Pharmacokinetics of acyclovir in adult horses

Objective: To determine the pharmacokinetics of acyclovir administered intravenously (IV) and orally to healthy adult horses. Design: Random cross-over with an approximate 1-week washout period between trials. Setting: University veterinary medical teaching hospital. Animals: Six healthy adult research herd horses. Interventions and main results: Acyclovir was administered IV (10 mg/kg in 1 L isotonic crystalloid solution over 60 minutes) and orally (20mg/kg) to healthy adult horses. Plasma samples were obtained and acyclovir concentrations were determined by high-pressure liquid chromatography. Peak concentration (mean ± SD) for IV acyclovir was 13.74 ± 5.88 μg/mL at the completion of the 1-hour infusion. The half-life of the distribution phase (a) was 0.16 hours while the half-life of the elimination phase (β) was 9.6 hours. The steady-state volume of distribution was 3.93 ± 1.21 L/kg. We were unable to measure pharmacokinetics after PO acyclovir as plasma concentrations were below the lower limits of detection in all 6 horses. Conclusions: IV administration of acyclovir to healthy adult horses achieves concentrations within the sensitivity range described for equine herpes virus-type 1. The oral bioavailability of acyclovir in horses is low and additional studies are required.