Detection of fenspiride and identification of in vivo metabolites in horse body fluids by capillary gas chromatography-mass spectrometry: administration, biotransformation and urinary excretion after a single oral dose.

This research article examines the biotransformation and urinary excretion of the drug fenspiride hydrochloride in horses. It highlights how the drug and its metabolites can be isolated and identified in horse body fluids through testing methods, and discusses the timeframes in which the unchanged drug and its major metabolite can be detected in post-administration samples. It also notes a significant difference in the urinary excretion of the major metabolite among different horse breeds.

Drug Administration and Metabolism

• The drug in focus is fenspiride hydrochloride, a bronchodilatory drug known to suppress cough reflexes, often used in treating respiratory diseases.
• After being administered orally to a horse, the drug is metabolized through a two-step process: Firstly, phase I functionalisation, followed by phase II conjugation pathways, which is a common metabolic pathway for drugs.

Metabolite Detection and Identification

• In post-administration biofluids, both fenspiride and its phase I metabolites were isolated using a technique called bonded co-polymeric mixed-mode solid-phase extraction, specifically meant for isolating basic compounds.
• After undergoing a process called trimethylsilylation (TMS), these elements were identified using a testing method known as capillary gas chromatography-mass spectrometry (GC-MS)
• In the urine, the researchers detected fenspiride along with seven metabolites resulting from the drug’s oxidation. They acquired important data about fenspiride’s metabolism through the positive ion electron ionisation mass spectra of the TMS derivatives of the drug and its metabolites.

Excretion Timeframe and Detection in Horse Species

• The unchanged drug can be identified in both plasma and urine samples up to 24 hours after administration.
• The major phenolic metabolite (designated as ‘G’) is the primary element surveyed in post-racing tests looking for the presence of fenspiride in equine urine samples. This metabolite can be detected up to 72 hours post administration.
• A key observation was the noticeable difference in the excretion of metabolite G between different breeds of horses. Specifically, the thoroughbred horses used in the UK study displayed a different metabolic response compared to the quarterbred and standardbred horses used in the USA.