Gentamicin sulfate plus carbenicillin disodium is used in the treatment of Pseudomonas infections because the two drugs are often synergistic in vitro. Incubation of this combination of drugs, however, results in the inactivation of gentamicin. The rate of inactivation is influenced by the relative concentration of the two drugs, the fluid into which they are placed, and the temperature. Usual doses of the drugs, when placed in solutions for intravenous administration, result in rapid inactivation, but in serum the half-life of gentamicin with carbenicillin is 24 hours. In treated patients, the rate of inactivation of gentamicin was the same as in serum. Only in patients with severe renal failure was inactivation of gentamicin by carbenicillin observed. Gentamicin with carbenicillin can be recommended for treatment if physical mixing of the drugs in vitro is avoided.
Riff LJ, Jackson GG. Laboratory and Clinical Conditions for Gentamicin Inactivation by Carbenicillin. Arch Intern Med. 1972;130(6):887–891. doi:https://doi.org/10.1001/archinte.1972.03650060077013
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