Extracellular vs Intracellular Delivery of CO: Does It Matter for a Stable, Diffusible Gasotransmitter?

Carbon monoxide (CO) is a gasotransmitter produced in humans. An essential unanswered question in the design of carbon monoxide releasing molecules (CORMs) is whether the delivery molecule should be localized extra- or intracellularly to produce desired biological effects. Herein we show that extracellular CO release is less toxic and is sufficient to produce an anti-inflammatory effect similar to that of intracellular CO release at nanomolar concentrations. This information is valuable for the design of CORMs.

Design and Biological Evaluation of Manganese- and Ruthenium-Based Hybrid CO-RMs (HYCOs)

Interest in the therapeutic effects of carbon monoxide (CO), a product of heme degradation catalyzed by the enzyme heme oxygenase-1 (HO-1), has led to the development of CO-releasing molecules (CO-RMs) for the controlled delivery of this gas in vivo. We recently proposed conjugating a cobalt-based CO-RM with various activators of nuclear factor erythroid 2-related factor 2 (Nrf2), the transcription factor that regulates HO-1 expression, in order to exploit the beneficial effects of exogenous and endogenous CO. In this study, we describe the preparation of hybrid molecules (termed HYCOs) conjugating a fumaric acid derivative as an Nrf2 activator to a Mn- or a Ru-based CO-RM known to be pharmacologically active. With the exception of an acyl-manganese complex, these hybrids were obtained by associating the two bioactive entities by means of a linker of variable structure. X-ray diffraction analyses and preliminary biological investigations are also presented.

CO release with ratiometric fluorescence changes: a promising visible-light-triggered metal-free CO-releasing molecule

The first visible-light-triggered metal-free and ratiometric fluorescent CORM is reported. This CORM can be used to release CO with distinct ratiometric fluorescence changes in aqueous solution, living cells, zebrafish, and mice, which provided an excellent controllable and trackable CORM for living systems.

Synthesis, Structural Characterization and Photodecarbonylation Study of a Dicarbonyl Ruthenium(II)-Bisquinoline Complex

A photoactivatable ruthenium(II) carbonyl complex mer,cis-[Ru(II)Cl(BisQ)(CO)2]PF6 2 was prepared using a tridentate bisquinoline ligand (BisQ=(2,6-diquinolin-2-yl)pyridin). Compound 2 was thoroughly characterized by standard analytical methods and single crystal X-ray diffraction. The crystal structure of the complex cation reveals a distorted octahedral geometry. The decarbonylation upon exposure to 350 and 420 nm light was monitored by UV/VIS absorbance and Fourier transform infrared spectroscopies in acetonitrile and 1 % (v/v) DMSO in water, respectively. The kinetic of the photodecarbonylation has been elucidated by multivariate curve resolution alternating least-squares analysis. The stepwise decarbonylation follows a serial mechanism. The first decarbonylation occurs very quickly whereas the second decarbonylation step proceeds more slowly. Moreover, the second rate constant is lower in 1 % (v/v) DMSO in water than in acetonitrile. In comparison to 350 nm irradiation, exposure to 420 nm light in acetonitrile results in a lower second rate constant.

Spectroscopic and 1O2 Sensitization Characteristics of a Series of Isomeric Re(bpy)(CO)3Cl Complexes Bearing Pendant BODIPY Chromophores

Two new Re(I)bipyridyltricarbonyl chloride complexes, Re(BB3)(CO)₃Cl and Re(BB4)(CO)₃Cl, featuring BODIPY groups appended to the 5,5'- or 6,6'-positions of the bipyridine ligand, respectively, were synthesized as structurally isomeric compliments to a previously reported 4,4'-substituted homologue, Re(BB2)(CO)₃Cl. X-ray crystal structures of the compounds show that the 4,4'-, 5,5'-, and 6,6'-substitution patterns place the BODIPY groups at progressively shorter distances of 9.43, 8.39, and 5.56 Å, respectively, from the complexes' Re centers. The photophysical properties of the isomeric complexes were investigated to ascertain the manner in which the heavy rhenium atom might induce intersystem crossing of the pendant BODIPY moieties positioned at progressively shorter through-space distances. Electronic absorption spectroscopy revealed that the three metal complexes retain the strong visible absorption features characteristic of the bpyBODIPY (BB2-BB4) ligands; however, the fluorescence of the parent borondipyrromethane appended ligands is attenuated by more than an order of magnitude in Re(BB2)(CO)₃Cl and Re(BB3)(CO)₃Cl and by more than two orders of magnitude in Re(BB4)(CO)₃Cl. Furthermore, phosphorescence from Re(BB4)(CO)₃Cl is observed under a nitrogen atmosphere, consistent with highly efficient ISC to the triplet-excited state. Singlet oxygen sensitization studies confirm that all three complexes produce singlet oxygen with quantum yields that increase as the distance of the BODIPY groups to the heavy rhenium center is decreased. The trends observed across the series of rhenium complexes with respect to emission and ¹O₂ sensitization properties can be rationalized in terms of the varied distal separation between the metal center and BODIPY groups in each system.

Tracking CO release in cells via the luminescence of donor molecules and/or their by-products

Carbon monoxide (CO) is a bioactive signalling molecule that is produced endogenously via the breakdown of heme. Beneficial health effects associated with the delivery of CO gas have spurred the development of CO-releasing molecules (CORMs) that can be used to provide specific amounts of the gas. In addition to their potential use as therapeutics, CORMs are needed to provide insight into the biological targets of CO. In this regard, light-activated CO-releasing molecules (photoCORMs), are valuable for examining the effects of localized CO release. Herein we examine luminescent CORMs and photoCORMs that have been reported for tracking CO delivery in cells. A variety of motifs are available that exhibit differing luminescence properties and cover a wide range of wavelengths. Trackable CO donors have been successfully applied to targeting CO delivery to mitochondria, thus demonstrating the feasibility of using such molecules in detailed investigations of the biological roles of CO.

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.

Electrospray ionization study of tricarbonyl fac-[Re(CO)3 (PO)(X)]-type complexes: influence of ancillary co-ligands in the release of carbon monoxide

RATIONALE

fac-[Re(CO)₃ (PO)(X)]-type complexes (PO = chelated bidentate tertiary phosphine (1-), X = various neutral, mono-dentate ligands) represent a class of compounds that meets the synthetic criteria for the preparation of potential carbon monoxide (CO) release molecules (CORMs) for medicinal application. The aim of our investigation was to achieve qualitative information whether the nature of the ancillary X ligand might influence the release of CO.

METHODS

The release of CO has been investigated by means of product ion spectrometry of electrospray ionization (ESI)-generated [M + H]⁺ species, produced by multiple collisional experiments, using an ion trap mass spectrometer.

RESULTS

Tandem mass spectrometry applied to the protonated species [Re(CO)₃ (PO)(X) + H]⁺ of seven complexes (those including X = OH₂ (1), isonitrile (2, 3), imidazole (4), pyridine (5) and phosphine (6, 7)) shows initial loss of coordinated water (1) or pyridine (5), whereas the majority of investigated entries display initial, sequential release of CO groups. The energetics of CO release have been investigated by breakdown curves for selected collisionally activated decomposition processes involving CO, and compared with those involving X groups.

CONCLUSIONS

The nature of the co-ligand X drives the primary loss in the MSⁿ processes of [Re(CO)₃ (PO)(X) + H]⁺ compounds. When X = solvent, the energetics of these decompositions follow the trend H₂ O < MeOH < CO. In each case, loss of CO is a favored fragmentation route with associated energies following the trend: N-py ≤ P-phosphine < C-isonitrile. Overall, MSⁿ pathways indicate that [Re(PO)] (Re with chelated PO phosphine) constitutes the residual moiety. This behavior indicates that the presence of a functionalized phosphine is essential for a sequential, controlled release of CO.

An Overview of the Potential Therapeutic Applications of CO-Releasing Molecules

Carbon monoxide (CO) has long been known as the “silent killer” owing to its ability to form carboxyhemoglobin—the main cause of CO poisoning in humans. Its role as an endogenous neurotransmitter, however, was suggested in the early 1990s. Since then, the biological activity of CO has been widely examined via both the direct administration of CO and in the form of so-called “carbon monoxide releasing molecules (CORMs).” This overview will explore the general physiological effects and potential therapeutic applications of CO when delivered in the form of CORMs.

Visible-Light-Activated Quinolone Carbon-Monoxide-Releasing Molecule: Prodrug and Albumin-Assisted Delivery Enables Anticancer and Potent Anti-Inflammatory Effects

The delivery of controlled amounts of carbon monoxide (CO) to biological targets is of significant current interest. Very few CO-releasing compounds are currently known that can be rigorously controlled in terms of the location and amount of CO released. To address this deficiency, we report herein a new metal-free, visible-light-induced CO-releasing molecule (photoCORM) and its prodrug oxidized form, which offer new approaches to controlled, localized CO delivery. The new photoCORM, based on a 3-hydroxybenzo[ g]quinolone framework, releases 1 equiv of CO upon visible-light illumination under a variety of biologically relevant conditions. This nontoxic compound can be tracked prior to CO release using fluorescence microscopy and produces a nontoxic byproduct following CO release. An oxidized prodrug form of the photoCORM is reduced by cellular thiols, providing an approach toward activation in the reducing environment of cancer cells. Strong noncovalent affinity of the nonmetal photoCORM to albumin enables use of an albumin:photoCORM complex for targeted CO delivery to cancer cells. This approach produced cytotoxicity IC₅₀ values among the lowest reported to date for CO delivery to cancer cells by a photoCORM. This albumin:photoCORM complex is also the first CO delivery system to produce significant anti-inflammatory effects when introduced at nanomolar photoCORM concentration.

Is IQG-607 a Potential Metallodrug or Metallopro-Drug With a Defined Molecular Target in Mycobacterium tuberculosis?

The emergence of strains of Mycobacterium tuberculosis resistant to isoniazid (INH) has underscored the need for the development of new anti-tuberculosis agents. INH is activated by the mycobacterial katG-encoded catalase-peroxidase, forming an acylpyridine fragment that is covalently attached to the C4 of NADH. This isonicotinyl-NAD adduct inhibits the activity of 2-trans-enoyl-ACP(CoA) reductase (InhA), which plays a role in mycolic acid biosynthesis. A metal-based INH analog, Na₃[Fe^II(CN)₅(INH)]·4H₂O, IQG-607, was designed to have an electronic redistribution on INH moiety that would lead to an intramolecular electron transfer to bypass KatG activation. HPLC and EPR studies showed that the INH moiety can be oxidized by superoxide or peroxide yielding similar metabolites and isonicotinoyl radical only when associated to IQG-607, thereby supporting redox-mediated drug activation as a possible mechanism of action. However, IQG-607 was shown to inhibit the in vitro activity of both wild-type and INH-resistant mutant InhA enzymes in the absence of KatG activation. IQG-607 given by the oral route to M. tuberculosis-infected mice reduced lung lesions. Experiments using early and late controls of infection revealed a bactericidal activity for IQG-607. HPLC and voltammetric methods were developed to quantify IQG-607. Pharmacokinetic studies showed short half-life, high clearance, moderate volume of distribution, and low oral bioavailability, which was not altered by feeding. Safety and toxic effects of IQG-607 after acute and 90-day repeated oral administrations in both rats and minipigs showed occurrence of mild to moderate toxic events. Eight multidrug-resistant strains (MDR-TB) were resistant to IQG-607, suggesting an association between katG mutation and increasing MIC values. Whole genome sequencing of three spontaneous IQG-607-resistant strains harbored katG gene mutations. MIC measurements and macrophage infection experiments with a laboratorial strain showed that katG mutation is sufficient to confer resistance to IQG-607 and that the macrophage intracellular environment cannot trigger the self-activation mechanism. Reduced activity of IQG-607 against an M. tuberculosis strain overexpressing S94A InhA mutant protein suggested both the need for KatG activation and InhA as its target. Further efforts are suggested to be pursued toward attempting to translate IQG-607 into a chemotherapeutic agent to treat tuberculosis.

Vesicles Functionalized with a CO-Releasing Molecule for Light-Induced CO Delivery

In this paper, a new type of methodology to deliver carbon monoxide (CO) for biological applications has been introduced. An amphiphilic manganese carbonyl complex (1.Mn) incorporated into the 1,2-distearoyl-sn-glycero-3-phosphocholine lipid vesicles has been reported first time for the photoinduced release of CO. The liposomes (Ves-1.Mn) gradually released CO under light at 365 nm over a period of 50 min with a half-time of 26.5 min. The CO-releasing ability of vesicles appended with 1.Mn complexes has been confirmed by myoglobin assay and infrared study. The vesicles appended with 1.Mn have the advantages of biocompatibility, water solubility, and steady and slow CO release. This approach could be a rational approach for applying various water-insoluble photoinduced CO donors in aqueous media by using vesicles as a nanocarrier for CO release.

Diiron Hexacarbonyl Complex Induces Site-Specific Release of Carbon Monoxide in Cancer Cells Triggered by Endogenous Glutathione

In this study, we have evaluated a water-soluble, nontarget reagent and a carrier-free diiron hexacarbonyl complex, [Fe₂{μ-SCH₂CH(OH)CH₂(OH)}₂(CO)₆] (TG-FeCORM), that can induce the site-specific release of carbon monoxide (CO) in cancer cells triggered by endogenous glutathione (GSH). The releasing rate of CO was dependent on the amount of endogenous GSH. Being the amount of endogenous GSH higher in cancer cells than in normal cells, the CO-releasing rate resulted faster in cancer cells. Moreover, the anti-inflammatory properties related to the intracellular CO release of TG-FeCORM were also confirmed in the living HeLa cells.

Mono- and bimetallic manganese-carbonyl complexes and clusters bearing imidazol(in)ium-2-dithiocarboxylate ligands

Five complexes with the generic formula fac-[MnBr(CO)₃(S₂C·NHC)] were obtained by reacting [MnBr(CO)₅] with a set of representative imidazol(in)ium-2-dithiocarboxylate zwitterions. These ligands are the adducts of N-heterocyclic carbenes (NHCs) and carbon disulfide. The mononuclear Mn(i) derivatives were coupled with Na[Mn(CO)₅] to afford bimetallic [Mn₂(CO)₆(S₂C·NHC)] clusters. Yet, the most convenient strategy to access these dinuclear Mn(0) products implied a direct carbonyl substitution from the [Mn₂(CO)₁₀] dimer. The molecular structures of three monometallic and four bimetallic compounds were elucidated by single crystal X-ray diffraction analysis. In the monometallic complexes, the NHC·CS₂ ligands exhibited a bidentate κ²-S,S' coordination mode with an S-C-S bite angle of about 116°. In the dinuclear clusters, the CS₂^- unit acted as a chelate toward one manganese center and as a pseudoallylic ligand toward the other one. The S-C-S bite angle was reduced to ca. 104°. Thus, the zwitterions displayed a remarkable flexibility, which also permitted a staggered arrangement of the carbonyl groups in the bimetallic systems. Examination of the [small nu, Greek, macron]_CO absorption bands on IR spectroscopy helped identify the presence of fac-Mn(CO)₃ or Mn₂(CO)₆ motifs, while the ¹³C NMR chemical shift of the CS₂^- moiety was a reliable indicator for monitoring its hapticity. Whereas the dinuclear clusters were air- and moisture-stable crystalline solids, mononuclear halido derivatives displayed only a limited stability under aerobic conditions. Both types of compounds underwent rather unselective, extensive fragmentations in the gas phase, in sharp contrast with the analogous rhenium derivatives that led to clean sequential decarbonylation processes upon collision-induced dissociation.

Acetoxymethyl Concept for Intracellular Administration of Carbon Monoxide with Mn(CO)3 -Based PhotoCORMs

Targeted administration of carbon monoxide with CO releasing molecules (CORMs) inside of cells proved to be very challenging. Consequently, there are only very few reports on intracellular uptake of CORMs requiring high extracellular CORM loading because an equilibrium between extra- and intracellular concentrations can be assumed. Here we present a strategy for a targeted intracellular administration of manganese(I)-based CORMs that are altered inside of cells to trap these complexes. Thereafter, carbon monoxide can be liberated by irradiation (photoCORMs). To achieve this innovative task, acetoxymethyl (AM) groups are attached at the periphery of the hydrophobic manganese(I) carbonyl complexes to not influence the CO release behavior. Inside of cells these AM substituents are cleaved by esterases yielding hydrophilic manganese(I) carbonyl compounds which are captured inside of cells. This objective is realized by using the bidentate bases 4-(acetoxymethoxycarbonyl)phenyl-bis(3,5-dimethylpyrazolyl)methane (1) and 4-(acetoxymethoxy)phenyl-bis(3,5-dimethylpyrazolyl)methane (4) at facial (OC)₃ MnBr fragments yielding CORM-AM1 (2) and CORM-AM2 (5), respectively. Besides synthesis, crystal structures and spectroscopic properties we present targeted administration and intracellular accumulation of these AM-containing CORMs.

Visible-light-activated photoCORMs: rational design of CO-releasing organic molecules absorbing in the tissue-transparent window

Rational design of visible-light-activatable transition-metal-free CO-releasing molecules with an emphasis on mechanistic details of the CO release.

Red Light-Triggered CO Release from Mn2(CO)10 Using Triplet Sensitization in Polymer Nonwoven Fabrics

Applicability of phototherapeutic CO-releasing molecules (photoCORMs) is limited because they are activated by harmful and poorly tissue-penetrating near-ultraviolet light. Here, a strategy is demonstrated to activate classical photoCORM Mn2(CO)10 using red light (635 nm). By mixing in solution a triplet photosensitizer (PS) with the photoCORM and shining red light, energy transfer occurs from triplet excited-state 3PS* to a photolabile triplet state of Mn2(CO)10, which, like under near-UV irradiation, led to complete release of carbonyls. Crucially, such "triplet-sensitized CO-release" occurred in solid-state materials: when PS and Mn2(CO)10 were embedded in electrospun nonwoven fabrics, CO was liberated upon irradiation with low-intensity red light (≤36 mW 635 nm).

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.

Spin-Forbidden Excitation: A New Approach for Triggering Photopharmacological Processes with Low-Intensity NIR Light

Exposure to low-intensity radiation in the near-infrared (NIR) spectral region matching the optically transparent "phototherapeutic window" of biological tissues can be applied to directly populate spin-restricted excited states of light-responsive compounds. This unconventional and unprecedented approach is introduced herein as a new strategy to overcome some of the major unresolved problems observed in the rapidly emerging fields of photopharmacology and molecular photomedicine, where practical applications in living cells and organisms are still limited by undesired side reactions and insufficient light penetration. Water-soluble and biocompatible metal complexes with a significant degree of spin-orbit coupling were identified as target candidates for testing our new hypothesis. As a first example, a dark-stable manganese carbonyl complex acting as a visible-light-triggered CO-releasing molecule (Photo-CORM) is shown to be photoactivated by NIR radiation, although apparently no spectroscopically evident absorption bands are detectable in this low-energy region. This quite remarkable effect is ascribed to a strongly restricted, but obviously not completely forbidden optical population of the lowest triplet excited state manifold of the diamagnetic complex from the singlet ground state.

Light-responsive paper strips as CO-releasing material with a colourimetric response

Carbon monoxide (CO) is known for its multifaceted role in human physiology, and molecules that release CO in a controlled way have been proposed as therapeutic drugs. In this work, a light-responsive CO-releasing molecule (CORM-Dabsyl) showed a strong colourimetric response upon photochemical CO-release, owing to the tight conjugation of a Mn(i) tricarbonyl centre to a dabsyl chromophoric ligand (L). Whereas the complex was very stable in the dark in nitrogen-purged aqueous media, CO-release was effectively triggered using 405 nm irradiation. CORM-Dabsyl, L and the inactive product iCORM-Dabsyl have been investigated by DFT and TD-DFT calculations. Only mild toxicity of CORM-Dabsyl was observed against LX-2 and HepaRG® human cell lines (IC50 ∼ 30 μM). Finally, to develop a CO storage and release material that is readily applicable to therapeutic situations, CORM-Dabsyl was loaded on low-cost and easily disposable paper strips, from which the light triggered CO-release was conveniently visible with the naked eye.

Studies of Carbon Monoxide Release from Ruthenium(II) Bipyridine Carbonyl Complexes upon UV-Light Exposure

The UV-light-induced CO release characteristics of a series of ruthenium(II) carbonyl complexes of the form trans-Cl[RuLCl₂(CO)₂] (L = 4,4'-dimethyl-2,2'-bipyridine, 4'-methyl-2,2'-bipyridine-4-carboxylic acid, or 2,2'-bipyridine-4,4'-dicarboxylic acid) have been elucidated using a combination of UV-vis absorbance and Fourier transform infrared spectroscopies, multivariate curve resolution alternating least-squares analysis, and density functional theory calculations. In acetonitrile, photolysis appears to proceed via a serial three-step mechanism involving the sequential formation of [RuL(CO)(CH₃CN)Cl₂], [RuL(CH₃CN)₂Cl₂], and [RuL(CH₃CN)₃Cl]⁺. Release of the first CO molecule occurs quickly (k₁ ≫ 3 min^-1), while release of the second CO molecule proceeds at a much more modest rate (k₂ = 0.099-0.17 min^-1) and is slowed by the presence of electron-withdrawing carboxyl substituents on the bipyridine ligand. In aqueous media (1% dimethyl sulfoxide in H₂O), the two photodecarbonylation steps proceed much more slowly (k₁ = 0.46-1.3 min^-1 and k₂ = 0.026-0.035 min^-1, respectively) and the influence of the carboxyl groups is less pronounced. These results have implications for the design of new light-responsive CO-releasing molecules ("photoCORMs") intended for future medical use.

Co-Registered Molecular Logic Gate with a CO-Releasing Molecule Triggered by Light and Peroxide

Co-registered molecular logic gates combine two different inputs and outputs, such as light and matter. We introduce a biocompatible CO-releasing molecule (CORM, A) as Mn(I) tricarbonyl complex with the ligand 5-(dimethylamino)-N, N-bis(pyridin-2-ylmethyl) naphthalene-1-sulfonamide (L). CO release is chaperoned by turn-on fluorescence and can be triggered by light (405 nm) as well as with hydrogen peroxide in aqueous phosphate buffer. Complex A behaves as a logic "OR" gate via co-registering the inputs of irradiation (light) and peroxide (matter) into the concomitant outputs fluorescence (light) and CO (matter). Cell viability assays confirm the low toxicity of A toward different human cell lines. The CORM has been used to track the inclusion of A into cancer cells.

pH-Sensitive metal-free carbon monoxide prodrugs with tunable and predictable release rates

Carbon monoxide prodrugs with triggered release profiles are highly desirable for targeted CO delivery to minimize their untoward side-effects.

Nonmetallic carbon monoxide releasing molecules (CORMs)

Recent progress on nonmetallic carbon monoxide releasing molecules (CORMs) is reviewed.

Core–shell materials bearing iron(ii) carbonyl units and their CO-release via an upconversion process

A core–shell nanoplatform was constructed with upconversion nanomaterials onto which iron carbonyl units were chemically loaded. The materials with excellent biocompatibility release CO upon irradiation with a NIR laser.

Properties of a flavonol-based photoCORM in aqueous buffered solutions: influence of metal ions, surfactants and proteins on visible light-induced CO release

A flavonol-based photoCORM exhibits reliable visible light-induced CO release in aqueous buffer environments containing constituents of relevance to biological environments.