Mass Spectrometric Characterization of Circulating Covalent Protein Adducts Derived from Epoxide Metabolites of Carbamazepine in Patients

Carbamazepine (CBZ) is an effective anti epileptic drug that has been associated with hypersensitivity reactions. The pathogenesis of those reactions is incompletely understood but is postulated to involve a complex interplay between the drug's metabolism, genetic variation in human leukocyte antigens, and adverse activation of the immune system. Multiple T-cell activation mechanisms have been hypothesized including activation by drug peptide conjugates derived from proteins haptenated by reactive metabolites. However, definitive evidence of the drug protein adducts in patients has been lacking. In this study, mass spectrometry was used to characterize protein modifications by microsomally-generated metabolites of CBZ and in patients taking CBZ therapy. CBZ 10,11-epoxide (CBZE), a major electrophilic plasma metabolite of CBZ, formed adducts with glutathione-S-transferase pi (GSTP; Cys47) and human serum albumin (HSA; His146 and His338, but not Cys34) in vitro via notably divergent side-chain selectivity. Both proteins were adducted at the same residues by undefined monoxygenated metabolites ([0]CBZ) when they were incubated with human liver microsomes, NADPH and CBZ. There was also evidence for formation of a CBZ adduct at His146 and His338 of HSA derived via dehydration from an intermediate arene oxide adduct. Glutathione trapping of reactive metabolites confirmed microsomal production of CBZE and indicated simultaneous production of arene oxides. In 15 patients prescribed CBZ therapy, [0]CBZ-modified HSA (His146) was detected in all subjects. The relative amount of adduct was moderately positively correlated with plasma concentrations of CBZ (r(2) = 0.44, p = 0.002) and CBZE = 0.35, p = 0.006). Our results have provided the first chemical evidence for microsomal production of [0]CBZ species that are able to escape the microsomal domain to react covalently with soluble proteins. This study has also demonstrated the presence of circulating [0] CBZ-modified HSA in patients without hypersensitivity reactions who were receiving standard CBZ therapy. The implications of those circulating adducts for susceptibility to CBZ hypersensitivity merit further immunological investigation in hypersensitive patients.