The pharmacokinetics of a fentanyl matrix patch applied at three different anatomical locations in horses.
Authors: Skrzypczak Heather, Reed Rachel, Brainard Benjamin, Sakai Daniel, Barletta Michele, Quandt Jane, Smyth Carly, Ruch Melanie, Knych Heather
Journal: Equine veterinary journal
Summary
# Editorial Summary: Transdermal Fentanyl Patches in Horses Matrix fentanyl patches offer a non-invasive alternative to catheter-dependent analgesia, yet their use in equine medicine remained unexplored until this investigation. Six horses received two 100 μg/h patches applied to three different sites (inguinal region, metacarpus, and ventral tail base) in a randomised crossover design, with blood sampling and physiological monitoring continuing for 96 hours post-application. Fentanyl was consistently detectable in plasma for the full monitoring period; peak plasma concentrations ranged from 1.55–2.07 ng/mL, achieved within 10 hours of application, with no clinically significant differences between anatomical locations. Notably, heart rate, respiratory rate and rectal temperature remained stable across all treatments, suggesting an absence of systemic adverse effects at this dosage. The critical limitation is that without intravenous administration data, bioavailability cannot be calculated—meaning we cannot yet determine whether the plasma levels achieved are sufficient to produce analgesia in horses. Practitioners considering matrix patches for extended analgesia should recognise that whilst safety appears favourable and absorption is predictable, controlled efficacy studies are essential before recommending these patches as a clinical analgesic strategy.
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Practical Takeaways
- •Matrix fentanyl patches provide extended analgesia in horses without requiring venous catheterisation, with plasma concentrations maintained for up to 96 hours post-application
- •Application site (inguinal, metacarpal, or ventral tail base) does not significantly affect fentanyl pharmacokinetics, allowing flexibility in patch placement based on clinical preference
- •Further studies are needed to establish what plasma fentanyl concentration actually produces analgesia in horses before routine clinical use can be recommended
Key Findings
- •Matrix fentanyl patches showed rapid absorption with time to maximum plasma concentration of 10±3.79, 14.3±5.13, and 10±3.79 hours for inguinal, metacarpal, and tail base applications respectively
- •No significant difference in pharmacokinetic parameters (Cmax, AUC, Tmax) among the three anatomical application sites
- •Fentanyl persisted in plasma for up to 96 hours with area under curve of 46.6±9.3, 44.6±6.0, and 46.2±7.68 ng·hours/mL for the three sites
- •No adverse effects observed on heart rate, respiratory rate, or rectal temperature across all treatments