Urinary excretion profiles of the orexin receptor antagonist suvorexant and its metabolites

Suvorexant is an orexin receptor antagonist used in the treatment of insomnia. In this study, we investigated the urinary excretion profiles of suvorexant and its major metabolites, including conjugates, to obtain fundamental information for proving exposure to suvorexant in criminal cases. Urine specimens were collected from three subjects for maximum 168 h after a single oral ingestion of suvorexant (10 mg), and suvorexant and its metabolites in urine were determined using liquid chromatography-tandem mass spectrometry with a C18 semi-micro column. The carboxylic and hydroxy metabolites (M4 and M9) were identified with authentic standards synthesized in our laboratory, and their glucuronides and other hydroxy metabolites (M8 and M10) were tentatively detected based on measured exact masses and product ion spectra of them. Suvorexant, M4 and M9 would be detectable for 20-34 h, 6-7 days and 42-61 h after intake, respectively. The quantitative results demonstrated that the molar ratios of accumulated amounts of M4 and M9 including their glucuronides excreted in urine to dose ranged about 2.6-6.2% and 0.37-0.51%, respectively, while that of the unchanged parent was much lower (0.011-0.013%). The ratios of the amount of glucuronide to the total amount of M4 and M9 excreted in urine was less than 10% and approximately 90%, respectively. The urinary excretion profiles indicated that M4 and M9 would be effective indicators for proving suvorexant intake, and M4 could be detected until one week after intake even without enzymatic hydrolysis (limit of detection: 0.05 ng/mL).