Bioavailability and pharmacokinetics of oral and injectable formulations of methadone after intravenous, oral, and intragastric administration in horses.

This research article investigates the bioavailability and pharmacokinetics of different forms of methadone administered to horses in various ways. The study suggests that oral administration of methadone resulted in higher bioavailability compared to intragastric administration, an insight that could guide better pain management strategies in horses.

Overview of the Study

• The research involved administering either oral or injectable forms of methadone to six healthy adult horses through intravenous, oral, or intragastric methods.
• Each horse received a single dose of the drug, with a two-week washout period between each treatment to allow the drug to be entirely flushed out of their systems before commencing the next round of testing.

Methodology

• A blood sample was taken from each horse before and after the drug administration at different time points over a 360-minute period. These samples were analyzed using gas chromatography-mass spectrometry to track the concentration of the drug in the horse’s system.
• The pharmacokinetic parameters were estimated using a non-compartmental model, which is a mathematical model based on drug concentrations over time.

Key Findings

• Results indicated that the concentration levels of methadone in the horses’ blood were equivalent to or higher than the effective range for humans, and there were no adverse effects recorded.
• It was found that oral pharmacokinetics in horses had a short half-life (approximately 1 hour), high total body clearance, and a small apparent volume of distribution. These data suggested the fast metabolism and quick excretion of the drug.
• Oral administration of methadone showed about three times higher bioavailability than that of intragastric administration, meaning the orally taken drug was more easily absorbed and utilized in the body than the one directly delivered to the stomach.

Conclusion and Implications

• Based on the findings, the study concludes that absorption of methadone in the small intestine might be limited due to the lower bioavailability after intragastric administration.
• The insights from this study can improve the understanding of methadone’s disposition in the body and potentially contribute to developing better pain management strategies in the veterinary field, particularly for horses. A more effective route of administration could optimize analgesia while minimizing potential side effects.