Once-daily dosing decreases renal accumulation of gentamicin and netilmicin.

The pathogenesis of aminoglycoside nephrotoxicity is intimately related to the extent of drug accumulated in the renal cortex. In the framework of searching for preventive measures of aminoglycoside-induced nephrotoxicity, we investigated the influence of dosage regimen on the renal cortical accumulation of gentamicin and netilmicin in humans. Patients with a tumor partly involving one kidney, with normal renal function, and scheduled for nephrectomy received one dose of either gentamicin (4.5 mg/kg) or netilmicin (5 mg/kg) as a single short-term infusion or as 24-hour continuous infusion. Treatment started 24 hours before surgery. Serum aminoglycoside pharmacokinetics were examined during treatment and renal cortical tissue was sampled at the moment of operation for drug determination. The short-term infusion schedule yielded cortical concentrations of 103.2 +/- 36.3 and 137.4 +/- 34.6 micrograms/gm for gentamicin and netilmicin, respectively. Tissue levels after continuous infusion were 158.1 +/- 52.9 and 178.5 +/- 21.8 micrograms/gm for gentamicin and netilmicin, respectively. For each aminoglycoside, a single short-term infusion resulted in significantly lower renal drug levels than did a continuous infusion of the same dose. From the nephrotoxicity point of view, these data support the administration of gentamicin and netilmicin as once-daily injections. This also supports the appropriateness of further studies to determine clinical efficacy of once-a-day dosing for aminoglycosides.

Effect of hyperfiltration, proteinuria and diabetes mellitus on the uptake kinetics of gentamicin in the kidney cortex of rats.

The influence of hyperfiltration-hypertrophy, proteinuria and glucosuria on the renal cortical uptake of gentamicin was studied in several experimental models. Two groups of remnant kidney rats, one fed a standard protein diet and one a low protein diet, heavy proteinuric rats (adriamycin) and diabetic rats, each with their own control group, were treated with increasing doses of gentamicin, given as a continuous infusion over 6 hr. The relationship between increasing steady-state serum levels (ranged from 1 to 100 micrograms/ml) and the cortical gentamicin concentrations at the end of the infusions was examined by means of Michaelis-Menten kinetics. The uptake curves were compared to their respective control curves. It was demonstrated that gentamicin uptake was reduced in remnant kidney rats fed standard diet (showing hyperfiltration and heavy proteinuria), in adriamycin rats (showing heavy proteinuria in the absence of hyperfiltration) and in diabetic rats. Uptake of gentamicin in remnant kidney rats fed low protein diet (showing hyperfiltration and slight proteinuria) was comparable to controls. It appeared that among the pathophysiological factors examined, proteinuria is the most important in decreasing the cortical uptake of gentamicin. It is suggested that high levels of proteins in the proximal tubular fluid interfere with the adsorptive endocytic process, involved in the uptake of both proteins and gentamicin.

Pharmacokinetics and tolerance after repeated doses of imipenem/cilastatin in patients with severe renal failure.

The pharmacokinetics of imipenem and cilastatin after repeated doses have been studied in six patients with severe renal impairment (mean creatinine clearance 10.4 ml/min/1.73 m2). The patients received nine iv injections of imipenem/cilastatin sodium (500/500 mg) at 12-hour intervals. The imipenem plasma concentration-time profile and the pharmacokinetic parameters on day 5 were similar in all respects to those on day 1. Therapeutic plasma levels of imipenem (greater than or equal to 4 mg/l) were maintained for 8-10 h after administration. Most pharmacokinetic parameters of cilastatin were similar on both days. However, the area under the plasma concentration curve (AUC) was significantly increased on day 5, as a result of some accumulation, but the trough levels stabilized after the third injection. Twice daily administration of imipenem/cilastatin 500/500 mg was felt to be a well tolerated and optimal dose regimen in patients with severe renal failure.

The effect of dosing strategy on kidney cortical accumulation of aminoglycosides in rats.

The influence of three different dosage schedules on kidney cortical accumulation of aminoglycosides was studied in rats. A daily dose of 10 mg/kg of gentamicin or tobramycin or 33.3 mg/kg of amikacin were given, either in single injections, three injections eight hours apart, or by continuous infusion. Kidney cortical levels of aminoglycosides were examined at regular intervals up to eight days. The pattern for cortical accumulation under different dose regimens was drug-dependent. Continuous infusion of gentamicin resulted in remarkably higher cortical concentrations than after intermittent administrations. In contrast, cortical levels of tobramycin were independent of the regimen used. For amikacin, higher accumulation resulted from continuous administration from the second day. However, the differences in tissue levels after one or the other dose regimen were less striking than for gentamicin. These observations were explained by differences in cortical uptake kinetics for aminoglycosides previously reported. Elevations in serum concentrations of gentamicin were associated with nonlinear uptake. Accordingly, uptake of gentamicin is more "efficient" at low serum concentrations. Uptake of tobramycin was linearly related to increases in serum levels. Amikacin showed a mixed kinetics pattern, saturation at low serum levels, and linear kinetics at high serum concentrations. This explains the higher cortical levels reached with continuous infusion of amikacin compared with intermittent administration. Uptake kinetics play an essential role in cortical aminoglycoside concentrations achieved after different dosing strategies. The design of regimens minimizing cortical uptake may decrease the risk of nephrotoxicity.

Renal cortical uptake kinetics of gentamicin in rats with impaired renal function.

The renal cortical uptake kinetics of the aminoglycoside antibiotic gentamicin was determined in the remnant kidney model. Renal failure was induced by partial ablation of the right kidney followed by left nephrectomy in female Wistar rats. The animals received a six-hour gentamicin infusion at a constant rate yielding steady-state serum concentrations ranging from 0.5 to 150 micrograms/mL. The renal cortical gentamicin concentrations were determined and related to the serum concentrations achieved. This relationship was nonlinear and followed Michaelis-Menten kinetics. Gentamicin cortical uptake rate, however, did not show clear saturation in the range of gentamicin serum levels studied as was observed in rats with normal renal function. The Michaelis-Menten parameters determined by nonlinear regression were Km = 15.0, 73.9, and 135.7 micrograms/mL; and Vmax = 149.9, 213.7, and 239.2 micrograms/g cortex/h, respectively, for controls, rats with serum creatinine levels between 0.9 and 1.2 mg/dL, and those with levels between 1.3 and 1.8 mg/dL. It is concluded that at serum levels below 100 micrograms/mL, the gentamicin renal cortical uptake is diminished in rats with renal failure. This decrease in renal cortical uptake is more pronounced in the group of rats with more severe renal failure.

Renal cortical kinetics of gentamicin after implantation of gentamicin-polymethylmethacrylate beads in rats.

Beads of gentamicin-polymethylmethacrylate (Septopal), each containing 4.5 mg of gentamicin base, were implanted intraperitoneally in rats. Each rat received one bead. Serum levels and urinary excretion of gentamicin were maximal in the first day of treatment (0.6 micrograms/ml in serum 3 h after implantation of the bead and 525 micrograms per 24-h urine sample) and decreased thereafter. Kidney cortical concentrations of gentamicin progressively increased and peaked after 4 days, reaching 117 micrograms/g. Tissue levels decreased thereafter in spite of the persistence of the drug in urine, and this occurred in the absence of cell damage leading to cell death. Release of gentamicin from intact proximal tubular cells, despite continuous uptake, prevented intracellular drug concentrations from reaching a nephrotoxic level. This experimental study provides a rational basis for the previous clinical observation that nephrotoxicity due to treatment with gentamicin-polymethylmethacrylate beads is improbable.

Choice of drug and dosage regimen. Two important risk factors for aminoglycoside nephrotoxicity.

Since the clinical use of aminoglycosides may be limited by the development of nephrotoxicity, it is important to be aware of those risk factors associated with a greater incidence of renal damage. Some of these factors are related to the drug and its administration and others are related to the patient's clinical condition. In the human kidney, the toxicity mechanism is very likely the same for all aminoglycosides, although the risk of nephrotoxicity increases for a given aminoglycoside as cortical concentrations increase. Kinetic studies in the rat demonstrated a nonlinear increase in renal cortical uptake of gentamicin and netilmicin, a linear relationship for tobramycin uptake, and a mixed kinetic pattern for amikacin, that is, saturation kinetics at low serum concentrations and a linear pattern at high serum levels. At comparable steady-state low serum levels, amikacin and tobramycin showed lower cortical concentrations than gentamicin or netilmicin, demonstrating a lower affinity for the uptake of amikacin and tobramycin in the rat. Since drug uptake kinetics determine the extent of cortical concentrations achieved, dosing strategies may affect cortical accumulation of aminoglycosides. Our kinetic data show that continuous infusions of low doses of gentamicin and amikacin resulted in higher cortical levels, but the differences between regimens were more remarkable for gentamicin than for amikacin. For tobramycin, however, cortical concentrations were similar regardless of the dosing strategy used. In addition, our data show that dosage regimens also determine cortical accumulation in humans. A second major determinant of nephrotoxicity is intrinsic toxicity. At therapeutic doses, gentamicin, tobramycin, netilmicin, and amikacin induce an early lysosomal phospholipidosis in the human kidney cortex comparable to that observed in animals treated with low doses of these drugs. However, animal and human studies have shown that amikacin induces significantly less lysosomal overloading than the other aminoglycosides with no loss of phospholipase A1 activity. Based on the examination of cortical drug levels and the detection of early biochemical and morphologic alterations induced by aminoglycosides, the data suggest that amikacin has less pronounced nephrotoxic effects than gentamicin, netilmicin, or tobramycin, when used in strictly comparable clinical conditions.

In vivo uptake kinetics of aminoglycosides in the kidney cortex of rats.

The renal cortical uptake kinetics of four aminoglycosides were studied in vivo. Gentamicin, netilmicin, tobramycin or amikacin were administered to rats by continuous infusion over 6 hr achieving constant serum levels ranging from 0.2 to 100 micrograms/ml. Renal cortical concentrations at the end of the infusion were plotted against the steady-state serum levels. Steady-state elevations of serum gentamicin and netilmicin were associated with nonlinear increases in cortical levels, suggesting saturable uptake. Analysis of the data using Michaelis-Menten kinetics indicates that the apparent Km for gentamicin and netilmicin were 15.01 and 23.84 micrograms/ml and Vmax 149.83 and 178.36 micrograms/g of cortex per hr, respectively. The "initial" rate of uptake (at serum levels below 15 micrograms/ml) was highest for gentamicin. The cortical uptake of tobramycin was linearly related to elevations in serum levels [cortex concentration (conc) = 9.24 + 1.40 serum conc]. The initial rate of tobramycin uptake was considerably lower than that for gentamicin and netilmicin. For amikacin, the initial rate of uptake followed Michaelis Menten kinetics and the second phase of the titration curve was linear. The equation for total amikacin uptake was: cortex conc = 12.98 + 1.71 serum conc. Aminoglycosides exhibit differing kinetics for renal cortical uptake in the rat during constant infusions. These results indicate that more than one mechanism probably mediates the uptake of each aminoglycoside. Depending on which mechanism predominates, the kinetic pattern may be saturable, linear or mixed.

Cyclosporine nephrotoxicity: comparative cytochemical study of rat kidney and human allograft biopsies.

The aim of this study was to detect early renal changes in the rat. Female Wistar rats received oral doses of cyclosporine (12.5, 25 or 50 mg/kg daily). The duration of the experiment was 1, 2, and 3 weeks. Controls received the vehicle only (olive oil). The following alterations were seen by light microscopy: Hypertrophy of the juxtaglomerular apparatus (PAS stain). Cytoplasmic droplets of neutral fat (Oil Red 0) in clusters of cortical tubules, probably belonging to the same nephron. Both the above phenomena increased with dosage and duration of treatment and were absent in controls. In the fat containing tubulus (FCT) brush border staining (alkaline phosphatase) was decreased or absent. Since after PAS the brush border was visualized in many FCT, it is concluded that many FCT were proximal tubulus (PT) of which the brush border has been damaged. In FCT mitochondrial staining (Cytochrome oxidase activity) was strongly decreased or absent. Mean lysosomal volume (acid phosphatase and dipeptidase II) is increased in the PT; in some cyclosporine animals, lysosomes were enlarged, while in others they were comparable to controls. Electron microscopy showed in some PT cells an increased number of empty vacuoles and focal alteration of mitochondria. Normal mitochondria were present next to grossly altered mitochondria. Autophagocytosis of mitochondria was clearly present. The lysosomes appeared swollen and contained electron dense material, not organised in the typical 50 A pattern of myeloid figures. These morphological changes suggest a defect of mitochondrial metabolism, leading to lipid deposition in PT. The mitochondrial metabolism can be disturbed by a direct toxic effect of cyclosporine or indirectly via ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemoperfusion-hemodialysis ineffective for paraquat removal in life-threatening poisoning?

We report on a patient treated with hemoperfusion-hemodialysis (HP-HD) for severe paraquat poisoning. This procedure was adopted since the combination of adsorption and dialysis may improve overall drug removal. On admission blood paraquat was 15.8 micrograms/ml. He received conventional treatment and combined HP-HD which started within 3 hours after ingestion of the chemical and lasted 5 hours. Blood samples were obtained during and after HP-HD. The samples during HP-HD were taken before the charcoal column, between the charcoal column and the artificial kidney and after the artificial kidney. Blood clearances of paraquat were 116 +/- 32 ml/min (n=6) for the charcoal column (HP), 90 +/- 54 ml/min (n=6) for the artificial kidney (HD) and 151 +/- 37 ml/min (n=6) for the combined systems (HP-HD). After HP-HD a limited rebound of blood paraquat level was seen. One day after admission renal and hepatic failure had developed, and the patient died after 5 days. Tissue paraquat levels (microgram/g wet tissue) were: skeletal muscle 9.4, pancreas 6.0, prostate 5.6, thyroid 4.2, lungs 4.0, bone marrow 4.0, kidney 3.1, spleen 2.9, adrenal 2.9, heart 2.8, liver 2.3, stomach and testis below 1.0. Measurements of blood levels demonstrated the efficient clearances of paraquat with HP-HD from the central (plasma) compartment. However, the present results confirmed those previously reported which suggest that the efficiency of short HP-HD in treating severe paraquat poisoning is questionable since paraquat levels in the peripheral (tissue) compartment remain elevated.

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.

Improved procedure for extracting aminoglycosides from renal cortical tissue.

An efficient and reproducible procedure was developed for extracting aminoglycosides from renal cortical tissue. It involves a double homogenization and two rinsings with trichloroacetic acid. A higher recovery is obtained compared with that of other previously reported methods.