Objective: 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.
Animals: 6 healthy adult cats.
Procedures: 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.
Results: 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.
Conclusions and clinical relevance: 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.