Disposition of transdermally administered fentanyl in dogs

To evaluate the disposition of fentanyl after i.v. and transdermal administrations. The hypothesis was that transdermal administration of fentanyl would result in a measurable plasma opioid concentration.

Each dog received 2 treatments in a randomized, crossover design.

6 clinically normal Beagles.

2 treatments consisting of i.v. fentanyl (50 micrograms/kg of body weight) and transdermal fentanyl (50 micrograms/h) administrations. Plasma fentanyl concentrations were measured at fixed times, and pharmacokinetic values were calculated.

Intravenous pharmacokinetics of fentanyl was similar to those previously described in dogs and provided the distribution and clearance data necessary to calculate the rate of absorption of the transdermally administered opioid. The transdermal fentanyl patch produced average steady-state concentrations of 1.6 ng/ml. The actual rate of delivery of transdermal fentanyl was 35.7 (range, 13.7 to 49.8) micrograms/h, which represented 71.48% (range, 27.45 to 99.56%) of the theoretical rate of delivery. The mean elimination half-life of fentanyl after patch removal was 1.39 hours.

Transdermally administered fentanyl resulted in fairly constant plasma concentrations, in the range generally considered to be analgesic, from 24 to 72 hours after application of the patch. The rate of drug delivery was less than expected, and there was substantial individual variation.

Transdermally administered fentanyl has the potential to be a clinically useful analgesic regimen in dogs, and further evaluation of its analgesic actions and potential side effects warranted