Ceftobiprole Efficacy In Vitro against Panton-Valentine Leukocidin Production and In Vivo against Community-Associated Methicillin-Resistant Staphylococcus aureus Osteomyelitis in Rabbits

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) can cause osteomyelitis with severe sepsis and/or local complications in which a Panton-Valentine leukocidin (PVL) role is suspected. In vitro sub-MIC antibiotic effects on growth and PVL production by 11 PVL+ MRSA strains, including the major CA-MRSA clones (USA300, including the LAC strain; USA400; and USA1000), and 11 PVL+ methicillin-susceptible S. aureus (MSSA) strains were tested in microplate culture. Time-kill analyses with ceftobiprole at its MIC were also run with LAC. Efficacies of ceftobiprole (40 mg/kg of body weight subcutaneously [s.c.] four times a day [q.i.d.]) or vancomycin (60 mg/kg intramuscularly [i.m.] twice a day [b.i.d.]) alone or combined with rifampin (10 mg/kg b.i.d.) against rabbit CA-MRSA osteomyelitis, induced by tibial injection of 3.4 x 10(7) CFU of LAC, were compared. Treatment, started 14 days postinoculation, lasted 14 days. In vitro, 6/11 strains cultured with sub-MICs of ceftobiprole produced 1.6- to 4.8-fold more PVL than did the controls, with no link to specific clones. Rifampin decreased PVL production by all tested strains. In time-kill analyses at the LAC MIC (0.75 mg/liter), PVL production rose transiently at 6 and 8 h and then declined 2-fold at 16 h, concomitant with a 2-log(10)-CFU-count decrease. In vivo, the mean log(10) CFU/g of bone for ceftobiprole (1.44 +/- 0.40) was significantly lower than that for vancomycin (2.37 +/- 1.22) (P = 0.034), with 7/10 versus 5/11 bones sterilized, respectively. Combination with rifampin enhanced ceftobiprole (1.16 +/- 0.04 CFU/g of bone [P = 0.056], 11/11 sterile bones) and vancomycin (1.23 +/- 0.06 CFU/g [P = 0.011], 11/11 sterile bones) efficacies. Ceftobiprole bactericidal activity and the rifampin anti-PVL effect could play a role in these findings, which should be of interest for treating CA-MRSA osteomyelitis.