Pharmacokinetics and Efficacy of a Long-lasting, Highly Concentrated Buprenorphine Solution in Rats

Buprenorphine (Bup) is an opioid analgesic that is commonly used in laboratory rodents to provide postoperative analgesia. However, dosing every 4 to 6 h is necessary to maintain an analgesic plasma concentration of the drug. A long lasting, highly concentrated veterinary formulation of Bup (LHC-Bup) has been used to provide prolonged analgesia in cats and nonhuman primates. In the current study, we evaluated the duration of efficacy of LHC-Bup to determine if this formulation would provide a similarly prolonged analgesia in rats. Drug concentrations were measured after subcutaneous injection of 0.5 mg/kg LHC-Bup in both male and female rats. Plasma levels were measured at 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, and 72 h. Male and female rats had peak plasma levels of LHC-Bup at 90 ng/and 34 ng/mL, respectively, at 15 min after administration, with a steady decrease by 24 h to 0.7 ng/mL in males and 1.3 ng/mL in females. Mechanical pain tolerance was evaluated after LHC-Bup administration using a Randall-Selitto analgesiometer to assess paw withdrawal. Male rats had a significantly longer paw withdrawal time for up to 12 h after administration, and females had longer paw withdrawal times for up to 24 h. An experimental laparotomy model was then used to assess the clinical efficacy of LHC-Bup at 0.5 mg/kg. LHC-Bup treatment was associated with a greater total distance traveled, reduced time to retrieve a food treat, and reduced grooming from 3 to 12 h after surgery as compared with saline controls. Groups receiving LHC-Bup showed coprophagy whereas other rats did not. These results suggest that administering LHC-Bup at 0.5 mg/kg provides therapeutic plasma concentrations for 12 to 24 h after administration and analgesic efficacy for at least 12 h after dosing. As such, LHC-Bup is a suitable alternative to Bup-HCl.