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

Modulation of Amyloidogenic Peptide Aggregation by Photoactivatable CO-Releasing Ruthenium(II) Complexes

Three Ru(II)-based CO-releasing molecules featuring bidentate benzimidazole and terpyridine derivatives as ligands were investigated for their ability to modulate the aggregation process of the second helix of the C-terminal domain of nucleophosmin 1, namely nucleophosmin 1 (NPM1)_264-277, a model amyloidogenic system, before and after irradiation at 365 nm. Thioflavin T (ThT) binding assays and UV/Vis absorption spectra indicate that binding of the compounds to the peptide inhibits its aggregation and that the inhibitory effect increases upon irradiation (half maximal effective concentration (EC₅₀) values in the high micromolar range). Electrospray ionization mass spectrometry data of the peptide in the presence of one of these compounds confirm that the modulation of amyloid aggregation relies on the formation of adducts obtained when the Ru compounds react with the peptide upon releasing of labile ligands, like chloride and carbon monoxide. This mechanism of action explains the subtle different behavior of the three compounds observed in ThT experiments. Overall, data support the hypothesis that metal-based CO releasing molecules can be used to develop metal-based drugs with potential application as anti-amyloidogenic agents.

Aging and Progression of Beta-Amyloid Pathology in Alzheimer's Disease Correlates with Microglial Heme-Oxygenase-1 Overexpression

Neuroinflammation and oxidative stress are being recognized as characteristic hallmarks in many neurodegenerative diseases, especially those that portray proteinopathy, such as Alzheimer’s disease (AD). Heme-oxygenase 1 (HO-1) is an inducible enzyme with antioxidant and anti-inflammatory properties, while microglia are the immune cells in the central nervous system. To elucidate the brain expression profile of microglial HO-1 in aging and AD-progression, we have used the 5xFAD (five familial AD mutations) mouse model of AD and their littermates at different ages (four, eight, 12, and 18 months). Total brain expression of HO-1 was increased with aging and such increase was even higher in 5xFAD animals. In co-localization studies, HO-1 expression was mainly found in microglia vs. other brain cells. The percentage of microglial cells expressing HO-1 and the amount of HO-1 expressed within microglia increased progressively with aging. Furthermore, this upregulation was increased by 2–3-fold in the elder 5xFAD mice. In addition, microglia overexpressing HO-1 was predominately found surrounding beta-amyloid plaques. These results were corroborated using postmortem brain samples from AD patients, where microglial HO-1 was found up-regulated in comparison to brain samples from aged matched non-demented patients. This study demonstrates that microglial HO-1 expression increases with aging and especially with AD progression, highlighting HO-1 as a potential biomarker or therapeutic target for AD.

Enhancing the Activity of Drugs by Conjugation to Organometallic Fragments

Resistance to chemotherapy is a current clinical problem, especially in the treatment of microbial infections and cancer. One strategy to overcome this is to make new derivatives of existing drugs by conjugation to organometallic fragments, either by an appropriate linker, or by direct coordination of the drug to a metal. We illustrate this with examples of conjugated organometallic metallocene sandwich and half-sandwich complexes, RuII and OsII arene, and RhIII and IrIII cyclopentadienyl half-sandwich complexes. Ferrocene conjugates are particularly promising. The ferrocene-chloroquine conjugate ferroquine is in clinical trials for malaria treatment, and a ferrocene-tamoxifen derivative (a ferrocifen) seems likely to enter anticancer trails soon. Several other examples illustrate that organometallic conjugation can restore the activity of drugs to which resistance has developed.

Controlled Release of Reactive Gases: A Tale of Taming Carbon Monoxide

This Personal Account describes the development of air-stable and solid precursors for on-demand release of carbon monoxide. In combination with the development of a two-chamber reactor, COware®, CO liberation can be achieved under safe working conditions, as well as allowing transition metal-mediated carbonylations with stoichiometric carbon monoxide. Particularly appealing is the adaptability of this chemical technology for the preparation of carbon isotope labeled bioactive compounds.

A multifunctional anti-inflammatory drug that can specifically target activated macrophages, massively deplete intracellular H2O2, and produce large amounts CO for a highly efficient treatment of osteoarthritis

Specifically inhibiting the proliferation of activated macrophages and clearing the high levels of reactive oxygen species (ROS) secreted by macrophages is crucial for osteoarthritis (OA) treatment. Moreover, if the clearance of these high levels of ROS can be simultaneously used to induce oxidation-responsive release of anti-inflammatory drugs, the therapeutic effect of OA may be further improved. Here, a multifunctional anti-inflammatory drug (CPHs) based on a peptide dendrimer nanogel was constructed by physically encapsulating CORM-401 and wrapping its surface with folic acid (FA)-modified hyaluronic acid (HA). CPHs is capable of efficiently entering activated macrophages via FA- and HA-mediated specific targeting effects and then rapidly release large amounts of CO by massive consumption of H₂O₂. The generated CO effectively suppresses the secretion of interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α by inhibiting cell proliferation; inducing the activation of heme oxygenase (HO-1), and downregulating the expression of p38 MAPK, NF-kB (p50/p65) and TLR-2. In vivo experiments further confirmed that CPHs can massively deplete ROS in OA joints and effectively suppress the degradation of articular cartilage and their extracellular matrix. More importantly, CPHs is non-toxic to normal macrophages, and the high levels of CO generated in the joints do not result in notable changes in the HbCO levels in blood. Together, these results show that CPHs is an effective and safe anti-inflammatory drug and has essential application prospects in OA treatment.

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.

Heme is required for carbon monoxide activation of mitochondrial BKCa channel

Carbon monoxide (CO) is an endogenously synthesized gaseous mediator and is involved in the regulation of numerous physiological processes. Mitochondria, in which hemoproteins are abundant, are among the targets for CO action. Large-conductance calcium-activated (mitoBK_Ca) channels in the inner mitochondrial membrane share multiple biophysical similarities with the BK_Ca channels of the plasma membrane and could be a potential target for CO. To test this hypothesis, the activity of the mitoBK_Ca channels in human astrocytoma U-87 MG cell mitochondria was assessed with the patch-clamp technique. The effects of CO-releasing molecules (CORMs), such as CORM-2, CORM-401, and CORM-A1, were compared to the application of a CO-saturated solution to the mitoBK_Ca channels in membrane patches. The applied CORMs showed pleiotropic effects including channel inhibition, while the CO-containing solution did not significantly modulate channel activity. Interestingly, CO applied to the mitoBK_Ca channels, which were inhibited by exogenously added heme, stimulated the channel. To summarize, our findings indicate a requirement of heme binding to the mitoBK_Ca channel for channel modulation by CO and suggest that CORMs might have complex unspecific effects on mitoBK_Ca channels.

CO Sense and Release Flavonols: Progress toward the Development of an Analyte Replacement PhotoCORM for Use in Living Cells

Carbon monoxide (CO) is a signaling molecule in humans. Prior research suggests that therapeutic levels of CO can have beneficial effects in treating a variety of physiological and pathological conditions. To facilitate understanding of the role of CO in biology, molecules that enable fluorescence detection of CO in living systems have emerged as an important class of chemical tools. A key unmet challenge in this field is the development of fluorescent analyte replacement probes that replenish the CO that is consumed during detection. Herein, we report the first examples of CO sense and release molecules that involve combining a common CO-sensing motif with a light-triggered CO-releasing flavonol scaffold. A notable advantage of the flavonol-based CO sense and release motif is that it is trackable via fluorescence in both its pre- and postsensing (pre-CO release) forms. In vitro studies revealed that the PdCl₂ and Ru(II)-containing CORM-2 used in the CO sensing step can result in metal coordination to the flavonol, which minimizes the subsequent CO release reactivity. However, CO detection followed by CO release is demonstrated in living cells, indicating that a cellular environment mitigates the flavonol/metal interactions.

A multistage assembly/disassembly strategy for tumor-targeted CO delivery

CO gas molecule not only could selectively kill cancer cells but also exhibits limited anticancer efficacy because of the lack of active tumor-targeted accumulation capability. In this work, a multistage assembly/disassembly strategy is developed to construct a new intelligent nanomedicine by encapsulating a mitochondria-targeted and intramitochondrial microenvironment-responsive prodrug (FeCO-TPP) within mesoporous silica nanoparticle that is further coated with hyaluronic acid by step-by-step electrostatic assembly, realizing tumor tissue-cell-mitochondria-targeted multistage delivery and controlled release of CO in a step-by-step disassembly way. Multistage targeted delivery and controlled release of CO involve (i) the passive tumor tissue-targeted nanomedicine delivery, (ii) the active tumor cell-targeted nanomedicine delivery, (iii) the acid-responsive prodrug release, (iv) the mitochondria-targeted prodrug delivery, and (v) the ROS-responsive CO release. The developed nanomedicine has effectively augmented the efficacy and safety of CO therapy of cancer both in vitro and in vivo. The proposed multistage assembly/disassembly strategy opens a new window for targeted CO therapy.

Carbon Monoxide Inhibits the Expression of Proteins Associated with Intestinal Mucosal Pyroptosis in a Rat Model of Sepsis Induced by Cecal Ligation and Puncture

BACKGROUND Carbon monoxide (CO) has anti-inflammatory effects and protects the intestinal mucosal barrier in sepsis. Pyroptosis, or cell death associated with sepsis, is mediated by caspase-1 activation. This study aimed to investigate the role of CO on the expression of proteins associated with intestinal mucosal pyroptosis in a rat model of sepsis induced by cecal ligation and puncture (CLP). MATERIAL AND METHODS The rat model of sepsis was developed using CLP. Male Sprague-Dawley rats (n=120) were divided into six study groups: the sham group (n=20); the CLP group (n=20); the hemin group (treated with ferric chloride and heme) (n=20); the zinc protoporphyrin IX (ZnPPIX) group (n=20); the CO-releasing molecule 2 (CORM-2) group (n=20); and the inactive CORM-2 (iCORM-2) group (n=20). Hemin and CORM-2 were CO donors, and ZnPPIX was a CO inhibitor. In the six groups, the seven-day survival curves, the fluorescein isothiocyanate (FITC)-labeled dextran 4000 Da (FD-4) permeability assay, levels of intestinal pyroptosis proteins caspase-1, caspase-11, and gasdermin D (GSDMD) were measured by confocal fluorescence microscopy. Proinflammatory cytokines interleukin (IL)-18, IL-1ß, and high mobility group box protein 1 (HMGB1) were measured by Western blot and enzyme-linked immunosorbent assay (ELISA). RESULTS CO reduced the mortality rate in rats with sepsis and reduced intestinal mucosal permeability and mucosal damage. CO also reduced the expression levels of IL-18, IL-1ß, and HMGB1, and reduced pyroptosis by preventing the cleavage of caspase-1 and caspase-11. CONCLUSIONS In a rat model of sepsis induced by CLP, CO had a protective role by inhibiting intestinal mucosal pyroptosis.

Metal-free carbon monoxide-releasing micelles undergo tandem photochemical reactions for cutaneous wound healing

Carbon monoxide (CO) has shown broad biomedical applications. The site-specific delivery and controlled release of CO is of crucial importance to achieve maximum therapeutic benefits. The development of carbon monoxide (CO)-releasing polymers (CORPs) can increase the stability, optimize pharmacokinetic behavior, and reduce the side effects of small molecule precursors. However, almost all established CORPs were synthesized through a post functional approach, although the direct polymerization strategy is more powerful in controlling the chain compositions and architectures. Herein, a direct polymerization strategy is proposed toward metal-free CO-releasing polymers (CORPs) based on photoresponsive 3-hydroxyflavone (3-HF) derivatives. Such CO-releasing amphiphiles self-assemble into micelles, having excellent water-dispersity. Intriguingly, photo-triggered tandem photochemical reactions confer successive fluorescence transitions from blue-to-red-to-colorless, enabling self-reporting CO release in vitro and in vivo as a result of the incorporation of 3-HF derivatives. More importantly, the localized CO delivery of CORPs by taking advantage of the spatiotemporal control of light stimulus outperformed conventional metal carbonyls such as CORMs in terms of anti-inflammation and cutaneous wound healing. This work opens a novel avenue toward metal-free CORPs for potential biomedical applications.

Nitro-imidazole-based ruthenium complexes with antioxidant and anti-inflammatory activities

Inflammation is a physiological process triggered in response to tissue damage, and involves events related to cell recruitment, cytokines release and reactive oxygen species (ROS) production. Failing to control the process duration lead to chronification and may be associated with the development of various pathologies, including autoimmune diseases and cancer. Considering the pharmacological potential of metal-based compounds, two new ruthenium complexes were synthesized: cis-[Ru(NO₂)(bpy)₂(5NIM)]PF₆ (1) and cis-[RuCl(bpy)₂(MTZ)]PF₆ (2), where bpy = 2,2'-bipyridine, 5NIM = 5-nitroimidazole and MTZ = metronidazole. Both products were characterized by spectroscopic techniques, followed by Density Functional Theory (DFT) calculations in order to support experimental findings. Afterwards, their in vitro cytotoxic, antioxidant and anti-inflammatory activities were investigated. Compounds 1 and 2 presented expressive in vitro antioxidant activity, reducing lipid peroxidation and decreasing intracellular ROS levels with comparable effectiveness to the standard steroidal drug dexamethasone or α-tocopherol. These complexes showed no noticeable cytotoxicity on the tested cancer cell lines. Bactericidal assay against metronidazole-resistant Helicobacter pylori, a microorganism able to disrupt oxidative balance, unraveled compound 1 moderate activity over that strain. Besides this, it was able to inhibit interleukin-6 (IL-6) and tumor necrosis factor-α (TNF- α) production as well as interleukin-1β (IL-1β) and cyclooxygenase-2 (COX-2) expression in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophages. This latter activity is remarkable, which has not been reported for other ruthenium-based complexes. Altogether, these results suggest cis-[Ru(NO₂)(bpy)₂(5NIM)]PF₆ complex has potential pharmacological application as an anti-inflammatory agent that deserve further biological investigation.

Ultrafast Spectroscopy of [Mn(CO)3] Complexes: Tuning the Kinetics of Light-Driven CO Release and Solvent Binding

Manganese tricarbonyl complexes are promising catalysts for CO₂ reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)₃(^Rbpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (^Rbpy, where R = ^tBu, H, CF₃, NO₂). Transient absorption spectroscopy measurements for Mn(CO)₃(^Rbpy)Br coordination compounds with R = ^tBu, H, and CF₃ in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale. The Mn(CO)₃(^NO₂bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or subsequent solvent coordination. The kinetics of photolysis and solvent coordination for light-sensitive complexes depend on the electronic properties of the disubstituted bipyridyl ligand. The results indicate that both metal-to-ligand charge-transfer (MLCT) and dissociative ligand-field (d-d) excited states play a role in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.

Classification of Metal-based Drugs According to Their Mechanisms of Action

Metal-based drugs and imaging agents are extensively used in the clinic for the treatment and diagnosis of cancers and a wide range of other diseases. The current clinical arsenal of compounds operate via a limited number of mechanisms, whereas new putative compounds explore alternative mechanisms of action, which could potentially bring new chemotherapeutic approaches into the clinic. In this review, metal-based drugs and imaging agents are characterized according to their primary mode of action and the key properties and features of each class of compounds are defined, wherever possible. A better understanding of the roles played by metal compounds at a mechanistic level will help to deliver new metal-based therapies to the clinic, by providing an alternative, targeted and rational approach, to supplement non-targeted screening of novel chemical entities for biological activity.

Porous materials as carriers of gasotransmitters towards gas biology and therapeutic applications

This review highlights the strategies employed to load and release gasotransmitters such as NO, CO and H₂S from different kinds of porous materials, including zeolites, mesoporous silica, metal–organic frameworks and protein assemblies.

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.

Carbon monoxide: An emerging therapy for acute kidney injury

Treating acute kidney injury (AKI) represents an important unmet medical need both in terms of the seriousness of this medical problem and the number of patients. There is also a large untapped market opportunity in treating AKI. Over the years, there has been much effort in search of therapeutics with minimal success. However, over the same time period, new understanding of the underlying pathobiology and molecular mechanisms of kidney injury have undoubtedly helped the search for new therapeutics. Along this line, carbon monoxide (CO) has emerged as a promising therapeutic agent because of its demonstrated cytoprotective, and immunomodulatory effects. CO has also been shown to sensitize cancer, but not normal cells, to chemotherapy. This is particularly important in treating cisplatin-induced AKI, a common clinical problem that develops in patients receiving cisplatin therapies for a number of different solid organ malignancies. This review will examine and make the case that CO be developed into a therapeutic agent against AKI.

Emerging Delivery Strategies of Carbon Monoxide for Therapeutic Applications: from CO Gas to CO Releasing Nanomaterials

Carbon monoxide (CO) therapy has emerged as a hot topic under exploration in the field of gas therapy as it shows the promise of treating various diseases. Due to the gaseous property and the high affinity for human hemoglobin, the main challenges of administrating medicinal CO are the lack of target selectivity as well as the toxic profile at relatively high concentrations. Although abundant CO releasing molecules (CORMs) with the capacity to deliver CO in biological systems have been developed, several disadvantages related to CORMs, including random diffusion, poor solubility, potential toxicity, and lack of on-demand CO release in deep tissue, still confine their practical use. Recently, the advent of versatile nanomedicine has provided a promising chance for improving the properties of naked CORMs and simultaneously realizing the therapeutic applications of CO. This review presents a brief summarization of the emerging delivery strategies of CO based on nanomaterials for therapeutic application. First, an introduction covering the therapeutic roles of CO and several frequently used CORMs is provided. Then, recent advancements in the synthesis and application of versatile CO releasing nanomaterials are elaborated. Finally, the current challenges and future directions of these important delivery strategies are proposed.

Heme oxygenase-1/carbon monoxide as modulators of autophagy and inflammation

Heme oxygenase-1 (HO-1) catalyzes heme degradation to generate biliverdin-IXα, carbon monoxide (CO), and iron. The HO-1/CO system confers cytoprotection in animal models of organ injury and disease, via modulation of inflammation and apoptosis. Recent studies have uncovered novel anti-inflammatory targets of HO-1/CO including regulation of the autophagy and inflammasome pathways. Autophagy is a lysosome-dependent program for the turnover of cellular organelles such as mitochondria, proteins, and pathogens; which may downregulate inflammatory processes. Therapeutic modulation of autophagy by CO has been demonstrated in models of sepsis. The nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3 (NLRP3) inflammasome regulates the maturation of pro-inflammatory cytokines. CO can regulate NLRP3 inflammasome activation and associated pro-inflammatory cytokines production and promote the resolution of inflammation by upregulating the synthesis of specialized pro-resolving mediators (SPMs). Mitochondria may represent a proximal target of HO-1/CO action. HO-1 may localize to mitochondria in response to stress, while CO can moderate mitochondrial dysfunction and regulate mitochondrial autophagy (mitophagy) and biogenesis. The interplay between mitochondrial autophagy, mitochondrial dysfunction, and the regulation and resolution of inflammation may make important contributions to the protection afforded by HO-1/CO in cellular and organ injury models. Recent studies have continued to explore the potential of CO for clinical applications.

Photo-crosslinkable elastomeric protein-derived supramolecular peptide hydrogel with controlled therapeutic CO-release

As an attempt to establish a method for efficient and safe administration of therapeutic carbon monoxide (CO) to the human body, supramolecular nanoplatforms incorporated with CO-releasing molecules (CORMs) have recently been developed. In particular, hydrogel scaffolds have attracted considerable attention due to the possibility of site-specific and controlled liberation of CO. However, it would be greatly beneficial to enhance the mechanical strength of hydrogels to widen their applicability in biomedical, pharmaceutical, and surgical sectors. Herein, we report a visible light-mediated crosslinkable supramolecular CO-releasing hydrogel (CORH), based on the fibrillar assembly of elastomeric protein-derived tyrosine-containing short peptides. A photo-driven dimerization of tyrosine moieties located on the fibrillar surface of CORH, accelerated by a Ru-based catalyst, results in the entanglement and bundling of nanofibrils that significantly increases the mechanical strength and stability of the CORH, which allows prolonged CO-liberation through limiting the contact of CORMs with water molecules. The contact probability of a CORM with water determined by the spatial position of the CORM on the fibrils containing a crosslinkable tyrosine moiety that affects CO-releasing behavior was confirmed by adjusting the CORM position closer to or farther from the tyrosine in the peptide sequence. A bulky CORM closely located to the tyrosine in a peptide inhibited the effective dityrosine formation of tyrosine on the fibril surface, resulting in loose bundling of nanofibrils in the CORH and facilitating the release of CO through the exchange with water. The photo-crosslinked CORH demonstrated a potent cytoprotective effect on oxidatively stressed cardiomyocytes, as expected. This work could provide a useful insight for the practical application of gasotransmitters as functional nanomaterials in pharmaceutical and biomedical fields.

Carbon Monoxide Attenuates Lipopolysaccharides (LPS)-Induced Acute Lung Injury in Neonatal Rats via Downregulation of Cx43 to Reduce Necroptosis

Background

Acute lung injury (ALI) is one of major causes of death in newborns, making it urgent to improve therapy. Administration of low dose carbon monoxide (CO) plays a protective role in ALI but the mechanisms are not fully understood. This study was designed to test the therapeutic effect of monoxide-releasing molecule 3 (MORM3) in lipopolysaccharide (LPS) induced neonatal ALI and the possibly associated molecular mechanisms.

Material/Methods

For this study, 3- to 8-day old Newborn Sprague-Dawley rats were subjected to intraperitoneal injection of 3 mg/kg LPS to induce ALI. Then animals received intraperitoneal injection of carbon monoxide-releasing molecules 3 (CORM3) (8 mg/kg) or inactive CORM3 (iCORM3) for 7 consecutive days. Lung tissues were collected for histological examination and total cell counts and protein content in bronchoalveolar lavage fluid (BALF) were measured. Expression of Cx43 and necroptosis-related markers were detected by quantitative real-time polymerase chain reaction (qRT-PCR) and western blot.

Results

LPS exposure induced significant lung injury indicated by histological damage, increased lung wet/dry weight ratio (W/D) and increased total cell counts and protein concentration in BALF. These changes were significantly ameliorated by administration of CORM3 but not iCORM3. LPS also increased necroptosis-related markers RIP1, RIP3, and MLKL and their elevation was blocked by CORM3. CORM3 administration ameliorated LPS induced elevation of Cx43 expression and adenoviral overexpression of Cx43 abolished lung protective effect of CORM3. CORM3 administration attenuated LPS induced activation of extracellular-signal-regulated kinase (ERK) and its protection against necroptosis was abolished by ERK inhibitor U0126.

Conclusions

CORM3 attenuates LPS-Induced ALI in neonatal rats and its lung protective effect might be through downregulation of Cx43 to attenuate ERK signaling and ameliorate necroptosis, suggesting CORM3 as a potential therapeutic drug for ALI in neonates.

The Intercalation of CORM-2 with Pharmaceutical Clay Montmorillonite (MMT) Aids for Therapeutic Carbon Monoxide Release

The pharmaceutical clay montmorillonite (MMT) is, for the first time, explored as a carbon monoxide-releasing material (CORMat). MMT consists of silicate double layered structure; its exfoliation feature intercalate the CORM-2 [RuCl(μ-Cl)(CO)3]2 inside the layers to suppress the toxicity of organometallic segment. The infrared spectroscopy (IR) confirmed the existence of ruthenium coordinated carbonyl ligand in MMT layers. The energy-dispersive X-ray spectroscopy (EDX) analysis showed that ruthenium element in this material was about 5%. The scanning electron microscopy (SEM) and transmission electron microscope (TEM) images showed that the layer-structure of MMT has been maintained after loading the ruthenium carbonyl segment. Moreover, the layers have been stretched out, which was confirmed by X-ray diffraction (XRD) analysis. Thermogravimetric (TG) curves with huge weight loss around 100-200 °C were attributed to the CO hot-release of ruthenium carbonyl as well as the loss of the adsorbed solvent molecules and the water molecules between the layers. The CO-liberating properties have been assessed through myoglobin assay. The horse myoglobin test showed that the material could be hydrolyzed to slowly release carbon monoxide in physiological environments. The half-life of CO release was much longer than that of CORM-3, and it has an excellent environmental tolerance and slow release effect.

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.

Starvation-amplified CO generation for enhanced cancer therapy via an erythrocyte membrane-biomimetic gas nanofactory

Carbon monoxide (CO)-based gas therapy has emerged as an attractive therapeutic strategy for cancer therapy. However, the main challenges are the in situ-triggered and efficient delivery of CO in tumors, which limit its further clinical application. Herein, we developed an erythrocyte membrane-biomimetic gas nanofactory (MGP@RBC) to amplify the in situ generation of CO for combined energy starvation of cancer cells and gas therapy. This nanofactory was constructed by encapsulating glucose oxidase (GOx) and Mn₂(CO)₁₀ (CO-donor) into the biocompatible polymer poly(lactic-co-glycolic acid), obtaining MGP nanoparticles, which are further covered by red blood cell (RBC) membrane. Because of the presence of proteins on RBC membranes, the nanoparticles could effectively avoid immune clearance in macrophages (Raw264.7) and significantly prolong their blood circulation time, thereby achieving higher accumulation at the tumor site. After that, the GOx in GMP@RBC could effectively catalyze the conversion of endogenous glucose to hydrogen peroxide (H₂O₂) in the presence of oxygen. The concomitant generation of H₂O₂ could efficiently trigger CO release to cause dysfunction of mitochondria and activate caspase, thereby resulting in apoptosis of the cancer cells. In addition, the depletion of intratumoral glucose could starve tumor cells by shutting down the energy supply. Altogether, the in vitro and in vivo studies of our synthesized biomimetic gas nanofactory exhibited an augmentative synergistic efficacy of CO gas therapy and energy starvation to inhibit tumor growth. It provides an attractive strategy to amplify CO generation for enhanced cancer therapy in an accurate and more efficient manner. STATEMENT OF SIGNIFICANCE: Carbon monoxide (CO) based gas therapy has emerged as an attractive therapeutic strategy for cancer therapy. In this study, we developed an erythrocyte membrane biomimetic gas nanofactory to amplify the in-situ generation of CO for combined cancer starvation and gas therapy. It is constructed by coating glucose oxidase (GOx) and CO donor-loaded nanoparticles with erythrocyte membrane. Due to the erythrocyte membrane, it can effectively prolong blood circulation time and achieve higher tumor accumulation. After accumulated in tumor, endogenous glucose can be effectively catalyzed to hydrogen peroxide, in-situ amplified CO release to induce the apoptosis of cancer cells. In addition, depleting glucose can also starve tumor cells by shutting down the energy supply. Overall, our biomimetic gas nanofactory exhibits an augmentative synergistic efficacy of CO gas therapy and starvation to increased tumor inhibition. It provide a novel strategy to deliver CO in an accurate and more efficient manner, promising for combined cancer therapy in future clinical application.

An H2S-activated ratiometric CO photoreleaser enabled by excimer/monomer conversion

Based on the excimer-monomer conversion of a pyrene-flavone hybrid, a ratiometric CO photoreleaser, PFN, was constructed for simultaneous H2S quantification and CO release in inflammatory cells.

CO-Releasing Materials: An Emphasis on Therapeutic Implications, as Release and Subsequent Cytotoxicity Are the Part of Therapy

The CO-releasing materials (CORMats) are used as substances for producing CO molecules for therapeutic purposes. Carbon monoxide (CO) imparts toxic effects to biological organisms at higher concentration. If this characteristic is utilized in a controlled manner, it can act as a cell-signaling agent for important pathological and pharmacokinetic functions; hence offering many new applications and treatments. Recently, research on therapeutic applications using the CO treatment has gained much attention due to its nontoxic nature, and its injection into the human body using several conjugate systems. Mainly, there are two types of CO insertion techniques into the human body, i.e., direct and indirect CO insertion. Indirect CO insertion offers an advantage of avoiding toxicity as compared to direct CO insertion. For the indirect CO inhalation method, developers are facing certain problems, such as its inability to achieve the specific cellular targets and how to control the dosage of CO. To address these issues, researchers have adopted alternative strategies regarded as CO-releasing molecules (CORMs). CO is covalently attached with metal carbonyl complexes (MCCs), which generate various CORMs such as CORM-1, CORM-2, CORM-3, ALF492, CORM-A1 and ALF186. When these molecules are inserted into the human body, CO is released from these compounds at a controlled rate under certain conditions or/and triggers. Such reactions are helpful in achieving cellular level targets with a controlled release of the CO amount. However on the other hand, CORMs also produce a metal residue (termed as i-CORMs) upon degradation that can initiate harmful toxic activity inside the body. To improve the performance of the CO precursor with the restricted development of i-CORMs, several new CORMats have been developed such as micellization, peptide, vitamins, MOFs, polymerization, nanoparticles, protein, metallodendrimer, nanosheet and nanodiamond, etc. In this review article, we shall describe modern ways of CO administration; focusing primarily on exclusive features of CORM's tissue accumulations and their toxicities. This report also elaborates on the kinetic profile of the CO gas. The comprehension of developmental phases of CORMats shall be useful for exploring the ideal CO therapeutic drugs in the future of medical sciences.

Carbon monoxide releasing molecule 401 (CORM-401) modulates phase I metabolism of xenobiotics

Next to its well-studied toxicity, carbon monoxide (CO) is recognized as a signalling molecule in various cellular processes. Thus, CO-releasing molecules (CORMs) are of considerable interest for basic research and drug development. Aim of the present study was to investigate if CO, released from CORMs, inhibits cytochrome P450-dependent monooxygenase (CYP) activity and modulates xenobiotic metabolism. CORM-401 was used as a model CO delivering compound; inactive CORM-401 (iCORM-401), unable to release CO, served as control compound. CO release from CORM-401, but not from iCORM-401, was validated using the cell free myoglobin assay. CO-dependent inhibition of CYP activity was shown by 7-ethoxyresorufin-O-deethylation (EROD) with recombinant CYP and HepG2 cells. Upon CORM-401 exposure EROD activity of recombinant CYP decreased concentration dependently, while iCORM-401 had no effect. Treatment with CORM-401 decreased EROD activity in HepG2 cells at concentrations higher than 50 μM CORM-401, while iCORM-401 showed no effect. At the given concentrations cell viability was not affected. Amitriptyline was selected as a model xenobiotic and formation of its metabolite nortriptyline by recombinant CYP was determined by HPLC. CORM-401 treatment inhibited the formation of nortriptyline whereas iCORM-401 treatment did not. Overall, we demonstrate CO-mediated inhibitory effects on CYP activity when applying CORMs. Since CORMs are currently under drug development, the findings emphasize the importance to take into account that this class of compounds may interfere with xenobiotic metabolism.

Lights and shadows of electrophile signaling: focus on the Nrf2-Keap1 pathway

Targeted covalent modification is assuming consolidated importance in drug discovery. In this context, the electrophilic tuning of redox-dependent cell signaling is attracting major interest, as it opens prospect for treating numerous pathologic conditions. Herein, we discuss the rationale and the issues of electrophile-based approaches, focusing on the transcriptional Nrf2-Keap1 pathway as a test case. We also highlight relevant medicinal chemistry strategies researchers have devised to meet the ambitious goal, dwelling on the investigational and therapeutic potential of modulating redox-signaling networks through regulatory cysteine switches.

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.

Luminescent fac-[Re(CO)3(phen)] carboxylato complexes with non-steroidal anti-inflammatory drugs: synthesis and mechanistic insights into the in vitro anticancer activity of fac-[Re(CO)3(phen)(aspirin)]

Luminescent fac-[Re(CO)₃(phen)(aspirin)]: insights into in vitro anticancer activity and confocal microscopy imaging in HeLa cells.

Supramolecular Carbon Monoxide‐Releasing Peptide Hydrogel Patch

Carbon monoxide (CO) is recently accepted as a therapeutic molecule that exhibits remarkable biological actions, including anti‐inflammation, antiapoptosis, and cytoprotection, at a physiological level. For clinical use without the side effect of tissue hypoxia, which arises from the uncontrolled administration of CO in the human body, CO‐releasing molecules (CORMs) are developed to ensure safe and efficient CO‐delivery. Herein, a syringe‐injectable CO‐releasing peptide hydrogel (COH) and a corresponding bioadhesive hydrogel patch (COHP), developed by rational supramolecular chemistry, to enhance the therapeutic efficacy of CO with controllable CO‐release to a specific tissue is report. The injectable COH is prepared by self‐assembly of the CORM‐attached peptides with a gel‐forming diphenylalanine‐derivative, resulting in fibrillar networks and exhibiting prolonged CO‐release compared with CORMs. Furthermore, Ca2+‐chelating and mussel‐derived catechol‐functionalized peptides are introduced to afford a mechanically rigid, bioadhesive COHP that elicits cytoprotective and anti‐inflammatory activities. The supramolecular COHP can be utilized in the efficient CO‐delivery to the site of interest by conformal contacts, making it a promising scaffold for biomedical applications.

Gas Signaling Molecules and Mitochondrial Potassium Channels

Recently, gaseous signaling molecules, such as carbon monoxide (CO), nitric oxide (NO), and hydrogen sulfide (H₂S), which were previously considered to be highly toxic, have been of increasing interest due to their beneficial effects at low concentrations. These so-called gasotransmitters affect many cellular processes, such as apoptosis, proliferation, cytoprotection, oxygen sensing, ATP synthesis, and cellular respiration. It is thought that mitochondria, specifically their respiratory complexes, constitute an important target for these gases. On the other hand, increasing evidence of a cytoprotective role for mitochondrial potassium channels provides motivation for the analysis of the role of gasotransmitters in the regulation of channel function. A number of potassium channels have been shown to exhibit activity within the inner mitochondrial membrane, including ATP-sensitive potassium channels, Ca²⁺-activated potassium channels, voltage-gated Kv potassium channels, and TWIK-related acid-sensitive K⁺ channel 3 (TASK-3). The effects of these channels include the regulation of mitochondrial respiration and membrane potential. Additionally, they may modulate the synthesis of reactive oxygen species within mitochondria. The opening of mitochondrial potassium channels is believed to induce cytoprotection, while channel inhibition may facilitate cell death. The molecular mechanisms underlying the action of gasotransmitters are complex. In this review, we focus on the molecular mechanisms underlying the action of H₂S, NO, and CO on potassium channels present within mitochondria.

Phosphine oxide-based tricarbonylrhenium(i) complexes from phosphine/phosphine oxide and dihydroxybenzoquinones

Neutral phosphine oxide (P[double bond, length as m-dash]O) donor-based organometallic complexes [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (1), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (2), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (3), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (4), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PCy3}] (5), and [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PPh3}] (6) were assembled from phosphine/phosphine oxide, a dihydroxybenzoquinone donor and Re2(CO)10via a one-pot solvothermal approach. The soft phosphine donor was transformed into a hard phosphine oxide donor during the formation of 1, 3, 4, 5, and 6. The complexes 1-6 were air and moisture stable and were soluble in polar organic solvents. The complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopic methods. The molecular structures of 1, 2, 4, and 6 were analyzed by single-crystal X-ray diffraction analysis. The UV-Visible absorption studies indicated that 1-6 in THF display strong visible light absorption in the range of ∼350-700 nm.

Carbon monoxide ameliorates Staphylococcus aureus-elicited COX-2/IL-6/MMP-9-dependent human aortic endothelial cell migration and inflammatory responses

Staphylococcus aureus (S. aureus) can often lead to many life-threatening diseases. It has the ability to invade normal endovascular tissue. Acute inflammation and its resolution are important to ensure bacterial clearance and limit tissue injury. Carbon monoxide (CO) has been shown to exert anti-inflammatory effects in various tissues and organ systems. In our study, we investigated the effects and the mechanisms of carbon monoxide releasing molecule-2 (CORM-2) on S. aureus-induced inflammatory responses in human aortic endothelial cells (HAECs). We proved that S. aureus induced cyclooxygenase-2 (COX-2)/prostaglandin E₂ (PGE₂)/interleukin-6 (IL-6)/matrix metallopeptidase-9 (MMP-9) expression and cell migration, which were decreased by CORM-2. Moreover, CORM-2 had no effects on TLR2 mRNA levels in response to S. aureus. Interestingly, we proved that S. aureus decreased intracellular ROS generation, suggesting that the inhibition of ROS further promoted inflammatory responses. However, CORM-2 significantly inhibited S. aureus-induced inflammation by increasing intracellular ROS generation. S. aureus-induced NF-κB activation was also inhibited by CORM-2. Finally, we proved that S. aureus induced levels of the biomarkers of inflammation in cardiovascular diseases, which were inhibited by CORM-2. Taken together, these results suggest that CORM-2 inhibits S. aureus-induced COX-2/PGE₂/IL-6/MMP-9 expression and aorta inflammatory responses by increasing the ROS generation and reducing the inflammatory molecules levels.

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.

Carbon monoxide releasing molecule-2 attenuates Pseudomonas aeruginosa-induced ROS-dependent ICAM-1 expression in human pulmonary alveolar epithelial cells

Pseudomonas aeruginosa (P. aeruginosa) infection in the lung is common in patients with cystic fibrosis (CF). Intercellular adhesion molecule-1 (ICAM-1) is known to play a key role in lung inflammation. Acute inflammation and its timely resolution are important to ensure bacterial clearance and limit tissue damage. Carbon monoxide (CO) has been shown to exert anti-inflammatory effects in various tissues and organ systems. Here, we explored the protective effects and mechanisms of carbon monoxide releasing molecule-2 (CORM-2) on P. aeruginosa-induced inflammatory responses in human pulmonary alveolar epithelial cells (HPAEpiCs). We showed that P. aeruginosa induced prostaglandin E₂ (PGE₂)/interleukin-6 (IL-6)/ICAM-1 expression and monocyte adherence to HPAEpiCs. Moreover, P. aeruginosa-induced inflammatory responses were inhibited by transfection with siRNA of Toll-like receptor 4 (TLR4), PKCα, p47^phox, JNK2, p42, p50, or p65. P. aeruginosa also induced PKCα, JNK, ERK1/2, and NF-κB activation. We further demonstrated that P. aeruginosa increased intracellular ROS generation via NADPH oxidase activation. On the other hand, P. aeruginosa-induced inflammation was inhibited by pretreatment with CORM-2. Preincubation with CORM-2 had no effects on TLR4 mRNA levels in response to P. aeruginosa. However, CORM-2 inhibits P. aeruginosa-induced inflammation by decreasing intracellular ROS generation. P. aeruginosa-induced PKCα, JNK, ERK1/2, and NF-κB activation was inhibited by CORM-2. Finally, we showed that P. aeruginosa induced levels of the biomarkers of inflammation in respiratory diseases, which were inhibited by pretreatment with CORM-2. Taken together, these data suggest that CORM-2 inhibits P. aeruginosa-induced PGE₂/IL-6/ICAM-1 expression and lung inflammatory responses by reducing the ROS generation and the inflammatory pathways.

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CORM-2 inhibits P. aeruginosa-induced PGE₂/IL-6/ICAM-1 expression.

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CORM-2 reduced PKCα phosphorylation in response to P. aeruginosa.

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We provide molecular mechanisms for antibacterial effects of CORM-2.

CatroxMP-II: a heme-modulated fibrinogenolytic metalloproteinase isolated from Crotalus atrox venom

It has been recently demonstrated that the hemotoxic venom activity of several species of snakes can be inhibited by carbon monoxide (CO) or a metheme forming agent. These and other data suggest that the biometal, heme, may be attached to venom enzymes and may be modulated by CO. A novel fibrinogenolytic metalloproteinase, named CatroxMP-II, was isolated and purified from the venom of a Crotalus atrox viper, and subjected to proteolysis and mass spectroscopy. An ion similar to the predicted singly charged m/z of heme at 617.18 was identified. Lastly, CORM-2 (tricarbonyldichlororuthenium (II) dimer, a CO releasing molecule) inhibited the fibrinogenolytic effects of CatroxMP-II on coagulation kinetics in human plasma. In conclusion, we present the first example of a snake venom metalloproteinase that is heme-bound and CO-inhibited.

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.

Photoactivated in Vitro Anticancer Activity of Rhenium(I) Tricarbonyl Complexes Bearing Water-Soluble Phosphines

Fifteen water-soluble rhenium compounds of the general formula [Re(CO)3(NN)(PR3)]+, where NN is a diimine ligand and PR3 is 1,3,5-triaza-7-phosphaadamantane (PTA), tris(hydroxymethyl)phosphine (THP), or 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA), were synthesized and characterized by multinuclear NMR spectroscopy, IR spectroscopy, and X-ray crystallography. The complexes bearing the THP and DAPTA ligands exhibit triplet-based luminescence in air-equilibrated aqueous solutions with quantum yields ranging from 3.4 to 11.5%. Furthermore, the THP and DAPTA complexes undergo photosubstitution of a CO ligand upon irradiation with 365 nm light with quantum yields ranging from 1.1 to 5.5% and sensitize the formation of 1O2 with quantum yields as high as 70%. In contrast, all of the complexes bearing the PTA ligand are nonemissive and do not undergo photosubstitution upon irradiation with 365 nm light. These compounds were evaluated as photoactivated anticancer agents in human cervical (HeLa), ovarian (A2780), and cisplatin-resistant ovarian (A2780CP70) cancer cell lines. All of the complexes bearing THP and DAPTA exhibited a cytotoxic response upon irradiation with minimal toxicity in the absence of light. Notably, the complex with DAPTA and 1,10-phenanthroline gave rise to an IC50 value of 6 μM in HeLa cells upon irradiation, rendering it the most phototoxic compound in this library. The nature of the photoinduced cytotoxicity of this compound was explored in further detail. These data indicate that the phototoxic response may result from the release of both CO and the rhenium-containing photoproduct, as well as the production of 1O2.

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.