Pharmacokinetics and pharmacodynamics of linezolid in obese patients with cellulitis

Linezolid is an oxazolidinone antimicrobial with excellent oral bioavailability and tissue penetration and is active against multidrug-resistant skin/soft tissue pathogens.

To study the pharmacokinetics and antibacterial activity of linezolid against selective skin/soft tissue pathogens in obese patients.

We obtained multiple serum samples from 7 obese patients (>50% over their calculated ideal body weight) receiving oral linezolid 600 mg every 12 hours for treatment of cellulitis. Following a minimum of 3 doses, serum concentrations of linezolid were measured in each subject prior to (trough) and 1 and 6 hours after a dose. These samples were then tested against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) (linezolid minimum inhibitory concentrations [MICs] 1.0, 2.0, 4.0 microg/mL) and one strain each of vancomycin-resistant Enterococcus faecium (VRE) (MIC 2.0 microg/mL), Bacteroides fragilis (MIC 2.0 microg/mL), and Peptostreptococcus magnus (MIC 1.0 microg/mL). Serum inhibitory titers (SITs) and bactericidal titers (SBTs) were measured at each time point, and the median activity for these 7 patients was calculated.

Mean linezolid serum concentrations were 4.2, 12.3, and 7.2 microg/mL at these respective time points. Median SITs for 12 hours (100% of the dosing interval) were observed against each organism with the exception of the least susceptible strain of MRSA (MIC 4.0 microg/mL); serum inhibitory activity was observed only at the one-hour time point against this isolate. Furthermore, prolonged (> or =6 h) median SBTs were observed against one isolate of MRSA (MIC 1.0 microg/mL) as well as the strain of VRE and P. magnus.

Serum concentrations of oral linezolid in this patient population were diminished compared with those of healthy volunteers, but still provided prolonged serum inhibitory activity against common pathogens associated with skin/soft tissue infections. One treatment concern would be an obese patient receiving oral linezolid who was infected with a less susceptible (MIC > or =4.0 microg/mL) strain of S. aureus. Bactericidal activity was also observed against selective pathogens.