Evaluation of novel antipseudomonal drugs using the serum bactericidal activity test

Serum bactericidal activity against Pseudomonas aeruginosa was determined in six volunteers 1 and 4 h after administration of 2 g ceftazidime, 4 g piperacillin, 500 mg imipenem, 80 mg tobramycin and four combinations of these agents. Ceftazidime produced the highest serum bactericidal titers, killing 100% and 86% of the 50 Pseudomonas aeruginosa strains tested after 1 and 4 h respectively at a serum dilution of 1:8. Imipenem had lower serum bactericidal titers than ceftazidime, killing 88% of the isolates after 1 h at a serum dilution of 1:8. The combination showed only slightly higher titers. Killing curves were determined for nine strains of Pseudomonas aeruginosa using undiluted volunteer serum drawn 1 h after administration of the antibiotics. The combinations ceftazidime/tobramycin and piperacillin/tobramycin exhibited higher killing activity than the single drugs. As the activity of the aminoglycosides could be underestimated on the basis of their low serum bactericidal titers, it is concluded that determination of these titers is inappropriate for evaluating the efficacy of the aminoglycosides.

Comparative study of serum bactericidal activity of cefotaxime alone or in combination with tobramycin

The objectives of this study were to investigate the bactericidal activity in serum of cefotaxime alone or in combination with tobramycin against clinical strains and to determine the influence of tobramycin on the pharmacokinetics of cefotaxime. The peak bactericidal activity in serum of cefotaxime alone against Klebsiella oxytoca, Enterobacter aerogenes, Serratia marcescens, Pseudomonas cepacia, and Listeria monocytogenes varied between 1:4 and 1:256. Bactericidal activity could still be detected at 6 h against K. oxytoca and L. monocytogenes. The addition of tobramycin increased the bactericidal activity of cefotaxime against E. aerogenes from 1:16 to 1:128 (P less than 0.01). Cefotaxime recovery from urine was significantly decreased when tobramycin was added. Our data are comparable with those of other investigators who have shown a limited increase in the bactericidal activity of cefotaxime when aminoglycosides are added.

Studies on systemic absorption of tobramycin in polyethylene glycol ointment applied to wounds of burn patients

The penetration of tobramycin ointment (0.2 per cent) through human burn wounds was studied in two patients with superficial dermal burn (SDB) and deep dermal burn (DDB), two patients with DDB, one patient with full thickness burn (FB) and one patient with burn ulcer (BU). After application of the ointment, the absorption of tobramycin occurred promptly with a peak level at 2-6 h in the sera of patients with SDB + DDB and DDB only. In the BU patient it was found at 2 h; while in the FB, at 10-12 h. The excretion of tobramycin into the urine was also studied. The absorption rate constants (ka) of the two SDB + DDB patients were 0.31 and 0.74, of the two DDB patients, 0.079 and 0.18, and those of the DB and the BU patients, 0.053 and 0.95, respectively. The absorption ratios of tobramycin (amount excreted into the urine during 24 h/applied dose) were different, depending on the condition of the burn wound, and ranged from a low of 10.3 per cent (FB) to a high of 44.0 per cent (BU). These results show that the treatment of burn patients with tobramycin ointment to control burn wound infection should be carefully supervised, especially in the case of BU.

Tobramycin pharmacokinetics in premature identical twins during newborn period

Tobramycin is frequently used in premature infants but little is known about its pharmacokinetics in identical twins during the newborn period. We studied 6 twin infants (gestational age 29-31 weeks; postnatal age 3-4 days; birth weight 1.0-1.3 kg) receiving tobramycin 2.5 mg/kg i.v. over 20 min every 12-18 h. Steady-state peak and trough serum concentrations of tobramycin were in the range of 5.3-8.4 and 1.2-2.0 micrograms/ml, respectively. Total clearance (ClT) ranged from 0.74 to 1.19 ml/min/kg, distribution volume (V) from 0.74 to 0.94 liter/kg, and elimination half-life (t1/2) from 8.2 to 12.8 h. Comparison of data between infants in three identical twin pairs showed that despite a similar infusion method: (a) the time to achieve peak serum concentration ranged from 0.5 to 2.0 h; (b) the peak and trough concentrations normalized for dose varied from 0 to 50%, and (c) the ClT, V and t1/2 varied from 3 to 20%. These data should be considered in therapeutic drug monitoring of tobramycin in premature, identical twins during the newborn period.

Comparison of venous and capillary blood sampling for the clinical determination of tobramycin serum concentrations

Tobramycin concentrations have been determined in serum from capillary, venous and arterial blood samples taken from 16 patients during and after surgery. In 73 paired samples the concentrations in capillary samples were not significantly different from those measured in venous samples. The small concentration differences were neither dependent upon sampling time nor core-peripheral temperature differences. In 26 paired samples, concentrations in capillary samples were not significantly different from those determined in arterial samples. We conclude that concentrations in capillary samples are precise and unbiased estimators of venous concentrations and may be used in the adjustment of tobramycin dosage regimens.

In vitro inactivation of tobramycin by cephalosporins

The in vitro inactivation of tobramycin when combined with each of six cephalosporins in samples of human serum was investigated. Each of six cephalosporins (cefazolin sodium, cefoxitin sodium, cefamandole nafate, moxalactam disodium, cefoperazone sodium, and cefotaxime sodium) was added to human serum samples containing tobramycin sulfate 8 micrograms/mL to produce final cephalosporin concentrations of approximately 250 and 1000 micrograms/mL. Duplicate solutions were prepared and stored at either 0 or 21 degrees C. Solutions containing tobramycin 8 micrograms/mL alone and with carbenicillin disodium in four concentrations were prepared as controls. Samples were assayed using a fluorescence polarization immunoassay (TDX) at 0, 2, 4, 8, 12, 24, and 48 hours to determine tobramycin concentration; two of the carbenicillin-tobramycin solutions were frozen immediately for assay 53 hours later. Tobramycin concentrations in the admixtures were compared with those in tobramycin reference samples. At both temperatures, samples containing tobramycin with cefamandole 250 micrograms/mL or cefotaxime 250 micrograms/mL showed less than 10% inactivation of tobramycin for at least 48 hours. At 0 degrees C, tobramycin retained greater than 90% activity when combined with cefoperazone 250 and 1000 micrograms/mL. In samples containing cefazolin 250 micrograms/mL at 0 degrees C and cefoperazone 250 micrograms/mL at 21 degrees C, tobramycin was stable for 24 hours. Only samples containing moxalactam stored at 21 degrees C showed greater than 16% inactivation of tobramycin at 48 hours. Under these study conditions, tobramycin is only moderately inactivated in vitro when combined with clinically achievable concentrations of the tested cephalosporins (excluding moxalactam) and then stored for up to 48 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and extravascular penetration of aztreonam in patients with abdominal sepsis

Patients with abdominal sepsis were enrolled in a clinical trial of aztreonam vs. tobramycin. All were given clindamycin concomitantly. The pharmacokinetics of aztreonam in 21 patients randomly assigned to receive treatment with aztreonam are reported. The mean age of these patients was 68 years; most had underlying disorders such as malnutrition and cardiac or pulmonary disease. Creatinine clearance (Clcr) ranged from 11.2 to 133.1 ml/min. The usual dose of aztreonam was 2.0 g every 8-12 hr. A single pharmacokinetic study was performed over one dosing interval after steady-state conditions were achieved. In approximately one-half of the patients, peritoneal fluid was collected during the interval between doses. Penetration of aztreonam, as expressed as the ratio of concentration in the peritoneal fluid to that in serum, was higher for aztreonam (0.95:1) than for tobramycin (0.46:1). The ratio of the concentration in peritoneal fluid to the minimum inhibitory concentration (MIC) of the infecting bacteria was also higher for aztreonam. Serum pharmacokinetic data were analyzed by both two-compartment and moment analysis. For both the steady-state volume of distribution (Vdss) and total body clearance (TBC), the values determined by both methods were highly correlated (r = .96, .99, respectively). Average values for Vdss and TBC were 0.28 liters/kg and 80 ml/min. TBC for aztreonam correlated strongly with CLcr and was described by the regression equation TBC = 1.1 (Clcr) + 1.6, r = .87, P less than .01.

Tobramycin therapy--two or three doses per day?

11 patients with moderate infections were treated with bolus injections of tobramycin in a fixed dose of 4.5 mg/kg and day for 8 days. The patients were randomized to injections every 8 h (TID) for 4 days followed by injection every 12 h (BID) or vice versa. Serum concentrations were measured on day 2 of each regimen. The TID regimen gave peak levels less than 4 micrograms/ml in 5/11 patients, mean value 4.2 micrograms/ml. With the BID regimen 2 patients had peaks less than 4 micrograms/ml, mean value 5.9 micrograms/ml. The BID regimen seems preferable when treating patients with normal renal function. The bolus injections were well tolerated.

Comparison of ciprofloxacin with azlocillin plus tobramycin in the therapy of experimental Pseudomonas aeruginosa endocarditis

The efficacy of ciprofloxacin (Bay o 9867), a promising new quinolone, was compared with the efficacy of azlocillin plus tobramycin in rabbits with experimentally induced Pseudomonas aeruginosa endocarditis. The MBCs of ciprofloxacin, azlocillin, and tobramycin against the test strain were 0.5, 8, and 4 micrograms/ml respectively. Ciprofloxacin at a concentration of 50 mg/kg or azlocillin at a concentration of 200 mg/kg in combination with tobramycin at a concentration of 5 mg/kg was administered intramuscularly at 8-h intervals for 4 days. Both regimens produced median peak serum bactericidal titers of 1:8. The concentrations of ciprofloxacin, azlocillin, and tobramycin in serum, 1.8 +/- 0.7, 154 +/- 48, and 9.1 +/- 2.4 micrograms/ml (mean +/- standard deviation), respectively, closely approximated concentrations found in humans after accepted dosages. At the end of treatment, the titers of P. aeruginosa were 3.0 +/- 1.6 log10 CFU/g of vegetation (mean +/- standard deviation) for recipients of ciprofloxacin and 3.2 +/- 1.3 log10 CFU/g of vegetation for recipients of azlocillin plus tobramycin. These values compared with control titers of 7.3 +/- 1.6 CFU/g. These data indicate that at the doses used, ciprofloxacin was as effective as azlocillin plus tobramycin in the treatment of P. aeruginosa endocarditis in rabbits. Since the latter drug combination has proven efficacy, ciprofloxacin deserves further evaluation in the therapy of systemic infections in animal models and in humans.

Pharmacokinetics of tobramycin in patients with cystic fibrosis. Implications for the dosing interval

The pharmacokinetics of tobramycin were evaluated in 15 patients (8 to 22 years of age) with cystic fibrosis (CF). A dose of 3.0 to 3.3 mg/kg of body weight was given intravenously over 20 minutes, and concentrations in serum were followed up to eight hours after initiation of the infusion. In the calculation of pharmacokinetic parameters, a two-compartment open model was used. The elimination half-life of the drug was highly inversely correlated with age (p less than 0.0004), and body weight (p less than 0.00002). Total body clearance (TBC), and volume of distribution at steady state (VDSS) were directly correlated with age and body weight. However, when TBC and VDSS were corrected for BSA, no correlation could be demonstrated. The mean one-hour and eight-hour serum concentrations of tobramycin were 5.40 and 0.45 microgram X ml-1, respectively. Between patients, considerable differences were found in the time after administration at which the serum concentration decreased below 1 microgram X ml-1. This interpatient variation has clinical implications for tobramycin therapy in CF, in particular for the dosing interval.

Dosage of tobramycin in cystic fibrosis--a short report

A 15 kg, 7-year-old girl with cystic fibrosis was admitted with a chest infection. It was found that a tobramycin, 12 mg/kg/day (standard recommended dose 6-7.5 mg/kg/day), was required to obtain acceptable plasma levels and that the volume of distribution varied in line with the patient's weight changes.

Attenuation of experimental tobramycin nephrotoxicity by ticarcillin

It is well known that in vitro the combination of carbenicillin, ticarcillin, or other antipseudomonal penicillins with gentamicin, tobramycin, or other aminoglycoside antibiotics results in the inactivation of the antibacterial activity of the aminoglycoside. To assess the influence of the in vivo interaction of tobramycin and ticarcillin on experimental nephrotoxicity, male Fischer 344 rats were given either tobramycin alone (120 mg/kg per day), tobramycin (120 mg/kg per day) and ticarcillin (250 mg/kg per day) concomitantly, or the combination of these drugs at the same doses that had been preincubated for 24 h and at the time of delivery contained but 63 and 25%, respectively, of the initial concentrations of tobramycin and ticarcillin as measured by conventional analytical procedures. Initial experiments were conducted to determine the concentrations of the antibiotics in serum achieved after administration of each test solution. After a single dose of the test solution, ticarcillin concentrations in serum were higher and more prolonged in rats given tobramycin plus ticarcillin than in rats given ticarcillin alone. After 7 days of exposure to the test solutions, inulin clearance in animals given tobramycin alone was 0.15 +/- 0.1 (mean +/- 2 standard errors) ml/min per 100 g of body weight as compared with 0.53 +/- 0.1 in rats given tobramycin and ticarcillin concomitantly, 0.59 +/- 0.1 in animals given the partially inactivated tobramycin-ticarcillin mixture, and 0.79 +/- 0.1 in control rats. Although there was some improvement in inulin clearance in the group containing tobramycin alone, the three treatment groups maintained the same rank relationship in inulin clearance through 14 days of treatment. Real histology confirmed the attenuation of tubular injury in animals given tobramycin and ticarcillin concomitantly. There was no evidence of toxicity from the presumed inactivation complexes of tobramycin-ticarcillin. These results document an in vivo protective effect of ticarcillin on experimental tobramycin nephrotoxicity.

Measurement of tobramycin concentrations in saliva

Paired serum and saliva specimens were obtained from 16 patients and three volunteers who had received tobramycin. In addition, saliva was collected from ten controls who had not taken any drugs in the previous week. Tobramycin concentrations were measured in duplicate by the Emit assay modified to obtain precise results at concentrations greater than 0.25 microgram/ml. The drug was undetectable in 32 of 34 saliva specimens and only trace amounts were present in the other two, even though corresponding steady-state serum concentrations were greater than or equal to 4.0 micrograms/ml for 29 of 34 specimens. Tobramycin values were zero for all ten saliva specimens from controls. These results probably can be extrapolated to other aminoglycoside antibiotics since most are hydrophilic and ionized at physiologic pH, which disfavors penetration into saliva. We conclude that saliva cannot be used as a noninvasive indirect method of measuring aminoglycoside serum concentrations to guide dosage adjustments.

Aminoglycoside dosing in obese puerperal women

We compared serum aminoglycoside levels in two groups of obese puerperal women being treated for endometritis. In these obese puerperal women (weights as much as 100 kg), the aminoglycoside dose did not have to be adjusted for excess weight initially, but it seems that determination of antibiotic levels should be performed in view of the large doses.

Quantitative determination of dibekacin using radioimmunoassay, substrate-labelled fluorescent immunoassay and rate nephelometric inhibition immunoassay for tobramycin

Radioimmunoassay, rate nephelometric inhibition immunoassay and substrate-labelled fluorescent immunoassay were employed for the quantitative determination of dibekacin in serum. The cross-reactivity of the antibody provided with each assay allowed the use of tobramycin assay procedures for measuring dibekacin concentrations. With radioimmunoassay and nephelometric immunoassay, a dibekacin calibration curve was required, whereas fluorescent immunoassay was directly suitable for dibekacin assay, with cross-reactivity of nearly 100%. This allows the purchase of one assay kit for testing two antibiotics and thus reduces the cost to medical laboratories.

Pharmacokinetics of ceftazidime, alone or in combination with piperacillin or tobramycin, in the sera of cancer patients

We administered 2 g of ceftazidime intravenously every 8 h to cancer patients for the empiric therapy of febrile episodes. Ceftazidime was administered as monotherapy for patients with granulocyte counts in excess of 1,000/microliter. Febrile, neutropenic patients were randomized to also receive either piperacillin or tobramycin. The pharmacokinetic profile of ceftazidime during a steady-state dosing interval was ascertained in 21 patients. No differences were seen between groups for any of the pharmacokinetic parameters examined. As expected, the observed half-life was longer, the serum clearance was smaller, and the volumes of distribution were larger than in previously reported studies of volunteers. Serum concentrations remained above the MIC for inhibition of 90% of strains of the most common bacteremic pathogens seen in our cancer center for the entire 8-h dosing interval.

Comparative ototoxicity of netilmicin, gentamicin, and tobramycin in cats

Netilmicin, a semisynthetic aminoglycoside antibiotic, is less ototoxic in a variety of species than other aminoglycosides currently in therapeutic use. In this study, mixed-breed cats (four/group) were given daily sc injections of netilmicin (20, 40, and 80 mg/kg), gentamicin (20 and 40 mg/kg), or tobramycin (20, 40, and 80 mg/kg) for up to 30 weeks or until ototoxicity was observed. The animals were examined throughout the study for effects on cochlear and vestibular function. Hematologic, serum chemical, and drug-serum (24-hr postdose) assays were performed at approximate monthly intervals during the dosing period. The cochleae, kidneys, and liver were examined microscopically. The mean number of dose days required to produce vestibulotoxic effects, demonstrated by impaired righting reflex or locomotor ataxia, was from 41 to 61 in cats dosed with tobramycin (40 and 80 mg/kg) or gentamicin. No vestibular dysfunction was observed in any of the netilmicin 20-mg/kg-dosed cats, in two cats each of the tobramycin 20-mg/kg and netilmicin 40-mg/kg groups, and in one netilmicin 80-mg/kg-dosed animal. Histologic examination of the cochleae revealed degeneration of the hair cells and supporting sensory structures in the majority of cats dosed with gentamicin at 20 and 40 mg/kg and tobramycin at 40 and 80 mg/kg. Less than 50% of the tissues from cats of the tobramycin 20-mg/kg and netilmicin 40- and 80-mg/kg-dosed groups had similar degenerative cochlear changes. No cochlear damage was noted in any of the cats given netilmicin at 20 mg/kg. Results of the clinical laboratory determinations were generally unremarkable. Proximal tubular degeneration was the principal finding observed in the kidneys of the animals. Under the conditions of this study, at least a twofold (vestibular) to fourfold (cochlear) relative safety margin for ototoxicity was established in favor of netilmicin over tobramycin and gentamicin.

Effect of various storage conditions on a fluorescence polarization immunoassay for tobramycin

The effects of various storage conditions on the results of a fluorescence polarization immunoassay for tobramycin were studied. Two venous blood samples (150 mL each) were drawn one hour and six hours after a single intramuscular dose of tobramycin. From each of these samples, which represented peak (6 micrograms/mL) and trough (1 microgram/mL) concentrations, aliquots of whole blood and of serum were prepared and stored in both glass and polypropylene containers. Serum samples were stored at -20 degrees C and assayed for tobramycin at intervals of 1-372 days. Samples of serum and whole blood were stored at 4 and 25 degrees C and assayed on days 1, 3, and 7. Mean tobramycin concentrations over time and between-run coefficients of variation were calculated for each set of samples. There was no substantial variation in tobramycin concentrations over time. Significant differences between tobramycin concentrations were noted only for peak serum samples in glass versus plastic containers at -20 degrees C and for trough serum samples stored in glass at -20 degrees C versus 25 degrees C. However, these differences were small and are unlikely to be clinically important. Under the conditions tested, the results of a fluorescence polarization immunoassay for tobramycin do not appear to be affected by storage time, storage temperature, container material, or storage medium (whole blood versus serum).

A Bayesian feedback method of aminoglycoside dosing

We assessed the accuracy of a Bayesian method in providing dosing regimens to achieve desired serum aminoglycoside concentrations. This method calculates individual kinetics based on serum drug concentration data. Performance was analyzed by determining accuracy, bias, correlations of observed to desired serum drug concentrations, and the ability to achieve a target serum drug concentration. We also compared results from the Bayesian method with those resulting from the use of the predictive algorithm portion of the computer program and with routine physician dosing. The Bayesian method resulted in a high correlation coefficient (r = 0.913) between observed and predicted serum concentrations. Analysis of peak aminoglycoside concentrations indicated that the Bayesian method was more accurate and less biased than the predictive algorithm portion of the program or routine physician dosing. A similar trend occurred for trough concentrations. Finally, there were no statistically significant differences between the predicted and observed peak (6.4 +/- 1.5 and 5.9 +/- micrograms/ml) and trough (1.2 +/- 0.9 and 1.4 +/- 0.8 micrograms/ml) serum aminoglycoside concentrations with the Bayesian dosing method. There were significant differences for peak concentrations with the predictive algorithm portion of the program and for peak and trough concentrations with physician dosing. These data demonstrate the accuracy of the Bayesian dosing method in attaining desired peak and trough serum aminoglycoside concentrations.

Comparison of the Predictions of One- and Two-Compartment Microcomputer Programs for Long-Term Tobramycin Therapy

A retrospective study of patients receiving tobramycin compared the accuracy of predictions of actual trough serum concentrations using two commercially available microcomputer software programs. Twelve patients met the study criteria of intravenous tobramycin treatment for more than 10 days with serum concentration monitoring within the first 5 days and after 10 days of therapy. No patients received dialysis. Twenty-five serum concentrations were compared. Predictions within 0.2 microgram/ml were considered clinically "exact." No significant differences were found by chi-square analysis for any of the four possible choices (p less than 0.3). One of the programs, distributed by Dista Pharmaceuticals, offers a one-compartment model, a two-compartment model, and a two-compartment prenephrotoxic option. SIMKIN, a program marketed by Medical Engineering, Inc., uses a two-compartment model. Overall, the predictions errors were small, but occasionally were clinically significant. Further evaluation of microcomputer programs for therapeutic drug monitoring is necessary to document their impact on predicting drug efficacy and toxicity.

Hydroxylamine technique for in vitro prevention of penicillin inactivation of tobramycin

Hydroxylamine was evaluated and found to be a highly effective agent for the in vitro prevention of penicillin inactivation of tobramycin. This inactivation reaction resulted in an underestimation of tobramycin concentrations and was dependent on time, temperature, amount and type of penicillin, and amount of tobramycin. Plasma samples containing tobramycin and three clinically relevant concentrations of ticarcillin, carbenicillin, azlocillin, or piperacillin were incubated with and without hydroxylamine, and tobramycin concentrations were monitored at 0, 12, 24, 48, and 72 h. The inactivation reaction was found to be completely inhibited by hydroxylamine (1 mg/ml) compared with a 27 to 50% loss of measured tobramycin concentration in the unprotected tobramycin-penicillin samples. Hydroxylamine did not interfere with the Emit enzyme immunoassay (Syva Co.) at either high or low tobramycin concentrations. Hydroxylamine was effective in inhibiting the tobramycin inactivation at both room and refrigerator temperatures and was 100% effective in protecting tobramycin on a 1:1 molar basis.