Significance of intrarenal concentrations of gentamicin for the outcome of experimental pyelonephritis in rats

Pharmacologic Basis for the Treatment of Pyelonephritis

Urinary tract infections (UTIs) are among the most common bacterial infections in humans. Even though physicians have been treating UTIs for 60 years, there has been no standardized approach regarding the rational choice of antimicrobial agents and optimal treatment duration for these infections. This review discusses the pharmacologic basis for the treatment of UTIs. Although most antibiotics concentrate well in the urine and can eradicate most of the sensitive uropathogens that cause lower UTI, antibiotics given for the treatment of pyelonephritis must concentrate and kill bacteria embedded within the renal parenchyma. Investigators once believed that antibiotics must concentrate in sufficient amounts in the urine of infected patients to be effective in treating pyelonephritis. In fact, the efficacy of an antibiotic in the treatment of pyelonephritis is proportional to its capacity to converge in high concentration not only in urine but also in the renal parenchyma because serum and urine levels of antibiotics are poor predictors of the intrarenal levels. Other factors should also be taken into consideration in the management of UTIs, such as the time of day antibiotics are given because significant time-dependent differences have been observed in the pharmacokinetics and rate of excretion in urine of several antibiotics. Finally, the authors review the recent development in the inflammatory response in the urinary tract that may explain the clinical features of UTI and may be useful in the diagnosis as well as better management of UTI.

L-651,392, a potent leukotriene inhibitor, controls inflammatory process in Escherichia coli pyelonephritis

In this study, the relationship between leukotrienes, peritubular cell infiltration with polymorphonuclear cells (PMNs) and renal tubular damage was investigated in a rat model of acute ascending pyelonephritis. Infection was induced by the injection of 10(5) CFU of Escherichia coli into the bladder and occlusion of the left ureter for 24 h. Treatment of infected animals was started 24 h after the induction of pyelonephritis with either hydrocortisone (25 mg/kg of body weight per day), the leukotriene inhibitor L-651,392 (10 mg/kg/day), or the vehicle of L-651,392 and was maintained for 5 days. At the end of treatment, the animals were killed, serum was collected, and both kidneys were removed for colony counts and histopathology. Renal function was evaluated by the measurement of blood urea nitrogen levels and creatinine clearance. The numbers of PMNs and mononuclear cells (MNs) in the cortex and medulla were recorded for all groups on plastic sections done from the left kidney. Infection alone (vehicle of L-651,392) resulted in intensive interstitial infiltration and a severe tubular destruction in the cortex. Treatment with hydrocortisone did not prevent PMN migration and tissue damage. By contrast, treatment with L-651,392 resulted in a significant reduction in PMNs (P < 0.001 in comparisons with all other groups) and greater preservation of the tubular structure despite identical bacterial counts than in the group receiving hydrocortisone. We conclude that L-651,392 prevents inflammatory cells from reaching the site of infection and protects the kidney from tubular damage associated with inflammation during pyelonephritis. Inhibitors of leukotrienes should be further investigated for their potential benefit as adjuvants to antibiotherapy in the treatment of pyelonephritis.

Renal pharmacokinetic changes of gentamicin during enterococcal pyelonephritis

In the present study, the renal pharmacokinetics of gentamicin was investigated in pyelonephritic rats infected with Streptococcus (Enterococcus) faecalis. Four days after the induction of infection, animals were given either a single dose of gentamicin or two daily injections for 7 days. The treated animals were evaluated from 1 h to 6 months posttreatment. After a single injection, limited pharmacokinetic variations were observed, whereas after 14 injections infected kidneys demonstrated significantly higher concentrations and a more extended renal elimination phase of the antibiotic. Analysis of the area under the curve of the concentration in kidney versus time showed more marked renal accumulation by the infected and long-term treated animals than by normal animals or those receiving only one injection of aminoglycoside. Renal function remained normal in both the infected and normal animals treated with this aminoglycoside. These results demonstrate that S. faecalis pyelonephritis disturbs the renal handling of gentamicin and may increase the susceptibility of the kidney to aminoglycosides.

Benefit from high intrarenal levels of gentamicin in the treatment of E. coli pyelonephritis

The importance of high intrarenal levels of gentamicin on the outcome of experimental pyelonephritis was studied in rats receiving either a short course (three days) of gentamicin (G) alone or combined with a longer course (14 days) of ampicillin (A), cephalothin (C), or trimethoprim (T), or two weeks of therapy with ampicillin, cephalothin, trimethoprim and gentamicin given alone. While ampicillin, cephalothin and trimethoprim were undetectable in the medulla within six hours of cessation of therapy, gentamicin was still detectable in levels six folds above the MIC up to six months after treatment had ceased. Six months after the end of treatment, the percentage of sterile left kidneys in animals treated with ampicillin (50%), cephalothin (15%), trimethoprim (20%) was lower than the percentage of animals receiving 14 days of gentamicin (100%), or the combinations AG:89%, CG:67% and TG:60%, P less than 0.01. Following three days of gentamicin, 50% of the left kidneys were sterilized. When compared to ampicillin, cephalothin or trimethoprim alone, combined therapies significantly reduced the number of CFU in the kidneys P less than 0.01. These combinations were almost as effective as two weeks of therapy with gentamicin. Short-term therapy (three days) with an aminoglycoside which concentrates in the renal parenchyma, combined with an antibiotic which will accumulate in other parts of the nephron, may result in "pharmacological synergy". This new approach to therapy of pyelonephritis may be promising.

Tissue penetration of antibiotics

To be effective, antibiotics must be active against the offending pathogen(s) and must reach sufficient concentrations at the site of infection where microorganisms have induced severe inflammation. Such inflammation may, depending on the infected tissue, increase or decrease antibiotic penetration. Once it has reached the infected site an antibiotic may be inactivated locally. In this work, we have reviewed: 1. Factors modulating the penetration of antibiotics in tissues of normal and infected humans and animals; 2. The relationship between concentrations and efficacy of antimicrobials in experimental and human infections; 3. The mechanisms by which high tissue levels of antibiotics may be toxic; 4. New antibiotic targetting delivery systems which may increase tissue concentrations of antibiotics. To better understand the interaction between antimicrobial agents, pathogens in specific infected sites and the host, more experimentation dealing simultaneously with tissue penetration and antibiotic efficacy in acute and chronic infection is required. Innovative approaches to the therapy of human infections are needed.

Influence of endotoxin on the intrarenal distribution of gentamicin, netilmicin, tobramycin, amikacin, and cephalothin

Multiple factors may modify the pharmacokinetics of aminoglycosides and increase their nephrotoxic potential. In this study, we investigated the influence of Escherichia coli endotoxin on the renal handling of several aminoglycosides and one cephalosporin. Drug levels in the renal parenchyma, as well as several parameters of renal function and histology, were compared in rats treated with endotoxin (0.25 mg/kg) and normal rats treated with either gentamicin (10 mg/kg), netilmicin (10 mg/kg), tobramycin (10 mg/kg), amikacin (50 mg/kg), or cephalothin (100 mg/kg). Blood pressure and pulse rate were recorded. Endotoxin was associated with a decrease in the half-life and in the apparent volume of distribution of gentamicin. The endotoxin-injected animals accumulated significantly (P less than 0.05) more aminoglycosides in their kidneys than the normal animals. The amount of cephalothin recovered in the renal parenchyma was identical in both groups. Slight decreases in the glomerular filtration rate and renal plasma flow were observed after endotoxin treatment. Blood pressure and cardiac frequency were minimally affected by endotoxin. No histological lesions were observed by light microscopy in animals receiving endotoxin. Thus, endotoxin modifies the renal handling of aminoglycosides in the absence of any major physiological disturbance or histological change. By increasing the total amount of drug within the kidneys, endotoxin might increase the nephrotoxic potential of aminoglycosides.

Dose-response studies of gentamicin nephrotoxicity in rats with experimental renal dysfunction. II. Polyvinyl alcohol glomerulopathy

Animal studies involving concurrent pathophysiologic states, including experimental renal dysfunction, are useful for a proper understanding of the mechanisms of gentamicin nephrotoxicity and acute renal failure. This study examined gentamicin nephrotoxicity in a model of glomerular dysfunction in rats. Administration of medium molecular weight polyvinyl alcohol (PVA) produced a glomerulopathy, with characteristic accumulation of macromolecular PVA in the glomerular mesangium without altering glomerular filtration or causing proteinuria. Subsequent daily doses of gentamicin ranging from 0 to 120 mg/kg elicited a dose-response nephrotoxicity in both control and PVA-pretreated rats after 6 or 12 days of drug. Based on statistical analysis of renal clearances of creatinine, urea, sodium, and potassium; serum creatinine and urea nitrogen; urinary N-acetyl-beta-D-glucosaminidase excretion (6 days only); in vitro renal cortical transport of tetraethylammonium (TEA) (6 days); and quantified light-microscopic data (12 days), PVA induced an early (6 days) sensitivity to gentamicin nephrotoxicity. By 12 days, there were no differences in the responses of control and PVA rats to gentamicin. Single-dose gentamicin clearance, volume of distribution, and half-life were not altered by PVA and renal concentrations at 6 days were generally lower in these rats. Results of TEA transport studies tend to rule out PVA-induced metabolic lesions in the proximal tubular epithelium as the mechanism for the early sensitivity. This investigation demonstrates altered gentamicin nephrotoxicity in rats with an otherwise benign glomerulopathy and, combined with similar conclusions from a related study in subtotally nephrectomized rats, presents further evidence that the underlying pathophysiologic state of the kidney is an important factor in the renal response to nephrotoxins.

Increased nephrotoxicity of gentamicin in pyelonephritic rats

Multiple factors may increase the nephrotoxic potential of aminoglycosides. We studied gentamicin susceptibility of kidneys infected with E. coli. Several parameters of renal function, histological changes on light and electron microscopy, and drug levels in renal parenchyma were compared in pyelonephritic and normal rats treated with low doses (10 mg/kg/Q8 hr for 3 days), or high doses (60 mg/kg/day for 14 days), of gentamicin. A significant increase (P less than 0.01) in beta-galactosidase and protein excreted in urine over a period of 17 days associated with severe changes in diuresis and osmolality was noted in the infected treated rats (low doses) compared with normal, treated, infected or control animals. Histological modifications compatible with gentamicin nephrotoxicity were more persistent in the infected treated animals. A significant decrease in 14C inulin (P less than 0.01) and 3H-PAH clearance and secretion (P less than 0.02) was observed in the infected treated rats receiving high doses of antibiotics. Cellular necrosis and tubular desquamation also were more severe in this group. Gentamicin levels in the cortex and medulla of infected animals were significantly higher than in the normals (P less than 0.01) and might have been responsible for the increased toxicity noted in the pyelonephritic animals. Infected kidneys appeared to be more susceptible to the nephrotoxic potential of gentamicin.