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ABSTRACT:Covalent binding of reactive electrophilic intermediates to proteins is considered to play an important role in the processes leading to adverse drug reactions and idiosyncratic drug reactions. Consequently, both for the discovery and the development of new drugs, there is a great interest in sensitive methodologies that enable the detection of covalent binding of drugs and drug candidates in vivo. In this work, we present a strategy for the generation and analysis of drug adducts to human serum albumin. Our methodology is based on the isolation of albumin from blood, its digestion to peptides by pronase E, and the sensitive detection of adducts to the characteristic cysteine-proline-phenylalanine (CPF) tripeptide by liquid chromatography/tandem mass spectrometry. We chose acetaminophen (APAP) as a model compound because this drug is known to induce covalent binding to proteins when bioactivated by cytochromes P450 to its reactive N-acetyl-p-benzoquinoneimine metabolite. First, by microsomal incubations of APAP in presence of CPF and/or intact albumin, in vitro reference adducts were generated to determine the mass spectrometric characteristics of the expected CPF adducts and to confirm their formation on pronase E digestion of the alkylated protein. When applying this methodology to albumin isolated from blood of patients exposed to APAP, we were indeed able to detect the corresponding CPF adducts. Therefore, this strategy could be seen as a potential biomonitoring tool to detect in vivo reactive intermediates of drugs and drug candidates, e.g., in the preclinical and clinical development phase.

Trimethoprim (TMP) is a widely used antibacterial agent that is usually considered as a safe drug. TMP has, however, been implicated in rare adverse drug reactions (ADRs) in humans. Bioactivation to a reactive iminoquinone methide intermediate has been proposed as a possible cause for the toxicity of the drug. However, little is known about the cytochrome P450s (P450s) involved in this bioactivation and in the metabolism of TMP in general. In this study, we have investigated the metabolism and bioactivation of TMP by human liver microsomes (HLM) and rat liver microsomes, by recombinant human cytochrome P450s, and by the bacterial P450 BM3 mutant M11(his). In addition to non GSH-dependent metabolites, five GSH adducts were identified in the HLM incubations. Next to two major GSH adducts probably originating from the iminoquinone methide intermediate described previously, three minor GSH adducts were also identified, indicating that other types of reactive intermediates are formed by HLM, such as ortho-quinones and para-quinone methide intermediates. The major GSH adducts were produced by P450 1A2 and P450 3A4, while the minor GSH adducts were mainly formed by P450 1A2, P450 3A4, and P450 2D6. Although preliminary, these results might implicate that genetic polymorphisms in P450 enzymes could play a role in the onset of TMP-related ADRs in humans.

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