Opioids are the most effective class of analgesic drugs for management of moderate to severe pain. Based on the results of previous studies, kappa opioid receptor agonist drugs have been used predominantly in birds.4,5,7,8 However, recent studies have shown that the mu agonists fentanyl and hydromorphone may have analgesic properties in raptors.2,3,6 Buprenorphine is a semi-synthetic partial mu opioid receptor agonist; in mammals it has a long-lasting, moderate analgesic effect and high margin of safety.1 Preliminary data using a thermal stimulus in American kestrels (Falco sparverius) suggests that buprenorphine has analgesic properties in this species (Guzman, unpublished data). In the study reported here, the pharmacokinetics of buprenorphine hydrochloride following IM and IV administration were determined in kestrels (n=16) in a complete crossover design, with a 2-week washout period. Blood was collected from 3 or 4 birds at each of 9 time points, from 5 minutes to 9 hours. Plasma buprenorphine concentrations were measured by liquid chromatography–mass spectrometry, and pharmacokinetic parameters were determined using non-compartmental analysis. After IV administration, the steady-state volume of distribution was 4,024 mL/kg, with a clearance of 49 mL/kg/min and an elimination half-life of 105 minutes. After IM administration, bioavailability was 94.5%, with a maximum plasma concentration of 243 ng/mL at 5 minutes and an elimination half-life of 93 minutes. IM administration of buprenorphine hydrochloride to American kestrels results in rapid absorption, high bioavailability, and similar kinetics to IV administration.
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