Pharmacodynamic comparisons of levofloxacin, ciprofloxacin, and ampicillin against Streptococcus pneumoniae in an in vitro model of infection.

The increasing frequency of penicillin-resistant pneumococcus continues to be of concern throughout the world. Newer fluoroquinolone antibiotics, such as levofloxacin, have shown enhanced in vitro activity against Streptococcus pneumoniae. In this study, the bactericidal characteristics and pharmacodynamic profiles of levofloxacin, ciprofloxacin, and ampicillin against four isolates of S. pneumoniae were compared by using an in vitro model of infection. Standard antibiotic dosing regimens which simulated the pharmacokinetic profile observed in humans were used. Control and treatment models were sampled for bacterial CFU per milliliter over the duration of each 24- or 48-h experiment. In addition, treatment models were sampled for MIC determinations and drug concentration. Regrowth of all isolates as well as an increase in MICs throughout the study period was observed in the ciprofloxacin experiments. A limited amount of regrowth was noted during levofloxacin therapy for one isolate; however, no change in MIC was detected for any isolate. Ampicillin showed rapid and sustained bactericidal activity against all isolates. In this study, ratios of effective fluoroquinolone area under the concentration-time curve (AUC):MIC values ranged from 30 to 55. Levofloxacin, owing to its larger AUC0-24 values, has excellent and sustained activity against different pneumococcal strains superior to that of ciprofloxacin.