Bactericidal activity of moxifloxacin compared to grepafloxacin and clarithromycin against Streptococcus pneumoniae and Streptococcus pyogenes investigated using an in vitro pharmacodynamic model

The aim of the present investigation was to study and compare the killing a ctivity of two new fluoroquinolone compounds, moxifloxacin and grepafloxaci n, and a new generation macrolide, clarithromycin, against three clinical i solates of Streptococcus pneumoniae (penicillin-susceptible, -intermediate and -resistant) and two Streptococcus pyogenes (erythromycin-susceptible an d -resistant) strains by simulating their human pharmacokinetics in a pharm acodynamic model, Results were achieved by measuring the reduction in viabl e bacterial count during the 24-h experimental period. All three antimicrobials led to a continuous reduction in the bacterial cou nts of penicillin-susceptible S. pneumoniae and erythromycin-susceptible S, pyogenes strains, the maximal reduction observed after 8-10 hours being 5- 6 logs for moxifloxacin and 3 logs for grepafloxacin; clarithromycin exhibi ted a similar reduction of 5 logs only after 24 h. No regrowth was observed for any strain after 24 h with any of the antibiotics. The bactericidal activity of both the fluoroquinolones was not affected by penicillin resistance of S, pneumoniae and erythromycin resistance of S, py ogenes. In contrast, clarithromycin was not able to reduce the bacterial co unt of penicillin-resistant S, pneumoniae and erythromycin-resistant S. pyo genes strains. Moxifloxacin exhibited, within 24 h, higher and faster bactericidal activit y than grepafloxacin and clarithromycin against S. pneumoniae, and was not affected by penicillin resistance, These results suggest that moxifloxacin is a promising new agent for treatment of streptococcal infections.