3-Methoxytyrosine as an indicator of dopaminergic manipulation in equine plasma.

This study presents a novel method for detecting the misuse of dopaminergic agents in racehorses by isolating and measuring the levels of 3-methoxytyrosine, a metabolite of levodopa, in their plasma. It suggests a threshold level of 3-methoxytyrosine, beyond which it may indicate doping.

Objective of the Study

• The aim of the study was to develop a method to detect the misuse of doping agents in racehorses. Specifically, it focused on levodopa, a compound that stimulates the production of the neurotransmitter dopamine.
• Levodopa and dopamine can be concealed with catechol-O-methyltransferase inhibitors, hence a reliable detection method was urgently needed.

Means of Detection

• The research team focused on isolating 3-methoxytyramine and 3-methoxytyrosine from horse plasma. These compounds are metabolites of dopamine and levodopa respectively, hence their presence can indicate the use of levodopa.
• The researchers used a protein precipitation method to isolate these compounds from 100 µL equine plasma.

Setting a Threshold

• 3-methoxytyramine already had an established threshold in urine to detect misuse of dopaminergic agents. However, no such threshold existed for plasma samples.
• With their method, the researchers proposed a conservative threshold level of 1,000 ng/mL in plasma for 3-methoxytyrosine, at a 99.995% confidence level.
• This metric was based on analysis of samples from over one-thousand equine athletes.

Validation of Method

• To validate their method, an administration study was conducted on twelve horses. They were given Stalevo®, which contains 800 mg of L-DOPA along with other compounds.
• The 3-methoxytyrosine level in the horses’ plasma was elevated for 24 hours post-administration, thus confirming the validity of the research team’s method.