Detection of levamisole and its metabolites in horses after oral levamisole administration over seven days.

Levamisole is a regulated substance sometimes administered to racehorses to treat equine protozoal myelitis. Metabolites include compound II, aminorex, and pemoline. Aminorex and pemoline are Horseracing Integrity and Safety Authority-banned substances. Previous studies have examined single doses of the drug. This study examined the disposition of levamisole after 7 days of dosing. 6 healthy Thoroughbred geldings. Horses were treated with 500 mg (approx 0.91 to 1 mg/kg) of compounded levamisole hydrochloride paste PO every 12 hours for a total of 13 doses over 7 days. Serum and urine samples were analyzed for levamisole and its metabolites over a 28-day period. The terminal half-life of levamisole in serum was variable between horses. Following the last dose of levamisole on day 7, serum levamisole levels took 3 to 14 days (days 10 to 21) to fall below the limit of detection (LOD) in 5 of 6 horses. Serum from 1 horse remained over LOD on the last testing day (day 28). In urine, following the final dose (day 7), levamisole was below LOD on day 13 (6 days after final dose) and aminorex was below LOD on day 10 (3 days after final dose). Compound II was above LOD in 4 of 5 horses sampled on the last sampling day (day 28). Levamisole and its metabolites can be detected for variable lengths of time in horses, with detection lasting for days to weeks following multiple doses. This study supports the Racing Medication and Testing Consortium Advisory on levamisole, which suggests that clearance sample testing should be conducted on treated horses to verify elimination of levamisole and its metabolites.