Visible-Light-Induced CO Release from a Therapeutically Viable Tryptophan-Derived Manganese(I) Carbonyl (TryptoCORM) Exhibiting Potent Inhibition againstE. coli

Discovery of antibacterial manganese(i) tricarbonyl complexes through combinatorial chemistry

The continuous rise of antimicrobial resistance is a serious threat to human health and already causing hundreds of thousands of deaths each year. While natural products and synthetic organic small molecules have provided the majority of our current antibiotic arsenal, they are falling short in providing new drugs with novel modes of action able to treat multidrug resistant bacteria. Metal complexes have recently shown promising results as antimicrobial agents, but the number of studied compounds is still vanishingly small, making it difficult to identify promising compound classes or elucidate structure-activity relationships. To accelerate the pace of discovery we have applied a combinatorial chemistry approach to the synthesis of metalloantibiotics. Utilizing robust Schiff-base chemistry and combining 7 picolinaldehydes with 10 aniline derivatives, and 6 axial ligands, either imidazole/pyridine-based or solvent, we have prepared a library of 420 novel manganese tricarbonyl complexes. All compounds were evaluated for their antibacterial properties and 10 lead compounds were identified, re-synthesised and fully characterised. All 10 compounds showed high and broad activity against Gram-positive bacteria. The best manganese complex displayed low toxicity against human cells with a therapeutic index of >100. In initial mode of action studies, we show that it targets the bacterial membrane without inducing pore formation or depolarisation. Instead, it releases its carbon monoxide ligands around the membrane and inhibits the bacterial respiratory chain. This work demonstrates that large numbers of metal complexes can be accessed through combinatorial synthesis and evaluated for their antibacterial potential, allowing for the rapid identification of promising metalloantibiotic lead compounds.

Harnessing Transition Metal Scaffolds for Targeted Antibacterial Therapy

Antimicrobial resistance, caused by persistent adaptation and growing resistance of pathogenic bacteria to overprescribed antibiotics, poses one of the most serious and urgent threats to global public health. The limited pipeline of experimental antibiotics in development further exacerbates this looming crisis and new drugs with alternative modes of action are needed to tackle evolving pathogenic adaptation. Transition metal complexes can replenish this diminishing stockpile of drug candidates by providing compounds with unique properties that are not easily accessible using pure organic scaffolds. We spotlight four emerging strategies to harness these unique properties to develop new targeted antibacterial agents.

Type-I Photosensitizer-Triggered Controllable Carbon Monoxide Release for Effective Treatment of Staph Skin Infection

Staphylococcus aureus (S. aureus) infection is a major infectious skin disease that is highly resistant to conventional antibiotic treatment and host immune defense, leading to recurrence and exacerbation of bacterial infection. Herein, we developed a photoresponsive carbon monoxide (CO)-releasing nanocomposite by integrating anion-π+ type-I photosensitizer (OMeTBP) and organometallic complex (FeCO) for the treatment of planktonic S. aureus and biofilm-associated infections. After optimizing the molar ratio of FeCO and OMeTBP, the prepared nanoparticles, OMeTBP@FeCONPs, not only ensured sufficient loading of CO donors and efficient CO generation but also showed negligible free ROS leakage under light irradiation, which helped to avoid tissue damage caused by excessive ROS. Both in vitro and in vivo results demonstrated that OMeTBP@FeCONPs could effectively inhibit S. aureus methicillin-resistant S. aureus (MRSA), and bacterial biofilm. Our design has the potential to overcome the resistance of conventional antibiotic treatment and provide a more effective option for bacterial infections.

The Triple Crown: NO, CO, and H2S in cancer cell biology

Nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) are three endogenously produced gases with important functions in the vasculature, immune defense, and inflammation. It is increasingly apparent that, far from working in isolation, these three exert many effects by modulating each other's activity. Each gas is produced by three enzymes, which have some tissue specificities and can also be non-enzymatically produced by redox reactions of various substrates. Both NO and CO share similar properties, such as activating soluble guanylate cyclase (sGC) to increase cyclic guanosine monophosphate (cGMP) levels. At the same time, H2S both inhibits phosphodiesterase 5A (PDE5A), an enzyme that metabolizes sGC and exerts redox regulation on sGC. The role of NO, CO, and H2S in the setting of cancer has been quite perplexing, as there is evidence for both tumor-promoting and pro-inflammatory effects and anti-tumor and anti-inflammatory activities. Each gasotransmitter has been found to have dual effects on different aspects of cancer biology, including cancer cell proliferation and apoptosis, invasion and metastasis, angiogenesis, and immunomodulation. These seemingly contradictory actions may relate to each gas having a dual effect dependent on its local flux. In this review, we discuss the major roles of NO, CO, and H2S in the context of cancer, with an effort to highlight the dual nature of each gas in different events occurring during cancer progression.

Water-Soluble Carbon Monoxide-Releasing Molecules (CORMs)

Carbon monoxide-releasing molecules (CORMs) are promising candidates for producing carbon monoxide in the mammalian body for therapeutic purposes. At higher concentrations, CO has a harmful effect on the mammalian organism. However, lower doses at a controlled rate can provide cellular signaling for mandatory pharmacokinetic and pathological activities. To date, exploring the therapeutic implications of CO dose as a prodrug has attracted much attention due to its therapeutic significance. There are two different methods of CO insertion, i.e., indirect and direct exogenous insertion. Indirect exogenous insertion of CO suggests an advantage of reduced toxicity over direct exogenous insertion. For indirect exogenous insertion, researchers are facing the issue of tissue selectivity. To solve this issue, developers have considered the newly produced CORMs. Herein, metal carbonyl complexes (MCCs) are covalently linked with CO molecules to produce different CORMs such as CORM-1, CORM-2, and CORM-3, etc. All these CORMs required exogenous CO insertion to achieve the therapeutic targets at the optimized rate under peculiar conditions or/and triggering. Meanwhile, the metal residue was generated from i-CORMs, which can propagate toxicity. Herein, we explain CO administration, water-soluble CORMs, tissue accumulation, and cytotoxicity of depleted CORMs and the kinetic profile of CO release.

Synergetic Antimicrobial Activity and Mechanism of Clotrimazole-Linked CO-Releasing Molecules

Several metal-based carbon monoxide-releasing molecules (CORMs) are active CO donors with established antibacterial activity. Among them, CORM conjugates with azole antibiotics of type [Mn(CO)₃(2,2′-bipyridyl)(azole)]⁺ display important synergies against several microbes. We carried out a structure–activity relationship study based upon the lead structure of [Mn(CO)₃(Bpy)(Ctz)]⁺ by producing clotrimazole (Ctz) conjugates with varying metal and ligands. We concluded that the nature of the bidentate ligand strongly influences the bactericidal activity, with the substitution of bipyridyl by small bicyclic ligands leading to highly active clotrimazole conjugates. On the contrary, the metal did not influence the activity. We found that conjugate [Re(CO)₃(Bpy)(Ctz)]⁺ is more than the sum of its parts: while precursor [Re(CO)₃(Bpy)Br] has no antibacterial activity and clotrimazole shows only moderate minimal inhibitory concentrations, the potency of [Re(CO)₃(Bpy)(Ctz)]⁺ is one order of magnitude higher than that of clotrimazole, and the spectrum of bacterial target species includes Gram-positive and Gram-negative bacteria. The addition of [Re(CO)₃(Bpy)(Ctz)]⁺ to Staphylococcus aureus causes a general impact on the membrane topology, has inhibitory effects on peptidoglycan biosynthesis, and affects energy functions. The mechanism of action of this kind of CORM conjugates involves a sequence of events initiated by membrane insertion, followed by membrane disorganization, inhibition of peptidoglycan synthesis, CO release, and break down of the membrane potential. These results suggest that conjugation of CORMs to known antibiotics may produce useful structures with synergistic effects that increase the conjugate’s activity relative to that of the antibiotic alone.

Recent Advances on Carbon Monoxide Releasing Molecules for Antibacterial Applications

Carbon monoxide (CO) has been known as an endogenous signaling molecule in addition to an air pollutant. It plays a critical role in many physiological and pathological processes. Therefore, CO has been recognized as a potent therapeutic agent for the treatment of numerous diseases such as cancers, rheumatoid arthritis, and so on. Instead of direct CO inhalation, two main categories of CO-releasing molecules (CORMs) (i. e., metal carbonyls and nonmetallic CO donors) have been developed to safely and locally deliver CO to target tissues. In this minireview, we summarize the recent achievements of CORMs on antibacterial applications. It appears that the antibacterial activity of CORMs is different from CO gas, which is tightly correlated to not only the types of CORMs applied but also the tested bacterial strains. In some circumstances, the antibacterial mechanisms are debated and need to be clarified. We hope more attention can be paid to this emerging field and new antibacterial agents with a low risk of drug resistance can be developed.

Red Light-Triggered Intracellular Carbon Monoxide Release Enables Selective Eradication of MRSA Infection

Carbon monoxide (CO) is an important gaseous signaling molecule. The use of CO-releasing molecules such as metal carbonyls enables the elucidation of the pleiotropic functions of CO. Although metal carbonyls show a broad-spectrum antimicrobial activity, it remains unclear whether the bactericidal property originates from the transition metals or the released CO. Here, we develop nonmetallic CO-releasing micelles via a photooxygenation mechanism of 3-hydroxyflavone derivatives, enabling CO release under red light irradiation (e.g., 650 nm). Unlike metal carbonyls that non-specifically internalize into both Gram-positive and Gram-negative bacteria, the nonmetallic micelles are selectively taken up by S. aureus instead of E. coli cells, exerting a selective bactericidal effect. Further, we demonstrate that the CO-releasing micelles can cure methicillin-resistant S. aureus (MRSA)-infected wounds, simultaneously eradicating MRSA pathogens and accelerating wound healing.

Hydrogen Sulfide and Carbon Monoxide Tolerance in Bacteria

Hydrogen sulfide and carbon monoxide share the ability to be beneficial or harmful molecules depending on the concentrations to which organisms are exposed. Interestingly, humans and some bacteria produce small amounts of these compounds. Since several publications have summarized the recent knowledge of its effects in humans, here we have chosen to focus on the role of H2S and CO on microbial physiology. We briefly review the current knowledge on how bacteria produce and use H2S and CO. We address their potential antimicrobial properties when used at higher concentrations, and describe how microbial systems detect and survive toxic levels of H2S and CO. Finally, we highlight their antimicrobial properties against human pathogens when endogenously produced by the host and when released by external chemical donors.

Development of photoactivable phenanthroline-based manganese(I) CO-Releasing molecules (PhotoCORMs) active against ESKAPE bacteria and bacterial biofilms

The synthesis and biological evaluation of a series of phenanthroline-based visible-light-activated manganese(I) carbon-monoxide-releasing molecules (PhotoCORMs) against ESKAPE bacteria and bacterial biofilms is reported. Four carbonyl compounds of general formula fac-[Mn(N^∧N)(CO)₃(L)] have been synthesized and characterized. Despite being thermally stable in the absence of light, these PhotoCORMs readily release CO upon blue (435-450 nm) LED light irradiation as confirmed by spectrophotometric CO releasing experiments (Mb Assay). The antibacterial activity of the four PhotoCORMs has been investigated against a panel of ESKAPE bacteria. The compounds 1-3 were found to be effective antibacterials at low concentrations against multidrug-resistant Klebsiella pneumoniae and Acinetobacter baumannii when photoactivated with blue-light. In addition, the PhotoCORMs 1-2 were found to inhibit the formation of Klebsiella pneumoniae and Acinetobacter baumannii bacterial biofilms at low concentrations (MIC = 4-8 μg/mL), turning out to be promising candidates to combat antimicrobial resistance. The antibacterial and biofilm inhibitory effect of the PhotoCORMs is plausibly due to the release of CO as well as the formation of phenanthroline photo-by-products as revealed by spectroscopy and microbiology experiments.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Diversification of quinoline-triazole scaffolds with CORMs: Synthesis, in vitro and in silico biological evaluation against Plasmodium falciparum

A discrete series of tricarbonyl manganese and rhenium complexes conjugated to a quinoline-triazole hybrid scaffold were synthesised and their inhibitory activities evaluated against Plasmodium falciparum. In general, the complexes show moderate activity with improved inhibitory activities for the photoactivatable manganese(I) tricarbonyl complexes in the malaria parasite. All complexes are active in the dark against the NF54 CQS (chloroquine-sensitive) and K1 MDR (multidrug-resistant) strains of Plasmodium falciparum, with IC₅₀ values in the low micromolar range. Of significance, the complexes retain their activity in the MDR strain with resistance indices ranging between 1.1 and 2.1. The Mn(I) analogues display photodissociation of all three CO ligands upon irradiation at 365 nm. More importantly, the complexes show increased antimalarial activity in vitro upon photoactivation, something not observed by the clinically used reference drug, chloroquine. As a purported mechanism of action, the compounds were evaluated as β-haematin inhibitors. To further understand the interactions of the complexes, in silico hemozoin docking simulations were performed, attesting to the fact that CO-release could be vital for blocking the hemozoin formation pathway. These results show that this strategy may be a valuable, novel route to design antimalarial agents with higher efficacy.

Phototherapy-based combination strategies for bacterial infection treatment

The development of nanomedicine is expected to provide an innovative direction for addressing challenges associated with multidrug-resistant (MDR) bacteria. In the past decades, although nanotechnology-based phototherapy has been developed for antimicrobial treatment since it rarely causes bacterial resistance, the clinical application of single-mode phototherapy has been limited due to poor tissue penetration of light sources. Therefore, combinatorial strategies are being developed. In this review, we first summarized the current phototherapy agents, which were classified into two functional categories: organic phototherapy agents (e.g., small molecule photosensitizers, small molecule photosensitizer-loaded nanoparticles and polymer-based photosensitizers) and inorganic phototherapy agents (e.g., carbo-based nanomaterials, metal-based nanomaterials, composite nanomaterials and quantum dots). Then the development of emerging phototherapy-based combinatorial strategies, including combination with chemotherapy, combination with chemodynamic therapy, combination with gas therapy, and multiple combination therapy, are presented and future directions are further discussed. The purpose of this review is to highlight the potential of phototherapy to deal with bacterial infections and to propose that the combination therapy strategy is an effective way to solve the challenges of single-mode phototherapy.

Direct Measurement of the Visible to UV Photodissociation Processes for the PhotoCORM TryptoCORM

PhotoCORMs are light-triggered compounds that release CO for medical applications. Here, we apply laser spectroscopy in the gas phase to TryptoCORM, a known photoCORM that has been shown to destroy Escherichia coli upon visible-light activation. Our experiments allow us to map TryptoCORM's photochemistry across a wide wavelength range by using novel laser-interfaced mass spectrometry (LIMS). LIMS provides the intrinsic absorption spectrum of the photoCORM along with the production spectra of all of its ionic photoproducts for the first time. Importantly, the photoproduct spectra directly reveal the optimum wavelengths for maximizing CO ejection, and the extent to which CO ejection is compromised at redder wavelengths. A series of comparative studies were performed on TryptoCORM-CH3 CN which exists in dynamic equilibrium with TryptoCORM in solution. Our measurements allow us to conclude that the presence of the labile CH3 CN facilitates CO release over a wider wavelength range. This work demonstrates the potential of LIMS as a new methodology for assessing active agent release (e.g. CO, NO, H2 S) from light-activated prodrugs.

Biological activity of manganese(i) tricarbonyl complexes on multidrug-resistant Gram-negative bacteria: From functional studies to in vivo activity in Galleria mellonella

Three new manganese(i) tricarbonyl complexes [Mn(bpqa-κ3N)(CO)3]Br, [Mn(bqpa-κ3N)(CO)3]Br, and [Mn(CO)3(tqa-κ3N)]Br as well as the previously described compound [Mn(CO)3(tpa-κ3N)]Br with bpqa = bis(2-pyridinylmethyl)(2-quinolinylmethyl)amine, bqpa = bis(2-quinolinylmethyl)(2-pyridinylmethyl)amine, tqa = tris(2-quinolinylmethyl)amine, and tpa = tris(2-pyridinylmethyl)amine were examined for their antibacterial activities on 14 different multidrug-resistant clinical isolates of Acinetobacter baumannii and Pseudomonas aeruginosa, in recognition of the current antimicrobial resistance (AMR) concerns with these pathogens. Minimal inhibitory concentrations (MIC) of the most potent tqa compound were in the mid-micromolar range and generally lower than that of the free ligand. Activity against both bacterial species increased with the number of quinolinylmethyl groups and lipophilicity in the order of tpa < bpqa < bqpa ≈ tqa, consistent with measured increases in release of ATP, a uniquely cytoplasmic biomolecule and induced permeability to exogenous fluorescent intercalating compounds. [Mn(CO)3(tqa-κ3N)]Br was also evaluated in the Galleria mellonella model of infection, and displayed a lack of host toxicity combined with effective bacterial clearance.

Ultra-efficient Antibacterial System Based on Photodynamic Therapy and CO Gas Therapy for Synergistic Antibacterial and Ablation Biofilms

In recent years, with the emergence of various kinds of drug-resistant bacteria, existing antibiotics have become inefficient in killing these bacteria, and the formation of biofilms has further weakened the therapeutic effect. More problematically, the massive use and abuse of antibiotics have caused severe side effects. Thus, the development of ultra-efficient and safe antibacterial systems is urgently needed. Herein, a photodynamic therapy (PDT)-driven CO-controlled delivery system (Ce6&CO@FADP) is developed for synergistic antibacterial and ablation biofilms. Ce6&CO@FADP is constructed using a fluorinated amphiphilic dendritic peptide (FADP) and physically loaded with Ce6 and CORM-401. After efficiently entering the bacteria, Ce6&CO@FADP can rapidly release CO intracellularly by the massive consumption of the H₂O₂ generated during the PDT process, without affecting the generation of singlet oxygen (¹O₂). As such, the combination of CO and ¹O₂ exerts notable synergistic antibacterial and biofilm ablation effects both in vitro and in vivo (including subcutaneous bacterial infection and biofilm catheter models) experiments. More importantly, all biosafety assessments suggest the good biocompatibility of Ce6&CO@FADP. Together, these results reveal that Ce6&CO@FADP is an efficient and safe antibacterial system, which has essential application prospects for the treatment of bacterial infections and ablation of biofilms in vivo.

Recent Studies on the Antimicrobial Activity of Transition Metal Complexes of Groups 6–12

Antimicrobial resistance is an increasingly serious threat to global public health that requires innovative solutions to counteract new resistance mechanisms emerging and spreading globally in infectious pathogens. Classic organic antibiotics are rapidly exhausting the structural variations available for an effective antimicrobial drug and new compounds emerging from the industrial pharmaceutical pipeline will likely have a short-term and limited impact before the pathogens can adapt. Inorganic and organometallic complexes offer the opportunity to discover and develop new active antimicrobial agents by exploiting their wide range of three-dimensional geometries and virtually infinite design possibilities that can affect their substitution kinetics, charge, lipophilicity, biological targets and modes of action. This review describes recent studies on the antimicrobial activity of transition metal complexes of groups 6–12. It focuses on the effectiveness of the metal complexes in relation to the rich structural chemical variations of the same. The aim is to provide a short vade mecum for the readers interested in the subject that can complement other reviews.

Metal Complexes, an Untapped Source of Antibiotic Potential?

With the widespread rise of antimicrobial resistance, most traditional sources for new drug compounds have been explored intensively for new classes of antibiotics. Meanwhile, metal complexes have long had only a niche presence in the medicinal chemistry landscape, despite some compounds, such as the anticancer drug cisplatin, having had a profound impact and still being used extensively in cancer treatments today. Indeed, metal complexes have been largely ignored for antibiotic development. This is surprising as metal compounds have access to unique modes of action and exist in a wider range of three-dimensional geometries than purely organic compounds. These properties make them interesting starting points for the development of new drugs. In this perspective article, , the encouraging work that has been done on antimicrobial metal complexes, mainly over the last decade, is highlighted. Promising metal complexes, their activity profiles, and possible modes of action are discussed and issues that remain to be addressed are emphasized.

A biotin-conjugated photo-activated CO-releasing molecule (biotinCORM): efficient CO-release from an avidin–biotinCORM protein adduct

A biotinylated carbon monoxide-releasing molecule (BiotinCORM) releases CO by photoirradiation at 400 nm; an avidin–biotinCORM adduct is an effective CO-releasing molecule.

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.

Ultrafast Intersystem Crossing vs Internal Conversion in α-Diimine Transition Metal Complexes: Quantum Evidence

Whereas third row transition metal carbonyl α-diimine complexes display luminescent properties and possess low-lying triplet metal-to-ligand charge transfer (MLCT) states efficiently accessible by a spin-vibronic mechanism, first row analogues hold low-lying metal-centered (MC) excited states that could quench these properties. Upon visible irradiation, different functions are potentially stimulated, namely, luminescence, electron transfer, or photoinduced CO release, the branching ratio of which is governed by the energetics, the character, and the early time dynamics of the photoactive excited states. Simulations of ultrafast nonadiabatic quantum dynamics, including spin-vibronic effects, of [M(imidazole)(CO)₃(phenanthroline)]⁺ (M = Mn, Re) highlight the role of the metal atom. An ultrafast intersystem crossing process, driven by spin-orbit coupling, populates the low-lying triplet states of [Re(imidazole)(CO)₃(phen)]⁺ within the first tens of fs. In contrast, efficient internal conversion between the two lowest ¹MLCT states of [Mn(imidazole)(CO)₃(phen)]⁺ is mediated within 50 fs by vibronic coupling with upper MC and MLCT states.

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.

The Antimicrobial Activity of a Carbon Monoxide Releasing Molecule (EBOR-CORM-1) Is Shaped by Intraspecific Variation within Pseudomonas aeruginosa Populations

Carbon monoxide releasing molecules (CORMs) have been suggested as a new synthetic class of antimicrobials to treat bacterial infections. Here we utilized a novel EBOR-CORM-1 ([NEt4][MnBr2(CO)4]) capable of water-triggered CO-release, and tested its efficacy against a collection of clinical Pseudomonas aeruginosa strains that differ in infection-related virulence traits. We found that while EBOR-CORM-1 was effective in clearing planktonic and biofilm cells of P. aeruginosa strain PAO1 in a concentration dependent manner, this effect was less clear and varied considerably between different P. aeruginosa cystic fibrosis (CF) lung isolates. While a reduction in cell growth was observed after 8 h of CORM application, either no effect or even a slight increase in cell densities and the amount of biofilm was observed after 24 h. This variation could be partly explained by differences in bacterial virulence traits: while CF isolates showed attenuated in vivo virulence and growth compared to strain PAO1, they formed much more biofilm, which could have potentially protected them from the CORM. Even though no clear therapeutic benefits against a subset of isolates was observed in an in vivo wax moth acute infection model, EBOR-CORM-1 was more efficient at reducing the growth of CF isolate co-culture populations harboring intraspecific variation, in comparison with efficacy against more uniform single isolate culture populations. Together these results suggest that CORMs could be effective at controlling genetically diverse P. aeruginosa populations typical for natural chronic CF infections and that the potential benefits of some antibiotics might not be observed if tested only against clonal bacterial populations.

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.

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.

Toxicity of tryptophan manganese(i) carbonyl (Trypto-CORM), against Neisseria gonorrhoeae

The potential for carbon monoxide-releasing molecules (CO-RMs) as antimicrobials represents an exciting prospective in the fight against antibiotic resistance. Trypto-CORM, a tryptophan-containing manganese(i) carbonyl, is toxic against E. coli following photo-activation. Here, we demonstrate that Trypto-CORM is toxic against Neisseria gonorrhoeae in the absence of photoactivation. Trypto-CORM toxicity was reversed by the high CO affinity globin leg-haemoglobin (Leg-Hb), indicating that the toxicity is due to CO release. Release of CO from Trypto-CORM in the dark was also detected with Leg-Hb (but not myoglobin) in vitro. N. gonorrhoeae is more sensitive to CO-based toxicity than other model bacterial pathogens, and may serve as a viable candidate for antimicrobial therapy using CO-RMs.

Examining the antimicrobial activity and toxicity to animal cells of different types of CO-releasing molecules

Transition metal carbonyl complexes used as CO-releasing molecules (CORMs) for biological and therapeutic applications may exhibit interesting antimicrobial activity. However, understanding the chemical traits and mechanisms of action that rule this activity is required to establish a rationale for the development of CORMs into useful antibiotics. In this work the bactericidal activity, the toxicity to eukaryotic cells, and the ability of CORMs to deliver CO to bacterial and eukaryotic cells were analysed for a set of seven CORMs that differ in the transition metal, ancillary ligands and the CO release profile. Most of these CORMs exhibited bactericidal properties that decrease in the following order: CORM-2 > CORM-3 > ALF062 > ALF850 > ALF186 > ALF153 > [Fe(SBPy3)(CO)](BF4)2. A similar yet not entirely coincident decreasing order was found for their induction of intracellular reactive oxygen species (ROS) in E. coli. In contrast, studies in model animal cells showed that for any given CORM, the level of intracellular ROS generated was negligible when compared with that measured inside bacteria. Importantly, these CORMs were in general not toxic to eukaryotic cells, namely murine macrophages, kidney LLC-PK1 epithelial cells, and liver cell line HepG2. CORM-2 and CORM-3 delivered CO to the intracellular space of both E. coli and the two types of tested eukaryotic cells, yet toxicity was only elicited in the case of E. coli. CO delivered by ALF186 into the intercellular space did not enter E. coli cells and the compound was not toxic to either bacteria or to eukaryotic cells. The Fe(ii) carbonyl complex [Fe(SBPy3)(CO)](2+) had the reverse, undesirable toxicity profile, being unexpectedly toxic to eukaryotic cells and non-toxic to E. coli. ALF153, the most stable complex in the whole set, was essentially devoid of toxicity or ROS induction ability in all cells. These results suggest that CORMs have a relevant therapeutic potential as antimicrobial drugs since (i) they can show opposite toxicity profiles towards bacteria and eukaryotic cells; (ii) their activity can be modulated through manipulation of the ancillary ligands, as shown with the three {Ru(CO)3}(2+) and two zerovalent Mo based CORMs; and (iii) their toxicity to eukaryotic cells can be made acceptably low. With this new approach, this work contributes to the understanding of the roots of the bactericidal action of CORMs and helps in establishing strategies for their development into a new class of antibiotics.

Antimicrobial Activity of the Manganese Photoactivated Carbon Monoxide-Releasing Molecule [Mn(CO)3(tpa-κ(3)N)](+) Against a Pathogenic Escherichia coli that Causes Urinary Infections

AIMS

We set out to investigate the antibacterial activity of a new Mn-based photoactivated carbon monoxide-releasing molecule (PhotoCORM, [Mn(CO)3(tpa-κ(3)N)](+)) against an antibiotic-resistant uropathogenic strain (EC958) of Escherichia coli.

RESULTS

Activated PhotoCORM inhibits growth and decreases viability of E. coli EC958, but non-illuminated carbon monoxide-releasing molecule (CORM) is without effect. NADH-supported respiration rates are significantly decreased by activated PhotoCORM, mimicking the effect of dissolved CO gas. CO from the PhotoCORM binds to intracellular targets, namely respiratory oxidases in strain EC958 and a bacterial globin heterologously expressed in strain K-12. However, unlike previously characterized CORMs, the PhotoCORM is not significantly accumulated in cells, as deduced from the cellular manganese content. Activated PhotoCORM reacts avidly with hydrogen peroxide producing hydroxyl radicals; the observed peroxide-enhanced toxicity of the PhotoCORM is ameliorated by thiourea. The PhotoCORM also potentiates the effect of the antibiotic, doxycycline.

INNOVATION

The present work investigates for the first time the antimicrobial activity of a light-activated PhotoCORM against an antibiotic-resistant pathogen. A comprehensive study of the effects of the PhotoCORM and its derivative molecules upon illumination is performed and mechanisms of toxicity of the activated PhotoCORM are investigated.

CONCLUSION

The PhotoCORM allows a site-specific and time-controlled release of CO in bacterial cultures and has the potential to provide much needed information on the generality of CORM activities in biology. Understanding the mechanism(s) of activated PhotoCORM toxicity will be key in exploring the potential of this and similar compounds as antimicrobial agents, perhaps in combinatorial therapies with other agents. Antioxid. Redox Signal. 24, 765-780.

Red-light activated photoCORMs of Mn(i) species bearing electron deficient 2,2′-azopyridines

A family of carbonyl Mn(i) complexes capable of releasing CO when triggered with red light (≥ 625 nm) is reported. The species are activated by irradiation of their MLCT bands in the 630–690 nm range.

Photoinduced Carbon Monoxide Release from Half-Sandwich Iron(II) Carbonyl Complexes by Visible Irradiation: Kinetic Analysis and Mechanistic Investigation

Three half-sandwich iron(II) complexes, [Fe(η(5) -Cp)(cis-CO)2 X] (X(-) =Cl(-) , Br(-) , I(-) ), were synthesized and characterized. The kinetics of the CO-releasing behaviour of these complexes upon illumination by visible irradiation in various media was investigated. Our results indicated that the CO release was significantly affected by the auxiliary ligands. Of the three light sources used (blue, green, and red), blue light exhibited the highest efficiency. In the photoinduced CO release, the solvents and exogenous nucleophiles in the media were involved, which allowed their CO-releasing reaction to comply with pseudo first-order model rather than the characteristic zero-order model for a photochemical reaction. In aqueous media (D2 O), an intermediate bearing the core of {Fe(II) (cis-CO)2 } involving cleavage of cyclopentadiene was detected. Despite the non-absorption of the red light, its illumination combined with nucleophilic substitution did cause considerable CO release. Assessment of the cytotoxicity of the three complexes indicated that they showed good biocompatibility.

CO-releasing Metal Carbonyl Compounds as Antimicrobial Agents in the Post-antibiotic Era

The possibility of a “post-antibiotic era” in the 21st century, in which common infections may kill, has prompted research into radically new antimicrobials. CO-releasing molecules (CORMs), mostly metal carbonyl compounds, originally developed for therapeutic CO delivery in animals, are potent antimicrobial agents. Certain CORMs inhibit growth and respiration, reduce viability, and release CO to intracellular hemes, as predicted, but their actions are more complex, as revealed by transcriptomic datasets and modeling. Progress is hindered by difficulties in detecting CO release intracellularly, limited understanding of the biological chemistry of CO reactions with non-heme targets, and the cytotoxicity of some CORMs to mammalian cells.

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.

Quantum-CORMs: quantum dot sensitized CO releasing molecules

Quantum dot sensitized photoactive CORMs show a 2 to 6-fold increase of the photodecomposition rate upon irradiation with visible light.