Single and multiple dose pharmacokinetics of gentamicin administered intravenously and intramuscularly in adult conditioned thoroughbred mares.

The research investigated how the drug gentamicin is absorbed and processed in the body of adult Thoroughbred mares. The study found that the rate of absorption and processing was consistent regardless of whether the drug was given intravenously or intramuscularly, and whether it was administered once or multiple times.

About the Study

• In this experiment, the researchers looked at the pharmacokinetics (how drugs are absorbed, distributed, metabolized, and excreted by the body) of the antibiotic gentamicin in 6 adult Thoroughbred mares. This was done in a two-phase, randomized cross-over study, where the horses were administered the drug either intravenously (directly into their blood) or intramuscularly (into their muscles).
• The doses were given at a ratio of 3.3 mg per kg of body weight every 12 hours, for five consecutive treatments. Equal numbers of mares were subjected to each method of administration in each phase.
• Between the two phases, a wash-out period of 5 days was observed to ensure the drug was completely removed from the horses’ bodies before starting the next phase. Multiple blood samples were taken from each mare during the process to monitor how the drug was processed in their bodies.

Results and Findings

• The results showed that plasma gentamicin concentration versus time data for both single and multiple doses given via either route was best described by a two-compartmental open model with first order rate constants. Essentially, this means that gentamicin was distributed and eliminated from the body in two phases.
• The first phase (distribution phase), which happens rapidly, had a half-life of 0.1 +/- 0.1 hours. The second (elimination phase), which is slower, had a half-life of 1.2 +/- 0.2 hours. The mean residence time (the average time the drug stays in the body) was 1.4 +/- 0.1 hours, and the total clearance rate was 1.4 +/- 0.2 ml/kg/min.
• No significant differences were observed for any of these parameters between single or multiple doses or different routes of administration. However, there were significant differences between multiple intravenous and intramuscular treatments for all the pharmacokinetic parameters determined, except for the Area Under the Curve (AUC) – a measure of total drug exposure over time.