Organic carbon monoxide prodrug, BW-CO-111, in protection against chemically-induced gastric mucosal damage

Review on recent advancements in understanding acetylsalicylic acid-induced gastrointestinal injury: mechanisms, medication, and dosage refinement

Acetylsalicylic acid (ASA) is a clinical drug with multiple effects, including prevention of cardiovascular adverse events and anti-cancer effects. However, gastrointestinal side effects, such as gastrointestinal ulcers and bleeding, limit the use of ASA and reduce patient compliance. Various studies have investigated the mechanisms of ASA-induced gastrointestinal injury, and many medicines have been reported to be effective in preventing and treating the adverse gastrointestinal effects of ASA. New formulations of ASA have demonstrated milder gastrointestinal injury than ASA alone. In this article, we summarized the mechanisms of ASA-induced gastrointestinal injury, drugs that resist gastrointestinal side effects of ASA, and progress in research on formulation improvement of ASA to help resolve the clinical dilemma of ASA usage.

Carbon Monoxide as a Potential Therapeutic Agent: A Molecular Analysis of Its Safety Profiles

Carbon monoxide (CO) is endogenously produced in mammals, with blood concentrations in the high micromolar range in the hemoglobin-bound form. Further, CO has shown therapeutic effects in various animal models. Despite its reputation as a poisonous gas at high concentrations, we show that CO should have a wide enough safety margin for therapeutic applications. The analysis considers a large number of factors including levels of endogenous CO, its safety margin in comparison to commonly encountered biomolecules or drugs, anticipated enhanced safety profiles when delivered via a noninhalation mode, and the large amount of safety data from human clinical trials. It should be emphasized that having a wide enough safety margin for therapeutic use does not mean that it is benign or safe to the general public, even at low doses. We defer the latter to public health experts. Importantly, this Perspective is written for drug discovery professionals and not the general public.

On the Question of CO's Ability to Induce HO-1 Expression in Cell Culture: A Comparative Study Using Different CO Sources

With the recognition of the endogenous signaling roles and pharmacological functions of carbon monoxide (CO), there is an increasing need to understand CO's mechanism of actions. Along this line, chemical donors have been introduced as CO surrogates for ease of delivery, dosage control, and sometimes the ability to target. Among all of the donors, two ruthenium-carbonyl complexes, CORM-2 and -3, are arguably the most commonly used tools for about 20 years in studying the mechanism of actions of CO. Largely based on data using these two CORMs, there has been a widely accepted inference that the upregulation of heme oxygenase-1 (HO-1) expression is one of the key mechanisms for CO's actions. However, recent years have seen reports of very pronounced chemical reactivities and CO-independent activities of these CORMs. We are interested in examining this question by conducting comparative studies using CO gas, CORM-2/-3, and organic CO donors in RAW264.7, HeLa, and HepG2 cell cultures. CORM-2 and CORM-3 treatment showed significant dose-dependent induction of HO-1 compared to "controls," while incubation for 6 h with 250-500 ppm CO gas did not increase the HO-1 protein expression and mRNA transcription level. A further increase of the CO concentration to 5% did not lead to HO-1 expression either. Additionally, we demonstrate that CORM-2/-3 releases minimal amounts of CO under the experimental conditions. These results indicate that the HO-1 induction effects of CORM-2/-3 are not attributable to CO. We also assessed two organic CO prodrugs, BW-CO-103 and BW-CO-111. BW-CO-111 but not BW-CO-103 dose-dependently increased HO-1 levels in RAW264.7 and HeLa cells. We subsequently studied the mechanism of induction with an Nrf2-luciferase reporter assay, showing that the HO-1 induction activity is likely due to the activation of Nrf2 by the CO donors. Overall, CO alone is unable to induce HO-1 or activate Nrf2 under various conditions in vitro. As such, there is no evidence to support attributing the HO-1 induction effect of the CO donors such as CORM-2/-3 and BW-CO-111 in cell culture to CO. This comparative study demonstrates the critical need to consider possible CO-independent effects of a chemical CO donor before attributing the observed biological effects to CO. It is also important to note that such in vitro results cannot be directly extrapolated to in vivo studies because of the increased level of complexity and the likelihood of secondary and/or synergistic effects in the latter.

Gastroprotective effect of eupatilin, a polymethoxyflavone from Artemisia argyi H.Lév. & Vaniot, in ethanol-induced gastric mucosal injury via NF-κB signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Artemisia argyi H.Lév. & Vaniot (AA) has been extensively utilized as an important medicine and food homology in China, Japan, Korea, and eastern parts of Russia, owing to its pharmacological effects, which include anti-inflammatory, antibacterial, antitussive, and antiallergic properties. Despite the extract of AA can significantly alleviate gastric mucosal injury, its precise material basis for effectiveness is not yet clear. As one of the polymethoxy flavonoids with high content in AA, the gastroprotective activity and molecular mechanism of eupatilin (EUP) require further investigation.

AIM OF THE STUDY

This study aims to investigate the gastroprotective effects and possible mechanisms of EUP by using an ethanol-induced gastric mucosal injury model in rats.

MATERIALS AND METHODS

EUP was isolated from 95% ethanol extract of AA using a systematic phytochemical method. The gastroprotective activity of EUP was evaluated using a male SD rat model with ethanol-induced gastric mucosa injury. Histopathology evaluation of gastric tissues was performed using hematoxylin and eosin (H&E) staining. The levels of cytokines in the plasma and tissues were tested using the ELISA kits, while western blot analysis was employed to assess the expressions of COX-2, iNOS, and NF-κB pathway proteins.

RESULTS

A sufficient amount of EUP was obtained from AA through chromatographic methods and identified by NMR experiment. In vivo, experimental results proved that EUP could significantly alleviate pathological features, increased SOD, GSH, and IL-10 levels, and decreased the contents of MDA, TNF-α, IL-1β, and IL-6. Further in vitro and in vivo Western blot experimental results showed that EUP significantly down-regulates the expressions of the NF-κB signal pathway to relieve inflammatory responses.

CONCLUSION

This study demonstrated that EUP could exert gastroprotective effects by inhibiting inflammation, enhancing gastric mucosal defense, and ameliorating oxidative stress, which is beneficial for providing scientific data for the development of gastric protection.

Chemical Strategies Toward Prodrugs and Fluorescent Probes for Gasotransmitters

Three gaseous molecules are widely accepted as important gasotransmitters in mammalian cells, namely NO, CO and H2S. Due to the pharmacological effects observed in preclinical studies, these three gasotransmitters represent promising drug candidates for clinical translation. Fluorescent probes of the gasotransmitters are also in high demand; however, the mechanisms of actions or the roles played by gasotransmitters under both physiological and pathological conditions remain to be answered. In order to bring these challenges to the attention of both chemists and biologists working in this field, we herein summarize the chemical strategies used for the design of both probes and prodrugs of these three gasotransmitters.

Hydrogen Sulfide-Releasing Indomethacin-Derivative (ATB-344) Prevents the Development of Oxidative Gastric Mucosal Injuries

Hydrogen sulfide (H2S) emerged recently as an anti-oxidative signaling molecule that contributes to gastrointestinal (GI) mucosal defense and repair. Indomethacin belongs to the class of non-steroidal anti-inflammatory drugs (NSAIDs) and is used as an effective intervention in the treatment of gout- or osteoarthritis-related inflammation. However, its clinical use is strongly limited since indomethacin inhibits gastric mucosal prostaglandin (PG) biosynthesis, predisposing to or even inducing ulcerogenesis. The H2S moiety was shown to decrease the GI toxicity of some NSAIDs. However, the GI safety and anti-oxidative effect of a novel H2S-releasing indomethacin derivative (ATB-344) remain unexplored. Thus, we aimed here to compare the impact of ATB-344 and classic indomethacin on gastric mucosal integrity and their ability to counteract the development of oxidative gastric mucosal injuries. Wistar rats were pretreated intragastrically (i.g.) with vehicle, ATB-344 (7-28 mg/kg i.g.), or indomethacin (5-20 mg/kg i.g.). Next, animals were exposed to microsurgical gastric ischemia-reperfusion (I/R). Gastric damage was assessed micro- and macroscopically. The volatile H2S level was assessed in the gastric mucosa using the modified methylene blue method. Serum and gastric mucosal PGE2 and 8-hydroxyguanozine (8-OHG) concentrations were evaluated by ELISA. Molecular alterations for gastric mucosal barrier-specific targets such as cyclooxygenase-1 (COX)-1, COX-2, heme oxygenase-1 (HMOX)-1, HMOX-2, superoxide dismutase-1 (SOD)-1, SOD-2, hypoxia inducible factor (HIF)-1α, xanthine oxidase (XDH), suppressor of cytokine signaling 3 (SOCS3), CCAAT enhancer binding protein (C/EBP), annexin A1 (ANXA1), interleukin 1 beta (IL-1β), interleukin 1 receptor type I (IL-1R1), interleukin 1 receptor type II (IL-1R2), inducible nitric oxide synthase (iNOS), tumor necrosis factor receptor 2 (TNFR2), or H2S-producing enzymes, cystathionine γ-lyase (CTH), cystathionine β-synthase (CBS), or 3-mercaptopyruvate sulfur transferase (MPST), were assessed at the mRNA level by real-time PCR. ATB-344 (7 mg/kg i.g.) reduced the area of gastric I/R injuries in contrast to an equimolar dose of indomethacin. ATB-344 increased gastric H2S production, did not affect gastric mucosal PGE2 content, prevented RNA oxidation, and maintained or enhanced the expression of oxidation-sensitive HMOX-1 and SOD-2 in line with decreased IL-1β and XDH. We conclude that due to the H2S-releasing ability, i.g., treatment with ATB-344 not only exerts dose-dependent GI safety but even enhances gastric mucosal barrier capacity to counteract acute oxidative injury development when applied at a low dose of 7 mg/kg, in contrast to classic indomethacin. ATB-344 (7 mg/kg) inhibited COX activity on a systemic level but did not affect cytoprotective PGE2 content in the gastric mucosa and, as a result, evoked gastroprotection against oxidative damage.

Toward "CO in a Pill": Silica-Immobilized Organic CO Prodrugs for Studying the Feasibility of Systemic Delivery of CO via In Situ Gastrointestinal CO Release

Carbon monoxide (CO), an endogenous signaling molecule, is known to exert a range of pharmacological effects, including anti-inflammation, organ protection, and antimetastasis in various animal models. We have previously shown the ability of organic prodrugs to deliver CO systemically through oral administration. As part of our efforts for the further development of these prodrugs, we are interested in minimizing the potential negative impact of the "carrier" portion of the prodrug. Along this line, we have previously published our work on using benign "carriers" and physically trapping the "carrier" portion in the gastrointestinal (GI) tract. We herein report our feasibility studies on using immobilized organic CO prodrugs for oral CO delivery while minimizing systemic exposure to the prodrug and the "carrier portion." In doing so, we immobilize a CO prodrug to silica microparticles, which are generally recognized as safe by the US FDA and known to provide large surface areas for loading and water accessibility. The latter point is essential for the hydrophobicity-driven activation of the CO prodrug. Amidation-based conjugation with silica is shown to provide 0.2 mmol/g loading degree, effective prodrug activation in buffer with comparable kinetics as the parent prodrug, and stable tethering to prevent detachment. One representative silica conjugate, SICO-101, is shown to exhibit anti-inflammation activity in LPS-challenged RAW264.7 cells and to deliver CO systemically in mice through oral administration and GI CO release. We envision this strategy as a general approach for oral CO delivery to treat systemic and GI-specific inflammatory conditions.

Mutual leveraging of proximity effects and click chemistry in chemical biology

Nature has the remarkable ability to realize reactions under physiological conditions that normally would require high temperature and other forcing conditions. In doing so, often proximity effects such as simultaneous binding of two reactants in the same pocket and/or strategic positioning of catalytic functional groups are used as ways to achieve otherwise kinetically challenging reactions. Though true biomimicry is challenging, there have been many beautiful examples of how to leverage proximity effects in realizing reactions that otherwise would not readily happen under near-physiological conditions. Along this line, click chemistry is often used to endow proximity effects, and proximity effects are also used to further leverage the facile and bioorthogonal nature of click chemistry. This review brings otherwise seemingly unrelated topics in chemical biology and drug discovery under one unifying theme of mutual leveraging of proximity effects and click chemistry and aims to critically analyze the biomimicry use of such leveraging effects as powerful approaches in chemical biology and drug discovery. We hope that this review demonstrates the power of employing mutual leveraging proximity effects and click chemistry and inspires the development of new strategies that will address unmet needs in chemistry and biology.

De Novo Construction of Fluorophores via CO Insertion-Initiated Lactamization: A Chemical Strategy toward Highly Sensitive and Highly Selective Turn-On Fluorescent Probes for Carbon Monoxide

Extensive studies in the last few decades have led to the establishment of CO as an endogenous signaling molecule and subsequently to the exploration of CO's therapeutic roles. In the current state, there is a critical conundrum in CO-related research: the extensive knowledge of CO's biological effects and yet an insufficient understanding of the quantitative correlations between the CO concentration and biological responses of various natures. This conundrum is partially due to the difficulty in examining precise concentration-response relationships of a gaseous molecule. Another reason is the need for appropriate tools for the sensitive detection and concentration determination of CO in the biological system. We herein report a new chemical approach to the design of fluorescent CO probes through de novo construction of fluorophores by a CO insertion-initiated lactamization reaction, which allows for ultra-low background and exclusivity in CO detection. Two series of CO detection probes have been designed and synthesized using this strategy. Using these probes, we have extensively demonstrated their utility in quantifying CO in blood, tissue, and cell culture and in cellular imaging of CO from exogenous and endogenous sources. The probes described will enable many biology and chemistry labs to study CO's functions in a concentration-dependent fashion with very high sensitivity and selectivity. The chemical and design principles described will also be applicable in designing fluorescent probes for other small molecules.

Identification and verification of ferroptosis-related genes in gastric intestinal metaplasia

Background: Gastric intestinal metaplasia (IM) is the key link of gastric precancerous lesions. Ferroptosis is a novel form of programmed cell death. However, its impact on IM is unclear. The focus of this study is to identify and verify ferroptosis-related genes (FRGs) that may be involved in IM by bioinformatics analysis. Materials and methods: Differentially expressed genes (DEGs) were obtained from microarray dataset GSE60427 and GSE78523 downloaded from Gene Expression Omnibus (GEO) database. Differentially expressed ferroptosis-related genes (DEFRGs) were obtained from overlapping genes of DEGs and FRGs got from FerrDb. DAVID database was used for functional enrichment analysis. Protein-protein interaction (PPI) analysis and Cytoscape software were used to screen hub gene. In addition, we built a receiver operating characteristic (ROC) curve and verified the relative mRNA expression by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Finally, the CIBERSORT algorithm was used to analyze the immune infiltration in IM. Results: First, a total of 17 DEFRGs were identified. Second, a gene module identified by Cytoscape software was considered as hub gene: PTGS2, HMOX1, IFNG, and NOS2. Third, ROC analysis showed that HMOX1 and NOS2 had good diagnostic characteristics. qRT-PCR experiments confirmed the differential expression of HMOX1 in IM and normal gastric tissues. Finally, immunoassay showed that the proportion of T cells regulatory (Tregs) and macrophages M0 in IM was relatively higher, while the proportion of T cells CD4 memory activated and dendritic cells activated was lower. Conclusion: We found significant associations between FRGs and IM, and HMOX1 may be diagnostic biomarkers and therapeutic targets for IM. These results may enhance our understanding of IM and may contribute to its treatment.

Ultrasound-Mediated Antitumor Therapy via Targeted Acoustic Release Carrier of Carbon Monoxide (TARC-CO)

As one of the most valuable endogenous gas signaling molecules, carbon monoxide (CO) has been demonstrated in numerous studies to show excellent promise in the treatment of diseases, such as cancer. However, for many years, the inherent high affinity of CO for hemoglobin severely impeded the clinical transformation of CO-based treatments. Therefore, the controlled delivery of CO to target tissues has become a common challenge. Herein, an efficient ultrasonic-triggered and targeted CO release strategy was constructed based on a novel targeted acoustic release carrier of carbon monoxide (TARC-CO) that we synthesized in this study. The designed TARC-COs could afford a safe, stable, and ultrasound-guided delivery of CO in vivo by loading a specified dose of CO inside microbubbles, resulting in breast tumor suppression. Taking advantage of the high loading capacity of microbubbles, the unit volume of TARC-CO suspension could encapsulate up to 337.1 ± 8.0 (×10³ ppm) of CO. In addition, the satisfactory ultrasound contrast-enhanced ability of TARC-COs achieved real-time interactive guidance and visual policing of CO delivery. For the in vitro antitumor study, TARC-COs with ultrasonic irradiation were demonstrated to effectively induce mitochondrial dysfunction by reducing mitochondrial membrane potential, leading to the apoptosis of 4T1 cells. In addition, we realized that TARC-CO-based treatment could significantly slow the growth rate of tumors by inducing apoptosis, inhibiting the proliferation of cancer cells, and limiting tumor angiogenesis. In summary, this proof-of-concept study demonstrates the feasibility and tremendous potential of TARC-COs for controlled release of CO, which can be expected to provide new inspirations and a promising perspective for therapy based on active gases.

Carbon monoxide and its role in human physiology: A brief historical perspective

Carbon monoxide is a molecule with notoriety in modern culture and extensive documentation regarding its toxic physiological effects, long predating its formal discovery in the 18th century. Upon its discovery as a molecule in 1772, subsequent investigations into its properties have provided mechanisms describing its toxicity and insights into its function as an endogenously produced molecule and as a therapeutic agent. This brief review aims to provide a historical perspective on this molecule and recognize research regarding its physiological functions and therapeutic applications, often overshadowed by its reputation as a lethal substance. Historicizing science is an acknowledgment of the pioneers and helps us better conceptualize the issues.

External stimuli-responsive gasotransmitter prodrugs: Chemistry and spatiotemporal release

Gasotransmitters like nitric oxide, carbon monoxide, and hydrogen sulfide with unique pleiotropic pharmacological effects in mammals are an emerging therapeutic modality for different human diseases including cancer, infection, ischemia-reperfusion injuries, and inflammation; however, their clinical translation is hampered by the lack of a reliable delivery form, which delivers such gasotransmitters to the action site with precisely controlled dosage. The external stimuli-responsive prodrug strategy has shown tremendous potential in developing gasotransmitter prodrugs, which affords precise temporospatial control and better dose control compared with endogenous stimuli-sensitive prodrugs. The promising external stimuli employed for gasotransmitter activation range from photo, ultrasound, and bioorthogonal click chemistry to exogenous enzymes. Herein, we highlight the recent development of external stimuli-mediated decaging chemistry for the temporospatial delivery of gasotransmitters including nitric oxide, carbon monoxide, hydrogen sulfide and sulfur dioxide, and discuss the pros and cons of different designs.

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

Delivery of therapeutic carbon monoxide by gas-entrapping materials

Carbon monoxide (CO) has long been considered a toxic gas but is now a recognized bioactive gasotransmitter with potent immunomodulatory effects. Although inhaled CO is currently under investigation for use in patients with lung disease, this mode of administration can present clinical challenges. The capacity to deliver CO directly and safely to the gastrointestinal (GI) tract could transform the management of diseases affecting the GI mucosa such as inflammatory bowel disease or radiation injury. To address this unmet need, inspired by molecular gastronomy techniques, we have developed a family of gas-entrapping materials (GEMs) for delivery of CO to the GI tract. We show highly tunable and potent delivery of CO, achieving clinically relevant CO concentrations in vivo in rodent and swine models. To support the potential range of applications of foam GEMs, we evaluated the system in three distinct disease models. We show that a GEM containing CO dose-dependently reduced acetaminophen-induced hepatocellular injury, dampened colitis-associated inflammation and oxidative tissue injury, and mitigated radiation-induced gut epithelial damage in rodents. Collectively, foam GEMs have potential paradigm-shifting implications for the safe therapeutic use of CO across a range of indications.

NO, CO and H2S: A Trinacrium of Bioactive Gases in the Brain

Oxygen and carbon dioxide are time honored gases that have direct bearing on almost all life forms, but over the past thirty years, and in large part due to the Nobel Prize Award in Medicine for the elucidation of nitric oxide (NO) as a bioactive gas, the research and medical communities now recognize other gases as critical for survival. In addition to NO, hydrogen sulfide (H₂S) and carbon monoxide (CO) have emerged as a triumvirate or Trinacrium of gases with analogous importance and that serve important homeostatic functions. Perhaps, one of the most intriguing aspects of these gases is the functional interaction between them, which is intimately linked by the enzyme systems that produce them. Despite the need to better understand NO, H₂S and CO biology, the notion that these are environmental pollutants remains ever present. For this reason, incorporating the concept of hormesis becomes imperative and must be included in discussions when considering developing new therapeutics that involve these gases. While there is now an enormous literature base for each of these gasotransmitters, we provide here an overview of their respective physiologic roles in the brain.

Carbon monoxide (CO)/heme oxygenase (HO)-1 in gastrointestinal tumors pathophysiology and pharmacology - possible anti- and pro-cancer activities

Gastrointestinal (GI) tract cancers pose a significant pharmacological challenge for researchers in terms of the discovery of molecular agents and the development of targeted therapies. Although many ongoing clinical trials have brought new perspectives, there is still a lack of successful long-term treatment. Several novel pharmacological and molecular agents are being studied in the prevention and treatment of GI cancers. On the other hand, pharmacological tools designed to release an endogenous gaseous mediator, carbon monoxide (CO), were shown to prevent the gastric mucosa against various types of injuries and exert therapeutic properties in the treatment of GI pathologies. In this review, we summarized the current evidence on the role of CO and heme oxygenase 1 (HO-1) as a CO producing enzyme in the pathophysiology of GI tumors. We focused on a beneficial role of HO-1 and CO in biological systems and common pathological conditions. We further discussed the complex and ambiguous function of the HO-1/CO pathway in cancer cells with a special emphasis on molecular and cellular pro-cancerous and anti-cancer mechanisms. We also focused on the role that HO-1/CO plays in GI cancers, especially within upper parts such as esophagus or stomach.

Carbon monoxide signaling and soluble guanylyl cyclase: Facts, myths, and intriguing possibilities

The endogenous signaling roles of carbon monoxide (CO) have been firmly established at the pathway level. For CO's molecular mechanism(s) of actions, hemoproteins are generally considered as possible targets. Importantly, soluble guanylyl cyclase (sGC) is among the most widely referenced molecular targets. However, the affinity of CO for sGC (K_d: 240 μM) is much lower than for other highly abundant hemoproteins in the body, such as myoglobin (K_d: 29 nM) and hemoglobin (K_d: 0.7 nM-4.5 μM), which serve as CO reservoirs. Further, most of the mechanistic studies involving sGC activation by CO were based on in-vitro or ex-vivo studies using CO concentrations not readily attenable in vivo and in the absence of hemoglobin as a competitor in binding. As such, whether such in-vitro/ex-vivo results can be directly extrapolated to in-vivo studies is not clear because of the need for CO to be transferred from a high-affinity binder (e.g., hemoglobin) to a low-affinity target if sGC is to be activated in vivo. In this review, we discuss literature findings of sGC activation by CO and the experimental conditions; examine the myths in the disconnect between the low affinity of sGC for CO and the reported activation of sGC by CO; and finally present several possibilities that may lead to additional studies to improve our understanding of this direct CO-sGC axis, which is yet to be convincingly established as playing generally critical roles in CO signaling in vivo.

Targeted carbon monoxide delivery combined with chemodynamic, chemotherapeutic and photothermal therapies for enhanced antitumor efficacy

Polydopamine-coated hollow mesoporous copper sulfide loaded with DHA and CO-releasing molecules selectively delivered DHA and CO to tumor cells under 808 nm light irradiation, demonstrating multimodal synergistic antitumor efficacy.

Glucagon-like peptide-2 protects the gastric mucosa via regulating blood flow and metabolites

INTRODUCTION

Refractory peptic ulcers lead to perforation and hemorrhage, which are fatal. However, these remain a therapeutic challenge. Gastric mucosal blood flow is crucial in maintaining gastric mucosal health. It's reported that Glucagon-like peptide-2 (GLP-2), a gastrointestinal hormone, stimulated intestinal blood flow. However, the direct role of GLP-2 in gastric mucosal blood flow and metabolites remain unclear. Here, we speculated that GLP-2 might protect the gastric mucosa by increasing gastric mucosal blood flow and regulating metabolites. This study was conducted to evaluate the role of GLP-2 in gastric mucosal lesions and its underlying mechanism.

METHODS

We analyzed endogenous GLP-2 during gastric mucosal injury in the serum. Rats were randomly divided into two groups, with 36 rats in each group as follows: (1) normal control group (NC1); (2) ethanol model group (EC1); rats in EC1 and NC1 groups were intragastrically administered ethanol (1 ml/200 g body weight) and distilled water (1 ml/200 g body weight). The serum was collected 10 min before intragastric administration and 15, 30, 60, 90, and 120 min after intragastric administration. Furthermore, additional male Sprague-Dawley rats were randomly divided into three groups, with six rats in each group as follows: (1) normal control group (NC); (2) ethanol model group (EC); (3) 10 μg/200 g body weight GLP-2 group (GLP-2). Rats in the NC and EC groups were intraperitoneally injected with saline. Those in the GLP-2 group were intraperitoneally injected with GLP-2. Thirty minutes later, rats in the EC and GLP-2 groups were intragastrically administered ethanol (1 ml/200 g body weight), and rats in the NC group were intragastrically administered distilled water (1 ml/200 g body weight). After the intragastric administration of ethanol for 1 h, the animals were anesthetized and gastric mucosal blood flow was measured. Serum were collected for ultra performance liquid chromatography/tandem mass spectrometry (UPLC-MS/MS) metabolomics.

RESULTS

There were no significant change in endogenous GLP-2 during gastric mucosal injury (P<0.05). Pretreatment with GLP-2 significantly reduced ethanol-induced gastric mucosal lesions by improving the gastric mucosal blood flow, as examined using a laser Doppler flow meter, Guth Scale, hematoxylin-eosin staining, and two-photon microscopy. UPLC-MS/MS analyses showed that GLP-2 also maintained a steady state of linoleic acid metabolism.

CONCLUSIONS

Taken together, GLP-2 protects the gastric mucosa against ethanol-induced lesions by improving gastric mucosa blood flow and affecting linoleic acid metabolism.

B-Ring-extended flavonol-based photoCORM: activated by cysteine-ratiometric fluorescence sensing and accurate control of linear CO release

The first B-ring-extended (to biphenyl) flavonol-based Cys-ratiometric fluorescent probe B-bph-fla-acr (2-([1,1'-biphenyl]-4-yl)-4-oxo-4H-chromen-3-yl acrylate) is developed. B-bph-fla-acr can ratiometrically sense and non-ratiometrically image endogenous and exogenous cysteine (Cys) in living HeLa cells and zebrafish rapidly (45 s), selectively (vs. homocysteine and glutathione), sensitively (detection limit: 18.5 nM), and with a large Stokes shift (186 nm). Quantitatively released (from the reaction of B-bph-fla-acr with Cys) fluorophore B-bph-fla-OH (2-([1,1'-biphenyl]-4-yl)-3-hydroxy-4H-chromen-4-one) is designed as a photoCORM (photo-triggered CO releasing molecule). Under O₂ and visible light irradiation, the amount of CO released by B-bph-fla-OH can be accurately controlled linearly by adjusting the light irradiation intensity, irradiation time, or photoCORM dose. This process is accompanied by fluorescence quenching; therefore, the location of the photoCORM and the CO release process can be monitored in real time. B-bph-fla-acr and all reaction products exhibit good membrane permeability and low toxicity for living HeLa cells. In living HeLa cells and zebrafish, B-bph-fla-acr can image endogenous and exogenous Cys, and the released B-bph-fla-OH can photo-release CO under O₂ at room temperature. This study is the first to combine a B-ring-extended flavonol-based fluorescent probe (for the effective ratiometric sensing and non-ratiometric imaging of endogenous and exogenous Cys in vitro and in vivo) with a photoCORM (Cys-activated, visible light-triggered linear CO release under O₂). Our study provides important insights into the biological roles of Cys and CO, as well as a reliable method for safely supplying accurately controlled amounts of CO to living systems, thereby facilitating the development of convenient clinical diagnostic molecular tools and therapeutic prodrugs.

Redox and catalase-like activities of four widely used carbon monoxide releasing molecules (CO-RMs)

The pathophysiological roles of the endogenous signaling molecule, carbon monoxide (CO), have been extensively studied and validated in cell culture and animal models. Further, evidence supporting the therapeutic effects of CO in various human diseases has been mounting over the last two decades. Along this line, there has been intensive interest in developing various delivery forms including CO gas, CO in solution, metal–carbonyl complexes widely known as CO-releasing molecules (CO-RMs), and organic CO prodrugs. Among them, two ruthenium-based carbonyl complexes, CORM-2 and -3, occupy a very special place because they have been used in over 500 published studies. One of the mechanisms for CO's actions is known to be through attenuation of oxidative stress and regulation of production of reactive oxygen species (ROS). For this reason, it is important that CO delivery forms do not have intrinsic chemical redox properties. Herein, we describe our findings of catalase-like activities of CORM-2 and -3 in a CO-independent fashion, leading to the rapid degradation of hydrogen peroxide (H₂O₂) in PBS buffer (pH = 7.4) and in cell culture media. Further, we have found that CORM-2 and CORM-3 possess potent radical scavenging abilities. We have also studied two other widely used CO donors: CORM-401 and CORM-A1. Both showed chemical reactivity with ROS, but to a lesser degree than CORM-2 and -3. Because of the central role of ROS in some of the proposed mechanisms of actions for CO biology, the discovery of intrinsic chemical redox properties for these CO-RMs means that additional attention in designing proper controls is needed in future biological experiments using these CO-RMs for their CO-donating functions. Further, much more work is needed to understand the true implications of the chemical reactivity of these CO-RMs in cell-culture and animal-model studies of CO biology.

Four CO-releasing molecules are found to degrade H₂O₂ and free radicals either catalytically (CORM-2 and -3) or through direct reactions (CORM-401 and -A1) in solution under near-physiological conditions.

"CO in a pill": Towards oral delivery of carbon monoxide for therapeutic applications

Along with the impressive achievements in understanding the endogenous signaling roles and mechanism(s) of action of carbon monoxide (CO), much research has demonstrated the potential of using CO as a therapeutic agent for treating various diseases. Because of CO's toxicity at high concentrations and the observed difference in toxicity profiles of CO depending on the route of administration, this review analyzes and presents the benefits of developing orally active CO donors. Such compounds have the potential for improved safety profiles, enhancing the chance for developing CO-based therapeutics. In this review, the difference between inhalation and oral administration in terms of toxicity, CO delivery efficiency, and the potential mechanism(s) of action is analyzed. The evolution from CO gas inhalation to oral administration is also extensively analyzed by summarizing published studies up to date. The concept of "CO in a pill" can be achieved by oral administration of novel formulations of CO gas or appropriate CO donors.

Adapting decarbonylation chemistry for the development of prodrugs capable of in vivo delivery of carbon monoxide utilizing sweeteners as carrier molecules

Carbon monoxide as an endogenous signaling molecule exhibits pharmacological efficacy in various animal models of organ injury. To address the difficulty in using CO gas as a therapeutic agent for widespread applications, we are interested in developing CO prodrugs through bioreversible caging of CO in an organic compound. Specifically, we have explored the decarboxylation-decarbonylation chemistry of 1,2-dicarbonyl compounds. Examination and optimization of factors favorable for maximal CO release under physiological conditions led to organic CO prodrugs using non-calorific sweeteners as leaving groups attached to the 1,2-dicarbonyl core. Attaching a leaving group with appropriate properties promotes the desired hydrolysis-decarboxylation-decarbonylation sequence of reactions that leads to CO generation. One such CO prodrug was selected to recapitulate the anti-inflammatory effects of CO against LPS-induced TNF-α production in cell culture studies. Oral administration in mice elevated COHb levels to the safe and efficacious levels established in various preclinical and clinical studies. Furthermore, its pharmacological efficacy was demonstrated in mouse models of acute kidney injury. These studies demonstrate the potential of these prodrugs with benign carriers as orally active CO-based therapeutics. This represents the very first example of orally active organic CO prodrugs with a benign carrier that is an FDA-approved sweetener with demonstrated safety profiles in vivo.

Click, release, and fluoresce: In-vivo generation of CO with concomitant synthesis of a fluorescent reporter

Delivering a therapeutically active gaseous molecule represents very unique challenges in terms of both precise dosing and concentration assessment. To overcome these obstacles, there have been recent reports of using prodrug approaches for the in-vitro and in-vivo generation of carbon monoxide (CO), which is an endogenous signaling molecule with validated therapeutic efficacy in a range of animal models. Some key components of these approaches include the use of a hydrophobicity-driven Diels-Alder reaction under physiological conditions followed by a cheletropic reaction of the corresponding norbornadien-7-one intermediate, leading to extrusion of CO. With proper design, the same approach also leads to the formation of a fluorescent reporter, allowing for quantitative assessment of the amount of CO released. All these allow for a strategy of "click, release, and fluoresce" in delivering a precise dose of carbon monoxide with the ability to "self-report" delivery quantity and efficiency. This strategy has also been further refined to construct a CO delivery platform with additional functionalities such as bioorthogonal labeling, targeting, triggered release, and simultaneously delivery of more than one payload. This review highlights recent developments in this area.

Carbon Monoxide and Nitric Oxide as Examples of the Youngest Class of Transmitters

The year 2021 is the 100th anniversary of the confirmation of the neurotransmission phenomenon by Otto Loewi. Over the course of the hundred years, about 100 neurotransmitters belonging to many chemical groups have been discovered. In order to celebrate the 100th anniversary of the confirmation of neurotransmitters, we present an overview of the first two endogenous gaseous transmitters i.e., nitric oxide, and carbon monoxide, which are often termed as gasotransmitters.

Nature's marvels endowed in gaseous molecules I: Carbon monoxide and its physiological and therapeutic roles

Nature has endowed gaseous molecules such as O2, CO2, CO, NO, H2S, and N2 with critical and diverse roles in sustaining life, from supplying energy needed to power life and building blocks for life's physical structure to mediating and coordinating cellular functions. In this article, we give a brief introduction of the complex functions of the various gaseous molecules in life and then focus on carbon monoxide as a specific example of an endogenously produced signaling molecule to highlight the importance of this class of molecules. The past twenty years have seen much progress in understanding CO's mechanism(s) of action and pharmacological effects as well as in developing delivery methods for easy administration. One remarkable trait of CO is its pleiotropic effects that have few parallels, except perhaps its sister gaseous signaling molecules such as nitric oxide and hydrogen sulfide. This review will delve into the sophistication of CO-mediated signaling as well as its validated pharmacological functions and possible therapeutic applications.

Novel Hydrogen Sulfide (H2S)-Releasing BW-HS-101 and Its Non-H2S Releasing Derivative in Modulation of Microscopic and Molecular Parameters of Gastric Mucosal Barrier

Hydrogen sulfide (H2S) is an endogenously produced molecule with anti-inflammatory and cytoprotective properties. We aimed to investigate for the first time if a novel, esterase-sensitive H2S-prodrug, BW-HS-101 with the ability to release H2S in a controllable manner, prevents gastric mucosa against acetylsalicylic acid-induced gastropathy on microscopic and molecular levels. Wistar rats were pretreated intragastrically with vehicle, BW-HS-101 (0.5-50 μmol/kg) or its analogue without the ability to release H2S, BW-iHS-101 prior to ASA administration (125 mg/kg, intragastrically). BW-HS-101 was administered alone or in combination with nitroarginine (L-NNA, 20 mg/kg, intraperitoneally) or zinc protoporphyrin IX (10 mg/kg, intraperitoneally). Gastroprotective effects of BW-HS-101 were additionally evaluated against necrotic damage induced by intragastrical administration of 75% ethanol. Gastric mucosal damage was assessed microscopically, and gastric blood flow was determined by laser flowmetry. Gastric mucosal DNA oxidation and PGE2 concentration were assessed by ELISA. Serum and/or gastric protein concentrations of IL-1α, IL-1β, IL-2, IL-4, IL-6, IL-10, IL-13, VEGF, GM-CSF, IFN-γ, TNF-α, and EGF were determined by a microbeads/fluorescent-based multiplex assay. Changes in gastric mucosal iNOS, HMOX-1, SOCS3, IL1-R1, IL1-R2, TNF-R2, COX-1, and COX-2 mRNA were assessed by real-time PCR. BW-HS-101 or BW-iHS-101 applied at a dose of 50 μmol/kg protected gastric mucosa against ASA-induced gastric damage and prevented a decrease in the gastric blood flow level. H2S prodrug decreased DNA oxidation, systemic and gastric mucosal inflammation with accompanied upregulation of SOCS3, and EGF and HMOX-1 expression. Pharmacological inhibition of nitric oxide (NO) synthase but not carbon monoxide (CO)/heme oxygenase (HMOX) activity by L-NNA or ZnPP, respectively, reversed the gastroprotective effect of BW-HS-101. BW-HS-101 also protected against ethanol-induced gastric injury formation. We conclude that BW-HS-101, due to its ability to release H2S in a controllable manner, prevents gastric mucosa against drugs-induced gastropathy, inflammation and DNA oxidation, and upregulate gastric microcirculation. Gastroprotective effects of this H2S prodrug involves endogenous NO but not CO activity and could be mediated by cytoprotective and anti-inflammatory SOCS3 and EGF pathways.

Gaseous Mediators as a Key Molecular Targets for the Development of Gastrointestinal-Safe Anti-Inflammatory Pharmacology

Non-steroidal anti-inflammatory drugs (NSAIDs) represent one of the most widely used classes of drugs and play a pivotal role in the therapy of numerous inflammatory diseases. However, the adverse effects of these drugs, especially when applied chronically, frequently affect gastrointestinal (GI) tract, resulting in ulceration and bleeding, which constitutes a significant limitation in clinical practice. On the other hand, it has been recently discovered that gaseous mediators nitric oxide (NO), hydrogen sulfide (H₂S) and carbon monoxide (CO) contribute to many physiological processes in the GI tract, including the maintenance of GI mucosal barrier integrity. Therefore, based on the possible therapeutic properties of NO, H₂S and CO, a novel NSAIDs with ability to release one or more of those gaseous messengers have been synthesized. Until now, both preclinical and clinical studies have shown promising effects with respect to the anti-inflammatory potency as well as GI-safety of these novel NSAIDs. This review provides an overview of the gaseous mediators-based NSAIDs along with their mechanisms of action, with special emphasis on possible implications for GI mucosal defense mechanisms.

Chemical Reactivities of Two Widely Used Ruthenium-Based CO-Releasing Molecules with a Range of Biologically Important Reagents and Molecules

Ruthenium-based CO-releasing molecules (CO-RMs), CORM-2 and CORM-3, have been widely used as surrogates of CO for studying its biological effects in vitro and in vivo with much success. However, several previous solution-phase and in vitro studies have revealed the ability of such CO-RMs to chemically modify proteins and reduce aromatic nitro groups due to their intrinsic chemical reactivity under certain conditions. In our own work of studying the cytoprotective effects of CO donors, we were in need of assessing chemical factors that could impact the interpretation of results from CO donors including CORM-2,3 in various in vitro assays. For this, we examined the effects of CORM-2,3 toward representative reagents commonly used in various bioassays including resazurin, tetrazolium salts, nitrites, and azide-based H₂S probes. We have also examined the effect of CORM-2,3 on glutathione disulfide (GSSG), which is a very important redox regulator. Our studies show the ability of these CO-RMs to induce a number of chemical and/or spectroscopic changes for several commonly used biological reagents under near-physiological conditions. These reactions/spectroscopic changes cannot be duplicated with CO-deleted CO-RMs (iCORMs), which are often used as negative controls. Furthermore, both CORM-2 and -3 are capable of consuming and reducing GSSG in solution. We hope that the results described will help in the future design of control experiments using Ru-based CO-RMs.

Role of Carbon Monoxide in Host-Gut Microbiome Communication

Nature is full of examples of symbiotic relationships. The critical symbiotic relation between host and mutualistic bacteria is attracting increasing attention to the degree that the gut microbiome is proposed by some as a new organ system. The microbiome exerts its systemic effect through a diverse range of metabolites, which include gaseous molecules such as H₂, CO₂, NH₃, CH₄, NO, H₂S, and CO. In turn, the human host can influence the microbiome through these gaseous molecules as well in a reciprocal manner. Among these gaseous molecules, NO, H₂S, and CO occupy a special place because of their widely known physiological functions in the host and their overlap and similarity in both targets and functions. The roles that NO and H₂S play have been extensively examined by others. Herein, the roles of CO in host-gut microbiome communication are examined through a discussion of (1) host production and function of CO, (2) available CO donors as research tools, (3) CO production from diet and bacterial sources, (4) effect of CO on bacteria including CO sensing, and (5) gut microbiome production of CO. There is a large amount of literature suggesting the "messenger" role of CO in host-gut microbiome communication. However, much more work is needed to begin achieving a systematic understanding of this issue.