The Role of Tumor Necrosis Factor Alpha (TNFα)in Hearing Loss and Vestibular Schwannomas

Abstract

Purpose of review:

The aim of this review is to highlight relevant literature on the role of tumor necrosis factor alpha (TNFα) in sensorineural hearing loss (SNHL) and vestibular schwannomas (VS).

Recent Findings:

A comprehensive review of publically available databases including PubMed was performed. The mechanism by which hearing loss occurs in VS is still unknown and likely multifactorial. Genetic differences between VSs and tumor secreted proteins may be responsible, at least in part, for VS-associated SNHL. TNFα has pleotropic roles in promoting inflammation, maintaining cellular homeostasis, inducing apoptosis, and mediating ototoxicity in patients with sporadic VS. TNFα-targeted therapies have shown efficacy in both animal models of sensorineural hearing loss and clinical trials in patients with immune-mediated hearing loss. Efforts are underway to develop novel nanotechnology-based methods to target TNFα and other pathogenic molecules in VS.

Summary:

Development of molecularly targeted therapies against TNFα represents an important area of research in ameliorating VS-associated hearing loss.

Introduction

Vestibular schwannomas (VSs) are the most common tumors of the cerebellopontine angle (CPA) and the fourth most common intracranial tumors overall [1]. They arise from Schwann cells lining cranial nerve VIII (vestibular nerve). Although benign in nature, they are associated with substantial morbidity due to their location within the CPA and proximity to the brainstem. These include asymmetric sensorineural hearing loss (SNHL) which affects up to 95% of patients, tinnitus, dizziness, cranial neuropathies such as facial palsy, brainstem compression, and hydrocephalus [2]. Today, surgical resection and stereotactic radiation remain the main therapeutic modalities for growing VSs; however, neither can address VS-associated SNHL. While several recent clinical trials on systemic therapies for neurofibromatosis type 2 (NF2)-associated VSs have shown promising results, there are currently no FDA-approved drugs to halt VS growth or ameliorate VS-associated hearing loss [3, 4].

Tumor necrosis factor alpha (TNFα) is a pro-inflammatory cytokine with important key functions in diverse cellular processes, such as regulation of pro-inflammatory responses and maintenance of cellular homeostasis. Aberrant signaling in TNFα or in its cognate receptor (TNFR) has been implicated in diseases including rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, and inflammatory bowel disease. Dysregulation of TNF function has also been shown to be associated with autoimmunity and cancer. While much is known about the role of TNF in these disorders and several therapeutics targeting TNFα have been approved for clinical use, little is known about the association of TNF signaling with hearing loss.

The functions of the TNFα cytokine in cancer are complex and have been subjects of intensive laboratory investigation. Since its initial discovery as a serum factor that induced necrosis of tumor cells, TNFα was thought to exert its downstream signaling effects by acting on the tumor vasculature to compromise its blood supply. Recombinant TNFα exhibited cytostatic effects on certain cancer cell lines in vitro, an effect that was augmented by interferon-gamma (IFN-γ) [5]. More recent work suggested that effects of TNF on tumor cells may also be secondary to induction of senescence, a state of permanent growth arrest in cancer cells observed in animal models [6]. In the context of established tumors, TNFα has been shown to promote tumorigenesis by activating survival signaling via upregulation of NFκB, contribute to immunosuppression by enabling escape from immunosurveillance, and facilitate metastasis by inducing the synthesis of matrix metalloproteinases [7, 8].

In this report, the pathophysiology of VS tumorigenesis and VS-associated SNHL are reviewed, in the context of TNF signaling. The roles of TNF in mediating inflammation and hearing loss are examined. Finally, studies aiming to develop targeted TNF-based therapies for treatment of VS-associated hearing loss are summarized.

Vestibular Schwannomas and Hearing Loss

The overall incidence of VS is estimated at approximately 1.09 per 100,000 population in the US. While the vestibular nerve, not the cochlear nerve, typically serves as a site of predilection for tumor growth, up to 95% of patients with VSs are affected by SNHL [9, 10]. VS was first observed at an autopsy in 1777 and the first case of VS and associated SNHL was described by Sir Charles Bell in 1833 [11]. Nevertheless, the precise mechanism by which SNHL occurs is still unknown and likely multifactorial. One hypothesis suggests that SNHL is caused, at least in part, by mechanical compression of the adjacent auditory nerve and compromise of the cochlear vascular supply. However, while some correlations exist between tumor size and the degree of SNHL, especially for NF2-associated VSs, neither the overall radiographic dimension nor the intracannalicular extent of VSs fully correlates with the degree of SNHL. Furthermore, a subset of patients with non-growing VSs develop progressive hearing loss [12, 13]. Therefore, nerve compression does not appear to fully account for the degree of SNHL in sporadic VS.

A second hypothesis postulates that cochlear damage occurs because of ototoxic and neurotoxic tumor metabolites reaching the cochlea. This is supported by the findings that decreased distortion product otoacoustic emissions (DPOAEs), generated by outer hair cells (OHCs), are observed in VS patients with mild SNHL [14], suggesting that OHC dysfunction could be a primary event rather than secondary to blockade of auditory neurons. Furthermore, damage to hair cells and cochlear neurons is a common post-mortem finding, affecting ~90% of human temporal bones with ipsilateral untreated VS [9].

More recently, studies have suggested that differences in the inherent genetic landscape of VSs and secretions from VS may be responsible in the hearing loss observed in patients. When our laboratory compared gene expression profiles between VS with good hearing versus poor hearing, we found several differences, including in the expression of genes associated with peroxisomal biogenesis [15]. The expression of platelet-derived growth factor (PDGF-A) was found to be inversely correlated with the degree of SNHL in VS patients [16]. Focusing on VS tumor secretions, the overall protein levels in the perilymphatic fluid was 5–15 times higher in patients with VS than healthy controls, the basis for the diagnosis of VS prior to the advent of MRI scans [17, 18]. Our laboratory and others have used liquid chromatography tandem mass-spectrometry based proteomic analysis to reveal a specific pattern of expression of proteins in the perilymph and differential expression between patients with VSs and those without, suggesting that proteins in VS secretions may modulate cochlear degeneration [19, 20].

Tumor angiogenesis governed by secreted pro-angiogenic factors such as vascular endothelial growth factor (VEGF) and inflammation mediated by TNF are two distinct but closely interlinked processes in the context of tumor survival and growth. TNFα signaling is dependent on the VEGF pathway to recruit inflammatory macrophages and activate lymphatic endothelial cells, thereby promoting lymphangiogenesis and metastasis [21]. Therapeutics targeting VEGF may therefore reduce the level of TNFα secreted by VS, and mitigate cochlear hair cell and spiral ganglion neuron damage caused by ototoxic cytokines [13, 22]. The most promising results reported in the literature involved NF2 patients treated with Bevacizumab, a monoclonal antibody against VEGF. Hearing improvement was observed in approximately 57% of NF2 patients which was independent from a concomitant decrease in tumor size [23, 24].

Molecular Biology of TNFα

In a landmark paper from Carswell et al. in 1975, a factor made by host cells in response to endotoxin destroyed tumors in animals with experimental cancers. This factor was named “tumor necrosis factor” due to its functional activity [25]. The gene was cloned and its full spectrum of activity was characterized in the mid-1980s [26–28]. TNFα is a member of the TNF superfamily (TNFSF) that consists of 19 ligands, which interacts with 29 receptors as a part of the TNF receptor superfamily (TNFRSF) including TNF receptor 1 (TNFR1) and TNFR2. Newly synthesized TNF is initially expressed as a trimeric transmembrane protein, which is cleaved by the metalloproteinase TNF-converting enzyme to release soluble extracellular TNF. The generation of soluble TNF is a tightly regulated process that occurs as a response to various types of stimuli, and is controlled in part by rhomboid protein 2.

TNFR1 is expressed ubiquitously in almost every cell type, bears conserved death-domain motifs, and is activated by both soluble and membrane-bound TNF. Binding of TNF to TNFR1 recruits the adapter molecule TNFR1-associated death domain protein (TRADD) and leads to activation of transcription factors including NFκB and mitogen-activated protein kinases which induce inflammation, tissue degeneration, and apoptosis. TNFR2, which lacks a death-domain motif, is only expressed in specific cell types such as endothelial cells, immune cells, and neurons. Activation of TNFR2 is primarily by transmembrane TNF via cell-cell interactions. TNFR2 recruits TNFR-associated factor 2 (TRAF2) and triggers activation of NFκB, MAPKs, and AKT, which is mainly responsible for promoting cellular homeostasis including survival and proliferation [29].

TNF plays a critical role in diseases characterized by chronic inflammation and thereby serves as an attractive drug target. Currently, five molecular therapeutics against TNFα have been approved for the treatment of rheumatoid arthritis (RA), inflammatory bowel disease (IBD) such as Crohn disease and ulcerative colitis, psoriasis, psoriatic arthritis, juvenile idiopathic arthritis, ankylosing spondylitis, and hiradenitis suppurativa (Table 1). Overproduction or dysregulated hyper-function of TNF activates vascular endothelium, recruits immune cells and ultimately results in tissue destruction. In addition, TNF signaling has also been implicated in cellular homeostatic functions, including defense against pathogens and induction of tissue regeneration such as neuronal remyelination and cartilage repair [30, 31]. By contrast, the complex role TNF plays in the organs of hearing and balance, both in maintaining normal functions and in conditions of hearing loss, is less well-understood.

Table 1.

Currently FDA-approved TNF-based therapeutics

Drug (Trade name)	Description	Indications
Etanercept (Enbrel)	TNFR2 fused to IgG1 Fc	AS, JIA, plaque psoriasis, PsA, RA
Adalimumab (Humira)	Human whole monoclonal antibody against TNF	AS, Crohn disease, hidradenitis suppurativa, JIA, plaque psoriasis, PsA, RA
Certolizumab pegol (Cimzia)	PEGylated Fab’ fragment of a humanized monoclonal antibody against TNF	AS, Crohn disease, PsA, RA
Golimumab (Simponi)	Human whole monoclonal antibody against TNF	AS, PsA, RA, ulcerative colitis
Infliximab (Remicade)	Chimeric whole monoclonal antibody against TNF	AS, Crohn disease, plaque psoriasis, PsA, RA, ulcerative colitis

Abbreviations: AS – ankylosing spondylytis, JIA – juvenile idiopathic arthritis, PsA – psoriatic arthritis, RA – rheumatoid arthritis

TNFα in Animal Models of Hearing Loss

The expression of TNFα and the efficacy of TNFα blockade in animal models of hearing loss have been investigated extensively (Table 2). TNFα is typically undetectable in the normal cochlea [32], but is up-regulated with aging, inflammation, or acoustic trauma. In mice, increased expression of HIF-1 targets, including TNFα, was found in the modiolus, spiral ganglion, and stria vascularis at 4 weeks of age, possibly related to hypoxic stress-induced inflammation [33]. Intrachcolear perfusion of keyhole limpet hemocyanin (KLH) in a mouse model of immune-mediated labyrinthitis led to a rapid accumulation of TNFα-producing inflammatory cells, which was reduced with systemic injection of Etanercept, a TNFα receptor blocker [34]. In a mouse model of acoustic trauma, TNFα and IL-6 were elevated in the cochlea within hours after noise exposure, and followed a time course similar to that in other CNS organs after ischemia or trauma [35]. Similarly, transcanal vibration in guinea pigs led to increased cochlear sheer stress and increased expression of TNFα, TNFR1 and TNFR2 [32].

Table 2.

TNFα blockade in animal models of hearing loss

Model [Ref]	Indication	Drug	Route	Treatment	Outcome
Mouse [34]	KLH induced labyrinthitis	Etanercept	Intraperitoneal	Pre (10 min before)	Decreased number of inflammatory cells
Rat [41]	Cisplatin induced ototoxicity	Etanercept	Transtympanic	Pre (30 min before)	20–37 dB improvement in ABR thresholds at day 3
Rat [43]	Gentamicin induced ototoxicity	Dexamethasone	Intrachcolear perfusion via RW	Simultaneous	20–25 dB improvement in ABR thresholds at day 20
Guinea pig [51]	TNFα cochlear perfusion	Etanercept	Intracochlear perfusion	Pre (5 min before)	Cochlear blood flow maintained
Guinea pig [53]	Cochlear implantation	Etanercept	Perilymph infusion	Post (immediately after)	28–48 dB improvement in ABR threshold at day 28
Guinea pig [52]	Noise-induced trauma	Etanercept	Intraperitoneal	Post (15 min after)	18 dB improvement in ABR thresholds; cochlear blood flow maintained

TNFα is also up-regulated in acute or chronic otitis media (COM) and thereby contributes to inflammation-related middle ear remodeling. Specifically, both TNFα and IL-1 were significantly elevated by over four-fold in the middle ear and over two-fold in the inner ear of mice with COM [36]. Similarly, middle ear TNFα levels doubled following trans-tympanic injections with Haemophilus influenza to mimic acute otitis [37]. Direct innoculation of cerebral spinal fluid (CSF) with S. pneumoniae in rats led to bacterial meningitis and subsequent rise in numerous pro-inflammatory cytokines, with TNFα increased by over 10-fold. Auditory brainstem response (ABR) hearing thresholds were also elevated and correlated with TNFα levels in the CSF. Histopathological analyses showed that both spiral ganglion neurons and OHCs are affected, especially in the cochlear basal turn [38].

Activation of TNFα was also seen with treatment with cisplatin, a platinum-based chemotherapeutic and gentamicin, an aminoglycoside antibiotic. The ototoxic effects of cisplatin were thought to be due to, at least in part, by the generation and release of pro-inflammatory cytokines. In immortalized cochlear cell culture, cisplatin decreased cell viability and increased synthesis of TNFα, IL-1, and IL-6 [39]. Immunohistochemical staining demonstrated that TNFα expression was localized to the spiral ligament, spiral limbus and organ or Corti [39, 40]. These cytokines were also elevated in the serum of cisplatin-injected rodents, an effect that was rescued by administration of Etanercept. Interestingly, pre-treatment with Etanercept via intra-tympanic instillation protected OHC damage and reduced ABR threshold shifts [41]. In rat organ or Corti explants, treatment with dexamethasone led to reduction of both TNFα and TNFR1, decreased generation of reactive oxygen species and expression of pro-apoptosis genes [42]. In vivo infusion of Rat cochleae with dexamethasone protected against gentamicin induced OHC loss and hearing damage [43].

While substantial research has attempted to elucidated the role of TNFα in mediating inflammatory responses in auditory hair cells in vitro, little is known about the in vivo auditory phenotype in transgenic animals where TNF signaling is altered. Two transgenic mouse models have been established [44–47]. Mice deficient in TNFα developed more variable and incomplete frequency representations in the cortex, weaker cortical response to tones, and less refinement after exposure to multiple frequencies compared to wild type [48]. TNFα-deficient homozygous mutant mice exhibit higher ABR thresholds and reduced DPOAE amplitudes, which was attributed to defects in OHC function as evidenced by sporadic absence of stereocilia in the cochlear basal turn and distortion in the middle turn on electron microscopy [49]. Nevertheless, hearing in mice deficient in TNFR1 or TNFR2 has not been well characterized.

TNFα may also affect hearing via a vascular-based mechanism through modulation of the cochlear microcirculation, and inhibition of TNFα may improve cochlear blood flow. Blood flow in the cochlear lateral wall vessels in guinea pigs was reduced in a dose-dependent fashion after perfusion with TNFα as measured by intravital microscopy, which was abrogated when the animals received Etanercept either as pretreatment prophylaxis or post-treatment rescue [50, 51]. In guinea pigs exposed to acoustic trauma, administration of Etanercept led to increased cochlear blood flow and preservation of ABR thresholds [50, 52]. In a separate study, guinea pigs undergoing cochlear implantation received instillation of Etanercept into the perilymph via an osmotic mini-pump. This resulted in preservation and recovery of hearing thresholds as early as 3 days post-operatively, with maximal benefits at 28 days [53].

TNFα in Hearing Disorders

Much of the existing literature has focused on the efficacy of TNF antagonist therapies in disorders of the inner ear where autoimmunity may play an important role, including autoimmune inner ear disease (AIED), idiopathic sudden sensorineural hearing loss (ISSNHL), and Meniere’s disease (MD). AIED is a syndrome of progressive, often fluctuating, rapid bilateral SNHL. In AIED, immunosuppressive therapies such as high-dose corticosteroids [54], methotrexate [55], and cyclophosphamide [56] have been shown to have some benefit albeit with significant toxicity. In ISSNHL, Demirhan et al. reported that patients who did not respond to glucocorticoids had higher serum levels of TNFα 6-weeks after treatment, suggesting that TNFα may play a role in the steroid response [57]. In a small group of subjects with non-glucocorticoid responsive sudden hearing loss, treatment with Etanercept led to some recovery of hearing within two weeks [58]. Nonetheless, controversies remain regarding the levels of pro-inflammatory cytokines, including TNFα, IL-6 and IL-8, in patients with SSNHL [59].

Meniere’s disease (MD) is an inner ear disorder characterized by intermittent episodes of vertigo, fluctuating SNHL, tinnitus, and aural fullness. While the precise etiology of MD is still unclear, autoimmunity appeared to be closely associated with its pathogenesis. To support this hypothesis, elevated levels of circulating immune complexes were found in a group of 30 patients with MD [60]. Elsewhere, elevated thyroid autoantibodies were found in a higher proportion of MD patients than healthy subjects [61]. In a large series of patients with both MD and SNHL, there was a higher prevalence of autoimmune disorders including RA, systemic lupus erythematosus and ankylosing spondylitis than expected for the general population. However, no significant difference in the serum levels of pro-inflammatory cytokines including TNFα was observed in patients with MD versus those without [62].

In patients with VS, TNFα has been identified as a putative ototoxic molecule in tumor secretions. This was demonstrated in several ways by Dilwali et al. [63]. There was a positive correlation between serum TNFα level and the degree of SNHL in patients with sporadic unilateral VS. When tumor secretions were applied to murine cochlear explants, the extent of cochlear cellular damage correlated with the severity of hearing loss. Secretions from tumors with the most substantial SNHL led to the most significant damage to hair cells and spiral ganglion neuronal fibers, whereas secretions from matched tumors without hearing loss led to minimal damage. Finally, to investigate whether tumor-secreted TNFα has a direct role in mediating ototoxicity, TNFα was neutralized in tumor secretions with a blocking antibody, which significantly reduced secretion-induced damage in cochlear explants. Applying recombinant TNFα directly to cochlear explants resulted in neurite loss and disorganization in the cochlear basal turn. Together, these results suggested that TNFα plays a key role in mediating SNHL in VS by directly damaging hair cells and neurites.

Anti-TNFα Therapy in Hearing Restoration and Vestibular Schwannomas

Given the pleotropic role of TNFα in orchestrating the inflammatory response in animal models of hearing loss, there have been numerous investigations on the efficacy of pharmacological targeting of TNF, albeit with mixed results (Table 3). In a series of 9 patients with AIED dependent on steroid treatment, intratympanic administration of Infliximab, a monoclonal anti-TNFα antibody, allowed tapering of steroids and improvement in pure tone average (PTA) thresholds [64]. Elsewhere in a series of 10 patients with steroid-dependent AIED, intratympanic injection of Golimumab, an antibody that binds both soluble and membrane-bound TNFα, resulted in hearing stabilization [65]. There is a case report of a patient with both RA and steroid-refractory SNHL where simultaneous resolution of arthritis and improvement of hearing were seen after the administration of Adalimumab, an antibody that binds TNFα and blocks receptor interactions [66]. In a retrospective small series of 12 patients with bilateral, long-standing immune-mediated SNHL treated with Etanercept, 92% of patients had stabilization or improvement of hearing and 90% had improvement of vertigo over short-term follow-up [67]. Similar results were reported in an open-label study of Etanercept in immune-mediated hearing loss and Meniere’s disease, where hearing was stabilized in 87% of patients [68]. However, conflicting results were reported in a randomized, placebo-controlled study of 20 patients with AIED, where 8-week treatment with Etanercept did not show any significant benefit in hearing restoration [69]. Therefore, larger series are still needed to validate the efficacy of TNF biologics in immune-mediated hearing loss.

Table 3.

Anti-TNFα therapy for hearing loss

Study (ref)	Indication	N	Agent	Route	Outcomes
Prospective, non-randomized, open-label [64]	AIED	9	Infliximab	Intratympanic (4 weeks)	80% tapered off steroids; 75% PTA improved to 22.6 dB; reduced recurrence of hearing loss episodes
Prospective, non-randomized, open-label [65]	AIED	10	Golimumab	Intratympanic (5 weeks)	71% stable PTA, 57% stable WR; 28% improved WR
Randomized, placebo-controlled [69]	AIED	20	Etanercept	Subcutaneous (8 weeks)	Treatment no better than placebo (1 of 8 treatment vs. 2 of 9 placebo improved PTA >10 dB and/or improved WR>12%)
Case report [66]	AIED; RA	1	Infliximab	Systemic (indefinite)	Long-term stabilization of hearing loss
Retrospective [67]	IMCVD	12	Etanercept	Subcutaneous (28 weeks)	92% improved hearing and tinnitus, 88% improved vertigo, 89% improved aural fullness
Prospective, non-randomized, open-label [68]	IMCVD; MD	23	Etanercept	Subcutaneous (24 weeks)	30% improved PTA > 10dB at least one ear; 26% improved WR > 12% at least one ear
Prospective, non-randomized, open-label [58]	ISSNHL	12	Etanercept	Subcutaneous (12 weeks)	85% with acute hearing loss improved PTA by 30.3 dB
N/A [63]	VS	2	Anti- TNFα antibody	Applied to cochlear explants	Prevented IHC and OHC loss

Latest work from our laboratory and others have begun to uncover the ototoxic properties of TNFα in VS tumor secretions as well as therapeutic strategies targeting TNFα for hearing loss, both in cochlear explants in vitro and in animal models in vivo. In one study, over 70% of VS tissues expressed TNFα, whereas only 5% of healthy vestibular nerve controls stained positive by immunohistochemistry [70]. To ascertain whether neutralization of TNFα in VS tumor secretions can rescue hair cell damage and neurite loss, an anti-human TNFα antibody was added to secretions from patients with VS and significant SNHL that also contained high levels of TNFα. Reduction of TNFα by nearly 80% prevented the loss of IHCs in the apical and basal turns of murine cochlear explants but did not affect the fiber organization of neurites [63].

Recently, we have also begun to develop a nanotechnology-based platform for preclinical testing of therapeutics in VS [71]. Fresh tumor cells derived from patients with sporadic VS were found to express two unique receptors, αvβ3/β5 integrins and neuropilin-1, which distinguish them from normal tissue counterparts. Nanoparticles were generated from a mixture of tandem peptides consisting of a targeting domain that selectively binds αvβ3/β5 integrins, and a membrane-penetrating domain that complexes with short interfering RNA (siRNA) against TNFα. The nanoparticles were taken up by primary VS cultures, delivered TNFα-specific siRNA, and resulted in potent suppression of TNFα secretion by VSs when exposed to a pro-inflammatory stimulus. This is the first study that demonstrated tumor-targeting nanotechnology and RNA interference can be leveraged synergistically to mitigate the secretion of ototoxic molecules including TNFα. As more promising therapeutic targets for VS and VS-associated hearing loss emerge from large-scale genome-wide screens [72], the nanoparticle system can be adapted to validate other genes of interest, and ultimately identify those suitable for in vivo testing and clinical translation.

Furthermore, targeting TNFα in VS may also have beneficial effects on tumor growth through interactions with other molecular pathways, which in turn regulate TNFα levels. The transcription factor NFκB mediates the induction of COX-2 expression and production of reactive oxygen species via proinflammatory cytokines including TNFα [73, 74]. Specifically, anti-inflammatory medications that signal through NFκB [75] or COX-2 [76] were found to reduce VS cellular proliferation in vitro, in line with our findings that aspirin intake correlated with reduced growth of VS in vivo [77]. Taken together, our findings have not only motivated the ongoing prospective, randomized, placebo-controlled phase II clinical trial of aspirin for VSs, but also further investigation in the role of TNF inhibition as an anti-tumor therapy to suppress VS growth [78].

Given the immunosuppressive nature of anti-TNF therapy, there is concern of increased incidence of adverse events such as serious and opportunistic infections including tuberculosis, and malignancies such as skin cancers and lymphomas in patients undergoing therapy. Importantly, studies suggest that chronic inflammation in these conditions is itself associated with an increased risk for lymphoma [79–81]. Indeed, long-term safety studies from clinical trials showed the overall incidence of lymphoma was approximately 105.9 per 100,000 person-years, similar to the population of RA patients not on anti-TNF biologics, but greater than age- and gender-matched general populations without RA [82–84].

Conclusions

TNFα is a pleiotropic cytokine with diverse functions, including maintaining cellular homeostasis and mediating the inflammatory response in various disease states. Insights into the underlying biology of TNF signaling have led to the development of molecularly-targeted therapeutics in inflammatory conditions where aberrant TNF activation plays an important role. Selective inhibition of TNF has yielded moderate benefits in disorders of hearing loss although results are limited to animal models and early clinical trials.

Recent insights into VS biology have further enriched our understanding of TNF-mediated cellular injury, especially in the context of VS-associated cochlear damage and hearing loss. Further, large-scale genome-wide screens have led to the discovery of additional pathways aberrantly activated to promote tumor survival and hearing loss. Deployment of new delivery strategies such as tumor-targeted nanotechnology to address these targets, including TNFα, represents a new therapeutic paradigm for the treatment of VS and VS-associated hearing loss.

Many important challenges remain in our understanding of the complex roles TNFα plays in hearing loss, and how to best translate this knowledge into better treatments. Looking forward, several important research directions include development of a deeper understanding of the pathways that underlie and regulate TNFα responses, discovery of TNF-induced molecules that may be responsible for sustaining chronic inflammation and cochlear damage, and identification of additional markers in patients that may predict clinical response to anti-TNF therapy. Future challenges ahead lie in creating biologics that selectively blunt the deleterious effects of TNFα and can be delivered specifically to tumor cells in patients with VS and debilitating hearing loss.