In vivo biotransformation of metoprolol in the horse and on-column esterification of the aminocarboxylic acid metabolite by alcohols during solid phase extraction using mixed mode columns.

The research article explores the biochemical transformations of the drug metoprolol tartrate in racehorses upon oral ingestion. It specifically looks at how the drug and its resulting metabolites are extracted, analysed and how long it stays detectable in a horse’s system.

Extraction and Analysis of Metoprolol and its Metabolites

• The researchers administer a single oral dose of metoprolol tartrate to a thoroughbred racehorse. The in vivo transformation of this drug is closely studied.
• The drug’s transformation is analysed in the horse’s urine and plasma. To achieve this, a solid phase extraction (SPE) technique is used, which leverages mixed mode SPE cartridges. These cartridges help isolate metoprolol and its phase I metabolites.
• Once extracted, the isolates get converted into trimethylsilyl ethers. These ethers get analysed further using techniques such as capillary column gas chromatography, positive ion electron ionisation, and ammonia chemical ionisation mass spectrometry.

Understanding Metoprolol Metabolism in Horses

• The analysis finds that metoprolol primarily undergoes oxidative transformations, particularly of its p-(2-methoxy)ethyl substituent.
• The researchers discover five different phase I metabolites in the horse’s urine. This, along with the detection of the zwitterionic compound H117/04, mirrors the metabolisation seen in humans, dogs, and rats.
• Interestingly, the researchers observe an unusual on-column esterification of the carboxylic acid moiety by alcohols during the SPE process.

Detection Periods of Metoprolol and its Metabolites

• The research finds that metoprolol and its major aliphatic acid metabolite remain detectable in the horse’s urine for about 10 and 40 hours respectively, without the need for enzymatic hydrolysis.
• Post enzymatic hydrolysis, these detection periods extend to 15 and 60 hours, respectively. This suggests that metoprolol and its metabolites undergo some phase II metabolism in horses.