Absolute bioavailability of ceftriaxone after intramuscular administration to healthy volunteers

The disposition and bioavailability of ceftriaxone, a novel cephalosporin, was studied in 7 healthy volunteers after a single 1.5-gram intramuscular administration in comparison with the same dose given by the intravenous route. The plasma levels were measured by means of a high-performance liquid chromatography method up to 48 h after administration. Comparable areas under plasma concentration-time curve (AUC) and biological half-life were obtained proving that ceftriaxone, if assessed by conventional methods, is completely bioavailable when given by the intramuscular route. However, taking into account its nonlinear pharmacokinetics it becomes obvious that direct comparison of AUC values generally leads to a certain overestimation of bioavailability after extravascular administration. This aspect is briefly discussed.