Antioxidant protection in a new animal model of cisplatin-induced ototoxicity

Dexamethasone reduces cisplatin-induced hair cell damage by inducing cisplatin resistance through metallothionein-2

PURPOSE

Hair cell damage is a common side effect caused by the anticancer drug cisplatin (CDDP), which reduces patient quality of life. One CDDP resistance mechanism that occurs in recurrent cancers is heavy metal detoxification by metallothionein-2 (mt2). Here, we show that in zebrafish larvae, dexamethasone (DEX) reduces CDDP-induced hair cell damage by enhancing mt2 expression.

METHODS

Transgenic zebrafish (cldn: gfp; atoh1: rfp) that express green and red fluorescent proteins in neuromasts and hair cells, respectively, were used. The zebrafish were pretreated with DEX at 52 h post-fertilization (hpf) for 8 h, followed by CDDP treatment for 12 h. The lateral line hair cells of CDDP-treated zebrafish at 72 hpf were observed by fluorescence microscopy.

RESULTS

Reporting odds ratio (ROR) analysis using an adverse event database indicated an association between a decrease in CDDP-induced ototoxicity and DEX as an antiemetic treatment for cancer chemotherapy. Pretreatment with DEX protected 72 hpf zebrafish hair cells from CDDP-induced damage. The expression of mt2 mRNA was significantly increased by the combination of 10 µM DEX with CDDP. Gene editing of mt2 reversed the protective effect of DEX against CDDP-induced damage in hair cells.

CONCLUSION

DEX protects hair cells from CDDP-induced damage through increased mt2 expression, which is a resistance mechanism for platinum-based anticancer drugs.

Hear and Now: Ongoing Clinical Trials to Prevent Drug-Induced Hearing Loss

Each year over half a million people experience permanent hearing loss caused by treatment with therapeutic drugs with ototoxic side effects. There is a major unmet clinical need for therapies that protect against this hearing loss without reducing the therapeutic efficacy of these lifesaving drugs. At least 17 clinical trials evaluating 10 therapeutics are currently underway for therapies aimed at preventing aminoglycoside- and/or cisplatin-induced ototoxicity. This review describes the preclinical and clinical development of each of these approaches, provides updates on the status of ongoing trials, and highlights the importance of appropriate outcome measures in trial design and the value of reporting criteria in the dissemination of results.

Microneedle-mediated drug delivery for cutaneous diseases

Microneedles have garnered significant interest as transdermal drug delivery route owing to the advantages of nonselective loading capacity, minimal invasiveness, simple operation, and good biocompatibility. A number of therapeutics can be loaded into microneedles, including hydrophilic and hydrophobic small molecular drugs, and macromolecular drugs (proteins, mRNA, peptides, vaccines) for treatment of miscellaneous diseases. Microneedles feature with special benefits for cutaneous diseases owing to the direct transdermal delivery of therapeutics to the skin. This review mainly introduces microneedles fabricated with different technologies and transdermal delivery of various therapeutics for cutaneous diseases, such as psoriasis, atopic dermatitis, skin and soft tissue infection, superficial tumors, axillary hyperhidrosis, and plantar warts.

The protective effect of aspirin-induced temporary threshold shift in an animal model of cisplatin-related ototoxicity

PURPOSE

The purpose of this study was to evaluate whether induction of temporary threshold shift (TTS) with aspirin prior to cisplatin exposure can prevent or minimize cisplatin detrimental effects on hearing.

METHODS

We randomly divided BALB mice into three groups: (1) cisplatin only, (2) aspirin only, and (3) combined aspirin/cisplatin. Cisplatin was administered as a single intraperitoneal injection of 14 mg/kg. Aspirin was administered for three weeks via intraperitoneal injection of 200 mg/kg sodium salicylate, twice daily. Air conduction thresholds were recorded using Auditory Brainstem Responses (ABR). Cochleae were harvested and cochlear hair cells were counted using a scanning electron microscope (SEM).

RESULTS

Aspirin-induced TTS have reached an average of 30.05±16.9 dB after 2 weeks. At 60 days, cisplatin-only treated mice experienced an average threshold shifts of 50.7 dB at 4 kHz, 35.16 dB at 8 kHz, 70 dB at 16 kHz, 53.1 dB at 32 kHz. All threshold shifts were significantly worse than for cisplatin/aspirin treated mice with TTS of 11.85 dB at 4 kHz, 3.58 dB at 8 kHz, 16.58  dB at 16 kHz, 20.41 dB at 32 kHz (p < 0.01). Cochlear cell count with SEM has shown reduction in the number of both inner and outer hair cells in the mid-turn in cisplatin treated mice.

CONCLUSION

Aspirin induced TTS can protect from cisplatin-induced ototoxicity. This beneficial effect was demonstrated by auditory thresholds as well as SEM. Larger pre-clinical and clinical studies are still needed to confirm these findings.

Strategies to reduce the risk of platinum containing antineoplastic drug-induced ototoxicity

INTRODUCTION

Cisplatin is a highly effective chemotherapeutic agent against a variety of solid tumors in adults and in children. Unfortunately, a large percentage of patients suffer permanent sensorineural hearing loss. Up to 60% of children and at least 50% of adults suffer this complication that seriously compromises their quality of life. Hearing loss is due to damage to the sensory cells in the inner ear. The mechanisms of cochlear damage are still being investigated. However, it appears that inner ear damage is triggered by reactive oxygen species (ROS) formation and inflammation 34.

AREAS COVERED

We discuss a number of potential therapeutic targets that can be addressed to provide hearing protection. These strategies include enhancing the endogenous antioxidant pathways, heat shock proteins, G protein coupled receptors and counteracting ROS and reactive nitrogen species, and blocking pathways that produce inflammation, including TRPV1 and STAT1 36.

EXPERT OPINION

Numerous potential protective agents show promise in animal models by systemic or local administration. However, clinical trials have not shown much efficacy to date with the exception of sodium thiosulfate. There is an urgent need to discover safe and effective protective agents that do not interfere with the efficacy of cisplatin against tumors yet preserve hearing 151.

Cisplatin-induced ototoxicity in organotypic cochlear cultures occurs independent of gap junctional intercellular communication

Cisplatin is a very effective chemotherapeutic, but severe and permanent hearing loss remains a prevalent side effect. The processes underpinning cisplatin-induced ototoxicity are not well understood. Gap junction channels composed of connexin (Cx) subunits allow for the passage of small molecules and ions between contacting neighboring cells. These specialized channels have been postulated to enhance cisplatin-induced cell death by spreading “death signals” throughout the supporting cells of the organ of Corti. This study sought to investigate the role of Cx43 in cisplatin-induced ototoxicity using organotypic cochlear cultures from control and two Cx43-mutant mouse strains harboring either a moderate (Cx43^I130T/+) or severe (Cx43^G60S/+) reduction of Cx43 function. Cochlear cultures from Cx43-mutant mice with a severe reduction in Cx43-based gap junctional intercellular communication (GJIC) had an enhanced number of hair cells that were positive for cleaved caspase 3, a marker of active apoptosis, after cisplatin treatment. In cisplatin-treated organotypic cochlear cultures, there was a decrease in the co-localization of Cx26 and Cx30 compared with untreated cultures, suggesting that cisplatin causes reorganization of connexin composition in supporting cells. Both Cx26 and Cx30 protein expression as well as GJIC were decreased in organotypic cochlear cultures treated with the gap-junction blocker carbenoxolone. When cisplatin and carbenoxolone were co-administered, there were no differences in hair cell loss compared with cisplatin treatment alone. Using cisplatin-treated control and Cx43-ablated organ of Corti derived HEI-OC1 mouse cells, we found that greatly reducing GJIC led to preferential induction of an ER stress pathway. Taken together, this study strongly suggests that inhibition of GJIC in organ of Corti cells does not lead to differential susceptibility to cisplatin-induced ototoxicity. Although cisplatin causes the same degree of cell death in gap junction competent and incompetent cochlear cells, the engagement of the mitochondrial dysregulation and ER stress differs.

Increased oxidative stress, inflammation, and glutamate: Potential preventive and therapeutic targets for hearing disorders

Hearing disorders constitute one of the major health concerns in the USA. Decades of basic and clinical studies have identified numerous ototoxic agents and investigated their modes of action on the inner ear, utilizing tissue culture as well as animal and human models. Current preventive and therapeutic approaches are considered unsatisfactory. Therefore, additional modalities should be developed. Many studies suggest that increased levels of oxidative stress, chronic inflammation, and glutamate play an important role in the initiation and progression of damage to the inner ear leading to hearing impairments. To prevent these cellular deficits, antioxidants, anti-inflammatory agents, and antagonists of glutamate receptor have been used individually or in combination with limited success. It is essential, therefore, to simultaneously enhance the levels of antioxidant enzymes by activating the Nrf2 (a nuclear transcriptional factor) pathway, dietary and endogenous antioxidant compounds, and B12-vitamins in order to reduce the levels of oxidative stress, chronic inflammation, and glutamate at the same time. This review presents evidence to show that increased levels of these cellular metabolites, biochemical or factors are involved in the pathogenesis of cochlea leading to hearing impairments. It presents scientific rationale for the use of a mixture of micronutrients that may decrease the levels of oxidative damage, chronic inflammation, and glutamate at the same time. The benefits for using oral administration of proposed micronutrient mixture in humans are presented. Animal and limited human studies indirectly suggest that orally administered micronutrients can accumulate in the inner ear. Therefore, this route of administration may be useful in prevention, and in combination with standard care, in improved management of hearing problems following exposure to well-recognized and studied ototoxic agents, such as noise, cisplatin, aminoglycoside antibiotics, and advanced age.

Dexamethasone-loaded chitosan-based genipin-cross-linked hydrogel for prevention of cisplatin induced ototoxicity in Guinea pig model

OBJECTIVES

The aim of this study was to investigate the protective effects of a sustained release form of dexamethasone (dex) loaded chitosan-based genipin-cross-linked hydrogel (CBGCH) in a guinea pig model of cisplatin (CP) induced hearing loss.

METHODS

Implantation of CBGCH was made by intratympanic (IT) injection. Ototoxicity was produced by intraperitoneal (IP) single dose of 14 mg/kg CP. Animals were randomly divided into four groups with 6 guinea pigs in each. Group 1 received only IP CP; group 2 received only IT dex-loaded CBGCH injections. Group 3 and group 4 received IP CP, plus IT nondrug CBGCH and IT dex-loaded CBGCH respectively 24 h prior to IP CP injections. Distortion product otoacoustic emissions (DPOAEs) and auditory brainstem response (ABR) measurements were obtained before the treatments and solely ABR measurements were done after 3 and 10 days. The ultrastructural effects were investigated by scanning electron microscopy (SEM) analysis.

RESULTS

The postCP ABR thresholds at 4, 8, 12, 16, 32 kHz frequencies were significantly better in group 4 than groups 1 and 3 (p < 0.05). The comparison of time effective ABR thresholds between groups 1 and 4 and between groups 3 and 4 showed significantly lower ABR thresholds in group 4 (p < 0.05). The SEM analysis showed that stereocilia of inner and outer hair cells were preserved in group 4, almost like group 2, whereas cytotoxic degenerations were noted in groups 1 and 3.

CONCLUSIONS

Intratympanic administration of dex-loaded CBGCH has been shown to provide functional and structural protection against CP-induced ototoxicity.

An optimized, clinically relevant mouse model of cisplatin-induced ototoxicity

Cisplatin-induced ototoxicity results in significant, permanent hearing loss in pediatric and adult cancer survivors. Elucidating the mechanisms underlying cisplatin-induced hearing loss as well as the development of therapies to reduce and/or reverse cisplatin ototoxicity have been impeded by suboptimal animal models. Clinically, cisplatin is most commonly administered in multi-dose, multi-cycle protocols. However, many animal studies are conducted using single injections of high-dose cisplatin, which is not reflective of clinical cisplatin administration protocols. Significant limitations of both high-dose, single-injection protocols and previous multi-dose protocols in rodent models include high mortality rates and relatively small changes in hearing sensitivity. These limitations restrict assessment of both long-term changes in hearing sensitivity and effects of potential protective therapies. Here, we present a detailed method for an optimized mouse model of cisplatin ototoxicity that utilizes a multi-cycle administration protocol that better approximates the type and degree of hearing loss observed clinically. This protocol results in significant hearing loss with very low mortality. This mouse model of cisplatin ototoxicity provides a platform for examining mechanisms of cisplatin-induced hearing loss as well as developing therapies to protect the hearing of cancer patients receiving cisplatin therapy.

Study of the protective effect of dexamethasone on cisplatin-induced ototoxicity in rats

PURPOSE

To evaluate the ability of dexamethasone to protect against cisplatin (CDDP)-induced ototoxicity.

METHODS

Male Wistar rats were divided into the following three groups: 1) Control (C): 6 animals received intraperitoneal (IP) saline solution, 8 ml/kg/day for four days; 2) C + CDDP: 11 animals received 8 ml/kg/day of IP saline and, 90 min after saline administration, 8 mg/kg/day of IP CDDP for four days; and 3) DEXA15 + CDDP: 11 animals received IP dexamethasone 15 mg/kg/day and, 90 min after dexamethasone administration, received 8 mg/kg/day of IP CDDP for four days.

RESULTS

It was found that dexamethasone did not protect against weight loss in CDDP-exposed animals. The mortality rate was comparable with that previously reported in the literature. The auditory threshold of animals in the DEXA15 + CDDP group was not significantly altered after exposure to CDDP. The stria vascularis of animals in the DEXA15 + CDDP group was partially preserved after CDDP exposure.

CONCLUSIONS

Dexamethasone at the dose of 15 mg/kg/day partially protected against CDDP-induced ototoxicity, based on functional evaluation by brainstem evoked response audiontry (BERA) and morphological evaluation by optical microscopy. However, dexamethasone did not protect against systemic toxicity.

Cisplatin is retained in the cochlea indefinitely following chemotherapy

Cisplatin chemotherapy causes permanent hearing loss in 40–80% of treated patients. It is unclear whether the cochlea has unique sensitivity to cisplatin or is exposed to higher levels of the drug. Here we use inductively coupled plasma mass spectrometry (ICP-MS) to examine cisplatin pharmacokinetics in the cochleae of mice and humans. In most organs cisplatin is detected within one hour after injection, and is eliminated over the following days to weeks. In contrast, the cochlea retains cisplatin for months to years after treatment in both mice and humans. Using laser ablation coupled to ICP-MS, we map cisplatin distribution within the human cochlea. Cisplatin accumulation is consistently high in the stria vascularis, the region of the cochlea that maintains the ionic composition of endolymph. Our results demonstrate long-term retention of cisplatin in the human cochlea, and they point to the stria vascularis as an important therapeutic target for preventing cisplatin ototoxicity.

Permanent hearing loss occurs in many cancer patients treated with cisplatin. In this study, the authors examine cisplatin pharmacokinetics in the cochleae of mice and humans showing that cisplatin is retained for months to years after treatment.

Dose-dependent cochlear and vestibular toxicity of trans-tympanic cisplatin in the rat

In vivo studies are needed to study cisplatin ototoxicity and to evaluate candidate protective treatments. Rats and mice are the preferred species for toxicological and pharmacological pre-clinical research, but systemic administration of cisplatin causes high morbidity in these species. We hypothesized that trans-tympanic administration of cisplatin would provide a good model for studying its auditory and vestibular toxicity in the rat. Cisplatin was administered by the trans-tympanic route in one ear (50μl, 0.5-2mg/ml) of rats of both sexes and two different strains. Cochlear toxicity was corroborated by histological means. Vestibular toxicity was demonstrated by behavioral and histological analysis. Cisplatin concentrations were assessed in inner ear after trans-tympanic and i.v. administration. In all experiments, no lethality and only scant body weight loss were recorded. Cisplatin caused dose-dependent cochlear toxicity, as demonstrated by hair cell counts in the apical and middle turns of the cochlea, and vestibular toxicity, as demonstrated by behavioral analysis and hair cell counts in utricles. High concentrations of cisplatin were found in the inner ear after trans-tympanic administration. In comparison, i.v. administration resulted in lower inner ear concentrations. We conclude that trans-tympanic administration provides an easy, reproducible and safe model to study the cochlear and vestibular toxicity of cisplatin in the rat. This route of exposure may be useful to address particular questions on cisplatin induced ototoxicity and to test candidate protective treatments.

The in vitro protective effect of salicylic acid against paclitaxel and cisplatin-induced neurotoxicity

Paclitaxel (PAC) and cisplatin (CIS) are two established chemotherapeutic drugs used in combination for the treatment of various solid tumors. However, the usage of PAC and CIS are limited because of the incidence of their moderate or severe neurotoxic side effects. In this study, we aimed to assess the protective role of salicylic acid (SA) against neurotoxicity caused by PAC and CIS. For this purpose, newborn Sprague Dawley rats were decapitated in sterile atmosphere and primary cortex neuron cultures were established. On the 10th day SA was added into culture plates. PAC and CIS were added on the 12th day. The cytotoxicity was determined by using the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. Oxidative alterations were assessed using total antioxidant capacity and total oxidative stress assays in rat primary neuron cell cultures. It was shown that both concentrations of PAC and CIS treatments caused neurotoxicity. Although SA decreased the neurotoxicity by CIS and PAC, it was more effective against the toxicity caused by CIS rather than the toxicity caused by PAC. In conclusion it was clearly revealed that SA decreased the neurotoxic effect of CIS and PAC in vitro.

Cisplatin-Induced Ototoxicity: Effects, Mechanisms and Protection Strategies

Cisplatin is a highly effective chemotherapeutic agent that is widely used to treat solid organ malignancies. However, serious side effects have been associated with its use, such as bilateral, progressive, irreversible, dose-dependent neurosensory hearing loss. Current evidence indicates that cisplatin triggers the production of reactive oxygen species in target tissues in the inner ear. A variety of agents that protect against cisplatin-induced ototoxicity have been successfully tested in cell culture and animal models. However, many of them interfere with the therapeutic effect of cisplatin, and therefore are not suitable for systemic administration in clinical practice. Consequently, local administration strategies, namely intratympanic administration, have been developed to achieve otoprotection, without reducing the antitumoral effect of cisplatin. While a considerable amount of pre-clinical information is available, clinical data on treatments to prevent cisplatin ototoxicity are only just beginning to appear. This review summarizes clinical and experimental studies of cisplatin ototoxicity, and focuses on understanding its toxicity mechanisms, clinical repercussions and prevention strategies.

Curcuma longa (curcumin) decreases in vivo cisplatin-induced ototoxicity through heme oxygenase-1 induction

HYPOTHESIS

To investigate whether curcumin may have in vivo protective effects against cisplatin ototoxicity by its direct scavenger activity and/or by curcumin-mediated upregulation of HO-1.

BACKGROUND

Cisplatin-induced ototoxicity is a major dose-limiting side effect in anticancer chemotherapy. A protective approach to decrease cisplatin ototoxicity without compromising its therapeutic efficacy remains a critical goal for anticancer therapy. Recent evidences indicate that curcumin exhibits antioxidant, anti-inflammatory, and chemosensitizer activities.

METHODS

In male adult Wistar rats, a curcumin dose of 200 mg/kg, selected from a dose-response curve, was injected 1 hour before cisplatin administration and once daily for the following 3 days. A single dose of cisplatin (16 mg/kg) was administered intraperitoneally. Rats were divided as follows: 1) control, 2) curcumin control, 3) vehicle control, 4) cisplatin, 5) cisplatin+ vehicle, and 6) curcumin+cisplatin. ABRs were measured before and at Days 3 and 5 after cisplatin administration. Rhodamine-phalloidin staining, 4-hydroxy-2-nonenal and heme-oxigenase-1 immunostainings, and Western blot analyses were performed to assess and quantify OHC loss, lipid peroxidation, and the endogenous response to cisplatin-induced damage and to curcumin protection.

RESULTS

Curcumin treatment attenuated hearing loss induced by cisplatin, increased OHC survival, decreased 4-HNE expression, and increased HO-1 expression.

CONCLUSION

This preclinical study demonstrates that systemic curcumin attenuates ototoxicity and provides molecular evidence for a role of HO-1 as an additional mediator in attenuating cisplatin-induced damage.

Sound preconditioning therapy inhibits ototoxic hearing loss in mice

Therapeutic drugs with ototoxic side effects cause significant hearing loss for thousands of patients annually. Two major classes of ototoxic drugs are cisplatin and the aminoglycoside antibiotics, both of which are toxic to mechanosensory hair cells, the receptor cells of the inner ear. A critical need exists for therapies that protect the inner ear without inhibiting the therapeutic efficacy of these drugs. The induction of heat shock proteins (HSPs) inhibits both aminoglycoside- and cisplatin-induced hair cell death and hearing loss. We hypothesized that exposure to sound that is titrated to stress the inner ear without causing permanent damage would induce HSPs in the cochlea and inhibit ototoxic drug–induced hearing loss. We developed a sound exposure protocol that induces HSPs without causing permanent hearing loss. We used this protocol in conjunction with a newly developed mouse model of cisplatin ototoxicity and found that preconditioning mouse inner ears with sound has a robust protective effect against cisplatin-induced hearing loss and hair cell death. Sound therapy also provided protection against aminoglycoside-induced hearing loss. These data indicate that sound preconditioning protects against both classes of ototoxic drugs, and they suggest that sound therapy holds promise for preventing hearing loss in patients receiving these drugs.

Oxidative stress and neurodegenerative disorders

Living cells continually generate reactive oxygen species (ROS) through the respiratory chain during energetic metabolism. ROS at low or moderate concentration can play important physiological roles. However, an excessive amount of ROS under oxidative stress would be extremely deleterious. The central nervous system (CNS) is particularly vulnerable to oxidative stress due to its high oxygen consumption, weakly antioxidative systems and the terminal-differentiation characteristic of neurons. Thus, oxidative stress elicits various neurodegenerative diseases. In addition, chemotherapy could result in severe side effects on the CNS and peripheral nervous system (PNS) of cancer patients, and a growing body of evidence demonstrates the involvement of ROS in drug-induced neurotoxicities as well. Therefore, development of antioxidants as neuroprotective drugs is a potentially beneficial strategy for clinical therapy. In this review, we summarize the source, balance maintenance and physiologic functions of ROS, oxidative stress and its toxic mechanisms underlying a number of neurodegenerative diseases, and the possible involvement of ROS in chemotherapy-induced toxicity to the CNS and PNS. We ultimately assess the value for antioxidants as neuroprotective drugs and provide our comments on the unmet needs.

Dexamethasone otoprotection in a multidose cisplatin ototoxicity mouse model

OBJECTIVE

To develop a murine model for multidose administration of cisplatin that produces significant hearing threshold elevations and to use this model to assess the protective properties of intratympanic (IT) dexamethasone against cisplatin ototoxicity.

STUDY DESIGN

Controlled repeated measures design.

SETTING

Translational research laboratory.

SUBJECTS AND METHODS

Intraperitoneal (IP) cisplatin, 2 or 3 mg/kg/day, was administered for a total of 5 or 10 days in young CBA/J mice. Pure-tone evoked auditory brainstem response (ABR) thresholds were performed on days 7, 14, 21, and 28 to evaluate hearing threshold shifts. After development of the optimal dosing regimen, 15 mice received IT dexamethasone (24 mg/ml) in one ear and IT saline in the contralateral ear.

RESULTS

Significant threshold elevations were obtained for the 2 and 3 mg/kg/day 10 day groups, but both had high mortality rates and were excluded as potential multidose murine models. The 3 mg/kg/day 5 day group had a lower mortality rate and significant ABR threshold elevations for all frequencies on days 7, 14, 21, and 28. Using this dosing model, no statistically significant difference between IT dexamethasone and saline treated ears was found.

CONCLUSIONS

Unlike previous single dose models, IT dexamethasone did not have an otoprotective effect in a multidose murine model of cisplatin ototoxicity.

Comparison of cochlear cell death caused by cisplatin, alone and in combination with furosemide

Establishment of appropriate animal models is an important step in exploring the mechanisms of drug-induced ototoxicity. In the present study, using guinea pigs we compared cochlear lesions induced by cisplatin administered in two regimens: consecutive application alone and in combination with furosemide. The effects of furosemide alone were also evaluated; it was found to cause temporary hearing loss and reversible damage to the stria vascularis. Consecutive application of cisplatin alone appeared to be disadvantageous because it resulted in progressive body weight loss and higher mortality compared to the combined regimen, which used a smaller cisplatin dose. The combined regimen resulted in comparable hearing loss and hair cell loss but a markedly lower mortality. However, their coadministration failed to cause similar damage to spiral ganglion neurons (SGN), as seen in animals that received cisplatin alone. This difference suggests that the combined regimen did not mimic the damage to cochlear neuronal innervation caused by the clinical application of cisplatin. The difference also suggests that the SGN lesion is not caused by cisplatin entering the cochlea via the stria vascularis.

Ototoxicity in dogs and cats

A variety of drugs in veterinary use have side effects that can potentially damage the senses of hearing or balance in animals. A large body of literature exists on the incidence and mechanisms of ototoxicity in experimental animals and in humans, but little is documented in domestic dogs and cats. However, the generality of these adverse actions across species allows one to extrapolate and provide the veterinarian with insight into possible complications of chemotherapy.

Review: ototoxic characteristics of platinum antitumor drugs

Cisplatin, carboplatin, nedaplatin, and oxaliplatin are widely used in contemporary oncology; however, their ototoxic and neurotoxic side effects are quite different as discussed in this review. Cisplatin is considered the most ototoxic, but despite its reputation, the magnitude of hair cell loss that occurs with a single, large drug bolus is limited and confined to the base of the cochlea. For all of these platinum compounds, a major factor limiting damage is drug uptake from stria vascularis into the cochlear fluids. Disrupting the blood-labyrinth barrier with diuretics or noise exposure enhances drug uptake and significantly increases the amount of damage. Combined treatment with ethacrynic acid (a loop diuretic) and cisplatin results in rapid apoptotic hair cell death characterized by upregulation of initiator caspase-8 and membrane death receptor, TRADD, followed by downstream executioners, caspase-3 and caspase-6. Unlike cisplatin, nedaplatin and oxaliplatin are highly neurotoxic when applied to cochlear cultures preferentially damaging auditory nerve fibers at low concentrations and hair cells at high concentrations. Carboplatin, considered far less ototoxic than cisplatin, is paradoxically highly toxic to chinchilla inner hair cells and type I spiral ganglion neurons; however, at high doses it also damages outer hair cells. Hair cell death from cisplatin and carboplatin is characterized in its early stages by upregulation of p53; blocking p53 expression with pifithrin-α prevents hair cell death. Major differences in the toxicity of these four platinum compounds may arise from several different metal transporters that selectively regulate the influx, efflux, and sequestration of these drugs.

Lycium barbarum polysaccharides attenuate cisplatin-induced hair cell loss in rat cochlear organotypic cultures

The aim of the present study was to investigate the effects of Lycium barbarum polysaccharides (LBP) on cisplatin-induced hair cell damage in the organ of Corti explant. The neonatal (P2–3) rat organ of Corti explant was exposed to cisplatin (20 μM; 48 h) with or without LBP pretreatment (150 and 600 μg/mL; 24 h). Hair cell loss was indicated by FITC-labeled phalloidin staining. The level of reactive oxygen species (ROS) and alteration of mitochondrial membrane potential (ΔΨ_m) in hair cells were analyzed using fluorescent probes 2′,7′-dichlorofluorescein diacetate and JC-1, respectively. The results showed that LBP significantly attenuated hair cell loss (p < 0.01). Hair cells pretreated with LBP showed significant reduction in ROS production and the decline of ΔΨ_m compared with cisplatin alone group (p < 0.01), indicating the protective effect of LBP on cisplatin-induced hair cell loss. Taken together, these results indicate that LBP was effective in attenuating cisplatin-induced hair cell loss by reducing the production of ROS and maintaining mitochondrial ΔΨ_m.

Mechanisms Involved in Ototoxicity

The modern era of evidence-based ototoxicity emerged in the 1940s following the discovery of aminoglycosides and their ototoxic side effects. New classes of ototoxins have been identified in subsequent decades, notably loop diuretics, antineoplastic drugs, and metal chelators. Ototoxic drugs are frequently nephrotoxic, as both organs regulate fluid and ion composition. The mechanisms of ototoxicity are as diverse as the pharmacological properties of each ototoxin, though the generation of toxic levels of reactive oxygen species appears to be a common denominator. As mechanisms of cytotoxicity for each ototoxin continue to be elucidated, a new frontier in ototoxicity is emerging: How do ototoxins cross the blood-labyrinth barrier that tightly regulates the composition of the inner ear fluids? Increased knowledge of the mechanisms by which systemic ototoxins are trafficked across the blood-labyrinth barrier into the inner ear is critical to developing new pharmacotherapeutic agents that target the blood-labyrinth barrier to prevent trafficking of ototoxic drugs and their cytotoxic sequelae.

Antioxidant micronutrient impact on hearing disorders: concept, rationale, and evidence

PURPOSE

Although auditory disorders are complex conditions, device-related modalities dominate current treatment. However, dysfunction from the central cortex to the inner ear apparatus is increasingly thought to be related to biochemical pathway abnormalities and to free radical-induced oxidative damage and chronic inflammation. Therefore, considering appropriate biologic therapy as an adjunct to standard care against these damaging factors may provide rational expansion of treatment options for otolaryngologists and audiologists.

METHODS

This review outlines the biologic concepts related to some auditory and vestibular conditions and details the current rationale for utilizing antioxidants for a spectrum of hearing disorders. The strategy is based on the authors' collective experience in antioxidant science and supported with published research, pilot animal data and preliminary clinical observations.

RESULTS

A comprehensive micronutrient approach was developed to exploit these pathways, and demonstrated safety and efficacy against oxidative damage and inflammation and clinically relevant neuroprotection. Cooperative research with Department of Defense institutions used prospective, randomized designs to show (1) reduction in oxidative damage measured in plasma and urine over six months, (2) protection against oxidative damage during 12 weeks of intense military training, (3) protection against inflammation after total body blast exposure (rodents), (4) strong neuroprotection against chemically-induced Parkinson's disease (rodents), (5) nerve VIII function improvement after concussive head injury in military personnel, and (6) tinnitus improvement in majority of patients after 90-day evaluation.

CONCLUSION

This systematic review of biologic strategies against hearing disorders combined with new animal and human observations may provide a rational basis for expanding current practice paradigms.

Auditory late effects of childhood cancer therapy: a report from the Children's Oncology Group

Children treated for malignancies may be at risk for early- or delayed-onset hearing loss that can affect learning, communication, school performance, social interaction, and overall quality of life. Survivors at particular risk include those treated with platinum compounds (cisplatin and/or carboplatin) for neuroblastoma, hepatoblastoma, osteosarcoma, or germ-cell tumors and/or those treated with radiation that affects the ear at doses of >30 Gy for pediatric head and neck tumors. The aims of the Auditory/Hearing Late Effects Task Force of the Children's Oncology Group in this report were to (1) review ototoxicity resulting from childhood cancer therapy including platinum compounds (cisplatin and carboplatin) and radiation, (2) describe briefly cochlear pathophysiology and genetics of cisplatin-related hearing loss, (3) explain the impact of hearing loss resulting from chemotherapy and radiation, and (4) offer recommendations regarding evaluation and management of pediatric patients who are at risk for treatment-related hearing loss. A questionnaire is included as a tool to assist pediatricians in assessment.

Effectiveness of different approaches for establishing cisplatin-induced cochlear lesions in mice

CONCLUSIONS

Mouse cochleae are highly resistant to systemically administered cisplatin. However, cochlear lesions can be produced effectively in mice when cisplatin is applied locally through the round window niche or tympanum.

OBJECTIVE

To explore the optimal approach for creating cisplatin-induced cochlear lesions in mice.

MATERIALS AND METHODS

Cisplatin was administered to adult C57BL/6J mice via four approaches: (1) transtympanic injection, (2) round window niche injection, (3) intraperitoneal injection (i.p.) at 4 mg/kg/day for 4 consecutive days, and (4) one 15 mg/kg dose i.p. The hearing was monitored using frequency-specific auditory brainstem responses (ABRs) and distortion-product otoacoustic emissions (DPOAEs). Cochlear pathology was observed in cochleograms with Harris' hematoxylin staining.

RESULTS

Cisplatin applied systemically did not cause any significant ABR threshold elevation across the frequencies tested (2-32 kHz), whereas local application of cisplatin through the round window niche or tympanum resulted in significant ABR threshold elevations from high to medium frequencies. The functional changes were consistent with the cochlear pathology across groups.

Distortion-product otoacoustic emissions and auditory brainstem responses sensitivity assessment in cisplatin-induced ototoxicity in rats

Cisplatin (cis-diamminedicloroplatinum) is an antineoplastic drug used in the treatment of a variety of cancers, especially head-and-neck cancer. Its ototoxicity, however, has been noted as a common side-effect which limits its use and causes significant morbidity.

Aim

to assess distortion-product otoacoustic emissions (DPOAE) and brainstem evoked response audiometry (BERA) sensitivity to detect secondary ototoxicity caused by different doses and means of administration of cisplatin in rats.

Study Design

Experimental.

Materials and Methods

Male Wistar rats were intraperitoneally (i.p.) injected with 24 mg/kg cisplatin, divided into three equal doses (8mg/kg) or a single i.p. injection of 16 mg/kg. The animals were evaluated by distortion product otoacoustic emission (DPOAE) or brainstem evoked response audiometry (BERA) on the 3rd and 4th days after the cisplatin injection.

Results

Treatment with cisplatin 24 mg/kg resulted in significant DPOAE decrease and it raised the BERA electrophysiological threshold. The 16mg/kg dose could not significantly reduce the DPOAE amplitude, but it raised the animals' hearing thresholds – detected by the BERA.

Conclusion

In rats, BERA was more sensitivity than DPOAE at detecting cisplatin-induced ototoxicity in rats considering different doses and means of administration.

DNA repair proteins and telomerase reverse transcriptase in the cochlear lateral wall of cisplatin-treated rats

Cochlear lateral wall damage is a side effect of cisplatin chemotherapy. Recent studies have shown that cisplatin treatment precipitates platinated DNA adducts in the cochlear lateral wall which suggest that DNA damage may contribute to ototoxicity. Platinated adducts are high-affinity substrates for the global genomic nucleotide excision repair (GG-NeR) pathway which is facilitated by xeroderma pigmentosum (Xp) complementing proteins, such as XpC, XpD and XpA. tumor biology has shown that in addition to stimulating GG-NeR, cisplatin may deplete telomerase reverse transcriptase (teRt). in the current study Fischer344 rats were treated with cisplatin (2 mg/kg/4 days, i.p.) and their cochleae harvested for immunohistochemistry. XpC, XpD and XpA expression increased while teRt expression decreased among cisplatin treated animals compared to vehicle control. these findings suggest that in addition to forming platinated adducts, cisplatin chemotherapy may up-regulate DNA repair proteins and modify teRt expression in the cochlear lateral wall.

Roscovitine up-regulates p53 protein and induces apoptosis in human HeLaS(3) cervix carcinoma cells

Exposure of human HeLaS(3) cervix carcinoma cells to high doses of conventional cytostatic drugs, e.g. cisplatin (CP) strongly inhibits their proliferation. However, most cytostatic agents are genotoxic and may generate a secondary malignancy. Therefore, therapeutic strategy using alternative, not cytotoxic drugs would be beneficial. Inhibition of cyclin-dependent kinases (CDKs) by pharmacological inhibitors became recently a promising therapeutic option. Roscovitine (ROSC), a selective CDK inhibitor, efficiently targets human malignant cells. ROSC induces cell cycle arrest and apoptosis in human MCF-7 breast cancer cells. ROSC also activates p53 protein. Activation of p53 tumor suppressor protein is essential for induction of apoptosis in MCF-7 cells. Considering the fact that in HeLaS(3) cells wt p53 is inactivated by the action of HPV-encoded E6 oncoprotein, we addressed the question whether ROSC would be able to reactivate p53 protein in them. Their exposure to ROSC for 24 h induced cell cycle arrest at G(2)/M and reduced the number of viable cells. Unlike CP, ROSC in the used doses did not induce DNA damage and was not directly cytotoxic. Despite lack of detectable DNA lesions, ROSC activated wt p53 protein. The increase of p53 levels was attributable to the ROSC-mediated protein stabilization. Further analyses revealed that ROSC induced site-specific phosphorylation of p53 protein at Ser46. After longer exposure, ROSC induced apoptosis in HeLaS(3) cells. These results indicate that therapy of HeLaS(3) cells by ROSC could offer an advantage over that by CP due to its increased selectivity and markedly reduced risk of generation of a secondary cancer.

Resveratrol Protects Auditory Hair Cells from Gentamicin Toxicity

Resveratrol is a naturally occurring polyphenol that is synthesized by a variety of plant species. It is abundant in grapes and grape products (e.g., red wine). Resveratrol has demonstrated reactive oxygen species (ROS) scavenger activity, and it has been linked to nuclear factor-kappa B (NF-κB) activity. We recently demonstrated that NF-κB is important to the survival of immature mammalian hair cells. Therefore, we undertook an in vitro experiment to determine if resveratrol is able to exert some protective influence against gentamicin-induced damage to and death of auditory hair cells. To accomplish this, we dissected the organ of Corti (OC) from newborn Sprague-Dawley rats and cultured the OCs in medium overnight for recovery. We treated two groups of OC explants with different concentrations of resveratrol plus gentamicin for 24 hours; for comparison and control purposes, we also treated a group of explants with gentamicin only and we left a group untreated. We found that resveratrol in both concentrations had a moderate but statistically significant protective effect against gentamicin-induced toxicity in vitro.

Cisplatin induces cytoplasmic to nuclear translocation of nucleotide excision repair factors among spiral ganglion neurons

Genomic DNA is a high-affinity target for the antineoplastic molecule cisplatin. Cell survival from cisplatin DNA damage is dependent on removal of cisplatin-DNA adducts by nucleotide excision repair (NER) pathways. The rate-limiting steps in the NER pathways are DNA damage identification and verification. These steps are accomplished by xeroderma pigmentosum complementation group C and A (XPC and XPA) and RNA polymerase II. Unlike RNA polymerase II, XPC and XPA have no known cellular function beyond DNA repair. Cisplatin is known to damage spiral ganglion neurons at the basal coil of the cochlea therefore it was posited that cisplatin may target their DNA and mobilize XPC and XPA. Female Fisher344 rats were given two, four day cycles of cisplatin (2mg/kg) or saline, separated by a 10day rest period. A 2 x 3 x 2 factorial design, consisting of two treatment conditions (cisplatin and saline treatment), three survival times (5, 19 and 22 days) and two analysis methods (quantitative RT-PCR and immunohistochemistry) was employed to evaluate the expression and distribution of XPC and XPA. Quantitative RT-PCR revealed statistically significant differences in cochlear XPC and XPA mRNA levels after cisplatin treatment at all times except day 22 for XPA. Immunohistochemistry revealed that a proportion ( approximately 50%) of spiral ganglion neurons in control rats showed cytoplasmic expression of XPC and XPA. After cisplatin treatment, a similar proportion ( approximately 50%) of spiral ganglion neurons showed increased nuclear expression of XPC and XPA, which appears to represent translocation from the cytoplasm. Basal coil spiral ganglion neurons translocated XPC and XPA at later treatment cycles and with less magnitude than apical coil neurons after cisplatin treatment. Therefore, it is suggested that cisplatin treatment induces nuclear translocation of NER proteins among spiral ganglion neurons and that this nuclear translocation is less efficient at the base relative to the apex.

Cisplatin-induced hair cell loss in zebrafish (Danio rerio) lateral line

We have used time-lapse imaging to study cisplatin-induced hair cell death in lateral line neuromasts of zebrafish larvae in vivo. We found that cisplatin-induced hair cell death occurred much more slowly than had been shown to occur in aminoglycoside-induced hair cell death. By prelabeling hair cells with FM1-43FX, and assessing hair cell damage, it was established that cisplatin causes hair cell loss in the lateral line in a dose-dependent fashion. The kinetics of hair cell loss during exposure to different concentrations of cisplatin was also assessed and it was found that the onset of hair cell loss correlated with the accumulated dose of cisplatin. These data demonstrate the feasibility and repeatability of cisplatin damage protocols in the zebrafish lateral line and set the stage for future evaluations of modulation of cisplatin-induced hair cell death.

Cell death after co-administration of cisplatin and ethacrynic acid

Ethacrynic acid (EA) significantly enhances the ototoxic effects of cisplatin. To gain insights into the mechanisms underlying Cis/EA ototoxicity, cochleas were labeled with several apoptotic markers. Cis/EA treatment caused extensive outer hair cell (OHC) and inner hair cell (IHC) damage; OHC lesions decreased from the base towards apex of the cochlea whereas the IHC lesion was relatively constant (25-60%) along the length of the cochlea. Propidium iodide labeled OHC nuclei appeared relatively normal at 6h post-treatment, were condensed and fragmented at 12h post-treatment and were frequently missing 48 h post-treatment. Initiator caspase 8, associated with membrane death receptors, and TRADD, a protein that recruits caspase 8, were present in OHC at 6h post-treatment. Caspase 8 labeling increased from 6 to 24h, but was largely absent at 48 h post-treatment. Executioner caspase 3 and caspase 6, which lie downstream of caspase 8, were expressed in OHC 12-24h post-treatment. Initiator caspase 9, associated with mitochondrial damage, was only expressed at low levels at 48 h post-treatment. These results suggest that the rapid onset of Cis/EA induced programmed cell death is initiated by membrane death receptors associated with TRADD and caspase 8.

Protection Against Cisplatin-Induced Ototoxicity by Adeno-Associated Virus-Mediated Delivery of the X-Linked Inhibitor of Apoptosis Protein Is Not Dependent on Caspase Inhibition

HYPOTHESIS

Gene therapy with an adeno-associated viral (AAV) vector encoding the X-linked inhibitor of apoptosis protein (XIAP) in an animal model of cisplatin-induced ototoxicity can elucidate apoptotic pathways in the inner ear.

BACKGROUND

Cisplatin is limited clinically by ototoxicity associated with apoptosis in the inner ear. The relevant intracellular apoptotic pathways, however, are unknown. XIAP is an antiapoptotic protein that both inhibits caspases and reciprocally regulates the proapoptotic Smac/Omi proteins. AAV-mediated delivery of various XIAP mutants could distinguish between these antiapoptotic pathways in the ear and further the development of specific reagents for gene therapy- mediated prevention of cisplatin-induced ototoxicity.

METHODS

We administered unilaterally through the round-window AAV-harboring genes encoding wild-type dXIAP, yellow fluorescent protein, or either of two dXIAP point mutants-one deficient in caspase inhibition (dXIAP-d) and the other additionally deficient in the binding of Smac/Omi (dXIAP-t). All rats received a 3-day systemic course of cisplatin. Functional hearing loss was measured by shifts in auditory brainstem response (ABR) thresholds after cisplatin treatment, and hair-cell loss was assessed by whole-mount phalloidin staining of cochlear turns.

RESULTS

Uninjected ears universally displayed high-frequency-specific hair-cell loss and ABR threshold shifts upon cisplatin treatment. Although yellow fluorescent protein had no effect, ears injected with dXIAP exhibited 68% less ABR threshold shift at 32 kHz and 50% less basal-turn outer-hair-cell loss compared with contralateral untreated ears. This protection was maintained in ears injected with dXIAP-d but was abolished in those expressing dXIAP-t, which is incapable of blocking Smac/Omi.

CONCLUSION

Hair-cell apoptosis induced by cisplatin involves the Smac/Omi pathway. Thus, gene therapy with either wild-type dXIAP or Smac/Omi-selective dXIAP-d may be effective to protect against cisplatin-mediated ototoxicity.

Future opportunities in preventing cisplatin induced ototoxicity

Cisplatin is one of the most commonly used cytotoxic agents. Ototoxicity is an important and dose-limiting side-effect of cisplatin therapy. It is believed that cisplatin suppresses the formation of endogenous anti-oxidants that normally prevent the inner ear against reactive oxygen species (ROS). These ROS affect the outer hair cells (OHCs) in the organ of Corti. Results from clinical trials with amifostine, an anti-oxidant with possible otoprotective action during cisplatin therapy, were disappointing. A variety of agents with chemoprotective action against cisplatin-induced ototoxicity were successfully tested in animal models. It is important to translate these promising results from animal models into clinical practice. The possible routes of administration are systemic and transtympanic. An important condition when using such an agent systemically is that the compound may not affect the anti-tumor effect of cisplatin. The critical step at transtympanic administration is the diffusion of the compound through the round window membrane (RWM). This diffusion depends on the characteristics of the medication as on the properties of the RWM. Positive results of an otoprotector in clinical practice may increase the effectiveness of cisplatin therapy and can improve the quality of life for a large group of patients.

Central role of supporting cells in cochlear homeostasis and pathology

HYPOTHESIS

Supporting cells have a crucial role in degenerative and regenerative events of primary sensorial hair cells of the organ of Corti. This new role should determine future studies about pathophysiology of hearing loss and its regenerative treatment.

SUPPORTING EVIDENCE

Recent findings suggest an active role of supporting cells in the maintenance of hair cell function and structure. Evidences of high energy consumption and close proximity to auditory nervous fibers suggesting K+ active exchange, preferential expression of specific proteins and antigens, presence of glucocorticoids receptors, affinity for cisplatin and regenerative potential give the supporting cells an important role in homeostasis of the organ of Corti and in some specific diseases affecting this structure.

CONCLUSION

As well as glial cells provide protection and regeneration to neural tissues, supporting cells may provide the necessary metabolic and electrolitic conditions for hair cells mechanical and bioelectrical function. This opens new possibilities for the treatment of apparently "irreversible" destruction of the inner ear.

High accumulation of platinum-DNA adducts in strial marginal cells of the cochlea is an early event in cisplatin but not carboplatin ototoxicity

Ototoxicity is a typical dose-limiting side effect of cancer chemotherapy with cisplatin but much less so with carboplatin. To elucidate the underlying molecular pathological mechanisms, we have measured the formation and persistence of drug-induced DNA adducts in the nuclei of inner ear cells of guinea pigs after short-term exposure to either cisplatin or carboplatin using immunofluorescence staining and quantitative image analysis. After application of carboplatin, all cells of the cochlea exhibited a similar burden of guanine-guanine intrastrand cross-links in DNA. In contrast, we observed a pronounced 3- to 5-fold accumulation of this cytotoxic adduct exclusively in the marginal cells of the stria vascularis between 8 and 48 h after treatment with cisplatin. In the kidney, the other critical target tissue of cisplatin toxicity, a similar high preferential formation of cytotoxic DNA adducts was measured in the tubular epithelial cells but not in other renal cell types. As for the ear, this excessive formation of DNA damage in a particular cell type was seen in animals treated with cisplatin but not those treated with carboplatin. Because cisplatin ototoxicity is often attributed to oxidative stress mediated by the generation of radical oxygen species (ROS), we have measured in parallel the levels of the lead DNA oxidation product 8-oxoguanine (8-oxoG) in cochlear cryosections. Compared with basal levels in untreated control cochleas, no additional formation of 8-oxoG was detectable up to 48 h after cisplatin treatment in the DNA of either inner-ear cell type. This suggests that the generation of ROS may be a secondary event in cisplatin ototoxicity.

Ototoxicity: therapeutic opportunities

Two major classes of drugs currently in clinical use can cause permanent hearing loss. Aminoglycoside antibiotics have a major role in the treatment of life-threatening infections and platinum-based chemotherapeutic agents are highly effective in the treatment of malignant disease. Both damage the hair cells of the inner ear, resulting in functional deficits. The mechanisms underlying these troublesome side effects are thought to involve the production of reactive oxygen species in the cochlea, which can trigger cell-death pathways. One strategy to protect the inner ear from ototoxicity is the administration of antioxidant drugs to provide upstream protection and block the activation of cell-death sequences. Downstream prevention involves the interruption of the cell-death cascade that has already been activated, to prevent apoptosis. Challenges and opportunities exist for appropriate drug delivery to the inner ear and for avoiding interference with the therapeutic efficacy of both categories of ototoxic drugs.