Bioactivation Reactions Research Articles

Postcoiling syndrome including headache and fever after endovascular cerebral aneurysmal coil embolization: A narrative review.

Endovascular cerebral aneurysmal coil embolization is becoming more popular than direct aneurysmal neck clipping due to its noninferiority in long-term outcomes and being less invasive. Neuroradiologists often find postoperative symptoms such as headache and fever after unruptured aneurysmal coil embolization, however, they have not paid much attention because symptoms almost always resolve spontaneously within a few days. Since the concept of this syndrome has not been standardized, we named it postcoiling syndrome (PCS). In this short review, we reviewed the criteria, risk factors, mechanisms, significance, and treatment of PCS based on a few pieces of literature. Almost all literature has regarded that some kind of bioactive reaction might be involved in PCS. Preliminary data showed the possibility of inhibition of PCS by histamine-2 receptor antagonists. PCS also might have the potential of more predictive maker than previously reported risk factors for recurrence after aneurysm coil embolization. Further investigation is needed in the future, including the accumulation of cases, unification of concepts, and mid-to-long-term follow-up.

Glutathione-Mediated Conjugation of Anticancer Drugs: An Overview of Reaction Mechanisms and Biological Significance for Drug Detoxification and Bioactivation.

The effectiveness of many anticancer drugs depends on the creation of specific metabolites that may alter their therapeutic or toxic properties. One significant route of biotransformation is a conjugation of electrophilic compounds with reduced glutathione, which can be non-enzymatic and/or catalyzed by glutathione-dependent enzymes. Glutathione usually combines with anticancer drugs and/or their metabolites to form more polar and water-soluble glutathione S-conjugates, readily excreted outside the body. In this regard, glutathione plays a role in detoxification, decreasing the likelihood that a xenobiotic will react with cellular targets. However, some drugs once transformed into thioethers are more active or toxic than the parent compound. Thus, glutathione conjugation may also lead to pharmacological or toxicological effects through bioactivation reactions. My purpose here is to provide a broad overview of the mechanisms of glutathione-mediated conjugation of anticancer drugs. Additionally, I discuss the biological importance of glutathione conjugation to anticancer drug detoxification and bioactivation pathways. I also consider the potential role of glutathione in the metabolism of unsymmetrical bisacridines, a novel prosperous class of anticancer compounds developed in our laboratory. The knowledge on glutathione-mediated conjugation of anticancer drugs presented in this review may be noteworthy for improving cancer therapy and preventing drug resistance in cancers.

Roles of selected non-P450 human oxidoreductase enzymes in protective and toxic effects of chemicals: review and compilation of reactions.

This is an overview of the metabolic reactions of drugs, natural products, physiological compounds, and other (general) chemicals catalyzed by flavin monooxygenase (FMO), monoamine oxidase (MAO), NAD(P)H quinone oxidoreductase (NQO), and molybdenum hydroxylase enzymes (aldehyde oxidase (AOX) and xanthine oxidoreductase (XOR)), including roles as substrates, inducers, and inhibitors of the enzymes. The metabolism and bioactivation of selected examples of each group (i.e., drugs, “general chemicals,” natural products, and physiological compounds) are discussed. We identified a higher fraction of bioactivation reactions for FMO enzymes compared to other enzymes, predominately involving drugs and general chemicals. With MAO enzymes, physiological compounds predominate as substrates, and some products lead to unwanted side effects or illness. AOX and XOR enzymes are molybdenum hydroxylases that catalyze the oxidation of various heteroaromatic rings and aldehydes and the reduction of a number of different functional groups. While neither of these two enzymes contributes substantially to the metabolism of currently marketed drugs, AOX has become a frequently encountered route of metabolism among drug discovery programs in the past 10–15 years. XOR has even less of a role in the metabolism of clinical drugs and preclinical drug candidates than AOX, likely due to narrower substrate specificity.

Induced hemocompatibility of polyethersulfone (PES) hemodialysis membrane using polyvinylpyrrolidone: Investigation on human serum fibrinogen adsorption and inflammatory biomarkers released

When blood comes in contact with dialyzer membranes, the interfacial bioactivation reactions occurring at blood/membrane boundaries detect the gross mechanism of thrombogenicity. In the presence of biocompatible polymers, the adhesion and subsequent interactions of platelets, as well as other serum components (e.g., proteins), are altered. It delays thrombus formation. In this study, the hemocompatibility of polyvinylpyrrolidone (PVP) modified polyethersulphone (PES) hemodialysis membrane fabricated using UV-assisted photochemical synthesis was investigated. To obtain varying degrees of PVP modification on each membrane, each coated surface was aged within the viscous polymer suspension for a predetermined duration. The correlation between the membrane induced hemocompatibility with membrane roughness, chemical composition, and surface charge at different aging duration of coating. In our study, we performed a comprehensive investigations on membrane morphology, hydrophilicity and its chemical composition using Atomic Force Microscopy (AFM), Fourier Transform Infra-Red (FTIR) Spectroscopy, Nuclear Magnetic Resonance (NMR), Zeta potential, and Scanning Electron Microscopy (SEM). The coated samples were analyzed using Luminex assays for the inflammatory biomarkers of Serpin/Antithrombin-III, C5a, 1L-1α, 1L-1β, IL6, and C5b-9. Modified membranes showed significant resistance against fibrinogen adsorption compared to their bare membrane counterpart. This inherent antifouling activity against fibrinogen adsorption was observed more on modified membranes that aged longer within the PVP suspension during synthesis. A longer ageing period contributed to the enhanced PVP modification, hence, more stable polymeric hydration layers. In addition, both bare and coated membranes triggered both complement system activation and inflammatory responses, but to a mild degree in the latter case. Coated membranes at longer coating duration, around ∼4 min, had smoother surfaces and lower magnitudes of surface charge and consequently exhibited reduced release of all biomarkers (except IL-1α), indicating enhanced biocompatibility. Higher levels of C5a were observed in membranes coated at 1 and 2 min, promoting modest reduction toward the release of other components.

Identification of Key Genes and Pathways in Osteoarthritis via Bioinformatic Tools: An Updated Analysis.

Osteoarthritis (OA) is a severe and common degenerative disease; however, the exact pathology of OA is undefined. Our study is designed to investigate the underlying molecular mechanism of OA with bioinformatic tools. Three updated GEO datasets: GSE55235, GSE55457, and GSE82107 were selected for data analyzing. R software was utilized to screen and confirm the candidate differentially expressed genes in the development of OA. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes pathway were performed to identify the enriched GO terms and signaling pathways. Protein and protein interaction (PPI) models were built to observe the connected relationship among each potential protein. A total of 113 upregulated genes and 161 downregulated genes were found by integrating 3 datasets. GO enrichment indicated that cell differentiation, cellular response to starvation, and negative regulation of phosphorylation were important biological processes. KEGG enrichment indicated that FoxO, IL-17 signaling pathways, and osteoclast differentiation mainly participated in the progression of OA. Combining the molecular function and PPI results, ubiquitylation was identified as a pivotal bioactive reaction involved in OA. Our study provided updated candidate genes and pathways of OA, which may benefit further research and treatment for OA.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update.

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites.Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

Identification and characterization of in vitro, in vivo, and reactive metabolites of tandutinib using liquid chromatography ion trap mass spectrometry.

Tandutinib (TND) is a novel, oral small molecule designed for treating acute myeloid leukemia (AML) by inhibiting type III receptor tyrosine kinases. This study reports the use of in silico, in vivo, and in vitro methods to investigate the metabolism and possible metabolic bioactivation of TND. First, in silico metabolism of TND was assessed using the WhichP450™ module of the StarDrop® software to determine labile sites of metabolism in the TND chemical structure. Second, the XenoSite reactivity model, a web-based metabolism prediction software, was used to determine probable bioactive centers. Based on the in silico outcomes, a list of predicted metabolites and reactive intermediates were prepared. Third, in vitro and in vivo experiments were performed. In vitro TND metabolites were generated through incubation of TND with rat liver microsomes (RLMs). Another incubation of TND with RLMs was separately performed in the presence of GSH and KCN to check for the generation of reactive intermediates (soft and hard electrophiles). In vitro phase II metabolism was assessed by incubation of TND with isolated perfused rat hepatocytes. In vivo metabolism was investigated by oral gavage of TND (37 mg kg-1) in Sprague Dawley rats. Five in vitro phase I metabolites, one in vitro phase II and five reactive iminium intermediates (cyano adducts), six in vivo phase I, and one in vivo phase II metabolites of TND were characterized. The in vitro and in vivo metabolic pathways involved were O-dealkylation, α-hydroxylation, α-carbonyl formation, reduction, glucuronide, and sulfate conjugation. No GSH conjugate or its catabolic products were detected either in vitro or in vivo. Two cyclic tertiary rings of TND (piperazine and piperidine) were metabolically bioactivated to generate reactive iminium intermediates forming cyano adducts with KCN. The formed reactive intermediates may be the reason behind TND toxicity. In silico toxicological studies were performed for TND and its related (in vitro and in vivo) metabolites were evaluated using the DEREK software tool.

Dual mechanisms suppress meloxicam bioactivation relative to sudoxicam

Thiazoles are biologically active aromatic heterocyclic rings occurring frequently in natural products and drugs. These molecules undergo typically harmless elimination; however, a hepatotoxic response can occur due to multistep bioactivation of the thiazole to generate a reactive thioamide. A basis for those differences in outcomes remains unknown. A textbook example is the high hepatotoxicity observed for sudoxicam in contrast to the relative safe use and marketability of meloxicam, which differs in structure from sudoxicam by the addition of a single methyl group. Both drugs undergo bioactivation, but meloxicam exhibits an additional detoxification pathway due to hydroxylation of the methyl group. We hypothesized that thiazole bioactivation efficiency is similar between sudoxicam and meloxicam due to the methyl group being a weak electron donator, and thus, the relevance of bioactivation depends on the competing detoxification pathway. For a rapid analysis, we modeled epoxidation of sudoxicam derivatives to investigate the impact of substituents on thiazole bioactivation. As expected, electron donating groups increased the likelihood for epoxidation with a minimal effect for the methyl group, but model predictions did not extrapolate well among all types of substituents. Through analytical methods, we measured steady-state kinetics for metabolic bioactivation of sudoxicam and meloxicam by human liver microsomes. Sudoxicam bioactivation was 6-fold more efficient than that for meloxicam, yet meloxicam showed a 6-fold higher efficiency of detoxification than bioactivation. Overall, sudoxicam bioactivation was 15-fold more likely than meloxicam considering all metabolic clearance pathways. Kinetic differences likely arise from different enzymes catalyzing respective metabolic pathways based on phenotyping studies. Rather than simply providing an alternative detoxification pathway, the meloxicam methyl group suppressed the bioactivation reaction. These findings indicate the impact of thiazole substituents on bioactivation is more complex than previously thought and likely contributes to the unpredictability of their toxic potential.

Potentiometric and Speciation Studies on the Complex Formation Reactions of [Pd(2-methylaminomethyl)-pyridine)(H2O)2]2+ with Some Bio-active Ligands and Displacement Reaction of Coordinated Inosine

The stoichiometry and stability constants of the complexes formed between [Pd(MAMP)(H2O)2]2+ and various biologically relevant ligands containing different functional groups were investigated. The ligands used are amino acids, peptides and DNA constituents. The results show the formation of 1:1 complexes with amino acids and peptides and the corresponding deprotonated amide species. Structural effects of peptides on amide deprotonation were investigated. The purine and pyrimidine bases uracil, uridine, cytosine, inosine, inosine 5′-monophosphate (5′-IMP) and thymine form 1:1 and 1:2 complexes. The concentration distribution of the various complex species was calculated as a function of pH. The effect of chloride ion concentration on the formation constant of CBDCA with Pd(MAMP)2+ was also reported. The results show ring opening of CBDCA and monodentate complexation of the DNA constituent with the formation of [Pd(MAMP)(CBDCA-O)DNA], where (CBDCA-O) represents cyclobutane dicarboxylate coordinated by one carboxylate oxygen. The equilibrium constant of the displacement reaction of coordinated inosine, as a typical DNA constituent, by SMC and/or methionine was calculated. The results are expected to contribute to the chemistry of antitumor agents. The calculated parameters of the optimized complexes support the measured formation constants.

Toward the laboratory identification of [O,N,S,S] isomers: Implications for biological NO chemistry.

Benchmark ab initio calculations are performed to investigate the stable isomers of [O,N,S,S]. These computations are carried out using coupled cluster (RCCSD(T)) and explicitly correlated coupled cluster methods (RCCSD(T)-F12). In addition to the already known cis isomer of SSNO, nine other stable forms are predicted. The most stable isomer is cis-OSNS. Nine structures are chain bent-bent with relatively large dipole moments which make them detectable, as cis-SSNO, by infrared, far-infrared, and microwave spectroscopies. We found also a C2v isomer (NS2O). Since these species are strongly suggested to play an important role as intermediates during the bioactive reaction products of the NO/H2S interaction, the rotational and vibrational spectroscopic parameters are presented to help aid the in vivo identification and assignment of these spectra. Results from this work show that [O,N,S,S] may play key roles during nitric oxide transport and deliver in biological media, as well as, provide an explanation for the weak characteristic of disulfide bridges within proteins.

Cannabinoids and Cytochrome P450 Interactions.

This review consists of three parts, representing three different possibilities of interactions between cannabinoid receptor ligands of both exogenous and endogenous origin and cytochrome P450 enzymes (CYPs). The first part deals with cannabinoids as CYP substrates, the second summarizes current knowledge on the influence of various cannabinoids on the metabolic activity of CYP, and the third outline a possible involvement of the endocannabinoid system and cannabinoid ligands in the regulation of CYP liver activity. We performed a structured search of bibliographic and drug databases for peer-reviewed literature using focused review questions. Biotransformation via a hydrolytic pathway is the major route of endocannabinoid metabolism and the deactivation of substrates is characteristic, in contrast to the minor oxidative pathway via CYP involved in the bioactivation reactions. Phytocannabinoids are extensively metabolized by CYPs. The enzymes CYP2C9, CYP2C19, and CYP3A4 catalyze most of their hydroxylations. Similarly, CYP represents a major metabolic pathway for both synthetic cannabinoids used therapeutically and drugs that are abused. In vitro experiments document the mostly CYP inhibitory activity of the major phytocannabinoids, with cannabidiol as the most potent inhibitor of many CYPs. The drug-drug interactions between cannabinoids and various drugs at the CYP level are reported, but their clinical relevance remains unclear. The direct activation/inhibition of nuclear receptors in the liver cells by cannabinoids may result in a change of CYP expression and activity. Finally, we hypothesize the interplay of central cannabinoid receptors with numerous nervous systems, resulting in a hormone-mediated signal towards nuclear receptors in hepatocytes.

Rapid Identification of Bioactive Carbohydrazide Reaction Products by an LC-DAD-SPE-NMR Approach

On-line coupling of high performance liquid chromatography with diode array detection to solid phase extraction combined with nuclear magnetic resonance (LC-DAD-SPE-NMR) was used to monitor carbohydrazide condensation reaction progress. First, a chromatographic method was developed and optimised and individual peak separation was readily achieved by using an isocratic acetonitrile-phosphate buffer mobile phase. Subsequently, separated compounds were trapped on SPE cartridges and dried with nitrogen gas. Peak elution was then performed with deuterated acetonitrile and sent for NMR analysis. Single and multiple trapping options were applied. One- and two-dimensional NMR spectra were recorded using a Prodigy cryoprobe. It was demonstrated that LC-DAD-SPE-NMR setup was proved very useful for rapid and unambiguous identification of the reaction products and for determination of their structure. By using Prodigy cryoprobe in NMR measurements we were able to detect and identify compounds present at microgram level thus proving a high sensitivity of this methodology for monitoring reactions of bioactive molecules and drugs.

Key bioactive reaction products of the NO/H2S interaction are S/N-hybrid species, polysulfides, and nitroxyl

Experimental evidence suggests that nitric oxide (NO) and hydrogen sulfide (H2S) signaling pathways are intimately intertwined, with mutual attenuation or potentiation of biological responses in the cardiovascular system and elsewhere. The chemical basis of this interaction is elusive. Moreover, polysulfides recently emerged as potential mediators of H2S/sulfide signaling, but their biosynthesis and relationship to NO remain enigmatic. We sought to characterize the nature, chemical biology, and bioactivity of key reaction products formed in the NO/sulfide system. At physiological pH, we find that NO and sulfide form a network of cascading chemical reactions that generate radical intermediates as well as anionic and uncharged solutes, with accumulation of three major products: nitrosopersulfide (SSNO(-)), polysulfides, and dinitrososulfite [N-nitrosohydroxylamine-N-sulfonate (SULFI/NO)], each with a distinct chemical biology and in vitro and in vivo bioactivity. SSNO(-) is resistant to thiols and cyanolysis, efficiently donates both sulfane sulfur and NO, and potently lowers blood pressure. Polysulfides are both intermediates and products of SSNO(-) synthesis/decomposition, and they also decrease blood pressure and enhance arterial compliance. SULFI/NO is a weak combined NO/nitroxyl donor that releases mainly N2O on decomposition; although it affects blood pressure only mildly, it markedly increases cardiac contractility, and formation of its precursor sulfite likely contributes to NO scavenging. Our results unveil an unexpectedly rich network of coupled chemical reactions between NO and H2S/sulfide, suggesting that the bioactivity of either transmitter is governed by concomitant formation of polysulfides and anionic S/N-hybrid species. This conceptual framework would seem to offer ample opportunities for the modulation of fundamental biological processes governed by redox switching and sulfur trafficking.

Melatonin, its metabolites and its synthetic analogs as multi-faceted compounds: antioxidant, prooxidant and inhibitor of bioactivation reactions.

It is known that melatonin (MLT) and some of its metabolites act as antioxidants by scavenging free radicals as well as increasing the activity of antioxidant enzymes in the body. MLT is suggested to exert beneficial effects via various mechanisms in the treatment of many diseases, such as cancer, neurodegenerative diseases, epilepsy, diabetes mellitus and obesity. People working in nightshift exhibit decreased MLT levels that are suggested to be related with increased risk of hormone-related diseases. Similarly blind people were found to have increased MLT levels protecting against many diseases. This review briefly summarizes the published reports supporting these beneficial effects of MLT. Furthermore the present review involves recent developments related to the antioxidant effect of remarkable and multi-faceted molecule MLT as well as its metabolites and its synthesized analogues. The role of MLT as an inhibitor of bioactivation reactions is also discussed.

Expression of a novel mRNA transcript for human microsomal epoxide hydrolase (EPHX1) is regulated by short open reading frames within its 5′-untranslated region

Microsomal epoxide hydrolase (mEH, EPHX1) is a critical xenobiotic-metabolizing enzyme, catalyzing both detoxification and bioactivation reactions that direct the disposition of chemical epoxides, including the carcinogenic metabolites of several polycyclic aromatic hydrocarbons. Recently, we discovered that a previously unrecognized and primate-specific EPHX1 transcript, termed E1-b, was actually the predominant driver of EPHX1 expression in all human tissues. In this study, we identify another human EPHX1 transcript, designated as E1-b'. Unusually, both the E1-b and E1-b' mRNA transcripts are generated from the use of a far upstream gene promoter, localized ∼18.5 kb 5'-upstream of the EPHX1 protein-coding region. Although expressed at comparatively lower levels than E1-b, the novel E1-b' transcript is readily detected in all tissues examined, with highest levels maintained in human ovary. The E1-b' mRNA possesses unusual functional features in its 5'-untranslated region, including a GC-rich leader sequence and two upstream AUGs that encode for short peptides of 26 and 17 amino acids in length, respectively. Results from in vitro transcription/translation assays and direct transfection in mammalian cells of either the E1-b' transcript or the encoded peptides demonstrated that the E1-b' upstream open reading frames (uORFs) are functional, with their presence markedly inhibiting the translation of EPHX1 protein, both in cis and in trans configurations. These unique uORF peptides exhibit no homology to any other known uORF sequences but likely function to mediate post-transcription regulation of EPHX1 and perhaps more broadly as translational regulators in human cells.

Exploiting endobiotic metabolic pathways to target xenobiotic antioxidants to mitochondria

Oxidative stress plays a role in a range of human disease entities. Hence, strategies to target antioxidants to mitochondria are an active area of investigation. Triphenylphosphonium cation-based antioxidants and SS-peptides have been described and show significant uptake by mitochondria and effectiveness in animal models of conditions linked to oxidative stress. We tested the hypothesis that the mitochondrial β-oxidation pathway could be exploited to activate the antioxidant phenolic and methimazole prodrugs. Most compounds studied underwent mitochondrial biotransformation to release their antioxidant moieties, and some were cytoprotective in a hypoxia–reoxygenation model in rat cardiomyocytes. These results demonstrate the feasibility of exploiting mitochondrial bioactivation reactions for targeted drug delivery.

Role of Cyclic Tertiary Amine Bioactivation to Reactive Iminium Species: Structure Toxicity Relationship

Cytochrome P450-mediated bioactivation of drugs to reactive metabolites has been reported to be the first step in many adverse drug reactions. Metabolic activation of cyclic tertiary amines often generates a number of oxidative products including N-dealkylation, ring hydroxylation, α-carbonyl formation, N-oxygenation, and ring opening metabolites that can be formed through iminium ion intermediates. Therapeutic pharmaceuticals and their metabolites containing a cyclic tertiary amine structure have the potential to form iminium intermediates that are reactive toward nucleophilic macromolecules. Examples of cyclic tertiary amines that have the potential for forming reactive iminium intermediates include the piperazines, piperidines, 4-hydroxypiperidines, 4-fluoropiperidines and related compounds, pyrrolidines and N-alkyltetrahydroquinolines. Major themes explored in this review include bioactivation reactions for cyclic tertiary amines, which are responsible for the formation of iminium intermediates, together with some representative examples of drugs and guidance for discovery scientists in applying the information to minimize the bioactivation potential of cyclic amine-based compounds in drug discovery. Potential strategies to abrogate reactive iminium intermediate formation are also discussed.

Putting bioactivation reactions to work: Targeting antioxidants to mitochondria

The goal of this research was to test the hypothesis that bioactivation reactions could be exploited to deliver and activate mitochondria-targeted antioxidant prodrugs. The concept that bioactivation reactions could be used for prodrug delivery and activation has received little attention. Most bioactivation reactions result in the conversion of the parent drug to a reactive electrophilic metabolite, but bioactivating enzymes that catalyze elimination or hydrolytic reactions may offer potential for targeted drug delivery. Because mitochondria are the major cellular source of reactive oxygen species, there is much interest in targeting antioxidants to mitochondria. Previous studies showed that the mitochondrial fatty acid β-oxidation pathway biotransforms a range of xenobiotic alkanoates, including ω-(phenyl)alkanoates and ω-(phenoxy)alkanoates. 5,6-Dichloro-4-thia-5-hexenoate, the desamino analog of S-(1,2-dichlorovinyl)- l-cysteine, is biotransformed by the fatty acid β-oxidation pathway. Hence, the prodrugs ω-(phenoxy)alkanoates, 3-(phenoxy)acrylates, and ω-(1-methyl-1 H-imidazol-2-ylthio)alkanoates were expected to undergo biotransformation by the mitochondrial β-oxidation pathway to release phenolic antioxidants and the antioxidant methimazole (Roser et al., Bioorg. Med. Chem. 18 (2010) 1441–1448). The rates of biotransformation of ω-(phenoxy)alkanoates varied with the structure, and bulky substituents on the phenoxy moiety reduced rates of biotransformation; this was attributed to substrate limitations imposed by the medium-chain acyl-CoA dehydrogenase. Hence, 3-(2,6-dimethylphenoxy)acrylate was prepared; it was expected that, after conversion to its CoA thioester, 3-(2,6-dimethylphenoxy)acryloyl-CoA would be a substrate for enoyl-CoA hydratase. This expectation was correct: 3-(2,6-dimethylphenoxy)acrylate was an excellent substrate. ω-(1-Methyl-1 H-imidazol-2-ylthio)alkanoates were also good substrates for the β-oxidation pathway. Significantly, 3-(2,6-dimethylphenoxy)propanoate, 3-(2,6-dimethylphenoxy)acrylate, and 3-(1-methyl-1 H-imidazol-2-ylthio)propanoate were cytoprotective in a hypoxia-reoxygenation model in rat cardiomyocytes. These results demonstrate the feasibility of exploiting bioactivation reactions for targeted drug delivery.

Drug bioactivation and protein adduct formation in the pathogenesis of drug-induced toxicity

Adverse drug reactions (ADRs) remain a major complication of drug therapy and can be classified as ‘on-target’ or ‘off-target’ (idiosyncratic) reactions. On-target reactions can be predicted from the known primary or secondary pharmacology of the drug and often represent an exaggeration of the pharmacological effect of the drug. In contrast, off-target adverse reactions cannot be predicted from knowledge of the basic pharmacology of the drug. The exact mechanisms of idiosyncratic drug reactions are still unclear; however it is believed that they can be initiated by chemically reactive drug metabolites. It is well known that xenobiotics can undergo metabolic bioactivation reactions which have the potential to cause cellular stress and damage. Bioactivation of drugs is thought to have the potential of initiating covalent linkages between cellular protein and drugs which can be recognised by the adaptive immune system in the absence of detectable cellular stress. This process cannot yet be predicted in pre-clinical models or discovered in clinical trials. Because of this hazard perception, the formation of chemically reactive metabolites in early drug discovery remains a serious impediment to the development of new medicines and can lead to withdrawal of an otherwise effective therapeutic agent. The fear of such reactions occurring at the post-licensing stage – when such problems first become evident – is a major contribution to drug attrition. The first step towards such methodology has been the development of chemically reactive metabolite screens. The chemical basis of drug bioactivation can usually be rationalised and synthetic strategies put in place to prevent such bioactivation. However, there is no simple correlation between drug bioactivation in vitro and adverse drug reactions in the clinic. Such a chemical approach is clearly limited by the facts that (a) not all drugs that can undergo bioactivation by human drug-metabolising enzymes are associated with hypersensitivity in the clinic and (b) drug bioactivation may not always be a mandatory step in drug hypersensitivity. To predict such reactions in early drug development, it will require an integrated understanding of the chemical, immunological and genetic basis of adverse drug reactions in patients, which in turn will depend on the development of novel in vitro experimental systems.

Effect of two storage solutions on surface topography of two root‐end fillings

The effect of different storage solutions on surface topography of mineral trioxide aggregate (MTA) and new experimental cement (NEC) as root-end fillings was investigated. Twenty-four single-rooted teeth were cleaned, shaped and obturated in a same manner. After root-end resection, 3-mm deep root-end cavities were ultrasonically prepared. Samples were randomly divided into four test groups (A1-A2-B1-B2, n = 6). Root-end cavities in groups A and B were filled with MTA and NEC, respectively, and were then stored in 100% humidity for 24 h. The samples of groups 1 and 2 were, respectively, immersed in normal saline (NS) and phosphate buffer saline solutions for 1 week. The samples were imaged under stereomicroscope before and after immersion and were then investigated and analysed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA). Results showed significant difference among studied groups. Surface topography of all samples was altered by crystal formation and precipitation on root-end fillings except for group A1 (MTA-NS). SEM and EDXA results showed that the composition and structure of precipitated crystals were comparable with that of standard hydroxyapatite. It was concluded that biocompatibility, sealing ability, and cementogenic activity of MTA and probably NEC may be attributed to this fundamental bioactive reaction.