Pharmacokinetics of a Sustained Release Formulation of Buprenorphine After Intramuscular and Subcutaneous Administration to American Kestrels (Falco sparverius)

Previous studies have validated the clinical use of opioids with μ-receptor affinities for pain management in raptors. Buprenorphine appears to have a longer duration of action and minimal adverse effects when compared to other opioids in American kestrels (Falco sparverius). To determine the pharmacokinetics of a sustained release formulation of buprenorphine in kestrels, we administered a commercially available product (Buprenorphine SR-LAB; Wildlife Pharmaceuticals, Windsor, CO, USA) intramuscularly and subcutaneously to adult kestrels in a partial-crossover experimental design study. A total of 12 birds (6 males and 6 females) were assigned randomly to 3 groups of 4 birds each. A single dose of Buprenorphine SR-LAB (1.8 mg/kg) was administered intramuscularly (IM), and blood samples were collected at 0.25, 3, and 24 hours (n = 4); 1, 6, and 48 hours (n = 4); and 2, 12, and 72 hours (n = 4) after drug administration. Plasma buprenorphine concentrations were measured by tandem liquid chromatography–mass spectrometry. Pharmacokinetic parameters were determined by use of least squares linear regression and noncompartmental analysis of naïve pooled data. After 1 year, the same dose of buprenorphine was administered subcutaneously (SC) to 12 birds divided into 3 groups as previously, and blood samples were collected at the same times after drug administration. Maximum plasma buprenorphine concentration was measured at 15 minutes after IM and SC administration. Mean plasma buprenorphine concentrations were >1 ng/mL for 48 hours after IM and SC administration. The elimination half-life was 13.5 and 11.1 hours for IM and SC administration, respectively. Depending on the severity and type of pain, adjunctive therapy, and the individual response, Buprenorphine SR-LAB administered at 1.8 mg/kg IM or SC to American kestrels would require administration every 12 to 72 hours to manage pain. Further pharmacodynamic and clinical evaluations are warranted in kestrels and other raptors to establish accurate dosing recommendations.