ACTA oTorhinolAryngologiCA iTAliCA 2012;32:380-385
Audiology
The effect of alpha-lipoic acid on temporary threshold shift in humans: a preliminary study
Effetto dell’acido alfa-lipoico sulla deriva temporanea di soglia nell’uomo:
studio preliminare
N. QuaraNta, a. Dicorato, V. Matera, a. D’elia, a. QuaraNta
otolaryngology unit, Department of Neuroscience and Sensory organs, university of Bari “a. Moro”, italy SummAry
noise-induced hearing loss (nhil) is a significant source of hearing loss in industrialized countries. recent research on the cellular bases of nihl has led to new avenues for protection through prophylactic drugs. Although in experimental animal models several compounds have shown a protective effect in nihl, limited data are available in humans. many authors are focusing their attention on the role of anti-oxidant on hearing protection. Alpha-lipoic acid (AlA), an essential cofactor in mitochondrial enzymes, is a novel
biological antioxidant and a potent free radical scavenger and, in animal models, it has been shown to protect from age-induced and cisplatin-induced hearing loss. The aim of our study was to evaluate the effect of alpha-lipoic acid on temporary threshold shift measured 2 minutes after the end of
exposure (TTS
2) induced by a 3 khz tone in young normally hearing subjects. Thirty young normal hearing volunteers served as control
subjects. individuals were randomly assigned to three groups. group A (10 subjects) subjects were exposed to a 90 dB hl 3 khz pure tone for 10 min. group B (10 subjects) subjects were exposed to a 90 dB hl 3 khz pure tone one hour after oral ingestion of 600 mg of AlA. group C (10 subjects) were exposed to a 90 dB hl 3 khz pure tone after 10 days of oral ingestion of 600 mg of AlA. Statistical analysis
showed that prior to the exposure the hearing thresholds did not differ significantly among the three groups. TTS
2 of group C was signifi-
cantly lower that TTS
2 of groups A and B at 6 khz (p 0.03), and TEoAEs amplitude change after noise exposure was lower for group C
compared to groups A (p = 0.089) and B (p = 0.03). AlA is a powerful lipophilic antioxidant and free radical scavenger currently used
in clinical practice. A single dose of 600 mg of dose AlA did not induce any protection on the TTS
2 induced by a 90 dB hl
3 khz tone,
while 10 days of therapeutic dosage assumption of AlA was associated with significant protection at 6 khz. The results of this study show
that a short course of AlA protects from TTS
2 in humans, and therefore further studies are needed to better define the role of AlA in the
prevention of noise induced hearing loss.
KEy wordS: Noise-induced hearing loss • Hearing protection • Antioxidants • Alpha-lipoic acid
riASSunTo
L’ipoacusia da rumore è un’importante causa di sordità nei paesi industrializzati. Recenti ricerche sulle basi biologiche della sordità da rumore hanno condotto a nuove prospettive per quanto riguarda la prevenzione farmacologica. Sebbene gli esperimenti condotti su animali abbiano dimostrato l’efficacia di numerosi composti nella prevenzione del danno da rumore, pochi dati sono ad oggi disponibili nell’uomo. Molti autori stanno concentrando la loro attenzione sul ruolo di protezione degli antiossidanti dal danno da rumore. L’acido alfa-lipoico (ALA), un importante cofattore degli enzimi mitocondriali, è un nuovo antiossidante biologico ed un potente scavenger dei radicali liberi. In modelli animali è stata dimostrata la sua efficacia nella protezione dall’ipoacusia età-correlata e cisplatino-indotta. L’obiettivo del nostro studio è stato quello di valutare l’effetto dell’acido alfa lipoico sull’innalzamento temporaneo di soglia indotto da un tono puro a 3 kHz in giovani soggetti normoudenti. Trenta giovani volontari sono stati utilizzati come soggetti di studio e suddivisi in tre gruppi. Il Gruppo A (10 soggetti) è stato esposto ad un tono puro di 90 dB HL a 3 kHz per dieci minuti. I
l Gruppo B (10 soggetti) è stato esposto ad un tono puro di 90 dB HL a 3 kHz un’ora dopo l’assunzione per os di 600 mg di acido alfa-lipoico (ALA). Il Gruppo C (10 soggetti) é stato esposto ad un tono puro di 90 dB HL a 3 kHz dopo dieci giorni dall’assunzione per os di 600 mg di ALA. Prima
dell’esposizione a rumore la soglia uditiva dei tre gruppi non mostrava differenze statisticamente significative. La TTS
2 del Gruppo C é
risultata invece significativamente piú bassa di quella dei Gruppi A e B a 6 kHz (p 0.03) e la variazione di ampiezza delle TEOAEs per il Gruppo C é risultata significativamente piú bassa di quella del Gruppo A (p = 0.089) e del gruppo B (p = 0.03). L’ALA é pertanto un potente antiossidante lipofilico e scavenger dei radicali liberi attualmente utilizzato nella pratica clinica. Una singola dose di ALA di 600
mg non induce alcuna protezione sulla TTS
2 provocata dall’esposizione ad un tono puro di 90 dB HL a
3 kHz, ma é possibile ottenere una
protezione statisticamente significativa attraverso l’assunzione per dieci giorni di ALA a dosaggio terapeutico. Il nostro studio dimostra quindi che un breve trattamento con ALA protegge dal danno da rumore nell’uomo e pertanto ulteriori ricerche sono necessarie per meglio definire il ruolo di ALA nella prevenzione del danno da rumore.
PArolE ChiA vE: Ipoacusia da rumore • Otoprotezione • Antiossidanti • Acido alfa lipoico
Acta Otorhinolaryngol Ital 2012;32:380-385
380
Alpha lipoic acid and TTS
381
Introduction
noise-induced hearing loss (nhil) is a significant source of hearing loss in industrialized countries 1. Currently, pre-vention of nihl focuses on the use of hearing protecting devices, frequent hearing screening for at risk populations and education on the causes of hearing loss 2 3.
recently, research on the cellular bases of nihl has led to new avenues for protection through prophylactic drugs. Although in experimental animal models several com-pounds have shown a protective effect in nihl 4-7, only limited data are available in humans 8 9.
noise exposure causes a broad set of physical changes in the major cellular system of the cochlea that lead to temporary threshold shift (TTS) and permanent threshold shift (PTS) 4. The reversible nature of TTS indicates that the underlying mechanisms associated with hearing loss are most probably of a metabolic nature 4 8 10 11.
The generators of this oxidative stress likely include acoustically-induced ischaemia reperfusion, glutamate excitotoxicity and an increase in mitochondrial free radi-cal production due to higher metabolic demand as well as through less efficient energy production due to mitochon-drial damage 12-14.
Several studies have shown that during and after noise ex-posure reactive oxygen species (roS), reactive nitrogen species (rnS) and lipid peroxides all increase 4 5. yamane et al. 15 showed an increase of roS in marginal cells of stria vascularis, while nicotera et al. 16 were able to local-ize roS within outer hair cells (ohC) after noise expo-sure. Furthermore, Ciorba et al. 17 provided evidence, by
spectrophotometric analyses, of the presence and produc-tion of superoxide in inner ear perilymph of human sub-jects affected by profound hearing loss and treated with cochlear implantation.
The origin of increased roS into the cochlea is some-what speculative. however, the increased energy demand induced by the noise exposure may lead to the use of large amount of oxygen and the formation of peroxide as a by-product of phosphorylation 18. in addition, noise exposure decreases cochlear blood flow, and its reduction is associ-ated with increased roS formation 4.
Several molecules with antioxidant and scavenging prop-erties including α-tocopherol 19-21, idebenone 22 23, the water-soluble formulation of coenzyme Q10 24 25, gluta-thione 26, n acetylcysteine 27, d-methionine 28, ferulic acid (FA, 4-hydroxy 3-methoxycinnamic acid) 29 30 have been tested for their ability to reduce oxidative stress-induced hair cell death after intense sound exposure in animals.Alpha-lipoic acid (AlA), an essential cofactor in mito-chondrial enzymes, is a novel biological antioxidant and a potent free radical scavenger 31-33. in animal models, AlA has been shown to protect from both age-induced and cisplatin-induced hearing loss 34 35. The aim of this study was to evaluate the effects of AlA on reversible cochlear alterations using a TTS model in humans.
Materials and methods
A total of 30 volunteers served as subjects were ran-domly divided into three groups. mean age was 23.9 years (range 20-30 years). group A subjects were only exposed to noise, group
B subjects were exposed to noise 1 hour after assumption of 600 mg of AlA (Alfa wassermann S.p.A., Alanno-PE, italy) and group
C sub-jects were exposed to noise after 10 days of oral daily assumption of 600 mg of AlA. All subjects signed an informed consent and the drug was kindly provided by Alfa wassermann S.p.A. The study was approved by the local review Board.
Subjects
There were 15 males and 15 females. Subject selection was based on age (20-30 years old) and hearing level (mean threshold for frequencies from 0.5 to 8 khz bet-ter than 20 dB hl). All subjects were carefully screened for negative history of otological disease, noise exposure, ototoxic drugs, metabolic disease associated with hearing loss and family history of hearing loss.
Auditory threshold and otoacoustic emissions measurement
Pure tone audiometry was performed in a soundproof cabin using pure tones (250 msec duration, 25
msec rise/fall time, 50% duty cycle) at octave frequencies from 125 hz to 8000 hz with a maximum intensity of 120 dB SPl with an Amplaid 319 audiometer.
TEoAEs were recorded for click stimulation. Clicks were generated by very short electrical pulses (< 50 µsec) with a wideband spectrum using an ilov6 sys-tem in the standard non-linear mode. The click level was 0.3 Pa in the ear canal. The intrameatal stimulus inten-sity was measured directly in the outer auditory canal and was adjusted by the software. The noise rejection level was set at 4.6 mPa corresponding to 47.3 dB SPl. The spectrum analyzer was triggered at 4 msec after the stimulus presentation to avoid acoustic ringing of the in-put stimuli, and the temporal window was set at 20 msec. 260 averages were recorded. TEoAEs were considered present when the wave reproducibility was greater than 70%. All recordings were performed in a soundproof booth.
Noise exposure and drug assumption
Subjects were randomly assigned to three groups. group A (10 subjects) was exposed to a 90 dB hl 3 khz pure tone for 10 min. group B (10 subjects) was exposed to a 90 dB hl 3 khz pure tone 1 hour after the oral assump-tion of 600 mg of AlA. group C (10 subjects) was ex-posed to a 90 dB hl 3 khz pure tone for 10 min following 10 days of 600 mg AlA. in all subjects, the right ear was exposed to noise.
Fig. 1. Mean TTS2 for the three groups.
Alpha lipoic acid and TTS
383
Discussion
The results of the present study show that a 10 day course, and not a single administration, of 600 mg of AlA can protect the cochlea from subsequent acoustic trauma. The protection was evident on both subjective (TTS2 at 6 khz) and objective (TEAoEs) measures of hearing. in reality, an increased sensitivity of oAEs compared to pure tone audiometry in the monitoring of preclinical cochlear dam-age, has been demonstrated in literature 29 36 37.
Acoustic overstimulation is associated with increased oxidative stress due to ischaemia reperfusion, glutamate excitotoxicity and an increase in mitochondrial free radi-cal production 12-15. AlA, an essential cofactor in mito-chondrial enzymes, may protect against cochlear damage through a variety of mechanisms, such as providing a sub-strate for cochlear gSh synthesis, free radical scavenging and inhibition of cell death pathway activation 38.
AlA is readily absorbed from the diet, transported, tak-en up by cells and reduced to dihydrolipoate in various tissues, including the brain 38. Both lipoate and dihydro-lipoate are antioxidants 19 and participate in reactions neutralizing roS as well as in reduction of the oxidized forms of other antioxidants such as endogenous vitamin E, vitamin C and glutathione 18. AlA administration has been shown to increase intracellular glutathione levels by 30-70% both in cell culture and in vivo 39, scavenge no and inhibit the formation of nF-κB (20), a transcription factor whose nuclear transport has recently been shown to increase after acoustic trauma 40.
in animal models, AlA has been shown to protect from age-induced and cisplatin-induced hearing loss 34 35. Seid-man et al. 34 supplemented Fischer rats aged 24 months for 6 weeks with either placebo, acetyl-l-carnitine or AlA. Animals treated with AlA showed a delay in progression of hearing loss, which was, however, significant only at 3 khz; in addition, qualitative and quantitative analysis of mitochondrial dnA (mt-dnA) showed that in treat-ed animals the common aging deletion was present to a lesser degree 34. Even better results were obtained in rats supplemented with acetyl-l-carnitine. The authors con-cluded that mitochondrial metabolites such as AlA or acetyl-l-carnitine may reduce age-associated deteriora-tion in auditory sensitivity and improve cochlear function by their ability to protect and repair age-induced mt-dnA damage.
rybak et al. 35 investigated the protective effect of AlA against cisplatin-induced ototoxicity in wistar rats, which has been shown to be mediated by roS. rats treated with cisplatin plus AlA did not show significant elevations of hearing thresholds, and AlA further prevented the deple-tion of glutathione as well as the decrease of superoxide dismutase, catalase, glutathione peroxidase and glutathi-
one reductase activities 35. The protective effect of AlA was related to its action as a roS scavenger and a chelator of platinum from the cochlea. interestingly, the authors noted that the dose of AlA required to protect from cis-platin toxicity was 3 to 12 times lower than that of other chelating agents.
Animal and human studies have demonstrated that exog-enous administration of AlA has therapeutic potential in diabetes and diabetic neuropathy, neurodegenerative dis-orders and heavy metal toxicity 38 41. in clinical practice, AlA is administered orally at a daily dosage of 600 mg. Although blood levels of lipoate and dihydrolipoate were not dosed in this study, the drug is rapidly absorbed in the stomach, crosses the blood-brain barrier 41 and reaches a peak plasma concentration 60 min after oral assump-tion 42.
in the present study, the administration of a single dose of AlA 60 min before acoustic overstimulation did not have any protective effect, while a 10 day course led to sig-nificant protection at 6 khz and on T
EoAEs. we believe that the main effect of AlA in these patients is related to an increase in gSh synthesis and free radical scavenging that prevented the TTS.
To date, very few clinical studies have examined the pro-tective effects of compounds in nihl in humans. Attias et al. 8 have shown that the oral assumption of 122 mg of magnesium for 10 days can significantly reduce the TTS induced by a 90 dB Sl white noise in normal subjects.A recent study looked at the protective effects of n-acetyl-cysteine (nAC), a drug commonly used in clinical prac-tice on TTS in voluntary discotheque attendees 43; an oral dose of 900 mg of nAC or placebo were administered 1 hour prior to attending a local discotheque. upon exiting, audiometric measures were performed in a soundproof van, and no difference was found between the drug and placebo. Confounding factors in this study were the small number of subjects and the variability in noise exposure levels, and that only one dose of nAC favourable preliminary results have been reported by Kopke et al. 9 in military subjects who were supple-mented with 2.7 gm/day of nAC, a dose three times high-er than that used in discotheque attendees.
in this preliminary study, AlA appears to be protective for the ear from noise in humans. The dosage needed to obtain a protective effect is not higher than that proposed in clinical practice, and therefore AlA seems to have the characteristics of the “ideal” pharmacologic prophylaxis. in fact, it specifically ad
dresses known mechanisms of acoustic trauma, is orally administered and is safe, effec-tive and inexpensive 9. Further research is however war-ranted to evaluate the effects of AlA in a larger study population and its effects on TTS and permanent cochlear damage.
n. Quaranta et al.
384
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