Evaluation of the analgesic and pharmacokinetic properties of transdermally administered fentanyl in goats

Objective Evaluate the analgesic properties and pharmacokinetics of transdermal fentanyl patches (TFPs) in goats. Design Prospective, randomized study. Setting Preclinical Testing Facility at a University Teaching Hospital. Animals Thirty-four adult female Boer-cross goats. Interventions Goats underwent surgery as part of a concurrent orthopedic research study. Twelve hours prior to surgery, each goat received a TFP (target dosage of 2.5 μg/kg/h), or a placebo patch with analgesia provided by buprenorphine (0.01 mg/kg, IM, q 6 h). Patches were removed after 72 hours. Blood was sampled at specified intervals, up to 84 hours following TFP placement. Plasma concentrations of fentanyl (FEN) were determined using liquid chromatography–mass spectrometry. Postoperative pain assessments were performed by two independent blinded observers. Measurements and Main Results TFPs were applied at a mean (± standard deviation, SD) dose of 2.54 ± 0.36 μg/kg/h. No adverse events occurred. Pain scores between TFP and BUP groups were not significantly different at any time point. Mean plasma FEN concentration (± SD) 2 hours following patch application was 1.06 ± 0.85 ng/mL, and remained above 0.5 ng/mL for 40 hours. Maximum mean plasma FEN concentration (Cmax) was 1.84 (ranging from 0.81 to 3.35) ng/mL with average time to maximum concentration (Tmax) of 12 hours after patch application. Conclusions TFP resulted in consistent FEN absorption and plasma concentrations within the human and ovine therapeutic ranges. Pain scores for goats administered TFP were not different than those administered buprenorphine. Ease of administration, duration of analgesia, and decreased dosing frequency make TFPs an attractive option for pain management in goats.