Penetration and effectiveness of prophylactic fluoroquinolones in experimental methicillin-sensitive or methicillin-resistant Staphylococcus aureus anterior chamber infections

PURPOSE: To determine the effectiveness of moxifloxacin and besifloxacin prophylactic therapy for experimental Staphylococcus aureus infections originating in the rabbit anterior chamber. SETTING: Microbiology Department, University of Mississippi Medical Center, Jackson, Mississippi, USA. DESIGN: Experimental study. METHODS: Minimum inhibitory concentrations (MICs) of moxifloxacin 0.5% and besifloxacin 0.6% for methicillin-sensitive Staphylococcus aureus (MSSA) and methicillin-resistant Staphylococcus aureus (MRSA) strains were determined. Eyes were treated with moxifloxacin, a moxifloxacin alternative formulation 0.5%, or besifloxacin (45 μL) 30 minutes or 60 minutes before anterior chamber infection (10(6) colony-forming units [CFUs]). Aqueous humor was removed 30 minutes after infection for quantification of antibiotic and bacteria. RESULTS: The MIC for both organisms was 0.06 μg/mL for moxifloxacin and 0.03 μg/mL for besifloxacin. In MSSA infections, the untreated eyes contained 5.18 log CFU/mL, which was similar to besifloxacin-treated eyes with either treatment (P≥.1091). Eyes treated with moxifloxacin or moxifloxacin alternative formulation contained significantly fewer CFUs than untreated controls or besifloxacin-treated eyes with either treatment (P≤.0020). The aqueous humor in eyes treated with moxifloxacin or moxifloxacin alternative formulation contained significantly more drug than besifloxacin-treated eyes at both prophylactic time points (P≤.0012). In MRSA infections, the untreated eyes contained 4.91 log CFU/mL, which was similar to besifloxacin-treated eyes with either treatment (P≥.5830). Eyes treated with moxifloxacin or moxifloxacin alternative formulation contained significantly fewer CFUs than untreated controls or besifloxacin-treated eyes at both prophylactic time points (P≤.0008). CONCLUSIONS: Moxifloxacin had greater in vivo effectiveness against MSSA and MRSA than besifloxacin. The aqueous antibiotic concentrations suggest limited penetration by besifloxacin, accounting for its lack of effectiveness.