This study aims to investigate whether the addition of fosfomycin or cloxacillin to daptomycin provides better outcomes in the treatment of methicillin-resistant Staphylococcus aureus (MRSA) experimental aortic endocarditis in rabbits. Five MRSA strains were used to perform in vitro time-kill studies at standard (105) and high (108) inocula. Combined therapy was compared with daptomycin monotherapy treatment in the MRSA experimental endocarditis model. A human-like pharmacokinetics model was applied and the equivalents of cloxacillin 2g/4hiv, fosfomycin 2g/6h iv and daptomycin 6-10mg/kg/d iv were administered. The combination of daptomycin and fosfomycin or cloxacillin was synergistic in the five strains tested at both inocula. A bactericidal effect was detected in four out of five strains tested with both combinations. The MRSA-277 strain (vancomycin MIC, 2 mcg/mL) was used for the experimental endocarditis model. Daptomycin plus fosfomycin significantly improved the efficacy of daptomycin monotherapy at 6 mg/kg/d in terms of both the proportion of sterile vegetations (100% vs. 72%, P=.046) and the decrease in the density of bacteria within the vegetations (P=.025). Daptomycin plus fosfomycin was as effective as daptomycin monotherapy at 10 mg/kg/d (100% vs. 93%, P=1.00) and had similar activity to daptomycin plus cloxacillin when daptomycin was administered at 6 mg/kg/d (100% vs. 88%, P=0.48). Daptomycin non-susceptibility was not detected in any of the isolates recovered from vegetations. In conclusion, for the treatment of MRSA experimental endocarditis, the combination of daptomycin plus fosfomycin showed synergistic and bactericidal activity.
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