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Remifentanil in the horse: identification and detection of its major urinary metabolite
A F Lehner1, P Almeida, J Jacobs
1Maxwell H. Gluck Equine Research Center and the Department of Veterinary Science, University of Kentucky, Lexington 40546, USA. alehner@pop.uky.edu
Abstract:
Remifentanil (4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid methyl ester) is a mu-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 HCl (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.
Insights
The major equine urinary metabolite of remifentanil, an ester hydrolysis product, was identified. This metabolite is present in higher concentrations than remifentanil, aiding in forensic detection in racing horses.
Area of Science:
- Veterinary Pharmacology
- Analytical Chemistry
- Forensic Toxicology
Background:
- Remifentanil, a potent mu-opioid agonist, is used in horses but has abuse potential.
- Accurate detection methods are crucial for anti-doping control in equine sports.
Purpose of the Study:
- To identify and characterize the major equine urinary metabolite of remifentanil.
- To establish the utility of this metabolite for forensic detection in racing horses.
Main Methods:
- Administration of remifentanil HCl to horses.
- ELISA for detecting fentanyl equivalents in urine.
- Mass spectrometry with solid-phase extraction and TMS derivatization for metabolite identification.
- Base hydrolysis of remifentanil for comparison.
Main Results:
- Parent remifentanil was detected in low concentrations in equine urine.
- A major metabolite, 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid (an ester hydrolysis product), was identified.
- This metabolite peaked at 1 hour post-administration and was present at 10-fold higher concentrations than remifentanil.
- The identified metabolite matched the product of base hydrolysis of remifentanil.
Conclusions:
- Remifentanil administration leads to significant urinary excretion of its ester hydrolysis metabolite.
- Screening and confirmation tests for this metabolite are essential for effective forensic control of remifentanil use in racing horses.