Design, Characterization and In Vivo Evaluation of a Microparticulate Depot Formulation of Buprenorphine for Veterinary Use

Buprenorphine is a semisynthetic opioid derivative commonly used to ameliorate pain in laboratory and companion animals after surgical interventions. While buprenorphine is a highly potent analgesic agent, its potency for severe side effects like respiratory depression is low. A major drawback however is buprenorphine’s short terminal half-life of 3-5 hours in rodents, which necessitates repeated injections several times per day. This leads to considerable stress and pain for animals through recurring animal handling and increases workload for caretakers and researchers. Even though alternative formulations and administration routes of buprenorphine for veterinary use have been proposed, none of the suggested methods are devoid of drawbacks, side effects or other problems associated with reliable pain alleviation. The aim of this work was therefore to develop and characterize an easy to use, safe and effective depot formulation of buprenorphine to prolong the analgesic effect. Poly (lactic-co-glycolic acid) (PLGA) is one of the most studied synthetic polymers for controlled release drug delivery. Its excellent biodegradable and biocompatible characteristics make it a highly valuable excipient for sustained release applications. This work proposes therefore a novel size-controlled PLGA based microparticulate depot formulation for prolonged and controlled pain reduction. Different PLGA polymers were used to produce various microparticulate buprenorphine formulations. Characterization was done regarding size, morphology, drug load and in vitro release. Lead formulation was identified as a product with a burst release of roughly 30% and a controlled drug release of up to three days. Pharmacokinetic studies in naive, female, adult C57BL/6J mice revealed fast onset of action and exposure above therapeutic threshold of 1 ng/ml in plasma and brain for 12 and 72 hours, respectively sustained-release formulation was further assessed with the hotplate assay. Thereby, significant effect was shown for at least 24 hours in mice. Furthermore, analgesic effect was evaluated after sham-ovariectomy, to simulate real-life surgical set-up. Mouse Grimace Scale revealed that one injection of novel depot formulation was equivalent to several injections of commercial non-retard formulation regarding pain alleviation post-surgery. No side effects or impairments appraised by nest building behavior and clinical parameters (e.g. body weight, food, and water intake) were identified after surgery. Based on the duration of action and the capability to alleviate pain reliably after surgical intervention without any side effects, this depot product is considered a valuable alternative to commercial non-retard formulations. Further evaluation of depot formulation revealed sensitivity of buprenorphine towards X-rays during terminal sterilization process, necessitating aseptic manufacturing to ensure sterility. Further characterization in terms of potential future industrial production showed, that sustained-release formulation complies with requirements regarding bacterial endotoxin burden, residual moisture levels, shelf life after reconstitution and shelf life of final product. This work therefore indicates, that the proposed manufacturing procedures allow for industrial production and future commercialization. A usage of buprenorphine depot formulation is therefore proposed as a safe and effective product for prolonged pain management in laboratory mice.