In Vivo Metabolic Profiling of Suzetrigine in Thoroughbred Horses Following Oral Administration: Implications for Equine Anti‐Doping Control

Suzetrigine (VX‐548), a selective NaV1.8 sodium channel blocker recently approved for pain management, was investigated for its metabolic fate in Thoroughbred horses using liquid chromatography coupled to high‐resolution mass spectrometry (LC‐HRMS). Equine urine and plasma samples were collected following administration and processed prior to analysis. Combining the present in vivo data with earlier reported in vitro liver incubations, a total of 12 metabolites ( M1 – M12 ) were identified and characterised, comprising nine Phase I and three Phase II biotransformation products. Five Phase I metabolites ( M1 – M4 and M9 ) were detected in vivo, formed through O‐demethylation, C‐demethylation, monohydroxylation, and hydrolysis of the amide function. Four further Phase I metabolites ( M5 – M8 ), previously reported by our team, were observed solely in in vitro liver incubations and were not detected in any in vivo sample. Phase II metabolism yielded three conjugated metabolites, comprising two glycosidation products ( M10 and M11 ) detected in both urine and plasma, and one sulfonate conjugate ( M12 ) confined to urine. Intact suzetrigine was the most abundant plasma analyte at every sampling point, followed by the O‐demethylated metabolite M1 , whereas in urine, the amide hydrolysis product M9 was overwhelmingly dominant and the parent drug accounted for less than 1% of its response. Intact suzetrigine remained detectable in plasma throughout a 192‐h period, while M1 and M9 persisted in urine to the final sampling points. These findings define the metabolic disposition of suzetrigine in the horse and identify M9 and M1 as the most suitable analytical targets for the detection of suzetrigine misuse in equine doping control.