Abstract
Objective—To compare pharmacokinetic and pharmacodynamic characteristics of fentanyl citrate after IV or transdermal administration in cats.
Animals—6 healthy adult cats with a mean weight of 3.78 kg.
Procedure—Each cat was given fentanyl IV (25 mg/cat; mean ± SD dosage, 7.19 ± 1.17 mg/kg of body weight) and via a transdermal patch (25 µg of fentanyl/h). Plasma concentrations of fentanyl were measured by use of radioimmunoassay. Pharmacokinetic analyses of plasma drug concentrations were conducted, using an automated curvestripping process followed by nonlinear, leastsquares regression. Transdermal delivery of drug was calculated by use of IV pharmacokinetic data.
Results—Plasma concentrations of fentanyl given IV decreased rapidly (mean elimination half-life, 2.35 ± 0.57 hours). Mean ± SEM calculated rate of transdermal delivery of fentanyl was 8.48 ± 1.7 mg/h (< 36% of the theoretical 25 mg/h). Median steadystate concentration of fentanyl 12 to 100 hours after application of the transdermal patch was 1.58 ng/ml. Plasma concentrations of fentanyl < 1.0 ng/ml were detected in 4 of 6 cats 12 hours after patch application, 5 of 6 cats 18 and 24 hours after application, and 6 of 6 cats 36 hours after application.
Conclusions and Clinical Relevance—In cats, transdermal administration provides sustained plasma concentrations of fentanyl citrate throughout a 5- day period. Variation of plasma drug concentrations with transdermal absorption for each cat was pronounced. Transdermal administration of fentanyl has potential for use in cats for long-term control of pain after surgery or chronic pain associated with cancer. (Am J Vet Res 2000;61:672–677)
