Pharmacokinetics of intra-articular morphine in horses with lipopolysaccharide-induced synovitis.

The research paper describes a study conducted to understand the effects and longevity of morphine when administered directly into the joint, specifically in the case of horses with a certain type of joint inflammation. The study found that the morphine remained within the joint for at least 24 hours while maintaining very low serum concentrations, suggesting the possibility of longer lasting pain relief through this method of administration.

Methodology

• The study consisted of eight adult mixed breed horses with an average age of 6.5 years and weight of 535 kilogrammes.
• To induce synovitis, a form of joint inflammation, the horses were given an injection of 3 micrograms of Lipopolysaccharide (LPS) in the radiocarpal (wrist) joint.
• This procedure was performed twice, once on each wrist, separated by a three-week period to allow for any effects to wear off.
• Four hours after the LPS was administered, the horses were then given a treatment. One group received morphine delivered directly into the joint (intra-articularly) with a saline solution delivered via intravenous (IV), and the other group was given a saline IA along with the morphine IV.

Findings

• The results showed that all of the horses developed synovitis as intended after the LPS was injected.
• Morphine was found in the synovial fluid of all eight joints 24 hours after treatment, and in six out of eight joints 28 hours after treatment, indicating that it remained in the joint for an extended period of time. The estimated half-life of the morphine was 2.6 hours.
• The mean serum concentrations of the morphine were below 5 ng mL(-1) from 2 to 28 hours after treatment, suggesting the presence of very low levels of morphine in the body outside of the treated joint.

Conclusion

• The findings of this study suggest that the administration of morphine directly into the joint may provide pain relief lasting at least 24 hours.
• The cause of this extended relief is likely due to the fact that the morphine remained within the joint while producing very low serum concentrations, emphasizing a primarily peripheral effect.
• This evidence strengthens the theory that the analgesic (pain-relief) and anti-inflammatory effects of intra-articularly administered morphine in horses are primarily mediated at the site of injection.