[Experimental investigation on various administration methods of gentamicin. Comparison of the effect of gentamicin on phagocytosis].

We compared the efficacy of intravenously, intramuscularly administered or intravenously infused over 45 minutes gentamicin (GM) by the experimental system in rabbits using diffusion chambers. Serum concentrations of GM obtained with 3 administration methods were different, while the peak values after intramuscular injection and intravenous drip infusion were similar. Similar concentration time curves of GM in the chambers were revealed after intramuscular injection and intravenous infusion. No marked difference in 3 administration methods was showed in the effects on the growth of P. aeruginosa in the chamber. Viable bacterial number in the chambers decreased when the GM concentration in the chamber was 4--5 times of MIC, and thereafter regrowth was observed after the decrease of GM concentration in the chambers to 2--3 times of MIC. About 8 hours were required for growth to the base line value. In the leucocyte containing chambers, viable bacterial number similarly decreased, but the rate of regrowth was slow. About 12 hours were required for growth to the base line value. The regrowth rate in the GM containing chamber was similar to that in the antibiotic free chamber. From this result, it is suggested that, when the antibiotic concentration in the chamber is 1--2 times of MIC, antibiotic does not show the growth suppressive effect and the activating effect on phagocytosis of leucocytes in the environment closed to the practical pathological condition such as this model. For the patients with qualitative or quantitative abnormal changes of phagocyte, short interval of drug administration might be needed. For this purpose, intravenous drip infusion under monitoring of serum concentration is more suitable than intramuscular injection, which might be accompanied with severe pain and contracture of injected muscle.