Renal tubular transport of gentamicin in the rat

Organic Cation Transporter 2 Overexpression May Confer an Increased Risk of Gentamicin-Induced Nephrotoxicity

Nephrotoxicity is a relevant limitation of gentamicin, and obese patients have an increased risk for gentamicin-induced kidney injury. This damage is thought to depend on the accumulation of the drug in the renal cortex. Obese rats showed substantially higher levels of gentamicin in the kidney than did lean animals. This study characterized the role of organic cation transporters (OCTs) in gentamicin transport and elucidated their possible contribution in the increased renal accumulation of gentamicin in obesity. The mRNA and protein expression levels of the organic cation transporters Oct2 (Slc22a2) and Oct3 (Slc22a3) were increased in kidney samples from obese mice fed a high-fat diet. Similarly, OCT2 (∼2-fold) and OCT3 (∼3-fold) showed increased protein expression in the kidneys of obese patients compared with those of nonobese individuals. Using HEK293 cells overexpressing the different OCTs, human OCT2 was found to transport [(3)H]gentamicin with unique sigmoidal kinetics typical of homotropic positive cooperativity (autoactivation). In mouse primary proximal tubular cells, [(3)H]gentamicin uptake was reduced by approximately 40% when the cells were coincubated with the OCT2 substrate metformin. The basolateral localization of OCT2 suggests that gentamicin can enter proximal tubular cells from the blood side, probably as part of a slow tubular secretion process that may influence intracellular drug concentrations and exposure time. Increased expression of OCT2 may explain the higher accumulation of gentamicin, thereby conferring an increased risk of renal toxicity in obese patients.

Entry of aminoglycosides into renal tubular epithelial cells via endocytosis-dependent and endocytosis-independent pathways

Aminoglycoside antibiotics such as gentamicin and amikacin are well recognized as a clinically important antibiotic class because of their reliable efficacy and low cost. However, the clinical use of aminoglycosides is limited by their nephrotoxicity and ototoxicity. Nephrotoxicity is induced mainly due to high accumulation of the antibiotics in renal proximal tubular cells. Therefore, a lot of studies on characterization of the renal transport system for aminoglycosides so far reported involved various in-vivo and in-vitro techniques. Early studies revealed that aminoglycosides are taken up through adsorptive endocytosis in renal epithelial cells. Subsequently, it was found that megalin, a multiligand endocytic receptor abundantly expressed on the apical side of renal proximal tubular cells, can bind aminoglycosides and that megalin-mediated endocytosis plays a crucial role in renal accumulation of aminoglycosides. Therefore, megalin has been suggested to be a promising molecular target for the prevention of aminoglycoside-induced nephrotoxicity. On the other hand, recently, some reports have indicated that aminoglycosides are transported via a pathway that does not require endocytosis, such as non-selective cation channel-mediated entry, in cultured renal tubular cells as well as cochlear outer hair cells. In this commentary article, we review the cellular transport of aminoglycosides in renal epithelial cells, focusing on endocytosis-dependent and -independent pathways.

Comparison of four renal function estimation equations for pharmacokinetic modeling of gentamicin in geriatric patients

Most aminoglycoside pharmacokinetic models include an index of renal function, such as creatinine clearance, to describe drug clearance. However, the best clinical descriptor of renal function for the pharmacokinetic modeling of aminoglycosides has not been established. This analysis was based on 412 gentamicin concentrations from 92 geriatric patients who received intravenous gentamicin for various infectious diseases. Four two-compartment population models were fitted to gentamicin concentrations in a learning set of 64 patients using the nonparametric adaptive grid (NPAG) algorithm. Each model included an index of renal function, namely, the Cockcroft-Gault (CG), Jelliffe (JEL), modification of diet in renal disease (MDRD), or modified MDRD (MDRDm; adjusted to individual body surface area) equation as a covariate influencing gentamicin serum clearance. Goodness of fit and predictive performance of the four models were compared using standard criteria in both the learning set and in a validation set of 28 patients. A final analysis was performed to estimate the population pharmacokinetic parameter values of the entire 92-patient group. In the learning set, the CG-based model best fit the data, followed by JEL-, MDRD-, and MDRDm-based models, with relative reductions of the Akaike information criterion of 29.4, 20.2, 14.2, and 4.2, respectively. Bias and precision of population predictions were significantly different among the four models. In the validation set, individual predictions from the four models showed marginally different biases. The final estimation confirmed the previous results. Specifically, the CG-based model showed predictive performance that was comparable to or better than that of the MDRD-based model at each stage of the analysis. This study shows that methods used to estimate renal function should not be considered interchangeable for the model-based estimation of gentamicin concentrations.

Decreased Expression of Na/K-ATPase, NHE3, NBC1, AQP1 and OAT in Gentamicin-induced Nephropathy

The present study was aimed to determine whether there is an altered regulation of tubular transporters in gentamicin-induced nephropathy. Sprague-Dawley male rats (200~250 g) were subcutaneously injected with gentamicin (100 mg/kg per day) for 7 days, and the expression of tubular transporters was determined by immunoblotting and immunohistochemistry. The mRNA and protein expression of OAT was also determined. Gentamicin-treated rats exhibited significantly decreased creatinine clearance along with increased plasma creatinine levels. Accordingly, the fractional excretion of sodium increased. Urine volume was increased, while urine osmolality and free water reabsorption were decreased. Immunoblotting and immunohistochemistry revealed decreased expression of Na(+)/K(+)-ATPase, NHE3, NBC1, and AQP1 in the kidney of gentamicin-treated rats. The expression of OAT1 and OAT3 was also decreased. Gentamicin-induced nephropathy may at least in part be causally related with a decreased expression of Na(+)/K(+)-ATPase, NHE3, NBC1, AQP1 and OAT.

The influence of lipoic acid on adriamycin induced nephrotoxicity in rats

Adriamycin, which is widely used in the treatment of various neoplastic conditions, exerts toxic effects in several organs. Adriamycin nephrotoxicity has been recently documented in a variety of animal species. The present study was designed to investigate the effect of lipoic acid on the nephrotoxic potential of adriamycin. The study was carried out with adult male albino rats of Wistar strain. Test animals were divided into four groups of six rats each as follows: Group I (control) received only normal saline throughout the course of the experiment. Group II (ADR) received intravenous injections of adriamycin through the tail vein (1 mg kg(-1) body wt day(-1)) once a week for a period of 12 weeks. Group III (LA) received lipoic acid (35 mg kg(-1) body wt day(-1)) intraperitoneally once a week for a period of 12 weeks. Group IV (ADR + LA) received a single injection of lipoic acid intraperitoneally 24 h prior to the administration of adriamycin through the tail vein once a week for a period of 12 weeks. Intravenous injections of adriamycin resulted in decreased activities of the glycolytic enzymes; hexokinase, phosphoglucoisomerase, aldolase and lactate dehydrogenase in the rat renal tissue. The gluconeogenic enzymes, glucose-6-phosphatase and fructose-1,6-diphosphatase, showed a decline in their activities on adriamycin administration. The transmembrane enzymes namely the Na+,K+-ATPase, Ca2+-ATPase, Mg2+-ATPase and the brush-border enzyme alkaline phosphatase also showed a decrease in their activities. This decrease in the activities of ATPases and alkaline phosphatase suggests basolateral and brush-border membrane damage. Decreased activities of the TCA cycle enzymes isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase, suggest a loss in mitochondrial function and integrity. Nephrotoxicity was evident from the increased excretions of N-acetyl-beta-D-glucosaminidase and gamma-glutamyl transferase in the urine of adriamycin administered rats. These biochemical disturbances were effectively counteracted on pre-treatment with lipoic acid, which brought about an increase in the activities of glycolytic enzymes, ATPases and the TCA cycle enzymes. On the other hand, the gluconeogenic enzymes showed a further decrease in their activities on lipoic acid pretreatment. LA pretreatment also restored the activities of the urinary enzymes to normal. These observations shed light on the nephroprotective action of lipoic acid rendered against experimental aminoglycoside toxicity.

Clusterin protects renal tubular epithelial cells from gentamicin-mediated cytotoxicity

Clusterin is a heterodimeric secreted glycoprotein that is upregulated after acute renal injury. In aminoglycoside nephrotoxicity, clusterin is induced in the tubular epithelium and increased levels are found in the urine. In this study, we developed an in vitro model of gentamicin-induced cytotoxicity in renal proximal tubule cells and tested whether clusterin protected these cells from injury. LLC-PK(1) cells were incubated with varying concentrations of gentamicin in serum-free media, and cytotoxicity was quantified by lactate dehydrogenase release and confirmed by vital dye exclusion. A dose-dependent increase in cytotoxicity occurred with gentamicin concentrations up to 27 mg/ml. Clusterin decreased cytotoxicity in a dose- and time-dependent manner at 6, 12, and 24 h, whereas albumin, used as a control protein, had no effect. In contrast to the aminoglycoside model, when cells were injured by depletion of ATP, clusterin had only a minimally protective effect. LLC-PK(1) cells did not express megalin, a receptor that can mediate the uptake of both clusterin and aminoglycosides into proximal tubule cells. Uptake of gentamicin into LLC-PK(1) cells was observed despite the absence of megalin. In conclusion, clusterin specifically protects against gentamicin-induced renal tubular cell injury by a megalin-independent mechanism.

Comparison of the effects of amikacin and kanamycins A and B on dimyristoylphosphatidylglycerol bilayers. An infrared spectroscopic investigation

Aminoglycoside antibiotics are very effective against severe Gram-negative infections, but their clinical use is associated with nephrotoxic side-effects. The cascade of events leading to acute renal failure involves an impairment of lysosomal phospholipase activity, which is thought to result from the direct interaction of the drugs with the head group of negative phospholipids. Fourier transform infrared spectroscopy was used to study the effects of three aminoglycosides from the kanamycin family (amikacin and kanamycins A and B) on dimyristoylphosphatidylglycerol (DMPG) bilayers at lysosomal pH. The results obtained were consistent with a tightening of the lipidic network caused by the neutralization of the negative head groups of DMPG by the positive charges of the aminoglycosides. These antibiotics induced an increase of the transition temperature of DMPG, a decrease of both the frequency and relative intensity of the hydrogen-bonded carbonyl component, and a decrease of the phosphate antisymmetric band frequency. Kanamycin B, which is known to be the most nephrotoxic drug of the three, exhibited the greatest effects on the transition temperature and on the carbonyl stretching band. A comparison of the nature and extent of the spectral changes led us to conclude that amikacin lies flat on the bilayer surface, whereas kanamycin B is located between the lipidic head groups and quite close to some of the carbonyl groups. Finally, a possible correlation between the importance of bilayers perturbation and the respective inhibitory potency against phospholipases was examined.

Early defect in branching morphogenesis of the ureteric bud in induced nephron deficit

Development of the metanephric kidney during embryogenesis can be altered both in vivo and in vitro by exposure to gentamicin, which may lead to oligonephronia. To study the role of the ureteric bud in nephron deficit genesis, we used metanephros organ cultures exposed to gentamicin as a model of impaired nephrogenesis. Ultrastructural localization of the antibiotic showed that by eight hours it was already present within the epithelial cells of the ureteric bud and in its growing ends, and also trapped in the adjacent blastema. Using confocal microscopy and image analysis, we devised a quantitative approach to analyze the branching pattern of the ureteric bud, and showed that by 24 hours of culture, despite no change of explants growth, gentamicin had significantly decreased the number of branching points. This effect involved the early branching events and was limited to end buds that had no nephron anlagen nearby. Our findings indicate that impaired branching morphogenesis of the ureteric bud is the likely event of gentamicin-induced nephron deficit.

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.

Nephrotoxicity of xenobiotics

Nephrotoxicity can be grouped by the xenobiotics place of action, by the clinical presentation or by the generic toxic effect. The latter can be dose related, indirect, idiosyncratic or allergic. Nephrotoxicity of lithium, demeclocycline, aminoglycosides, cyclosporine, mercuric ion, nonsteroidal anti-inflammatory drugs, methoxyflurane, ethylene glycol, D-penicillamine and methicillin is reviewed in light of all these three viewpoints, but emphasis is on toxic mechanisms.

Selective exposure of human proximal tubule cells to gentamicin provides evidence for a basolateral component of toxicity

The goal of the present study was to determine if cultured human proximal tubule (HPT) cells could provide evidence for a basolateral component of gentamicin toxicity. Six isolates of HPT cells were grown on Millicell filters and exposed to gentamicin either apically, basolaterally, or both apically and basolaterally. Toxicity was determined by the release of lactate dehydrogenase into the growth media. The results clearly demonstrated that basolateral exposure and combined apical and basolateral exposure to gentamicin resulted in significant levels of cell toxicity. In contrast, apical exposure to gentamicin elicited only marginal toxicity. The transepithelial flux of gentamicin was shown to be the same in either the apical to basolateral or the basolateral to apical direction. A two-step mechanism of gentamicin toxicity is proposed in order to integrate basolateral toxicity with known features of aminoglycoside nephrotoxicity.

Renal handling of tobramycin in the isolated perfused rat kidney

The renal handling of tobramycin (TOB), an aminoglycoside antibiotic (AG), was studied using a single-pass isolated perfused rat kidney with moment analysis. In the bolus administration study at tracer concentration (7.4 microM), 32% of the glomerular-filtrated TOB remained in the lumen, but no TOB was found in the vein. This ratio of the luminal uptake was reduced in a dose-dependent manner. Other aminoglycosides such as gentamicin inhibited this uptake, but tetraethylammonium and glucosamine had no effect. In addition, under the alkalinuria condition, TOB uptake was decreased to 67% of the control value. This indicated that TOB has mainly been taken into the renal epithelial cells from their luminal site and that this uptake process was saturable and specific for AGs which have more than one cationic group. The present findings should be helpful in developing a method to reduce the nephrotoxicity of AGs and to identify their toxicity mechanisms.

Aminoglycoside antibiotics alter the paracellular transport properties of cultured human proximal tubule cells

Cell cultures retaining properties of the human proximal tubule were utilized to determine whether or not the aminoglycoside antibiotics alter paracellular transport. The transepithelial resistance (RT) of the human proximal tubule (HPT) cell monolayers was determined by Ussing chamber analysis of cells grown on permeable supports. This analysis revealed that RT was reduced as a result of aminoglycoside exposure and that the reduction corresponded to the known clinical nephrotoxicities of the aminoglycosides. Variation in the aminoglycoside concentration necessary to elicit this response was documented using 14 individual cell isolates. Ultrastructural analysis provided evidence indicating that the alterations in RT were not associated with general damage to the HPT cells. An examination of the structure of the tight junctions by freeze-fracture analysis demonstrated only minimal alteration of the sealing strands as a result of aminoglycoside exposure. Consequently, the reductions in RT were not directly associated with discernible tight junction structural alterations. Alteration in the paracellular route of transport, as indicated by altered RT values, was clearly documented as a result of aminoglycoside exposure. In addition, this alteration was accompanied by an increased density of intramembrane particles within the apical cell membrane.

Apically and basolaterally internalized aminoglycosides colocalize in LLC-PK1 lysosomes and alter cell function

Aminoglycosides bind to apical and basolateral (BL) membranes of renal epithelial cells. However, little is known regarding differential uptake and intracellular processing after internalization across these distinct surface membrane domains. To examine these processes independently, LLC-PK1 cells were grown on porous filters, which allow selective access to both domains. Apical and BL membrane uptakes of gentamicin (0.5 mM), quantified using [3H]gentamicin, were linear from 2 to 24 h (r = 0.99). The 4-h apical gentamicin uptake was 667 +/- 59 pmol/mg protein, the BL 748 +/- 26 pmol/mg protein, and concurrent apical and BL uptake 1,389 +/- 22 pmol/mg protein. Aminoglycoside uptake, documented using indirect immunogold techniques, occurred via the apical and BL endocytic systems and colocalized with cationic ferritin. Aminoglycosides internalized via the apical (gentamicin) and BL (tobramycin) membrane converged at the lysosomal level. Gentamicin incorporated via either domain significantly decreased lysosomal N-acetylglucosaminidase below control values (P < 0.05). We conclude that, after binding, aminoglycosides are internalized equally across apical and BL membranes of LLC-PK1 cells via receptor-mediated endocytosis, colocalize within the lysosomal compartment, and alter cellular function similarly.

A comparative study of enzymuria, in the rat, of the drug combinations amikacin/vancomycin and amikacin/teicoplanin

The aim of this study was to compare nephrotoxicity of the combinations amikacin/vancomycin and amikacin/teicoplanin. Eighteen male Wistar rats were divided into 3 groups of 6 animals each. The first group received 50 mg.kg-1 of amikacin (i.m. route) and 100 mg.kg-1 of vancomycin (i.p. route). The second group received 50 mg.kg-1 of amikacin (i.m. route) and 40 mg.kg-1 of teicoplanin (i.p. route). The third group received an isotonic solution of sodium chloride. The antibiotics were injected for a period of 6 days. Urine samples of animals were taken 24 h before the beginning of the experiment, then every day, throughout the duration of the treatment (6 days), continuing for an additional 3 days following completion of the administration of the drugs. There were no significant modifications in the urinary excretions of alanine aminopeptidase and the creatinine between the 3 groups; but in the group receiving amikacin/teicoplanin, we observed between days 3 and 8 an increase in the excretion of N-acetyl-beta-D- glucosaminidase when compared to the group receiving amikacin/vancomycin (p < or = 0.05) and to the control group (p < or = 0.01).

Subcellular distribution of gentamicin in proximal tubular cells, determined by immunogold labeling

The subcellular distribution of gentamicin in rat renal proximal tubular cells was evaluated by immunogold labeling. The distribution of the drug was monitored from 10 min to 10 days following single (40 mg/kg of body weight) and multiple (5 and 20 mg/kg/12 h) injections of gentamicin. Animals were killed on day 11, and cubes of renal cortex tissue were fixed overnight in cold phosphate-buffered glutaraldehyde (0.5%), dehydrated in ethanol, and embedded in Araldite 502 epoxy resin. Ultrathin sections were made and incubated with sheep antigentamicin and then with protein A-gold (15 nm) complex. At 10 min after a single injection, the labeling was found over the brush border membrane and over the membranes of endocytic apical vesicles of proximal tubular cells. After 1 h, a similar distribution was observed and the labeling was also seen over small lysosomes located close to the brush border membrane. At 24 h, gold particles were found over large lysosomes of proximal tubular cells. Following 10 days of treatment, lysosomes of proximal tubular cells were densely labeled with gold particles. The labeling was distributed uniformly over the lysosomes, although a lower density of labeling was observed over the myeloid bodies inside the lysosomes. Necrotic proximal tubular cells showed labeling over intact lysosomes and also in the cytoplasms of the cells, in the mitochondria, and in the nucleoli. The various control experiments demonstrated the high specificity of these results. The present immunocytochemical study better documents the subcellular disposition of gentamicin in proximal tubular cells, as previously evaluated by subcellular fractionation and autoradiography. This technique will be useful for better understanding the relationship between drug disposition and drug-induced toxicity.

Alterations in lysosomal enzymes of the proximal tubule in gentamicin nephrotoxicity

Gentamicin accumulates in proximal tubule lysosomes, increases their number, and changes their structure. An important lysosomal function is degradation of intracellular proteins. To evaluate the effect of gentamicin on this lysosomal function, we measured the activity of the key lysosomal proteinases, cathepsin B and L, in microdissected S1, S2, and S3 segments of rat proximal tubules by means of a fluorometric microassay. The cathepsin activities were decreased in S1 and S2 following one and four gentamicin injections of 100 mg/kg body weight. The lysosomal enzyme, acid phosphatase, was also measured and was not decreased by gentamicin. The urine excretion of cathepsins B and L was decreased after gentamicin. This excludes an increase in urinary loss of cathepsins as the cause of decreased tubule activity. Structural changes of the lysosomes per se were excluded as the factor responsible for the reduced cathepsin activity by demonstrating increased cathepsin B and L activity in proximal tubule segments from rats injected with dextran, since dextran induces an increase in number and size of proximal tubule lysosomes. In vitro incubation of urine and tubule segments with gentamicin demonstrated a concentration-dependent reversible inhibition of cathepsin B and L. We conclude that gentamicin per se decreased cathepsin B and L activities in proximal tubule segments as early as 24 hours following one injection due to either enzyme inhibition or reduced generation of active intralysosomal cathepsin B and L. Gentamicin may, therefore, reduce renal protein catabolism by decreasing the activity of the key proteolytic enzymes, cathepsin B and L. Since cathepsin B and L are proteolytic activators of other lysosomal enzymes, their reduced activity may also decrease the activities of other lysosomal enzymes.

The effect of guanidine on the accumulation of amikacin in guinea pig renal cortical slices

The role of a recently identified organic ion transport system in the accumulation of the aminoglycoside (AG), amikacin (AK) in the kidney was investigated in the present study. Because this transport system has been characterized as being a carrier for the organic cation, guanidine, the effect of guanidine on the uptake of AK into renal slices from guinea pig was examined. Renal slices incubated in medium containing AK concentrated the drug against a concentration gradient (i.e. slice:medium ratio (S/M) greater than 1.0). This uptake was significantly reduced when an equimolar concentration (1 x 10(-5) M) of another AG, gentamicin was added to the incubation medium. In contrast, AK uptake was relatively insensitive to the presence of the cation, tetraethylammonium (TEA) in the medium. Guanidine was also ineffective at inhibiting AK uptake into slices and reduced AK uptake by only 22% at guanidine concentrations of 1 x 10(-2) M. In comparison, TEA was slightly more sensitive to the presence of guanidine in the incubation media since TEA uptake was reduced by 22% at guanidine concentrations of 1 x 10(-3) M and reduced by approximately 70% at guanidine concentrations of 1 x 10(-2) M. Thus, the results of the present study suggest that the guanidine transport system does not play a role in the renal accumulation of AK since the presence of guanidine in the incubation medium had little effect on the accumulation of AK into renal cortical slices.

Effect of dextran sulfate on renal accumulation of gentamicin

The effect of dextran sulfate of three molecular weights (1000, 5000, and 90,000) on the accumulation of gentamicin in rat kidney was investigated using a continuous infusion technique. During the infusions of both gentamicin and gentamicin-dextran sulfate mixtures, the gentamicin plasma concentration was maintained at 10 microgram/ml. The renal cortical accumulation of gentamicin was significantly lower when dextran sulfate (1000, 5000) was coadministered. The inhibition of cortical gentamicin accumulation increased with increasing dextran sulfate dose, and it was proportional to the amount of dextran sulfate excreted into the urine. Analysis by electrophoresis on cellulose acetate membrane indicated that gentamicin binds to dextran sulfate in rat urine. Therefore, gentamicin-dextran sulfate binding within the lumen of the proximal tubules may reduce the renal reabsorption and possibly the renal toxicity of gentamicin.

Inhibition of tobramycin reabsorption in nephron segments by metabolic alkalosis

Chronic metabolic acidosis appears to potentiate aminoglycoside induced nephrotoxicity and renal cortical accumulation while some, but not all, studies show that bicarbonate loading reduces nephrotoxicity. The purpose of the present study was to determine if reabsorption of tobramycin in proximal and distal nephron segments in vivo is altered by systemic pH. To test this possibility on the single nephron level, 24 nl samples of [3H] tobramycin were micro-injected into proximal and distal nephron segments, and its recovery was compared to that of [14C] inulin in rats undergoing osmotic diuresis with NaHCO3 or Na2SO4 containing HCl. Results were obtained in 66 tubules in 20 rats. Although plasma and urine pH were altered as anticipated in bicarbonate-infused and acid-infused animals, urine and late proximal tubule flow rates were similar. When animals were acid infused, 24.8 +/- 1.90% of [3H] tobramycin injected into proximal nephrons was reabsorbed compared to 7.5 +/- 1.56% (P less than 0.001) when the animals were bicarbonate-infused. When [3H] tobramycin was injected into distal tubules, 5.9 +/- 0.75% was reabsorbed in acid-infused rats while virtually none of the injected tobramycin, 0.43 +/- 1.59%, was reabsorbed by the distal nephron of bicarbonate-loaded animals. Our results provide evidence that tobramycin reabsorption by both proximal and distal nephron segments is substantially reduced by bicarbonate-infusion.

Model of gentamicin-induced nephrotoxicity and its amelioration by calcium and thyroxine

The exact mechanism of gentamicin-induced acute renal failure is presently unknown; various mechanisms have been proposed but there is no proposed commonality between them. In animals, dietary calcium loading and L-thyroxine administration have been shown to ameliorate toxicity, with again no common process. A mechanism of competitive displacement of calcium and other cations from anionic phospholipids at the plasma and organelle membrane level, resulting in a decrease in Na+ -K+ ATPase, adenylate cyclase, mitochondrial function and ATP production, protein synthesis, solute reabsorption and overall cellular function is proposed. A further proposal is dietary calcium loading and thyroxine (which increases intracellular calcium) reverse gentamicin-induced acute renal failure by increasing the calcium and solute flux, thereby competitively inhibiting the primary lesion: anionic phospholipid binding.

Absence of tobramycin pharmacokinetic and creatinine clearance variation during the menstrual cycle: implied absence of variation in glomerular filtration rate

During the menstrual cycle, a 20% increase in creatinine clearance (CL(CR] has previously been reported between the menstrual (phase 1) and late luteal (phase 4) phases. Tobramycin pharmacokinetics and CL(CR) were studied in eight healthy women with documented, regular, ovulatory menses. During the first and fourth phases of the menstrual cycle (as determined by urinary luteinizing hormone peak and basal body temperature shift), subjects received tobramycin by intravenous bolus. Tobramycin half-life, total body clearance, and volume of distribution were not significantly different between the two study phases. No significant change in total urinary creatinine excretion or CL(CR) was seen between phases. Total 24 hour urinary recovery of tobramycin was 98-99.7%. We conclude that no significant changes in renal function, as evaluated by tobramycin pharmacokinetics and CL(CR), occur between these hormonally different phases of the menstrual cycle, and that urinary recovery of a single dose of tobramycin is nearly complete within 24 hours in premenopausal women with normal renal function.

Mechanisms of aminoglycoside nephrotoxicity

Aminoglycosides continue to be widely used for the treatment of serious Gram-negative infections. Ten to fifteen per cent of all courses of therapy are complicated by declines in renal function, despite close monitoring of serum drug levels. The proposed pathogenesis and biochemical mechanisms of renal dysfunction caused by these commonly used therapeutic agents are discussed.

Enhanced urinary trypsin inhibitory activity in gentamicin-induced nephrotoxicity in rats

We delineated in rats, the relationship between trypsin inhibitory activity in the urine and the nephrotoxic effects of gentamicin, eg, proteinuria and deterioration of glomerular filtration rate (GFR), measured by creatinine clearance. Gentamicin, 70 mg/kg per day, was injected intraperitoneally for 6-10 successive days. Serum and urine gentamicin levels were determined by a microbiological test. Trypsin inhibitory activity was assayed by the casein digestion method. The results showed a steady increase in urinary trypsin inhibitory activity starting from the fourth injection day. The increased levels of urinary trypsin inhibitory activity were associated with increased levels of urinary gentamicin excretion (r = 0.36, p less than 0.02, n = 50 after the fourth injection day), and were significantly higher than in control groups (p less than 0.001). The urinary trypsin inhibitory activity was inversely correlated with the GFR (r = -0.45, p less than 0.01, after the second injection day). The serum trypsin inhibitory activity remained unchanged throughout the study period in all groups. These data suggest that increased urinary trypsin inhibitory activity may be involved in the pathogenesis of gentamicin-induced nephrotoxicity.

Protective effect of piperacillin against nephrotoxicity of cephaloridine and gentamicin in animals

The protective effect of piperacillin against the nephrotoxicity of cephaloridine and gentamicin was examined in experimental animals. In rabbits, piperacillin was infused at a dose of 1 mg/kg (body weight) per min over 225 min and cephaloridine (300 mg/kg) was intravenously administered as a bolus 45 min after the start of a drip infusion. Blood urea nitrogen, serum creatinine, and N-acetyl-beta-D-glucosaminidase (NAG) in urine were measured as the renal toxicological parameters before and 24 h after cephaloridine dosing. Although the single administration of cephaloridine significantly elevated these parameters, the elevation was prevented by the concomitant administration of piperacillin. The protective effect of piperacillin was superior to those of cephalothin and fosfomycin. In rats, piperacillin (1,000 mg/kg) was intravenously administered and immediately followed by the intramuscular administration of gentamicin (100 mg/kg) every 24 h for 5 days. When piperacillin was concomitantly administered with gentamicin, the elevations of blood urea nitrogen, serum creatinine, and urinary NAG were significantly lower than when gentamicin was given alone. The concomitant administration of piperacillin resulted in a significant protective effect against the nephrotoxicity of cephaloridine in rabbits and of gentamicin in rats. Histopathological observation also supported the protective effect of piperacillin. The protective mechanism of piperacillin might be the inhibition of transport from the peritubular side to tubular cells for cephaloridine and from both the peritubular and luminal sides for gentamicin.

Inhibition of alkaline phosphatase activity and D-glucose uptake in rat renal brush-border membrane vesicles by aminoglycosides

The binding of aminoglycoside antibiotics to, and their effects on, the plasma membrane were studied using isolated rat renal brush-border membrane vesicles. Dibekacin was noted to bind with brush-border membrane vesicles having a single class of many binding sites. 3H-labeled dibekacin binding was inhibited competitively by unlabeled dibekacin, gentamicin or amikacin. The inhibition constants obtained from the Dixon plots followed the order of gentamicin approximately equal to dibekacin greater than amikacin. The alkaline phosphatase activity of brush-border membrane vesicles was inhibited by gentamicin significantly, as was also observed by a histochemical study. Sodium-dependent D-glucose uptake by brush-border membrane vesicles was significantly inhibited by the addition of gentamicin.

Autoradiographic study of tobramycin uptake by proximal and distal tubules of normal and pyelonephritic rats

Multiple factors may modify the pharmacokinetics of aminoglycosides and affect their nephrotoxic potential. In the present study, the influence of Escherichia coli pyelonephritis on the renal handling of [3H]tobramycin was investigated. The accumulation of [3H]tobramycin in proximal and distal tubules in both normal and infected rats was compared. Following induction of pyelonephritis, disturbed intrarenal localization of the drug was noted. Grain counts were affected in both proximal and distal tubules. Decreased labeling was observed at all time intervals in the proximal tubules. Electron microscopy showed that radioactivity was associated mostly with lysosomes in both normal and infected rats 1 and 24 h following the injection of the drug. We could detect significantly higher amounts of drug in the distal tubules of the pyelonephritic kidney than the normal levels at 10 min and 24 h postinjection. The drug did not seem to be associated with any particular organelle and was evenly distributed within the distal tubular cells. The present study shows that the transport of tobramycin within the infected nephron is disturbed. These data might shed some light on the influence of infection on the intrarenal pharmacology of aminoglycosides.

Correlation between renal membrane binding and nephrotoxicity of aminoglycosides

The kinetics of aminoglycoside binding to renal brush border and basolateral membrane vesicles from rat renal cortex were studied by using [3H]amikacin. [3H]amikacin binding to renal membranes was found to be a rapid, saturable process with a fourfold greater affinity for basolateral membranes than for brush border membranes (Kd basolateral = 607 microM; Kd brush border = 2,535 microM). Renal membranes prepared from immature rats (2 to 3 weeks old) exhibited a significantly lower affinity compared with membranes from adults (Kd basolateral = 2,262 microM; Kd brush border = 6,216 microM). Additionally, the inhibitory behavior of several aminoglycosides versus [3H]amikacin binding to brush border membranes revealed the following rank order of potency: neomycin greater than tobramycin approximately gentamicin approximately netilmicin greater than amikacin approximately neamine greater than streptomycin. The relative insensitivity of immature rats to aminoglycoside-induced nephrotoxicity in vivo and the comparative nephrotoxicity of the various aminoglycosides suggest that renal membrane-binding affinity is closely correlated to the nephrotoxic potential of these antibiotics.

Characteristics of gentamicin uptake in the isolated crista ampullaris of the inner ear of the guinea pig

The characteristics of aminoglycoside uptake in the inner ear were investigated in the isolated crista ampullaris. The organ was incubated with radiolabeled gentamicin, and the stable, nonexchangeable radioactivity was considered the intracellular pool of the drug. Gentamicin was transported against a concentration gradient, and the resulting tissue to medium ratios ranged from 1.4 to 18.6. Transport was inhibited by reduction in temperature or by the addition of metabolic blockers, indicative of an energy-dependent component. The uptake system showed at least two sites, the first a high-affinity site with a dissociation constant KD = 39 nM and a capacity of n = 0.2 pmole/crista; the second had a KD = 16 microM and a capacity of n = 11 pmoles/crista. Aminoglycosides competed with uptake of gentamicin in the order netilmicin greater than or equal to neomycin greater than tobramycin, and polyamines competed in the order spermine greater than spermidine greater than putrescine; glucosamine and the basic amino acids lysine and asparagine were ineffective, excluding the participation of glucose and basic amino acid transport systems in gentamicin uptake. These results along with direct comparisons with some characteristics of putrescine uptake suggest that aminoglycoside and polyamine transport have common features in the crista ampullaris.

Uptake of aminoglycoside antibiotics into brush-border membrane vesicles and inhibition of (Na+ + K+)-ATPase activity of basolateral membrane

The effects of aminoglycoside antibiotics on plasma membranes were studied using rat renal basolateral and brush-border membrane vesicles. 3',4'-Dideoxykanamycin was bound to the basolateral membrane and brush-border membrane vesicles. They had a single class of binding sites with nearly the same constant, and the basolateral membrane vesicles had more binding sites than those of the brush-border membrane. Dideoxykanamycin B was transported into the intravesicular space of brush-border membrane vesicles, but not into that of basolateral membrane vesicles. The (Na+ + K+)-ATPase activity of the plasma membrane fraction prepared from the kidney of rat administered with dideoxykanamycin B intravenously decreased significantly. Aminoglycoside antibiotics entrapped in the basolateral membrane vesicles inhibited (Na+ + K+)-ATPase activity, but those added to the basolateral membrane vesicles externally failed to do so. The activity of (Na+ + K+)-ATPase was non-competitively inhibited by gentamicin. It is thus concluded that aminoglycoside antibiotics are taken up into the renal proximal tubular cells across the brush-border membrane and inhibit the (Na+ + K+)-ATPase activity of basolateral membrane. This inhibition may possibly disrupt the balance of cellular electrolytes, leading to a cellular dysfunction, and consequently to the development of aminoglycoside antibiotics' nephrotoxicity.

Studies on the nephrotoxicity of aminoglycoside antibiotics and protection from these effects (3). Protective effect of latamoxef against tobramycin nephrotoxicity and its protective mechanism

Effect of latamoxef (LMOX) against tobramycin (TOB)-induced nephrotoxicity was studied in rats. Treatment with TOB (90 mg/kg/day, s.c.) alone resulted in marked increases in the activities of urinary enzymes such as lactate dehydrogenase, N-acetyl-beta-D-glucosaminidase and lysozyme, urinary protein content and blood urea nitrogen, which peaked on the 7th or 10th day. The combination with LMOX (500, 1000 or 2000 mg/kg/day, s.c.) significantly suppressed increases in the parameters with TOB alone. The extent of this suppression roughly depended on the LMOX dosage. Although TOB alone caused pronounced histological changes such as extensive cortical proximal tubular cell necrosis, residual tubular basement membrane and cast formations in the renal cortex and medulla on the 7th day, these changes were apparently suppressed by combination with LMOX. In addition, intrarenal TOB concentrations in the rat given TOB alone were about 350, 500 and 1000 micrograms/g tissue wet weight at 3 hr, on day 3 and on day 5, respectively. On the other hand, there was a significant reduction (30-60%) in intrarenal TOB concentration by combination with LMOX. These results indicate that combination with LMOX obviously protects the rat kidney from TOB nephrotoxicity, and the protective effect may be partially due to suppression of intrarenal accumulation of TOB by LMOX.

Gentamicin inhibits Na+-dependent D-glucose transport in rabbit kidney brush-border membrane vesicles

We studied the effect of gentamicin on Na+-dependent D-glucose transport into brush-border membrane vesicles isolated from rabbit kidney outer cortex (early proximal tubule) and outer medulla (late proximal tubule) in vitro. We found the same osmotically active space and nonspecific binding between control and gentamicin-treated brush-border membrane vesicles. There was no difference in the passive permeability properties between control and gentamicin-treated brush-border membrane vesicles. Kinetic analyses of D-glucose transport into 1 mM gentamicin-treated brush-border membrane vesicles demonstrated that gentamicin decreased Vmax in the outer cortical preparation, while it did not affect Vmax in the outer medullary preparation. With regard to Km, there was no effect of gentamicin in any vesicle preparation. When brush-border membrane vesicles were incubated with higher concentrations of gentamicin, Na+-dependent D-glucose transport was inhibited dose-dependently in both outer cortical and outer medullary preparations. Dixon plots yield inhibition constant Ki = 4 mM in the outer cortical preparation and Ki = 7 mM in the outer medullary preparation. These results indicate that the Na+-dependent D-glucose transport system in early proximal tubule is more vulnerable to gentamicin toxicity than that in late proximal tubule.

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.

The use of renal tubule fragments isolated from the rat to investigate aspects of gentamicin nephrotoxicity

A suspension of renal tubule fragments from the rat was prepared by a method involving collagenase digestion of the excised renal cortex and dispersion of the digest by passage through a nylon mesh. Through the use of scanning electronmicroscopy it was confirmed that the tubular lumena were patent, thus ensuring access of medium to both the luminal and the contraluminal membranes of the tubular cells. The viability of the tubule fragments was ascertained by measuring the rate of formation of glucose from pyruvate as substrate and the uptake of [14C] L-lysine against a concentration gradient. The uptake of L-lysine was unimpaired in the presence of gentamicin (10(-3) M), suggesting that there is no competition between this basic amino acid and the polycationic aminoglycoside for transport into tubular cells. The uptake of [3H] gentamicin was studied and found to be reduced in the presence of 2,4-dinitrophenol, potassium cyanide, and ouabain. The reduced uptake in the presence of ouabain was interpreted to mean that a component of gentamicin uptake, which occurs at the luminal membrane, may be driven by the Na+ gradient created by Na+-K+ATPase activity at the contraluminal membrane. This renal tubule preparation offers advantages over the kidney-slice technique for studies into the mechanisms of aminoglycoside nephrotoxicity.

Aminoglycoside nephrotoxicity

Aminoglycosides are life-saving antibiotics in patients with gram negative sepsis. Renal dysfunction occurs in approximately 10% of all clinical courses of aminoglycosides. Because of close pharmacokinetic and toxicologic similarities, rats are excellent human surrogates for comparing the nephrotoxic potentials of these antibiotics. Comparisons in rats are also more sensitive than clinical comparisons due to the insensitivities of clinical renal function tests, the confounding influences present in seriously-ill patients and the inability to make morphologic comparisons in the clinic. The pathogenesis of aminoglycoside nephrotoxicity is still evolving despite extensive world-wide investigations. However, these investigations have facilitated the identification of several inhibitors of aminoglycoside nephrotoxicity. The clinical usefulness of these inhibitors must still be established.

Dose-response studies of gentamicin nephrotoxicity in rats with experimental renal dysfunction. I. Subtotal surgical nephrectomy

Gentamicin pharmacokinetics and nephrotoxicity have not been widely studied in animals with preexisting renal dysfunction, despite the fact that nephrotoxicity is a continuing manifestation of clinical therapy. The present study contrasted the dose-response nephrotoxicity of gentamicin in control rats with that of rats with renal insufficiency secondary to subtotal (2/3) surgical nephrectomy. Total daily doses ranging from 0 to 120 mg/kg were given in a divided regimen, every 8 hr and doses were reduced by doubling the interval in subtotally nephrectomized (Nx) rats, in proportion to impaired renal elimination on the first day of gentamicin administration. Estimates of renal function, including creatinine clearance, fractional sodium and potassium excretion, and serum creatinine and urea nitrogen, were collected after 6 and 12 days of dosing. In addition, urinary N-acetyl-beta-D-glucosaminidase excretion (6 days), in vitro renal cortical slice accumulation of tetraethylammonium (TEA) (6 days), quantified morphological lesions (12 days), and renal gentamicin concentrations (6 days) were examined. Pharmacokinetic data collected immediately after the first dose revealed a reduced gentamicin clearance and slightly reduced volume of distribution, with a corresponding prolonged half-life in the Nx rats. Based on statistical analysis of the dose-response relationships, Nx rats were functionally resistant to gentamicin nephrotoxicity after 6 days of dosing. This resistance was partially reversed by 12 days dosing, despite light-microscopic evidence of greater structural damage in the control rats. Renal parenchymal gentamicin concentrations were lower at some doses in the Nx rats, in contrast to the higher fractional reabsorption found in these rats at all doses. TEA transport was depressed at all doses in control rats but not at the lower doses in Nx rats, indicating that resistance was partially mediated at the level of the proximal tubular epithelium. This study demonstrates altered gentamicin pharmacokinetics and nephrotoxicity in a surgical model of renal dysfunction in rats.

Relationship between rat renal accumulation of gentamicin, tobramycin, and netilmicin and their nephrotoxicities

Gentamicin, tobramycin, and netilmicin were given to rats in daily doses of either 5 or 20 mg/kg for 30 days to determine the renal accumulation kinetics of the compounds and to correlate steady-state renal parenchymal concentrations with nephrotoxicity. Four rats from each group were sacrificed daily and renal parenchymal tissue concentrations were determined microbiologically. Nephrotoxicity was assessed by changes in creatinine values in serum, renal creatinine clearances, and pathological scores. There was no indication of aminoglycoside-induced nephrotoxicity in any tests performed. The following steady-state levels resulted: 36, 148, and 176 micrograms/g after 5 mg/kg per day and 148, 260, and 510 micrograms/g after 20 mg/kg per day for tobramycin, gentamicin, and netilmicin, respectively. We conclude that aminoglycoside parenchymal accumulation in rats follows this order: tobramycin less than gentamicin less than netilmicin. Therefore, differences in the relative toxicities of gentamicin, tobramycin, and netilmicin do not correlate with the renal parenchymal accumulation of these agents and may be more dependent on intrinsic toxicity to the renal proximal tubule than to the concentration of the aminoglycoside in the kidney.

Recovery of cortical phospholipidosis and necrosis after acute gentamicin loading in rats

The recovery from gentamicin-induced phospholipidosis in the rat kidney cortex was characterized both morphologically and biochemically after a single 12-hr drug infusion. Total dosages administered were 10, 60, or 140 mg/kg, achieving constant serum concentrations of 3, 11, and 27 micrograms/ml, respectively. At the end of the 12-hr infusion, the cortical drug concentrations corresponding to the three dosages were 124, 450, and 993 micrograms/g of wet tissue. At the low dose (10 mg/kg), myeloid bodies were seen inside lysosomes of proximal tubular cells, along with a modest decrease of lysosomal sphingomyelinase activity. The cortical drug level declined steadily following first-order kinetics along with a disappearance of myeloid bodies and return of sphingomyelinase activity to control levels. At the high dose (140 mg/kg), we observed a sustained loss of sphingomyelinase activity (37% of controls), a subsequent increase of phospholipid concentration in the kidney cortex (up to 117% of controls 2 days after) and a prominent accumulation of myeloid bodies inside the lysosomes of proximal tubular cells (up to 4% of cell volume). Tubular regeneration and interstitial infiltration became detectable by histology and the increase of DNA synthesis as from day 1, along with an apparent reduction of the phospholipidosis at days 3 and 4. Drug cortical concentrations showed a sharp decline 2 days after infusion. An intermediate behavior was observed at 60 mg/kg. It is concluded that the proximal tubular cells behave in a fundamentally different way after gentamicin loading with low and high doses. At the low dose there is a regression of the drug-induced changes in the absence of any sign of necrosis-regeneration. Above a threshold in cortical drug concentration there is further development of the alterations leading to cell death-regeneration.

Gentamicin, netilmicin, dibekacin, and amikacin nephrotoxicity and its relationship to tubular reabsorption in rabbits

The role of the tubular reabsorption of aminoglycosides in nephrotoxicity was considered. The tubular reabsorption rate, fractional reabsorption, and net balance, expressed as the excreted to infused aminoglycoside ratio, were concomitantly studied in male rabbits by continuous infusion of gentamicin, netilmicin, dibekacin, and amikacin. Aminoglycoside nephrotoxicity was evaluated by creatinine levels in serum and pathological renal damage after 14 days of a low- or high-dose regimen, comprising either eight, hourly intramuscular injections of gentamicin, netilmicin, or dibekacin (4 mg/kg) or amikacin (16 mg/kg); twelve, hourly intramuscular injections of gentamicin, netilmicin, or dibekacin (15 mg/kg) or amikacin (60 mg/kg); or injections of saline for the control group. Aminoglycosides exhibited three degrees of tubular reabsorption: gentamicin had the highest, netilmicin had the lowest, and dibekacin and amikacin had intermediate degrees of reabsorption. Nephrotoxicity associated with alteration in renal histology was observed with gentamicin and, to a lesser extent, with dibekacin in the high-dose regiment. No nephrotoxicity was noted with netilmicin or amikacin compared with the control group. Concentrations of the aminoglycosides in renal cortex and serum were not predictive of renal toxicity. Except for amikacin, which appeared to exhibit the lowest intrinsic renal toxicity, nephrotoxicity was correlated with the tubular reabsorption of each aminoglycoside. It was concluded that aminoglycoside renal toxicity can be determined by two major factors: importance of transport into tubular cells and intrinsic intracellular toxicity.