Microbiologic effectiveness of time- or concentration-based dosing strategies in Streptococcus pneumoniae

This in vitro study evaluated the pharmacodynamic performance of levofloxacin using different dosing strategies against both a levofloxacin-sensitive (MIC = 1 mg/liter) and -resistant (MIC = 16 mg/liter) strain of Streptococcus pneumoniae. The strain was genotypically characterized by a mutation in gyrA and two mutations in parE; resistance was shown not to be efflux-mediated. The purpose of this study was to determine if simulated levofloxacin dosing strategies focused either on time or concentration would affect microbiologic outcome. Differing peak concentration/MIC ratios (1,2, and 10), T>MIC (3.6,9.6,15.6, and 24 h corresponding to 15, 40, 65, and 100% of the 24-h dosing interval), and AUC/MIC ratios (13-180) were generated by varying dosing strategies. Initial bacterial inocula were decreased by 99.9% in each experiment conducted. Despite the wide variation in exposure levels, in terms of AUC/MIC, Cp-max/MIC, and T>MIC, the kill portions of the bacterial density curves were super-imposable between all permutations of antibiotic exposure. However, there appeared to be an AUC/MIC breakpoint (35-40) defining bacterial regrowth. Over a 10-fold concentration range, levofloxacin appeared to kill S. pneumoniae in a concentration-independent fashion. When given in concentrations suitable to achieve specified pharmacodynamic endpoints (AUC/MIC >/=35), levofloxacin demonstrated the ability to eradicate both a levofloxacin-resistant and levofloxacin-sensitive strain of S. pneumoniae in the in vitro model.