Phenylbutazone and oxyphenbutazone distribution into tissue fluids in the horse.

This research investigates the distribution of a drug called phenylbutazone and its active metabolite oxyphenbutazone in horses. The researchers measure the drug concentrations in various body fluids after administering it and suggest how these findings could explain the drug’s extended period of effectiveness.

Research Methodology

• The study involved injecting the commonly recommended dosage of phenylbutazone into five Welsh Mountain ponies.
• Scientists then examined how the drug and its active metabolite oxyphenbutazone spread into different body fluids. They conducted this by measuring the concentrations in the plasma, tissue-cage fluid, peritoneal fluid, and acute inflammatory exudate collected from a model of inflammation.

Key Findings

• Phenylbutazone was found to easily penetrate into the inflammatory exudate, as demonstrated by the relatively high mean maximum concentration (Cmax) of 12.4 micrograms/ml occurring at around 4.6 hours, and a mean area under the curve (AUC0-24) of 128 microgram X h/ml.
• The average exudate to plasma ratio was also high, indicating easy passage of the drug between these two mediums.
• The drug ratios were initially higher in plasma but subsequently more concentrated in the exudate, suggesting slower clearance from the exudate. This was indicated by the half-life values for plasma and exudate at approximately 4.8 hours and 24 hours respectively.
• The lower values for Cmax and AUC0-24 for phenylbutazone passing into peritoneal fluid demonstrate a more limited penetration compared to the tissue cage fluid.
• Concentrations of oxyphenbutazone were lower than phenylbutazone at all times in all fluids, yet they were still found to readily penetrate, particularly into inflammatory exudate; suggesting it possibly contributes to the anti-inflammatory effect of the parent drug.

Conclusion and Implications

• The research suggests that the extended duration of action of phenylbutazone could be due to its slower clearance from the exudate, despite its plasma elimination half-life being less than 5 hours.
• Scientists also hypothesize that the increased blood flow (hyperaemia) seen in acute inflammation and high protein levels in inflammatory exudate may facilitate the passage of phenylbutazone and oxyphenbutazone into the exudate.