Pharmacokinetics and pharmacodynamics of butorphanol in llamas after intravenous and intramuscular administration

Objective-To evaluate disposition of butorphanol after IV and IM administration, effects on physiologic variables, and analgesic efficacy after IM administration in llamas. Design-Nonrandomized crossover study. Animals-6 healthy adult male llamas. Procedure-Butorphanol (0.1 mg/kg [0.045 mg/lb] of body weight) was administered IM first and IV 1 month later. Blood samples were collected intermittently for 24 hours after administration. Plasma butorphanol versus time curves were subjected to pharmacokinetic analysis. Two months later, butorphanol (0.1 mg/kg) was administered IM, and physiologic variables and analgesia were assessed. Results-Extrapolated peak plasma concentrations after IV and IM administration were 94.8 +/- 53.1 and 34.3 +/- 11.6 ng/ml, respectively. Volume of distribution at steady state after IV administration was 0.822 +/- 0.329 L/kg per minute and systemic clearance was 0.050 +/- 0.014 L/kg per minute. Slope of the elimination phase was significantly different, and elimination half-life was significantly shorter after IV (15.9 +/- 9.1 minutes) versus IM (66.8 +/- 13.5 minutes) administration. Bioavailability was 110 +/- 49% after IM administration. Heart rate decreased and rectal temperature increased. Somatic analgesia was increased for various periods. Two llamas became transiently sedated, and 2 became transiently excited after butorphanol administration. Conclusions and Clinical Relevance-Although IV administration of butorphanol results in a short half-life that may limit its analgesic usefulness, the elimination half-life of butorphanol administered IM is likely to be clinically useful. The relationship among plasma butorphanol concentration, time, and analgesia differed with the somatic analgesia model; clinically useful analgesia may occur at lower plasma concentrations than those reported here.