Factors affecting absorption of non-steroidal anti-inflammatory agents in the horse.

The study examined the absorption of phenylbutazone, a non-steroidal anti-inflammatory drug, in horses and ponies under different conditions. It found that absorption varied among individual animals and was affected by factors such as fasting and time of administration.

Research Method

• The researchers studied the absorption of orally administered phenylbutazone (5 mg/kg) in ten thoroughbreds, eight ponies, and four pony foals. They examined the area under the curve (AUC), which is a measure of drug exposure, and peak plasma concentrations.
• They tested the effects of different conditions, such as fasting overnight before drug administration and repeated twice-daily administration. Phenylbutazone (5 mg/kg) was given after an overnight fast in various situations. The absorption characteristics of two different preparations of oral phenylbutazone (Equipalazone by Arnolds and Prodynam by Leo Laboratories) were also tested under the same conditions.

Key Findings

• The study identified large variations in drug absorption within the same animal and among different animals. The mean AUC values varied among the three groups of animals but were not significantly different after an overnight fast.
• The absorption characteristics of the two phenylbutazone preparations tested were found to be similar.
• Feeding before drug administration reduced the AUC and peak plasma concentration of the drug. It also extended the range of time taken to achieve peak concentration.
• Repeated administration of phenylbutazone twice daily led to faster absorption in the morning than in the afternoon. The same was found true with the use of another NSAID, meclofenamic acid (4.4 mg/kg), with higher peak plasma concentrations and AUC following overnight fast.
• When phenylbutazone was administered through intramuscular injections, the absorption was slower than after oral administration in fasted animals.

Implications

• The findings suggest that feeding schedules, the time of administration, and the method of drug delivery can all affect the absorption of phenylbutazone in horses. This could have practical implications for veterinarians and horse owners who are trying to manage pain and inflammation in these animals.
• Given the high variability seen both within and among individuals, dosing may need to be adjusted based on individual response rather than a standard dose for all horses.