Pharmacokinetics of Cephalexin in Dogs After Intramuscular Administration of a Long Acting Formulation

The kinetics of cephalexin was studied in four mixed-breed adult dogs after administration of a novel (long-acting) formulation of this cephalosporine. A single dose (1 ml/kg b.w.) of the product, a suspension of cephalexin monohydrate (12%) and cephalexin benzathine (8%) in ethyl-oleate, was administered intramuscularly. Blood samples were taken at intervals up to 36 h after treatment. Cephalexin was extracted from plasma using solid-phase extraction C18 cartridges, and the concentrations measured by means of HPLC/UV analysis. The plasma concentration-time data were analysed by compartmental pharmacokinetics and non-compartmental methods. The disposition curve for the drug was best described by a triexponential equation (two compartment model). The elimination half-life was 6.05 ± 3.58 h, the area under the curve was 148.59 ± 59.35 µg h/mL and the Cmax was 15.54 ± 3.03 µg/mL. Results were compared to the already available data on the pharmacokinetics of conventional cephalexine formulation administered intramuscularly to dogs. Comparison shows that, considering the MICs of cephalexin for the sensible micro-organisms, 1 ml/kg b.w. of the new long-acting formulation can be administered at 12-h intervals, instead of the 8-h intervals necessary using the conventional formulation. At the dosage level adopted, the new formulation of cephalexin cannot be considered as a real long-acting product; however, the administration of larger doses should allow longer treatment intervals.