046 EFFICACY OF DAPTOMYCIN IN MONOTHERAPY OR COMBINED WITH RIFAMPICIN IN A RABBIT MODEL OF EXPERIMENTAL ENDOCARDITIS DUE TO E. FAECIUM

Background: Endocarditis due to multi-resistant enterococci is a major therapeutic concern. Daptomycin (DAP) is a lipopeptide antibiotic with bactericidal activity against vancomycin-resistant (Van-R) and highlevel aminoglycoside-R enterococci in vitro. DAP is currently licensed for the treatment of bacteremia and right-sided endocarditis due to Staphylococcus aureus. DAP, however, is not approved for Enterococcus faecium or Van-R enterococcal infections. Indeed, clinical failures associated with selection of resistant strains, although rare, have been described in patients with enterococcal endocarditis treated with DAP alone. One strategy to prevent resistance emergence is the use of antibiotics in combination. We evaluated the potential ability of ampicillin (AMP), gentamicin (GEN) or rifampicin (RIF) to synergize with DAP and to prevent the selection of DAP-R enterococci in vitro. Methods: Five E. faecalis (4 Van-susceptible and 1 Van-R) and 5 E. faecium (3 Van-susceptible and 2 Van-R) were tested for antibiotic synergism by time-kill assay. Of these, two E. faecalis and 2 E. faecium were used for resistance studies. Spontaneous DAP resistance was checked by plating 108 CFU on agar+50mg/l calcium and increasing DAP concentrations. Bacteria were also serially exposed in broth+50mg/l calcium to two-fold increasing concentrations of DAP alone or in combination with a fixed amount (0.25×MIC) of the second agent for 7 days. The MIC was examined after each cycle. Results: Baseline DAP MICs were 0.25 2mg/l. Combining DAP with AMP or GEN acted in synergy in 70% and 100% isolates, respectively. Combining DAP with RIF was indifferent. Spontaneous DAP-R mutants (growth on >4mg/l) on plates inoculated with 108 CFU were not found. On the other hand, sequential exposure of enterococci to DAP alone led to the enrichment of resistant mutants (MIC > 4mg/l) within 3 4 cycles of exposure. Addition of AMP prevented or delayed the selection of DAP-R enterococci. Addition of GEN or RIF had no effect. Conclusions: DAP-resistant enterococci did not emerge spontaneously and were only obtained by serial passage on the drug. Combining DAP with AMP prevented the selection of DAP-R isolates. This combination has the potential to overcome the issues encountered when DAP was used alone for the treatment of E. faecium or Van-R enteroccocal infections such as endocarditis. Further investigations, particularly animal studies, are needed to confirm the benefit of these findings.