Auditory function in guinea pigs treated with netilmicin and other aminoglycoside antibiotics

Sensitive developmental periods for kanamycin ototoxic effects on distortion-product otoacoustic emissions

The developing rat is hypersensitive to aminoglycoside toxicity, which is expressed early on as a destruction of outer hair cells (OHC). In the current study, distortion-product otoacoustic emissions (DPOAE), which specifically measure the micromechanical activity of OHCs, were used to assess functional effects of administering a regimen of kanamycin to three groups of neonatal rats representing discrete postnatal developmental periods. In this manner, pigmented rats were treated at postnatal days 1-10, 11-20, and 21-30. A series of input-output (I/O) functions obtained for the 2f1-f2 DPOAE during the post-treatment period indicated that detection thresholds were significantly elevated for the animals treated on postnatal days 1-10 and 11-20, with the greatest elevations observed at the higher test frequencies.

High-performance liquid chromatographic determination of netilmicin in guinea-pig and human serum by fluorodinitrobenzene derivatization with spectrophotometric detection

A high-performance liquid chromatographic procedure for netilmicin determination in guinea-pig and human serum using pre-column derivatization with 1-fluoro-2,4-dinitrobenzene and UV detection is described. Linearity was established over the range 0.5-40 micrograms/ml using only 50 microliters of serum. Accuracy and precision were good, with a mean coefficient of variation less than 5% and a mean relative error less than 4%. This procedure correlates well with an enzyme multiplied immunoassay technique and has a sensitivity similar to those of published fluorescence derivatization methods.

Comparative uptake of gentamicin, netilmicin, and amikacin in the guinea pig cochlea and vestibule

The kinetics of the entry of three aminoglycosides into inner-ear tissues of the guinea pig after acute and chronic administration were compared: gentamicin toxic to the cochlea and the vestibule, amikacin preferentially cochleotoxic, and netilmicin of low ototoxic liability. During constant intravenous infusion, levels of the three drugs in plasma tended to reach a plateau after 1 h, while levels in perilymph did not reach a plateau within 6 h. The drug concentrations in both vestibular and cochlear tissues quickly reached saturation. Amikacin and gentamicin concentrations were similar in vestibular and cochlear tissues, while netilmicin values were somewhat lower. After 1 week of chronic treatment (100 mg of drug per kg of body weight daily subcutaneously), levels of gentamicin and amikacin in tissue were similar to each other and were not significantly different between cochlear and vestibular tissues. Netilmicin concentrations again were somewhat lower in the tissues, but identical to those of the other drugs in the perilymph. After 3 weeks of treatment, all of the drugs were equally distributed in the inner-ear tissues. Release of the drug from the tissues after the 3-week treatment was faster for amikacin (83% decrease after 20 days) than for netilmicin and gentamicin (approximately 50% decrease). There was no correlation, under any of the experimental conditions, between the drug concentrations and their degrees of toxicity. These results demonstrate that selective aminoglycoside ototoxicity cannot be explained by a preferential uptake or accumulation of drugs in the afflicted tissues or in the perilymph.

Comparative oto-vestibular effects in the pigmented guinea pig after dibekacin and netilmicin treatment

We treated groups of pigmented guinea pigs with either intramuscular netilmicin or dibekacin at 100 and 150 mg/kg per day for 3 weeks. Saline was used as the control solution. All animals were tested weekly for both vestibular and auditory functions. The vestibular function was evaluated by the duration of post-rotatory nystagmus (PRN) elicited by interrupting the rotation of the animal around the vertical axis; auditory function was evaluated by the threshold response for the Preyer's pinna reflex (PPR). All animals were then sacrificed and either their labyrinths or Corti organs were processed for further investigations using the scanning electron microscope (SEM). The duration of PRN decreased over the treatment period in all of the groups as a result of adaptation. However, 150 mg/kg dibekacin produced a significant decrease of the PRN responses as compared to the control and other groups. This effect also continued during the recovery period. Likewise, the PPR threshold of the animals receiving 150 mg/kg dibekacin showed a significant increase at the end of the treatments and during the recovery period, while the other dibekacin group had no significant auditory impairment. Netilmicin at both doses did not significantly affect responses following either vestibular or auditory stimulations. SEM observations demonstrated that the sensory epithelia of the labyrinths and Corti organs affected by 150 mg/kg dibekacin had great losses of stereocilia, while comparable doses of netilmicin (150 mg/kg) had only very moderate losses of stereocilia in the labyrinths but not in the Corti organs.

Principles in cochlear toxicity

The hair cells of the cochlea (neuroepithelium) represent the primary target in most drug-induced ototoxic adverse effects on hearing (e.g. aminoglycoside antibiotics). To what extent an exogenically-induced morphologic damage to hair cells is reversible is not known. In aging structurally altered hair cells can persist for years likewisely not any longer participating in sensory transduction as the hair cells degenerate, secondary changes occur in the spiral ganglion cells and the neuronal pathways. Following heavy metal poisoning an adverse effect is observed on both central and peripheral innervation of the cochlea and only minor primary changes occur in the receptor cells. The link between function and morphology in the cochlea is very obvious regarding the high and middle frequencies with a distinct tonotopic localisation whereas for low frequencies (below 1 khz) such a specific morphologic correlation is lacking. Ototoxic effects primarily affecting the source for the production of endolymph, i.e. the stria vascularis, become manifest at all frequencies and at a rather early stage. Independent of type of substance penetrating into the inner ear, the substance has a considerably slower elimination rate as compared with all other compartments in the body. The toxicity of the drugs seems to be more related to its tissue binding capacity and saturation of receptor sites than related to the concentration of the drug in endo-or perilymph.

Comparative ototoxicity of dibekacin and netilmicin in guinea pigs

The cochleo- and vestibulotoxicity of dibekacin and netilmicin were compared in a guinea pig model. Both aminoglycosides were administered subcutaneously for 21 days at the dose level of 150 mg/kg/day. Control animals were injected with saline. Dibekacin-treated animals showed a significant (P less than 0.05) increase in the thresholds of the Preyer pinna reflex and the VIIIth nerve compound action potential in response to sound click stimulation. Moreover, a deterioration of the electrophysiologic auditory response and an almost complete suppression of the post-rotatory nystagmus were detected. In contrast, netilmicin did not induce any significant change in auditory and vestibular functions as compared to the control group. Our results demonstrated that netilmicin was devoid of ototoxicity in the guinea pig, while dibekacin provoked mild cochlear and severe vestibulotoxicity.

Comparative pharmacokinetics of aminoglycoside antibiotics in guinea pigs

The pharmacokinetics of netilmicin, gentamicin, and tobramycin in plasma and in perilymph of guinea pigs were studied after a single intravenous injection of 40 mg/kg. Detailed pharmacokinetic analysis of the plasma drug concentration-time data up to 36 h after the intravenous dose revealed that the pharmacokinetics of the aminoglycoside antibiotics can be best described as a three-compartment open model. The disposition half-lives (t1/2) in plasma of the three antibiotics were comparable and within the following ranges: t1/2 alpha of 0.09 to 0.16 h; t1/2 beta of 0.88 to 1.01 h; and t1/2 gamma of 7.87 to 8.29 h. The volume of distribution in the central compartment and the total body clearance of netilmicin (294 ml/kg, 5.74 ml/min per kg) were greater than those of gentamicin (160 ml/kg, 3.40 ml/min per kg) and tobramycin (204 ml/kg, 4.63 ml/min per kg). Pharmacokinetic analysis of the perilymph drug concentration-time data indicated that all three antibiotics penetrated the perilymph readily, but netilmicin cleared from the perilymph compartment faster than gentamicin and tobramycin. The maximum perilymph drug concentrations were 4.17, 8.05, and 6.78 micrograms/ml and occurred at 1, 2, and 4 h for netilmicin, gentamicin, and tobramycin, respectively. The ratio of area under the curve of perilymph to plasma was lowest for netilmicin (0.27), followed by gentamicin (0.39) and tobramycin (0.57). These results suggest that the differences in pharmacokinetics and concentrations of netilmicin in the perilymph may account for less ototoxic liability of netilmicin compared with gentamicin and tobramycin.

Histological evaluation of ototoxic reaction due to some aminoglycoside antibiotics

Twenty-one days s.c. treatment with netilmicin, gentamicin, sisomicin, and kanamicin at different doses were performed in albino guinea pigs of both sexes. Block surface preparation of Corti's organ was carried out and missing OHCs and IHCs counted in selected areas of each turn. The animals treated with gentamicin and sisomicin showed a dose-depending missing of hair cells, preferably relative to the outer ones and quantitatively the same for both the antibiotics. Furthermore, a characteristic distribution of the damage progressively increasing from the apex to the basal turn of the cochlea was detected. Kanamicin at 267 and 400 mg/kg gave the complete destruction of OHCs and IHCs along the whole cochlea while at 178 mg/kg did not show any significant damage in comparison to the control group. Finally, netilmicin neither damaged Corti's organ nor statistically determined missing OHCs and IHCs at all the doses used. These histological results and the previous electrophysiological and reflexological observations obtained in the guinea pig show a higher safety of netilmicin in comparison to the other aminoglycosides used for the problem relative to ototoxicity.