Nephrotoxicity of dactimicin, a novel pseudo-disaccharide aminoglycoside possessing the N-formimidoyl group, compared with that of astromicin, amikacin and other aminoglycoside antibiotics in animals

The nephrotoxicity of dactimicin, the first aminoglycoside possessing the N-formimidoyl group, was compared with that of astromicin and, in part, amikacin, ribostamycin, kanamycin and gentamicin as reference aminoglycoside antibiotics. When a dose of 200 mg/kg was given intramuscularly to dehydrated mice, dactimicin caused no change of BUN and serum creatinine, while reference aminoglycosides caused significant elevations of the parameters. In the urinalysis of rats at doses of 40 and 80 mg/kg per day for 11 days or 21 days, dactimicin caused little changes in urinary parameters except for nucleated cells and NAG. In a detailed comparison between dactimicin and astromicin at 20, 40, 80, 120, 180 and 270 mg/kg for 11 or 30 days, dactimicin induced fewer changes in nucleated cells and NAG at high dosages. While dactimicin and astromicin caused no significant changes in BUN and serum creatinine at dosages of 20-270 mg/kg, histological observations using light and electron microscopes revealed that dactimicin consistently showed fewer lesions on the proximal tubular cells than those of astromicin for all dosages. When injected intramuscularly in rats, dactimicin and astromicin showed a similar distribution in the blood and main organs, except for the kidney, in which renal accumulation of dactimicin was about 60% of that of astromicin. Dactimicin slowly degraded in vitro and in vivo to give fortimicin B as a main product which was accumulated in the kidney. Through comparative studies with astromicin, it was disclosed that the N-formimidoyl group of dactimicin did not increase but decreased the nephrotoxicity, probably by suppressing reabsorption of dactimicin via proximal tubular cells.     

Prostaglandins and aminoglycoside nephrotoxicity

The role of prostaglandins in the development of aminoglycoside-induced acute renal failure was studied in CD-COBS rats (200 to 250 g). The animals were treated with gentamicin (80 mg/kg), acetylsalicylic acid (ASA, 100 or 200 mg/kg), or both drugs or saline for 5 or 10 days. Renal function was studied measuring creatinine clearance, blood urea nitrogen (BUN), and serum electrolytes, urine osmolality, and maximal urinary concentrating capacity after water deprivation and vasopressin administration. Gentamicin toxicity on the proximal tubule was evaluated by measuring urinary excretion of the lysosomal enzyme N-acetylglucosaminidase (NAG). Renal prostaglandin (PG) production was evaluated measuring the concentration of PGE2, PGD2, PGF2 alpha, 6-keto-PGF1 alpha, and thromboxane B2 (TXB2) in whole renal homogenate after a 15-min incubation at 37 degrees C using gas chromatography-mass spectrometry. Gentamicin alone reduced the glomerular filtration rate (GFR) 20 to 30% after 5 and 10 days of treatment. Combination with ASA potentiated the toxic effect of the aminoglycoside after 10 but not after 5 days of treatment. Similarly, gentamicin reduced the urinary concentrating capacity and addition of ASA worsened the effects. Gentamicin markedly increased NAG excretion but this effect was reduced by ASA, probably as a result of lysosomal stabilization. ASA alone inhibited the production of prostaglandins in renal tissue by 70 to 90% after single or multiple doses. The animals treated with gentamicin alone presented a significant, specific increase in PGE2 production after 10 days of treatment but this increase did not occur when the two compounds were given together. Since PGE2 has a vasodilatory effect in the kidney these results suggest that it may play a specific role in maintaining normal renal blood flow and GFR during the development of aminoglycoside nephrotoxicity. The inhibition of prostaglandin production by nonsteroid anti-inflammatory drugs prevents this compensatory mechanism and worsens the renal damage.     

Comparative effects of gentamicin, amikacin and dactimicin on excretion of N-acetyl-beta-D-glucosaminidase (NAG) and kidney histological pattern in rats

Dactimicin (DC) is a new pseudodisaccharide aminoglycoside antibiotic containing a formimidoyl group in its molecule (1). DC exhibits a greater antibacterial activity than other aminoglycosides against the clinical isolates of Serratia marcescens and is active against many gentamicin- and amikacin-resistant bacteria. This characteristic of the drug appears to be linked with a probable protective action exerted by the formimidoyl group in its structure. The greatest limitation in the clinical use of aminoglycosides is their potential for nephrotoxicity. The present study compares the renal effects of DC versus gentamicin (GT) and amikacin (AK), respectively the most and the least nephrotoxic pseudotrisaccharide aminoglycosides in present use (2), evaluating the urinary NAG excretion and the histological changes induced in the kidney.     

Molecular mechanisms underlying renal accumulation of aminoglycoside antibiotics and mechanism-based approach for developing nonnephrotoxic aminoglycoside therapy

Aminoglycoside antibiotics, such as gentamicin and amikacin, are a class of clinically important antibiotics used worldwide in the treatment of infections caused by Gram-positive and Gram-negative bacteria. However, nephrotoxicity and ototoxicity are serious problems in the use of aminoglycosides and are the major dose-limiting side effects. Most of the intravenously administered dose is excreted into the urine, whereas some of the aminoglycoside injected (about 10% of the dose) is selectively accumulated in the renal cortex, leading to renal injury. Aminoglycosides are taken up into the epithelial cells of the renal proximal tubules by an endocytic pathway. Acidic phospholipids, broadly distributed in the plasma membranes in various tissues, were considered to be the binding site of aminoglycosides. Recently, megalin, a giant endocytic receptor abundantly expressed in renal proximal tubules, has been reported to bind aminoglycosides. Therefore we first examined whether megalin plays an important role in the renal accumulation of aminoglycosides under in vivo and in vitro conditions. We then attempted to develop new strategies for preventing the nephrotoxicity of aminoglycosides based on the molecular mechanisms of aminoglycoside accumulation in the kidney. This review summarizes our recent findings ol the role of megalin in the renal accumulation of aminoglycosides and our approach to develop nonnephrotoxic aminoglycoside therapy.     

Enzymuria in aminoglycoside-induced kidney damage. Comparative study of gentamicin, amikacin, sisomicin and netilmicin

Forty-one patients with urinary tract infections were randomly assigned to receive for six days gentamicin, amikacin, sisomicin or netilmicin. The dose for each patient was calculated according to creatinine clearance and lean body mass in order to avoid overdosages. Urinary enzymes (alpha-glucosidase, gamma-glutamyltranspeptidase and muramidase), serum creatinine and creatinine clearance, proteinuria and urinary sediment were evaluated for nephrotoxicity. None of the patients developed nephrotoxicity, but urinary enzymes rose significantly in all. The statistical analysis of enzymuria during the treatment permitted the definition of a rank order of the nephrotoxic potential of the aminoglycosides studied.     

In vitro response of rat and human kidney lysosomes to aminoglycosides

To study aminoglycoside nephrotoxicity, renal cortical lysosomes were prepared from rat kidneys and from healthy portions of five human kidneys removed for tumor. The renal cortex was homogenized in 1 mM EDTA with 0.3M sucrose, and the lysosomes were separated by differential centrifugation. Lysosomes were incubated in isotonic sucrose solution with various drug concentrations for 1 hr at 37°. They were resedimented adn the N-acetyl-β-glucosaminidase (NAG) activity was measured in the supernatant fraction and in the disrupted pellet. Incucation with four aminoglycosides at therapeutic plasma concentrations lowered the percentage of NAG released into the supernatant fraction in a dose-related fashion. Incubation with the polyamines spermine and spermidine also produced this effect, with spermine and gentamicin being additive. This apparent lysosomal stabilization at clinically achieved plasma concentrations was also observed after substituting isotonic glycine for sucrose in the incubation mixture. High concentrations of aminoglycoside consistent with those accumulated in the renal cortex of patients and rats produced a dose-dependent release of lysosomal NAG with a rank order of potency paralleling their clinically observed potential for producing nephrotoxicity. Rats were treated with 20 mg/kg gentamicin twice a day for 28 days producing kidneys resistant to aminoglycoside nephrotoxicity. Lysosomes prepared from these animals compared to saline-treated controls showed decreased response to gentamicin at 2 and 4μg/ml and to apermine. Human renal cortical lysososomes also exhibited aminoglycoside- and polyamine-induced changes in NAG release. We conclude taht the lysosome is a site of action for aminoglycoside nephrotoxicity. We propose that aminoglycoside stabilization of this lysosomal membrane may lead to eventual disruption of the proximal tubular lysosomal system and cell injury.     

Netilmicin Sulfate: A Comparative Evaluation of Antimicrobial Activity,Pharmacokinetics, Adverse Reactions and Clinical Efficacy

Netilmicin, the 1-N-ethyl derivative of sisomicin, is a new aminoglycoside antibiotic that was recently marketed in the United States. Its role in therapeutics is not yet established. The pharmacokinetic profile of netilmicin is very similar to that of gentamicin. Its antimicrobial spectrum and clinical efficacy is similar to that of gentamicin, tobramycin and amikacin. It is less active in vitro against Pseudomonas aeruginosa than gentamicin and tobramycin, but in clinical trials the efficacy of netilmicin against this organism has been similar to other aminoglycosides. Netilmicin is active against some gentamicin and tobramycin-resistant strains of gram-negative bacilli, particularly those harboring adenylating and phosphorylating enzymes. Most of these strains are sensitive to amikacin as well, and amikacin is also active against most netilmicin-resistant strains of these bacteria. Therefore, amikacin remains the aminoglycoside of choice against gentamicin tobramycin and netilmicin-resistant gram-negative bacilli. In comparison to other currently available aminoglycosides, a lower frequency of nephrotoxicity and ototoxicity has been observed in laboratory animals given netilmicin. This has not been unequivocally demonstrated in humans. The frequency of nephrotoxicity in humans has been similar to that of other aminoglycosides. The frequency of ototoxicity associated with netilmicin in humans has been low but not significantly less than with other aminoglycosides, except in one trial. If further studies document a significantly lower frequency of ototoxicity with netilmicin, it may become the aminoglycoside of choice for patients with significant risk factors for ototoxicity, such as advanced age, renal impairment, concomitant ototoxic drug therapy and prolonged aminoglycoside administration.     

Acute effects of gentamicin on thick ascending limb function in the rat

It is well established that the aminoglycoside antibiotics can adversely affect proximal tubule function. Predominantly indirect evidence suggests that aminoglycosides may also affect function of more distal nephron segments. The present study utilized whole kidney clearance, in vivo micropuncture and in vitro microperfusion to directly determine whether acute gentamicin treatment affects sodium chloride transport in the thick ascending limb of the loop of Henle. Gentamicin (25 mg/kg) significantly increased urine flow, as well as sodium, potassium and chloride excretion within 15 min of intravenous injection. Glomerular filtration rate and proximal tubule fluid reabsorption were not altered by acute gentamicin treatment. In contrast, both fractional and absolute loop chloride transport was significantly decreased. In the in vitro microperfused medullary thick ascending limb, luminal but not basolateral administration of gentamicin (1 mM) significantly decreased chloride reabsorption when compared to time controls. These data suggest that the increased urine and electrolyte excretion associated with acute gentamicin treatment is, at least in part, a consequence of decreased transport in the thick ascending limb of Henle's loop.     

Aminoglycoside nephrotoxicity. I. Effects of aminoglycoside antibiotics on iodohippurate accumulation in rabbit renal cortical slices.

The effects of aminoglycoside antibiotics on the accumulation of O-125I-hippurate (OIH) in rabbit renal cortical slices were assessed in an attempt to establish an in vitro model for aminoglycoside nephrotoxicity. Accumulation of OIH was measured after incubation of cortex slices in media containing aminoglycosides in different concentrations. All aminoglycosides depressed OIH accumulation in the following minimum concentrations: Dihydrostreptomycin and kanamycin, 2,000 microgram/ml (P less than 0.01); streptomycin and neomycin, 1,000 microgram/ml (P less than 0.05 and P less than 0.01); amikacin and tobramycin, 300 microgram/ml (P less than 0.05); gentamicin, 100 microgram/ml (P less than 0.05). A concentration of 2,000 microgram/ml caused the following reduction in OIH accumulation: Dihydrostreptomycin, 19.3%; streptomycin, 28.9%; kanamycin, 23.8%; neomycin, 62.5%; gentamicin, 68.0%; amikacin and tobramycin, 100%. Changes in pH of the incubation media after addition of aminoglycosides were only partially responsible for the observed depression of OIH accumulation and there was no evidence of substrate competition between aminoglycosides and OIH. The in vitro model described here appears to be inadequate as a sole predictor of aminoglycoside nephrotoxicity, but may provide a supplementary tool in the investigation of aminoglycoside proximal tubular cell toxicity.     

Monitoring aminoglycoside use in patients with severely impaired renal function.

Twenty patients with severely impaired renal function, 17 of wnom had recently transplanted kidneys, were treated with aminoglycosides for severe infections acquired in hospital. Serum aminoglycoside concentrations were closely monitored and dosages adjusted individually to obtain peak and trough concentrations that ensured adequate treatment while avoiding toxicity. Causative organisms were susceptible to treatment in 21 out of 26 episodes of infection (81%), and 12 of the 17 patients (71%) in whom organisms were isolated were cured. Nephrotoxicity attributable to aminoglycosides alone was not observed during the 35 courses of treatment. Ototoxicity occurred in only one patient, who had excessively high serum concentrations of amikacin. Serum aminoglycoside concentrations were directly affected by carbenicillin and flucytosine. The concurrent administration of cephradine and cephalexin with gentamicin may have produced nephrotoxicity. We conclude that aminoglycosides, when carefully monitored, are effective and safe in patients with severely impaired renal function.     

Measurement of total phospholipids in urine of patients treated with gentamicin

Aims The excretion of phospholipids in urine may be a marker of the early renal toxicity of the aminoglycoside antibiotics. Urinary phospholipids are formed in myeloid bodies which develop in the lysosomes of proximal tubules during treatment with the aminoglycosides, and overflow into the urine.
Methods Published assays were modified in order to measure the total phospholipid concentrations in human urine. Phospholipids were extracted from freeze-dried urine samples, digested in concentrated sulphuric acid, and the inorganic phosphorus content determined by complexing with ammonium molybdate and measuring the absorbance at 820  nm. Ten septicaemic patients treated with gentamicin for 5–7 days had significantly higher urine phospholipid concentrations than 10 healthy untreated control subjects ( P <0.0001). There was a negative linear relationship between phospholipid excretion and creatinine clearance ( r ²=0.71).
Results In 34 patients with acute pyelonephritis, increased phospholipid concentrations were observed prior to treatment compared with healthy controls ( P <0.001) and did not alter during treatment with gentamicin. However, the phospholipid concentrations decreased significantly after treatment was completed ( P <0.03).
Conclusions These studies suggest that urinary phospholipids may indicate early aminoglycoside toxicity but with poor specificity, as many of the infections being treated may themselves be associated with phospholipiduria.     

Comparative nephrotoxicity and tissue accumulation of dactimicin, amikacin and gentamicin

Dactimicin (ST 900) is a new pseudo-disaccharide aminoglycoside antibiotic which has been shown to be active against systemic infections in mice. Few data have so far been reported on dactimicin tissue accumulation or its potential nephrotoxicity. In this study, nephrotoxicity and renal tissue concentrations of gentamicin, amikacin and dactimicin were compared in Wistar rats. Liver, heart and lung accumulation of these drugs were also evaluated. Groups of 5 rats were respectively injected with 100 mg/kg body weight of the different drugs daily for 7 days. Five control rats were also injected with saline. Twenty-four hours after the last injection, all rats were sacrificed and bled to death. Blood samples were taken for BUN and serum creatinine assay. Kidney, liver, heart and lung tissues, as well as blood, were removed and processed for microbiological assay of gentamicin, amikacin and dactimicin. The results of this study showed that dactimicin, as well as amikacin, did not induce any significant increase in BUN and serum creatinine, while gentamicin administration resulted in severe uraemia in all rats. Consequently a much higher accumulation of gentamicin than amikacin and dactimicin was achieved in serum and tissues.     

Drug usage evaluation of aminoglycoside-induced nephrotoxicity in a community hospital

A DUE was conducted at this institution to determine the incidence of aminoglycoside-induced nephrotoxicity. The charts of all patients (113) who received an aminoglycoside during the first quarter of 1989 were reviewed. Information gathered included patient age, aminoglycoside used, loading and maintenance doses, serum peak and trough concentrations, changes in serum creatinine during aminoglycoside administration, and culture and sensitivity results. Physicians were inconsistent in prescribing loading doses, while all patients dosed by the pharmacy received an initial dose of 1.5 to 1.75 mg/kg of ideal body weight for gentamicin and tobramycin. Ninety percent of maintenance doses were calculated by the pharmacy. All patients had serum peak concentrations between 3 and 10 micrograms/ml, and only three patients had serum trough concentrations greater than 2 micrograms/ml. No patient demonstrated changes in serum creatinine suggestive of clinically apparent nephrotoxicity. This study suggests that with routine pharmacist intervention (via a pharmacist-managed dosing service), aminoglycosides can be prescribed with a low incidence of nephrotoxicity.     

庆大霉素应用在气腹时的肾毒性实验研究

目的：研究气腹是否会加强大鼠的庆大霉素的肾毒性。方法：收集28只大鼠中24h的尿液。气腹组（n=7),在15mmHg压力下持续2h腹腔注入二氧化碳。庆大霉素组（n=7),静脉注射9mg/kg庆大霉素。气腹结合庆大霉素组（n=7),在气腹前10min给相同剂量的庆大霉素。操作后,收集24h尿液,7d后检测血液肌酐并收集最后24h尿液。对所有的尿液样品进行肌酐和NAG（N-acetyl-beta-glucosaminidase)的定量分析。结果：庆大霉素组和气腹结合庆大霉素这两组表现出第一的NAG排泄明显大于同一天空白组或操作前的量。第一天和第七天尿液的剩余肌氨酸酐和NAG的量同一天空白组和操作前对应的量没有显著的差别。在这些组中第七天的血清肌酐和肌酐清除率没有显著差异。结论：给大鼠静脉注射庆大霉素会使尿液的NAG排出量暂时性增加,NAG排出量在7d内达到稳定。单独2h 15mmHg气腹或合并庆大霉素治疗不会增加大鼠的尿液NAG。而且肯定了庆大霉素加气腹不会使肌酐清除量减少超过60％。这些结果不支持“气腹会加强庆大霉素的肾毒性”的假设。     

Protective effects of fosfomycin on cisplatin-induced nephrotoxicity in patients with lung cancer

SUBJECTS, MATERIAL AND METHODS: Protective effects of fosfomycin on cisplatin-induced nephrotoxicity have been previously reported, however, the proper time, duration and dosage of its administration were uncertain. Therefore, we investigated the protective effect of concurrent administration of twice-daily doses of 2 g fosfomycin for 5 days in 13 cisplatin-na?ve lung cancer patients who were due to receive a single dose per cycle of 100 mg/m2 cisplatin. On each chemotherapeutic cycle, patients were randomly given cisplatin alone or cisplatin plus fosfomycin every 4 weeks for a maximum of 4 consecutive cycles. Indicators of nephrotoxicity, urinary N-acetyl-beta-D-glucosaminidase (NAG) activity, serum creatinine (Scr) and creatinine clearance (Clcr) were determined the day before and at day 3 and day 6 after cisplatin administration. Results were compared and statistically analyzed by the non-parametric Mann-Whitney's test. We found that the NAG activities obtained on day 0, day 3 and day 6 of the fosfomycin cycles were comparable to values obtained during the control cycles (p > 0.05). Moreover, the NAG activities on day 3 of both treatment cycles were significantly elevated from baseline (p < 0.01) and had normalized on day 6. There were no significant changes in serum creatinine and creatinine clearance. CONCLUSION: High-dose cisplatin induced reversible elevation of urinary NAG and concurrent administration of low-dose fosfomycin for 5 days had no effect on the enzymuria. In the prevention of cisplatin nephrotoxicity, a further study using dose escalation (8 to 12 g/d) of fosfomycin administered 2 to 3 days prior to cisplatin are required to demonstrate its nephroprotective effects.     

Diltiazem enhances gentamicin nephrotoxicity in rats

The effect of the calcium antagonist, diltiazem, was examined in gentamicin-induced nephrotoxicity states in rats. Animals were injected for 5 days with diltiazem intraperitoneally (40 mg/kg/day), or gentamicin subcutaneously (100 mg/kg/day) or simultaneously with both preparations using the same doses. At the time of sacrifice, the urea and creatinine clearances, as well as urine osmolality were determined and the renal tissues were processed for examination by light microscopy. Gentamicin-injected rats demonstrated the typical pattern of aminoglycoside nephrotoxicity characterized by poliuric renal failure and necrosis of the proximal tubular epithelium. Rats injected with diltiazem revealed only mild depression of urine osmolality. There was no elevation of blood urea nitrogen and serum creatinine or depression of urea and creatinine clearances, and no focal tubular cell necrosis was detected. However, concomitant administration of both compounds considerably increased nephrotoxicity by according both histological indications and renal function measurements. Thus, we conclude that the combination of diltiazem and gentamicin must be used carefully in human clinical practice.     

Effect of glucarolactam on ototoxicity of aminoglycoside antibiotics in guinea-pigs

Intramuscular administration of glucarolactam in the form of aminoglycoside salt to guinea-pigs protected the experimental ototoxicity caused by high dosing of aminoglycoside antibiotics. The protection was evidenced by the pinna reflex threshold and histochemical examinations of hair cells of cochlea as well as body weight gain. The degree of protection differed with the aminoglycosides, and high protection was observed for dibekacin, gentamicin, tobramycin, followed by kanamycin and bekanamycin. However, protection was weak or not observed when glucarolactam was administered as a mixture of glucarolactam potassium and aminoglycoside sulfate. Serum analysis of the guinea-pigs on day 14 post-administration as a measure of nephrotoxicity revealed that glucarolactam suppressed the elevation of BUN and serum creatinine caused by the aminoglycosides. The protective effect of glucarolactam on the aminoglycoside-induced nephrotoxicity in the dehydrated rats did not differ between the salt and the mixture. No difference in the in vivo antibiotic activity against bacterial infections of mice was observed between the salt and the mixture.     

Distribution of gentamicin among subcellular fractions from rat renal cortex

A substantial amount of data is available to suggest that lysosomal sequestration of aminoglycoside antibiotics plays a role in the pathogenesis of aminoglycoside-induced renal tubule cell injury; however, relatively little information is available on the subcellular distribution of aminoglycosides in the kidney during treatment protocols of the type that ultimately go on to produce extensive lethal renal tubule cell injury and acute renal failure in experimental animals. This study assessed the distribution of gentamicin and subcellular membranes on a discontinuous sucrose density gradient after in vivo exposure of rats to four daily 100 mg/kg doses of gentamicin as compared to in vitro exposure of normal rat renal cortex to gentamicin during tissue homogenization at drug levels comparable to those seen after in vivo treatment. After both in vivo and in vitro exposure, major localization of gentamicin, the lysosomal marker enzyme N-acetyl-beta-D-glucosaminidase (NAG), and the endoplasmic reticulum marker enzyme NADPH-cytochrome c reductase, occurred in a very light membrane fraction. Within this membrane fraction, gentamicin was more closely associated with the NAG than with the NADPH-cytochrome c reductase. The results could not be explained by complete lysosomal disruption during subcellular fractionation after in vivo gentamicin. These data provide additional insights into both the possibilities for subcellular interactions of aminoglycosides in the kidney, and into the methodology required to optimally assess such interactions.     

Aminoglycoside therapy Current use and future prospects

The microbiological, pharmacokinetic, toxicological and clinical aspects of aminoglycosides are reviewed. Aminoglycosides still have an important place in serious infections in neutropenic patients, endocarditis andPseudomonas aeruginosa infections, all in combination with beta-lactams. Monotherapy (with streptomycin) is indicated in less common diseases like tularaemia and bubonic plague. Several experimental studies support a oncedaily dosing regimen for aminoglycosides (comparable or better efficacy with less ototoxicity and nephrotoxicity). Only a very limited number of prospective comparative studies have been performed, and much more data on efficacy, development of resistance and toxicity is needed before once-daily administration can be recommended. The choice of an aminoglycoside should be based primarily on the local sensitivity patterns and cost. Differences in ototoxicity and nephrotoxicity are usually minor. If the acquisition costs of amikacin decline, it is to be expected that amikacin will be the aminoglycoside of choice.     

Further studies of the response of kidney Lysosomes to aminoglycosides and other cations

Rat renal cortical lysosomes were isolated in 0.3 M sucrose containing 1 mM EDTA by differential centrifugation. Lysosomes were incubated in isotonic sucrose or isotonic glycine with various concentrations of endogenous and exogenous compounds at 37° for 1 hr. Lysosomes were resedimented, and the N-acetyl-β-glucosaminidase (NAG) activity was measured in the supernatant fraction and the disrupted pellet and the percentage of total NAG released was calculated. Gentamicin and its C₁ and C₂ components had similar potencies for inhibiting NAG release from lysosomes at low concentrations. The release of alpha-galactosidase and beta-galactosidase from lysosomes was also inhibited by streptomycin and gentamicin. Mepacrine at low concentrations stabilized lysosomes and at high concentrations disrupted lysosomes. This drug also enhanced the effect of low concentrations of gentamicin on lysosomes. Inositol hexaphosphoric acid was a potent antagonist of the effect of low concentrations of gentamicin and mepacrine on lysosomes. Rats were treated with gentamicin at doses of 40, 80 and 160 mg/kg for 1 and 3 days. NAG excretion in gentamicin-treated groups as compared to saline controls was unchanged at day 1. Only the 160 mg/kg treatment group showed a tendency toward elevated renal cortical NAG at day 1 (P < 0.06). All treatment groups had elevated renal cortical NAG at day 3, while the 160 mg/kg group also had elevated NAG excretion. Lysine, arginine, l-canavanine and polymyxin B all affected NAG release from lysosomes in vitro. Lysine enhanced the disruptive effect of high gentamicin concentrations on lysosomes. Ferric and ferrous ions, tested over widely varied concentrations, inhibited NAG release at low concentrations while enhancing NAG release at high concentrations. We therefore conclude that the nephrotoxicity of aminoglycoside and other endogenous and exogenous renally excreted cationic compounds may be produced by their effects on lysosomes in the proximal renal tubule.