Pharmacokinetics of phenylbutazone in two age groups of ponies: a preliminary study.

The study examined the metabolic behaviour of a drug, phenylbutazone, in ponies of two different age groups. It revealed that the drug’s clearance from the body was quicker in younger ponies, while older ones showed higher amounts of the drug in their plasma post-oral administration.

Experiment Method

• A group of six Welsh mountain ponies were given a clinical dose of phenylbutazone. These ponies were divided into two age groups: three were three years old, and three were between eight to ten years old.
• The drug was administered in two different ways: intravenously (directly into the bloodstream) and orally.
• The subsequent concentration of the drug in the ponies’ plasma was monitored over time. This data was used to examine the pharmacokinetics (how the drug moves through the body) and bioavailability (extent and rate at which the drug is absorbed and becomes available at the site of action).

Findings

• The drug was found to clear from the plasma of the younger ponies almost twice as fast as it did from the older ponies when administered intravenously. The plasma drug clearance refers to the rate at which the drug is removed from the plasma.
• The higher plasma levels of phenylbutazone in older ponies did not result from more efficient absorption, but from a slower rate of plasma clearance after oral administration.
• The fractional absorption (the proportion of the drug that is absorbed into the system) of phenylbutazone was quite similar in both young and older ponies.
• Around 25% of the administered intravenous dose was found excreted as phenylbutazone and its by-products, oxyphenbutazone and gamma-hydroxyphenylbutazone, in the ponies’ urine in the first 24 hours. This was consistent between both age groups.
• The fate of the remaining 75% of the drug is discussed but not mentioned explicitly in the abstract.

Implications

• The study provides preliminary evidence that age influences how a horse’s body metabolizes phenylbutazone. This research could help equine veterinarians prescribe appropriate phenylbutazone dosages for their patients, considering the age and metabolic response of the animal.
• This provides a basis for further investigation into how factors such as age and method of administration affect the pharmacokinetics and bioavailability of drugs in horses.