Drug interactions in the horse: effect of furosemide on plasma and urinary levels of phenylbutazone.

The study tests the effects of a drug called furosemide on the plasma and urinary concentrations of another drug, phenylbutazone, in horses. The research found that furosemide does impact the detectability of phenylbutazone in urine, but it doesn’t affect its presence in plasma.

Experimental Procedure

• The experiment involved horses which were initially treated with a specific dose (6.6 mg/kg) of phenylbutazone. This drug is often used in horses for pain relief and as an anti-inflammatory agent.
• Following this, these horses were injected with a dose (1 mg/kg) of furosemide intravenously. Furosemide is a diuretic that’s commonly used in horses to prevent exercise-induced pulmonary hemorrhage.
• The researchers then measured the levels of phenylbutazone in both the plasma and urine of the horses.

Key Findings

• The study found that furosemide did not have a significant influence on the plasma levels or the plasma half-life of phenylbutazone. In other words, its presence in the blood remained steady and unaltered by the administration of furosemide.
• However, the urinary levels of phenylbutazone reduced significantly (by 18 times) due to furosemide administration. This suggests that furosemide increases the excretion of phenylbutazone through urine, making it difficult to trace in urinalysis tests.

Implications of the Research

• This study implies that it’s not possible to accurately monitor the use of phenylbutazone in horses through urine testing alone if furosemide is also being used. It’s because furosemide drastically lowers the urinary levels of phenylbutazone, potentially resulting in false negatives in drug detection tests.
• However, because furosemide does not affect the phenylbutazone levels in plasma, plasma level determinations can still be a reliable method for monitoring its usage in horses.