Metabolite Study and Structural Authentication for the First-in-Human Use Sphingosine-1-phosphate Receptor 1 Radiotracer

The sphingosine-1-phosphate receptor 1 (S1PR1) radiotracer [11C]CS1P1 has shown promise in proof-of-concept PET imaging of neuroinflammation in multiple sclerosis (MS). Our HPLC radiometabolite analysis of human plasma samples collected during PET scans with [11C]CS1P1 detected a radiometabolite peak that is more lipophilic than [11C]CS1P1. Radiolabeled metabolites that cross the blood-brain barrier complicate quantitative modeling of neuroimaging tracers; thus, characterizing such radiometabolites is important. Here, we report our detailed investigation of the metabolite profile of [11C]CS1P1 in rats, nonhuman primates, and humans. CS1P1 is a fluorine-containing ligand that we labeled with C-11 or F-18 for preclinical studies; the brain uptake was similar for both radiotracers. The same lipophilic radiometabolite found in human studies also was observed in plasma samples of rats and NHPs for CS1P1 labeled with either C-11 or F-18. We characterized the metabolite in detail using rats after injection of the nonradioactive CS1P1. To authenticate the molecular structure of this radiometabolite, we injected rats with 8 mg/kg of CS1P1 to collect plasma for solvent extraction and HPLC injection, followed by LC/MS analysis of the same metabolite. The LC/MS data indicated in vivo mono-oxidation of CS1P1 produces the metabolite. Subsequently, we synthesized three different mono-oxidized derivatives of CS1P1 for further investigation. Comparing the retention times of the mono-oxidized derivatives with the metabolite observed in rats injected with CS1P1 identified the metabolite as N-oxide 1, also named TZ82121. The MS fragmentation pattern of N-oxide 1 also matched that of the major metabolite in rat plasma. To confirm that metabolite TZ82121 does not enter the brain, we radiosynthesized [18F]TZ82121 by the oxidation of [18F]FS1P1. Radio-HPLC analysis confirmed that [18F]TZ82121 matched the radiometabolite observed in rat plasma post injection of [18F]FS1P1. Furthermore, the acute biodistribution study in SD rats and PET brain imaging in a nonhuman primate showed that [18F]TZ82121 does not enter the rat or nonhuman primate brain. Consequently, we concluded that the major lipophilic radiometabolite N-oxide [11C]TZ82121, detected in human plasma post injection of [11C]CS1P1, does not enter the brain to confound quantitative PET data analysis. [11C]CS1P1 is a promising S1PR1 radiotracer for detecting S1PR1 expression in the CNS.

Poly(vinylpyridine-co-vinylpyridine N-oxide) Excipients Mediate Rapid Dissolution and Sustained Supersaturation of Posaconazole Amorphous Solid Dispersions

The chemical structure of excipients molecularly mixed in an amorphous solid dispersion (ASD) has a significant impact on properties of the ASD including dissolution behavior, physical stability, and bioavailability. Polymers used in ASDs require a balance between hydrophobic and hydrophilic functionalities to ensure rapid dissolution of the amorphous dispersion as well as sustained supersaturation of the drug in solution. This work demonstrates the use of postpolymerization functionalization of poly(vinylpyridine) excipients to elucidate the impact of polymer properties on the dissolution behavior of amorphous dispersions containing posaconazole. It was found that N-oxidation of pyridine functionalities increased the solubility of poly(vinylpyridine) derivatives in neutral aqueous conditions and allowed for nanoparticle formation which supplied posaconazole into solution at concentrations exceeding those achieved by more conventional excipients such as hydroxypropyl methylcellulose acetate succinate (HPMCAS) or Eudragit E PO. By leveraging these functional modifications of the parent poly(vinylpyridine) excipient to increase polymer hydrophilicity and minimize the effect of polymer on pH, a new polymeric excipient was optimized for rapid dissolution and supersaturation maintenance for a model compound.

Monitoring stability indicating impurities and aldehyde content in lipid nanoparticle raw material and formulated drugs

Lipid nanoparticles (LNPs) are designed to protect and transport sensitive payloads or active pharmaceutical ingredients as part of new therapeutic modalities. As a multi-component particle, a high degree of quality control is necessary to ensure raw materials are free of critical impurities that could adversely impact the drug product. In this study, we demonstrate a reversed phase liquid chromatography method hyphenated with a single quadrupole mass spectrometer (RPLC-MS) as an alternative platform to methods that incorporate evaporative light scattering or charged aerosol detectors in the detection and quantitation of critical impurities associated with LNPs. The proposed RPLC-MS method offers an increase of up to 2 orders of magnitude in dynamic range and 3 orders of magnitude in sensitivity in the analysis of impurities associated with LNPs compared to conventional detectors. Access to complementary mass data enabled the detection and identification of stability indicating impurities as part of stress studies carried out on an ionizable lipid. In addition to confirmation of peak identity, complementary mass data was also used to assess residual aldehydes in raw material and formulated LNPs in accordance with regulatory guidance. Following derivatization using 2,4-dinitrophenylhydrazine, aldehyde content in the ionizable lipid raw material was determined to exceed the reporting threshold of 0.05% in 30% of the test cases. The experimental findings observed in this study demonstrate the utility of the proposed RPLC-MS method in the identification and monitoring of stability-indicating attributes associated with LNPs as part of current Good Manufacturing Practices for improved consumer safety in drug products.

N-oxidation Regioselectivity and Risk Prediction Using DFT-ALIE Calculations

INTRODUCTION

The formation of N-oxide degradants is a major concern in development of new drugs due to potential effects on a compound's pharmacological activity. Such effects include but are not limited to solubility, stability, toxicity, and efficacy. In addition, these chemical transformations can impact physicochemical properties that affect drug manufacturability. Hence identification and control of N-oxide transformations is of critical importance in the development of new therapeutics.

OBJECTIVE

This study describes the development of an in-silico approach to identify N-oxide formation in APIs with respect to autoxidation.

METHODS

Average Local Ionization Energy (ALIE) calculations were carried out using molecular modeling techniques and application of Density Functional Theory (DFT) at the B3LYP/6-31G(d,p) level of theory. A total of 257 nitrogen atoms and 15 different oxidizable nitrogen types were used in developing this method.

RESULTS

The results show that ALIE could be reliably used to predict the most susceptible nitrogen for N-oxide formation. A risk scale was developed that rapidly categorizes nitrogen's oxidative vulnerabilities as small, medium, or high.

CONCLUSIONS

The developed process presents a powerful tool to identify structural susceptibilities for N-oxidation as well as enabling rapid structure elucidation in resolving potential experimental ambiguities.

Bioinspired N-Oxide-Based Zwitterionic Polymer Brushes for Robust Fouling-Resistant Surfaces

Fouling-resistant surfaces are needed for various environmental applications. Inspired by superhydrophilic N-oxide-based osmolytes in saltwater fish, we demonstrate the use of a trimethylamine N-oxide (TMAO) analogue for constructing fouling-resistant surfaces. The readily synthesized N-oxide monomer of methacrylamide is grafted to filtration membrane surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP). Successful grafting of the amine N-oxide brush layer as confirmed by material characterization endows the surface with increased hydrophilicity, reduced charge, and decreased roughness. Notably, the introduction of the N-oxide layer does not compromise transport properties, i.e., water permeability and water-salt selectivity. Moreover, the modified membrane exhibits improved antifouling properties with a lower flux decline (32.1%) and greater fouling reversibility (18.55%) than the control sample (45.4% flux decline and 3.26% fouling reversibility). We further evaluate foulant-membrane interaction using surface plasmon resonance (SPR) to relate the reduced fouling tendency to the synergic effects of surface characteristic changes after amine N-oxide modification. Our results demonstrate the promise and potential of the N-oxide-based polymer brushes for the design of fouling resistance surfaces for a variety of emerging environmental applications.

Transformation Products of Tire Rubber Antioxidant 6PPD in Heterogeneous Gas-Phase Ozonation: Identification and Environmental Occurrence

6PPD, a tire rubber antioxidant, poses substantial ecological risks because it can form a highly toxic quinone transformation product (TP), 6PPD-quinone (6PPDQ), during exposure to gas-phase ozone. Important data gaps exist regarding the structures, reaction mechanisms, and environmental occurrence of TPs from 6PPD ozonation. To address these data gaps, gas-phase ozonation of 6PPD was conducted over 24-168 h and ozonation TPs were characterized using high-resolution mass spectrometry. The probable structures were proposed for 23 TPs with 5 subsequently standard-verified. Consistent with prior findings, 6PPDQ (C18H22N2O2) was one of the major TPs in 6PPD ozonation (∼1 to 19% yield). Notably, 6PPDQ was not observed during ozonation of 6QDI (N-(1,3-dimethylbutyl)-N'-phenyl-p-quinonediimine), indicating that 6PPDQ formation does not proceed through 6QDI or associated 6QDI TPs. Other major 6PPD TPs included multiple C18H22N2O and C18H22N2O2 isomers, with presumptive N-oxide, N,N'-dioxide, and orthoquinone structures. Standard-verified TPs were quantified in roadway-impacted environmental samples, with total concentrations of 130 ± 3.2 μg/g in methanol extracts of tire tread wear particles (TWPs), 34 ± 4 μg/g-TWP in aqueous TWP leachates, 2700 ± 1500 ng/L in roadway runoff, and 1900 ± 1200 ng/L in roadway-impacted creeks. These data demonstrate that 6PPD TPs are likely an important and ubiquitous class of contaminants in roadway-impacted environments.

Complete 1 H, 13 C, and 15 N assignments of the minor isomer of codeine N-oxide

Codeine N-oxide 2 is an active metabolite of codeine obtained by oxidation and observed as a degradant in codeine drug products such as syrups. Oxidation of codeine's N-methyl function can deliver two regio-isomers due to chirality of the tetra-substituted nitrogen. Hydrogen peroxide oxidation of codeine was performed and induced two different isomers in a 9:1 ratio; these isomers were isolated using preparative high performance liquid chromatography (HPLC) and fully characterized by nuclear magnetic resonance (NMR) techniques. We describe the complete assignment of the minor isomer of codeine N-oxide 3 and attribute a (S) configuration (N-methyl axial) of the tetra-substituted nitrogen. The effects of N-oxidation on the ¹⁵ N chemical shifts of the codeine are presented. The ¹⁵ N shifts were determined using the CIGAR-HMBC experiment at natural abundance, and the nitrogen resonance of codeine shifted downfield from 42.8 to 118.7 ppm for both N-oxide isomers.

Mitochondria-targeted polymer-celastrol conjugate with enhanced anticancer efficacy

Celastrol, a natural triterpene extracted from traditional Chinese medicine, shows anticancer effects on various cancer cells. However, its poor water-solubility, short plasma half-life, and high systemic toxicity impede its applications in vivo, necessitating a stable drug delivery system to overcome these critical drawbacks. We present here a block copolymer, poly(2-(N-oxide-N,N-dimethylamino)ethyl methacrylate)-block-poly(2-hydroxyethyl methacrylate) (OPDMA-HEMA), as the carrier for celastrol delivery. The amphiphilic polymer-celastrol conjugate can self-assemble into nanoparticles in aqueous solutions. The OPDMA outer shell confers the nanoparticles with improved pharmacokinetics and efficient mitochondria targeting capacity, and profoundly potentiates celastrol's induction of immunogenic cell death, which collectively contribute to the enhanced therapeutic effects of celastrol in vivo. This mitochondria-targeted polymer-celastrol conjugate may promise the applications of celastrol in cancer treatment.

A Dansyl Amide N-Oxide Fluorogenic Probe Based on a Bioorthogonal Decaging Reaction

A smart fluorescence "turn-on" probe which contained a dansyl amide fluorophore and an N-oxide group was designed based on the bioorthogonal decaging reaction between N-oxide and the boron reagent. The reaction proceeds in a rapid kinetics (k2 =57.1±2.5 m-1  s-1 ), and the resulting reduction product showcases prominent fluorescence enhancement (up to 72-fold). Time dependent density functional theoretical (TD-DFT) calculation revealed that the process of photoinduced electron transfer (PET) from the N-oxide moiety to the dansyl amide fluorophore accounts for the quenching mechanism of N-oxide. This probe also showed high selectivity over various nucleophilic amino acids and good biocompatibility in physiological conditions. The successful application of the probe in HaloTag protein labeling and HepG2 live-cell imaging proves it a valuable tool for visualization of biomolecules.

Synthesis and Characterization of 1,10-Phenanthroline-mono-N-oxides

N-oxides of N-heteroaromatic compounds find widespread applications in various fields of chemistry. Although the strictly planar aromatic structure of 1,10-phenanthroline (phen) is expected to induce unique features of the corresponding N-oxides, so far the potential of these compounds has not been explored. In fact, appropriate procedure has not been reported for synthesizing these derivatives of phen. Now, we provide a straightforward method for the synthesis of a series of mono-N-oxides of 1,10-phenanthrolines. The parent compounds were oxidized by a green oxidant, peroxomonosulfate ion in acidic aqueous solution. The products were obtained in high quality and at good to excellent yields. A systematic study reveals a clear-cut correlation between the basicity of the compounds and the electronic effects of the substituents on the aromatic ring. The UV spectra of these compounds were predicted by DFT calculations at the TD-DFT/TPSSh/def2-TZVP level of theory.

Crystal structure and Hirshfeld surface analysis of N-[(Z)-(2-hy-droxy-phen-yl)methyl-idene]aniline N-oxide

The conformation of the title compound, C13H11NO2, is partially determined by a strong, intra-molecular O-H⋯O hydrogen bond. The crystal packing consists of strongly corrugated layers parallel to the ac plane and associated through C-H⋯π(ring) inter-actions. A Hirshfeld surface analysis of the crystal structure indicates that the most significant contributions to the crystal packing are from H⋯H (44.1%), C⋯H/H⋯C (29.4%) and O⋯H/H⋯O (17.3%) contacts.

Aporphine Alkaloids from Ocotea puberula with Anti-Trypanosoma Cruzi Potential - Activity of Dicentrine-β-N-Oxide in the Plasma Membrane Electric Potentials

One new aporphine, dicentrine-β-N-oxide (1), together with five related known alkaloids dehydrodicentrine (2), predicentrine (3), N-methyllaurotetanine (4), cassythicine (5), and dicentrine (6) were isolated from the leaves of Ocotea puberula (Lauraceae). Antiprotozoal activity of the isolated compounds was evaluated in vitro against trypomastigote forms of Trypanosoma cruzi. Among the tested compounds, alkaloid 1 exhibited higher potential with EC₅₀ value of 18.2 μM and reduced toxicity against NCTC cells (CC₅₀ >200 μM - SI>11.0), similar to positive control benznidazole (EC₅₀ of 17.7 μM and SI=10.7). Considering the promising results of dicentrine-β-N-oxide (1) against trypomastigotes, the mechanism of parasite death caused by this alkaloid was investigated. As observed, this compound reached the plasma membrane electric potential directly after 2 h of incubation and triggered mitochondrial depolarization, which probably leads to trypomastigote death. Therefore, dicentrine-β-N-oxide (1), reported for the first time in this work, can contribute to future works for the development of new trypanocidal agents.

Benzofuroxan Derivatives as Potent Agents against Multidrug-Resistant Mycobacterium tuberculosis

Tuberculosis (TB) is currently the leading cause of death related to infectious diseases worldwide, as reported by the World Health Organization. Moreover, the increasing number of multidrug-resistant tuberculosis (MDR-TB) cases has alarmed health agencies, warranting extensive efforts to discover novel drugs that are effective and also safe. In this study, 23 new compounds were synthesized and evaluated in vitro against the drug-resistant strains of M. tuberculosis. The compound 6-((3-fluoro-4-thiomorpholinophenyl)carbamoyl)benzo[c][1,2,5]oxadiazole 1-N-oxide (5 b) was particularly remarkable in this regard as it demonstrated MIC₉₀ values below 0.28 μM against all the MDR strains evaluated, thus suggesting that this compound might have a different mechanism of action. Benzofuroxans are an attractive new class of anti-TB agents, exemplified by compound 5 b, with excellent potency against the replicating and drug-resistant strains of M. tuberculosis.

Rapid transformation of H1-antihistamines cetirizine (CET) and diphenhydramine (DPH) by direct peroxymonosulfate (PMS) oxidation

With growing interest in advanced oxidation processes (AOPs), the number of research studies on peroxymonosulfate (PMS) mediated pollutant degradation has increased significantly due to its high radical generation potential upon activation. However, rare studies have focused on the non-radical based PMS reactions. In this study, degradation of model H₁-antihistamines cetirizine (CET) and diphenhydramine (DPH) by unactivated PMS was investigated. Addition of scavengers to the reaction mixture ruled out the involvement of hydroxyl radical (OH), sulfate radical (SO₄^-), singlet oxygen (¹O₂) and superoxide anion radical (O₂^-), indicating direct PMS oxidation as the predominant reaction path. Such a mechanism was further supported by the N-oxide products identified by mass spectrometry and nuclear magnetic resonance (NMR) analyses. Solution pH had a pronounced influence on the degradation kinetics regardless the presence or absence of transition metal Fe(II). The highest species dependent second order rate constants were k_HSO5-/DPH0 of 175 ± 15.9 M^-1 s^-1 and k_HSO5-/CET- of 36.6 ± 0.16 M^-1 s^-1. The addition of 100 μM Fe(II) promoted OH mediated degradation of H₁-antihistamines and their N-oxide products. This study demonstrated selective transformation with the potential for extensive degradation employing both the direct and catalytic PMS oxidative processes.

The presence of a cationic center is not alone decisive for the cytotoxicity of triterpene carboxylic acid amides

3-O-Acetyl-ursolic acid (2) and 3-O-acetyl oleanolic acid (8) were converted into piperazinylamides holding a distal NH, NMe or a NMe₂ group. These compounds as well as the corresponding N-methyl-N-oxides were accessed. Their cytotoxicity was assessed in SRB assays employing a panel of human tumor cell lines and non-malignant fibroblasts (NIH 3T3). As a result, compounds holding a quaternary distal N-substituent were less cytotoxic that those holding a NH-moiety. Hence, the presence of a distal cationic center seems not to be a sufficient criterion for obtaining triterpenoids of high cytotoxicity and selectivity.

Influence of the position of the methyl substituent and N-oxide formation on the geometry and intermolecular interactions of 1-(phenoxyethyl)piperidin-4-ol derivatives

Aminoalkanol derivatives have attracted much interest in the field of medicinal chemistry as part of the search for new anticonvulsant drugs. In order to study the influence of the methyl substituent and N-oxide formation on the geometry of molecules and intermolecular interactions in their crystals, three new examples have been prepared and their crystal structures determined by X-ray diffraction. 1-[(2,6-Dimethylphenoxy)ethyl]piperidin-4-ol, C₁₅H₂₃NO₂, 1, and 1-[(2,3-dimethylphenoxy)ethyl]piperidin-4-ol, C₁₅H₂₃NO₂, 2, crystallize in the orthorhombic system (space groups P2₁2₁2₁ and Pbca, respectively), with one molecule in the asymmetric unit, whereas the N-oxide 1-[(2,3-dimethylphenoxy)ethyl]piperidin-4-ol N-oxide monohydrate, C₁₅H₂₃NO₃·H₂O, 3, crystallizes in the monoclinic space group P2₁/c, with one N-oxide molecule and one water molecule in the asymmetric unit. The geometries of the investigated compounds differ significantly with respect to the conformation of the O-C-C linker, the location of the hydroxy group in the piperidine ring and the nature of the intermolecular interactions, which were investigated by Hirshfeld surface and corresponding fingerprint analyses. The crystal packing of 1 and 2 is dominated by a network of O-H...N hydrogen bonds, while in 3, it is dominated by O-H...O hydrogen bonds and results in the formation of chains.

Optimization of pyrrolo[3,4-f]indole-5,7-dione and indole-5,6-dicarbonitrile derivatives as inhibitors of monoamine oxidase

In recent studies, we have investigated the monoamine oxidase (MAO) inhibition properties of pyrrolo[3,4-f]indole-5,7-dione and indole-5,6-dicarbonitrile derivatives. Since numerous high potency MAO inhibitors are present among these chemical classes, the present study synthesizes 44 additional derivatives in an attempt to further derive structure-activity relationships (SARs) and to establish optimal substitution patterns for MAO inhibition. The results show that, with the exception of one compound, all indole-5,6-dicarbonitrile derivatives (10) exhibit submicromolar IC₅₀ values for the inhibition of MAO, with the most potent MAO-A inhibitor exhibiting an IC₅₀ value of 0.006 μM while the most potent MAO-B inhibitor exhibits an IC₅₀ value of 0.058 μM. Interestingly, an N-oxide derivative (4c) also proved to be a potent and nonspecific MAO inhibitor. With the exception of one compound, all of the pyrrolo[3,4-f]indole-5,7-diones (28) also exhibit submicromolar IC₅₀ values for the inhibition of an MAO isoform. The most potent inhibitor exhibit an IC₅₀ value of 0.011 μM for MAO-A. This study proposes that high potency MAO inhibitors such as those investigated here, may act as lead compounds for the development of treatments for neurodegenerative and neuropsychiatric disorders such as Parkinson's disease and depression.

One-step synthesis of cellooligomer-conjugated gold nanoparticles in a water-in-oil emulsion system and their application in biological sensing

Monodisperse gold nanoparticles (GNPs) were synthesized in a water-in-oil emulsion system (reverse micelles) composed of 80% N-methylmorpholine N-oxide (NMMO)/20% H₂O and dodecane, stabilized with an anionic surfactant: bis(2-ethylhexyl)sulfosuccinate sodium salt. Cellooligomers with a degree of polymerization of 6 or 15 (βGlc6 or βGlc15, respectively), which were labeled at each reducing end group with thiosemicarbazide (TSC) and dissolved in the aqueous NMMO phase, were successfully conjugated to the surfaces of GNPs in situ during spontaneous NMMO-mediated gold reduction. As-synthesized βGlc6-GNPs and βGlc15-GNPs had average diameters of 11.3 ± 2.1 and 10.5 ± 0.7 nm, respectively, while their surface sugar densities were 0.21 and 0.51 chains nm^-2, respectively. Concanavalin A (ConA), a lectin that recognizes non-reducing end groups of glucose residues, aggregated with βGlc15-GNPs with higher sensitivity than it did with βGlc6-GNPs, possibly as a result of the sugar density on the GNP surfaces. The aggregates were rapidly re-suspended by adding methyl-β-d-glucopyranoside as a binding inhibitor. Other lectins and proteins showed no interaction with βGlc-GNPs. Therefore, clustering of glucose non-reducing ends on the GNP surfaces via strong intermolecular association of cellooligomers, possibly led to high affinity for ConA. This facile synthesis route to structural carbohydrate-decorated GNPs has potential applications in carbohydrate-nanometal conjugate nano-biosensor development.

Identification of N-Oxide-Containing Aromatic Heterocycles as Pharmacophores for Rumen Fermentation Modifiers

Different strategies have been used to mitigate greenhouse gas emissions from domesticated ruminants, including the removal of protozoa (defaunation). The objective of the present work was to analyze the potential of different N-oxide-containing aromatic heterocycles with known antiprotozoal activity as rumen-gas-abating agents. Nineteen pure compounds, belonging to seven different N-oxide chemotypes from our chemo-library were studied together with monensin in an in vitro rumen simulation assay. Fermentation profiles, i.e., gas production, pH, and short carboxylic acid concentrations, were compared to an untreated control at 96 h post inoculation. In our study, we investigated whole-ruminal fluid, with and without compound treatments, by NMR spectroscopy focusing on concentrations of the metabolites acetate, propionate, butyrate, and lactate. From data analysis, three of the compounds from different N-oxide chemotypes, including quinoxaline dioxide, benzofuroxan, and methylfuroxan, were able to diminish the production of gases such as monensin with similar gas production lag times for each of them. Additionally, unlike monensin, one methylfuroxan did not decrease the rumen pH during the analyzed incubation time, shifting rumen fermentation to increase the molar concentrations of propionate and butyrate. These facts suggest interesting alternatives as feed supplements to control gas emissions from dairy ruminants.

Crystallographic identification of spontaneous oxidation intermediates and products of protein sulfhydryl groups

In the absence of protective reducing agents, Cys residues in purified proteins can be oxidized spontaneously by oxygen in the air, as frequently observed in protein crystal structures. However, the formation of an O-bridge via dehydration mechanism between a peroxidized Cys side chain and a primary amine of Lys side chain in proteins has not yet been reported. When an electron density feature was observed for an extra group or an extra atom between side chains of Cys-245 and Lys-158 in the crystal structure of histidinol phosphate phosphatase, mass spectrometric analysis was carried out for its chemical identification. That analysis led to a conclusion that this extra density corresponded to a methylene group. It was then proposed that these two residues were able to absorb CO2 and reduced it to CH2 spontaneously. Further examination of other protein structures in the PDB showed that the formation of this cross-linking species was a widespread phenomenon. This claim is examined in this study using methods recently developed for quantification of electrons around nucleus as the means for direct chemical identification. It is found that an O-bridge is actually formed between Cys and Lys side chains, instead of a CH2 -bridge.

Removal of hERG potassium channel affinity through introduction of an oxygen atom: Molecular insights from structure-activity relationships of strychnine and its analogs

Nux vomica has been effectively used in Traditional Chinese Medicine. The processing of Nux vomica is necessary to reduce toxicity before it can be used in clinical practice. However, the mechanism for processing detoxification is unclear. hERG channels have been subjected to a routine test for compound cardiac toxicity in the drug development process. Therefore, we examined the effects and mechanisms of strychnine and brucine, two main ingredients of Nux vomica, and their N-oxides on hERG channels. Strychnine and brucine exhibited concentration-dependent inhibition of hERG channels with IC₅₀ values of 25.9 μM and 44.18 μM, respectively. However, their nitrogen oxidative derivatives produced by processing of Nux vomica, strychnine N-oxide and brucine N-oxide, lost their activity on hERG channels. Compared to their parent compounds, only an oxygen atom was introduced in the nitrogen oxidative isoforms to compensate for the N⁺ - charge, suggesting that the protonated nitrogen is the key group for strychnine and brucine binding to hERG channel. Alanine-mutagenesis identified Y652 is the most important residue for strychnine and brucine binding to hERG channel. Y652A mutation increased the IC₅₀ for strychnine and brucine by 21.64-fold and 29.78-fold that of WT I_hERG, respectively. Docking simulations suggested that the protonated nitrogen of strychnine and brucine formed a cation-π interaction with the aromatic ring of Y652. This study suggests that introduction of an oxygen to compensate for the N⁺ - charge could be a useful strategy for reducing hERG potency and increasing the safety margin of alkaloid-type compounds in drug development.

Spatially dependent alkyl quinolone signaling responses to antibiotics in Pseudomonas aeruginosa swarms

There is a general lack of understanding about how communities of bacteria respond to exogenous toxins such as antibiotics. Most of our understanding of community-level stress responses comes from the study of stationary biofilm communities. Although several community behaviors and production of specific biomolecules affecting biofilm development and associated behavior have been described for Pseudomonas aeruginosa and other bacteria, we have little appreciation for the production and dispersal of secreted metabolites within the 2D and 3D spaces they occupy as they colonize, spread, and grow on surfaces. Here we specifically studied the phenotypic responses and spatial variability of alkyl quinolones, including the Pseudomonas quinolone signal (PQS) and members of the alkyl hydroxyquinoline (AQNO) subclass, in P. aeruginosa plate-assay swarming communities. We found that PQS production was not a universal signaling response to antibiotics, as tobramycin elicited an alkyl quinolone response, whereas carbenicillin did not. We also found that PQS and AQNO profiles in response to tobramycin were markedly distinct and influenced these swarms on different spatial scales. At some tobramycin exposures, P. aeruginosa swarms produced alkyl quinolones in the range of 150 μm PQS and 400 μm AQNO that accumulated as aggregates. Our collective findings show that the distribution of alkyl quinolones can vary by several orders of magnitude within the same swarming community. More notably, our results suggest that multiple intercellular signals acting on different spatial scales can be triggered by one common cue.

Crystal structure of (Z)-N-benzyl-idene-1-phenyl-methanamine oxide hydrogen peroxide monosolvate

The title structure consists of a (Z)-N-benzyl­idene-1-phenyl­methanamine oxide and a hydrogen peroxide mol­ecule linked through both O—H groups into a one-dimensional chain structure.

The title adduct, C₁₄H₁₃NO·H₂O₂, consists of (Z)-N-benzyl­idene-1-phenyl­methanamine oxide and hydrogen peroxide mol­ecules in a 1:1 ratio. The organic coformer adopts a skew geometry with an inter-aryl-ring dihedral angle of 81.9 (2)°. In the crystal, the organic and peroxide mol­ecules are linked through both peroxide O—H donor groups to oxide O-atom acceptors, giving one-dimensional chains extending along the b axis. Present also are weak inter­molecular C—H⋯O hydrogen-bonding inter­actions.

Efficient Reduction of Antibacterial Activity and Cytotoxicity of Fluoroquinolones by Fungal-Mediated N-Oxidation

Extensive usage of fluoroquinolone antibiotics in livestock results in their occurrence in manure and subsequently in the environment. Fluoroquinolone residues may promote bacterial resistance and are toxic to plants and aquatic organisms. Moreover, fluoroquinolones may enter the food chain through plant uptake, if manure is applied as fertilizer. Thus, the presence of fluoroquinolones in the environment may pose a threat to human and ecological health. In this study, the biotransformation of enrofloxacin, marbofloxacin, and difloxacin by the fungus X. longipes (Xylaria) was investigated. The main metabolites were unequivocally identified as the respective N-oxides by mass spectrometry and nuclear magnetic resonance spectroscopy. Fungal-mediated N-oxidation of fluoroquinolones led to a 77-90% reduction of the initial antibacterial activity. In contrast to their respective parent compounds, N-oxides showed low cytotoxic potential and had a reduced impact on cell proliferation. Thus, biotransformation by X. longipes may represent an effective method for inactivating fluoroquinolones.

Characterization of in vitro metabolites of methylenedioxypyrovalerone (MDPV): An N-oxide metabolite formation mediated by flavin monooxygenase

Methylenedioxypyrovalerone (MDPV) has emerged in recent years as a recreational substance with psychostimulant properties. In this study, in vitro metabolites of MDPV were characterized based on liquid chromatography/quadrupole-time-of-flight mass spectrometry (LC/QTOF MS). MDPV was incubated with human liver microsomes, human recombinant cDNA-expressed cytochrome P450 enzymes and flavin monooxygenase (FMO). MDPV was metabolized to yield eight metabolites (M1-M8) with major metabolic reactions such as demethylenation and oxidation. Among them, M6 was assigned as an N-oxide metabolite. FMO was found to be a principal enzyme responsible for the formation of M6; FMO1 and FMO3 were the main enzymes involved in N-oxidation of MDPV.

Synthesis and Preliminary Evaluation of N-Oxide Derivatives for the Prevention of Atherothrombotic Events

Thrombosis is the main outcome of many cardiovascular diseases. Current treatments to prevent thrombotic events involve the long-term use of antiplatelet drugs. However, this therapy has several limitations, thereby justifying the development of new drugs. A series of N-oxide derivatives (furoxan and benzofuroxan) were synthesized and characterized as potential antiplatelet/antithrombotic compounds. All compounds (3a,b, 4a,b, 8a,b, 9a,b, 13a,b and 14a,b) inhibited platelet aggregation induced by adenosine-5-diphosphate, collagen, and arachidonic acid. All compounds protected mice from pulmonary thromboembolism induced by a mixture of collagen and epinephrine; however, benzofuroxan derivatives (13a,b and 14a,b) were the most active compounds, reducing thromboembolic events by up to 80%. N-oxide derivative 14a did not induce genotoxicity in vivo. In conclusion, 14a has emerged as a new antiplatelet/antithrombotic prototype useful for the prevention of atherothrombotic events.

The comparative toxicity of a reduced, crude comfrey (Symphytum officinale) alkaloid extract and the pure, comfrey-derived pyrrolizidine alkaloids, lycopsamine and intermedine in chicks (Gallus gallus domesticus)

Comfrey (Symphytum officinale), a commonly used herb, contains dehydropyrrolizidine alkaloids that, as a group of bioactive metabolites, are potentially hepatotoxic, pneumotoxic, genotoxic and carcinogenic. Consequently, regulatory agencies and international health organizations have recommended comfrey be used for external use only. However, in many locations comfrey continues to be ingested as a tisane or as a leafy vegetable. The objective of this work was to compare the toxicity of a crude, reduced comfrey alkaloid extract to purified lycopsamine and intermedine that are major constituents of S. officinale. Male, California White chicks were orally exposed to daily doses of 0.04, 0.13, 0.26, 0.52 and 1.04 mmol lycopsamine, intermedine or reduced comfrey extract per kg bodyweight (BW) for 10 days. After another 7 days chicks were euthanized. Based on clinical signs of poisoning, serum biochemistry, and histopathological analysis the reduced comfrey extract was more toxic than lycopsamine and intermedine. This work suggests a greater than additive effect of the individual alkaloids and/or a more potent toxicity of the acetylated derivatives in the reduced comfrey extract. It also suggests that safety recommendations based on purified compounds may underestimate the potential toxicity of comfrey.

A qualitative method for prediction of amine oxidation in methanol and water

We have developed a predictive method, based on quantum chemical calculations, that qualitatively predicts N-oxidation by hydrogen peroxides in drug structures. The method uses linear correlations of two complementary approaches to estimate the activation barrier without calculating it explicitly. This method can therefore be automated as it avoids demanding transition state calculations. As such, it may be used by chemists without experience in molecular modeling and provide additional understanding to experimental findings. The predictive method gives relative rates for N,N-dimethylbenzylamine and N-methylmorpholine in good agreement with experiments. In water, the experimental rate constants show that N,N-dimethylbenzylamine is oxidized three times faster than N-methylmorpholine and in methanol it is two times faster. The method suggests it to be two and five times faster, respectively. The method was also used to correlate experimental with predicted activation barriers, linear free-energy relationships, for a test set of tertiary amines. A correlation coefficient R(2) = 0.74 was obtained, where internal diagnostics in the method itself allowed identification of outliers. The method was applied to four drugs: caffeine, azelastine, buspirone, and clomipramine, all possessing several nitrogens. Both overall susceptibility and selectivity of oxidation were predicted, and verified by experiments.

DFT study on the oxygen transfer mechanism in nitroethenediamine based H2-receptor antagonists using the bis-dithiolene complex as the model catalyst for N-oxide reductase enzyme

Nitroethenediamine is an important functional unit, which is present in H2-receptor antagonists. These drugs show low bioavailability due to the bacterial degradation caused by the N-oxide reductase type of enzymes present in the human colon. Quantum chemical studies have been carried out to elucidate the mechanism of metabolic degradation of nitroethenediamine in the active site of N-oxide reductase. Three different pathways have been explored for the N-oxide bond cleavage by the model system, Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)2](-), (where mdt=1,2-dimethyl-ethene-1,2-dithiolate) using B3LYP/6-311+G(d,p) and M06/6-311+G(d,p) Density Functional Theory methods. The oxygen atom transfer from the nitrogen atom of nitroethenediamine to the Mo(IV) complex, involves simultaneous weakening of the N-oxide bond and the formation of Mo-O bond through a least motion path. During this transfer, Mo center is converted from a square pyramidal geometry to a distorted octahedral geometry, to facilitate the process of oxygen atom transfer. The energy barrier for the oxygen atom transfer from the imine tautomer has been estimated to be 25.9kcal/mol however, the overall reaction has been found to be endothermic. On the other hand, oxygen transfer reaction from the nitronic acid tautomer requires 30.5kcal/mol energy leading to a highly exothermic metabolite (M-1) directly hence, this path can be considered thermodynamically favorable for this metabolite. The alternative path involving the oxygen atom transfer from the enamine tautomer requires comparatively a higher energy barrier (32.6kcal/mol) and leads to a slightly endothermic metabolite. This study established the structural and energetic details associated with the Mo(IV) bis-dithiolene complex that catalyzes the degradation of nitroethenediamine based drug molecules.