Pharmacodynamics of moxifloxacin and levofloxacin against Staphylococcus aureus and Staphylococcus epidermidis in an in vitro pharmacodynamic model.

An in vitro pharmacokinetic model was used to compare the pharmacodynamics of moxifloxacin and levofloxacin against 3 Staphylococcus aureus and 3 Staphylococcus epidermidis strains. Logarithmic-phase cultures were inoculated into the peripheral compartment of hollow-fiber cartridges and exposed to the peak serum concentrations achieved in humans with oral doses of moxifloxacin (400 mg) and levofloxacin (500 mg). Drugs were added at 0 and 24 h, elimination kinetics were simulated, and changes in viable bacterial counts were evaluated over the course of 36 h. Moxifloxacin was bactericidal against all 6 staphylococci (times to 99.9% kill, 1-3 h). Against most strains, bacterial killing continued through 36 h, with total kills exceeding 5.5 logs. Levofloxacin was bactericidal against 5 of the strains, with similar times to 99.9% kill. In contrast to moxifloxacin, however, resistant subpopulations emerged in 4 strains during therapy with levofloxacin, and this could have important implications for treatment of staphylococcal infections. These in vitro observations warrant the clinical evaluation of moxifloxacin in the treatment of staphylococcal infections.