Lack of in vitro inactivation of tobramycin by imipenem/cilastatin

OBJECTIVE: Inactivation of aminoglycosides by beta-lactam antimicrobials both in vitro and in vivo has been documented. Such an interaction has not previously been documented between carbapenems and aminoglycosides. Examination of serum concentrations of tobramycin in a patient receiving both agents suggested that this interaction might exist. The purpose of this study was to look at this question in an in vitro model. METHODS: Low concentrations of tobramycin (10 micrograms/mL) were incubated with imipenem/cilastatin (concentrations of 10, 20, and 40 micrograms/mL) in human serum at 37 degrees C. Aliquots of these solutions were withdrawn at 0, 6, 24, 72, and 120 hours and assayed for tobramycin concentrations using a fluorescence polarization immunoassay. Aliquots of tobramycin 10 micrograms/mL and carbenicillin 200 micrograms/mL were analyzed in the same manner, as a positive control. High concentrations of tobramycin (800 micrograms/mL) and imipenem (5000 micrograms/mL)/cilastatin were incubated together at 21 degrees C and sampled at 0, 6, 24, and 72 hours for tobramycin concentrations. RESULTS: The degradation rates for low-concentration tobramycin and the various concentrations of imipenem/cilastatin were not statistically different from those of the controlled incubations. In contrast, carbenicillin significantly enhanced the degradation rate of tobramycin at this concentration (half-life 72 hours and a 34 percent loss at 24 hours, p = 0.0028). Higher in vitro concentrations of imipenem (5000 micrograms/mL)/cilastatin and tobramycin (800 micrograms/mL) resulted in significant, but moderate degradation over controlled incubations (half-life 80 hours and 10 percent loss at 12 hours, p = 0.0031). CONCLUSIONS: These results suggest that inactivation of tobramycin is not a problem at common clinically achievable imipenem serum concentrations in patients.

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