Concentrations of indomethacin and its metabolite desmethylindomethacin in plasma and urine after repeated indomethacin topical application to Thoroughbreds.

This research study investigated the concentrations of anti-inflammatory drug indomethacin and its metabolite desmethylindomethacin in horse plasma and urine after repeated topical applications, providing important pharmacokinetic data for its use in Thoroughbred racehorses.

Research Objective and Methods

• The aim of the study was to investigate the amount of indomethacin and its metabolite, desmethylindomethacin, present in the plasma and urine of racehorses after continuous topical application. This has been a common practice among Japanese racehorses despite insufficient data about its pharmacokinetics – the way the drug is absorbed, distributed, metabolized, and excreted in the body.
• Seven female Thoroughbreds underwent treatment with 50 grams of 1% indomethacin cream applied to their back and hips for three days. This equates to a dosage of 500 mg of indomethacin per horse over an area of 2400 cm², or 0.21 g/cm².
• To analyze the samples, protein precipitation was used for the plasma and a liquid-liquid extraction with ethyl acetate following a hydrolysis with hydrochloric acid was used for urine. The concentrations of both indomethacin and desmethylindomethacin were measured using liquid chromatography-mass spectrometry, a technique to identify and quantify molecules in mixture.

Results

• After the final application of the cream, indomethacin could be quantified in the horses’ plasma for 48-72 hours and in their urine for up to 96 hours. Similarly, desmethylindomethacin was quantifiable in plasma up to 48 hours and in urine for 72-96 hours.

Conclusions

• Unfortunately, the relationship between the concentrations of indomethacin locally (at the site of application) and systemically (in the body overall) wasn’t determined. However, valuable pharmacokinetic data was obtained for repeated topical administration of 1% indomethacin cream to Thoroughbreds.
• The research also found that using hydrochloric acid to hydrolyze urine samples was an effective method for analyzing indomethacin and its metabolite.
• The researchers suggest that indomethacin may be a useful marker for doping tests in horses.
• The estimated time required for indomethacin to reduce below the limit of detection, also known as the withdrawal time, was calculated to be 342 hours. This information is critically important in determining when a racehorse can safely participate in a race after treatment with this drug.