Dose-Dependent Pharmacokinetics of a New Oral Cephalosporin,Cefixime, in the Dog

Cefixime (CL 284,635; FK 027) is a new third-generation oral cephalosporin. To study dose-dependent pharmacokinetics of cefixime in dogs, two balanced four-way crossover studies were conducted. In the first study, oral doses of 50, 100, and 200 mg/kg and an intravenous dose of 50 mg/kg cefixime were administered. In the second study, oral doses of 6.25, 12.5, and 25 mg/kg and an intravenous dose of 12.5 mg/kg cefixime were administered to the same dogs. A period of 1 month separated the two studies. When the two intravenous doses were compared (i.e., 12.5 and 50 mg/kg), a twofold increase in clearance and volume of distribution was observed after the higher dose. The oral systemic bioavailability in the dose range 6.25–50 mg/kg was 55%. It decreased to 44% at 100 mg/kg and 27% at 200 mg/kg. The average peak serum concentrations ranged from 15.8 µg/ml at 6.25 mg/kg to 119 µg/ml at 200 mg/kg. Within this concentration range, the fraction of free drug in serum (unbound to proteins) increased from 7 to 25%. This concentration-dependent protein binding was primarily responsible for changes in total clearance, volume of distribution, and bioavailability of the drug in dogs.