Bioactivation of Indoles by Human Liver Microsomes with Glutathione as Trapping Agent: Formation of Novel Cyclic Cysteine and Cysteine‐Glycine Adducts

Abstract

Metabolic bioactivation of drug candidates to reactive electrophilic intermediates can result in covalent adducts with cellular macromolecules, potentially causing drug‐induced toxicity. Thus, awareness of “structural alerts” in drug design and understanding of reactive metabolite (RM) formation in lead optimization efforts are extremely important to minimize the potential of idiosyncratic drug toxicity. The indole motif is ubiquitously present in natural products and drugs. Although RMs are known to be involved in the chemical‐induced toxicity of some indole derivatives, a systemic study of RMs of the indole motif has not been performed. In this study, compounds containing indole moieties (10 μM) were incubated with 1 mg/mL human liver microsomes and 2 mM GSH. These samples were then analyzed by ultra‐performance liquid chromatography coupled to a quadrupole‐time‐of‐flight high definition mass spectrometer (UPLC/Q‐TOF HDMS). Novel oxidative cyclic cysteine and cysteine‐glycine adducts of a series of 2,3‐unsubstituted indole derivatives, including Pindolol, a marketed drug, were observed. These cyclic cysteine adducts were also observed with cysteine as the trapping agent. The mechanism for the formation of these thiazolidine adducts were investigated, in which cysteine could act as a binucleophile, and therefore comparable GSH adducts are not possible. In conclusion, the metabolic bioactivation of 2,3‐unsubstituted indoles forms a novel class of cyclic cysteine and cysteine‐glycine adducts when using GSH as trapping agent. These cyclic conjugates are likely to be a risk of false negative in a standard GSH conjugate screening process. The toxicological implications of these cyclic adducts are still unclear.