Investigation of the metabolism of the selective androgen receptor modulator LGD-4033 in equine urine, plasma and hair following oral administration

LGD-4033 is one of a number of selective androgen receptor modulators (SARMs) that are being developed by the pharmaceutical industry to provide the therapeutic benefits of anabolic androgenic steroids, without the less desirable side effects. Though not available therapeutically, SARMs are available for purchase online as supplement products. The potential for performance enhancing effects associated with these products makes them a significant concern with regards to doping control in sports. The purpose of this study was to investigate the metabolism of LGD-4033 in the horse following oral administration, in order to identify the most appropriate analytical targets for doping control laboratories. LGD-4033 was orally administered to two Thoroughbred horses and urine, plasma and hair samples were collected and analysed for parent drug and metabolites. LC-HRMS was used for metabolite identification in urine and plasma. Eight metabolites were detected in urine, five of which were excreted only as phase II conjugates, with the longest detection time being observed for di- and tri-hydroxylated metabolites. The parent compound could only be detected in urine in the conjugated fraction. Seven metabolites were detected in plasma along with the parent compound where mono-hydroxylated metabolites provided the longest duration of detection. Preliminary investigations with hair samples using LC-MS/MS analysis indicated the presence of trace amounts of the parent compound and one of the mono-hydroxylated metabolites. In vitro incubation of LGD-4033 with equine liver microsomes was also performed for comparison, yielding 11 phase I metabolites. All of the metabolites observed in vivo were also observed in vitro.