Improved detection of reactive metabolites with a bromine-containing glutathione analog using mass defect and isotope pattern matching.

Drug bioactivation leading to the formation of reactive species capable of covalent binding to proteins represents an important cause of drug-induced toxicity. Reactive metabolite detection using in vitro microsomal incubations is a crucial step in assessing potential toxicity of pharmaceutical compounds. The most common method for screening the formation of these unstable, electrophilic species is by trapping them with glutathione (GSH) followed by liquid chromatography/mass spectrometry (LC/MS) analysis. The present work describes the use of a brominated analog of glutathione, N-(2-bromocarbobenzyloxy)-GSH (GSH-Br), for the in vitro screening of reactive metabolites by LC/MS. This novel trapping agent was tested with four drug compounds known to form reactive metabolites, acetaminophen, fipexide, trimethoprim and clozapine. In vitro rat microsomal incubations were performed with GSH and GSH-Br for each drug with subsequent analysis by liquid chromatography/high-resolution mass spectrometry on an electrospray time-of-flight (ESI-TOF) instrument. A generic LC/MS method was used for data acquisition, followed by drug-specific processing of accurate mass data based on mass defect filtering and isotope pattern matching. GSH and GSH-Br incubations were compared to control samples using differential analysis (Mass Profiler) software to identify adducts formed via the formation of reactive metabolites. In all four cases, GSH-Br yielded improved results, with a decreased false positive rate, increased sensitivity and new adducts being identified in contrast to GSH alone. The combination of using this novel trapping agent with powerful processing routines for filtering accurate mass data and differential analysis represents a very reliable method for the identification of reactive metabolites formed in microsomal incubations.