Metabolite characterization of ambrisentan, in in vitro and in vivo matrices by UHPLC/QTOF/MS/MS: Detection of glutathione conjugate of epoxide metabolite evidenced by in vitro GSH trapping assay

Abstract The focus of the present study is on in vitro and in vivo metabolite identification of ambrisentan (AMBR) a selective endothelin type – A (ETA) receptor antagonist using quadruple time-of-flight mass spectrometry (QTOF/MS). in vitro metabolism study was conducted by incubating AMBR in rat liver microsomes (RLM), rat and human liver S9 fractions. In vivo study was carried out through the collection of urine, faeces and plasma samples at various time points after oral administration of AMBR in suspension form at a dose of 25 mg/kg to six male Sprague – Dawley (SD) rats. The samples were prepared using an optimized sample preparation techniques involving protein precipitation (PP), freeze liquid extraction (FLE) and solid phase extraction (SPE). The extracted samples were further concentrated and analyzed by developing a sensitive and specific liquid chromatography-mass spectrometry (LC–MS) method. A total of seventeen metabolites were identified in in vivo samples which includes hydroxyl, demethylated, demethoxylated, hydrolytic, decarboxylated, epoxide and glucuronide metabolites. Most of the metabolites were observed in faeces and urine matrices and few were observed in the plasma matrix. Only ten metabolites were identified in in vitro study which was commonly observed in in vivo study. The detailed structural elucidation of all the metabolites was done using UHPLC/QTOF/MS/MS in combination with accurate mass measurements. The toxicity profile of AMBR and its metabolites were predicted using TOPKAT software. In addition, a mass spectrometric method was developed for the detection and characterization of GSH-trapped reactive epoxide metabolitein human liver S9 fraction supplemented with glutathione (GSH) as trapping agent.