Pharmacokinetic and Histopathologic Study of an Extended-Release, Injectable Formulation of Buprenorphine in Sprague–Dawley Rats

Abstract

A novel buprenorphine (BUP) extended-release formulation (BUP-XR) produced as a lipid-encapsulated, low viscosity BUP suspension for SC injection to control pain was evaluated for pharmacokinetics and safety in Sprague–Dawley rats given either 0.65 mg/kg (low dose) or 1.30 mg/kg (high dose). The 2 dosage groups each contained 6 male and 6 female rats to determine whether BUP-XR behaved differently in male or female animals. Blood samples were obtained from each animal before BUP-XR administration and at 6, 24, 48, 72, 96, and 168 h after administration. For necropsy and injection-site histopathology evaluation, 3 animals of each sex from each test group were euthanized on day 8, with the remaining animals euthanized on day 15. Mean plasma BUP concentration peaked from 6 to 24 h in all test groups, then declined in a linear fashion. Quantifiable plasma BUP was measured in all male rats at all time points except for one low dose group sample taken at 168 h. Female rats had quantifiable plasma BUP at all time points except for 1 low dose group sample at 72 and 96 h, and 2 low dose group samples at 168 h. The low dose groups, whether male or female, had lower mean plasma BUP levels at all time points as compared with their high dose counterparts, and female rats had lower mean plasma BUP levels than male rats at all time points. Results indicate that a single BUP-XR dose at either dose concentration can reliably provide plasma levels of BUP reported in the literature to be therapeutically relevant for up to 72 h, although lower plasma BUP levels can be anticipated in female rats compared with male counterparts. Mild to moderate injection-site granulomatous inflammation was observed in 6 of 12 rats in the low dose group and 7 of 12 in the high dose group. This reaction is characteristic of lipid material designed to persist in situ.