Absorption and pharmacokinetics of phenylbutazone in Welsh Mountain ponies.

The researchers studied how phenylbutazone, a common pain reliever and anti-inflammatory drug, is absorbed and metabolized in Welsh Mountain ponies. They found that feeding schedules greatly influenced absorption rates, with double peaks in blood levels being common when food was consumed before and after drug administration, posing potential implications for its clinical efficacy and use in equestrian sports.

Research Methodology

• The team administered intravenous doses of phenylbutazone (4.4 mg/kg) to six Welsh Mountain ponies and observed that the drug’s disposition followed a two-compartment open model. They monitored the pharmacokinetic parameters to notice any significant differences between morning and afternoon dosing.
• The researchers then administered the same dose of phenylbutazone orally to these ponies under varying feeding schedules to track variations in peak concentration times.

Findings

• When ponies were given hay before and after dosing, there were marked variations in time to peak concentrations, and double peaks were common. These phenomena were still seen when ponies were deprived of food prior to dosing and allowed hay afterward, albeit with much shorter peak times.
• When food was withheld both before and after dosing, absorption was more regular, and double peaks were less prominent.
• The researchers tentatively suggested that this inconsistency in absorption could be due to some phenylbutazone being absorbed quickly while some might get adsorbed onto the food and subsequently released during digestion.

Implications

• The study suggested that delayed absorption might have implications for the drug’s toxicity and clinical efficacy. In sports equestrian scenarios where phenylbutazone is commonly used, this could play a significant role.
• Despite the delayed absorption in ponies given hay, there was no significant reduction in total absorption. The bioavailability (amount of drug that enters circulation after administration) was estimated to be approximately 69% in fed and 78% in unfed ponies.
• The estimates of bioavailability were similar for both morning (72%) and afternoon (71%) dosing, indicating that the time of day did not significantly impact the drug’s absorption.