The pharmacokinetics and locomotor activity of alfentanil in the horse.

This study explores the effects and pharmacokinetics of different doses of alfentanil in horses. It found that larger doses significantly increased locomotor activity, and there were notable differences in drug kinetics between conscious and anesthetized horses.

Research Methodology

• The research invovled conducting pharmacological studies with the opioid analgesic, alfentanil, on horses.
• Four different doses of alfentanil (4, 10, 20, and 40 micrograms/kg) were administered to four horses at different times.
• The locomotor activity of the horses was monitored, specifically after the administration of 20 and 40 micrograms/kg doses which led to a significant increase in movement.
• The blood plasma concentrations of alfentanil were also measured in six standing horses, providing the basis for calculating the drug’s pharmacokinetics.

Findings

• The decay curves of alfentanil concentration over time were best described by a biexponential equation.
• The average elimination half-life (t1/2 beta) of alfentanil was observed to be approximately 21.65 minutes, while the clearance rate (Cl) was about 14.1 ml/kg/min.
• There were noted differences between the kinetics of the drug in conscious horses and those that had been anesthetized with xylazine-ketamine and maintained under halothane in oxygen or isoflurane in oxygen.
• The elimination half-life and clearance rate of alfentanil also differed under the two forms of anesthesia mentioned above. The t1/2 beta for alfentanil under halothane and isoflurane anaesthesia were 55.95 min and 68.03 min respectively, while Cl values were 14 ml/kg/min and 13.6 ml/kg/min.

Implications

• The study shows that alfentanil, an opioid, has different behaviors in the body of horses under different circumstances. It leads to increased locomotor behavior at higher doses and reacts differently when the horse is conscious or anaesthetized.
• The variability of alfentanil’s half-life under different anesthesia implies that the type of anesthesia used could affect the duration for which alfentanil remains in the horse’s body.
• Presumably, this study could influence protocols for using alfentanil or similar opioids in horses, and underscores the need for considering the state of consciousness and type of anesthesia when predicting drug behaviors.