Pharmacokinetics of butorphanol in cats after intramuscular and buccal transmucosal administration

To determine the pharmacokinetics of butorphanol in cats following IM and buccal transmucosal (BTM) administration, to determine the relative bioavailability of butorphanol following BTM administration, and to extrapolate a plasma concentration associated with antinociception on the basis of existing data from pharmacologic studies of butorphanol in cats.

6 healthy adult cats.

Following IM or BTM butorphanol tartrate (0.4 mg/kg) administration to cats in a 2-way crossover trial, plasma samples were obtained from blood collected via a central venous catheter during a 9-hour period. Plasma butorphanol concentrations were determined by high-performance liquid chromatography.

Data from 1 cat contained outliers and were excluded from pharmacokinetic analysis. Mean+/-SD terminal half-life of butorphanol for the remaining 5 cats was 6.3+/-2.8 hours and 5.2+/-1.7 hours for IM and BTM administration, respectively. Peak plasma butorphanol concentrations were 132.0 and 34.4 ng/mL for IM and BTM administration, respectively. Time to maximal plasma concentration was 0.35 and 1.1 hours for IM and BTM administration, respectively. Extent of butorphanol absorption was 37.16% following BTM application. On the basis of data from extant pharmacologic studies of butorphanol in cats, mean+/-SD duration of antinociception was 155+/-130 minutes. The estimated plasma concentration corresponding to this time point was 45 ng/mL.

In cats, IM butorphanol administration at 0.4 mg/kg maintained a plasma concentration of >45 ng/mL for 2.7+/-2.2 hours, whereas BTM administration at the same dose was not effective at maintaining plasma concentrations at >45 ng/mL.