Pharmacokinetics and antinociceptive activity of sustained-release buprenorphine in sheep

Buprenorphine is a potent analgesic commonly administered to alleviate pain in sheep used in research. Sustained-release buprenorphine (SRB) is an alternative to conventional buprenorphine hydrochloride (which must be injected repeatedly). To compare SRB with a typical conventional buprenorphine regimen (0.03 mg/kg every 8 h for 72 h), we used a simple 1:1 conversion to calculate a total SRB dose of 0.27 mg/kg per injection. The pharmacokinetics and thermal nociceptive effects of SRB were analyzed in 4 healthy adult sheep after a single intramuscular injection plus a washout period then a single subcutaneous injection. For both routes in all 4 sheep, plasma buprenorphine concentrations exceeded 0.1 ng/mL, considered the minimal threshold for therapeutic benefit, after 12 h and maintained a steady state for at least 72 h Likewise, for both routes in all sheep, thermal thresholds increased significantly between baseline and 12 h; lack of response persisted for at least 72 h. The average maximal plasma buprenorphine concentrations and bioavailability were similar for both routes. No clinical adverse effects occurred. Using a dose equivalent to the total course of conventional buprenorphine, this pilot study suggests that SRB is a well-tolerated, effective, and long-acting analgesic that can be administered as a single intramuscular or subcutaneous injection. SRB confers steady plasma concentrations and continuous analgesia in thermal nociception for at least 72 h. When compared with conventional buprenorphine, SRB has considerable advantages in improving wellbeing by minimizing handling-associated stress of repeated injection and limiting the likelihood of end-of-dose breakthrough pain.