Pharmacokinetics of phenobarbital in the horse.

This research looks into how the drug phenobarbital is metabolized and processed by the body (pharmacokinetics) in horses. The findings detail the drug’s distribution and elimination rates, and propose an ideal dosage routine for horses.

Study Design and Objectives

• The researchers performed a pharmacokinetic analysis of the drug phenobarbital in six mature horses. Phenobarbital is a long-acting antiepileptic and sedative medication often used in veterinary medicine.
• The objective was to understand how the drug is distributed in a horse’s body, how long it takes for half of the drug to be eliminated, and how quickly the drug is cleared from the body. These parameters can help inform appropriate dosing guidelines.

Key Findings

• The experiment started with a drug infusion of 12 mg of phenobarbital per kg of the horse’s body weight, spanning over 20 minutes. Following this, researchers observed a biexponential decrease in the drug concentration in the horse’s blood.
• The distribution phase, which pertains to how quickly the drug is spread throughout the body, had a half-life of approximately 0.101 hours. The half-life refers to the amount of time it takes for the concentration of the drug to reduce by half in the body.
• The elimination phase, the time period during which the drug is removed from the body, had a half-life of roughly 18.3 hours. This implies that, on average, it took about 18.3 hours for horses to eliminate half of the drug from their bodies.
• The total clearance, which indicates the volume of blood from which the drug is completely removed per unit of time, was found to be 30.8 mL/h/kg. This high clearance led to a notably shorter half-life of phenobarbital in horses compared to other species.
• The volume of distribution at steady state, which defines the theoretical space in the body where the drug disperses, was calculated at approximately 0.803 L/kg.

Implications and Predictions

• Based on the observed pharmacokinetic parameters, the researchers proposed an optimal dosing regimen for horses. This involved an infusion of 6.65 mg of phenobarbital per kg, delivered over 20 minutes, and repeated every 12 hours.
• With this regimen, the researchers predicted approximate peak and trough concentrations of 29 micrograms/ml and 15 micrograms/ml respectively. These represent the highest and lowest concentrations of drug in the blood over the proposed period.
• A loading dose, which is a higher initial dose of a drug to quickly establish a certain level in the bloodstream, of 12 mg/kg was suggested as being appropriate for this regimen.