Pharmacokinetics of acyclovir after single intravenous and oral administration to adult horses

The purpose of the study reported here was to describe the bioavailability and pharmacokinetics of acyclovir after intravenous and oral administration to horses. Six healthy adult horses were used in a randomized cross-over study with a 3 x 3 Latin square design. Three treatments were administered to each horse: 10 mg of injectable acyclovir/kg of body weight in 1 L of normal saline delivered as an infusion over 15 minutes; 10 mg of acyclovir/kg in tablets by nasogastric intubation; and 20 mg of acyclovir/kg in tablets by nasogastric intubation. A 2-week washout period was provided between each treatment. Serum samples were obtained for acyclovir assay using reversed-phase, high-performance liquid chromatography with fluorescence detection. Deproteinated serum was injected onto a C18 column, and elution occurred under isocratic conditions. The limit of quantification was 0.04 microg/mL. The assay exhibited suitable accuracy, precision, and recovery. The IV data were analyzed by a 3-compartment model, and oral data were analyzed noncompartmentally. Intragastric acyclovir administration at either dose was associated with high variability in serum acyclovir-time profiles, low Cmax, and poor bioavailability. The dosage of 20 mg/kg was associated with mean (+/- SD) Cmax of 0.19 +/- 0.10 microg/mL, and bioavailability was 2.8%. Inhibition of equine herpesvirus has been reported to require significantly higher acyclovir concentrations than those obtained here. The results of this study do not support a therapeutic benefit for the oral administration of acyclovir up to doses of 20 mg/kg.