Pharmacokinetics and protein binding of morphine in horses.

This research paper examines the pharmacokinetics—or how the body processes—a dose of morphine in horses. It found that a 0.1 mg/kg dose can be detected for up to 48 hours in blood and 144 hours in urine, with no observable side effects, making it a possible painkiller dose.

Study Method

• The researchers administered to horses a dose of 0.1 mg of morphine per kg of body weight.
• They then monitored and measured the amount of morphine present in the horses’ blood and urine for extended periods.
• They also used computer analysis to create the most fitting model for the horses’ physiological processes and responses to the drug—finding that a three-compartment open system model was most appropriate.

Morphine Persistence

• This study found that morphine can be detected in a horse’s system for quite a while. In the blood, it is detectable for up to 48 hours. In the urine, the detection window extends to up to 144 hours (or six days).
• Despite the long presence in the body, the dose did not result in any observable effects on horses, suggesting it can be used as a non-disruptive analgesic.

Morphine Metabolism and Binding

• The metabolism of the drug was modeled using a three-compartment open system, which produced a serum half-life (t1/2(beta)) of 87.9 minutes and a urine half-life of 101.1 minutes.
• It was also found that morphine binds linearly to equine serum proteins across the drug concentration range studied, with an average binding of 31.6%.
• Red Blood Cell (RBC) partitioning experiments showed that 78.1% of the drug ended up in the plasma fraction, indicating the major location of the drug after administration.

Implications for Horse Racing

• The lingering detectability of morphine in the system of a horse has implications for horse racing. According to the data, a horse should not be administered morphine one week prior to a race due to the lengthy detection period.
• This is particularly relevant within the context of horse racing regulations, which prohibit the use of certain substances, including morphine, for a specified period before races to ensure fair competition.