D-Methionine attenuates inner hair cell loss in carboplatin-treated chinchillas

D-Amino Acids and Cancer: Friends or Foes?

α-amino acids exist in two configurations, named D-(dextro) and L-(levo) enantiomers. L-amino acids are used in protein synthesis and play a central role in cell metabolism. The effects of the L-amino acid composition of foods and the dietary modifications of this composition on the efficacy of cancer therapies have been widely investigated in relation to the growth and reproduction of cancerous cells. However, less is known about the involvement of D-amino acids. In recent decades, D-amino acids have been identified as natural biomolecules that play interesting and specific roles as common components of the human diet. Here, we focus on recent investigations showing altered D-amino acid levels in specific cancer types and on the various roles proposed for these biomolecules related to cancer cell proliferation, cell protection during therapy, and as putative, innovative biomarkers. Notwithstanding recent progress, the relationship between the presence of D-amino acids, their nutritional value, and cancer cell proliferation and survival represents an underrated scientific issue. Few studies on human samples have been reported to date, suggesting a need for routine analysis of D-amino acid content and an evaluation of the enzymes involved in regulating their levels in clinical samples in the near future.

Promising Application of D-Amino Acids toward Clinical Therapy

The versatile roles of D-amino acids (D-AAs) in foods, diseases, and organisms, etc., have been widely reported. They have been regarded, not only as biomarkers of diseases but also as regulators of the physiological function of organisms. Over the past few decades, increasing data has revealed that D-AAs have great potential in treating disease. D-AAs also showed overwhelming success in disengaging biofilm, which might provide promise to inhibit microbial infection. Moreover, it can effectively restrain the growth of cancer cells. Herein, we reviewed recent reports on the potential of D-AAs as therapeutic agents for treating neurological disease or tissue/organ injury, ameliorating reproduction function, preventing biofilm infection, and inhibiting cancer cell growth. Additionally, we also reviewed the potential application of D-AAs in drug modification, such as improving biostability and efficiency, which has a better effect on therapy or diagnosis.

The Use of Antioxidants in the Prevention and Treatment of Noise-Induced Hearing Loss

As of today, there are no therapeutic measures for the prevention or treatment of noise-induced hearing loss (NIHL). The current preventative measures, including avoidance and personal protective hearing equipment, do not appear to be sufficient because there is an increasing number of people with NIHL, especially in the adolescent population. Therefore, we must find a therapy that prevents the impact of noise on hearing. Antioxidants are a promising option in preventing the damaging effects of noise by targeting free radicals but further studies are needed to confirm their efficacy in humans.

Oral D-methionine protects against cisplatin-induced hearing loss in humans: phase 2 randomized clinical trial in India

Objective: This exploratory Phase 2 clinical trial is the first determining safety and efficacy of oral D-methionine (D-met) in reducing cisplatin-induced ototoxicity.Design: Randomised parallel double-blind placebo-controlled exploratory Phase 2 study.Study samples: Fifty adult cancer patients received oral D-met or placebo before each cisplatin dose. Physical examination, blood collection and audiometry occurred at baseline and subsequent visits plus post-treatment audiometry. After attrition, final analysis included 27 patients.Results: Significant treatment group by ear and time (baseline vs. post-treatment) interactions occurred at 10 kHz and 11.2 kHz. Placebo and D-met groups differed in threshold shift for left ear at 11.2 kHz (mean difference = 22.97 dB [9.59, 36.35]). Averaging across ears, placebo group showed significant threshold shifts from baseline to post-treatment at 10 kHz (mean shift= -13.65 dB [-21.32,-5.98]), 11.2 kHz (-16.15 dB [-25.19,-7.12]), and 12.5 kHz (-11.46 dB [-19.18,-3.74]) but not 8 kHz (-8.65 dB [-17.86, 0.55]). The D-met group showed no significant threshold shifts (8 kHz: -1.25 dB [-7.75, 5.25]; 10 kHz:-3.93 dB [-8.89, 1.03]; 11.2 kHz:-4.82 dB [-11.21, 1.57]; 12.5 kHz:-3.68 dB [-11.57, 4.21]). Side effects did not significantly differ between groups.Conclusion: Oral D-met reduces cisplatin-induced ototoxicity in humans.

Protection for medication-induced hearing loss: the state of the science

OBJECTIVE

This review will summarise the current state of development of pharmaceutical interventions (prevention or treatment) for medication-induced ototoxicity.

DESIGN

Currently published literature was reviewed using PubMed and ClinicalTrials.gov to summarise the current state of the science. Details on the stage of development in the market pipeline are provided, along with evidence for clinical safety and efficacy reported.

STUDY SAMPLE

This review includes reports from 44 articles and clinical trial reports regarding agents in clinical or preclinical trials, having reached approved Investigational New Drug status with the Federal Drug Administration.

RESULTS

Vitamins and antioxidants are the most common agents currently evaluated for drug-induced ototoxicity intervention by targeting the oxidative stress pathway that leads to cochlear cell death and hearing loss. However, other strategies, including steroid treatment and reduction of ototoxic properties of the primary drugs, are discussed.

CONCLUSIONS

Retention of hearing during and after a life threatening illness is a major quality-of-life issue for patients receiving ototoxic drugs and their families. The agents discussed herein, while not mature enough at this point, offer great promise towards that goal. This review will provide a knowledge base for hearing providers to inquiries about such options from patients and interdisciplinary care teams alike.

OTOTOXIC EFFECTS OF CARBOPLATIN IN ORGANOTYPIC CULTURES IN CHINCHILLAS AND RATS

Carboplatin, a second-generation platinum chemotherapeutic drug, is considerably less ototoxic than cisplatin. While common laboratory species such as mice, guinea pigs and rats are highly resistant to carboplatin ototoxicity, the chinchilla stands out as highly susceptible. Moreover, carboplatin causes an unusual gradient of cell death in chinchillas. Moderate doses selectively damage type I spiral ganglion neurons (SGN) and inner hair cells (IHC) and the lesion tends to be relatively uniform along the length of the cochlea. Higher doses eventually damage outer hair cells (OHC), but the lesion follows the traditional gradient in which damage is more severe in the base than the apex. While carboplatin ototoxicity has been well documented in adult animals in vivo, little is known about its in vitro toxicity. To elucidate the ototoxic effects of carboplatin in vitro, we prepared cochlear and vestibular organotypic cultures from postnatal day 3 rats and adult chinchillas. Chinchilla cochlear and vestibular cultures were treated with carboplatin concentrations ranging from 50 µM to 10 mM for 48 h. Consistent with in vivo data, carboplatin selectively damaged IHC at low concentrations (50-100 µM). Surprisingly, IHC loss decreased at higher doses and IHC were intact at doses exceeding 500 µM. The mechanisms underlying this nonlinear response are unclear but could be related to a decrease in carboplatin uptake via active transport mechanisms (e.g., copper). Unlike the cochlea, the carboplatin dose-response function increased with dose with the highest dose destroying all chinchilla vestibular hair cells. Cochlear hair cells and auditory nerve fibers in rat cochlear organotypic cultures were unaffected by carboplatin concentrations <10 µM; however, the damage in OHC were more severe than IHC once the dose reached 100 µM. A dose at 500 µM destroyed all the cochlear hair cells, but hair cell loss decreased at high concentrations and nearly all the cochlear hair cells were present at the highest dose, 5 mM. Unlike the nonlinear dose-response seen with cochlear hair cells, rat auditory nerve fiber and spiral ganglion losses increased with doses above 50 µM with the highest dose destroying virtually all SGN. The remarkable species differences seen in vitro suggest that chinchilla IHC and type I SGN posse some unique biological mechanism that makes them especially vulnerable to carboplatin toxicity.

Effect OfN-Acetylcysteine on Carboplatin-Induced Ototoxicity and Nitric Oxide Levels in a Rat Model

OBJECTIVE

The aim of the present study is to investigate the effect of N-acetylcysteine (NAC) given 30 minutes before carboplatin administration on carboplatin-induced ototoxicity and nitric oxide (NO) levels in a rat model.

STUDY DESIGN

Animal study.

METHODS

Eighteen Sprague-Dawley rats were divided into three groups that each contained six animals. Intraperitoneal injection of physiologic saline was performed in group 1 twice with an interval of 30 minutes. Group 2 was treated with a single bolus administration of carboplatin at a dose of 256 mg/kg 30 minutes after the intraperitoneal injection of physiologic saline. Group 3 was treated with a single bolus administration of carboplatin at a dose of 256 mg/kg 30 minutes after the intraperitoneal injection of NAC at a dose of 400 mg/kg. Pretreatment and posttreatment distortion product otoacoustic emissions (DPOAE) were performed in rats from all groups. Then, the animals were sacrificed on the fourth day, and cochlear tissue NO and glutathione peroxidase (GSH-Px) levels were measured.

RESULTS

The comparison of pre- and posttreatment DPOAE responses did not demonstrate any significant changes for groups 1 and 3. Results of group 2 showed a decrease of the DPOAE amplitude. Cochlear NO levels were significantly higher in rats treated with carboplatin than in controls and in those treated with carboplatin plus NAC (P < .05). Cochlear GSH-Px levels were higher in rats treated with carboplatin plus NAC than in those treated with carboplatin, but the difference did not reach statistical significance (P = .079).

CONCLUSIONS

The present study showed that carboplatin at higher doses induced hearing loss and increased NO levels in the cochlea of rats. NAC appears to have a protective effect against carboplatin-induced ototoxicity, which may be related to its inhibitory effect on NO production.

Prevention of noise- and drug-induced hearing loss with D-methionine

A number of otoprotective agents are currently being investigated. Various types of agents have been found in animal studies to protect against hearing loss induced by cisplatin, carboplatin, aminoglycosides, or noise exposure. For over a decade we have been investigating D-methionine (D-met) as an otoprotective agent. Studies in our laboratory and others around the world have documented D-met's otoprotective action, in a variety of species, against a variety of ototoxic insults including cisplatin-, carboplatin-, aminoglycoside- and noise-induced auditory threshold elevations and cochlear hair cell loss. For cisplatin-induced ototoxicity, protection of the stria vascularis has also been documented. Further D-met has an excellent safety profile. D-met may act as both a direct and indirect antioxidant. In this report, we provide the results of three experiments, expanding findings in D-met protection in three of our translational research areas: protection from platinum based chemotherapy-, aminoglycoside- and noise-induced hearing loss. These experiments demonstrate oral D-met protection against cisplatin-induced ototoxicity, D-met protection against amikacin-induced ototoxicity, and D-met rescue from permanent noise-induced hearing loss when D-met is initiated 1h after noise exposure. These studies demonstrate some of the animal experiments needed as steps to translate a protective agent from bench to bedside.

Deletion of SLC19A2, the high affinity thiamine transporter, causes selective inner hair cell loss and an auditory neuropathy phenotype

Mutations in the gene coding for the high-affinity thiamine transporter Slc19a2 underlie the clinical syndrome known as thiamine-responsive megaloblastic anemia (TRMA) characterized by anemia, diabetes, and sensorineural hearing loss. To create a mouse model of this disease, a mutant line was created with targeted disruption of the gene. Cochlear function is normal in these mutants when maintained on a high-thiamine diet. When challenged with a low-thiamine diet, Slc19a2-null mice showed 40-60 dB threshold elevations by auditory brainstem response (ABR), but only 10-20 dB elevation by otoacoustic emission (OAE) measures. Wild-type mice retain normal hearing on either diet. Cochlear histological analysis showed a pattern uncommon for sensorineural hearing loss: selective loss of inner hair cells after 1-2 weeks on low thiamine and significantly greater inner than outer hair cell loss after longer low-thiamine challenges. Such a pattern is consistent with the observed discrepancy between ABR and OAE threshold shifts. The possible role of thiamine transport in other reported cases of selective inner hair cell loss is considered.

Understanding drug ototoxicity: molecular insights for prevention and clinical management

Ototoxicity is a trait shared by aminoglycoside and macrolide antibiotics, loop diuretics, platinum-based chemotherapeutic agents, some NSAIDs and antimalarial medications. Because their benefits in combating certain life-threatening diseases often outweigh the risks, the use of these ototoxic drugs cannot simply be avoided. In this review, the authors discuss some of the most frequently used ototoxic drugs and what is currently known about the cell and molecular mechanisms underlying their noxious effects. The authors also provide suggestions for the clinical management of ototoxic medications, including ototoxic detection and drug monitoring. Understanding the mechanisms of drug ototoxicity may lead to new strategies for preventing and curing drug-induced hearing loss, as well as developing new pharmacological drugs with less toxic side effects.

Damage and threshold shift resulting from cochlear exposure to paraquat-generated superoxide

Superoxide has been implicated as a contributing factor to cochlear pathology from a number of sources, including noise and ototoxic drugs. The effects of NADPH oxidase-dependent superoxide on the cochlea were investigated in the current study using paraquat (PQ). PQ is a toxic herbicide that causes tissue damage by generating superoxide through reduction of molecular oxygen in a reaction catalyzed by NADPH oxidase. The current study examined the effects of round window PQ administration on inferior colliculus (IC) evoked potential thresholds (EVP) and hair cell damage. Using implanted IC electrodes, chinchillas were tested for IC EVP thresholds before and after PQ exposure. Ears were exposed to PQ at one of four concentrations: 10, 5, 3 mM, and vehicle control. Thresholds were increased in a dose-dependent manner, and peaked between one and seven days post-exposure. Thresholds then showed a small amount of recovery before reaching PTS by Day 22. Outer and inner hair cell losses were consistent with PTS. The similarities between PQ ototoxicity and noise-induced hearing loss suggest the possibility of similar biochemical pathways involving superoxide.

Intratympanic treatment of hearing loss with novel and traditional agents

As knowledge of the cellular and molecular pathophysiology behind otopathologies expands, the possibility exists of preventing sensorineural hearing loss and perhaps reversing the loss. Cellular and molecular mechanisms seem to be similar in hearing loss secondary to aging, drug ototoxicity, noise, or other mechanisms. A final common pathway may hinge upon apoptosis. It is likely that anti-apoptotic factors will increasingly be realized as an important intervention strategy for sensorineural hearing loss. Furthermore, it is also possible that mounting a staged attack at the various regions in the pathway leading to cellular damage using a combination of several protective substances such as steroids, antioxidants, neurotrophic factors, anti-apoptotic compounds, and mitochondrial enhancers may prevent hearing loss and even reverse it in some situations. This article has presented some of the molecular and cellular mechanisms for hearing loss and potential ways of treating them. In theory, the delivery of these medications to the inner ear transtympanically would decrease systemic side effects and be more target specific. Because most of the studies conducted to date have been animal studies, randomized, double-blind, placebo-controlled clinical trials would be necessary before the use of these therapies becomes common practice.

Round Window Application of D-Methionine, Sodium Thiosulfate, Brain-Derived Neurotrophic Factor, and Fibroblast Growth Factor-2 in Cisplatin-Induced Ototoxicity

HYPOTHESIS

In this study we tested the effect of local administration of D-methionine, sodium thiosulfate, brain-derived neurotrophic factor, and fibroblast growth factor-2 on cisplatin ototoxicity in guinea pigs to the round window membrane.

BACKGROUND

Cisplatin is an important antineoplastic agent in the therapy of many malignancies. Its clinical utility is limited by severe side effects, including ototoxicity. Recent studies have shown protection against cisplatin ototoxicity in animal experiments by the systemic administration of D-methionine and sodium thiosulfate. Growth factors such as brain-derived neurotrophic factor and fibroblast growth factor-2 also have shown otoprotective effects in in vitro studies.

METHODS

Osmotic pumps (Alzet) were implanted unilaterally in 30 guinea pigs. Five groups of six animals received either D-methionine, sodium thiosulfate, fibroblast growth factor-2, brain-derived neurotrophic factor, or saline 0.9%. Cisplatin was administered intraperitoneally for 5 consecutive days. Distortion product otoacoustic emissions were recorded every day. The animals were killed on day 6, and their cochleae were removed and analyzed by transmission electron microscopy.

RESULTS

Compared with control animals, guinea pigs treated with D-methionine showed better otoacoustic emissions on days 3 and 4 (Mann-Whitney test, p < 0.05). The differences were not evident on days 5 and 6. Sodium thiosulfate, brain-derived neurotrophic factor, and fibroblast growth factor-2 showed no significant protective effect.

CONCLUSION

Local application to the round window membrane can be used as an effective treatment in the prevention of cisplatin toxicity. Local application may avoid systemic side effects and reduce the antineoplastic effects of cisplatin.

Electrophysiological findings in the Sprague-Dawley rat induced by moderate-dose carboplatin

Carboplatin is a second generation platinum-containing anti-tumor drug which selectively alters the micromechanical function of the inner hair cells (IHCs) of the organ of Corti in the chinchilla. Data from a recent study [Wake et al., Acta Otolaryngol. 116 (1996) 374-381], using the chinchilla model, have suggested that a moderate dose of carboplatin alters the efferent feedback loop gain of the OHCs. The present study was designed to evaluate the possible 'efferent feedback alteration mechanism' in the Sprague-Dawley rat using distortion product otoacoustic emissions (DPOAEs). A moderate dose of carboplatin (50 mg/kg body weight) was administered by a 30 min i.p. infusion. Pre- and 72-h post-treatment DPOAE and auditory brainstem response (ABR) recordings were acquired from a group of 12 rats. The animals were anesthetized with a ketamine-atropin anesthesia administered in two consecutive phases. The DPOAE responses (cubic distortion products) were recorded with four asymmetrical protocols: P1=60-50, P2=50-40, P3=40-30 and P4=30-20 dB SPL (sound pressure level), in the frequency range from 4.0 to 16 kHz. ABR responses were obtained for bipolar clicks and tone pips at the frequencies 8.0, 10.0, 20.0 and 30 kHz using stimuli in the range from 100 to 30 dB SPL. Significant ABR threshold shifts of 15 dB were observed at 30 kHz, and shifts of 10 dB at 20, 16 and 10 kHz. The comparison of pre- and post-treatment DPOAE responses did not reveal any significant changes for protocols P1, P2 and P4. Data from the P3 protocol indicated a decrease of the DPOAE amplitude. The findings from the rat model suggest that (a) moderate doses of carboplatin do not affect the efferent feedback loop OHC function and (b) the cochlear susceptibility to carboplatin across species is different, even at moderate-dose regimes.

Carboplatin-induced early cochlear lesion in chinchillas

Carboplatin preferentially damages inner hair cells (IHC) and type I spiral ganglion neurons (SGNs) in the chinchilla; however, the temporal sequence of events leading to the destruction of these structures is poorly understood. To better understand the mechanisms leading up to the destruction of IHCs and type I SGNs, we measured the activity in single auditory nerve fibers for the first 8 h following carboplatin treatment and also monitored the development of histopathologies in SGNs and IHCs using a dose of carboplatin that killed approximately 50% of the IHCs. The spontaneous discharge rate (SDR) showed a slight increase around 3 h post carboplatin followed by a significant decline at 4-5 h. The saturation driven discharge rate (DDR) showed a significant increase 1-5 h post carboplatin. These physiological changes were associated with the formation of small vacuoles in type I afferent terminals and proximal nerve fibers 1-6 h post carboplatin; signs of IHC damage were first observed around 24-48 h. Thus, the neurotoxic effects of carboplatin occur approximately a day before the IHCs are damaged. The large fluctuations in SDR and DDR that occur several hours after carboplatin treatment are most likely due to the neurotoxic effects of carboplatin.

Strategies for prevention of toxicity caused by platinum-based chemotherapy: review and summary of the annual meeting of the Blood-Brain Barrier Disruption Program, Gleneden Beach, Oregon, March 10, 2001

OBJECTIVES

To summarize the findings relevant to otolaryngology from the annual meeting of the Blood-Brain Barrier Disruption Consortium in Gleneden Beach, Oregon, March 10, 2001.

STUDY DESIGN

Summaries are provided by the speakers, as well as related data from the published literature. Findings in otology and oncology regarding ototoxicity that were discussed at the meeting are included.

RESULTS

Data considered included physiological research, animal studies, and clinical trials that relate to platinum-based chemotherapy and prevention of toxicity.

CONCLUSIONS

The dose-limiting side effects of platinum-based chemotherapy are preventable, but questions about the effect of the protective agents on oncological efficacy remain. Strategies for prevention of chemotherapy-induced toxicity include temporal or anatomical separation of cisplatin or carboplatin from sodium thiosulfate, D-methionine, or N-acetyl-cysteine. Clinical application of these methods has begun. The mechanisms presumably involve free radicals or drug conjugation, or both. Understanding the role of free radicals in medicine is likely to become important in the future.