The isomeric metabolites of doxepin in equine serum and urine.

This research investigates how the antidepressant drug doxepin, which has potential for misuse as a horse doping substance, is metabolized in horses. The study identifies specific metabolites of doxepin and explores methods for detecting these metabolites to confirm instances of doping.

Research Purpose and Methodology

• The aim of the study was to identify metabolites of doxepin which would provide more specific Mass Spectrometry (MS) data to confirm positive drug screening results. Doping in sport horses is a serious issue and doxepin, due to its tranquilizing properties, is a potential candidate for misuse as a doping agent.
• The methodology involved administering doxepin hydrochloride (HCl) to a horse, then analyzing the horse’s blood and urine for free or conjugated metabolites using gas chromatography (GC) combined with electron impact (EI) and positive chemical ionization (PCI) MS.

Findings

• The study found that the isomers (different structural forms of a compound) of desmethyldoxepin could be detected in both blood and urine up to 48 hours after treatment. This was achieved using a technique known as trifluoracetylation and GC/EI-MS.
• After carrying out enzymic hydrolysis of urine samples and propionylation of extracts, the researchers were able to find four isomers each of hydroxy desmethyldoxepin and hydroxydoxepin up to 24 and 48 hours after treatment, respectively.
• These compounds were characterized by their EI- and PCI-mass spectra. Although the exact positions of the hydroxyl groups couldn’t be determined, it was inferred that there were two cis/trans-isomeric pairs of monohydroxylated metabolites each.

Implications

• The results suggest that to effectively screen for doxepin use in horses, the analysis should include checking for its major isomeric metabolites (desmethyldoxepin and hydroxydoxepin), as these provide MS data specific enough for confirmatory analysis.
• This research contributes to the continued effort for improving drug testing standards and procedures in equestrian sports, helping to ensure fair competition and the wellbeing of the animals involved.