Pharmacokinetics of butorphanol in horses after intramuscular injection.

This study examines how the drug butorphanol, administered to horses either intravenously or intramuscularly, behaves and breaks down in the body (a field known as pharmacokinetics). The research identified that the absorption of butorphanol was rapid when injected, but its systemic availability was low and its lifespan in the body differed depending on the method of administration.

Study Design

• The research was conducted as a two-way cross-over study involving six horses. A cross-over study is a type of research where each participant receives two or more treatments in a random order, allowing each participant to act as their own control. This design was chosen to compare the effects of the drug administration method (intravenous vs intramuscular) on each horse.
• The drug dosage given to each horse was 0.08 mg/kg, which was administered either intravenously (IV) or intramuscularly (IM). Blood samples were taken at several time intervals after drug administration to measure the drug concentration in their bodies.

Key Findings

• The study found that the absorption of the drug butorphanol into the horse’s system was very rapid when administered intramuscularly; the half-life of absorption was only 6 minutes.
• However, the study also found that the systemic availability of the drug — that is, the proportion of the drug that enters the circulation and hence has an active effect — was low, at only 37% after intramuscular administration. This finding suggests that less than half of the administered drug was effectively absorbed into the horse’s system for use.
• Furthermore, the study found that the terminal half-life (the time it takes for the concentration of the drug in the body to be reduced by half) varied depending on the method of administration. The intravenous administration resulted in a shorter half-life (0.57 hours) compared to intramuscular administration (7.7 hours). This difference was attributed to the detection of a ‘deep compartment’ after IV administration, which was not detected following IM administration.
• In terms of physical parameters, there were no significant changes after butorphanol administration except for an increase in respiratory rate 60 and 180 minutes after IV administration. This change could indicate the effect of the drug on the respiratory system.

Implications and Recommendations

• According to the study’s findings, to maintain a targeted plasma butorphanol concentration — above 10 ng/mL — the horses may need to be administered an intramuscular injection of 0.08 mg/kg every 3 hours. This recommendation is due to the short absorption half-life and the low systemic availability of the drug when administered intramuscularly.
• The study indicates that, even with rapid absorption, the delivery method of butorphanol may significantly impact its effectiveness due to varying systemic availability and half-life.