Antimicrobial pharmacodynamics against MRSA in an in vitro infection model: comparing monotherapy to combinations under standard and altered conditions
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Methicillin-resistant S.aureus (MRSA) is a highly virulent pathogen associated with serious healthcare-associated (HCA-MRSA) and community-associated (CA-MRSA) infections. MRSA is an increasingly important cause of skin and skin-structure, bloodstream and other invasive infections including pneumonia and endocarditis. The pharmacodynamics of existing treatments and anovel cephalosporin with activity against MRSA were studied in an in vitro infection model comparing the antibacterial effects of monotherapy and combination therapy under standard and altered environmental conditions. The activity of monotherapy with vancomycin, daptomycin, linezolid and ceftobiprole against clinical MRSA isolates were tested along with combinations of vancomycin-ceftobiprole, daptomycin-ceftobiprole and linezolid-ceftobiprole. Antibacterial response under standard conditions supporting optimal bacterial growth were compared to altered conditions with acidic pH 5.5, diluted nutrient broth (1:2) and increased temperature 40°C. Two clinical isolates including one HCA-MRSA (#81655) and one CA-MRSA (#79002) were studied in an in vitro pharmacodynamic model (IPDM) over 24 hours. Clinical dosing regimens equivalent to vancomycin 1500 mg intravenously every 12 hours (peak =24.4 mg/L, trough =7.4 mg/L), daptomycin 6 mg/kg (420 mg) intravenously every 24 hours (peak =8.2 mg/L, trough =0.8 mg/L) and linezolid 600 mg intravenously every 12 hours (peak =9.2 mg/L, trough =2.8 mg/L) were tested. Ceftobiprole was administrated as a bolus dose followed by constant infusion of 10 mg/L. Antibacterial effects were quantified as initial bacterial kill rate over 4 hours (KR4) and absolute bacterial kill at 24 hours (BK24). Minimum inhibitory concentrations (MIC) were measured via E-test® methods using initial isolates and those recovered after 24 hours of therapy. The KR4 with daptomycin and vancomycin were equivalent (P=0.14), yet daptomycin was more rapid than ceftobiprole (P=0.03) and linezolid (P<0.0001). The BK24 was greatest with ceftobiprole and vancomycin which were superior to linezolid (P<0.0001, P<0.0001, respectively) and daptomycin (P=0.0001, P=0.0001, respectively). Daptomycin was associated with bacterial re-growth and increasing MICs from 0.25 mg/L to 2-4 mg/L during therapy for isolate #79002 under standard conditions. Furthermore, daptomycin activity against both isolates was significantly reduced under altered conditions (KR4, P=0.0001; BK24, P=0.04). Combination therapy with vancomycin-ceftobiprole was indifferent compared with either agent alone. Although daptomycin-ceftobiprole prevented daptomycin non-susceptibility during therapy and resulted in significantly greater BK24 compared with daptomycin alone (BK24 difference of 4.07 log10 cfu/mL, P=0.0001), the combination was indifferent from ceftobiprole alone. Finally, linezolid-ceftobiprole was similar to linezolid but significantly less active than ceftobiprole alone (BK24 difference of 1.39 log10 cfu/mL, P=0.005) raising concerns of potential antagonism with this combination. In conclusion, this study provides important data regarding antimicrobial pharmacodynamics against MRSA. Overall, monotherapy with either ceftobiprole or vancomycin was most active. Combination therapy with ceftobiprole prevented the emergence of daptomycin non-susceptibility during therapy, but demonstrated potential antagonism with linezolid.