Pharmacokinetics of acyclovir after single intravenous and oral administration to adult horses.
Authors: Bentz Bradford G, Maxwell Lara K, Erkert Ronald S, Royer Christopher M, Davis Michael S, MacAllister Charles G, Clarke Cyril R
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Acyclovir Bioavailability in Horses Oral acyclovir has limited clinical utility in equine practice due to extremely poor gastrointestinal absorption and bioavailability. Bradford and colleagues administered acyclovir intravenously (10 mg/kg) and orally (10 and 20 mg/kg via nasogastric tube) to six horses in a crossover study, measuring serum concentrations using high-performance liquid chromatography to determine pharmacokinetic parameters and bioavailability. Oral dosing produced highly variable serum levels with a maximum concentration of only 0.19 μg/mL at the higher 20 mg/kg dose and an absolute bioavailability of just 2.8%—far below the concentrations required to inhibit equine herpesvirus replication. These findings indicate that oral acyclovir tablets, even at doses double the intravenous recommendation, cannot reliably achieve therapeutic serum concentrations in horses. For practitioners treating equine herpesvirus infections, this research reinforces that intravenous administration remains the only evidence-supported route; oral formulations should not be relied upon as an alternative, and oral dosing strategies warrant reconsideration in treatment protocols.
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Practical Takeaways
- •Do not rely on oral acyclovir tablets for treating equine herpesvirus infections in horses, as oral bioavailability is too poor and unpredictable to achieve therapeutic concentrations
- •If acyclovir treatment is clinically indicated for EHV, intravenous administration is necessary to achieve adequate serum concentrations—oral dosing will likely be ineffective even at high doses
- •Consider alternative antiviral agents or treatment approaches for equine herpesvirus, as oral acyclovir administration cannot be recommended based on this pharmacokinetic evidence
Key Findings
- •Oral acyclovir administration in horses resulted in poor bioavailability of 2.8% at 20 mg/kg dose with high variability between animals
- •Maximum serum concentration (Cmax) at 20 mg/kg oral dose was only 0.19 ± 0.10 microg/mL, significantly below concentrations required to inhibit equine herpesvirus
- •High variability in serum acyclovir-time profiles and low peak concentrations were observed with intragastric administration at both 10 mg/kg and 20 mg/kg doses
- •Intravenous administration provided superior pharmacokinetic parameters compared to oral dosing and should be the preferred route if acyclovir therapy is indicated