1323 - Substantial Doses of Daptomycin and Rifampin Eradicate S. epidermidis Biofilm in an In Vitro Pharmacodynamic (IVPD) Model

Background: The concentration of antibiotics at the site of action needed to eradicate biofilm is currently unknown. Studies have previously suggested that bacteria in biofilms are 1000-fold more resistant to antibiotics than free-floating planktonic bacteria. We sought to describe concentrations of daptomycin alone and in combination with rifampin in relation to the pharmacodynamic exposures for biofilm eradication.

Methods: We utilized a methicillin-resistant high biofilm-forming S. epidermidis strain RP62a (ATCC® 35984) over a 48-hour in vitro PD biofilm model. The Centers for Disease Control (CDC) Biofilm Reactor model was used with chromium cobalt materials to simulate an orthopedic device infection. The reactor was inoculated and underwent a 24-hr growth phase and 16-hr conditioning phase to form biofilm on the chromium cobalt coupons, and then a 48-hr PK-PD phase was run. The daptomycin MIC for RP62a was 0.5 mg/L and the rifampin MIC was 0.015 mg/L. We modeled a growth control of the isolate alone, a 12 mg/kg regimen of daptomycin (fCmax: 14.7 mg/L, Ke 0.09), a daptomycin concentration of 1000 mg/L (2000x MIC), and a combination model of daptomycin 1000 mg/L with rifampin 15 mg/L (1000x MIC). Coupons with bacteria embedded in biofilm were sonicated, vortexed, and plated on Tryptic Soy Agar for colony counts read at 24 hrs. Bactericidal activity was defined as ≥ 3-log10 CFU/mL reduction from the initial inoculum.

Results: The simulated humanized dosing regimen of daptomycin 12 mg/kg (fAUC_0-24/MIC: 204) was similar to the growth control model. Bactericidal kill was demonstrated at 24hr and 48hr in the daptomycin 1000 mg/L model (fAUC_0-24/MIC: 20,248) but did not fall beneath the limit of detection. The daptomycin and rifampin combination model demonstrated bactericidal kill at 24hr and 48hr and went below the limit of detection.

Conclusion: This study demonstrated that significantly higher concentrations of antibiotics are needed at the site of action to eradicate biofilm than what maximum systemic dosing can provide. Identifying these concentrations provides a foundation for localized antibiotic therapy and further studies are needed to elucidate these concentrations for a variety of antibiotics and biofilm-forming organisms.