A biochemical mechanism of the ototoxic interaction between neomycin and ethacrynic acid

Efficacy of the cat deafening method: Co-administration of ethacrynic acid and kanamycin

OBJECTIVE

This study was designed to determine if hearing status monitoring during intravenous infusion of EA reduces individual variability and to evaluate the correlation between EA dose and Bwt.

MATERIALS AND METHODS

Twenty-five cats with the mean age of 24 ± 3.7 weeks (range = 20.6-28.3) and a mean weight of 3.21 ± 0.84 kg (range = 1.9-5.1) were administered a subcutaneous injection of KM (300 mg/kg) followed by an intravenous infusion of EA (1 mg/min). Click-evoked auditory brainstem responses (ABRs) were recorded to monitor hearing during the infusion. When ABR thresholds exceeded a 90 dB sound pressure level, the infusion of EA was terminated. Histopathology forapex, middle, and base sections of the cochlea were examined after 6 months.

RESULTS

The dose of EA was optimized for deafening through simultaneous ABR measurements. Bwt was positively correlated with EA dose (mg) (p < 0.001, R(2) = 0.548), which was different from a study previously reported. Cochlear histopathology assessments revealed an absence of organ of Corti in the majority of cochleae.

CONCLUSION

Co-administration of kanamycin (KM) and ethacrynic acid (EA) was an easy and effective method for deafening procedures in adult animals. Body weight (Bwt) was positively correlated with EA dose (mg) and an optimal EA dose can be calculated.

Late dosing with ethacrynic acid can reduce gentamicin concentration in perilymph and protect cochlear hair cells

A key factor in the well-known interaction between ethacrynic acid (EA) and aminoglycoside antibiotics (AABs) is disruption of the blood-labyrinth barrier (BLB), leading to rapid entry of EA and AABs into the cochlear fluids. The idea that the blood-labyrinthine fluid concentration gradient might be utilized in a protective manner was tested in the current experiment. We hypothesized that administering EA when gentamicin (GM) levels are higher in the cochlea than in the blood might actually reduce cochlear damage by permitting efflux of GM from the cochlear fluids into the bloodstream, down a concentration gradient and across a temporarily disrupted BLB. Guinea pigs received 1, 11, 14 or 20 injections of GM (125 mg/kg i.m.). Approximately half of the animals also received a single injection of EA (40 mg/kg i.v.) either concurrently or 12-18 h after the last GM injection. Concurrent injection of EA significantly increased GM concentration in serum and perilymph at all time points sampled (2.5, 5-8, and 12 h post injection). Compared to animals that received GM only, animals that received a delayed injection of EA had a significantly lower GM concentration in perilymph, lower thresholds of the compound action potential, and less outer hair cell loss. Collectively, the evidence suggests that EA can reduce GM ototoxicity if it is administered 12-18 h after GM, but the mechanism remains to be elucidated. The results may have implications for the clinical management of aminoglycoside ototoxicity in humans, as well as for understanding the mechanisms underlying AAB/EA interactions.

Profound hearing loss in the cat following the single co-administration of kanamycin and ethacrynic acid

Co-administration of kanamycin (KA) with the loop diuretic ethacrynic acid (EA) has previously been shown to produce a rapid and profound hearing loss in guinea pigs. In the present study we describe a modified technique for developing a profound hearing loss in cats. By monitoring the animal's hearing status during the intravenous infusion of EA the technique minimizes the effects of individual variability to the drug regime. Seven cats received a subcutaneous injection of KA (300 mg/kg) followed by intravenous infusion of EA (1 mg/min). Click-evoked auditory brainstem responses (ABRs) were recorded to monitor the animal's hearing during the infusion. When the ABR thresholds rose rapidly to levels in excess of 90 dB SPL the infusion of EA was stopped. This occurred at EA doses of 10-25 mg/kg, indicating considerable individual variability to the deafening procedure. However, there was a strong negative correlation (r = -0.93) between the EA dose and body weight which accounted for much of this variability. Subsequent ABR monitoring showed that this profound hearing loss was both bilateral and permanent. Significantly, blood urea and creatinine levels, monitored for periods of up to three days after the procedure, remained within the normal range. Furthermore, there was no clinical evidence of renal dysfunction as indicated by weight loss or oliguria. Cochlear histopathology, examined after a two months to three year survival period, showed an absence of all inner and outer hair cells in the majority of cochleas. The extent of loss of spiral ganglion cells was dependent on their distance from the round window and the period of survival following the deafening procedure. Clearly, the degeneration of spiral ganglion cells continued for several years following the initial insult. Finally, we observed no evidence of renal histopathology. In conclusion, the co-administration of KA and EA produces a profound hearing loss in cats without evidence of renal impairment. Monitoring the animal's hearing status during the procedure ensures that the dose of EA can be optimised for individual animals. Moreover, it may be possible to adapt this procedure to produce animal models with controlled high frequency hearing losses.

Ethacrynic acid: outflow effects and toxicity in human trabecular meshwork in perfusion organ culture

The effect of ethacrynic acid, a potential outflow agent for the treatment of glaucoma, was studied in a series of 25 pairs of human eyes in perfusion organ culture. Concentrations varied from 0.01 mM to 2.4 mM and were used in single or repetitive doses. Intraocular pressure was continuously recorded for up to two weeks after exposure. Eyes were then fixed and the meshwork examined histologically. Ethacrynic acid in single doses of 0.05 mM, 0.3 mM, and 0.6 mM increased facility of outflow at least 40% when compared with fellow control eyes. The duration of effect was approximately 18 hours, during which time the intraocular pressure gradually returned to baseline. Histologic examination revealed dose related effects on the trabecular cells, ranging from clumping of nuclear chromatin in some eyes to cellular swelling, disruption of cytoplasmic membranes, and cell necrosis in other eyes at concentrations of 0.1 mM and higher. No recovery or reversal of these changes was noted with time, even two weeks after a single exposure to the drug. Although ethacrynic acid is effective in temporarily lowering intraocular pressure in the human eye, a low therapeutic index may limit its clinical usefulness.

Cochlear vessel permeability to horseradish peroxidase after diuretic administration in the chinchilla

The permeability of strial vessels to the small protein, HRP, was examined after administration of diuretics to determine if increased vascular permeability is a factor in the development of strial edema as it is in acoustic trauma. Chinchillas were injected with HRP and either ethacrynic acid, a loop-inhibiting diuretic, or mannitol, an osmotic diuretic. There was no increased vascular permeability to HRP. Therefore, unlike the increased vessel permeability of HRP seen after an acoustic insult, increased vessel permeability of HRP is not a factor in the formation of strial edema after either mannitol or ethacrynic acid administration.

Drug-induced ototoxicity. Pathogenesis and prevention

Ototoxicity is a disabling adverse effect of several widely used classes of drugs, such as diuretics, anti-inflammatory agents, antineoplastic agents and aminoglycoside antibiotics. High-dose therapy with either diuretics or anti-inflammatory agents is primarily associated with acute and transient impairment of hearing or tinnitus. In contrast, long term treatment with antineoplastic agents or aminoglycoside antibiotics is typically associated with delayed and irreversible loss of hearing; lesion in the organ of Corti include the destruction of auditory sensory cells. Vestibular function can also be compromised by ototoxic drugs. Occasional cases of ototoxicity have been reported for a variety of other therapeutic compounds and environmental toxins. In addition, the simultaneous administration of multiple agents which are potentially ototoxic can lead to synergistic loss of hearing. Exposure to loud noise may also potentiate the hearing loss due to cochleotoxic drugs. Ototoxic agents can impair the sensory processing of sound at many cellular or subcellular sites. However, the molecular mechanisms of ototoxicity have not been established for most of these drugs, and structure-toxicity relationships have not been determined. It has therefore been difficult to predict the ototoxic potential of new drugs, and rational approaches to the prevention of ototoxicity are still lacking. The clinical and experimental features of ototoxicity are reviewed for several classes of drugs, with an emphasis on current knowledge of the mechanism and the possibilities for the prevention of ototoxicity for each.

Abolition of the negative endocochlear potential as a consequence of the gentamicin-furosemide interaction

The DC endocochlear potential and the AC cochlear potential in response to a 4 kHz tone were recorded in pigmented guinea pigs before and during ototoxic damage induced by sequential administration of the aminoglycoside antibiotic, gentamicin, and the loop diuretic, furosemide. Within 4 h significant diminution of the amplitude of the AC cochlear potential was accompanied by an almost complete abolition of the negative diffusion potential revealed by either furosemide administration or terminal anoxia. Thus, one of the effects of this interaction appears to involve a reduction in the potassium permeability of the cochlear partitions.

The potentiation of ototoxicity when aminooxyacetic acid and kanamycin are co-administered

Aminooxyacetic acid (AOAA) has been shown to confer protection against noise-induced cochlear trauma [3]. We, therefore, decided to study the possible protective effect of AOAA against kanamycin (KM) ototoxicity and found, instead, that AOAA potentiated the toxicity. To produce ototoxicity in guinea pigs, KM is usually given in 10-14 daily doses of 400 mg/kg s.c. However, when combined with a single dose of AOAA (8, 11, 15, or 25 mg/kg) a single 400 mg/kg dose of KM is sufficient to cause cochlear damage. Such animals show a negative Preyer's reflex between 1 to 3 days post injection. 21 days later hearing thresholds as detected electrocochleographically at 2, 4, 8, 12 and 16 kHz have changed drastically sometimes to the point of being undetectable. The damage seen histologically at this time is destruction of both inner and outer hair cells. A pharmacokinetic analysis of this potentiation revealed a slight prolongation of KM's sojourn in the inner ear. The possible mechanisms of this unexpected, marked potentiation are discussed but remain unknown.

Alteration of strial capillary transport in kanamycin-treated guinea pigs

Ferritin transport through the strial capillary during kanamycin intoxication was examined under a transmission electron microscope. Twelve guinea pigs were treated intramuscularly with kanamycin (400 mg/kg/day) for 2-3 weeks. When full hearing loss was recognized by estimating the Preyer reflex or auditory brain-stem response (ABR), ferritin was given intravenously and animals were sacrificed 1/3, 1, and 2 h later. At 1/3 h, ferritin was present diffusely not only in the endothelial cell but also in the basal lamina and within vesicles in the strial cell. Alternatively, it was discharged into the endolymphatic space. At 2 h, ferritin was seen on the luminal surface and in the cytoplasm of the endolymphatic cells of Reissner's membrane. These results suggest that the basal lamina of the strial capillary was altered qualitatively by kanamycin administration.

Ethacrynic acid facilitates gentamicin entry into endolymph of the rat

Influence of ethacrynic acid (EA) upon gentamicin kinetics in perilymph and endolymph was studied in rats that were given a constant-infusion of gentamicin (150 micrograms/min) and EA (140 micrograms/min). Inner ear fluids and plasma were sampled up to 5 h. The purity of the endolymph was ensured by measurement of sodium and potassium concentrations. Gentamicin assay was done with a modified radioimmunoassay. Results show that EA facilitates the entry of gentamicin into endolymph, while it does not affect the kinetics of the drug in perilymph. Although the mechanism of this facilitation remains unclear, this result may account for the ototoxic potentiation reported between EA and aminoglycoside antibiotics.

"Is magnesium depletion the reason for ototoxicity caused by aminoglycosides?"

Aminoglycoside antibiotic drugs may cause ototoxicity and nephrotoxicity. Our hypothesis postulates that aminoglycosides cause ototoxicity by a mechanism of magnesium depletion in the hair cells of the cochlea. The same mechanism maybe responsible for nephrotoxicity caused by aminoglycosides.