The bioavailability of phenylbutazone in the horse.

This study presents new data on the bioavailability and excretion pathways of the drug phenylbutazone in horses, showing that the drug is rapidly absorbed and primarily excreted through the urine and faeces, which may have implications for its gastrointestinal toxicity.

Methodology

• The researchers used radioactive phenylbutazone to trace the absorption and elimination of the drug in two horses. This involved administering the drug either orally or through injection and then monitoring the horses’ plasma levels and urinary and faecal elimination.

Key Findings

• The study found that phenylbutazone is rapidly and extensively absorbed after oral administration, with a bioavailability of 91%. Bioavailability refers to the proportion of a drug that enters the circulation after being administered and is therefore able to exert an effect.
• The researchers calculated the bioavailability of phenylbutazone by comparing the Area Under the Curve (AUC) of the plasma concentration-time curve after oral and intravenous administration. The AUC measures the overall exposure of the body to the drug.
• The plasma elimination half-life of phenylbutazone was found to be 9.7 hours, regardless of the route of administration. The elimination half-life refers to the time taken for the plasma concentration of a drug to reduce by half, providing an indication of how quickly a drug is removed from the body.
• The pattern of elimination of phenylbutazone was also independent of the route of administration, with 55% of the dose found in the urine within three days and a further 39% found in the faeces within seven days.

Implications

• These findings are the first reports on the absolute bioavailability and excretion pathways of phenylbutazone in horses. Understanding the absorption, distribution, metabolism, and excretion properties of a drug is critical for predicting its bodily interactions and potential side effects.
• The researchers suggest that these data could be significant for understanding the gastrointestinal toxicity of phenylbutazone, although the study itself does not directly address this.