The importance of pharmacodynamics in determining the dosing interval in therapy for experimental pseudomonas...

The efficacy of ciprofloxacin, BMY-28142, and ceftazidime was compared in vitro and in experimental left-sided endocarditis due to Pseudomonas aeruginosa in the rat. The dose, dosing interval, and duration of therapy were varied, and the resulting antibiotic levels in serum and vegetations were correlated with bacterial clearance from vegetations. These studies demonstrated that beta-lactams such as BMY exhibited a slow rate of bactericidal action and had no postantibiotic effect against P. aeruginosa in vitro or in vivo. As a consequence, BMY had to be given in multiple doses at relatively short intervals during which concentrations of antibiotics in vegetations were continuously in excess of the MBC for the pathogen. The earlier onset of rapid bactericidal action and the prolonged postantibiotic effect of ciprofloxacin (demonstrated in vivo and in vitro) were, in all likelihood, the factors that allowed the successful use of fewer doses of this antimicrobial agent at relatively longer dosing intervals.