Pharmacokinetics of fentanyl after intravenous and transdermal administration in goats

Objective-To evaluate disposition of fentanyl in goats after IV and transde rmal administration. Animals-8 healthy 2-year-old goats weighing 31.8 to 53.6 kg (mean +/- SD, 4 0.4 +/- 7.5 kg). Procedure-Each goat was given 2 treatments consisting of fentanyl administe red IV (2.5 mu g/kg of body weight) and via a transdermal patch (50 mu g/h) . There was a 2-month interval between treatments. Blood samples were colle cted at specified times and analyzed in duplicate to determine plasma fenta nyl concentrations. Pharmacokinetic values were calculated, using a compute rized modeling program. Results-Administration of fentanyl was tolerated by all goats. intravenous administration of fentanyl resulted in a transitory increase in rectal temp erature that was not clinically important. Terminal elimination half-life a fter IV administration was 1.20 +/- 0.78 h, volume of distribution at stead y 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 co ncentrations, 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 rangi ng 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 mu g/kg) in goats results in a relatively short half-life that will lim it its use for management of pain. Transdermal administration of fentanyl ( 50 mu g/h) in goats results in variable plasma concentrations that may exce ed those anticipated on the basis of a theoretical delivery rate, but stabl e plasma concentrations of fentanyl may not be achieved.