Propensity to release endotoxin after two repeated doses of cefuroxime in an in vitro kinetic model: higher release after the second dose.

OBJECTIVES: To study endotoxin release from two strains of Escherichia coli after exposure to two repeated doses of cefuroxime in an in vitro kinetic model. METHODS: Cefuroxime in concentrations simulating human pharmacokinetics was added to the bacterial solution with a repeated dose after 12 h. In another experiment, tobramycin was given concomitantly with the second dose of cefuroxime. Samples for viable counts and endotoxin analyses were drawn before the addition of antibiotics and at 2 and 4 h after each dose. RESULTS: The propensity to release endotoxin, expressed as log10 endotoxin release (EU)/log10 killed bacteria, was higher after the second than after the first dose, 0.80+/-0.04 and 0.65+/-0.01, respectively, in the ATCC strain and 0.80+/-0.04 and 0.65+/-0.02, respectively, in the clinical strain (P<0.001). Endotoxin was released earlier after the second dose (P<0.001). Addition of tobramycin at the second dose reduced the endotoxin release in comparison with that of cefuroxime alone (P<0.001). CONCLUSIONS: The propensity to liberate endotoxin is higher after the second dose of cefuroxime than after the first, resulting in a higher release of endotoxin than expected from bacterial count. The release after the second dose can be reduced by the addition of tobramycin.

Endotoxin release from Escherichia coli after exposure to tobramycin: dose-dependency and reduction in cefuroxime-induced endotoxin release.

OBJECTIVE: To study the release of free endotoxin from Escherichia coli exposed to varying concentrations of the penicillin-binding protein (PBP) 3-specific beta-lactam antibiotic cefuroxime, the aminoglycoside tobramycin, and a combination of the two, and to test the relationship between bacterial killing rate and endotoxin release. METHODS: A clinical isolate of Escherichia coli in logarithmic phase was exposed to 0.1, 2, 10, and 50 x minimum inhibitory concentration (MIC) of cefuroxime, tobramycin, and a combination of the two. Samples for viable counts and endotoxin analysis were drawn immediately before and after the addition of the antibiotics and at 1, 2, 4, 6, and 24 h. All experiments were performed in triplicate. For the analysis of endotoxin, a chromogenic limulus amoebocyte lysate assay was used. RESULTS: Endotoxin liberation was found to be proportional to the number of killed bacteria for each antibiotic regimen at each concentration level justifying the endotoxin-liberating potential to be expressed as release of endotoxin per killed bacterium, an expression that was independent of the inoculum size. At all concentration levels there was a statistically significant difference between the treatments, with the highest release of endotoxin per killed bacterium for cefuroxime, lower for tobramycin and the lowest for the combination of the two drugs (P < 0.001). With increasing doses, there was a significant reduction (P < 0.001) in the propensity to release endotoxin. When the bacterial killing rate was correlated to the propensity to release endotoxin in bacteria exposed to tobramycin or the combination of tobramycin and cefuroxime, a significant negative correlation was found (P < 0.01). This reduction in endotoxin release was not caused by an unspecific endotoxin binding of tobramycin. CONCLUSIONS: Addition of tobramycin reduced the cefuroxime-induced endotoxin release per killed bacterium to a level which was even lower than that of tobramycin alone in spite of an increased killing rate. Increasing concentrations of tobramycin led to reduction in endotoxin release, which may be of benefit when dosing aminoglycosides once daily.