Italian researchers investigating the ability of performance horses to process the anti-inflammatory painkiller phenylbutazone suggest a withdrawal time of five days may be appropriate for a five-day course at a once-a-day dose rate of 4.4 mg/kg.
The FEI, in its Clean Sport documentation, notes a detection time in blood plasma of 168 hours (seven days) for the drug for a range of common course regimes and doses.
However, the world governing body stresses that detection times are different to withdrawal times. The detection time is the approximate period a drug (or its metabolite) can remain in a horse’s system at a level that can be detected by a testing laboratory. It is provided only as a guide.
The withdrawal time for a drug must be decided upon by the treating veterinarian and is likely to be based on the detection time plus a safety margin, chosen with professional judgment and discretion to allow for individual differences between horses such as size, metabolism, degree of fitness, recent illness or disease, and so on.
The research team from the Department of Veterinary Sciences at the University of Pisa, writing in the Journal of Equine Veterinary Science, noted that phenylbutazone, commonly referred to as bute, was one of the most frequently identified therapeutic medications found in equine anti-doping tests.
Valentina Meucci and her colleagues set out to evaluate plasma elimination of the drug in healthy horses following intravenous and oral administration of two commercial phenylbutazone products.
The researchers administered the drug to horses intravenously or orally at a dose rate of 4.4 mg/kg once daily for five consecutive days.
Ten millilitre blood samples were collected before the last dose and then after 1, 3, 6, 9, 12 and 24 hours; and every 12 hours for 10 days after treatment. The samples then underwent laboratory testing for phenylbutazone and its major metabolite, oxyphenbutazone.
A detection time of 84 hours was obtained in the study with both intravenous and oral phenylbutazone, with a limit of detection of 0.005 μg/ml.
“Using the dose regimen here reported, a withdrawal time of five days could be a correct approach to be adopted for the use of phenylbutazone in performance horses,” they said.
Meucci was joined in the study by Giacomo Luci, Michele Vanni, Micaela Sgorbini and Luigi Intorre.
Withdrawal time of phenylbutazone administered to healthy horses.
Valentina Meucci, Giacomo Luci, Michele Vanni, Micaela Sgorbini and Luigi Intorre.
The abstract can be read here.