η1-2-Pyrone metal carbonyl complexes as CO-releasing molecules (CO-RMs): A delicate balance between stability and CO liberation

An organic-organometallic CO-releasing material comprising 4,4'-bipyridine and molybdenum subcarbonyl building blocks

Over the past two decades, following the discovery of the important biological roles of carbon monoxide (CO), metal carbonyl complexes have been intensively studied as CO-releasing molecules (CORMs) for therapeutic applications. To improve the properties of "bare" low molecular weight CORMs, attention has been drawn to conjugating CORMs with macromolecular and inorganic scaffolds to produce CO-releasing materials (CORMAs) capable of storing and delivering large payloads of the gasotransmitter. A significant obstacle is to obtain CORMAs that retain the beneficial features of the parent CORMs. In the present work, a crystalline metal-organic framework (MOF) formulated as Mo(CO)3(4,4'-bipyridine)3/2 (Mobpy), with a structure based on Mo(CO)3 metallic nodes and bipyridine linkers, has been prepared in near quantitative yield by a straightforward reflux method, and found to exhibit CO-release properties that mimic those typically observed for molybdenum carbonyl CORMs. Mobpy was characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), FT-IR, FT-Raman and diffuse reflectance (DR) UV-vis spectroscopies, and 13C{1H} cross-polarization (CP) magic-angle spinning (MAS) NMR. The release of CO from Mobpy was studied by the deoxy-myoglobin (deoxy-Mb)/carbonmonoxy-myoglobin (MbCO) UV-vis assay. Mobpy liberates CO upon contact with a physiological buffer in the dark, leading to a maximum released amount of 1.3-1.5 mmol g-1, after 1.5 h at 37 °C, with half-lives of 0.5-1.0 h (time to transfer 0.5 equiv. of CO to Mb). In the solid-state and under open air, Mobpy undergoes complete decarbonylation over a period of 42 days, corresponding to a theoretical CO-release of 7.25 mmol g-1.

The monoiron anionfac-[Fe(CO)3I3]−and its organic aminium salts: their preparation, CO-release, and cytotoxicity

The anionfac-[Fe(CO)₃I₃]⁻undergoes rapid decomposition to release CO and involve iodine radical. The CO-release can be tuned by its cations. The radical causes severe cytotoxicity which may endow the anion a great potential as an anticancer drug.

Diiron(ii) pentacarbonyl complexes as CO-releasing molecules: their synthesis, characterization, CO-releasing behaviour and biocompatibility

Four diiron(ii) carbonyl complexes, [Fe2(μ-SR)3(CO)5X] (X- = Br-, I-; R = CH2CH3, CH2CH2CH3) were facilely synthesized by reacting [Fe(CO)4X2] with monothiolates. Their potential as carbon monoxide-releasing molecules (CORMs) was systematically investigated, revealing that their CO-releasing behaviour is highly solvent-dependent. Specifically, in dimethyl sulfoxide (DMSO), the CO-releasing kinetics were fast. Intermediates with a lower oxidation state might be involved in the reaction. By contrast, in less polar solvents such as methanol, acetonitrile and dichloromethane, intermediates featuring the triiron carbonyl cation, [Fe3(μ-SCH2CH3)6(CO)6]+, were isolated. The triiron intermediate underwent further decomposition to liberate CO. One of the iodo complexes was also examined for its CO-release in PBS solution when solubilised with DMSO in the presence of deoxy-Mb and the CO-release was found to be quantitative. Furthermore, kinetic analyses were performed and the CO-release in general obeyed a first-order kinetic model. Plausible CO-releasing pathways are proposed for the parent complexes and the triiron intermediate. Assessments in cytotoxicity indicated that the cytoxicity of the diiron(ii) complexes varied with both the halide and thiolate and those bearing bromide and the thiolate with longer chains were more biocompatible.

Insight into the mechanism of CO-release from trypto-CORM using ultra-fast spectroscopy and computational chemistry

Photolysis of trypto-CORM results in ultra-fast CO-dissociation and formation of a 16-e triplet followed by solvation.

Study of the interactions of bovine serum albumin with a molybdenum(II) carbonyl complex by spectroscopic and molecular simulation methods

Therapy with inhaled carbon monoxide (CO) is being tested in human clinical trials, yet the alternative use of prodrugs, CO-Releasing Molecules (CORMs), is conceptually advantageous. These molecules are designed to release carbon monoxide in specific tissues, in response to some locally expressed stimulus, where CO can trigger a cytoprotective response. The design of such prodrugs, mostly metal carbonyl complexes, must consider their ADMET profiles, including their interaction with transport plasma proteins. However, the molecular details of this interaction remain elusive. To shed light into this matter, we focused on the CORM prototype [Mo(η⁵-Cp)(CH₂COOH)(CO)₃] (ALF414) and performed a detailed molecular characterization of its interaction with bovine serum albumin (BSA), using spectroscopic and computational methods. The experimental results show that ALF414 partially quenches the intrinsic fluorescence of BSA without changing its secondary structure. The interaction between BSA and ALF414 follows a dynamic quenching mechanism, indicating that no stable complex is formed between the protein Trp residues and ALF414. The molecular dynamics simulations are in good agreement with the experimental results and confirm the dynamic and unspecific character of the interaction between ALF414 and BSA. The simulations also provide important insights into the nature of the interactions of this CORM prototype with BSA, which are dominated by hydrophobic contacts, with a contribution from hydrogen bonding. This kind of information is useful for future CORM design.

Confocal and fluorescence lifetime imaging sheds light on the fate of a pyrene-tagged carbon monoxide-releasing Fischer carbene chromium complex

The synthesis of a new pyrene-containing Fischer carbene complex is described. The complex has a broad absorbance spectrum between 300 and 400 nm and, on excitation at 345 nm in CH2Cl2 solution, emission is observed at 395 and 415 nm. Emission is also observed in PBS buffer, but in this case the resulting spectra are much broader. Confocal and fluorescence lifetime imaging indicate that emission occurs on treating HeLa cells with the complex and co-localisation studies demonstrate that this is from the mitochondria and lipid-rich regions of the cell.

Carbon monoxide--physiology, detection and controlled release

Carbon monoxide (CO) is increasingly recognized as a cell-signalling molecule akin to nitric oxide (NO). CO has attracted particular attention as a potential therapeutic agent because of its reported anti-hypertensive, anti-inflammatory and cell-protective effects. We discuss recent progress in identifying new effector systems and elucidating the mechanisms of action of CO on, e.g., ion channels, as well as the design of novel methods to monitor CO in cellular environments. We also report on recent developments in the area of CO-releasing molecules (CORMs) and materials for controlled CO application. Novel triggers for CO release, metal carbonyls and degradation mechanisms of CORMs are highlighted. In addition, potential formulations of CORMs for targeted CO release are discussed.

Diiron hexacarbonyl complexes as potential CO-RMs: CO-releasing initiated by a substitution reaction with cysteamine and structural correlation to the bridging linkage

Substitution-initiated CO-releasing rate of diiron hexacarbonyl complexes are highly dependent on their bridging linkages and the complexes of the “open” form release CO much faster than those of the “close” form.

CO and CO-releasing molecules in medicinal chemistry

Since the discovery that CO acts as a cytoprotective and homeostatic molecule, increasing research efforts have been devoted to the exploitation of its therapeutic effects. Both endogenous and exogenous CO improves experimental lung, vascular and cardiac injuries and protects against several inflammatory states. The technology is now in place to bring CO to clinical applications, but the use of the gaseous molecule poses several problems. The challenges associated with the clinical implementation of the gas have in part been answered by the development of CO-releasing molecules (CO-RMs). As stable solid forms of CO, these molecules represent an alternative to the administration of carbon monoxide (orally or by injection). In this article, we present insights into the biochemical action of CO and discuss the efficacy of CO and CO-RMs in preclinical disease models. Recent advances in the CO-RMs field are critically addressed.

Live-fibroblast IR imaging of a cytoprotective PhotoCORM Activated with Visible Light

Carbon monoxide releasing molecules (CORMs) are an emerging class of pharmaceutical compounds currently evaluated in several preclinical disease models. There is general consensus that the therapeutic effects elicited by the molecules may be directly ascribed to the biological function of the released CO. It remains unclear, however, if cellular internalization of CORMs is a critical event in their therapeutic action. To address the problem of cellular delivery, we have devised a general strategy which entails conjugation of a CO-releasing molecule (here a photoactivated CORM) to the 5'-OH ribose group of vitamin B12. Cyanocobalamin (B12) functions as the biocompatible water-soluble scaffold which actively transports the CORM against a concentration gradient into the cells. The uptake and cellular distribution of this B12-photoCORM conjugate is demonstrated via synchrotron FTIR spectromicroscopy measurements on living cells. Intracellular photoinduced CO release prevents fibroblasts from dying under conditions of hypoxia and metabolic depletion, conditions that may occur in vivo during insufficient blood supply to oxygen-sensitive tissues such as the heart or brain.

Synthesis, spectroscopic properties, and photoinduced CO-release studies of functionalized ruthenium(II) polypyridyl complexes: versatile building blocks for development of CORM-peptide nucleic acid bioconjugates

A series of ruthenium(II) dicarbonyl complexes of formula [RuCl2(L)(CO)2] (L = bpy(CH3,CH3) = 4,4'-dimethyl-2,2'-bipyridine, bpy(CH3,CHO) = 4'-methyl-2,2'-bipyridine-4-carboxyaldehyde, bpy(CH3,COOH) = 4'-methyl-2,2'-bipyridine-4-carboxylic acid, CppH = 2-(pyridin-2-yl)pyrimidine-4-carboxylic acid, dppzcH = dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylic acid), and [RuCl(L)(CO)2](+) (L = tpy(COOH) = 6-(2,2':6',2″-terpyridine-4'-yloxy)hexanoic acid) has been synthesized. In addition, a high-yield synthesis of a peptide nucleic acid (PNA) monomer containing the 2-(pyridin-2-yl)pyrimidine ligand was also developed, and this compound was used to prepare the first Ru(II) dicarbonyl complex, [RuCl2(Cpp-L-PNA)(CO)2],(Cpp-L-PNA = tert-butyl-N-[2-(N-9-fluorenylmethoxycarbonyl)aminoethyl]-N-[6-(2-(pyridin-2-yl)pyrimidine-4-carboxamido)hexanoyl]glycinate) attached to a PNA monomer backbone. Such metal-complex PNA-bioconjugates are attracting profound interest for biosensing and biomedical applications. Characterization of all complexes has been undertaken by IR and NMR spectroscopy, mass spectrometry, elemental analysis, and UV-vis spectroscopy. Investigation of the CO-release properties of the Ru(II) complexes in water/dimethyl sulfoxide (49:1) using the myoglobin assay showed that they are stable under physiological conditions in the dark for at least 60 min and most of them even for up to 15 h. In contrast, photoinduced CO release was observed upon illumination at 365 nm, the low-energy shoulder of the main absorption maximum centered around 300 nm, establishing these compounds as a new class of PhotoCORMs. While the two 2,2'-bipyridine complexes release 1 equiv of CO per mole of complex, the terpyridine, 2-(2'-pyridyl)pyrimidine, and dipyrido[3,2-a:2',3'-c]phenazine complexes are less effective CO releasers. Attachment of the 2-(2'-pyridyl)pyrimidine complex to a PNA backbone as in [RuCl2(Cpp-L-PNA)CO2] did not significantly change the spectroscopic or CO-release properties compared to the parent complex. Thus, a novel class of Ru(II)-based PhotoCORMs has been established which can be coupled to carrier delivery vectors such as PNA to facilitate cellular uptake without loss of the inherent CORM properties of the parent compound.

Developing drug molecules for therapy with carbon monoxide

The use of Carbon Monoxide (CO) as a therapeutic agent has already been tested in human clinical trials. Pre-clinically, CO gas administration proved beneficial in animal models of various human diseases. However, the use of gaseous CO faces serious obstacles not the least being its well-known toxicity. To fully realise the promise of CO as a therapeutic agent, it is key to find novel avenues for CO delivery to diseased tissues in need of treatment, without concomitant formation of elevated, toxic blood levels of carboxyhemoglobin (COHb). CO-releasing molecules (CO-RMs) have the potential to constitute safe treatments if CO release in vivo can be controlled in a spatial and temporal manner. It has already been demonstrated in animals that CO-RMs can release CO and mimic the therapeutic effects of gaseous CO. While demonstrating the principle of treatment with CO-RMs, these first generation compounds are not suitable for human use. This tutorial review summarises the biological and chemical behaviour of CO, the current status of CO-RM development, and derives principles for the creation of the next generation of CO-RMs for clinical applications in humans.

Differential antibacterial activity against Pseudomonas aeruginosa by carbon monoxide-releasing molecules

AIMS

Carbon monoxide (CO) delivered in a controlled manner to cells and organisms mediates a variety of pharmacological effects to the extent that CO-releasing molecules (CO-RMs) are being developed for therapeutic purposes. Recently, ruthenium-based CO-RMs have been shown to posses important bactericidal activity. Here we assessed the effect of fast CO releasers containing ruthenium (Ru(CO)(3)Cl(glycinate) [CORM-3] and tricarbonyldichlororuthenium(II) dimer [CORM-2]) and a novel slow manganese-based CO releaser ([Me(4)N][Mn(CO)(4)(thioacetate)(2)] [CORM-371]) on O(2) consumption and growth of Pseudomonas aeruginosa (PAO1). We then compared these effects with the action elicited by sodium boranocarbonate (CORM-A1), which lacks a transition metal but liberates CO with a rate similar to CORM-371.

RESULTS

CORM-2, CORM-3, and, to a lesser extent, CORM-371 exerted a significant bactericidal effect and decreased O(2) consumption in PAO1 in vitro. The effect appeared to be independent of reactive oxygen species production, but in the case of metal-containing compounds it was prevented by the thiol donor N-acetylcysteine. In contrast, CORM-A1 was bacteriostatic rather than bactericidal in vitro eliciting only a moderate and transient decrease in O(2) consumption.

INNOVATION

None of the tested CO-RMs was toxic to murine macrophages or human fibroblasts at the concentration impairing PA01 growth but only ruthenium-containing CO-RMs showed potential therapeutic properties by increasing the survival of mice infected with PA01.

CONCLUSION

CO carriers inhibit bacterial growth and O(2) consumption in vitro, but transition metal carbonyls appear more powerful than compounds spontaneously liberating CO. The nature of the metal in CO-RMs also modulates the anti-bacterial effect, with ruthenium-based CO-RMs being efficacious both in vitro and in vivo.

Ruthenium imidazole oxime carbonyls and their activities as CO-releasing molecules

Six ruthenium carbonyl complexes with imidazole and alkoxy carbonyl ligands were prepared and found to act as carbon monoxide releasing molecules (CORMs).

17 e- rhenium dicarbonyl CO-releasing molecules on a cobalamin scaffold for biological application

Cyanocobalamin (B(12)) offers a biocompatible scaffold for CO-releasing 17-electron dicarbonyl complexes based on the cis-trans-[Re(II)(CO)(2)Br(2)](0) core. A Co-C≡N-Re conjugate is produced in a short time and high yield from the reaction of [Et(4)N](2)[Re(II)Br(4)(CO)(2)] (ReCORM-1) with B(12). The B(12)-Re(II)(CO)(2) derivatives show a number of features which make them pharmaceutically acceptable CO-releasing molecules (CORMs). These cobalamin conjugates are characterized by an improved stability in aqueous aerobic media over the metal complex alone, and afford effective therapeutic protection against ischemia-reperfusion injury in cultured cardiomyocytes. The non-toxicity (at μM concentrations) of the resulting metal fragment after CO release is attributed to the oxidation of the metal and formation in solution of the ReO(4)(-) anion, which is among the least toxic of all of the rare inorganic compounds. Theoretical and experimental studies aimed at elucidating the aqueous chemistry of ReCORM-1 are also described.

Carbon monoxide-releasing micelles for immunotherapy

With the discovery of important biological roles of carbon monoxide (CO), the use of this gas as a therapeutic agent has attracted attention. However, the medical application of this gas has been hampered by the complexity of the administration method. To overcome this problem, several transition-metal carbonyl complexes, such as Ru(CO)(3)Cl(glycinate), [Ru(CO)(3)Cl(2)](2), and Fe(η(4)-2-pyrone)(CO)(3), have been used as CO-releasing molecules both in vitro and in vivo. We sought to develop micellar forms of metal carbonyl complexes that would display slowed diffusion in tissues and thus better ability to target distal tissue drainage sites. Specifically, we aimed to develop a new CO-delivery system using a polymeric micelle having a Ru(CO)(3)Cl(amino acidate) structure as a CO-releasing segment. The CO-releasing micelles were prepared from triblock copolymers composed of a hydrophilic poly(ethylene glycol) block, a poly(ornithine acrylamide) block bearing Ru(CO)(3)Cl(ornithinate) moieties, and a hydrophobic poly(n-butylacrylamide) block. The polymers formed spherical micelles in the range of 30-40 nm in hydrodynamic diameter. Further characterization revealed the high CO-loading capacity of the micelles. CO-release studies showed that the micelles were stable in physiological buffer and serum and released CO in response to thiol-containing compounds such as cysteine. The CO release of the micelles was slower than that of Ru(CO)(3)Cl(glycinate). In addition, the CO-releasing micelles efficiently attenuated the lipopolysaccharide-induced NF-κB activation of human monocytes, while Ru(CO)(3)Cl(glycinate) did not show any beneficial effects. Moreover, cell viability assays revealed that the micelles significantly reduced the cytotoxicity of the Ru(CO)(3)Cl(amino acidate) moiety. This novel CO-delivery system based on CO-releasing micelles may be useful for therapeutic applications of CO.