Idiosyncratic toxicity is one of the principal causes for withdrawal of marketed drugs after launch. Circumstantial evidence suggests that several drug-induced adverse effects are a result of transformation of a drug to electrophilic reactive metabolites (RMs) that can covalently bind to vital macromolecules in the body. Strategies have been implemented in early discovery to examine (and minimize) the formation of RMs. A common technique involves incubation of a new chemical entity with NADPH-supplemented human liver microsomes (HLMs) in the presence of soft nucleophilic trapping agents, such as glutathione (GSH) or N-acetylcysteine (NAC). Advances in mass spectrometry and the advent of very sensitive mass spectrometers ensure facile identification of the resulting GSH or NAC adducts of the reactive species. Detection of sulfhydryl conjugates in in vitro incubations, however, raise more questions regarding the path forward for RM-positive drug candidates. One approach that can assist in mitigating RM formation is assessment of their total body burden. Computation of dose using in vitro intrinsic clearance (Clint), potency data (Ceff) and the fractional contribution of RM pathway (frm), can provide an initial read of the daily burden of RM. This overview attempts to provide practical ways of assessing these factors and assist in putting the risk of RM formation into perspective.
Keywords: Bioactivation; P450; Reactive metabolites; glutathione conjugates; metabolism.