Gentamicin nephrotoxicity: failure of three cephalosporins to potentiate injury in rats

Impaired Renal Function Secondary to Gentamicin — Identifying the Special Risk Patient

Although gentamicin has been commercially available since 1969, reports concerning the incidence of nephrotoxicity from this drug and variables relating to this toxicity are still unclear and conflicting. In view of this, a prospective study of patients receiving gentamicin over a two month period was undertaken to determine the incidence of nephrotoxicity and to study the influence of several variables on the potential for developing gentamicin associated nephrotoxicity. The variables studied were patient's age; the total grams of gentamicin received; the total number of days the patient received gentamicin with a hemoglobin of less than 12 g%; sex; total days duration of therapy; hemoglobin prior to therapy; hematocrit prior to therapy; red blood cell count prior to therapy; albumin level prior to therapy; and if the patient received another potentially nephrotoxic drug concomitantly with gentamicin. Sixty patients in total were studied. However, in the “toxic” and “nontoxic” grouping process, seventeen patients were excluded from the study due to missing variables. Of the remaining sample, ten patients were classified as “toxic” and thirty-three were classified as “nontoxic.” Thus the incidence of nephrotoxicity was at least 10/60 or 16.7 percent. The data relating to the variables identified were analysed utilizing Chi-square, t-test, and multiple regression analyses. Two variables were found to be highly significant in relationship to the development of nephrotoxicity while receiving gentamicin therapy. These were (1) the albumin level prior to therapy (lower albumin level in the “toxic” group) and (2) the concomitant use of another potentially nephrotoxic drug. The mechanism behind the influence of albumin on gentamicin toxicity is unclear, but may be related to protein binding. The basis for nephrotoxicity relating to combined use of nephrotoxic drugs is probably additive or synergistic toxicity but this is also unclear.

Until larger prospective studies concerning gentamicin associated nephrotoxicity provide more meaningful information concerning the significance of the variables involved in this adverse reaction, caution is recommended when using this drug in the albumin deficient patient or in combination with nephrotoxic drugs. In addition, it is further recommended that in patients receiving gentamicin, renal function should be closely monitored and the dosage regimen determined accordingly.

Enhanced hexachloro-1:3-butadiene nephrotoxicity in rats with a preexisting adriamycin-induced nephrotic syndrome

Renal damage was assessed by histopathology and urinalysis in male Wistar rats treated with either hexachloro-1:3-butadiene (HCBD; a single 170-mg/kg ip dose that caused proximal tubule necrosis), adriamycin (ADR; a single 5-mg/kg ip dose that caused minimal glomerular changes up to 35 days), or HCBD given 2 wk after ADR and compared with age-matched control rats for 21 days. Urinalysis values in ADR-treated rats showed minimal renal changes. HCBD significantly elevated urine volume (10-fold), protein (5-fold), glucose (175-fold), and brush border enzymes (10-600-fold), indicating severe proximal tubular damage, but most parameters returned to pretreatment levels 6 days after treatment. In ADR-pretreated rats subsequently given HCBD, both the urinary alkaline phosphatase and the ratio of kidney: body weight were significantly higher for longer periods. Histopathology demonstrated that the HCBD-induced proximal tubular lesion was confined to the outer stripe of the outer medulla. Advanced regeneration and repair was evident 21 days after HCBD treatment. In the ADR-pretreated rats the HCBD-induced lesion was more severe and affected the entire cortex and was characterized by marked tubular epithelial calcification, with little evidence of repair and tubular restitution 21 days after treatment. Enzyme histochemistry showed gamma-glutamyltranspeptidase localized to the proximal tubules. After HCBD treatment the enzyme staining was lost and subsequently returned in parallel with histological recovery up to 21 days. The distribution and intensity of gamma-glutamyltranspeptidase was unchanged in ADR-treated rats. The distribution and intensity of gamma-glutamyltranspeptidase in kidneys of ADR-pretreated rats given HCBD had not returned to normal by day 21. The results of this study indicate that pretreatment with ADR increases HCBD-induced nephrotoxic damage and decreases renal cortical repair capacity.

Nephrotoxicity of gentamicin and co-trimoxazole combination in rats

1. The nephrotoxicity of gentamicin is well known. However, little information is available regarding the combined effects of gentamicin plus co-trimoxazole (sulfamethoxazole-trimethoprim). Therefore, Wistar rats were treated daily with 100 mg/kg gentamicin or 100 mg/kg gentamicin plus 30 mg/kg trimethoprim-150 mg/kg sulfamethoxazole for 14 days. 2. Serum biochemical parameters were measured on days 0, 8 and 15, and histopathological examinations of kidneys were performed on day 15, one day following end of treatment. Gentamicin treated rats exhibited a 63% increase in blood urea nitrogen (BUN), a 124% increase in uric acid, and a 63% decrease in serum potassium levels on day 15. 3. The combination of gentamicin plus co-trimoxazole partially ameliorated these effects. With the three drug combination no change occurred in BUN, and only a 30% decrease occurred in serum potassium levels. 4. While serum creatinine levels significantly increased following gentamicin, the co-administration of co-trimoxazole resulted in a significant decrease (30%) in creatinine. Histopathological examinations of kidneys suggested a lower degree of nephrotoxicity in rats treated with gentamicin plus co-trimoxazole as compared to animals treated with gentamicin alone. 5. The results support the importance of monitoring serum biochemical parameters when treating with gentamicin or gentamicin plus co-trimoxazole.

Does gentamicin induce acute renal failure by increasing renal TXA2 synthesis in rats?

Acute renal failure (ARF) induced with large doses of Gentamicin (GM) (an aminoglycoside) was associated with increased urinary TXB (TXA) excretion which provoked a decrease of the ratios of urinary PGE2/TXB2 and 6-keto-PGF1 alpha (PGI2)/TXB2 excretions. Furthermore, as indicated by light microscopy most of the epithelial cells lining the proximal tubules show obvious lesions varying from swelling of their cytoplasm to complete necrosis. Either the inhibitor, OKY-O46, of TXA-synthetase, or volume expansion (VE) with isotonic saline (IS) of the experimental animals diminished urinary TXB excretion which provoked 1) augmentation of the ratios of urinary PGE/TXB and 6-keto-PGF1 alpha/TXB excretions, 2) elevation of creatinine clearance (Ccr) and 3) diminution of proteinuria (PU). This protection against ARF-by OKY-O46 and VE can a can be seen in microscopic sections where necrosis of proximal tubules is almost absent. Only a few proximal tubules show swelling of their epithelial cells and some focal areas of tubule necrosis. We suggest that the metabolites of arachidonic acid (AA), TXA2 a (potent vasoconstrictor agent) and prostaglandins (PGE2 and PGI2), (potent vasodilator factors), play an important role in the development (TXA2) or in the prevention (PGs) of ARF induced by this antibiotic.

Comparative study of aminoglycoside nephrotoxicity in normal rats and rats with experimental cirrhosis

Several authors have suggested that the risk of developing aminoglycoside nephrotoxicity is greater in cirrhotic patients than in the noncirrhotic population. However, this has not been confirmed by other investigators. To compare the intensity and characteristics of aminoglycoside nephrotoxicity in cirrhotic and normal rats, 31 rats with carbon tetrachloride-induced cirrhosis with ascites and 35 control rats were treated with gentamicin. Each group of rats was divided into two subgroups in order to receive 10 or 40 mg per kg per day of gentamicin, and different subsets of animals were killed on Days 4, 8 and 12 of treatment for renal histological examination and determination of renal tissue gentamicin concentration. Urine volume, osmolality, sodium excretion and N-acetyl-beta-D-glucosaminidase activity were measured daily throughout the study. Creatinine clearance and trough plasma concentration of gentamicin were determined in each animal immediately before killing. There were no significant differences between cirrhotic and control rats in relation to the magnitude of changes in urine volume, osmolality, sodium excretion and N-acetyl-beta-D-glucosaminidase activity and creatinine clearance during gentamicin administration. The values of a histopathological score semiquantitatively assessing the renal morphological changes observed by light microscopy were not significantly different in cirrhotic and control rats. In addition, similar trough plasma and renal cortical tissue concentrations of gentamicin were observed in both groups of animals. These results suggest that, in this experimental model, cirrhosis does not increase the risk for aminoglycoside nephrotoxicity.

Adverse drug reactions in the horse

Adverse drug reactions occasionally occur in the horse. The majority can be anticipated and avoided. The practicing veterinarian should understand the various types of adverse reactions as well as their mechanisms so that should such a reaction occur, the practitioner can promptly recognize the problem and institute corrective measures.

Transmission electron microscopy of urinary sediment in aminoglycoside nephrotoxicity

Urinary sediment from 20 patients treated with aminoglycosides (AG) was studied using transmission electron microscopy. For the purpose of comparison, urinary sediment was also studied (control) from an additional 9 patients who had acute renal failure (ARF) but who did not receive AG (5 posttransplant, 4 postsurgical). Urinary myeloid bodies and renal tubule cells were analyzed semiquantitatively. The diagnosis of AG nephrotoxicity (or ARF) was made on the basis of a rise in serum creatinine greater than or equal to 0.5 mg/dL from the baseline levels. Among 20 patients, 12 developed AG-ARF, and 11 of these 12 showed myeloid bodies and necrotic renal tubule cells in their urinary sediment. Of the 8 patients that did not develop AG-ARF, 5 showed myeloid bodies and 2 of these also showed renal tubule cells in their urinary sediment. This incidence of necrotic renal tubule cells in the nephrotoxic group is significantly higher than in the nonnephrotoxic group (p less than 0.01). Although no statistical difference was found in the incidence of myeloid bodies between the two groups, the number of myeloid bodies was significantly (p less than 0.05) greater in the nephrotoxic group than in the nonnephrotoxic group. Furthermore, consecutive sediment studies revealed that the appearance of necrotic renal tubule cells (and not of myeloid bodies) coincided with the increase in serum creatinine. All control patients showed necrotic renal tubule cells but no myeloid bodies in their urinary sediment. Thus this study suggests that the presence of necrotic renal tubule cells signifies ARF, and when preceded or accompanied by large numbers of myeloid bodies that it indicates AG-ARF.

The influence of cephem antibiotics on gentamicin nephrotoxicity in normal, acidotic, dehydrated, and unilaterally nephrectomized rats

Normal, acidotic, dehydrated, or unilaterally nephrectomized rats (Sprague-Dawley) were injected with gentamicin (80 mg/kg/day) alone or in combination with cefoxitin (2 g/kg/day), cephalothin (2 g/kg/day), or cephaloridine (1 g/kg/day) once daily for 10 consecutive days. Serum creatinine levels and renal cortical concentrations of gentamicin were determined and histopathological examination of the kidneys was made. Gentamicin caused marked renal damage in all the groups of rats. In normal rats, all the cephem antibiotics significantly reduced both gentamicin nephrotoxicity and renal gentamicin levels. In acidotic, dehydrated, and unilaterally nephrectomized rats, cefoxitin still significantly reduced gentamicin nephrotoxicity without affecting renal gentamicin concentration, whereas cephalothin and cephaloridine did not show any significant reduction of nephrotoxicity or gentamicin concentration, but rather caused general deterioration of the animals. Hence, the intrinsic nephrotoxic potential of cefoxitin, cephalothin, and cephaloridine may contribute to the difference in the combined effect rather than the concentration of gentamicin in the kidney.

Specificity of renal tubular damage criteria for aminoglycoside nephrotoxicity in critically ill patients

Two populations of critical care patients were studied using indices of renal tubular damage (beta 2-microglobulin, enzymes, casts) and indices of glomerular filtration (creatinine, creatinine clearance). The purpose of these studies had been initially to elucidate the type of renal failure typical of the critically ill patient treated with aminoglycoside gentamicin or tobramycin, then to determine its frequency. The second study population included a control group of patients given the nonnephrotoxic cephalosporin moxalactam, in order to assess the specificity of the renal tubular damage criteria for aminoglycoside nephrotoxicity versus other types of renal injury in critical care patients. Creatinine rise occurred in approximately 30 per cent of each tobramycin-treated group and in only 12 per cent in the moxalactam control patients (P less than 0.05). Thus, the data indicate that aminoglycosides are associated with an approximate doubling of the renal damage in those older, critically ill patients. Renal tubular damage criteria appear specific for the aminoglycoside effect, but a substantial percentage of the renal damage in this population is not associated with detectable alterations in renal tubular status.

Clinical and pathophysiologic aspects of aminoglycoside nephrotoxicity

Aminoglycoside antibiotics continue to be a mainstay of therapy in the clinical management of gram negative infections, but a major factor in the clinical use of aminoglycosides is their nephrotoxicity. With gram negative organisms accounting for the majority of hospital acquired infections, the occurrence of aminoglycoside induced acute renal failure has become commonplace. Presently at least 10% of all cases of acute renal failure can be attributed to these antibiotics. This article will cover the renal handling of the aminoglycosides, the pathogenetic mechanisms of nephrotoxicity, and the clinical aspects of aminoglycoside induced acute renal failure with particular emphasis on recent data which have increased our understanding of the interaction of aminoglycosides with the renal tubular cell and the effects of this interaction on cellular function and integrity.

Tobramycin nephrotoxicity: failure of cefotaxime to potentiate renal toxicity

The aim of our prospective clinical study was to determine whether the combination of tobramycin plus cefotaxime is more nephrotoxic than tobramycin alone. The studies were carried out in 30 patients with serious infections and normal renal function. Groups of ten patients each received either 2 g cefotaxime or 1 mg/kg body weight tobramycin or cefotaxime and tobramycin in the same dosage every eight hours intravenously for at least seven days. Serum creatinine, creatinine clearance and alanine aminopeptidase (AAP) excretion in 24-hour urine were determined before, during and five days after the antibiotics had been discontinued. These were used as parameters for glomerulotubular injury. The plasma levels of tobramycin and cefotaxime (assayed by agar diffusion) did not differ when the drugs were given alone or in combination. None of the patients treated with cefotaxime alone showed any signs of renal damage. In contrast, tobramycin alone or in combination with cefotaxime caused an increase in urinary enzymes in all patients. This activity was a mean five to six times greater than the initial values prior to antibiotic therapy. There were no significant differences between the AAP increase during treatment with tobramycin and tobramycin plus cefotaxime. Four to five days after discontinuing antibiotic therapy, AAP activity decreased to values similar to those measured prior to therapy. In some of the patients receiving tobramycin alone or in combination with cefotaxime, increased creatinine levels, a reduction in creatinine clearance to 60 ml/min and an increase in renal enzyme excretion could be observed. Thus, treatment with high doses of cefotaxime does not seem to increase tobramycin nephrotoxicity in patients with normal renal function. The nephrotoxicity of this drug combination is obviously due to the aminoglycoside.

Cephalosporin-aminoglycoside synergistic nephrotoxicity: fact or fiction?

A review of synergistic nephrotoxicity associated with the concomitant administration of aminoglycoside and cephalosporin antibiotics is presented. A combination of these antibiotics is frequently administered in clinical practice as initial therapy in the treatment of gram-negative bacillary infection. The nephrotoxicity associated with cephalosporin/aminoglycoside administration in humans has been characterized as an acute tubular necrosis. Attempts to investigate this type of toxicity in animals using the rat as the model have revealed that the rat kidney is not affected by the aminoglycoside/cephalosporin combination in the same manner as the human kidney. The results from studies using the rat were not predictive of the nephrotoxicity encountered in humans, and cephalosporins actually appear to protect the rat from aminoglycoside-induced renal damage. The mechanism of the protective effect in rats and the toxic effects in man remain unknown. A species differentiation clearly exists between man and rats with respect to the nephrotoxic effects of aminoglycoside/cephalosporin combinations. The weight of present evidence indicates that, in man, an aminoglycoside/cephalosporin combination is synergistically nephrotoxic.

Pathogenesis of renal failure due to aminoglycosides and contrast media used in roentgenography

The etiology of acute renal failure has changed in recent years due to the recognition of drug nephrotoxicity as a more common cause. In this communication we emphasize recent information concerning the pathophysiology of nephrotoxic acute renal failure produced by aminoglycoside antibiotics and the contrast media used in roentgenography. The aminoglycosides are excreted primarily by glomerular filtration; however, net tubular reabsorption and renal parenchymal accumulation do occur. The exact mechanism of uptake is not clear, but the luminal membrane seems primarily involved. The pathogenesis of nephrotoxicity, although probably linked to cortical accumulation, is complex since experimental animals recover from gentamicin-induced renal failure despite continued administration of the drug. Knowledge of the precise cellular mechanisms of injury awaits further studies. Histologic damage is usually limited to proximal tubular necrosis and, clinically, the renal failure is nonoliguric. Although reports of the contrast media used in roentgenography producing acute renal failure have increased, the pathogenesis is unclear. Evidence supporting various theories is reviewed.

Protection from gentamicin nephrotoxicity by cephalothin and carbenicillin

In rats, cephalothin exerts a protective effect upon the nephrotoxicity of gentamicin. To examine the possibility that this effect is also observed with carbenicillin, we gave the following (milligrams per kilogram) to rats daily for 14 days: gentamicin alone, 60; gentamicin plus cephalothin, 100, 500, or 1,000; gentamicin plus carbenicillin, 50, 100, 250, 500, or 1,000. A 500-mg/kg dose of cephalothin afforded significant partial protection from gentamicin nephrotoxicity, as did a 100-mg/kg dose of carbenicillin. Increasing doses of either drug failed to increase protection. The data suggest that in rats not only does carbenicillin afford some protection from gentamicin nephrotoxicity, but also that it does so at a lower dose than cephalothin. These findings may in part explain the divergent observations regarding the nephrotoxicity of cephalothin-gentamicin combination therapy in rats and humans.

Cephalothin plus an aminoglycoside is more nephrotoxic than methicillin plus an aminoglycoside

In a prospective, randomised, double-blind trial to determine if cephalothin plus an aminoglycoside is more nephrotoxic than methicillin plus an aminoglycoside, patients were assigned to one of four treatment groups: cephalothin and gentamicin (C.G.), cephalothin and tobramycin (C.T.), methicillin and gentamicin (M.G.), or methicillin and tobramycin (M.T.). The incidence of definite nephrotoxicity was: C.G., 7/23 (30.4%); C.T., 5/24 (20.8%); M.G., 2/20 (10%); and M.T., 1/23 (4.3%). There was no statistically significant difference in nephrotoxicity between the combined gentamicin groups (C.G. and M.G.) and the combined tobramycin groups (C.T. and M.T.). Definite nephrotoxicity developed in 12/47 (25.5%) of the combined cephalothin groups (C.G. and C.T.) and in only 3/43 (7%) of the combined methicilllin groups (M.G. and M.T.). The combination of cephalothin plus an aminoglycoside is therefore more nephrotoxic than the combination of methicillin plus an aminoglycoside.

Effects of various cephalosporins on the proximal tubule of the human kidney

Cephamandol 6.0 g, cephazolin 6.0 g or cephacetrile or cephalothin 8.0 g were administered as short-term infusions on 3 consecutive days to informed volunteers, who had no history or evidence of impairment of renal function. There were 15 subjects in the cephamandol, cephacetrile and cephalothin groups and 14 subjects in the cephazolin group. Alanine-aminopeptidase, a characteristic tubule enzyme, was determined in a 24-hour urine 2 days before administration, during the 3 day administration and on the 4 subsequent days. In addition, alanine-aminopeptidase was also estimated immunologically in concentrated urine with the aid of an anti-brush border antibody. Cephamandol, cephazolin and cephalothin were completely without effect on the proximal tubule. Cephacetrile, on the other hand, showed clear reactions in 9 out of 15 subjects, in the form of elevated AAP activity in urine and in 6 of the cases membrane elimination was demonstrable immunologically. After withdrawal of the medication, the values of the responder group returned spontaneously to normal, i.e. no cumulative effect was detected. These investigations show that elimination of alanine-aminopeptidase in the urine is a very sensitive index of the action of cephalosporins on renal tubules.

Nephrotoxicity of netilmicin in combination with non-aminoglycoside antibiotics

To assess the possibility that non-aminoglycoside antibiotics may adversely affect the nephrotoxicity of the new semisynthetic aminoglycoside netilmicin, we gave ampicillin, carbenicillin, methicillin, cefamandole, and clindamycin, either singly or in combination with netilmicin, at two dose concentrations in rats. Results were compared as to the effect of netilmicin given singly and to saline-injected and noninjected controls. Antibiotic combinations resulted in no greater nephrotoxicity than did netilmicin alone. Netilmicin concentrations in renal tissue were high, and these levels were not consistently affected by the other drugs. The data suggest that in rats the nephrotoxicity of netilmicin is not affected adversely by the presence of non-aminoglycoside antibiotics.

Daily single-dose gentamicin therapy in experimental pyelonephritis

The therapeutic efficacy of gentamicin given once or three times a day was compared in a model of experimental renal infection in rats. The same amount of gentamicin given either in a single injection or three injections a day produced no statistically significant difference in the treatment of incipient infection. The effect of this mode of administration on advanced infection depended on the length of the therapy. After five days, administration of the same dose given in a single injection or three injections did not result in significant differences. After ten days the therapy proved more effective when gentamicin was injected three times a day. In comparison a single daily dose, amounting to two thirds of the total dose when given three times a day every eight hours, revealed after five days of therapy a statistically significantly lower bacterial count in the kidney than three daily injections of gentamicin.

Effect of cefamandole nafate on the toxicity of tobramycin

Because of the potential for an interaction between cephalosporins and aminoglycosides leading to renal injury, an evaluation of the effect of a new cephalosporin, cefamandole nafate, on the toxicity of the aminoglycoside tobramycin was performed in laboratory animals. High doses of cefamandole nafate did not increase the acute toxicity (lethality) of tobramycin in rats or mice. In a subacute experiment in rats, dose-related tobramycin nephrotoxicity, as evidenced by blood urea nitrogen changes, increased kidney weights, and histologically determined tubular nephrosis and necrosis, was observed. Concomitant treatment with cefamandole nafate, 500 mg/kg, did not increase tobramycin nephrotoxicity, but protected against the aminoglycoside-induced renal injury. Determination of tissue radioactivity after administration of [(14)C]tobramycin indicated that cefamandole nafate treatment resulted in uniformly lower tobramycin tissue concentrations compared with the control, suggesting that the protective effect was produced by enhanced excretion of tobramycin after cefamandole nafate treatment.

Gentamicin nephrotoxicity--morphologic and pharmacologic features

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Nephrotoxicity of cephalosporin-gentamicin combinations in rats

TO STUDY THE POSSIBILITY THAT CEPHALOSPORINS AUGMENT THE NEPHROTOXICITY OF GENTAMICIN, GROUPS OF RATS WERE GIVEN FOUR HOURLY SUBCUTANEOUS DOSES OF: gentamicin (5 mg/kg), gentamicin plus cephalothin (100 mg/kg), gentamicin plus cefazolin (20 mg/kg), gentamicin plus cefazolin (50 mg/kg), gentamicin plus cephaloridine (50 mg/kg), or saline diluent for 15 days. Periodic measurements were made of urine volume, urine osmolality, urine protein excretion and lysosomal enzymuria, as well as blood urea nitrogen, creatinine clearance, and drug concentrations in renal cortex and medulla. Tissue was examined by light and electron microscopy. Enzymuria and proteinuria increased early in the course of all treatment groups, whereas urine osmolality declined. No distinct patterns of these variables were discernable among the groups. Gentamicin alone, gentamicin plus cephalothin, and gentamicin plus cefazolin (20 mg/kg) caused the same significant fall in glomerular filtrate rate from control values by day 15 (P < 0.05). Gentamicin plus cefazolin (50 mg/kg) and gentamicin plus cephaloridine failed to cause a decline in glomerular filtration rate compared with controls (P > 0.05). Gentamicin concentrations in renal cortex were 5 to 10 times higher than those in medulla in all groups. Cephaloridine and cefazolin (50 mg/kg) also displayed a gradient pattern in renal cortex, whereas cephalothin and cefazolin (20 mg/kg) did not. Cytosegrosomes with myeloid figures were characteristic ultra-structural changes seen in all groups; however, they tended to be smaller with less numerous myeloid bodies in the groups receiving gentamicin plus cephalothin, cefazolin (50 mg/kg), or cephaloridine. Cephalosporins did not augment gentamicin toxicity. High doses of cefazolin and cephaloridine protected kidneys from gentamicin nephrotoxicity. The protection may involve intracellular drug interaction within the renal cortex.

Effect of cephalothin on renal cortical concentrations of gentamicin in rats

Renal cortical concentrations of gentamicin were significantly lower in rats given this aminoglycoside and cephalothin simultaneously than in animals given gentamicin alone. This effect may be responsible, in part, for the reduction in nephrotoxicity reported previously in animals given the combination of drugs.

Protective effect of cephalothin against gentamincin-induced nephrotoxicity in rats

The possibility that the nephrotoxicity of gentamicin may be potentiated by the concomitant administration of cephalothin was examined in a rat model. Cephalothin given once daily in dosages up to 800 mg/kg per day for 10 days produced no renal damage. Gentamicin, at 6 to 50 mg/kg per day, caused pathological changes which were dosage related and affected primarily the proximal tubular cells. Administration of the two drugs simultaneously resulted in a significant protective effect of cephalothin against gentamicin-related nephrotoxicity (P < 0.01). When the daily injections of the two agents were separated by an interval of 6 h, the protective effect was lost, and the resultant damage was the same as that due to gentamicin alone. The protective effect of cephalothin was reproduced by the administration of equiosmolar amounts of sulfate (sodium sulfate), suggesting that the phenomenon might be related to the presence of nonresorbable anion in the urine. These studies indicate that, in the rat, cephalothin does not potentiate, but, in fact, may prevent the nephrotoxic effects of gentamicin.