Pharmacokinetics of butorphanol following intravenous and intramuscular administration in donkeys: A preliminary study.
Authors: Ebner Lisa, O Odette, Simon Bradley, Lizarraga Ignacio, Smith Joe, Cox Sherry
Journal: Frontiers in veterinary science
Summary
# Editorial Summary Butorphanol is a commonly used opioid analgesic in equine practice, yet its pharmacokinetic behaviour in donkeys—a species with known metabolic differences from horses—remained poorly characterised until this preliminary investigation. Five healthy donkeys aged 6–12 years received 0.1 mg/kg butorphanol via intravenous or intramuscular injection in a crossover design, with plasma concentrations measured over 24–48 hours and analysed by high-performance liquid chromatography. The drug demonstrated notably rapid elimination in donkeys, with a mean half-life of only 0.83 hours following IV administration—substantially shorter than reported in horses—alongside moderate volume of distribution (322 mL/kg) and substantial total body clearance (400 mL/h/kg); intramuscular administration achieved peak concentrations of 369 ng/mL within approximately 29 minutes, with bioavailability of 133%, suggesting enhanced absorption or altered first-pass metabolism compared to intravenous dosing. These findings indicate that donkeys metabolise butorphanol considerably faster than horses, which has direct implications for dosing intervals and pain management protocols in this species, particularly for conditions requiring sustained analgesia where more frequent dosing or alternative agents may be warranted to maintain therapeutic plasma concentrations.
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Practical Takeaways
- •Donkeys metabolize butorphanol faster than horses, suggesting that pain management protocols in donkeys may require more frequent dosing or higher doses to maintain therapeutic effect
- •Intramuscular administration achieves reliable absorption in donkeys, making it a viable route for pain management in clinical settings where IV access is challenging
- •Species-specific pharmacokinetic data supports the need for donkey-tailored anesthesia and analgesia protocols rather than relying solely on equine dosing guidelines
Key Findings
- •Butorphanol elimination half-life in donkeys is 0.83 ± 0.318 h, faster than reported in horses
- •Intramuscular bioavailability of butorphanol in donkeys was 133 ± 45%, indicating near-complete absorption
- •Maximum plasma concentration after IM administration reached 369 ± 190 ng/mL at 0.48 h
- •Donkeys have a larger apparent volume of distribution (322 ± 50 mL/kg) and higher clearance (400 ± 114 mL/h/kg) compared to equine literature values