Tissue levels of kanamycin in correlation with oto- and nephrotoxicity

Mechanisms and pharmacokinetic/pharmacodynamic profiles underlying the low nephrotoxicity and ototoxicity of etimicin

Etimicin (ETM), a fourth-generation aminoglycosides (AGs), is now widely clinically used in China due to its high efficacy and low toxicity. However, the mechanisms underlying its low nephrotoxicity and ototoxicity remain unclear. In the present study we compared the antibacterial and toxicity profiles of etimicin, gentamicin (GM, a second-generation AG), and amikacin (AMK, a third-generation AG), and investigated their pharmacokinetic properties in the toxicity target organs (kidney and inner ear) and subcellular compartments. We first demonstrated that ETM exhibited superior antibacterial activities against clinical isolates to GM and AMK, and it exerted minimal nephrotoxicity and ototoxicity in rats following multi-dose administration. Then, we conducted pharmacokinetic studies in rats, showed that the three AGs accumulated in the kidney and inner ear with ETM being distributed to a lesser degree in the two toxicity target organs as compared with GM and AMK high-dose groups. Furthermore, we conducted in vitro experiments in NRK-52E rat renal tubular epithelial cells and HEI-OC1 cochlear hair cells, and revealed that all the three AGs were distributed predominantly in the mitochondria with ETM showing minimal accumulation; they not only directly inhibited the activity of mitochondrial complexes IV and V but also inhibited mitochondrial function and its related PGC-1α-NRF1-TFAM pathway; ETM caused minimal damage to the mitochondrial complex and mitochondrial biogenesis. Our results demonstrate that the minimal otonephrotoxicity of ETM results from its lesser accumulation in mitochondria of target cells and subsequently lesser inhibition of mitochondrial function. These results provide a new strategy for discovering novel AGs with high efficacy and low toxicity.

Effects of chronic administration of kanamycin on the basement membrane anionic sites in the crista ampullaris of guinea pigs

The authors investigated the effects of chronic kanamycin (KM) administration on the basement membrane (BM) anionic sites in the ampulla by studying the binding of cationic polyethyleneimine (PEI). KM sulfate was administered intramuscularly to guinea pigs with normal Preyer's reflexes daily for 10 or 17 days. The PEI distribution was unchanged on the subepithelial BM in the dark cell region and on the capillary BM in the crista ampullaris. However, PEI binding decreased significantly on the subepithelial BM in the sensory cell and transitional cell regions of those guinea pigs administered KM for 17 days. In the sensory cell region, the PEI distribution did not recover until 6 weeks after KM treatment. Findings suggest that chronic administration of KM severely alters the number of subepithelial BM anionic sites in the sensory cell region.

Lysosomal targeting and accumulation of aminoglycoside antibiotics in sensory hair cells

Our recent study demonstrated that aminoglycoside antibiotics are taken up into sensory hair cells of the inner ear by receptor-mediated endocytosis (E. Hashino, M. Shero, Endocytosis of aminoglycoside antibiotics in sensory hair cells, Brain Res. 704 (1995) 135-140). To elucidate the intracellular trafficking pathway of aminoglycosides following endocytotic uptake, we administered kanamycin to neonatal chicks for 1 or 5 days (400 mg/kg/day) and determined the location of kanamycin within the hair cells at various time points using immunogold electron microscopy. Quantitative and qualitative analysis of immunogold staining revealed that: (1) kanamycin was primarily localized in vesicles beneath the cuticular plate 27 h postinjection; (2) the number of vesicles per hair cell and the number of gold particles per vesicle increased over time; (3) individual vesicles tended to increase in size over time, presumably due to aggregation of smaller vesicles; and (4) in pathological hair cells, immunogold was dispersed throughout the entire subcellular region. Light microscopic observations of the basilar papilla stained with the same antibody confirmed the temporal changes in the kanamycin distribution. Moreover, results obtained from acid phosphatase cytochemistry indicated that vesicles accumulating kanamycin were mainly lysosomes. These results suggest that internalized aminoglycosides are transported via vesicular traffic into lysosomes where they accumulate over time and lead to disruption of lysosomes. The time of diffusion of kanamycin was closely related to the time of cell death, suggesting that lysosomal rupture could be a direct trigger for the hair cell degeneration.

Influence of chronic kanamycin administration on basement membrane anionic sites in the labyrinth

We studied the effect of chronic treatment with kanamycin on the basement membrane (BM) anionic sites in the cochlea and endolymphatic sac using polyethyleneimine (PEI) as a cationic tracer. Albino guinea pigs weighing 250-300 g received kanamycin (400 mg/kg/day, i.m.) for 10 or 17 consecutive days. The number of BM anionic sites as derived from the PEI area was not affected in Reissner's membrane, spiral prominence, basilar membrane or endolymphatic sac, whereas it was significantly decreased in the stria vascularis and spiral limbus, being more marked in the guinea pigs treated for 17 days than in those treated for 10 days. The number of BM anionic sites in these regions did not recover until 6 weeks after kanamycin treatment. These findings suggest that chronically administered kanamycin may selectively and progressively affect the BM anionic sites in the stria vascularis and spiral limbus, resulting in disruption of a barrier function in the cochlea, and that severely impaired BM anionic sites in the cochlea may not recover.

Regenerated hair cells exhibit a transient resistance to aminoglycoside toxicity

Recent studies have demonstrated that sensory hair cells in the avian inner ear are reproduced by cell proliferation in response to the death of the original hair cell population. The regenerated hair cells appear to construct functional synaptic contacts, thereby transmitting acoustic signals to the peripheral nervous system. One of the most extraordinary, but overlooked characteristics of these regenerated hair cells, is their ability to survive in a highly ototoxic environment. Here, we report that hair cells regenerated after kanamycin induced hair cell loss can survive for a substantially longer time period than their predecessors during prolonged exposure to aminoglycoside antibiotics. The prolonged survival, however, belongs solely to the immature status of regenerated hair cells. Once the regenerated hair cells reach morphological maturation, they become vulnerable to aminoglycoside toxicity. Immunohistochemical evaluation of kanamycin suggested that kanamycin may be taken up into hair cells via a receptor-mediated endocytosis at their apical surfaces. By contrast, kanamycin was rarely incorporated into the cytoplasm of the regenerated hair cells. These results suggest that the process of a receptor-mediated transmembrane transport at the apical surface of hair cells is developmentally regulated, and that the lack of some of the assembly involved in the transmembrane transport could be responsible for the inhibition of aminoglycoside uptake, leading immature hair cells to be aminoglycoside resistant.

Endocytosis of aminoglycoside antibiotics in sensory hair cells

Immuno-gold electron microscopy was used to assess the uptake pathways of aminoglycoside antibiotic kanamycin (KM) in sensory hair cells. Accumulation of gold particles was evident on the plasma membrane as well as in large smooth vesicles beneath the apical surfaces of hair cells 12 h after a systemic administration of KM. Immuno-gold was exclusively localized in the vesicles 27 h post-injection. Cationic ferritin, a membrane-bound insoluble marker, was colocalized with KM in the vesicle structures after their simultaneous in vitro application. These results strongly suggest that KM is taken up into sensory hair cells via receptor-mediated endocytosis at their apical surfaces. In addition, the profound time lag between KM uptake and hair cell death suggests involvement of targeting mechanisms in cytotoxic signalling pathways of the drugs.

Effects of kanamycin ototoxicity and hair cell regeneration on the DC endocochlear potential in adult chickens

High doses of aminoglycoside antibiotics cause massive damage to the avian basilar papilla. The resulting functional loss could conceivably arise from the reduction in the DC endocochlear potential (EP) due to impairment of the tegmentum vasculosum (TV) or to shunting of current through the damaged sensory epithelium. To test this hypothesis, the EP was measured in adult chickens after destroying hair cells in the basal half of the cochlea with a high dose (400 mg/kg per day for 10 days) of kanamycin (KM). KM treatment caused an increase in the steady-state EP from +18.1 to +23.3 mV and a decrease in the magnitude of the negative EP from -42.0 to -19.2 mV. The EP showed almost no change between 1 and 2 days and 1 week post-KM treatment. After 4 weeks of recovery, most hair cells had regenerated; however, the steady-state EP was still elevated by 13% and the negative EP was depressed by 37%. These results suggest that functional loss as shown by the large reduction in cochlear microphonic (CM) and the elevated thresholds of compound action potential (CAP) following KM treatment is not due to a reduction in the EP but may arise from functional deficits in the hair cells and/or the auditory nerve.

Evidence that amikacin ototoxicity is related to total perilymph area under the concentration-time curve regardless of concentration

Previous studies have failed to fully establish whether ototoxicity is related in any way to the levels of an aminoglycoside antibiotic in the perilymph. To study this we exposed guinea pigs to continuously infused amikacin at four different dosing rates under conditions parallel to those used in our previous study which related ototoxicity to total plasma area under the concentration-time curve regardless of the level in plasma. It was found that at all dosing rates, levels in the perilymph and ratios of levels in perilymph/plasma remained constant as the dosing duration increased from nonototoxic to strongly ototoxic. Plasma and perilymph amikacin levels were found to be linear functions of the dosing rate even at ototoxic dosing exposures, and ratios of levels in perilymph/plasma did not differ between dosing rates. The total perilymph area under the concentration-time curve was not different between dosing rates either for a total dose associated with threshold ototoxicity or for one associated with severe ototoxicity. The results suggest that amikacin ototoxicity is related to the integral of the concentration in the perilymph over the total time of amikacin exposure regardless of the level in the perilymph.

Total extraction of aminoglycosides from guinea pig and bullfrog tissues with sodium hydroxide or trichloroacetic acid

In most experiments when aminoglycoside antibiotic (AG) tissue levels are measured, the AG is extracted into buffered media. The data from this study reveal that buffer extraction results in only partial recovery of AG from tissues and that total recovery can be obtained after NaOH or trichloroacetic acid treatment. Tissues studied here included the whole bullfrog and guinea pig kidney and cochlear tissues after in vivo drug treatment. The AGs used were kanamycin, tobramycin, and gentamicin. Drug concentrations were determined by enzymatic assay and, in the case of tobramycin, also 3H-labeled radioactivity. Only a fraction of total concentration of AG in tissue was released into the supernatants of tissue homogenates. However, the remainder was recovered after NaOH solubilization of the residual pellet. Also, it was found that the G released from the pellet by NaOH was associated with protein. By contrast, trichloroacetic acid precipitation of tissue protein immediately released the drug into the supernatant.

Furosemide-induced alteration of drug pathway to cochlea

The ototoxicity of aminoglycosides is known to be enhanced by post-treatment with loop diuretics. The present experimental study was undertaken to elucidate the mechanism of this enhancement immunohistologically. Intravenously administered kanamycin (KM) reached the organ of Corti rapidly (within 10 min) via the capillaries of the basilar membrane and spiral limbus. Besides this direct pathway, KM penetrated through the perilymphatic space pathway. This penetration, however, took a much longer time (90 min). In contrast, in the case when KM was followed by intravenous furosemide (FM), KM reached the perilymphatic region within a short time (1-5 min). The KM content of the organ of Corti ascertained 1-5 min after FM injection seemed almost equivalent to that determined 90 min after injection of KM alone. This FM-induced change in the time-course penetration pattern of KM might have a bearing on the enhancement of its otoxicity.

Ototoxicity of neomycin and its penetration through the round window membrane into the perilymph

The ototoxicity of neomycin and its concentration in the perilymph after direct application on the round window membrane were studied. After placing 5 mg of neomycin on the round window membrane of guinea pigs for various time intervals, concentration of the drug in the perilymph was determined by high performance liquid chromatography, and the cochlea was examined by light microscopy. Neomycin penetrated the round window membrane quite easily, and its concentration in the perilymph became extremely high in a short time and then decreased gradually. This indicates that high concentration of neomycin in the perilymph can be attained by application of a small amount of the drug on the round window membrane. Ototoxicity of neomycin was observed after application for 4 hours. Cochlear damage increased as neomycin application time became longer, but no consistent relationship was noted between the concentration of neomycin and the amount of damage. This result is discussed from the point of concentration and persistence of the drug in the inner ear fluids.

Kinetics of gentamicin uptake and release in the rat. Comparison of inner ear tissues and fluids with other organs

The kinetics of entry and release of gentamicin was investigated in fluids and tissues of the inner ear of the rat, as well as in renal cortex, and in organs that do not share susceptibility to the toxic effects of aminoglycosides. Various modes of administration were used to achieve different patterns of drug plasma concentrations. Electrophysiological and histological examinations were performed to correlate pharmacokinetics and ototoxicity. Results show that: the uptake of the drug by the inner ear tissues is dose dependent and manifests a rapid saturation kinetics with a concentration plateau of about 1 micrograms/mg of protein. The low ratio of the perilymph and endolymph to plasma concentrations argues against the concept of an accumulation of the drug in the inner ear over drug levels in plasma, which has been considered as the basic mechanism of ototoxicity. In renal cortex, the kinetics appears similar to that of the inner ear but the concentrations achieved are 10-fold higher than in cochlear tissues. In other organs (liver, heart, lung, and spleen), no saturation could be demonstrated within the duration of the experiment. Ototoxicity seems to be related to the penetration of the drug into compartment(s) from which the half-life of disappearance is extremely slow. Rapid uptake, early saturation, and long exposure of the tissues to the drug may account for the development of toxicity in inner ear and kidney.

A study of the nerve endings of the outer hair cells in the organ of Corti--using scanning electron microscopy

The nerve endings on the outer hair cells of the guinea pig organ of Corti were studied using a scanning electron microscope. The nerve endings in the basal turn were entirely exposed in the space of Nuel, whereas in the upper turns almost all nerve endings were covered by the Deiters' cell bulge. The number and the size of the nerve endings gradually decreased from the basal turn toward the apical turn. The shape of the nerve endings was spindle-shaped in the basal turn, while it was round in the upper turn. Some nerve endings were found to locate close to the outer hair cell surface. No pathological change was observed in the nerve endings 25 hr after completion of series treatments of streptomycin or kanamycin. However, the outer hair cells consistently showed shrinkage, especially in the basal turn.

Mechanism of protective effect of fosfomycin against aminoglycoside ototoxicity

The purpose of this study was to clarify the mechanism of the protective effect of fosfomycin (FOM) against inner ear damage induced by an aminoglycoside dibekacin (DKB), when administered concurrently DKB and FOM. Rats were treated with 50 mg/kg of DKB with or without 500 mg/kg of FOM for short-term administration. No significant difference was seen in the serum peak level and in the area under the curve between the group receiving DKB alone and the combined administration group of DKB and FOM. On the other hand, the DKB level in the kidney was significantly lower in the combined administration group than in the group receiving DKB alone. The mechanism of protective effect of FOM against DKB-induced ototoxicity may be considered as follows: FOM inhibits the accumulation of DKB in the kidney and reduces its concentration in the kidney and serum. Consequently, the transferability of DKB into the inner ear is decreased, and finally inner ear damage is reduced.

Aminoglycoside ototoxicity in the chick (Gallus domesticus) inner ear: I. The effects of kanamycin and netilmicin on the basilar papilla

A single dose (100 mg/kg of egg weight) of kanamycin or netilmicin was injected into the yolk sacs of 7-day-old chick (Gallus domesticus) embryos. Embryos were collected every 24 hours and processed for light and transmission electron microscopy. Morphologic study of the medial basilar papilla disclosed that both kanamycin and netilmicin are toxic to the hair cells in this region. Intoxication was manifested by an increased number of dense osmiophilic bodies, swollen mitochondria, agglomerated chromatin, and occasional disorganization of the kinocilium basal bodies. The cytologic changes observed in the hair cells of embryos injected with netilmicin and kanamycin were similar. However, mitochondrial damage was more severe in the chicks after kanamycin than after netilmicin injection. Some of the cytologic alterations described here are comparable to those already reported for aminoglycoside-intoxicated hair cells in several mammalian species. This study and previous work indicate that the chick embryo provides a satisfactory developmental model for testing ototoxicity of drugs in vivo.

Neomycin concentrations in inner ear tissues and other organs of the guinea pig after chronic drug administration

The kinetics of neomycin entry into tissues, including those of the inner ear, was studied in the guinea pig. The animals received daily subcutaneous injections of 100 mg neomycin/kg body weight and were killed after 1, 3, and 6 days and 1, 2, and 3 weeks. In agreement with previous studies, kidney accumulated the drug rapidly and to a high level (450 micrograms neomycin/g tissue at day 7) while levels in heart, liver, lung, and spleen were lower by more than one order of magnitude. The drug was virtually absent from brain at all times. The salient finding was that neomycin content of cochlear tissues was relatively low. For instance, at 2 weeks, concentrations in organ of Corti (0.7 micrograms/mg protein) and lateral wall tissues (0.4 microgram) were similar to those in heart (0.3 microgram), liver (0.5 microgram), lung (0.8 microgram) and spleen (0.6 microgram) while the concentration in kidney was 6.8 micrograms neomycin/mg protein. The results show that mechanisms of drug entry are different for kidney and cochlea. They also show that the frequently discussed 'accumulation' of aminoglycosides in perilymph is apparently not reflected in tissue levels of the drug.

Effects of acute hypertension on the extravasation of macromolecule in the temporal bone--the possible involvement of the blood-inner ear barrier

In guinea pigs the radioactivity in the vascular beds was washed out by a cardiac perfusion of saline 15 min after the injection of 50 microCi of radioiodinated human serum albumin (RISA). The brain and temporal bones were dissected and a blood sample was obtained. To examine the degree of extravasation of RISA, the extravascular radioactivity in the tissues was measured and expressed as a ratio to that of the blood. Intravenous injection of amphetamine sulfate increased the extravasation in the brain and the temporal bone of the rat but did not increase it in either tissue of the guinea pig. It was proposed that this was because amphetamine did not cause the blood pressure of the guinea pig to reach the "critical level" which would cause the opening of the blood-tissue barrier. In the rat the degrees of extravasation in both the brain and the temporal bone paralleled the maximal mean arterial pressure caused by amphetamine. It is proposed on the basis of these data that there may exist a blood-inner ear barrier and that this barrier has the same characteristics as the blood-brain barrier.

Occurrence of non-ototoxic antibiotic (cephazolin) in the inner ear

Cephazolin, a non-ototoxic antibiotic, and kanamycin, an ototoxic antibiotic, were administered to the guinea pig systemically and their tissue concentration was measured. Not only kanamycin but also cephazolin did occur in the inner ear as well as in the kidney and other tissues. Therefore, it is most likely that the accumulation of an antibiotic inside the inner ear forms only a sine qua non for its ototoxicity. The mechanism of the ototoxicity should exist in the antibiotic-vulnerable cells (hair cells) themselves.

Ototoxicity of kanamycin in developing rats: relationship with the onset of the auditory function

In order to test the relationship between the ototoxicity of kanamycin and the onset of the auditory function, two groups of developing rats were intoxicated with kanamycin before and after the period of onset of cochlear potentials (8th postnatal day). Kanamycin was shown to have a weak ototoxic effect before the 8th postnatal day and a strong ototoxic effect after this period. These results indicate a critical period of sensitivity to ototoxic antibiotics during auditory development.

Effects of kanamycin on the auditory evoked responses during postnatal development of the hearing of the rat

The ototoxic effects of kanamycin were studied in rats during the early postnatal period and at an adult age. Brain stem potentials as well as auditory cortical potentials were used for the estimating of ototoxic damage. The auditory potentials decreased promptly and markedly in the animals which were treated daily with 400 mg/kg body weight of kanamycin starting from the 11th day after birth. In these animals, the auditory potentials were almost completely abolished within 10 days after the beginning of the kanamycin treatment. However, when the same amount of kanamycin was applied earlier or later than that, i.e., avoiding the period of the initial appearance and the greatest development of auditory functions (from the 11th to the 15th day after birth in the rat), the auditory potentials were not apparently damaged. In light and scanning electronmicroscopy, marked ototoxic changes were observed which underlay the functional damage. The meaning of these findings is discussed.