Common pasture weed linked to positive drug tests in some horses

Yellow Rocket growing in Ithaca, New York.  Photo: Maylin et al. doi:10.1186/s13620-019-0153-5

A brassica plant that can be found growing in pastures across the United States can result in horses failing a drug test, research findings suggest.

The culprit is Barbarea vulgaris, more commonly known as yellow rocket.

Two Standardbred horses fed yellow rocket in a study subsequently tested positive for Aminorex, an amphetamine-like appetite suppressant, which is listed as a Schedule 1 controlled substance by the United States Drug Enforcement Administration.

Aminorex is listed by the Association of Racing Commissioners International as a Class 1, Penalty Class A foreign substance.

It is considered to have high potential for influencing a horse’s racing performance due to its stimulating effects, with positive tests carrying a fine and suspension.

George Maylin and his colleagues, writing in the Irish Veterinary Journal, say it is possible that some other members of the brassica family may trigger a similar result.

Aminorex in horse urine is usually present as a metabolic byproduct of Levamisole.

Levamisole is commercially available as a dewormer for cattle, sheep, goats, and pigs. However, it also has conventional off-label uses in horses as an immunostimulant and as a medication for treating Equine Protozoal Myelitis .

While the identification of Levamisole as a source of Aminorex led to a sharp reduction in the number of Aminorex identifications, it has not completely eliminated such findings.

In North America, sporadic unexplained Aminorex identifications have occasionally been reported.

Additionally, several recent Aminorex identifications have been reported in English sport horses where there was no history of the horses receiving Levamisole, nor any sign of other metabolic markers suggesting the use of Levamisole.

The Kentucky-based study team said a careful review of substances co-identified with Aminorex in these samples showed several plant-related, low molecular weight nitrogenous substances.

These findings, and a review of the botanical literature, led to suspicions that GlucoBarbarin and Barbarin, contained in certain plants and originally identified in Barbarea vulgaris, might be the source of these Aminorex identifications.

istribution of Yellow Rocket, Barbarea vulgaris, in the United States; Data re-constructed from USDA reports [13]. The British Isles show similar distribution, with only parts of Scotland having none reported as of May, 2019. Image: Maylin et al. doi:10.1186/s13620-019-0153-5
GlucoBarbarin in Brassicaceae serves as a precursor to barbarin. When the plant is damaged, the enzyme myrosinase removes the glucose molecule from GlucoBarbarin, yielding an intermediate that spontaneously turns to Barbarin.

GlucoBarbarin, they believe, may have a role in protecting the plant from potential predators.

“Consistent with this protective function of Barbarin, the yellow rocket plant material was refused by the experimental horses until mixed with grass and sweet feed.”

Suspicions were further heightened by their English colleagues, who reported the identification of Aminorex in at least one presumably European member of the genus Barbarea in the Brassicaceae family.

Thus, attention turned to yellow rocket, a well-known member of the Brassicaceae family which flowers from late April to early May in Kentucky pasture. The plant has wide distribution in both the US and Britain, with only parts of Scotland considered clear.

For their study, the researchers identified and harvested flowering yellow rocket in Kentucky in May 2018 and gave the plant orally to the two horses.

Analysis of urine samples afterward yielded Aminorex, showing that consumption of Kentucky Barbarea vulgaris can give rise to Aminorex identifications in equine urine.

“Since horsepersons face up to a 1-year suspension and a $10,000 fine for an Aminorex identification, the existence of natural sources of Aminorex precursors in equine feedstuffs is of importance to both individual horsepersons and the industry worldwide,” the authors wrote.

“Multiple members of the Brassicaceae family, including Barbarea vulgaris, produce GlucoBarbarin, which may be responsible for ongoing sporadic Aminorex identifications reported in post-event equine urine samples in the absence of evidence of Levamisole administration.”

Conversion of GlucoBarbarin (via Myrosinase) to Barbarin and Aminorex structure. Image: Photo: Maylin et al. doi:10.1186/s13620-019-0153-5

The plant Reseda luteola, more commonly known as “weld”, or Dyer’s rocket, is similarly known to produce GlucoBarbarin. This plant is widely distributed throughout Europe and has been introduced to many parts of the US. Historically used as a source of yellow dye, it was also grown domestically for its sweet aromatic smell.

“Based on the results herein reported with respect to Barbarea vulgaris, it is reasonable to assume that if Reseda luteola were to be ingested a measurable level of Aminorex might also be detectable in equine urine.”

They continued: “At this time, the precise mechanism for the conversion of Barbarin to Aminorex is unclear, and it is possible that some other compound produced by Barbarea vulgaris is responsible.

“However, based on the substantial structural similarities between Aminorex and Barbarin, Barbarin is at this time the most likely candidate for the primary source of Aminorex in the Brassicaceae family.

“Given these circumstances and the widespread distribution of such plants in North America and elsewhere, yellow rocket or related Brassicaceae plants are likely sources of sporadic low-concentration Aminorex identifications in the sports horse worldwide.”

The study team said future research will focus on identifying biomarkers of yellow rocket or other Brassicaceae to definitively identify botanical origins for Aminorex identifications and the acquisition of enough field data to support or appropriately adjust a proposed 30 ng/mL environmental “cut-off” or Screening Limit of Detection (SLOD) for pharmacologically and forensically insignificant Aminorex identifications in equine drug testing.

The research team comprised Jacob Machin, Sucheta Kudrimoti, Jonathan Green and Thomas Tobin, with the University of Kentucky; Rodney Eisenberg, with Frontier BioPharm in Kentucky; Clara Fenger, with Equine Integrated Medicine in Kentucky, and George Maylin, with the New York Drug Testing and Research Program.

Maylin, G., Fenger, C., Machin, J. et al. Aminorex identified in horse urine following consumption of Barbarea vulgaris; a preliminary report. Ir Vet J 72, 15 (2019) doi:10.1186/s13620-019-0153-5

The study, published under a Creative Commons License, can be read here

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