Effectiveness of netilmicin and tobramycin against Pseudomonas aeruginosa in vitro and in an experimental tissue infection in mice

The activity of netilmicin and tobramycin against Pseudomonas aeruginosa was assessed in vitro in the presence of constant and exponentially declining concentra-tions, and in mice in an experimental thigh infection. The activity in vitro at constant concentrations was expressed as the maximal killing rate (ER) during 3 h of exposure. On the basis of the quantitative relation between £(R) and the drug concentration, the numbers of cfu expected at consecutive times, at constant as well as at declining concentrations, were predicted. The relationship between observed numbers and predicted values of ERt were similar under both conditions for both drugs. On the same basis the numbers of cfu expected in the experimental thigh infection were predicted. There was indeed a significant linear relationship between observed numbers of cfu in homogenized muscle and the values predicted on the basis of the pharmacokinetics of the aminoglycosides, but the slope of this relation-ship was only 0-22. There was no difference in this respect between the two antibiotics. It is concluded that the efficacy of netilmicin and tobramycin against P. aeruginosa is considerably less in vivo than in vitro, but the relation is about the same for the two drugs; therefore the slightly higher activity of tobramycin in vitro is relevant in the in-vivo situation