Pharmacokinetics of fentanyl delivered transdermally in healthy adult horses–variability among horses and its clinical implications.

The research investigates the safety and pharmacokinetics of transdermal fentanyl patches in adult horses. Differing responses among individual horses raise implications for clinical use.

Transdermal Fentanyl Patches: Administration and Quantification

• The study looked into administering fentanyl, a potent opioid analgesic, through transdermal patches to six healthy adult horses. The patches, with a dosage of 60-67 microg/kg, were placed on the mid-dorsal thorax of each horse and left for 72 hours.
• The researchers periodically measured plasma fentanyl concentrations during the application of the patch and for 12 hours post-removal.
• The initial 2-hour delay in absorption was observed after which the plasma fentanyl concentration increased fairly linearly, reaching its peak at around 12 hours. A subsequent decline was observed gradually over the next 72 hours.

Individual Variation in Horses

• There was significant variability in the responses among horses. The initial delay ranged between 0 to 5.1 hours, time to reach peak concentration varied from 8.5 to 14.5 hours, and the peak plasma concentration ranged from 0.67 to 5.12 ng/mL.
• In two horses, the plasma fentanyl concentration didn’t exceed 1 ng/mL, whereas it inflicted a higher concentration in the other four. In two of these four horses, the higher concentration of fentanyl was maintained for the full 72 hours.

Safety and Clinical Implications

• No adverse effects were linked to the usage of fentanyl, establishing that its dosage applied in the study is safe in healthy adult horses.
• However, the effectiveness of the drug as an analgesic is questionable. Around one-third of the horses did not achieve the plasma concentration considered effective for pain relief (1 ng/mL or more). This emphasizes the variability among individual horses and signals potential issues in achieving consistent clinical outcomes.