Candidate drugs that can be metabolically transformed into reactive electrophilic products, such as epoxides, quinones, and nitroso compounds, are of special concern because subsequent covalent binding to bio-macromolecules can cause adverse drug reactions, such as allergic reactions, hepatotoxicity, and genotoxicity. Several strategies have been reported for screening reactive metabolites, such as a covalent binding assay with radioisotope-labeled drugs and a trapping method followed by LC–MS/MS analyses. Of these, a trapping method using glutathione is the most common, especially at the early stage of drug development. However, the cysteine of glutathione is not the only nucleophilic site in vivo; lysine, histidine, arginine, and DNA bases are also nucleophilic. Indeed, the glutathione trapping method tends to overlook several types of reactive metabolites, such as aldehydes, acylglucuronides, and nitroso compounds. Here, we introduce an alternate way for screening reactive metabolites as follows: A mixture of the light and heavy isotopes of simplified amino acid motifs and a DNA motif is used as a biomimetic trapping cocktail. This mixture consists of [2H0]/[2H3]-1-methylguanidine (arginine motif, Δ 3 Da), [2H0]/[2H4]-2-mercaptoethanol (cysteine motif, Δ 4 Da), [2H0]/[2H5]-4-methylimidazole (histidine motif, Δ 5 Da), [2H0]/[2H9]-n-butylamine (lysine motif, Δ 9 Da), and [13C0,15N0]/[13C1,15N2]-2′-deoxyguanosine (DNA motif, Δ 3 Da). Mass tag triggered data-dependent acquisition is used to find the characteristic doublet peaks, followed by specific identification of the light isotope peak using MS/MS. Forty-two model drugs were examined using an in vitro microsome experiment to validate the strategy.
Mass spectrometry Activated drug Trapping reagent Cocktail reagent
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This work was supported in part by a Grant-in-Aid for Challenging Exploratory Research (to T.O., no. 25670008 for 2013–2014) from Japan Society for the Promotion of Science (JSPS) and TaNeDS 2014 (to T.O., for 2014–2017) from Daiichi Sankyo Co., Ltd. The authors thank Astellas Pharma Inc. (Tsukuba, Japan) for donating a used LCQ DECA. The authors also thank Mr. Reona Yoshiizumi of our laboratory for help in preparing KP-NHS.
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Conflict of interest
The authors declare that there are no conflicts of interest.
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