Synthesis and Characterization of 2-(1-Hydroxyethyl)Promazine Sulfoxide-d4 HCl as an Internal Standard for the Accurate Quantitation of 2-(1-Hydroxyethyl)Promazine Sulfoxide in Equine Urine.

Overview

• This study presents the synthesis and purification of a deuterated internal standard, 2-(1-hydroxyethyl)promazine sulfoxide-d4 hydrochloride (HEPS-d4 HCl), for accurately measuring the concentration of 2-(1-hydroxyethyl)promazine sulfoxide (HEPS) in horse urine.
• This development supports the precise quantitation of HEPS to monitor and regulate the use of acepromazine in performance horses.

Background

• Acepromazine: A phenothiazine derivative previously used as an antipsychotic in humans, now primarily used in veterinary medicine as a sedative/tranquilizer for animals, including horses.
• Regulatory Importance: The use of acepromazine in performance horses is regulated with a threshold concentration of 10 ng/mL for its major urinary metabolite, HEPS, in equine urine, to ensure compliance and fair competition.
• Analytical Challenge: Accurate quantitation of HEPS at very low concentrations (picograms per milliliter) requires a reliable internal standard that can correct for variability in detection methods.

Objective

• To synthesize and purify a stable isotopically labeled internal standard (HEPS-d4) to enable precise and accurate measurement of HEPS levels in equine urine using liquid chromatography-mass spectrometry (LC/MS).
• Address the lack of published synthetic procedures despite the commercial availability of labeled HEPS compounds.

Synthetic Procedure

• The synthesis involves four major steps:
• (i) Freebasing Acepromazine Maleate Salt: Transforming the salt form of acepromazine into its freebase form to prepare it for further chemical modification.
• (ii) H-D Exchange at Room Temperature: Introducing deuterium atoms by replacing hydrogen on Acepromazine, enhancing isotopic labeling for use as an internal standard.
• (iii) Reduction of the Ketone with Sodium Borodeuteride (NaBD₄): Converting the ketone group into its corresponding alcohol while incorporating deuterium atoms, critical for creating the labeled hydroxyethyl side chain.
• (iv) Oxidation of the Thioether: Using hydrogen peroxide and acetic acid to oxidize the sulfur atom in the molecule to a sulfoxide, completing the formation of the labeled 2-(1-hydroxyethyl)promazine sulfoxide-d4.
• This multi-step synthetic pathway ensures incorporation of four deuterium atoms, providing a distinguishable mass signature for LC/MS analysis.

Analytical and Practical Significance

• Internal Standard Role: The deuterated HEPS serves as a precise internal standard in LC/MS, compensating for sample preparation variability, matrix effects, and instrumental fluctuations.
• Regulatory Compliance: Enables accurate quantitation of HEPS at threshold levels relevant for equine medication monitoring programs, facilitating enforcement of drug use regulations in performance horses.
• Detection Sensitivity: Allows detection of minute concentrations (picogram/mL range) of HEPS in urine, essential for sensitive doping control.
• Scientific Contribution: Provides the first documented synthetic route for HEPS-d4, filling a gap in published methodology and supporting future research and regulatory analysis.

Summary

• This study successfully synthesizes a stable isotopically labeled internal standard for HEPS using a four-step chemical procedure.
• The internal standard improves the reliability of LC/MS quantification at low concentration levels critical for monitoring acepromazine use in horses.
• The work supports regulatory frameworks by enabling precise detection and adherence to medication thresholds, promoting fair competition in equestrian sports.