Pharmacokinetics of fentanyl after intravenous and transdermal administration in goats

Abstract Objective To evaluate disposition of fentanyl in goats after IV and transdermal administration. Animals 8 healthy 2-year-old goats weighing 31.8 to 53.6 kg (mean ± SD, 40.4 ± 7.5 kg). Procedure Each goat was given 2 treatments consisting of fentanyl administered IV (2.5 μg/kg of body weight) and via a transdermal patch (50 μg/h). There was a 2-month interval between treatments. Blood samples were collected at specified times and analyzed in duplicate to determine plasma fentanyl concentrations. Pharmacokinetic values were calculated, using a computerized modeling program. Results Administration of fentanyl was tolerated by all goats. Intravenous administration of fentanyl resulted in a transitory increase in rectal temperature that was not clinically important. Terminal elimination half-life after IV administration was 1.20 ± 0.78 h, volume of distribution at steady state was 1.51 ± 0.39 L/kg, and systemic clearance was 2.09 ± 0.62 L/kg/h. Transdermal administration of fentanyl resulted in variable plasma concentrations, with peak plasma concentrations ranging from 1.12 to 16.69 ng/ml (mean ± SD, 6.99 ± 6.03 ng/ml) and time to peak concentration ranging from 8 to 18 hours (mean ± SD, 13 ± 4.5 hours). After removal of the transdermal patch, mean ± SD terminal elimination half-life was 5.34 ± 5.34 hours. Conclusions and Clinical Relevance Intravenous administration of fentanyl (2.5 μg/kg) in goats results in a relatively short half-life that will limit its use for management of pain. Transdermal administration of fentanyl (50 μg/h) in goats results in variable plasma concentrations that may exceed those anticipated on the basis of a theoretical delivery rate, but stable plasma concentrations of fentanyl may not be achieved. ( Am J Vet Res 1999;60:986–991)