New developments in our understanding of ototoxicity.

Ototoxicity has been well described for many years, but for most of that time, its mechanism has been unclear. However, new studies are finally beginning to clarify the mechanism, and we now have at least a fairly good idea as to how ototoxicity occurs in response to aminoglycoside therapy.

Mechanism of aminoglycoside ototoxicity

As is the case with acquired sensorineural hearing loss secondary to other causes, ototoxicity-induced hearing loss tends to first manifest in the high frequencies and in the basal portion of the cochlea. It then progressively involves the lower frequencies and the apical portion of the cochlea.

Histopathologic studies have shown that outer hair cells are more sensitive to ototoxic injury than are inner hair cells. We also know that the aminoglycosides are transported into the hair cells themselves, but they probably do not concentrate there. It is unusual for aminoglycoside concentrates in the cochlea to exceed serum concentrations, but the length of the dwell time is orders of magnitude longer. (1,2) Aminoglycosides can be detected in perilymph 60 to 90 minutes after systemic administration, and they remain detectable in hair cells 3 to 4 months later. The mechanism of transport is still unknown. There are some suggestions that this mechanism may be similar to the mechanism of transport in renal tubular cells. (1,2)

In animal models, histologic findings resemble apoptotic cell death rather than necrosis. After a period of several weeks, the hair cells disappear. Supporting cells--at least in the short and immediate term--remain intact, and there is no evidence of the inflammation that you would expect to see with cell necrosis.

We now understand a fair amount about apoptotic cell death. In cases of aminoglycoside ototoxicity, it appears that the apoptotic cascade has been initiated. If you examine explants of inner ears that have been subjected to ototoxic drugs, you can detect a variety of free-radical species, including both oxygen and nitrogen free-radical species. Radical oxygen species and radical nitrogen species initiate the apoptotic cascade. Free-radical-initiated apoptosis is a mechanism of injury in a variety of other disease processes, including noise-induced hair cell loss.

So the question then becomes, How do aminoglycosides produce free radicals? Schacht has developed experimental evidence that shows that aminoglycosides can interact fairly efficiently with transition metals, including iron and copper, which can form free radicals. (3,4) Aminoglycosides also have a very high affinity for binding to various types of phosphate lipids, and this affinity correlates fairly well with their potential ototoxicity. In effect, aminoglycosides are binding iron or copper molecules into active complexes and forming radical oxygen species that cause ototoxicity. (3-5)

A number of antioxidant enzymes appear to reduce ototoxicity-induced hearing loss, which would appear to confirm the free-radical model of injury. One of the things we know about free radicals is that regardless of how they are formed, the type of injury they cause conforms to histologic findings seen in aminoglycoside-injured cochleae. (3-5)

Genetics of aminoglycoside ototoxicity

Fischel-Ghodsian et al discovered that aminoglycoside ototoxicity occurred in multiple members of some families and that the gene was passed only by the mothers, not the fathers. (6) They believed that the transmission was probably mitochondrial in origin. Indeed, they were the first to recognize a specific mutation associated with a high susceptibly to aminoglycoside ototoxicity. They found a substitution in adenine binding at position 1555 in the 12S subunit of mitochondrial RNA in a Chinese family; this gene was not found in 278 controls. It is interesting that this is the same portion of that strand of nucleic acid that is involved in conferring resistance to aminoglycosides in yeast. Therefore, it appears that this is the portion of the nucleic acid sequence that is responsible for determining sensitivity or resistance to aminoglycosides. (Subsequently, Fischel-Ghodsian identified another susceptibility mutation in the mitochondrial 12S ribosomal RNA gene delta T961Cn. (7) This defect is much less common than the defect at position 1555.)

Phenotypically, there is a difference in the way this gene is expressed in Asian populations and in Caucasian populations. In Chinese patients with this defect, hearing loss caused by aminoglycoside-induced ototoxicity is usually rapid and severe. Among whites in the United States, hearing loss tends to be less severe and slowly progressive, sometimes occurring over a period of years after aminoglycoside administration. It has been speculated that the difference between these two populations may be attributable to another genetic mutation--either in the mitochondrial gene or possibly in the nuclear gene--that modulates or modifies the severity of the defect.

Virtually no one with a genetically mediated, aminoglycoside-induced hearing loss has a vestibular loss. It is fairly clear that patients who experience ototoxicity-induced hearing loss as a result of aminoglycoside administration have a genetic defect. Among those who do not have such a defect, the hearing loss appears to be a dose-related phenomenon.

What this means clinically is that a good history might allow us to prevent ototoxicity in some cases. If you determine that a patient's first-degree relative has experienced an aminoglycoside-induced hearing loss, this is a clear suggestion that perhaps the patient should not receive an aminoglycoside.

Discussion

Dr. Rutka: Some evidence indicates that there might be a simple way to prevent aminoglycoside toxicity, especially from the basic science research concerning the prevention of oxygen and nitrogen radical species. (8) One aspirin a day might be all that is necessary.

Dr. Coates: Yes, Schacht has published data on guinea pigs that show that salicylates protect hearing during the administration of gentamicin. (8) And others have indicated that aspirin protects against cisplatin ototoxicity. (9)

Dr. Roland: Therapeutic doses of aspirin might protect patients from aminoglycoside-induced hearing loss, but the data implicating high-dose salicylates in hearing loss are pretty solid.

Dr. Ovesen: Several study groups have identified a very specific receptor that mediates aminoglycoside toxicity in both the kidney and inner ear. It's called megalin. It's found in the cytoplasm of at least some kidney cells, but we don't know which cells in the inner ear harbor it. Some researchers have reported that megalin resides in the outer hair cells, but others have not been able to confirm this. (10) In our preliminary studies, we have localized megalin to the vascular stria of the cochlear duct. There may be a paracrine pathway at work, which could explain the few days' delay before changes in the outer hair cells are registered.

In Denmark we are working with an antagonist recently developed by a research group at the Institute of Medical Biochemistry at Aarhus University. We are conducting animal studies in which we combine the antagonist with gentamicin; by measuring otoacoustic emissions and performing brainstem audiometry and cochleography, we are attempting to determine whether the antagonist protects against ototoxicity. Microscopic changes are observed in the outer hair cells but, surprisingly, the animals do not become deaf, as determined by cochleography. Of course, these results have to be repeated, but it may create a problem with respect to our understanding of these animal models because we can always achieve some changes in the hair cells. But these changes may not give rise to a functional change in the hearing level. We have to put some fixation material into the inner ear, which we can do in two ways--either systemically or via direct injection through the round window membrane. The injection route, however, places a great deal of mechanical stress on the hairs, and they disappear immediately. So it is better to deliver the fixation material systemically.

Dr. Rutka: I'd like to make just a quick comment on ciprofloxacin toxicity. Over a period of 2 or 3 years recently, Health and Welfare Canada received reports of 5 cases of deafness or decreased hearing that had occurred in patients who took systemic ciprofloxacin. Four of these patients had taken 1 gram/day orally and the other had received 800 mg/day intravenously. Of this group, 2 patients recovered completely and 1 continued to have partial tonal deafness; the outcomes of the other 2 patients were unknown. The government became concerned because if systemic ciprofloxacin can cause hearing loss, we have to wonder if topical ciprofloxacin delivered to the middle ear might do the same thing.

Through the Freedom of Information Act in Canada, I was able to obtain pertinent information on the patients who were involved. One thing that I found very interesting is that none of these patients had ever had an audiogram. Therefore, we don't know what their hearing was like before they took the ciprofloxacin. Moreover, 2 of these patients were documented to have taken other drugs that we know are ototoxic, and it is possible that the others might have taken an ototoxic drug, as well. Also, some of these patients had only a unilateral hearing loss, which is very unlikely to be caused by a systemic medication. These are the kinds of data that we need to know before we start raising alarms.

Another compelling bit of information is that since oral ciprofloxacin was introduced into Canada, approximately 21 million prescriptions have been written for this medication. Therefore, if only a handful of patients experienced hearing loss, it is highly unlikely that it was caused by this medication.

References

(1.) Govaerts PJ, Claes J, Van De Heyning PH, et al. Aminoglycoside-induced ototoxicity. Toxicol Lett 1990;52:227-51.

(2.) Nakashima T, Teranishi M, Hibi T, et al. Vestibular and cochlear toxicity of aminoglycosides--A review. Acta Otolaryngol 2000; 120:904-11.

(3.) Schacht J. Biochemistry and pharmacology of aminoglycoside-induced hearing loss. Acta Physiol Pharmacol Ther Latinoam 1999;49:251-6.

(4.) Schacht J. Mechanisms for aminoglycoside toxicity: Basic science research. In: Roland PS, Rutka JA, eds. Ototoxicity. Hamilton, Ont.: B.C. Decker, 2004.

(5.) Van De Water TR, Rybak LP. Treatment, prevention, and neural regeneration. In: Roland PS, Rutka JA, eds. Ototoxicity. Hamilton, Ont.: B.C. Decker, 2004.

(6.) Fischel-Ghodsian N, Prezant TR, Bu X, Oztas S. Mitochondrial ribosomal RNA gene mutation in a patient with sporadic aminoglycoside ototoxicity. Am J Otolaryngol 1993;14:399-403.

(7.) Fischel-Ghodsian N. Genetic factors in aminoglycoside toxicity. Ann N Y Acad Sci 1999;884:99-109.

(8.) Sha SH, Schacht J. Salicylate attenuates gentamicin-induced ototoxicity. Lab Invest 1999;79:807-13.

(9.) Li G, Sha SH, Zotova E, et al. Salicylate protects hearing and kidney function from cisplatin toxicity without compromising its oncolytic action. Lab Invest 2002;82:585-96.

(10.) Forge A, Schacht J. Aminoglycoside antibiotics. Audiol Neurootol 2000;5:3-22.