Effects of age on the pharmacokinetics and selected pharmacodynamics of intravenously administered fentanyl in foals.

REASONS FOR PERFORMING STUDY

The use of fentanyl is limited in adult horses, in part due to potential for central nervous system excitation. The pharmacokinetics and the plasma concentration-related behavioural actions of fentanyl have not been described for young foals.

OBJECTIVES

The goal of the present study was to describe the pharmacokinetics and behavioural effects of fentanyl following administration to the same group of foals at 3 different ages.

STUDY DESIGN

Experimental study in healthy foals.

METHODS

Fentanyl was administered i.v. (4 μg/kg bwt) to a group of 9 foals on 3 separate occasions at 6–8, 20–22 and 41–42 days of age. Blood samples were collected prior to administration and at multiple times until 24 h post administration. Blood samples were analysed for fentanyl concentrations and pharmacokinetics determined at each age. Behavioural and physiological effects were also assessed.

RESULTS

The average volume of distribution was 3.55, 1.53 and 1.82 l/kg bwt and clearance 50.2, 40.7 and 35.7 ml/min/kg bwt when foals were 6–8, 20–22 and 41–42 days of age, respectively. The elimination half-life was slightly prolonged (49.3 min) at 6–8 days relative to 20–22 and 41–42 days of age (25.8 and 33.7 min, respectively). The primary metabolite detected in blood samples was the same as for adult horses. While the onset and duration varied widely between foals, sedation was observed at all ages studied.

CONCLUSIONS

Fentanyl appears to be consistently well tolerated following i.v. administration of 4 μg/kg bwt to foals ranging in age from 1 to 6 weeks. The results of this study support further study of fentanyl for clinical use in foals.