Pharmacokinetics of butorphanol in cats after intramuscular and buccal transmucosal administration

Abstract

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.