Arene oxides and the NIH shift: the metabolism, toxicity and carcinogenicity of aromatic compounds

Theranostic Fluorescent Probes

The ability to gain spatiotemporal information, and in some cases achieve spatiotemporal control, in the context of drug delivery makes theranostic fluorescent probes an attractive and intensely investigated research topic. This interest is reflected in the steep rise in publications on the topic that have appeared over the past decade. Theranostic fluorescent probes, in their various incarnations, generally comprise a fluorophore linked to a masked drug, in which the drug is released as the result of certain stimuli, with both intrinsic and extrinsic stimuli being reported. This release is then signaled by the emergence of a fluorescent signal. Importantly, the use of appropriate fluorophores has enabled not only this emerging fluorescence as a spatiotemporal marker for drug delivery but also has provided modalities useful in photodynamic, photothermal, and sonodynamic therapeutic applications. In this review we highlight recent work on theranostic fluorescent probes with a particular focus on probes that are activated in tumor microenvironments. We also summarize efforts to develop probes for other applications, such as neurodegenerative diseases and antibacterials. This review celebrates the diversity of designs reported to date, from discrete small-molecule systems to nanomaterials. Our aim is to provide insights into the potential clinical impact of this still-emerging research direction.

Phenytoin - An anti-seizure drug: Overview of its chemistry, pharmacology and toxicology

Phenytoin is a long-standing, anti-seizure drug widely used in clinical practice. It has also been evaluated in the context of many other illnesses in addition to its original epilepsy indication. The narrow therapeutic index of phenytoin and its ubiquitous daily use pose a high risk of poisoning. This review article focuses on the chemistry, pharmacokinetics, and toxicology of phenytoin, with a special focus on its mutagenicity, carcinogenicity, and teratogenicity. The side effects on human health associated with phenytoin use are thoroughly described. In particular, DRESS syndrome and cerebellar atrophy are addressed. This review will help in further understanding the benefits phenytoin use in the treatment of epilepsy.

Chemo- and Regioselective Dihydroxylation of Benzene to Hydroquinone Enabled by Engineered Cytochrome P450 Monooxygenase

Hydroquinone (HQ) is produced commercially from benzene by multi-step Hock-type processes with equivalent amounts of acetone as side-product. We describe an efficient biocatalytic alternative using the cytochrome P450-BM3 monooxygenase. Since the wildtype enzyme does not accept benzene, a semi-rational protein engineering strategy was developed. Highly active mutants were obtained which transform benzene in a one-pot sequence first into phenol and then regioselectively into HQ without any overoxidation. A computational study shows that the chemoselective oxidation of phenol by the P450-BM3 variant A82F/A328F leads to the regioselective formation of an epoxide intermediate at the C3=C4 double bond, which departs from the binding pocket and then undergoes fragmentation in aqueous medium with exclusive formation of HQ. As a practical application, an E. coli designer cell system was constructed, which enables the cascade transformation of benzene into the natural product arbutin, which has anti-inflammatory and anti-bacterial activities.

Hydroxylation of anilides by engineered cytochrome P450BM3

Cytochrome P450_BM3mutants achieve selectivepara-hydroxylation of substitutedN-sulfonylanilines under mild conditions.

Metabolomics of Δ9-tetrahydrocannabinol: implications in toxicity

Cannabis sativa is the most commonly used recreational drug, Δ(9)-tetrahydrocannabinol (Δ(9)-THC) being the main addictive compound. Biotransformation of cannabinoids is an important field of xenobiochemistry and toxicology and the study of the metabolism can lead to the discovery of new compounds, unknown metabolites with unique structures and new therapeutic effects. The pharmacokinetics of Δ(9)-THC is dependent on multiple factors such as physical/chemical form, route of administration, genetics, and concurrent consumption of alcohol. This review aims to discuss metabolomics of Δ(9)-THC, namely by presenting all known metabolites of Δ(9)-THC described both in vitro and in vivo, and their roles in the Δ(9)-THC-mediated toxic effects. Since medicinal use is increasing, metabolomics of Δ(9)-THC will also be discussed in order to uncover potential active metabolites that can be made available for this purpose.

4-Hydroxyphenylpyruvate Dioxygenase and Hydroxymandelate Synthase: 2-Oxo Acid-Dependent Oxygenases of Importance to Agriculture and Medicine

Despite a separate evolutionary lineage, 4-hydroxyphenylpyruvate dioxygenase (HPPD) and hydroxymandelate synthase (HMS) are appropriately grouped with the 2-oxo acid-dependent oxygenase (2OADO) family of enzymes. HPPD and HMS accomplish highly similar overall chemistry to that observed in the majority of 2OADOs but require only two substrates rather than three. 2OADOs typically use the 2-oxo acid of 2-oxoglutarate (2OG) as a source of electrons to reduce and activate dioxygen in order to oxidize a third specific substrate. HPPD and HMS use instead the pyruvate substituent of 4-hydroxyphenylpyruvate to activate dioxygen and then proceed to also hydroxylate this substrate, each yielding a distinctly different aromatic product. HPPD catalyses the second and committed step of tyrosine catabolism, a pathway common to nearly all aerobes. Plants require the HPPD reaction to biosynthesize plastoquinones and therefore HPPD inhibitors can have potent herbicidal activity. The ubiquity of the HPPD reaction, however, has meant that HPPD-specific molecules developed as herbicides have other uses in different forms of life. In humans herbicidal HPPD inhibitors can be used therapeutically to alleviate specific inborn defects and also to retard the progress of certain bacterial and fungal infections. This review is intended as a concise overview of the contextual and catalytic chemistries of HPPD and HMS.

Bioaccumulation of hydroxylated polychlorinated biphenyls and pentachlorophenol in the serum of northern elephant seal pups (Mirounga angustirostris)

Northern elephant seals (NES) (Mirounga angustirostris) from the Año Nuevo State Reserve (CA, USA) were sampled at 1-, 4-, 7- and 10-week post-weaning. Concentrations of hydroxylated polychlorinated biphenyls (HO-PCBs) and their parent PCBs were measured in the serum of each individual. The ΣHO-PCB concentrations in the serum increased significantly between early and late fast (from 282 ± 20 to 529 ± 31 pg/mL). This increase might result from a mobilisation of HO-PCBs transferred from the mother during gestation and/or lactation and stored in the pup's liver. Food deprivation has been shown to exacerbate biotransformation capacities in mammals, birds and fish. The HO-penta-CBs was the predominant homologue group, followed by HO-hexa-CBs and HO-hepta-CBs. No preferential pathway for the metabolism of HO-PCBs (HO-direct insertion or NIH-shift of a chlorine atom) could be evidenced. The concentrations of pentachlorophenol (PCP) in the serum of weaned NES increased from 103 ± 7 pg/mL at early fast to 246 ± 41 pg/mL at late fast, which is within the range of PCP concentrations usually encountered in marine mammals.

Increasing power density and dye decolorization of an X-3B-fed microbial fuel cell via TiO2 photocatalysis pretreatment

With pretreatment via photocatalysis, the output power density of MFC increased and more X-3B was removed.

4-Hydroxyphenylpyruvate dioxygenase and hydroxymandelate synthase: exemplars of the α-keto acid dependent oxygenases

4-Hydroxyphenylpyruvate dioxygenase (HPPD) and hydroxymandelate synthase (HMS) are outliers within the α-keto acid dependent oxygenase (αKAO) family. HPPD and HMS catalyze the chemistry of the majority of enzymes within the αKAO family but are clearly mechanistically convergent, having a grossly different structural topology. Some of the unique characteristics of HPPD and HMS have elucidated select parts of the catalytic cycle that are obscured in other family members. Moreover, the inhibitory chemistry of HPPD is a phenomenon with ever-expanding relevance across all kingdoms of life. This review is a synopsis of the literature pertaining to HPPD and HMS. It is not intended as an exhaustive compilation of all observations made for these enzymes but rather a condensed narrative that connects those studies that have advanced the understanding of the chemistry of both enzymes.

Valence isomerization of cyclohepta-1,3,5-triene and its heteroelement analogues

The valence isomerization of the all-carbon and heteroelement analogues of cyclohepta-1,3,5-triene into the corresponding bicyclo[4.1.0]hepta-2,4-dienes is reviewed to show the impact of the heteroatom on the stability of both valence isomers. The focus is on the parent systems and their synthetic applications.

A fungal P450 (CYP5136A3) capable of oxidizing polycyclic aromatic hydrocarbons and endocrine disrupting alkylphenols: role of Trp(129) and Leu(324)

The model white rot fungus Phanerochaete chrysosporium, which is known for its versatile pollutant-biodegradation ability, possesses an extraordinarily large repertoire of P450 monooxygenases in its genome. However, the majority of these P450s have hitherto unknown function. Our initial studies using a genome-wide gene induction strategy revealed multiple P450s responsive to individual classes of xenobiotics. Here we report functional characterization of a cytochrome P450 monooxygenase, CYP5136A3 that showed common responsiveness and catalytic versatility towards endocrine-disrupting alkylphenols (APs) and mutagenic/carcinogenic polycyclic aromatic hydrocarbons (PAHs). Using recombinant CYP5136A3, we demonstrated its oxidation activity towards APs with varying alkyl side-chain length (C3-C9), in addition to PAHs (3–4 ring size). AP oxidation involves hydroxylation at the terminal carbon of the alkyl side-chain (ω-oxidation). Structure-activity analysis based on a 3D model indicated a potential role of Trp¹²⁹ and Leu³²⁴ in the oxidation mechanism of CYP5136A3. Replacing Trp¹²⁹ with Leu (W129L) and Phe (W129F) significantly diminished oxidation of both PAHs and APs. The W129L mutation caused greater reduction in phenanthrene oxidation (80%) as compared to W129F which caused greater reduction in pyrene oxidation (88%). Almost complete loss of oxidation of C3-C8 APs (83–90%) was observed for the W129L mutation as compared to W129F (28–41%). However, the two mutations showed a comparable loss (60–67%) in C9-AP oxidation. Replacement of Leu³²⁴ with Gly (L324G) caused 42% and 54% decrease in oxidation activity towards phenanthrene and pyrene, respectively. This mutation also caused loss of activity towards C3-C8 APs (20–58%), and complete loss of activity toward nonylphenol (C9-AP). Collectively, the results suggest that Trp¹²⁹ and Leu³²⁴ are critical in substrate recognition and/or regio-selective oxidation of PAHs and APs. To our knowledge, this is the first report on an AP-oxidizing P450 from fungi and on structure-activity relationship of a eukaryotic P450 for fused-ring PAHs (phenanthrene and pyrene) and AP substrates.

Rearrangement reactions catalyzed by cytochrome P450s

Cytochrome P450s promote a variety of rearrangement reactions both as a consequence of the nature of the radical and other intermediates generated during catalysis, and of the neighboring structures in the substrate that can interact either with the initial radical intermediates or with further downstream products of the reactions. This article will review several kinds of previously published cytochrome P450-catalyzed rearrangement reactions, including changes in stereochemistry, radical clock reactions, allylic rearrangements, "NIH" and related shifts, ring contractions and expansions, and cyclizations that result from neighboring group interactions. Although most of these reactions can be carried out by many members of the cytochrome P450 superfamily, some have only been observed with select P450s, including some reactions that are catalyzed by specific endoperoxidases and cytochrome P450s found in plants.

Bacterial nitration of 4-chlorobiphenyl

In the course of a study dealing with the biodegradation of 4-chlorobiphenyl by strain B-206, we noticed that the gram-negative bacterium accumulated different metabolic intermediates depending on the nitrogen source of the medium. Hence, in the presence of nitrate, strain B-206 produced four compounds which were identified as 2- and 4-hydroxy-4'-chlorobiphenyl and 2- and 4-hydroxy-mononitro-4'-chlorobiphenyl. The accumulation of these compounds in the culture medium indicated the presence of a monooxygenase in strain B-206 leading to the production of arene oxide intermediates. The possible transformation of 4-chlorobiphenyl to an arene oxide by this bacterial strain is a matter of concern because of the high reactivity of these arene oxides with biological material.

Metabolism-related liabilities of a potent histone deacetylase (HDAC) inhibitor and relevance of the route of administration on its metabolic fate

Compound A [1-methyl-N-{(1S)-1-[5-(2-naphthyl)-1H-imidazol-2-yl]-7-oxooctyl}piperidine-4-carboxamide is a potent class I histone deacetylase (HDAC) inhibitor that demonstrated good antiproliferative activity against human tumour cell lines of different origin. This compound showed high in vivo clearance in rats (160 ml min(-1) kg(-1)) due to metabolism. The main metabolite detected in urine after intravenous dosing was characterized as a dihydrohydroxy S-mercapturic acid conjugate. Following oral dosing, however, the mercapturic acid derivative was no longer the main metabolite but the major metabolites were mono- and di-glucuronide conjugates of oxidized species having a mass shift of +34 m/z with respect to the parent. Comparison of plasma concentration after intra-arterial infusion and intravenous infusion and incubation with microsomes from different tissues (liver, kidney, small intestine and lung) in the presence of beta-nicotinamide adenine dinucleotide phosphate (NADPH) indicated that the compound was highly cleared by the lung. Oxidation of the naphthalene moiety was demonstrated to be the cause of the high in vivo clearance of compound A and the potential for bioactivation of this group was flagged.

Metabolism of carcinogens, possibilities for modulation

One of the structural elements which are widely occurring in very many chemical mutagens and carcinogens are aromatic and olefinic moieties. These can be transformed into epoxides by microsomal monooxygenases. Such epoxides may spontaneously react with nucleophilic centers in the cell and thereby covalently bind to DNA, RNA and protein. Such a reaction may lead to cytotoxicity, allergy, mutagenicity and/or carcinogenicity, depending on the properties of the epoxide in question. An important contributing factor is the presence of enzymes controlling the concentration of such epoxides. There are several microsomal monooxygenases which differ in activity and substrate specificity. With large substrates, some monooxygenases preferentially attack at one specific site different from that attacked by others. Some of these pathways lead to reactive products, others are detoxification pathways. Also important are the enzymes which metabolize epoxides, such as epoxide hydrolases and glutathione transferases. Such enzymes can act as inactivating and in some specific cases also as co-activating enzymes. Moreover, precursor-sequestering enzymes such as dihydrodiol dehydrogenase, glucuronosyl transferases and sulphotransferases are important for the control of reactive epoxides. These enzymes themselves are subject to control by many endogenous and exogenous factors. By virtue of their contribution to the control of carcinogenic metabolites such modulators can act as modifiers of tumorigenesis and can be used experimentally to study the role of the various individual enzymes.

Biodegradation of polychlorinated biphenyls in Aroclor 1232 and production of metabolites from 2,4,4'-trichlorobiphenyl at low temperature by psychrotolerant Hydrogenophaga sp. strain IA3-A

AIMS

To determine the extent and pattern of degradation of polychlorinated biphenyls (PCBs) in Aroclor 1232 at 5 degrees C by a psychrotolerant bacterium, and to confirm the formation of intermediates of PCB metabolism at low temperature using 2,4,4'-trichlorobiphenyl (2,4,4'-TCB).

METHODS AND RESULTS

10 ppm of Aroclor 1232 or 100 micromol l(-1) 2,4,4'-TCB was incubated with biphenyl-grown cells at 5 degrees C or 30 degrees C for 48 or 72 h. Degradation of PCBs and the products of metabolism of 2,4,4'-TCB were confirmed by gas chromatography and mass spectrometry. Extents of degradation of many of the PCBs were similar at 5 degrees C and 30 degrees C. The extent of biodegradation of PCBs in Aroclor 1232 at 5 degrees C was dependent on chlorination pattern. The 14 chlorine-containing intermediates of 2,4,4'-TCB metabolism, which were detected, include several isomers of dihydrodiols, dihydroxy compounds and meta-cleavage compounds.

CONCLUSIONS

The bacterium will be useful for bioremediation of PCB-contaminated sites in cold climates; however, knowledge of the products of PCB metabolism is necessary, as they could be more toxic than the parent compounds.

SIGNIFICANCE AND IMPACT OF THE STUDY

Substantial degradation of some PCBs in Aroclor 1232 was demonstrated at low temperature within 48 h. The detection of several isomeric intermediates suggests that multiple pathways are used to transform PCBs in this strain. For the first time, formation of metabolic products from 2,4,4'-TCB at low temperature is confirmed.

High-Throughput Radiometric CYP2C19 Inhibition Assay Using Tritiated (S)-Mephenytoin

A rapid and sensitive radiometric assay for assessing the potential of drugs to inhibit cytochrome P450 (P450) 2C19 in human liver microsomes is described. The new assay, which does not require high-performance liquid chromatography (HPLC) separation or mass spectrometric detection, is based on the release of tritium as tritiated water that occurs upon CYP2C19-mediated 4'-hydroxylation of (S)-mephenytoin labeled with tritium in the 4' position. Because this reaction is subject to an NIH shift, tritium was also introduced into the 3'- and 5'-positions of the tracer to enhance formation of a tritiated water product. Tritiated water was separated from the substrate using 96-well solid-phase extraction plates. The reaction is NADPH-dependent and sensitive to CYP2C19 inhibitors. IC(50) values for 15 diverse drugs differed less than 2.5-fold from those determined by quantification of the unlabeled 4'-hydroxy-(S)-mephenytoin product, using HPLC coupled to mass spectrometric detection. All of the steps of the new assay, namely incubation, product separation, and radioactivity counting, are performed in a 96-well format and can be automated. This assay represents a non-HPLC, high-throughput version of the classic (S)-mephenytoin 4'-hydroxylation assay, which is the most widely used method to assess the potential for CYP2C19 inhibition of new chemical entities.

In vitro metabolic fate of a novel structural class: Evidence for the formation of a reactive intermediate on a benzothiophene moiety

The characterization of the metabolic pathways of new chemical entities with a special emphasis on detecting potentially reactive metabolites is increasingly being performed early in the drug discovery process. In the present study, the preliminary in vitro metabolic routes of a series of novel 2-substituted benzothiophene-containing discovery molecules were determined in fresh and cryopreserved hepatocyte suspensions. The objectives of this investigation were: (1) to use systematic LC/MS and LC/MS/MS analyses to provide a preliminary characterization of the in vitro metabolism of these compounds, with a particular focus on metabolites potentially arising from reactive intermediates, and (2) to identify potential lead molecules not associated with such metabolic pathways. This benzothiophene-containing series of compounds was characterized by the formation of five metabolites, at least two of which (dihydrodiol formation and glutathione adduct of the dihydrohydroxyl) were indicative of the formation of a reactive arene oxide intermediate. Tandem mass spectral analysis of the metabolites formed from a variety of structurally similar compounds demonstrated this reactive arene oxide intermediate to form on the 2-substituted benzothiophene moiety. Substitution of the benzothiophene with other functional groups eliminated these potentially toxic metabolites. The data presented here demonstrate the utility of performing metabolic route screens early in the drug discovery process prior to lengthy and costly radiolabeled studies, and furthermore, implicate a 2-substituted benzothiophene moiety as a substrate for formation of a reactive arene oxide intermediate.