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Efficacy of once- and thrice-daily dosing of aminoglycosides in in-vitro models of infection.

The bactericidal efficacy of amikacin, isepamicin and netilmicin was studied against Pseudomonas aeruginosa and Serratia marcescens over a treatment period of 30 h using two one-compartment in-vitro models with differently designed culture compartments. High bacterial inocula were exposed to fluctuating drug concentrations, simulating human serum concentrations (t1/2 = 2 h) during clinical treatment. The same daily dose was administered as 1 h infusions given every 8 h or every 24 h, resulting in peak concentrations of 8 and 24 mg/l for netilmicin, and 24 and 72 mg/l for amikacin and isepamicin, respectively. Once-daily dosing was more bactericidal during initial treatment in the in-vitro models (P less than 0.01) and at least as effective as thrice-daily dosing in preventing bacterial regrowth, despite a prolonged period of subinhibitory drug concentration before administration of the second dose. Lower ratios of peak concentration to MIC were needed to achieve bactericidal activity (greater than 99.9% reduction of cfu) after 24 h treatment against S. marcescens compared with P. aeruginosa (P less than 0.01). All nine regimens providing peaks of at least four times the MIC were bactericidal against S. marcescens after 24 h exposure. In contrast, a bactericidal effect against P. aeruginosa occurred only during two of six experiments with peaks of four to nine times the MIC. Similar results were obtained in both in-vitro models of infection. These data suggest insufficient intrinsic activity of the aminoglycosides studied for single drug treatment of P. aeruginosa in the absence of host-defence mechanisms.