Population pharmacokinetics of butorphanol following intramuscular administration to exercised thoroughbred horses.

This research article examines how the drug butorphanol behaves in the bodies of thoroughbred horses when administered intramuscularly. The results suggest that to avoid positive drug tests during racing, a wait time of approximately 96 hours after administering the medication is required.

Objective of the Research

• The main objective of this research is to study the pharmacokinetics of butorphanol—a drug frequently used for therapeutic purposes in racehorses—in the bodies of racing horses. This study aims to understand how the drug is absorbed, distributed, metabolised, and expelled from the horses’ bodies when it is given through an intramuscular route. This understanding is critical for framing guidelines on appropriate usage of the drug in racehorses to prevent inadvertent positive drug tests.

Methodology

• In this study, a single intramuscular dose of 0.1 mg/kg butorphanol was given to 12 exercised Thoroughbred horses.
• The research team collected serum and urine samples at various points of time post drug administration to measure the concentration of butorphanol. They used a technique known as liquid chromatography tandem mass spectrometry to perform this measurement, which is known for its high sensitivity.
• The maximum concentration of the drug in the serum and the time taken to reach this concentration were ascertained. Other pharmacokinetic parameters, such as the terminal half-life of the drug, were also determined.

Findings

• The findings showed that butorphanol reaches maximum concentration (139.9 ± 72.8 ng/mL) in the serum about 0.43 ± 0.44 hours after administration.
• Even though it might not be clinically significant, the research noted a prolonged terminal phase, which resulted in a longer terminal half-life (7.67 ± 1.86 hours). This observation is important for ensuring the drug is completely expelled from the horse’s system before a drug test.
• Based on the calculated pharmacokinetic parameters and the proposed blood screening limits by the Horseracing Integrity and Welfare Unit, the estimated detection time for butorphanol after intramuscular administration was found to be 96 hours.

Estimated detection time

• Research suggests a wait time of approximately 96 hours after administering the medication is required to avoid a positive drug test during racing