Evaluation of ototoxicity of tobramycin in guinea pigs

Ototoxic destruction by co-administration of kanamycin and ethacrynic acid in rats

It is well known that ethacrynic acid (EA) can potentiate the ototoxicity of aminoglycoside antibiotics (AmAn) such as kanamycin (KM), if they were applied at the same time. Currently, to create the model of EA-KM-induced cochlear lesion in rats, adult rats received a single injection of EA (75 mg/kg, intravenous injection), or followed immediately by KM (500 mg/kg, intramuscular injection). The hearing function was assessed by auditory brainstem response (ABR) measurement in response to click and/or tone bursts at 4, 8, 12, 16, 20, 24, and 32 kHz. The static microcirculation status in the stria vascularis after a single EA injection was evaluated with eosin staining. The pathological changes in cochlear and vestibular hair cells were also quantified after co-administration of EA and KM. After a single EA injection, blood flow in vessels supplying the stria vascularis rapidly diminished. However, the blood supply to the cochlear lateral wall partially recovered 5 h after EA treatment. Threshold changes in ABR were basically parallel to the microcirculation changes in stria vascularis after single EA treatment. Importantly, disposable co-administration of EA and KM resulted in a permanent hearing loss and severe damage to the cochlear hair cells, but spared the vestibular hair cells. Since the cochlear lateral wall is the important part of the blood-cochlea barrier, EA-induced anoxic damage to the epithelium of stria vascularis may enhance the entry of KM to the cochlea. Thus, experimental animal model of selective cochlear damage with normal vestibular systems can be reliably created through co-administration of EA and KM.

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.

Insensitivity of the chicken embryo to the ototoxicity of aminoglycoside antibiotics and a loop diuretic

Guinea pigs are routinely used in the histological evaluation of the cochlea as a method of testing for ototoxicity, but the procedures are very time-consuming. Because the avian cochlea is easier to examine and newly hatched chicks are sensitive to the ototoxic effects of gentamicin, birds may be useful in testing for ototoxicity. The use of chicken embryos would be even better for testing, but whether or not chicken embryos are sensitive to ototoxicants is unknown. In an attempt to determine whether or not chicken embryos may be used instead of guinea pigs in screening tests for ototoxicity, aminoglycoside antibiotics and a loop diuretic, ethacrynic acid, were administered to chicken embryos. A maximum-tolerated dose of gentamicin, kanamycin, streptomycin, ethacrynic acid, or a combination of gentamicin and ethacrynic acid was administered to fertile eggs of White Leghorn chickens on incubation days 10-17. To compare the effect of route of exposure on ototoxicity, gentamicin was administered by injection into the allantoic space, yolk sac, and air cell as well as by submerging the egg in gentamicin solution. With the preferred air cell route the effects of the ototoxic drugs kanamycin, streptomycin, ethacrynic acid, and a combination of ethacrynic acid and gentamicin were compared. On incubation day 18, cochleas were removed from the chicken embryos. Serial sections of these avian cochleas were examined and hair cells were counted. No significant difference was seen between the number of hair cells in cochleas of control chicken embryos and those from chicken embryos treated with drugs. Therefore, the chicken embryo appears to be insensitive to the ototoxicity of aminoglycoside antibiotics and a loop diuretic.

Intratympanic ototoxicity: influence of post-injection survival period

Many studies have been reported on the intratympanic ototoxicity of different drugs in animal models. The recovery periods of the animals following intratympanic drug applications varied among these studies. The present study compares the cochlear damage caused by intratympanic kanamycin following short (4 days) and long (30 days) post-injection survival periods, using the guinea pig as the animal model. The degree of cochlear damage 4 days after kanamycin injection was consistent among the tested animals. The degeneration was mainly confined to the outer hair cells and almost all inner hair were spared. The change 30 days after kanamycin injection was more variable among the animals and both inner and outer hair cells were damaged. This shows that, although the damage to the cochlea after intratympanic aminoglycoside injection is progressive, a short post-injection recovery period is suitable for comparative intratympanic ototoxicity studies.

Some consequences of drug choice and dosage regimen for patients with impaired kidney function

The object of this article is to discuss difficulties in extrapolating the performances of a drug for which the kinetic parameters are derived in healthy volunteers, to patients with severely impaired kidney function. The theoretical background of some actual or probable background is given, and a possible solution for these problems is offered, that is, choosing another drug from the same drug group. In patients without kidney function, metabolism is the only pathway of elimination. When the elimination of the metabolite formed occurs by means of renal excretion only, this metabolite accumulates in patients with impaired or absent kidney function. When a metabolic pathway of the parent drug is part of a metabolic equilibrium, the metabolic return reaction results in an "apparent parent compound," with a half-life identical to that of the accumulated metabolite. In this way, the concentration of the "apparent" parent compound increases and the half-life of the sum of parent and "apparent" parent drug will change. Examples of this drug behavior are given for sulfamethoxazole, sulfametrole, sulfamethizole, procainamide, and N4-acetylprocainamide.

Structure side-effect sorting of drugs. VI. Ototoxicities

From a literature survey, over 130 (about 7.8%) drugs and chemicals have been associated with ototoxicities. The major classes are basic aminoglycoside and other antibiotics, anti-inflammatory drugs, antimalarials, beta-blockers, antineoplastic agents, heavy metals, diuretics, some topical agents and various miscellaneous drugs. Possible mechanisms of action are presented and discussed. These include inhibition of protein synthesis, the glycolytic cycle, the TCA cycle, energy utilization, energy generation and the respiratory system within the mitochondria membrane of the hair cell, and also alteration of the permeability of the endolymphatic membrane or alteration of the excretion system for the basic aminoglycosides in the lateral wall of the membranous cochlea. The relative rank order of ototoxicity and reactivity toward mucopolysaccharides of five aminoglycosides is found to be related to the number of basic groups in each molecule.

Double-blind comparison of the nephrotoxicity and auditory toxicity of gentamicin and tobramycin

Two hundred fifty-eight patients with suspected sepsis were treated with tobramycin or gentamicin in a prospective, randomized, double-blind trial. One hundred forty-six patients received nine or more doses, had serial determinations of serum creatinine, and were evaluated for nephrotoxicity; 91 were able to cooperate with audiometry and were evaluated for auditory toxicity. Auditory toxicity developed in five of 47 (10 per cent) given gentamicin and five of 44 (11 per cent) given tobramycin. Nephrotoxicity developed in 19 of 72 (26 per cent) given gentamicin and nine of 74 (12 per cent) given tobramycin (P less than 0.025). The severity of the nephrotoxicity was not different; the mean increase in creatinine was 1.3 mg per 100 ml (114.9 mumol per liter) in both groups. Both the tobramycin and gentamicin groups had a similar mean age, initial serum creatinine level, total dose, serum aminoglycoside level, and duration of therapy. We conclude that tobramycin causes nephrotoxicity less frequently than does gentamicin.