Pharmacokinetics of acyclovir in adult horses

Authors

  • Pamela A. Wilkins DVM, MS, PhD, DACVIM, DACVECC,

    1. From the Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA,
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  • Mark Papich DVM, MS, DACVCP,

    1. North Carolina State College of Veterinary Medicine, Raleigh, NC (Papich).
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  • Raymond W. Sweeney VMD, DACVIM

    III
    1. From the Department of Clinical Studies, School of Veterinary Medicine, University of Pennsylvania, New Bolton Center, Kennett Square, PA,
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  • This study was funded by the American College of Veterinary Emergency and Critical Care 2003 Research Award and the Raymond Firestone Research Foundation. The results of this study have been presented in abstract at the 2004 International Veterinary Emergency and Critical Care Society Meeting in San Diego, CA.

Address correspondence and reprint requests to:
Pamela A. Wilkins, 382 West Street Road, Kennett Square, PA 19348.
E-mail: pwilkins@vet.upenn.edu

Abstract

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 (20 mg/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 (α) 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.

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