The Trouble With Aminorex

by | 04.17.2018 | 3:03pm

Aminorex has already caused a few headaches for regulators. If a recent Kentucky Equine Drug Research Council (EDRC) meeting is any indication, it's destined to have a few more reaching for the aspirin.

Aminorex is a stimulant that was once available in the human market as a weight loss aid. It was pulled from sale in the United States after it was found to cause pulmonary hypertension in people and is listed by the Association of Racing Commissioners International's Uniform Classification guidelines as a Class 1 drug.

Regulating aminorex has proven to be a challenge because it is also a metabolite of a legitimate substance. When drugs break down in the body they do so in pieces, and those pieces sometimes happen to be identical to other recognized substances. Levamisole, a drug sometimes used to treat equine protozoal myeloencephalitis (EPM), can break down into aminorex in the horse's body, making it appear someone has dosed the horse with aminorex when they didn't. The EDRC has funded research to learn more about the relationship in order to draft guidelines on safe levamisole use.

(If you're wondering why a racehorse would be on a drug for EPM, it's important to note the disease does not always present with profound hind-end weakness or lack of balance. Some mild discomfort may be attributed to an early case of EPM and it's not uncommon for a veterinarian to order treatment for the disease to cover all bases.)      

Members of the EDRC heard a presentation from Kentucky Horse Racing Commission Equine Medical Director Dr. Mary Scollay with a new perspective on the drug's presence in a horse's body. A laboratory in England studied several sport horse samples which were found to contain aminorex in addition to some unknown molecules. (Remember, mass spectronomy identifies drugs in a blood or urine sample by matching molecules to the weight of known substances.) One of those molecules matched the mass of a substance called barbarin. Barbarin can be a product of glucobarbarin and myrosinase. Glucobarbarin can be found naturally in several types of plants, including wintercress and watercress, while myrosinase is released from these same plants when they are chewed or crushed.

This suggests (though the connection is not yet well-understood) that a horse could have aminorex in its system as a result of having recently eaten hay or grain containing a relative of wintercress.

Scollay said there has only been one finding for aminorex in Central Kentucky in the past ten years, even though several wintercress relatives do grow in the area. It seems incidental exposure to the plants in hay isn't causing positive tests, but scientists still want to understand more about the relationship between aminorex and barbarin.

For now, Scollay said, we don't know how much of a glucobarbarin-containing plant a horse may need to eat to get an aminorex positive.

“An even more important question has been raised that asks if barbarin can degrade to aminorex after a sample has been collected from a horse,” she said. “More research is needed to answer these questions before regulators can understand how to respond to a laboratory's detecting aminorex and barbarin in a sample.”

Scollay anticipates more information about the presence of aminorex in the horse could change the way stewards adjudicate positives. Last week, the Kentucky Horse Racing Commission's rules committee discussed the possibility of giving stewards the ability to consider mitigating circumstances when assigning penalties to trainers for drug positives.

A discovery that an aminorex finding is related to a horse's consumption of wintercress would not invalidate the positive altogether, though – the horse's connections would still likely have to forfeit the purse.

“Realizing that the way it got into the horse does not negate the fact that it was in the horse,” said Scollay. “How it got into the horse speaks directly to the trainer's culpability or lack thereof. The withdrawal of a finding is not warranted simply because the trainer had no role in the substance's entry into the horse.”

The Blood-Horse reported after a Tuesday Kentucky Horse Racing Commission that a positive in the state was rescinded as a result of the new unknowns about aminorex.

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