Modified biovectors for the tuneable activation of anti-platelet carbon monoxide release

meso-Bromination of cyano- and aquacobalamins facilitates their processing into Co(II)-species by glutathione

Cyanocobalamin (CNCbl), a medicinal form of vitamin B12, is resistant to glutathione (GSH), and undergoes intracellular processing via reductive decyanation producing the Co(II)-form of Cbl (Cbl(II)) mediated by the CblC-protein. Alteration of the CblC-protein structure might inhibit CNCbl processing. Here, we showed that introducing a bromine atom to the C10-position of the CNCbl corrin ring facilitates its reaction with GSH leading to the formation of Cbl(II) and cyanide dissociation. In a neutral medium, the reaction between C10-Br-CNCbl and GSH proceeds via the complexation of the reactants further leading to dimethylbenzimidazole (DMBI) substitution and electron transfer from GSH to the Co(III)-ion. The reaction is accelerated upon the GSH thiol group deprotonation. The key factors explaining the higher reactivity of C10-Br-CNCbl compared with unmodified CNCbl towards GSH are increasing the electrode potential of CNCbl two-electron reduction upon meso-bromination and the substantial labilization of DMBI, which was shown by comparing their reactions with cyanide and the pKa values of DMBI protonation (pKa base-off). Aquacobalamin (H2OCbl) brominated at the C10-position of the corrin reacts with GSH to give Cbl(II) via GSH complexation and subsequent reaction of this complex with a second GSH molecule, whereas unmodified H2OCbl generates glutathionyl-Cbl, which is resistant to further reduction by GSH.

The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

Functionalisation of vitamin B12 derivatives with a cobalt β-phenyl ligand boosters antimetabolite activity in bacteria

This study describes the syntheses of four singly- and two doubly-modified vitamin B₁₂ derivatives for generating antimetabolites of Lactobacillus delbrueckii (L. delbrueckii). The two most potent antagonists, a Co_β-phenyl-cobalamin-c,8-lactam and a 10-bromo-Co_β-phenylcobalamin combine a c-lactam or 10-bromo modification at the “eastern” site of the corrin ring with an artificial organometallic phenyl group instead of a cyano ligand at the β-site of the cobalt center. These two doubly-modified B₁₂ antagonists (10 nM) inhibit fully B₁₂-dependent (0.1 nM) growth of L. delbrueckii. In contrast to potent 10-bromo-Co_β-phenylcobalamin, single modified 10-bromo-Co_β-cyanocobalamin lacking the artificial organometallic phenyl ligand does not show any inhibitory effect. These results suggest, that the organometallic β-phenyl ligand at the Co center ultimately steers the metabolic effect of the 10-bromo-analogue.

This study describes the syntheses of four singly- and two doubly-modified vitamin B₁₂ derivatives for generating antimetabolites of Lactobacillus delbrueckii (L. delbrueckii).

Anticancer and Antibiotic Rhenium Tri- and Dicarbonyl Complexes: Current Research and Future Perspectives

Organometallic compounds are increasingly recognized as promising anticancer and antibiotic drug candidates. Among the transition metal ions investigated for these purposes, rhenium occupies a special role. Its tri- and dicarbonyl complexes, in particular, attract continuous attention due to their relative ease of preparation, stability and unique photophysical and luminescent properties that allow the combination of diagnostic and therapeutic purposes, thereby permitting, e.g., molecules to be tracked within cells. In this review, we discuss the anticancer and antibiotic properties of rhenium tri- and dicarbonyl complexes described in the last seven years, mainly in terms of their structural variations and in vitro efficacy. Given the abundant literature available, the focus is initially directed on tricarbonyl complexes of rhenium. Dicarbonyl species of the metal ion, which are slowly gaining momentum, are discussed in the second part in terms of future perspective for the possible developments in the field.

Efficient Direct Nitrosylation of α-Diimine Rhenium Tricarbonyl Complexes to Structurally Nearly Identical Higher Charge Congeners Activable towards Photo-CO Release

The reaction of rhenium α-diimine (N-N) tricarbonyl complexes with nitrosonium tetrafluoroborate yields the corresponding dicarbonyl-nitrosyl [Re(CO)2(NO)(N-N)X]+ species (where X = halide). The complexes, accessible in a single step in good yield, are structurally nearly identical higher charge congeners of the tricarbonyl molecules. Substitution chemistry aimed at the realization of equivalent dicationic species (intended for applications as potential antimicrobial agents), revealed that the reactivity of metal ion in [Re(CO)2(NO)(N-N)X]+ is that of a hard Re acid, probably due to the stronger π-acceptor properties of NO+ as compared to those of CO. The metal ion thus shows great affinity for π-basic ligands, which are consequently difficult to replace by, e.g., σ-donor or weak π-acids like pyridine. Attempts of direct nitrosylation of α-diimine fac-[Re(CO)3]+ complexes bearing π-basic OR-type ligands gave the [Re(CO)2(NO)(N-N)(BF4)][BF4] salt as the only product in good yield, featuring a stable Re-FBF3 bond. The solid state crystal structure of nearly all molecules presented could be elucidated. A fundamental consequence of the chemistry of [Re(CO)2(NO)(N-N)X]+ complexes, it that the same can be photo-activated towards CO release and represent an entirely new class of photoCORMs.

Mechanism of cyanocobalamin chlorination by hypochlorous acid

Hypochlorous acid (HOCl) is a strong oxidant produced by myeloperoxidase. Previous work suggested that HOCl modifies the corrin ring of cobalamins to yield chlorinated species via mechanisms that are incompletely understood. Herein, we report a mechanistic study on the reaction between cyanocobalamin (CNCbl, vitamin B₁₂) and HOCl. Under weakly acidic, neutral and weakly alkaline conditions, the reaction produces the c-lactone derivative of CNCbl chlorinated at the C10-position of corrin ring (C10-Cl-CNCbl-c-lactone). Formation of C10-Cl-CNCbl-c-lactone was not observed at pH ≥ 9.9. The chlorination of CNCbl by HOCl proceeds via two pathways involving one and two HOCl molecules: the reaction is initiated by the very fast formation of a complex between CNCbl and HOCl, which either undergoes slow transformation to chlorinated species, or rapidly reacts with a second HOCl molecule to produce C10-Cl-CNCbl. Subsequent reaction of C10-Cl-CNCbl with HOCl proceeds rapidly toward lactone ring formation by H-atom abstraction at position C8. This work uncovered mechanisms and products of the reaction of a biologically active and therapeutically used cobalamin, CNCbl and the endogenous oxidant HOCl. Binding and reactivity studies of C10-Cl-CNCbl and C10-Cl-CNCbl-c-lactone with relevant proteins of the cobalamin pathway and with cultured cells are necessary to elucidate the potential physiological effects of these species.

Distinct Pharmacological Properties of Gaseous CO and CO-Releasing Molecule in Human Platelets

Carbon monoxide (CO)—gaseous or released by CO-RMs—both possess antiplatelet properties; however, it remains uncertain whether the mechanisms involved are the same. Here, we characterise the involvement of soluble guanylate cyclase (sGC) in the effects of CO—delivered by gaseous CO–saturated buffer (CO_G) and generated by CORM-A1—on platelet aggregation and energy metabolism, as well as on vasodilatation in aorta, using light transmission aggregometry, Seahorse XFe technique, and wire myography, respectively. ODQ completely prevented the inhibitory effect of CO_G on platelet aggregation, but did not modify antiplatelet effect of CORM-A1. In turn, CO_G did not affect, whereas CORM-A1 substantially inhibited energy metabolism in platelets. Even though activation of sGC by BAY 41-2272 or BAY 58-2667 inhibited significantly platelet aggregation, their effects on energy metabolism in platelets were absent or weak and could not contribute to antiplatelet effects of sGC activation. In contrast, vasodilatation of murine aortic rings, induced either by CO_G or CORM-A1, was dependent on sGC. We conclude that the source (CO_G vs. CORM-A1) and kinetics (rapid vs. slow) of CO delivery represent key determinants of the mechanism of antiplatelet action of CO, involving either impairment of energy metabolism or activation of sGG.

Aerobically stable and substitutionally labile α-diimine rhenium dicarbonyl complexes

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes are described. The molecules are prepared in high yield from the cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br₂]⁻ anion (2, where ^tBu₂bpy is 4,4′-di-tert-butyl-2,2′-bipyridine), which can be isolated from the one electron reduction of the corresponding 17-electron complex (1). Compound 2 is stable in the solid state, but in solution it is oxidized by molecular oxygen back to 1. Replacement of a single bromide of 2 by σ-donor monodentate ligands (Ls) yields stable neutral 18-electron cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO)₂(^tBu₂bpy)Br(L)] species can be further reacted with Ls to prepare stable cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes in good yield. Ligand substitution of Re(i) complexes proceeds via pentacoordinate intermediates capable of Berry pseudorotation. In addition to the cis–cis–trans-complexes, cis–cis–cis- (all cis) isomers are also formed. In particular, cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes establish an equilibrium with all cis isomers in solution. The solid state crystal structure of nearly all molecules presented could be elucidated. The molecules adopt a slightly distorted octahedral geometry. In comparison to similar fac-[Re(CO)₃]⁺complexes, Re(i) diacarbonyl species are characterized by a bend (ca. 7°) of the axial ligands towards the α-diimine unit. [Re(CO)₂(^tBu₂bpy)Br₂]⁻ and [Re(CO)₂(^tBu₂bpy)Br(L)] complexes may be considered as synthons for the preparation of a variety of new stable diamagnetic dicarbonyl rhenium cis-[Re(CO)₂]⁺ complexes, offering a convenient entry in the chemistry of the core.

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes offer a convenient entry in the chemistry of the cis-[Re(CO)₂]⁺ core.

Antivitamins B12 -Some Inaugural Milestones

The recently delineated structure- and reactivity-based concept of antivitamins B12 has begun to bear fruit by the generation, and study, of a range of such B12 -dummies, either vitamin B12 -derived, or transition metal analogues that also represent potential antivitamins B12 or specific B12 -antimetabolites. As reviewed here, this has opened up new research avenues in organometallic B12 -chemistry and bioinorganic coordination chemistry. Exploratory studies with antivitamins B12 have, furthermore, revealed some of their potential, as pharmacologically interesting compounds, for inducing B12 -deficiency in a range of organisms, from hospital resistant bacteria to laboratory mice. The derived capacity of antivitamins B12 to induce functional B12 -deficiency in mammalian cells and organs also suggest their valuable potential as growth inhibitors of cancerous human and animal cells.

Luminescent PhotoCORMs: Enabling/Disabling CO Delivery upon Blue Light Irradiation

The new luminescent carbonyl compounds [Mn(Oxa-H)(CO)₃Br] (1) and [Mn(Oxa-NMe₂)(CO)₃Br] (2) were synthesized and fully characterized. Complexes 1 and 2 showed CO release under blue light (λ₄₅₃). Spectroscopic techniques and TD-DFT and SOC-TD-DFT calculations indicated that 1 and 2 release the Oxa-H and Oxa-NMe₂ coligands in addition to the carbonyl ligands, increasing the luminescence during photoinduction.

Antiplatelet Effect of Carbon Monoxide Is Mediated by NAD+ and ATP Depletion

Supplemental Digital Content is available in the text.

Objectives:

Carbon monoxide (CO) produced by haem oxygenases or released by CO-releasing molecules (CORM) affords antiplatelet effects, but the mechanism involved has not been defined. Here, we tested the hypothesis that CO–induced inhibition of human platelet aggregation is mediated by modulation of platelet bioenergetics.

Approach and Results:

To analyze the effects of CORM-A1 on human platelet aggregation and bioenergetics, a light transmission aggregometry, Seahorse XFe technique and liquid chromatography tandem-mass spectrometry–based metabolomics were used. CORM-A1–induced inhibition of platelet aggregation was accompanied by the inhibition of mitochondrial respiration and glycolysis. Interestingly, specific inhibitors of these processes applied individually, in contrast to combined treatment, did not inhibit platelet aggregation considerably. A CORM-A1–induced delay of tricarboxylic acid cycle was associated with oxidized nicotinamide adenine dinucleotide (NAD⁺) depletion, compatible with the inhibition of oxidative phosphorylation. CORM-A1 provoked an increase in concentrations of proximal (before GAPDH [glyceraldehyde 3-phosphate dehydrogenase]), but not distal glycolysis metabolites, suggesting that CO delayed glycolysis at the level of NAD⁺–dependent GAPDH; however, GAPDH activity was directly not inhibited. In the presence of exogenous pyruvate, CORM-A1–induced inhibition of platelet aggregation and glycolysis were lost, but were restored by the inhibition of lactate dehydrogenase, involved in cytosolic NAD⁺ regeneration, pointing out to the key role of NAD⁺ depletion in the inhibition of platelet bioenergetics by CORM-A1.

Conclusions:

The antiplatelet effect of CO is mediated by inhibition of mitochondrial respiration—attributed to the inhibition of cytochrome c oxidase, and inhibition of glycolysis—ascribed to cytosolic NAD⁺ depletion.

Cu-catalyzed click conjugation of cobalamin to a BODIPY-based fluorophore: A versatile tool to explore the cellular biology of vitamin B12

The Cu-catalyzed click conjugation of an azide-functionalized vitamin B₁₂ (cobalamin) and an alkyne-labeled 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) led to the formation of a highly stable fluorescent BODIPY-labeled vitamin B₁₂ (λ_ex/λ_em = 495/508 nm). The formation of what has been identified as an iodine adduct of the conjugate was also observed as a side-product during this reaction and could be removed using HPLC. BODIPY-labeled vitamin B₁₂ was characterized by NMR and HR-ESI-MS. In vitro studies on wild-type human fibroblasts indicated that BODIPY-labeled vitamin B₁₂ could internalize in a manner similar to that of untagged vitamin B₁₂. ATP-binding cassette sub-family D member 4 (ABCD4) is a lysosomal localized transporter required to export vitamin B₁₂ from the lysosomal lumen to the cytosol. Mutations in this transporter result in the accumulation of vitamin B₁₂ in lysosomes. In human fibroblasts harbouring a mutation in ABCD4, BODIPY-labeled vitamin B₁₂ accumulated in the lumen of lysosomes. Our data suggests the potential use of BODIPY-labeled vitamin B₁₂ to investigate the intracellular behavior of the vitamin in the context of disorders related to the abnormal cellular utilization of the vitamin. Moreover, results presented here demonstrate that click chemistry could be exploited for the conjugation of vitamin B₁₂ to various other fluorophores.

A comparison of multifunctional donors of carbon monoxide: Their anticoagulant, antioxidant, anti-aggregatory and cytotoxicity activities in an in vitro model

The study examines the effect of two water-soluble carbon monoxide (CO) donors, CORM-3 and CORM-A1, on selected parameters of oxidative stress and hemostasis in human plasma and blood platelets in vitro. It also compares their activity with that of the lipid-soluble CORM-2. The oxidation of amino acid residues in plasma proteins was evaluated by measuring the amounts of thiol and carbonyl groups. Plasma lipid peroxidation was measured as thiobarbituric acid reactive substance (TBARS) concentration. In addition, three haemostatic parameters of plasma were studied, viz. activated partial thromboplastin time (APTT), prothrombin time (PT) and thrombin time (TT), and one haemostatic parameter of platelets (platelet aggregation). Treatment with CORM-3 and CORM-A1 (all concentrations from 0.1 to 100 μM) decreased thiol group oxidation induced by H₂O₂/Fe. Incubation with CORM-3 and CORM-A1 also influenced plasma coagulation activity, e.g. CORM-3 and CORM-A1 significantly prolonged TT at the two highest tested concentrations (50 and 100 μM). Only CORM-2 at the highest tested concentration (100 μM) and CORM-3 (50 and 100 μM) reduced platelet aggregation induced by ADP. None of the tested CORMs caused platelet damage. The treatment of various diseases associated with oxidative stress, including cardiovascular diseases, may be enhanced by the administration of CO donors CORM-2 and CORM-3, these being modulators of oxidative stress and hemostasis.

Operando characterization of chemical reactions in single living cells using SERS

Operando probing of chemical reactions for the delivery of gaseous signaling molecules in living cells that is critical for understanding the physiological metabolic processes.

Novel Re(I) tricarbonyl coordination compounds based on 2-pyridyl-1,2,3-triazole derivatives bearing a 4-amino-substituted benzenesulfonamide arm: synthesis, crystal structure, computational studies and inhibitory activity against carbonic anhydrase I, II, and IX isoforms†

In this work, two bidentate 2-pyridyl-1,2,3-triazole ligands (3a and 3b) containing a 4-substituted benzenesulfonamide pharmacophore prepared by classical click chemistry procedures, as well as their corresponding rhenium complexes, 4a and 4b of general formula [ReCl(CO)3(L)] (L = 3a or 3b) were prepared and fully characterised by spectroscopic methods (IR, NMR, MS, UV-Vis), elemental analysis, X-ray diffraction, and theoretical studies using DFT and TD-DFT methods. In particular, we showed that, in the solid state, the pyridine and the triazole rings of 3b adopted an uncommon cis configuration which stems from intermolecular hydrogen bonds. Preliminary assays demonstrated a promising nanomolar inhibitory activity against carbonic anhydrase isoform IX for both ligands and complexes with a strong affinity Ki of 2.8 nM for ligand 3a. More interestingly, complex 4b exhibited a pronounced selectivity against hCA IX over the off-targets hCA I and hCA II which makes this compound a promising potential anticancer drug candidate.

Systemically Administered Plant Recombinant Holo-Intrinsic Factor Targets the Liver and is not Affected by Endogenous B12 levels

Precision targeting imaging agents and/or treatment agents to select cells or organs in the body remains a significant need and is an area of intense research. It has been hypothesized that the vitamin B12 (B12) dietary pathway, or components thereof, may be exploitable in this area. The question of whether gastric Intrinsic factor (IF), critical for B12 absorption in the GI tract via the cubilin receptor, could be used as a targeting moiety for the cubilin receptor systemically, has not been investigated. Cubilin is the only known receptor for holo-IF and is found primarily in the kidney and ear (outside of the ileum of the GI) offering significant scope for specific targeting. We utilized plant derived human gastric IF in fluorescent cell and PET based in vivo imaging and biodistribution studies and demonstrated that plant derived IF primarily targets the liver, likely a consequence of the unique glycosylation profile of the IF, and is not affected by endogenous B12 levels.

Modified vitamin B12 derivatives with a peptide backbone for biomimetic studies and medicinal applications

This short review highlights the author’s group research on modified vitamin B[Formula: see text] derivatives with a peptide backbone as (1) inhibitors of B[Formula: see text]-dependent enzymes and as (2) models of cofactor B[Formula: see text]-protein complexes.

Affinity-Enhanced Luminescent Re(I)- and Ru(II)-Based Inhibitors of the Cysteine Protease Cathepsin L

Two new Re(I)- and Ru(II)-based inhibitors were synthesized with the formulas [Re(phen)(CO)₃(1)](OTf) (7; phen = 1,10-phenanthroline, OTf = trifluoromethanesulfonate) and [Ru(bpy)₂(2)](Cl)₂ (8; bpy = 2,2'-bipyridine), where 1 and 2 are the analogues of CLIK-148, an epoxysuccinyl-based cysteine cathepsin L inhibitor (CTSL). Compounds 7 and 8 were characterized using various spectroscopic techniques and elemental analysis; 7 and 8 both show exceptionally long excited state lifetimes. Re(I)-based complex 7 inhibits CTSL in the low nanomolar range, affording a greater than 16-fold enhancement of potency relative to the free inhibitor 1 with a second-order rate constant of 211000 ± 42000 M^-1 s^-1. Irreversible ligation of 7 to papain, a model of CTSL, was analyzed with mass spectroscopy, and the major peak shown at 24283 au corresponds to that of papain-1-Re(CO)₃(phen). Compound 7 was well tolerated by DU-145 prostate cancer cells, with toxicity evident only at high concentrations. Treatment of DU-145 cells with 7 followed by imaging via confocal microscopy showed substantial intracellular fluorescence that can be blocked by the known CTSL inhibitor CLIK-148, consistent with the ability of 7 to label CTSL in living cells. Overall this study reveals that a Re(I) complex can be attached to an enzyme inhibitor and enhance potency and selectivity for a medicinally important target, while at the same time allowing new avenues for tracking and quantification due to long excited state lifetimes and non-native element composition.

Carbon Monoxide and Its Controlled Release: Therapeutic Application, Detection, and Development of Carbon Monoxide Releasing Molecules (CORMs)

Carbon monoxide (CO) is attracting increasing attention because of its role as a gasotransmitter with cytoprotective and homeostatic properties. Carbon monoxide releasing molecules (CORMs) are spatially and temporally controlled CO releasers that exhibit superior and more effective pharmaceutical traits than gaseous CO because of their chemistry and structure. Experimental and preclinical research in animal models has shown the therapeutic potential of inhaled CO and CORMs, and the biological effects of CO and CORMs have also been observed in preclinical trials via the genetic modulation of heme oxygenase-1 (HO-1). In this review, we describe the pharmaceutical use of CO and CORMs, methods of detecting CO release, and developments in CORM design and synthesis. Many valuable clinical CORMs formulated using macromolecules and nanomaterials are also described.