Plasma concentrations of transdermal fentanyl and buprenorphine in pigs (Sus scrofa domesticus)

Objective

To determine the absorption characteristics of fentanyl and buprenorphine administered transdermally in swine.

Study design

A randomized comparative experimental trial.

Animals

Twenty-four Yorkshire gilts weighing 27.8 ± 2.2 kg (mean ± standard deviation).

Methods

Animals were randomly assigned to different doses of transdermal patches (TPs) of fentanyl (50 μg hour^?1, 75 μg hour^?1 and 100 μg hour^?1) or buprenorphine (35 μg hour^?1 and 70 μg hour^?1), once or twice. Thirteen blood samples were obtained for each TP applied. Plasma concentrations were determined, and the area under the curve, peak serum concentration (C_max) and time to C_max were calculated.

Results

Fentanyl: C_max was observed at different time points: for the first TP application: 30 hours for 50 μg hour^?1, 6 hours for 75 μg hour^?1 and 100 μg hour^?1 patches; and for the second TP application: 30 hours for 50 μg hour^?1 and 36 hours for 75 μg hour^?1 patches. Buprenorphine: serum concentrations were not detected for the 35 μg hour^?1 patch; C_max was observed at different times for the 70 μg hour^?1 patch: 18 hours (n = 1), 24 hours (n = 3), 30 hours (n = 1) and 42 hours (n = 1) after application of the first patch and 12 hours after the second patch.

Conclusions and clinical relevance

A relevant serum concentration obtained with fentanyl TP dosed at 75 μg hour^?1 or 100 μg hour^?1suggests that TPs could represent an analgesia option for laboratory pigs weighing 25–30 kg. As concentrations of buprenorphine were variable, this study does not support the use of buprenorphine TPs in pigs. Consecutive fentanyl or buprenorphine TPs did not provide reliable serum concentrations. Further pharmacokinetic studies and analgesiometric tests in swine are needed to confirm the clinical adequacy of TPs.