Identification and potency of cyclodextrin-lipid inhibitors of Staphylococcus aureus α-toxin.

PURPOSE: Staphylococcus aureus, a leading cause of bacterial keratitis, secretes α-toxin, a cytotoxin active on the corneal epithelium. This study describes the production and testing of chemical inhibitors of α-toxin action. METHODS: Purified α-toxin was titered by its ability to lyse rabbit erythrocytes in buffered saline (PBS). To prepare potential toxin inhibitors, each of 18 lipids was incorporated into a complex with methyl-ß-cyclodextrin (MßCD) or hydroxypropyl-ß-cyclodextrin (HPßCD). Serial dilutions of each lipid-cyclodextrin (CD-lipid) complex were mixed with α-toxin prior to the addition of rabbit erythrocytes. Select CD-lipid complexes were mixed with 12 hemolytic units (HU) α-toxin and injected into the rabbit corneal stroma so the resulting corneal erosions could be measured at 4 and 8 hours post-injection (PI). Eyes injected with toxin alone, MßCD, or HPßCD alone served as controls. RESULTS: Neither form of CD alone inhibited α-toxin. Of the 36 complexes prepared, 6 lipid-CD complexes were found to inhibit >100 HU of α-toxin. Four lipid complexes able to inhibit >200 HU of α-toxin were tested in toxin-injected corneas; at 4 and 8 hours PI, the complexes of cholesterol or lanosterol with MßCD and squalene or desmosterol with HPßCD caused a significant reduction in the corneal erosion size as compared to eyes injected with α-toxin alone (P ≤ 0.05). CONCLUSIONS: Specific lipid inclusion complexes with either MßCD or HPßCD demonstrated a significant inhibition of α-toxin in both in vitro and in vivo assays. Changes in either the cyclodextrin or lipid of a complex affected the inhibitory activity.

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.