Ertapenem Pharmacokinetics in Equine Plasma and Synovial Fluid Following a Single Intravenous Dose

This study describes the pharmacokinetics of ertapenem, a carbapenem antimicrobial that has not been previously studied in equids. Administered as a 30 mg/kg intravenous bolus to six healthy horses, serial blood and synovial samples were obtained over 8 h after administration. Pharmacokinetic analysis of plasma and synovial fluid was performed. In plasma, the AUC was 353.10 h × μg/mL (CV = 49.02%), Vss 79.34 mL/kg (CV = 22.85%), CL 84.96 mL/h/kg (CV = 31.31%) and t1/2 2.03 h (CV = 15.32%). In synovial fluid, the AUC was 524.10 h × μg/mL (CV = 16.03%), VD 206.32 mL/kg (CV = 30.55%), CL 57.24 mL/h/kg (CV = 16.03%) and t1/2 2.50 h (CV = 32.47%). Synovial fluid maintained higher concentrations of ertapenem for longer, although exhibited a lower maximum concentration compared to plasma. The six most common surgical site infection isolates at a tertiary care center in equine orthopedic cases, and their susceptibility to ertapenem were evaluated. The isolates identified were largely consistent with previous reports. The drug was well tolerated, although it caused transient soft feces. Carbapenems remain a protected class of antimicrobials which should be reserved for cases where culture and susceptibility results support their administration. This study helps inform appropriate dosage and dosing intervals.