Pharmacokinetics and tolerance of transdermal fentanyl administration in foals

Objective To characterize the pharmacokinetics of fentanyl and the tolerance of foals to the drug following a single application of a commercially available transdermal system (TS). Study design Prospective experimental study. Animals Six (two male, four female) foals aged 4-8 days, weighing 56-74 kg. Methods After placement of a jugular sampling catheter, one fentanyl TS (FTS) containing 10.2 mg fentanyl, released at 100 μg hour^-1, was applied for 72 hours. Blood samples were withdrawn over the course of 90 hours for fentanyl plasma analysis. Before and after the study, weight, complete blood count and blood chemistry values were obtained. During the study, tolerance and safety were monitored by physical examination and assessment of behavior. Results Fentanyl was detected as early as 20 minutes after FTS placement. Peak plasma concentrations were variable (0.1-28.7 ng mL^-1), were reached after 14.3 ± 7.6 hours (mean ± SD), and returned to baseline concentrations 12 hours after FTS removal. All foals satisfactorily tolerated the FTS application and no significant adverse effects were observed. Rectal temperature increased above 38.5 °C (max. 39.0 °C) in all foals, although this did not correlate with fentanyl plasma concentrations. Results of hematological and biochemical analyses were within reference ranges. Conclusion and clinical relevance Our data show that 100 μg hour^-1 fentanyl administered by an FTS results in time-related but variable plasma concentrations in foals. The FTS was easy to apply and was well tolerated.