Quinone Methide Bioactivation Pathway: Contribution to Toxicity and/or Cytoprotection?

Stimuli-Responsive Prodrug Linkers That Simultaneously Release Cargo and Neutralize In Situ Generated (Aza)Quinone Methides

Self-immolative linkers that use p-amino/hydroxy-benzyloxycarbonyl (PABC/PHBC) spacers are essential to the mechanism of many prodrugs. However, a highly reactive (aza)quinone methide is generated as a potential toxic byproduct. To remove the methide as it forms, we synthesized a series of novel tripartite prodrugs, comprising different triggers (nitro, amide, azide, boronate) and a PABC/PHBC-type self-immolative spacer with an integrated nucleophile (amine). Upon reductive, hydrolytic, or oxidative-trigger activation, the release of the cargo is facilitated via a 1,6-elimination that generates a reactive (aza)quinone methide. With the built-in nucleophile, the (aza)quinone methide is rapidly self-quenched to generate tetrahydroisoquinolines (THIQs). One of the selected THIQs does not exhibit an anti-proliferative effect on the A431 mammalian tumor cell line. The new prodrug strategy has broad scope, enabling the use of a trigger that matches the targeted stimulus, while allowing for a diverse range of drug/cargo attachment. This proof-of-concept study adds a new linker strategy that quenches the electrophilic (aza)quinone methide generated in many self-immolative linker systems and could find applications in prodrug and antibody-drug conjugate strategies, or as a linker for probes in chemical biology.

Review of the genotoxicity of "Arvin compounds", drinking water contaminants formed by the degradation of antoxidants in polyolefin pipes

Polyolefin pipes used in drinking water distribution systems require a number of functional additives to ensure stability and durability. Some of these additives and/or their degradation products may migrate from the pipes into the drinking water. Previously, a number of branched chain alkylphenol degradants have been identified in drinking water; these were termed "Arvin substances" and numbered Arvin 1-10. As potential genotoxicity is a human health safety concern, the genotoxicity of Arvin substances is reviewed based on comprehensive in vitro and in vivo data available. Results obtained from genotoxicity studies addressing mutagenicity and clastogenicity are available for nine of the ten Arvin substances. These nine Arvin substances were consistently negative in bacterial mutagenicity studies. Divergent results were obtained in clastogenicity assays with some positive responses induced by the branched chain alkylphenols Arvin 1, 2, and 4, often accompanied by significant cytotoxicity. However, Arvin 1, 2, and 4 did not induce micronuclei or genotoxicity in vivo during follow-up testing. The other Arvin compounds did not show genotoxic activity in vitro. In conclusion, regarding human health risk characterization, the Arvin compounds are not considered genotoxic agents based on the available data.

Promising strategies for smart insulin delivery system: Glucose-sensitive microneedle

The diabetes treatment landscape is rapidly evolving towards intelligent and precise therapeutic interventions. Among these advancements, glucose-sensitive microneedle patches (GSMPs), which can automatically adjust the transdermal release rate of insulin based on glucose concentrations, are emerging as a promising strategy. In this work, a new classification method has been proposed for GSMPs, categorizing them into integrated, all-in-one, and core-shell structures. The working mechanism and performance of GSMPs are thoroughly analyzed to compare the advantages and disadvantages of these three forms. The correlation between glucose-sensitive performance and normal blood glucose maintenance time (NGT) is further explored. Our findings indicate that all-in-one GSMPs demonstrate a positive correlation between in vitro glucose-sensitive controlled-release performance and NGT, unlike assembled GSMPs, where the performance is influenced by the matrix material and crosslinking factors. Simultaneously, challenges in clinical translation and future development trends are discussed from a patient's perspective. In summary, the new classification method, in-depth explanation of mechanisms, and analysis of challenges in this work contribute to a better understanding of the field of GSMPs and provide guidance for the development of more advanced and efficient GSMPs.

Gas Delivery Relevant to Human Health using Porous Materials

Gases are essential for various applications relevant to human health, including in medicine, biomedical imaging, and pharmaceutical synthesis. However, gases are significantly more challenging to safely handle than liquids and solids. Herein, we review the use of porous materials, such as metal-organic frameworks (MOFs), zeolites, and silicas, to adsorb medicinally relevant gases and facilitate their handling as solids. Specific topics include the use of MOFs and zeolites to deliver H2S for therapeutic applications, 129Xe for magnetic resonance imaging, O2 for the treatment of cancer and hypoxia, and various gases for use in organic synthesis. This Perspective aims to bring together the organic, inorganic, medicinal, and materials chemistry communities to inspire the design of next-generation porous materials for the storage and delivery of medicinally relevant gases.

Base-Mediated Cascade Lactonization/1,3-Dipolar Cycloaddition Pathway for the One-Pot Assembly of Coumarin-Functionalized Pyrrolo[2,1-a]isoquinolines

An elegant and highly concise strategy for the construction of coumarin-functionalized pyrrolo[2,1-a]isoquinolines from available propargylamines and isoquinolinium N-ylides has been disclosed. In this reaction, isoquinolinium N-ylides acted as a C2 synthon to form a coumarin ring as well as a 1,3-dipole to construct a pyrrole ring in a single pot. This cascade process involves 1,4-conjugate addition/lactonization/1,3-dipolar cycloaddition to construct four chemical bonds (one C-O bond and three C-C bonds) and two new heterocyclic skeletons. Additionally, most of these compounds showed good fluorescence properties and exhibited high molar extinction coefficient and large Stokes shifts.

Current advances in photocatalytic proximity labeling

Understanding the intricate network of biomolecular interactions that govern cellular processes is a fundamental pursuit in biology. Over the past decade, photocatalytic proximity labeling has emerged as one of the most powerful and versatile techniques for studying these interactions as well as uncovering subcellular trafficking patterns, drug mechanisms of action, and basic cellular physiology. In this article, we review the basic principles, methodologies, and applications of photocatalytic proximity labeling as well as examine its modern development into currently available platforms. We also discuss recent key studies that have successfully leveraged these technologies and importantly highlight current challenges faced by the field. Together, this review seeks to underscore the potential of photocatalysis in proximity labeling for enhancing our understanding of cell biology while also providing perspective on technological advances needed for future discovery.

Latest advances in glucose-responsive microneedle-based systems for transdermal insulin delivery

The development of a self-regulated minimally invasive system for insulin delivery can be considered as the holy grail in the field of diabetes mellitus. A delivery system capable of releasing insulin in response to blood glucose levels would significantly improve the quality of life of diabetic patients, eliminating the need for frequent finger-prick tests and providing better glycaemic control with lower risk of hypoglycaemia. In this context, the latest advances in glucose-responsive microneedle-based transdermal insulin delivery are here compiled with a thorough analysis of the delivery mechanisms and challenges lying ahead in their clinical translation. Two main groups of microneedle-based systems have been developed so far: glucose oxidase-containing and phenylboronic acid-containing systems. Both strategies in combination have also been tested and two other novel strategies are under development, namely electronic closed-loop and glucose transporter-based systems. Results from preclinical studies conducted using these different types of glucose-triggered release systems are comprehensively discussed. Altogether, this analysis from both a mechanistic and translational perspective will provide rationale and/or guidance for future trends in the research hotspot of glucose-responsive microneedle-based insulin delivery systems.

A critical examination of human data for the biological activity of quercetin and its phase-2 conjugates

This critical review examines evidence for beneficial effects of quercetin phase-2 conjugates from clinical intervention studies, volunteer feeding trials, and in vitro work. Plasma concentrations of quercetin-3-O-glucuronide (Q3G) and 3'-methylquercetin-3-O-glucuronide (3'MQ3G) after supplementation may produce beneficial effects in macrophages and endothelial cells, respectively, especially if endogenous deglucuronidation occurs, and lower blood uric acid concentration via quercetin-3'-O-sulfate (Q3'S). Unsupplemented diets produce much lower concentrations (<50 nmol/l) rarely investigated in vitro. At 10 nmol/l, Q3'S and Q3G stimulate or suppress, respectively, angiogenesis in endothelial cells. Statistically significant effects have been reported at 100 nmol/l in breast cancer cells (Q3G), primary neuron cultures (Q3G), lymphocytes (Q3G and3'MQ3G) and HUVECs (QG/QS mixture), but it is unclear whether these translate to a health benefit in vivo. More sensitive and more precise methods to measure clinically significant endpoints are required before a conclusion can be drawn regarding effects at normal dietary concentrations. Future requirements include better understanding of inter-individual and temporal variation in plasma quercetin phase-2 conjugates, their mechanisms of action including deglucuronidation and desulfation both in vitro and in vivo, tissue accumulation and washout, as well as potential for synergy or antagonism with other quercetin metabolites and metabolites of other dietary phytochemicals.

NAD(P)H:Quinone Acceptor Oxidoreductase 1 (NQO1) Activatable Salicylamide H+ /Cl- Transporters

NAD(P)H:quinone acceptor oxidoreductase 1 (NQO1), a detoxifying enzyme overexpressed in tumors, plays a key role in protecting cancer cells against oxidative stress and thus has been considered an attractive candidate for activating prodrug(s). Herein, we report the first use of NQO1 for the selective activation of 'protransporter' systems in cancer cells leading to the induction of apoptosis. Salicylamides, easily synthesizable small molecules, have been effectively used for efficient H+ /Cl- symport across lipid membranes. The ion transport activity of salicylamides was efficiently abated by caging the OH group with NQO1 activatable quinones via either ether or ester linkage. The release of active transporters, following the reduction of quinone caged 'protransporters' by NQO1, was verified. Both the transporters and protransporters exhibited significant toxicity towards the MCF-7 breast cancer line, mediated via the induction of oxidative stress, mitochondrial membrane depolarization, and lysosomal deacidification. Induction of cell death via intrinsic apoptotic pathway was verified by monitoring PARP1 cleavage.

Hybrids of Sterically Hindered Phenols and Diaryl Ureas: Synthesis, Switch from Antioxidant Activity to ROS Generation and Induction of Apoptosis

The utility of sterically hindered phenols (SHPs) in drug design is based on their chameleonic ability to switch from an antioxidant that can protect healthy tissues to highly cytotoxic species that can target tumor cells. This work explores the biological activity of a family of 45 new hybrid molecules that combine SHPs equipped with an activating phosphonate moiety at the benzylic position with additional urea/thiourea fragments. The target compounds were synthesized by reaction of iso(thio)cyanates with C-arylphosphorylated phenols containing pendant 2,6-diaminopyridine and 1,3-diaminobenzene moieties. The SHP/urea hybrids display cytotoxic activity against a number of tumor lines. Mechanistic studies confirm the paradoxical nature of these substances which combine pronounced antioxidant properties in radical trapping assays with increased reactive oxygen species generation in tumor cells. Moreover, the most cytotoxic compounds inhibited the process of glycolysis in SH-SY5Y cells and caused pronounced dissipation of the mitochondrial membrane of isolated rat liver mitochondria. Molecular docking of the most active compounds identified the activator allosteric center of pyruvate kinase M2 as one of the possible targets. For the most promising compounds, 11b and 17b, this combination of properties results in the ability to induce apoptosis in HuTu 80 cells along the intrinsic mitochondrial pathway. Cyclic voltammetry studies reveal complex redox behavior which can be simplified by addition of a large excess of acid that can protect some of the oxidizable groups by protonations. Interestingly, the re-reduction behavior of the oxidized species shows considerable variations, indicating different degrees of reversibility. Such reversibility (or quasi-reversibility) suggests that the shift of the phenol-quinone equilibrium toward the original phenol at the lower pH may be associated with lower cytotoxicity.

Non-innocuous Consequences of Metabolic Oxidation of Alkyls on Arenes

Reactive metabolite (RM) formation is widely accepted as playing a pivotal role in causing adverse idiosyncratic drug reactions, with most attention paid to drug-induced liver injury. Mechanisms of RM formation are determined by the drug's properties in relation to human enzymes transforming the drug. This Perspective focuses on enzymatic oxidation of alkyl groups on aromatics leading to quinone methides and benzylic alcohol sulfates as RMs, a topic that has not received very much attention. Unlike previous overviews, we will include in our Perspective several fulvene-like methides such as 3-methyleneindole. We also speculate that a few older drugs may form non-reported methides of this class. In addition, we report a few guiding DFT calculations of changes in free energy on going from a benzylic alcohol to the corresponding methide. Particularly facile reactions of 2-aminothiazole-5-methanol and 4-aminobenzyl alcohol are noted.

Reactivity vs. selectivity of quinone methides: synthesis of pharmaceutically important molecules, toxicity and biological applications

Quinone methides (QMs) are considered to be highly reactive intermediates because of their aromatization both in chemical and biological systems. Being highly accessible, quinone methides (QMs) have been widely exploited and their concurrent use has been manifested for the synthesis of tertiary and quaternary carbon centers of bioactives, drugs and drug-like molecules. In this feature article, the synthetic routes, structure-reactivity relationships and synthetic applications of quinone methides are discussed. Formation of the intermediates during bioactivation of different chemical entities and possible chemical manifestations leading to their toxicity in biological systems are also covered.

Mapping Mechanistic Pathways of Acute Oral Systemic Toxicity Using Chemical Structure and Bioactivity Measurements

Regulatory agencies around the world have committed to reducing or eliminating animal testing for establishing chemical safety. Adverse outcome pathways can facilitate replacement by providing a mechanistic framework for identifying the appropriate non-animal methods and connecting them to apical adverse outcomes. This study separated 11,992 chemicals with curated rat oral acute toxicity information into clusters of structurally similar compounds. Each cluster was then assigned one or more ToxCast/Tox21 assays by looking for the minimum number of assays required to record at least one positive hit call below cytotoxicity for all acutely toxic chemicals in the cluster. When structural information is used to select assays for testing, none of the chemicals required more than four assays and 98% required two assays or less. Both the structure-based clusters and activity from the associated assays were significantly associated with the GHS toxicity classification of the chemicals, which suggests that a combination of bioactivity and structural information could be as reproducible as traditional in vivo studies. Predictivity is improved when the in vitro assay directly corresponds to the mechanism of toxicity, but many indirect assays showed promise as well. Given the lower cost of in vitro testing, a small assay battery including both general cytotoxicity assays and two or more orthogonal assays targeting the toxicological mechanism could be used to improve performance further. This approach illustrates the promise of combining existing in silico approaches, such as the Collaborative Acute Toxicity Modeling Suite (CATMoS), with structure-based bioactivity information as part of an efficient tiered testing strategy that can reduce or eliminate animal testing for acute oral toxicity.

Intramolecular tetrazine-acryloyl cycloaddition: chemistry and applications

An unprecedented intramolecular [4 + 2] tetrazine-olefin cycloaddition with α,β-unsaturated substrates was discovered. The reaction produces unique coumarin-dihydropyridazine heterocycles that exhibited strong fluorescence with large Stokes shifts and excellent photo- and pH-stability. This property can be used for reaction analysis. The rate of cycloaddition was found to be solvent dependent and was determined using experimental data with a kinetic modeling software (COPASI) as well as DFT calculations (k₁ = 0.64 ± 0.019 s⁻¹ and 4.1 s⁻¹, respectively). The effects of steric and electronic properties of both the tetrazine and α,β-unsaturated carbonyl on the reaction were studied and followed the known trends characteristic of the intermolecular reaction. Based on these results, we developed a “release-then-click” strategy for the ROS triggered release of methylselenenic acid (MeSeOH) and a fluorescent tracer. This strategy was demonstrated in HeLa cells via fluorescence imaging.

Tetrazines rapidly react with tethered acrylates/acrylamides to produce fused coumarin derivatives. This template can be used in prodrug designs by depleting toxic α,β-unsaturated byproducts while also producing an imaging agent.

Protein Adducts and Protein Oxidation as Molecular Mechanisms of Flavonoid Bioactivity

There are tens of thousands of scientific papers about flavonoids and their impacts on human health. However, despite the vast amount of energy that has been put toward studying these compounds, a unified molecular mechanism that explains their bioactivity remains elusive. One contributing factor to the absence of a general mechanistic explanation of their bioactivity is the complexity of flavonoid chemistry in aqueous solutions at neutral pH. Flavonoids have acidic protons, are redox active, and frequently auto-oxidize to produce an array of degradation products including electrophilic quinones. Flavonoids are also known to interact with specificity and high affinity with a variety of proteins, and there is evidence that some of these interactions may be covalent. This review summarizes the mechanisms of flavonoid oxidation in aqueous solutions at neutral pH and proposes the formation of protein-flavonoid adducts or flavonoid-induced protein oxidation as putative mechanisms of flavonoid bioactivity in cells. Nucleophilic residues in proteins may be able to form covalent bonds with flavonoid quinones; alternatively, specific amino acid residues such as cysteine, methionine, or tyrosine in proteins could be oxidized by flavonoids. In either case, these protein-flavonoid interactions would likely occur at specific binding sites and the formation of these types of products could effectively explain how flavonoids modify proteins in cells to induce downstream biochemical and cellular changes.

Interactions of the antioxidant enzymes NAD(P)H: Quinone oxidoreductase 1 (NQO1) and NRH: Quinone oxidoreductase 2 (NQO2) with pharmacological agents, endogenous biochemicals and environmental contaminants

NAD(P)H

Quinone Oxidoreductase 1 (NQO1) is an antioxidant enzyme that catalyzes the two-electron reduction of several different classes of quinone-like compounds (quinones, quinone imines, nitroaromatics, and azo dyes). One-electron reduction of quinone or quinone-like metabolites is considered to generate semiquinones to initiate redox cycling that is responsible for the generation of reactive oxygen species and oxidative stress and may contribute to the initiation of adverse drug reactions and adverse health effects. On the other hand, the two-electron reduction of quinoid compounds appears important for drug activation (bioreductive activation) via chemical rearrangement or autoxidation. Two-electron reduction decreases quinone levels and opportunities for the generation of reactive species that can deplete intracellular thiol pools. Also, studies have shown that induction or depletion (knockout) of NQO1 were associated with decreased or increased susceptibilities to oxidative stress, respectively. Moreover, another member of the quinone reductase family, NRH: Quinone Oxidoreductase 2 (NQO2), has a significant functional and structural similarity with NQO1. The activity of both antioxidant enzymes, NQO1 and NQO2, becomes critically important when other detoxification pathways are exhausted. Therefore, this article summarizes the interactions of NQO1 and NQO2 with different pharmacological agents, endogenous biochemicals, and environmental contaminants that would be useful in the development of therapeutic approaches to reduce the adverse drug reactions as well as protection against quinone-induced oxidative damage. Also, future directions and areas of further study for NQO1 and NQO2 are discussed.

A matched molecular pair (MMP) approach for selecting analogs suitable for structure activity relationship (SAR)-based read across

One of the most challenging aspects of SAR-based read across is the identification of structurally similar compounds suitable for use as data sources to cover the safety of a target chemical. Matched molecular pair analysis (MMPA) provides a systematic method for mining experimental data for chemical substitutions that may be interpreted in terms of changes in properties. Here we use the relationships between structural substitutions linking a target chemical with an analog determined to be suitable using the expert-judgment based P&G framework of Wu et al. (2010). The relationships are established by applying MMPA to a database of compounds with safety assessed using SAR-based read across to suitable analogs possessing toxicological data. The analysis revealed that only five categories of substitutions per chemical class (aromatic or aliphatic) were necessary to link all molecular pairs. These data are summarized in a workflow outlining a strategy for searching toxicological databases for potential analogs. This approach provides structural comparisons that are interpretable and sensitive to small differences in the local structure of two compounds that may be linked to suitability for read across in contrast to the use of quantitative similarity measures which show little correlation with analog suitability.

Catalytic Asymmetric Conjugate Addition/Hydroalkoxylation Sequence: Expeditious Access to Enantioenriched Eight-Membered Cyclic Ether Derivatives

A sequential enantioselective conjugate addition/hydroalkoxylation between in situ generated ortho-quinomethanes and ynones by combining bifunctional squaramide and DBU catalysis has been developed. A variety of eight-membered cyclic ethers with two contiguous tertiary stereocenters were obtained in high yields with excellent stereoselectivities. This reaction not only provides a new strategy for constructing enantioenriched eight-membered cyclic ethers but also demonstrates the practicability of ynones as C4-syntons for the synthesis of chiral medium-membered rings.

Regioselective synthesis and anticancer evaluation of H2O2-activable nucleosides

We describe here the design, synthesis, and biological evaluation of H2O2-activatable nucleosides via an efficient and regioselective functionalization of unprotected precursors. Biological evaluation of a H2O2-specific responsive prodrug of gemecitabin demonstrates an extremely fast activation, low toxicity and enhanced anticancer effects in two cell lines.

Pharmacoinformatic Investigation of Medicinal Plants from East Africa

Medicinal plants have widely been used in the traditional treatment of ailments and have been proven effective. Their contribution still holds an important place in modern drug discovery due to their chemical, and biological diversities. However, the poor documentation of traditional medicine, in developing African countries for instance, can lead to the loss of knowledge related to such practices. In this study, we present the Eastern Africa Natural Products Database (EANPDB) containing the structural and bioactivity information of 1870 unique molecules isolated from about 300 source species from the Eastern African region. This represents the largest collection of natural products (NPs) from this geographical region, covering literature data of the period from 1962 to 2019. The computed physicochemical properties and toxicity profiles of each compound have been included. A comparative analysis of some physico-chemical properties like molecular weight, H-bond donor/acceptor, logPo/w , etc. as well scaffold diversity analysis has been carried out with other published NP databases. EANPDB was combined with the previously published Northern African Natural Products Database (NANPDB), to form a merger African Natural Products Database (ANPDB), containing ∼6500 unique molecules isolated from about 1000 source species (freely available at http://african-compounds.org). As a case study, latrunculins A and B isolated from the sponge Negombata magnifica (Podospongiidae) with previously reported antitumour activities, were identified via substructure searching as molecules to be explored as putative binders of histone deacetylases (HDACs).

Hetero-Diels-Alder Reactions of Quinone Methides: The Origin of the α-Regioselectivity of 3-Methylene-1,2,4-naphthotriones

The regioselective formation of α- and β-lapachone via hetero-Diels-Alder reactions was investigated by experimental and computational approaches. The experimentally observed α-selectivity was explored in detail, revealing that the lower energy barrier for the formation of α-lapachone is a result of lower Pauli repulsion throughout the reaction path, when compared to the β-isomer. By comparing equivalent points on both α- and β-lapachone potential energy surfaces (PES), according to the activation strain model (ASM) and energy decomposition analysis (EDA), we were able to demonstrate that the Pauli repulsion term increases more significantly when going from reactants to TS_β than to TS_α, resulting in lower interaction energy in the early stages of the reaction path and in a later transition state for β-lapachone. Moreover, we confirmed that regio- and diastereoselectivity trends previously reported for other quinone methide intermediates are also observed for 3-methylene-1,2,4-naphthotriones, such as small endo/exo diastereoselectivity, as well as pronounced ortho/meta regioselectivity for reactions performed with dienophile containing electron-releasing groups. The results presented here provide a deeper understanding of the reactivity of quinone methide derivatives, aiding the future rational design of the reaction condition, structural modification of possible quinone methide intermediates, and the development of more selective synthetic routes for quinone derivatives.

Molecular and cellular basis of the anticancer activity of the prenylated flavonoid icaritin in hepatocellular carcinoma

The prenylated flavonoid icaritin (ICT) is currently undergoing phase 3 clinical trial for the treatment of advanced hepatocellular carcinoma (HCC), based on a solid array of preclinical and clinical data. The antitumor activity originates from the capacity of the drug to modulate several signaling effectors in cancer cells, mainly the estrogen receptor splice variant ERα36, the transcription factors STAT3 and NFκB, and the chemokine receptor CXCR4. Recent studies have implicated additional components, including different microRNAs, the generation of reactive oxygen species and the targeting of sphingosine kinase-1. ICT also engages the RAGE-HMGB1 signaling route and modulates the apoptosis/autophagy crosstalk to promote its anticancer activity. In addition, ICT exerts profound changes on the tumor microenvironment to favor an immune-response. Collectively, these multiple biochemical and cellular characteristics confer to ICT a robust activity profile which can be exploited to treat HCC, as well as other cancers, including glioblastoma and onco-hematological diseases such as chronic myeloid leukemia. This review provides an update of the pharmacological properties of ICT and its metabolic characteristics. It also addresses the design of derivatives, including both natural products and synthetic molecules, such as SNG1153 also in clinical trial. The prenylated flavonoid ICT deserves attention as a multifunctional natural product potentially useful to improve the treatment of advanced hepatocellular carcinoma.

Ectopic suicide inhibition of thioredoxin glutathione reductase

Selective suicide inhibitors represent a seductively attractive approach for inactivation of therapeutically relevant enzymes since they are generally devoid of off-target toxicity in vivo. While most suicide inhibitors are converted to reactive species at enzyme active sites, theoretically bioactivation can also occur in ectopic (secondary) sites that have no known function. Here, we report an example of such an "ectopic suicide inhibition", an unprecedented bioactivation mechanism of a suicide inhibitor carried out by a non-catalytic site of thioredoxin glutathione reductase (TGR). TGR is a promising drug target to treat schistosomiasis, a devastating human parasitic disease. Utilizing hits selected from a high throughput screening campaign, time-resolved X-ray crystallography, molecular dynamics, mass spectrometry, molecular modeling, protein mutagenesis and functional studies, we find that 2-naphtholmethylamino derivatives bound to this novel ectopic site of Schistosoma mansoni (Sm)TGR are transformed to covalent modifiers and react with its mobile selenocysteine-containing C-terminal arm. In particular, one 2-naphtholmethylamino compound is able to specifically induce the pro-oxidant activity in the inhibited enzyme. Since some 2-naphtholmethylamino analogues show worm killing activity and the ectopic site is not conserved in human orthologues, a general approach to development of novel and selective anti-parasitic therapeutics against schistosoma is proposed.

Safety of Aqueous Extract of Calea ternifolia Used in Mexican Traditional Medicine

There is a trend to use medicinal plants for primary medical care or as dietary supplements; however, the safety of many of these plants has not been studied. The objective of this work was to determine the toxic effect of the aqueous extract of Calea ternifolia (C. zacatechichi), known popularly as “dream herb” in vivo and in vitro in order to validate its safety. In vivo, the extract had moderate toxicity on A. salina. In vitro, the extract induced eryptosis of 73% at a concentration of 100 μg·mL⁻¹ and it inhibited CYP3A by 99% at a concentration of 375 μg/mL. After administering 8.5 mg/kg of C. ternifolia to rats, we found a reduction in platelets and leukocytes and an increase in urea and the liver enzymes alanine aminotransferase (ALT), aspartate aminotransferase (AST), and alkaline phosphatase (ALP). Histological analysis showed spongiform changes in the proximal tubules of renal tissue and a lymphoid infiltrate in liver tissue. This plant is used in the treatment of diabetes, and it is commercialized as a dietary supplement in several countries. Our results show renal and hepatic toxicity; therefore, more profound research on the toxicity of this plant is needed.

Migratory ability of quinone methide-generating acridine conjugates in DNA

Conversion of a bisquinone methide–acridine conjugate to its monofunctional analogue releases the constraints that limit migration of its reversible adducts within DNA.

Mild, Modular, and Convergent Synthesis of Helical Naphtho[2,1-c]chromenes via a Multistep Cyclization/Aromatization Cascade Sequence

Tetracyclic 6H-naphtho[2,1-c]chromenes are expeditiously synthesized through a BF₃·OEt₂-mediated, three-step cascade reaction, creating new central pyran and aromatic rings. The cascade involves the addition of phenol-derived alkynyl substrates to BF₃-activated aldehydes followed by alkyne-Prins cyclization, Friedel-Crafts reaction, and final elimination. Aliphatic and electron-deficient aromatic aldehydes afford the products in 50-74% isolated yields, but benzaldehyde and tolualdehyde resulted in lower yields. X-ray analysis of a p-bromophenyl derivative (5aA) shows the two aromatic moieties are twisted by 28° to create a helical backbone.

Purple pigment from Peltogyne mexicana heartwood as a potential colorant for food

ABSTRACT

Peltogyne mexicana heartwood might be a novel purple pigment source. The results of the present study demonstrate that the purple pigment is an important source of phenolic compounds (698.22 ± 2.99 mg GAE/g) and flavonoids (48.01 ± 0.51 mg EPE/g). UV-Vis spectrum and color parameters (L* a* b*) showed that purple pigment has different shades of purple-red (H° value 19.32 ± 0.02 in methanol and 22.85 ± 0.01 in ethanol) depending on the solvent and the pH. Also, the purple pigment did not exhibit acute oral toxicity at a single dose (2000 mg/kg body weight). No mutagenicity was observed in the Ames test with three Salmonella typhimurium strains. The purple pigment exhibited considerable coloring properties with a wider range of citric acid-dependent color hues in gelatin (H° from 280.3 to 319.9 and from 68.0 to 88.1), and higher color intensity than commercial anthocyanin. Minor variations in the hue were found in yogurt, for purple pigment with H° values from 317.5 to 315.0, and commercial anthocyanin from 82.6 to 88.7 and 276.9 to 295.5. However, purple pigment required lower concentrations to achieve superior effects. For gelatin and yogurt samples, similar variations in the color parameters L*, a*, b*, and pigment degradation were observed for purple pigment and commercial anthocyanin in the stability assay.

GRAPHICAL ABSTRACT

Lewis acid-catalyzed tandem cyclization of in situ generated o-quinone methides and arylsulfonyl hydrazides for a one-pot entry to 3-sulfonylbenzofurans

Lewis acid-mediated one-pot tandem cyclization of o-QMs with arylsulfonyl hydrazides was described for the first time and the corresponding 3-sulfonylbenzofuran products were obtained in moderate to good yields.

Aminobenzylated 4-Nitrophenols as Antibacterial Agents Obtained from 5-Nitrosalicylaldehyde through a Petasis Borono-Mannich Reaction

Multidrug-resistant bacteria are one of the current biggest threats to public health and are responsible for most nosocomial infections. Herein, we report the efficient and facile synthesis of antibacterial agents aminoalkylphenols, derived from 5-nitrosalicyladehyde and prepared through a Petasis borono-Mannich multicomponent reaction. Minimum inhibitory concentrations (MICs) as low as 1.23 μM for a chlorine derivative were determined for multidrug-resistant Gram-positive bacteria, namely, Staphylococcus aureus and Enterococcus faecalis, two of the main pathogens responsible for infections in a hospital environment. The most promising antibacterial agents were further tested against eight strains of four Gram-positive species in order to elucidate their antibacterial broadness. In vitro cytotoxicity assays of the most active aminoalkylphenol revealed considerably lower toxicity against mammalian cells, as concentrations one order of magnitude higher than the determined MICs were required to induce human keratinocyte cell death. The phenol moiety was verified to be important in deeming the antibacterial properties of the analyzed compounds, although no correlation between such properties and their antioxidant activity was observed. A density functional theory computational study substantiated the ability of aminoalkylphenols to serve as precursors of ortho-quinone methides.

Probing the Anticancer Action of Novel Ferrocene Analogues of MNK Inhibitors

Two novel ferrocene-containing compounds based upon a known MNK1/2 kinase (MAPK-interacting kinase) inhibitor have been synthesized. The compounds were designed to use the unique shape of ferrocene to exploit a large hydrophobic pocket in MNK1/2 that is only partially occupied by the original compound. Screening of the ferrocene analogues showed that both exhibited potent anticancer effects in several breast cancer and AML (acute myeloid leukemia) cell lines, despite a loss of MNK potency. The most potent ferrocene-based compound 5 was further analysed in vitro in MDA-MB-231 (triple negative breast cancer cells). Dose–response curves of compound 5 for 2D assay and 3D assay generated IC₅₀ values (half maximal inhibitory concentration) of 0.55 µM and 1.25 µM, respectively.

In Vivo Incorporation of Azide Groups into DNA by Using Membrane-Permeable Nucleotide Triesters

Metabolic incorporation of bioorthogonal functional groups into cellular nucleic acids can be impeded by insufficient phosphorylation of nucleosides. Previous studies found that 5azidomethyl-2'-deoxyuridine (AmdU) was incorporated into the DNA of HeLa cells expressing a low-fidelity thymidine kinase, but not by wild-type HeLa cells. Here we report that membrane-permeable phosphotriester derivatives of AmdU can exhibit enhanced incorporation into the DNA of wild-type cells and animals. AmdU monophosphate derivatives bearing either 5'-bispivaloyloxymethyl (POM), 5'-bis-(4-acetoxybenzyl) (AB), or "Protide" protective groups were used to mask the phosphate group of AmdU prior to its entry into cells. The POM derivative "POM-AmdU" exhibited better chemical stability, greater metabolic incorporation efficiency, and lower toxicity than "AB-AmdU". Remarkably, the addition of POM-AmdU to the water of zebrafish larvae enabled the biosynthesis of azide-modified DNA throughout the body.

Pharmacologic ATF6 activating compounds are metabolically activated to selectively modify endoplasmic reticulum proteins

Pharmacologic arm-selective unfolded protein response (UPR) signaling pathway activation is emerging as a promising strategy to ameliorate imbalances in endoplasmic reticulum (ER) proteostasis implicated in diverse diseases. The small molecule N-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide (147) was previously identified (Plate et al., 2016) to preferentially activate the ATF6 arm of the UPR, promoting protective remodeling of the ER proteostasis network. Here we show that 147-dependent ATF6 activation requires metabolic oxidation to form an electrophile that preferentially reacts with ER proteins. Proteins covalently modified by 147 include protein disulfide isomerases (PDIs), known to regulate ATF6 activation. Genetic depletion of PDIs perturbs 147-dependent induction of the ATF6-target gene, BiP, implicating covalent modifications of PDIs in the preferential activation of ATF6 afforded by treatment with 147. Thus, 147 is a pro-drug that preferentially activates ATF6 signaling through a mechanism involving localized metabolic activation and selective covalent modification of ER resident proteins that regulate ATF6 activity.

Formation and biological targets of botanical o-quinones

The formation of o-quinones from direct 2-electron oxidation of catechols and/or two successive one electron oxidations could explain the cytotoxic/genotoxic and/or chemopreventive effects of several phenolic botanical extracts. For example, poison ivy contains urushiol, an oily mixture, which is oxidized to various o-quinones likely resulting in skin toxicity through oxidative stress and alkylation mechanisms resulting in immune responses. Green tea contains catechins which are directly oxidized to o-quinones by various oxidative enzymes. Alternatively, phenolic botanicals could be o-hydroxylated by P450 to form catechols in vivo which are oxidized to o-quinones. Examples include, resveratrol which is oxidized to piceatannol and further oxidized to the o-quinone. Finally, botanical o-quinones can be formed by O-dealkylation of O-alkoxy groups or methylenedioxy rings resulting in catechols which are further oxidized to o-quinones. Examples include safrole, eugenol, podophyllotoxin and etoposide, as well as methysticin. Once formed these o-quinones have a variety of biological targets in vivo resulting in various biological effects ranging from chemoprevention - > no effect - > toxicity. This U-shaped biological effect curve has been described for a number of reactive intermediates including o-quinones. The current review summarizes the latest data on the formation and biological targets of botanical o-quinones.

Expression of Cyclooxygenase-2, Nitric Oxide Synthase 2 and Heme Oxygenase-1 mRNA Induced by Bis-Eugenol in RAW264.7 Cells and their Antioxidant Activity Determined Using the Induction Period Method

BACKGROUND/AIM

To clarify the mechanisms responsible for the anti-inflammatory/proinflammatory activities of eugenol-related compounds, we investigated the cytotoxicity and up-regulatory/down-refgulatory effects of the biphenols curcumin, bis-eugenol, magnolol and honokiol, and the monophenols eugenol and isoeugenol, on major regulators of cyclooxygenase-2 (Cox-2), nitric oxide synthase 2 (Nos2) and heme oxygenase-1 (HO-1) mRNA in RAW264.7 cells.

MATERIALS AND METHODS

mRNA expression was investigated using real-time reverse transcriptase-polymerase chain reaction (RT-PCR), and the theoretical parameters were calculated using the DFT/B3LYP/6-31* method. Also, the antioxidant activity of eugenol-related compounds in combination with 2-mercapto-1-methylimidazole (MMI, as a model for glutathione (GSH)) was investigated using the induction period method for polymerization of methyl methacrylate initiated by benzoyl peroxide (BPO).

RESULTS

The cytotoxicity of eugenol-related compounds showed a linear relationship with their softness (σ) and electrophilicity (ω). At a concentration of 50 μM, biphenols except for bis-eugenol elicited the expression of mRNA for both Cox-2 and Nos2, but monophenols did not. In contrast, bis-eugenol elicited Cox-2 gene expression, but down-regulated Nos2 gene expression. bis-Eugenol alone induced the expression of HO-1 mRNA, and when combined with MMI it showed a potent antagonistic effect on BPO-induced antioxidant activity. The ability of methoxyphenols to inhibit LPS-stimulated Cox-2 gene expression declined in the order curcumin >> isoeugenol > bis-eugenol >> eugenol, and the rank of ability was related to their ω value.

CONCLUSION

Most eugenol-related compounds had proinflammatory activity at high concentrations. However, they had also anti-inflammatory activity at lower concentrations. Eugenol-related compounds may exert antioxidant and anti-inflammatory activity in LPS-stimulated RAW264.7 cells possibly by inhibiting the activation of nuclear factor-kappa B (Nf-ĸB), whereas bis-eugenol requires induction of HO-1 expression. bis-Eugenol as well as curcumin, may have anti-inflammatory and anticancer therapeutic applications.

Cys34 Adductomes Differ between Patients with Chronic Lung or Heart Disease and Healthy Controls in Central London

Oxidative stress generates reactive species that modify proteins, deplete antioxidant defenses, and contribute to chronic obstructive pulmonary disease (COPD) and ischemic heart disease (IHD). To determine whether protein modifications differ between COPD or IHD patients and healthy subjects, we performed untargeted analysis of adducts at the Cys34 locus of human serum albumin (HSA). Biospecimens were obtained from nonsmoking participants from London, U.K., including healthy subjects (n = 20) and patients with COPD (n = 20) or IHD (n = 10). Serum samples were digested with trypsin and analyzed by liquid chromatography-high resolution mass spectrometry. Effects of air pollution on adduct levels were also investigated based on estimated residential exposures to PM_2.5, O₃ and NO₂. For the 39 adducts with sufficient data, levels were essentially identical in blood samples collected from the same subjects on two consecutive days, consistent with the 28 day residence time of HSA. Multivariate linear regression revealed 21 significant associations, mainly with the underlying diseases but also with air-pollution exposures (p-value < 0.05). Interestingly, most of the associations indicated that adduct levels decreased with the presence of disease or increased pollutant concentrations. Negative associations of COPD and IHD with the Cys34 disulfide of glutathione and two Cys34 sulfoxidations, were consistent with previous results from smoking and nonsmoking volunteers and nonsmoking women exposed to indoor combustion of coal and wood.

Recent advances in the application of Diels–Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis

This review summarizes recent advances in Diels–Alder reactions involving o-QDMs, o-QMs and aza-o-QMs. The power and potential of this strategy in organic synthesis and natural product total synthesis is highlighted.

Quercetin Oxidation Paradoxically Enhances its Antioxidant and Cytoprotective Properties

Quercetin oxidation is generally believed to ultimately result in the loss of its antioxidant properties. To test this assertion, quercetin oxidation was induced, and after each of its major metabolites was identified and isolated by HPLC-DAD-ESI-MS/MS, the antioxidant (dichlorodihydrofluorescein oxidation-inhibiting) and cytoprotective (LDH leakage-preventing) properties were evaluated in Hs68 and Caco2 cells exposed to indomethacin. Compared to quercetin, the whole mixture of metabolites (Q_OX) displayed a 20-fold greater potency. After resolution of Q_OX into 12 major peaks, only one (peak 8), identified as 2,5,7,3',4'-pentahydroxy-3,4-flavandione or its 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone tautomer, could account for the antioxidant and cytoprotective effects afforded Q_OX. Peak 8 exerted such effects at a 50 nM concentration, revealing a potency 200-fold higher than that of quercetin. The effects of peak 8 were seen regardless of whether it was added to the cells 40 min before or simultaneously with the oxygen-reactive species-generating agent, suggesting an intracellular ability to trigger early antioxidant responses. Thus, the present study is the first to reveal that in regard to the intracellular actions of quercetin, attention should be extended toward some of its oxidation products.

Glutathione homeostasis is significantly altered by quercetin via the Keap1/Nrf2 and MAPK signaling pathways in rats

Previously, we showed that 0.5% quercetin simultaneously decreased serum homocysteine and glutathione (GSH) levels in rats. The aim of the present study was to investigate the effects of 0.5% quercetin on GSH metabolism, related enzymes and signal pathways in rats. Rats were fed the control diet and 0.5% quercetin-supplemented diet for 6 weeks. The results showed that quercetin reduced serum and hepatic content of GSH and the ratio of GSH and oxidized glutathione (GSSG), enhanced hepatic activity and mRNA expression of glutathione S-transferase (GST), inhibited hepatic activity and mRNA expression of glutamate cysteine ligase (GCL), and decreased hepatic glutathione reductase (GR) mRNA expression. Levels of phosphorylated p38 and extracellular signal-regulated kinase (ERK) 1/2 mitogen-activated protein kinases (MAPKs) increased, while that of nuclear factor E2-like 2 (Nrf2) protein decreased after quercetin treatment. However, no significant hepatotoxicity was noted. We concluded that quercetin treatment altered hepatic GSH metabolism by modulating GSH metabolic enzyme activities and mRNA expression in rats, and p38, ERK1/2 MAPKs, and Nrf2 were involved in modulating GSH metabolism-related enzymes.

Lack of Cell Proliferative and Tumorigenic Effects of 4-Hydroxyestradiol in the Anterior Pituitary of Rats: Role of Ultrarapid O-Methylation Catalyzed by Pituitary Membrane-Bound Catechol-O-Methyltransferase

In animal models, estrogens are complete carcinogens in certain target sites. 4-Hydroxyestradiol (4-OH-E₂), an endogenous metabolite of 17β-estradiol (E₂), is known to have prominent estrogenic activity plus potential genotoxicity and mutagenicity. We report here our finding that 4-OH-E₂ does not induce pituitary tumors in ACI female rats, whereas E₂ produces 100% pituitary tumor incidence. To probe the mechanism, we conducted a short-term animal experiment to compare the proliferative effect of 4-OH-E₂ in several organs. We found that, whereas 4-OH-E₂ had little ability to stimulate pituitary cell proliferation in ovariectomized female rats, it strongly stimulates cell proliferation in certain brain regions of these animals. Further, when we used in vitro cultured rat pituitary tumor cells as models, we found that 4-OH-E₂ has similar efficacy as E₂ in stimulating cell proliferation, but its potency is approximately 3 orders of magnitude lower than that of E₂. Moreover, we found that the pituitary tumor cells have the ability to selectively metabolize 4-OH-E₂ (but not E₂) with ultrahigh efficiency. Additional analysis revealed that the rat pituitary expresses a membrane-bound catechol-O-methyltransferase that has an ultralow K_m value (in nM range) for catechol estrogens. On the basis of these observations, it is concluded that rapid metabolic disposition of 4-OH-E₂ through enzymatic O-methylation in rat anterior pituitary cells largely contributes to its apparent lack of cell proliferative and tumorigenic effects in this target site.

Compounds From Celastraceae Targeting Cancer Pathways and Their Potential Application in Head and Neck Squamous Cell Carcinoma: A Review

Squamous cell carcinoma of the head and neck is one of the most common cancer types worldwide. It initiates on the epithelial lining of the upper aerodigestive tract, at most instances as a consequence of tobacco and alcohol consumption. Treatment options based on conventional therapies or targeted therapies under development have limited efficacy due to multiple genetic alterations typically found in this cancer type. Natural products derived from plants often possess biological activities that may be valuable in the development of new therapeutic agents for cancer treatment. Several genera from the family Celastraceae have been studied in this context. This review reports studies on chemical constituents isolated from species from the Celastraceae family targeting cancer mechanisms studied to date. These results are then correlated with molecular characteristics of head and neck squamous cell carcinoma in an attempt to identify constituents with potential application in the treatment of this complex disease at the molecular level.