Effects of age on the pharmacokinetics and selected pharmacodynamics of intravenously administered fentanyl in foals.
Authors: Knych H K, Steffey E P, Mitchell M M, Casbeer H C
Journal: Equine veterinary journal
Summary
# Editorial Summary: Fentanyl Pharmacokinetics in Foals Fentanyl's clinical use in equine medicine remains limited, particularly in adult horses where CNS excitation is a recognised concern; however, its pharmacokinetic behaviour and safety profile in young foals have not been characterised. Researchers administered a single intravenous dose of fentanyl (4 µg/kg) to nine foals at three developmental timepoints—6–8 days, 20–22 days, and 41–42 days of age—collecting blood samples over 24 hours to determine plasma concentrations and assess behavioural responses. Volume of distribution was markedly elevated in the youngest foals (3.55 l/kg at 6–8 days versus 1.53 l/kg at 20–22 days), whilst clearance remained relatively stable across ages (50.2 to 35.7 ml/min/kg), resulting in a notably prolonged elimination half-life in neonates (49.3 minutes at 6–8 days compared to 25.8–33.7 minutes in older foals). Sedation was reliably observed at all ages despite variable onset and duration between individuals, with fentanyl metabolism proceeding via the same primary pathway as in adult horses. These findings suggest fentanyl is well-tolerated in foals up to six weeks of age, opening the door for controlled clinical trials investigating its utility as an analgesic and sedative in equine neonatal practice.
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Practical Takeaways
- •Fentanyl at 4 µg/kg IV is well tolerated and safe in foals aged 1-6 weeks old, with consistent sedation observed across all age groups
- •Pharmacokinetic parameters change significantly during early foal development; dosing regimens may need age-specific adjustment rather than using adult horse protocols
- •Unlike adult horses where fentanyl can cause CNS excitation, young foals showed predictable sedative effects without adverse excitatory reactions, suggesting potential for expanded clinical use in this population
Key Findings
- •Volume of distribution decreased from 3.55 l/kg at 6-8 days to 1.53 l/kg at 20-22 days of age, then increased slightly to 1.82 l/kg at 41-42 days
- •Elimination half-life was prolonged at 6-8 days (49.3 min) compared to 20-22 days (25.8 min) and 41-42 days (33.7 min)
- •Clearance ranged from 50.2 ml/min/kg at 6-8 days to 35.7 ml/min/kg at 41-42 days of age
- •Sedation was observed at all age groups following 4 µg/kg IV fentanyl administration with good tolerability and no CNS excitation reported