Pharmacokinetics, Pharmacodynamics, and Anti-Nociceptive Effects of Buprenorphine Following Transdermal Administration to Horses

Abstract

ObjectiveThis study describes the pharmacokinetics and pharmacodynamics, including antinociceptive effects, of a transdermal buprenorphine solution in horses. It was hypothesized that transdermal application would lead to sustained blood concentrations and antinociceptive effects with fewer adverse effects compared to intravenous (IV) injection. Study DesignProspective non-randomized 4-part parallel experimental study. AnimalsEight horses, three mares and five geldings, aged 6 to 12 years. MethodsHorses were given incremental doses of 15, 30, and 45 μg kg-1 buprenorphine transdermal solution and a single IV dose of 5 μg kg-1 buprenorphine with a two week washout period between treatments. Concentrations of buprenorphine were determined in plasma using liquid chromatography-tandem mass spectrometry and modeled using a nonlinear mixed effects population pharmacokinetic model to determine pharmacokinetic parameters. Pharmacodynamic effects, including changes in locomotor activity, heart rate, body temperature, gastrointestinal borborygmi, thermal and mechanical nociceptive thresholds were recorded. Mixed effects analysis of variance and post hoc comparisons were performed using a Bonferroni multiple comparison adjustment to assess differences in pharmacodynamic parameters between baseline and each time point within each dose group and between dose groups at the same time point. ResultsTransdermal application of buprenorphine resulted in low systemic concentrations relative to IV injection. Bioavailability following transdermal application was 11%. Thermal nociceptive thresholds were significantly (p < 0.05) increased (4.3–10.7% relative to baseline) for up to 72 hours in the IV dose group, but only sporadically in the transdermal dose groups (2.5–9.9% relative to baseline). Changes in locomotor activity, heart rate and borborygmi varied over time and with dose. Conclusions and Clinical RelevanceLimited thermal antinociceptive effects were observed at the transdermal doses studied likely due to limited absorption relative to IV dosing. Future studies may be directed toward investigating antinociceptive effects of higher transdermal doses and different application sites.