment of inner ear disorders are steroids prepared in conventional dosage forms commercially available on the market (for example, dexamethasone solution 4 mg/mL or 5 mg/mL; prednisolone solution 0.25 mg/mL; methylprednisolone solution 62.5 mg/mL; triamcinolone acetonide suspension 10 mg/mL or 40 mg/mL) (Lechner et al. 2019; Roßberg et al. 2020). However, these conventional formulations have limitations of a short residence time in the middle ear and a low degree of passage of drugs through the RWM, as described in the above section. To overcome these limitations, many advanced drug delivery systems have been developed with preeminent characteristics for the effective delivery of drugs into the inner ear. Hydrogels with a variety of tunable behaviors that allow prolonged retention in the middle ear cavity can serve as a reservoir for sustained release from the site of application to support drug transport through the RWM. A suspension of ciprofloxacin in poloxamer 407 hydrogel (OTO-201) was developed and approved by the FDA as Otiprio for otitis treatment (Edmunds 2017). In addition, some other products are currently in clinical trials and are expected to enter the market, such as AM-101 (clinical trial Phase III for treatment of Tinnitus of a hyaluronic hydrogel containing esketamine hydrochloride) (van de Heyning et al. 2014), AM-111 (clinical trial Phase III for treatment of acute unilateral sudden deafness of a hyaluronic hydrogel containing brimapitide – a JNK inhibitor) (Staecker et al. 2019), and OTO-104 (clinical trial Phase IIb for treatment of Meniere’s disease of a poloxamer 407 hydrogel containing dexamethasone) (Lambert et al. 2012). In addition to the hydrogel product, a lipid-based formulation of gacyclidine – an N-methyl-D-aspartic acid (NMDA) receptor antagonist (OTO-313) exhibiting sustained delivery was also undergoing phase 1/2 clinical evaluation for the treatment of tinnitus (Maxwell et al. 2021). Although no clinical trials are currently underway for nanoparticles for IT administration, these systems are still considered potential delivery vehicles for the future based on their ability to enhance stability, cellular uptake, and drug targeting to specific cells. In addition, the integration of various drug delivery systems to form a versatile platform with superior properties that increase the efficacy and accuracy of targeted drug delivery is also a promising approach.

Conclusion

In conclusion, ae profound understanding of the etiopathological mechanisms underlying inner ear impairments contributes to the development of new therapies as well as better strategies for the treatment and prevention of hearing loss. A deep understanding of the anatomical structure of the ear and inner ear helps researchers recognize the obstacles for delivering drugs into the inner ear through the IT administration route and to develop formulations with unique properties to overcome these obstacles to reach the targets more effectively. Various efforts have been made to develop an innovative injectable system that can replace conventional formulations for drug delivery into the inner ear with the goals of effectively treating inner ear damage with minimal invasiveness, improving the development of possible new therapeutic candidates, and utilizing the feasibility of IT administration. Many studies on advanced systems, such as hydrogels, nanoparticles and hydrogel-nanoparticles combinations, have accomplished many achievements, laying the foundation for future research concerning IT drug delivery systems.