Journal Article

Remifentanil in the Horse: Identification and Detection of its Major Urinary Metabolite*

A.F. Lehner, P. Almeida, J. Jacobs, J.D. Harkins, W. Karpiesiuk, W.E. Woods, L. Dirikolu, J.M. Bosken, W.G. Carter, J. Boyles, C. Holtz, T. Heller, C. Nattrass, M. Fisher and T. Tobin

in Journal of Analytical Toxicology

Volume 24, issue 5, pages 309-315
Published in print July 2000 | ISSN: 0146-4760
Published online July 2000 | e-ISSN: 1945-2403 | DOI:
Remifentanil in the Horse: Identification and Detection of its Major Urinary Metabolite*

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Remifentanil (4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid methyl ester) is a µ-opioid receptor agonist with considerable abuse potential in racing horses. The identification of its major equine urinary metabolite, 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid, an ester hydrolysis product of remifentanil is reported. Administration of remifentanil HCI (5 mg, intravenous) produced clear-cut locomotor responses, establishing the clinical efficacy of this dose. ELISA analysis of postadministration urine samples readily detected fentanyl equivalents in these samples. Mass spectrometric analysis, using solid-phase extraction and trimethylsilyl (TMS) derivatization, showed the urine samples contained parent remifentanil in low concentrations, peaking at 1 h. More significantly, a major peak was identified as representing 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid, arising from ester hydrolysis of remifentanil. This metabolite reached its maximal urinary concentrations at 1 h and was present at up to 10-fold greater concentrations than parent remifentanil. Base hydrolysis of remifentanil yielded a carboxylic acid with the same mass spectral characteristics as those of the equine metabolite. In summary, these data indicate that remifentanil administration results in the appearance of readily detectable amounts of 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid in urine. On this basis, screening and confirmation tests for this equine urinary metabolite should be optimized for forensic control of remifentanil.

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Subjects: Medical Toxicology ; Toxicology (Non-medical)

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