Disposition kinetics and bioavailability of piperacillin and cephapirin in mares.

The pharmacokinetics of piperacillin (430 mg/kg.b.wt.) and cephapirin (20 mg/kg.b.wt.) were investigated following a single intravenous and intramuscular injection in normal mares. The serum concentration-time curve following a single intravenous injection of both antibiotics obeyed a two-compartment open model. After intravenous dose, piperacillin and cephapirin were transferred from central to peripheral compartment (k12) with values 0.46 and 0.52 h-1, while their passages from peripheral to central compartment (k21) were equal to 0.56 and 0.49 h-1, respectively. The elimination half-lives [t0.5(beta)] following intravenous injection were 6.94 and 3.82 h for piperacillin and cephapirin, respectively. Piperacillin and cephapirin were cleared after intravenous injection by all clearance processes in the body (CItot.) at rates of 5.52 and 0.67 ml/min., respectively. The absorption half-lives [t0.5(ab)] of piperacillin and cephapirin following a single intramuscular injection were 0.46 and 0.88 h., respectively. The calculated maximum [Cmax.] and minimum [Cmin.] serum concentrations to establish a proper multiple dosage regimen were 52.94, 4.50 micromilligrams (for piperacillin) and 10.18, 2.20 micromilligrams (for cephapirin), respectively. The mean systemic bioavailabilities of piperacillin and cephapirin following a single intramuscular injection were 85.47% and 67.89%, respectively. The in-vitro protein binding percent was 19.23% for piperacillin and 52.62% for cephapirin.