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

Objective-To evaluate disposition of butorphanol after IV and IM administra tion, effects on physiologic variables, and analgesic efficacy after IM adm inistration 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 administ ered IM first and IV 1 month later. Blood samples were collected intermitte ntly for 24 hours after administration. Plasma butorphanol versus time curv es were subjected to pharmacokinetic analysis. Two months later, butorphano l (0.1 mg/kg) was administered IM, and physiologic variables and analgesia were assessed. Results-Extrapolated peak plasma concentrations after IV and IM administrat ion were 94.8 +/- 53.1 and 34.3 +/- 11.6 ng/ml, respectively. Volume of dis tribution 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. Slop e of the elimination phase was significantly different, and elimination hal f-life was significantly shorter after IV (15.9 +/- 9.1 minutes) versus IM (66.8 +/- 13.5 minutes) administration. Bioavailability was 110 +/- 49% aft er IM administration. Heart rate decreased and rectal temperature increased . Somatic analgesia was increased for various periods. Two llamas became tr ansiently sedated, and 2 became transiently excited after butorphanol admin istration. Conclusions and Clinical Relevance-Although IV administration of butorphano l results in a short half-life that may limit its analgesic usefulness, the elimination half-life of butorphanol administered IM is likely to be clini cally useful. The relationship among plasma butorphanol concentration, time , and analgesia differed with the somatic analgesia model; clinically usefu l analgesia may occur at lower plasma concentrations than those reported he re.