Remifentanil in the horse: identification and detection of its major urinary metabolite.

The study has identified the major urinary metabolite of a horse drug named Remifentanil and found that its presence can easily be detected in a horse’s urine. This discovery can help in establishing a methodology for controlling the misuse of this drug in horse racing.

Identification of Major Urinary Metabolite

• The study has identified the major urinary metabolite of Remifentanil as 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid. This compound is formed as a result of ester hydrolysis of Remifentanil.
• Horses were administered Remifentanil HCl intravenously and their locomotor responses were observed, validating the dose’s clinical efficacy.

Detection Methodology

• Post-administration urinalysis by ELISA (enzyme-linked immunosorbent assay) detected the presence of fentanyl equivalents in the samples, signifying that Remifentanil metabolizes into measurable derivatives of its parent compound.
• Mass spectrometric analysis was performed on the urine samples after treatment with a solid-phase extraction and TMS (trimethylsilyl) derivatization. Results showed the presence of parent Remifentanil, albeit in low quantities.

Remifentanil Metabolism

• A notable discovery was the presence of a peak, representing the major urinary metabolite, 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid. It appears as result of the ester hydrolysis of Remifentanil.
• The metabolite reached its maximum concentration in the urine after one hour, remaining present in up to ten times the concentration of parent Remifentanil.
• Base hydrolysis of Remifentanil yields a carboxylic acid with the same mass spectral characteristics as the equine metabolite, thus confirming the metabolic pathway.

Implications for Forensic Control

• These findings indicate that the administration of Remifentanil leads to the presence of readily detectable quantities of the major metabolite in horse urine.
• This opens the potential to optimize screening and confirmation tests for this metabolic signature, aiding in the forensic control of Remifentanil, which is a drug with considerable misuse potential in horse racing.