Pharmacokinetics and behavioral effects of methylphenidate in Thoroughbred horses.

The research article presents a study on the effects of the drug methylphenidate, commonly sold as Ritalin, on Thoroughbred horses. It explores how the drug is processed in the horses’ system and its impact on their behavior.

Pharmacokinetics of Methylphenidate in Horses

• Methylphenidate was administered to Thoroughbred horses intravenously (IV). The dosage given was 0.70 mg/kg.
• Upon administration, the initial plasma concentrations of the drug decreased quickly, with an alpha half-life of about 19 minutes. This means the drug was breaking down relatively quickly in the body.
• After this initial rapid decline, the drug concentration decreased more slowly, with a beta half-life of about 2.4 hours. This shows that the drug persisted in the horses’ systems for a period of time after the rapid initial breakdown occurred.
• The study used a 2-compartment open model to accurately depict how the drug was absorbed, distributed, metabolized, and then excreted by the horse’s body.
• About 40% of the methylphenidate in the horse’s blood was found in the plasma fraction, and of this, approximately 80% was bound to plasma-proteins.

Subcutaneous or Intramuscular Administration

• If the drug was administered either subcutaneously or intramuscularly, the plasma concentrations of methylphenidate peaked in about 1 hour.
• The drug was detectable in plasma up to 6 hours after administration, signifying that it takes that long for the drug to be cleared from the plasma.
• Urinary clearance time of the drug was longer, and a dose of methylphenidate would take between 12 and 24 hours to be cleared from urine.

Behavioral Effects of Methylphenidate

• The study also documented the behavioral impact of the drug using a variable-interval responding apparatus.
• Methylphenidate was able to stimulate the response rate of horses to up to six times the baseline rate, thereby classifying it as a potent central stimulant in the apparatus used in the study.
• Peak central stimulation was observed at 30 minutes after intravenous dosing when horses were given 0.4 to 1.0 mg of methylphenidate per kg of body weight.