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.

Acetylpromazine, 1-{10-[3-(dimethylamino)propyl]-10H-phenothiazin-2-yl}ethenone, C19H22N2OS, 326.46 g·mol-1 is a phenothiazine derivative at one time used in human medicine as an antipsychotic medication but now predominantly used in veterinary medicine as a sedative/tranquilizer and referred to as acepromazine. In performance horses its use is regulated by using a 10 ng/mL threshold for the major urinary metabolite 2-(1-hydroxyethyl) promazine-sulfoxide (HEPS) in equine urine. To enable accurate quantitation of HEPS in equine urine we have synthesized and purified hydroxyethylpromazine sulfoxide-d4 (HEPS-d4) to be used as a stable isotopically labeled internal standard. Although labeled HEPS is commercially available (CAS 1346605-30-8), to the best of our knowledge there is no published synthetic procedure in the scientific literature. Here we demonstrate a viable synthetic procedure consisting of four major steps: (i) freebasing the Acepromazine maleate salt, (ii) H-D exchange of Acepromazine at room temperature, (iii) reduction of the ketone with NaBD4, and (iv) oxidation of the thioether via hydrogen peroxide and acetic acid. This deuterated internal standard will allow for precise LC/MS quantitation of HEPS at regulatory threshold concentrations, enabling accurate detection and quantitation of picogram/mL concentrations in equine urine samples, thereby supporting regulatory compliance for equine medication control programs.