Drug Bioactivation Covalent Binding to Target Proteins and Toxicity Relevance
ABSTRACT A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.
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- "Liver S9 fraction, a mixture of microsomal and cytosolic fractions, is another important sample to study as it and is also often employed in toxicological and drug metabolism studies . Since an important goal in proteomics research aims at determining post-translational modifications (including covalent binding from reactive endogenous/exogenous species) , an ideal method would achieve very high sequence coverage of all potentially targeted proteins. We previously reported  an approach for the proteomic analysis of rat liver microsomes examining different combinations of proteases and solubilizing agents using single digestion, serial dual digestion and parallel dual digestion workflows. "
ABSTRACT: Liver plays a key role in metabolism and detoxification, therefore analysis of its proteome is relevant for toxicology and drug discovery studies. To optimize for high proteome coverage, protein and peptide-level ion exchange fractionation were assessed using rat liver microsomes and S9 fractions. 2D-(SCX-RP)-LC–MS/MS analysis with peptide fractionation was subsequently employed for rat, mouse and human samples, yielding between 1400 and 1939 identified proteins, 58% of which were shared between species, and with relatively high sequence coverage. This rich dataset is specifically interesting for the toxicology community, and could serve as an excellent source for targeted assay development.EuPA Open Proteomics 02/2015; 1259. DOI:10.1016/j.euprot.2015.01.003
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- "It is also worth mentioning that although reductive metabolic reactions are less frequent than oxidations, they can have dramatic toxic consequences.     Having in mind these last considerations, it is clear that several functional groups are clearly associated with toxic effects (toxicophoric groups), either directly or as the result of their metabolism.   Due to these reasons, it is not surprising that lists of functional groups involved in toxicity, as well as the main bioactivation pathways, are being considered in drug design and development programs. "
ABSTRACT: Cytochrome P450 (CYP) is a super family of phase I enzyme in the biotransformation of xenobiotics and medications. Most medications undergo deactivation by CYP, and then are eliminated through either bile or kidneys from the body. CYP isozymes play a crucial role in drug interactions that may result in enhanced toxicity, reduced efficacy or onset of adverse reactions. On the other hand, many agents affecting CYP expression and activity may alter metabolic rate of different medications co-administrated. Therefore, the molecular basis, regulation by inducers or inhibitors, and pharmacologic reaction of specific CYP isozymes are the key issues of biochemical mechanisms, pharmaceutical development and safe use of various medications. This book is to meet the needs from basic molecular biochemists, pharmacologists, pharmacists, medical students, clinical practitioners and scientists, as well as broad readers who wish to understand how an herbal extract, medication or natural supplement is metabolized or transformed in the liver or other sites for deactivation and elimination. Special focuses are paid to herbal extracts and medications in the treatment of neuro-psychiatric or cardiovascular disorders, diabetes and viral hepatitis. Detailed dissection of drug interactions in a particular field intends to provide rationales for useful guidance of safe drug use in daily practice. The contributing authors are basic scientists, pharmacists, pharmacologists and on-service physicians in cardiovascular, neuro-psychiatric, gastroenterologic and hepatologic fields from Europe (Germany, France, Portugal), Australia, the US and China. Thus, the book is the collection of master pieces by well-known experts from various regions of the world, and represents the current understanding of CYP enzyme reaction and a contemporary coverage of possible drug interactions in involved fields. The featured chapters are scientific elucidation of basic biochemistry, pharmacology and clinical investigations in the interest of drug metabolism, interaction and safe use guidance in the single focus of this microsomal enzyme with multi-facet metabolic function.First edited by Jian Wu, 09/2014; Nova Science Publishers, Inc.., ISBN: 978-1-61942-209-4
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- "Biotransformat ion reactions aim to facilitate elimination of lipophilic xenobioti cs from cells. However, drug-induced cytotoxicity is often caused by drug P450-depen dent activation into reactive metabolites in the course of Phase I reactions (Zhou et al., 2005; Gómez-Lechón et al., 2010; Tang and Lu, 2010 ). P450- catalyzed reactions can generate metabolites capable of reacting with nucleophiles, covalently binding to macromolec ules or initiating radical-chain reactions within cells (Uetrecht, 1999 ). "
ABSTRACT: Many adverse drug reactions leading to hepatotoxicity are caused by the cytochrome P450-dependent activation of non-toxic drugs or chemicals into reactive metabolites. To this end, adenoviruses were used as a tool to efficiently deliver specific CYP genes into cultured cells (i.e., human hepatoma cell line HepG2). Recombinant-defective adenoviral vectors encoding for genes CYP3A4 (Adv-CYP3A4), CYP2E1 (Adv-CYP2E1), CYP2A6 (Adv-CYP2A6) and CYP1A2 (Adv-CYP1A2) were used to confer specific CYP drug metabolic capabilities to HepG2 cells. Upgraded cells transiently expressed single specific cytochrome P450 enzymatic activities in terms of the number of the infecting virus particles used in their transduction. HepG2 cells transduced with adenoviruses and wild HepG2 cells cultured in 96 well-plates were incubated in the presence of model compounds, some of which can be metabolized to reactive metabolites. After compound exposure, cell viability was assessed by the commonly used MTT assay. The results confirm that the cell-based assay is a valuable tool in toxicology assessments and high-throughput screenings to detect cytotoxicity mediated by cytochrome P450 biotransformation in preclinical drug development. The assay also has a potential applicability in other industrial sectors such as the chemical industry.Toxicology in Vitro 08/2012; 27(4). DOI:10.1016/j.tiv.2012.08.001 · 2.90 Impact Factor