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

Recent advances in carbon monoxide-releasing nanomaterials

As an endogenous signaling molecule, carbon monoxide (CO) has emerged as an increasingly promising option regarding as gas therapy due to its positive pharmacological effects in various diseases. Owing to the gaseous nature and potential toxicity, it is particularly important to modulate the CO release dosages and targeted locations to elucidate the biological mechanisms of CO and facilitate its clinical applications. Based on these, diverse CO-releasing molecules (CORMs) have been developed for controlled release of CO in biological systems. However, practical applications of these CORMs are limited by several disadvantages including low stability, poor solubility, weak releasing controllability, random diffusion, and potential toxicity. In light of rapid developments and diverse advantages of nanomedicine, abundant nanomaterials releasing CO in controlled ways have been developed for therapeutic purposes across various diseases. Due to their nanoscale sizes, diversified compositions and modified surfaces, vast CO-releasing nanomaterials (CORNMs) have been constructed and exhibited controlled CO release in specific locations under various stimuli with better pharmacokinetics and pharmacodynamics. In this review, we present the recent progress in CORNMs according to their compositions. Following a concise introduction to CO therapy, CORMs and CORNMs, the representative research progress of CORNMs constructed from organic nanostructures, hybrid nanomaterials, inorganic nanomaterials, and nanocomposites is elaborated. The basic properties of these CORNMs, such as active components, CO releasing mechanisms, detection methods, and therapeutic applications, are discussed in detail and listed in a table. Finally, we explore and discuss the prospects and challenges associated with utilizing nanomaterials for biological CO release.

Investigating the Combined Toxicity of Cu(II) and Carbon Monoxide (CO); Cellular CO Delivery Using a Cu(II) Flavonolato Complex

Carbon monoxide (CO) delivery molecules are of significant current interest as potential therapeutics, including for anticancer applications. A recent approach toward generating new types of materials-based anticancer agents involves combining the Fenton reactivity of a redox active metal ion with CO delivery. However, small molecule examples of these types of entities have not been systematically studied to evaluate the combined effect on cellular toxicity. Herein we describe a Cu(II) flavonolato complex which produces anticancer effects through a combination of copper-mediated reactive oxygen species (ROS) generation and light-induced flavonol CO release. Confocal microscopy studies provide evidence of enhanced flavonol uptake in the copper flavonolato system relative to the free flavonol, which leads to an increased amount of CO delivery within cells. Importantly, this work demonstrates that a metal flavonolato species can be used to produce enhanced toxicity effects resulting from both metal ion-induced Fenton reactivity and increased cellular uptake of a flavonol CO donor.

H2O2-triggered CO release based on porphyrinic covalent organic polymers for photodynamic/gas synergistic therapy

A novel H2O2-responsive carbon monoxide nanogenerator was designed by effectively encapsulating a manganese carbonyl prodrug into porphyrinic covalent organic polymers for realizing the combined CO gas and photodynamic therapy under near infrared light irradiation.

A series of triphenylamine-derived fluorophores attached to a Cu-based MOF for gaseous CO optical sensing: synthesis, performance, and mechanism

A series of triphenylamine-derived fluorescent dyes were attached to a Cu2+-containing MOF (metal-organic framework), denoted as Pm@CuMOF. The molecular structures of these dyes were discussed by the single crystal structures. Their major absorption bands peaked at 410-450 nm, showing emission bands ranging from 556 to 586 nm with emission quantum yields ranging from 8.0 to 15.1%. It was found that the [-N(C2H5)2] group generally improved sensing performance, and the -OH group in the dyes helped the Cu2+ quenching effect. Pm@CuMOF was observed by SEM as nanorods with a width of ~100 nm and a length of 300 nm. Their XRD patterns and N2 adsorption/desorption isotherms were recorded to confirm their porous structure. A low probe loading level of ~4% was determined by TGA result. The CO sensing mechanism was revealed as a Cu2+/Cu+-involved sensing mechanism based on the result of NMR titration, IR, XPS, and EPR. The fluorescence of these triphenylamine-derived dyes was firstly quenched by CuMOF. In contact with CO, Cu2+ was reduced to Cu+, accompanied by the release and fluorescence recovery of the fluorescent dyes, showing emission turn-on effect towards CO gas. Pm@CuMOF showed increased emission intensity at CO level of 0.005% (versus N2), with response times ranging from 123 s to 280 s (depending on various temperatures). Good selectivity was observed over competing alkane gases, with stable emission for at least 5 days, but no linear calibration plots were observed.

Azoanthracene-core structure as Cu2+-assisted CO sensing probe: Characterization, performance, and bioimaging

Detection of endogenous CO (carbon monoxide) is an interesting topic in biology because it has been discovered as a messenger for signal transduction and therapeutic effects in vital biological activities. Fluorescence imaging has proven a powerful tool for detecting endogenous CO, which drives the development of low-cost and easy-to-use fluorescent probes. In this study, four azobenzene derivatives (A1, A2, A3, and A4) with various substituents were reported, including their geometric structures, photophysical parameters, and spectral responses to Cu2+ and CO. The relationship between substituent structure and performance was discussed along with Cu2+ quenching and CO sensing mechanisms. The optimal probe (A1), which had no substituent, efficiently quenched fluorescence in the presence of Cu2+, with its PLQY decreased from 0.33 to 0.02, PLQY = photoluminescence quantum yield. Upon CO deoxidization, A1's fluorescence could be recovered (PLQY recovered to 0.32) within 180 s. Its sensing mechanism was static by forming a non-fluorescent complex with Cu2+ (with a stoichiometric ratio of 1:1). The bioimaging performance of A1 for endogenous CO in HeLa cells was reported.

An injectable hydrogel dressing for controlled release of hydrogen sulfide pleiotropically mediates the wound microenvironment

The healing of scalded wounds faces many challenges such as chronic inflammation, oxidative stress, wound infection, and difficulties in vascular and nerve regeneration. Treating a single problem cannot effectively coordinate the complex regenerative microenvironment of scalded wounds, limiting the healing and functional recovery of the skin. Therefore, there is a need to develop a multi-effect treatment plan that can adaptively address the issues at each stage of wound healing. In this study, we propose a scheme for on-demand release of hydrogen sulfide (H2S) based on the concentration of reactive oxygen species (ROS) in the wound microenvironment. This is achieved by encapsulating peroxythiocarbamate (PTCM) in the ROS-responsive polymer poly(ethylene glycol)-poly(L-methionine) (PMet) to form nanoparticles, which are loaded into a thermosensitive injectable hydrogel, F127-poly(L-aspartic acid-N-hydroxysuccinimide) (F127-P(Asp-NHS)), to create a scald dressing. The H2S released by the hydrogel dressing on demand regulates the wound microenvironment by alleviating infection, reducing oxidative stress, and remodeling inflammation, thereby accelerating the healing of full-thickness scalded wounds. This hydrogel dressing for the adaptive release of H2S has great potential in addressing complex scalded wounds associated with infection and chronic inflammation.

RSS and ROS Sequentially Activated Carbon Monoxide Release for Boosting NIR Imaging-Guided On-Demand Photodynamic Therapy

Carbon monoxide shows great therapeutic potential in anti-cancer. In particular, the construction of multifunctional CO delivery systems can promote the precise delivery of CO and achieve ideal therapeutic effects, but there are still great challenges in design. In this work, a RSS and ROS sequentially activated CO delivery system is developed for boosting NIR imaging-guided on-demand photodynamic therapy. This designed system is composed of a CO releaser (BOD-CO) and a photosensitizer (BOD-I). BOD-CO can be specifically activated by hydrogen sulfide with simultaneous release of CO donor and NIR fluorescence that can identify H2S-rich tumors and guide light therapy, also depleting H2S in the process. Moreover, BOD-I generates 1O2 under long-wavelength light irradiation, enabling both PDT and precise local release of CO via a photooxidation mechanism. Such sequential activation of CO release by RSS and ROS ensured the safety and controllability of CO delivery, and effectively avoided leakage during delivery. Importantly, cytotoxicity and in vivo studies reveal that the release of CO combined with the depletion of endogenous H2S amplified PDT, achieving ideal anticancer results. It is believed that such theranostic nanoplatform can provide a novel strategy for the precise CO delivery and combined therapy involved in gas therapy and PDT.

Vacancy-Rich Bismuth-Based Nanosheets for Mitochondrial Destruction via CO Poisoning, Ca2+ Dyshomeostasis, and Oxidative Damage

Mitochondria are core regulators of tumor cell homeostasis, and their damage has become an arresting therapeutic modality against cancer. Despite the development of many mitochondrial-targeted pharmaceutical agents, the exploration of more powerful and multifunctional medications is still underway. Herein, oxygen vacancy-rich BiO2-x wrapped with CaCO3 (named BiO2-x@CaCO3/PEG, BCP) is developed for full-fledged attack on mitochondrial function. After endocytosis of BCP by tumor cells, the CaCO3 shell can be decomposed in the acidic lysosomal compartment, leading to immediate Ca2+ release and CO2 production in the cytoplasm. Near-infrared irradiation enhances the adsorption of CO2 onto BiO2-x defects, which enables highly efficient photocatalysis of CO2-to-CO. Meanwhile, such BiO2-x nanosheets possess catalase-, peroxidase- and oxidase-like catalytic activities under acidic pH conditions, allowing hypoxia relief and the accumulation of diverse reactive oxygen species (ROS) in the tumor microenvironment. Ca2+ overload-induced ion dyshomeostasis, CO-mediated respiratory chain poisoning, ROS-triggered oxidative stress aggravation, and cytosolic hyperoxia can cause severe mitochondrial disorders, which further lead to type I cell death in carcinoma. Not only does BCP cause irreversible apoptosis, but immunogenic cell death is simultaneously triggered to activate antitumor immunity for metastasis inhibition. Collectively, this platform promises high benefits in malignant tumor therapy and may expand the medical applications of bismuth-based nanoagents.

Pyrolysis of exhausted biochar sorbent: Fates of cadmium and generation of products

Biochar is a promising sorbent for Cd removal from water, while the disposal of the exhausted Cd-enriched biochar remains a challenge. In this study, pyrolysis was employed to treat the exhausted biochar under N2 and CO2 atmospheres at 600-900 °C, and the fate of Cd during pyrolysis and characteristics of high-valued products were determined. The results indicated that higher temperature and CO2 atmosphere favored the volatilization of Cd. Based on the toxicity characteristic leaching procedure (TCLP) results, the pyrolysis treatment under both atmospheres enhanced the stability of Cd, and the leached Cd concentration of regenerated biochar obtained at high temperatures (>800 °C) was lower than 1 mg/L. Compared with the pristine biochar, the regenerated biochar demonstrated higher carbon content and pH, whereas the contents of oxygen and hydrogen declined, and exhibited promising sorption properties (35.79 mg/g). The atmosphere played an important role in modifying biochar properties and syngas composition. The N2 atmosphere facilitated CH4 production, whereas the CO2 atmosphere increased the proportion of CO. These results implied that pyrolysis can be a valuable and environmental-friendly strategy for the treatment and reuse of exhausted biochar sorbent.

X-Ray-Triggered CO-Release from Gold Nanocluster: All-in-One Nanoplatforms for Cancer Targeted Gas and Radio Synergistic Therapy

Glycolysis-dominant metabolic pathway in cancer cells can promote their therapeutic resistance against radiotherapy (RT). Carbon monoxide (CO) as a glycolysis inhibitor can enhance the efficiency of RT. Herein, an X-ray responsive CO-releasing nanocomposite (HA@AuNC@CO) based on strong host-guest interactions between the radiosensitizer and CO donor for enhanced RT is developed. The encapsulated gold nanoclusters (CD-AuNCs) can effectively generate cytotoxic reactive oxygen species (ROS) under X-ray radiation, which not only directly inactivate cancer cells but also induce in situ CO gas generation from adamantane modified metal carbonyl (Ada-CO) for glycolysis inhibition. Both in vitro and in vivo results demonstrate that HA@AuNC@CO exhibits active targeting toward CD44 overexpressed cancer cells, along with excellent inhibition of glycolysis and efficient RT against cancer. This study offers a new strategy for the combination of gas therapy and RT in tumor treatment.

Current trends in gas-synergized phototherapy for improved antitumor theranostics

Phototherapy, such as photothermal therapy (PTT) and photodynamic therapy (PDT), has been considered an elegant solution to eradicate tumors due to its minimal invasiveness and low systemic toxicity. Nevertheless, it is still challenging for phototherapy to achieve ideal outcomes and clinical translation due to its inherent drawbacks. Owing to the unique biological functions, diverse gases have attracted growing attention in combining with phototherapy to achieve super-additive therapeutic effects. Specifically, gases such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) have been proven to kill tumor cells by inducing mitochondrial damage in synergy with phototherapy. Additionally, several gases not only enhance the thermal damage in PTT and the reactive oxygen species (ROS) production in PDT but also improve the tumor accumulation of photoactive agents. The inflammatory responses triggered by hyperthermia in PTT are also suppressed by the combination of gases. Herein, we comprehensively review the latest studies on gas-synergized phototherapy for cancer therapy, including (1) synergistic mechanisms of combining gases with phototherapy; (2) design of nanoplatforms for gas-synergized phototherapy; (3) multimodal therapy based on gas-synergized phototherapy; (4) imaging-guided gas-synergized phototherapy. Finally, the current challenges and future opportunities of gas-synergized phototherapy for tumor treatment are discussed. STATEMENT OF SIGNIFICANCE: 1. The novelty and significance of the work with respect to the existing literature. (1) Strategies to design nanoplatforms for gas-synergized anti-tumor phototherapy have been summarized for the first time. Meanwhile, the integration of various imaging technologies and therapy modalities which endow these nanoplatforms with advanced theranostic capabilities has been summarized. (2) The mechanisms by which gases synergize with phototherapy to eradicate tumors are innovatively and comprehensively summarized. 2. The scientific impact and interest. This review elaborates current trends in gas-synergized anti-tumor phototherapy, with special emphases on synergistic anti-tumor mechanisms and rational design of therapeutic nanoplatforms to achieve this synergistic therapy. It aims to provide valuable guidance for researchers in this field.

Hydrogels for Gasotransmitter Delivery: Nitric Oxide, Carbon Monoxide, and Hydrogen Sulfide

Gasotransmitters, gaseous signaling molecules including nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2 S), maintain myriad physiological processes. Low levels of gasotransmitters are often associated with specific problems or diseases, so NO, CO, and H2 S hold potential in treating bacterial infections, chronic wounds, myocardial infarction, ischemia, and various other diseases. However, their clinical applications as therapeutic agents are limited due to their gaseous nature, short half-life, and broad physiological roles. One route toward the greater application of gasotransmitters in medicine is through localized delivery. Hydrogels are attractive biomedical materials for the controlled release of embedded therapeutics as they are typically biocompatible, possess high water content, have tunable mechanical properties, and are injectable in certain cases. Hydrogel-based gasotransmitter delivery systems began with NO, and hydrogels for CO and H2 S have appeared more recently. In this review, the biological importance of gasotransmitters is highlighted, and the fabrication of hydrogel materials is discussed, distinguishing between methods used to physically encapsulate small molecule gasotransmitter donor compounds or chemically tether them to a hydrogel scaffold. The release behavior and potential therapeutic applications of gasotransmitter-releasing hydrogels are also detailed. Finally, the authors envision the future of this field and describe challenges moving forward.

Nanoreactor-based catalytic systems for therapeutic applications: Principles, strategies, and challenges

Inspired by natural catalytic compartments, various synthetic compartments that seclude catalytic reactions have been developed to understand complex multistep biosynthetic pathways, bestow therapeutic effects, or extend biosynthetic pathways in living cells. These emerging nanoreactors possessed many advantages over conventional biomedicine, such as good catalytic activity, specificity, and sustainability. In the past decade, a great number of efficient catalytic systems based on diverse nanoreactors (polymer vesicles, liposome, polymer micelles, inorganic-organic hybrid materials, MOFs, etc.) have been designed and employed to initiate in situ catalyzed chemical reactions for therapy. This review aims to present the recent progress in the development of catalytic systems based on nanoreactors for therapeutic applications, with a special emphasis on the principles and design strategies. Besides, the key components of nanoreactor-based catalytic systems, including nanocarriers, triggers or energy inputs, and products, are respectively introduced and discussed in detail. Challenges and prospects in the fabrication of therapeutic catalytic nanoreactors are also discussed as a conclusion to this review. We believe that catalytic nanoreactors will play an increasingly important role in modern biomedicine, with improved therapeutic performance and minimal side effects.

Tumor microenvironment-responsive delivery nanosystems reverse immunosuppression for enhanced CO gas/immunotherapy

Carbon monoxide (CO) gas therapy demonstrates great potential to induce cancer cell apoptosis and antitumor immune responses, which exhibits tremendous potential in cancer treatment. However, the therapeutic efficacy of CO therapy is inhibited by the immunosuppressive tumor microenvironment (TME). Herein, a facile strategy is proposed to construct hollow-structured rough nanoplatforms to boost antitumor immunity and simultaneously reverse immunosuppression by exploring intrinsic immunomodulatory properties and morphological optimization of nanomaterials. The TME-responsive delivery nanosystems (M-RMH) are developed by encapsulating the CO prodrug within hollow rough MnO2 nanoparticles and the subsequent surface functionalization with hyaluronic acid (HA). Rough surfaces are designed to facilitate the intrinsic properties of HA-functionalized MnO2 nanoparticles (RMH) to induce dendritic cell maturation and M1 macrophage polarization by STING pathway activation and hypoxia alleviation through enhanced cellular uptake. After TME-responsive degradation of RMH, controlled release of CO is triggered at the tumor site for CO therapy to activate antitumor immunity. More importantly, RMH could modulate immunosuppressive TME by hypoxia alleviation. After the combination with aPD-L1-mediated checkpoint blockade therapy, robust antitumor immune responses are found to inhibit both primary and distant tumors. This work provides a facile strategy to construct superior delivery nanosystems for enhanced CO/immunotherapy through efficient activation of antitumor immune responses and reversal of immunosuppression.

An Orally-Administered Nanotherapeutics with Carbon Monoxide Supplying for Inflammatory Bowel Disease Therapy by Scavenging Oxidative Stress and Restoring Gut Immune Homeostasis

Traditional drug-based treatments for inflammatory bowel disease (IBD) have significant limitations due to their potential off-target systemic side-effects. Currently, there is a lack of understanding on how to effectively address excessive oxidative stress, dysregulated immune homeostasis, and microbiota dysbiosis within the IBD microenvironment. Herein, we introduce a nanotherapeutic approach, named LBL-CO@MPDA, for IBD treatment. LBL-CO@MPDA is an orally administered formulation that supplies carbon monoxide (CO) for therapeutic purposes. To create the LBL-CO@MPDA nanocomposite, we developed a layer by layer (LBL) self-assembly strategy where we coated chitosan/alginate polyelectrolytes onto the surface of CO prodrug-loaded mesoporous polydopamine nanoparticles (CO@MPDA). Benefiting from the negatively charged surface of the LBL coating, it allows for targeted accumulation of LBL-CO@MPDA specifically onto the positively charged inflamed colon lesions through electrostatic interactions. Furthermore, in the oxidative microenvironment of the inflamed colon, the nanotherapeutic system releases CO in a responsive manner. Interestingly, CO@MPDA ameliorates inflammatory conditions by MPDA-mediated ROS-scavenging and CO-mediated immunomodulation. CO-supplying activates heme oxygenase-1, leading to macrophage M2 polarization via the Notch/Hes1/Stat3 signaling pathway, while suppressing the inflammatory response by down-regulating the p38 MAPK and NF-κB (p50/p65) signaling pathways. In the mice model of dextran sulfate sodium (DSS)-induced IBD, LBL-CO@MPDA effectively reverses the pro-inflammatory microenvironment and restores gut barrier functions through multiple mechanisms, including scavenging oxidative stress, restoring immune homeostasis, and modulating the gut microbiota. Collectively, our findings highlight the promising potential of this innovative nanotherapeutic strategy for the targeted treatment of IBD.

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

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

X-ray-controllable release of carbon monoxide potentiates radiotherapy by ultrastable hybrid nanoreservoirs

Carbon monoxide (CO) exhibits unique abilities in sensitizing cancer radiotherapy (RT). However, the development of a highly stable CO-delivery nanosystem with sustained CO release in tumor tissues and the prevention of CO leakage into normal tissues remains a challenge. Herein, an organic-inorganic hybrid strategy is proposed to create ultrastable CO nanoreservoirs by locking an unstable iron carbonyl (FeCO) prodrug in a stable mesoporous silica matrix. Different from traditional FeCO-loading nanoplatforms, FeCO-bridged nanoreservoirs not only tethered labile FeCO in the framework to prevent unwanted FeCO leakage, but also achieved sustained CO release in response to X-ray and endogenous H2O2. Importantly, FeCO-bridged nanoreservoirs exhibited the sequential release of CO and Fe2+, thereby performing highly efficient chemodynamic therapy. Such a powerful combination of RT, gas therapy, and chemodynamic therapy boosts robust immunogenic cell death, thus enabling the elimination of deeply metastatic colon tumors with minimal side effects. The proposed organic-inorganic hybrid strategy opens a new window for the development of stable nanoreservoirs for the on-demand delivery of unstable gases and provides a feasible approach for the sequential release of CO and metal ions from metal carbonyl complexes.

An ischemic area-targeting, peroxynitrite-responsive, biomimetic carbon monoxide nanogenerator for preventing myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion (MI/R) injury is common in patients who undergo revascularization therapy for myocardial infarction, often leading to cardiac dysfunction. Carbon monoxide (CO) has emerged as a therapeutic molecule due to its beneficial properties such as anti-inflammatory, anti-apoptotic, and mitochondrial biogenesis-promoting properties. However, its clinical application is limited due to uncontrolled release, potential toxicity, and poor targeting efficiency. To address these limitations, a peroxynitrite (ONOO^-)-triggered CO donor (PCOD585) is utilized to generate a poly (lactic-co-glycolic acid) (PLGA)-based, biomimetic CO nanogenerator (M/PCOD@PLGA) that is coated with the macrophage membrane, which could target to the ischemic area and neutralize proinflammatory cytokines. In the ischemic area, local produced ONOO^- triggers the continuous release of CO from M/PCOD@PLGA, which efficiently ameliorates MI/R injury by clearing harmful ONOO^-, attenuating the inflammatory response, inhibiting cardiomyocyte apoptosis, and promoting mitochondrial biogenesis. This study provides a novel insight into the safe therapeutic use of CO for MI/R injury by utilizing a novel CO donor combined with biomimetic technology. The M/PCOD@PLGA nanogenerator offers targeted delivery of CO to the ischemic area, minimizing potential toxicity and enhancing therapeutic efficacy.

Tumor Abnormality-Oriented Nanomedicine Design

Anticancer nanomedicines have been proven effective in mitigating the side effects of chemotherapeutic drugs. However, challenges remain in augmenting their therapeutic efficacy. Nanomedicines responsive to the pathological abnormalities in the tumor microenvironment (TME) are expected to overcome the biological limitations of conventional nanomedicines, enhance the therapeutic efficacies, and further reduce the side effects. This Review aims to quantitate the various pathological abnormalities in the TME, which may serve as unique endogenous stimuli for the design of stimuli-responsive nanomedicines, and to provide a broad and objective perspective on the current understanding of stimuli-responsive nanomedicines for cancer treatment. We dissect the typical transport process and barriers of cancer drug delivery, highlight the key design principles of stimuli-responsive nanomedicines designed to tackle the series of barriers in the typical drug delivery process, and discuss the "all-into-one" and "one-for-all" strategies for integrating the needed properties for nanomedicines. Ultimately, we provide insight into the challenges and future perspectives toward the clinical translation of stimuli-responsive nanomedicines.

A CO-mediated photothermal therapy to kill drug-resistant bacteria and minimize thermal injury for infected diabetic wound healing

With an increasing proportion of drug-resistant bacteria, photothermal therapy (PTT) is a promising alternative to antibiotic treatment for infected diabetic skin ulcers. However, the inevitable thermal damage to the tissues restricts its clinical practice. Carbon monoxide (CO), as a bioactive gas molecule, can selectively inhibit bacterial growth and promote tissue regeneration, which may be coordinated with PTT for drug-resistant bacteria killing and tissue protection. Herein, a CO-mediated PTT agent (CO@mPDA) was engineered by loading manganese carbonyl groups into mesoporous polydopamine (mPDA) nanoparticles via coordination interactions between the metal center and a catechol group. Compared to the traditional PTT, the CO-mediated PTT increases the inhibition ratio of the drug-resistant bacteria both in vitro and in diabetic wound beds by selectively inhibiting the co-chaperone of the heat shock protein 90 kDa (Hsp90), and lowers the heat resistance of the bacteria rather than the mammalian tissues. Meanwhile, the tissue-protective proteins, such as Hsp90 and vimentin (Vim), are upregulated via the WNT and PI3K-Akt pathways to reduce thermal injury, especially with a laser with a high-power density. The CO-mediated PTT unified the bacterial killing with tissue protection, which offers a promising concept to improve PTT efficiency and minimize the side-effects of PTT when treating infected skin wounds.

The importance of and difficulties involved in creating molecular probes for a carbon monoxide gasotransmitter

As one of the triumvirate of recognized gasotransmitter molecules, namely NO, H2S, and CO, the physiological effects of CO and its potential as a biomarker have been widely investigated, garnering particular attention due to its reported hypotensive, anti-inflammatory, and cytoprotective properties, making it a promising therapeutic agent. However, the development of CO molecular probes has remained relatively stagnant in comparison with the fluorescent probes for NO and H2S, owing to its inert molecular state under physiological conditions. In this review, starting from elucidating the definition and significance of CO as a gasotransmitter, the imperative for the advancement of CO probes, especially fluorescent probes, is expounded. Subsequently, the current state of development of CO probe methodologies is comprehensively reviewed, with an overview of the challenges and prospects in this burgeoning field of research.

Gas-assisted phototherapy for cancer treatment

Phototherapies, mainly including photodynamic and photothermal therapy, have made considerable strides in the field of cancer treatment. With the aid of phototherapeutic agents, reactive oxygen species (ROS) or heat are generated under light irradiation to selectively damage cancer cells. However, sole-modality phototherapy faces certain drawbacks, such as limited penetration of phototherapeutic agents into tumor tissues, inefficient ROS generation due to hypoxia, treatment-induced inflammation and resistance of tumor to treatment (e.g., high levels of antioxidants, expression of heat shock protein). Gas therapy, an emerging therapy approach that damages cancer cells by improving the level of certain gas at the tumor site, shows potential to overcome the challenges associated with phototherapies. In addition, with the rapid development of nanotechnology, gas-assisted phototherapy based on nanomedicines has emerged as a promising strategy to enhance the treatment efficacy. This review summarizes recent advances in gas-assisted phototherapy and discusses the prospects and challenges of this strategy in cancer phototherapy.

Carboxyhemoglobin (COHb): Unavoidable Bystander or Protective Player?

Carbon monoxide (CO) is a cytoprotective endogenous gas that is ubiquitously produced by the stress response enzyme heme-oxygenase. Being a gas, CO rapidly diffuses through tissues and binds to hemoglobin (Hb) increasing carboxyhemoglobin (COHb) levels. COHb can be formed in erythrocytes or in plasma from cell-free Hb. Herein, it is discussed as to whether endogenous COHb is an innocuous and inevitable metabolic waste product or not, and it is hypothesized that COHb has a biological role. In the present review, literature data are presented to support this hypothesis based on two main premises: (i) there is no direct correlation between COHb levels and CO toxicity, and (ii) COHb seems to have a direct cytoprotective and antioxidant role in erythrocytes and in hemorrhagic models in vivo. Moreover, CO is also an antioxidant by generating COHb, which protects against the pro-oxidant damaging effects of cell-free Hb. Up to now, COHb has been considered as a sink for both exogenous and endogenous CO generated during CO intoxication or heme metabolism, respectively. Hallmarking COHb as an important molecule with a biological (and eventually beneficial) role is a turning point in CO biology research, namely in CO intoxication and CO cytoprotection.

Tricarbonyl-Pyrazine-Molybdenum(0) Metal-Organic Frameworks for the Storage and Delivery of Biologically Active Carbon Monoxide

Metal-organic frameworks (MOFs) have high potential as nanoplatforms for the storage and delivery of therapeutic gasotransmitters or gas-releasing molecules. The aim of the present study was to open an investigation into the viability of tricarbonyl-pyrazine-molybdenum(0) MOFs as carbon monoxide-releasing materials (CORMAs). A previous investigation found that the reaction of Mo(CO)₆ with excess pyrazine (pyz) in a sealed ampoule gave a mixture comprising a major triclinic phase with pyz-occupied hexagonal channels, formulated as fac-Mo(CO)₃(pyz)_3/2·1/2pyz (Mo-hex), and a minor dense cubic phase, formulated as fac-Mo(CO)₃(pyz)_3/2 (Mo-cub). In the present work, an open reflux method in toluene has been optimized for the large-scale synthesis of the pure Mo-cub phase. The crystalline solids Mo-hex and Mo-cub were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), FT-IR and FT-Raman spectroscopies, and ¹³C{¹H} cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy. The release of CO from the MOFs was studied by the deoxy-myoglobin (deoxy-Mb)/carbonmonoxy-myoglobin (MbCO) UV-vis assay. Mo-hex and Mo-cub release CO upon contact with a physiological buffer in the dark, delivering 0.35 and 0.22 equiv (based on Mo), respectively, after 24 h, with half-lives of 3-4 h. Both materials display high photostability such that the CO-releasing kinetics is not affected by irradiation of the materials with UV light. These materials are attractive as potential CORMAs due to the slow release of a high CO payload. In the solid-state and under open air, Mo-cub underwent almost complete decarbonylation over a period of 4 days, corresponding to a theoretical CO release of 10 mmol per gram of material.

Gas sensors based on the oxide skin of liquid indium

Various non-stratified two-dimensional (2D) materials can be obtained from liquid metal surfaces that are not naturally accessible. Homogenous nucleation on atomically flat interfaces of liquid metals with air produces unprecedented high-quality oxide layers that can be transferred onto desired substrates. The atomically flat and large areas provide large surface-to-volume ratios ideal for sensing applications. Versatile crucial applications of the liquid metal-derived 2D oxides have been realized; however, their gas-sensing properties remain largely underexplored. The cubic In₂O₃ structure, which is nonlayered, can be formed as an ultrathin layer on the surface of liquid indium during the self-limiting Cabrera-Mott oxidation process in the air. The morphology, crystal structure, and band structure of the harvested 2D In₂O₃ nanosheets from liquid indium are characterized. Sensing capability toward several gases, both inorganic and organic, entailing NO₂, O₂, NH₃, H₂, H₂S, CO, and Methyl ethyl ketone (MEK) are explored. A high ohmic resistance change of 1974% at 10 ppm, fast response, and recovery times are observed for NO₂ at an optimum temperature of 200 °C. The sensing fundamentals are investigated for NO₂, and its performances and cross-selectivity to different gases are analyzed. The NO₂ sensing response from room temperature to 300 °C has been measured and discussed, and stability after 24 hours of continuous operation is presented. The results demonstrate liquid metal-derived 2D oxides as promising materials for gas sensing applications.

Carbon monoxide therapy: a promising strategy for cancer

Cancer is one of the acute life-threatening diseases endangering the whole of humanity. The treatment modalities for cancer are various. However, in most cases, a single treatment choice provides multiple side effects, poor targeting, and ineffective treatment. In recent years, the physiological regulatory function of carbon monoxide (CO) in the cancer process has been reported gradually, and CO-related nano-drugs have been explored. It shows better application prospects in cancer treatment and provides new ideas for treatment. The present review introduces the pathophysiological role of CO. The recent advances in cancer therapy, such as CO-mediated gas therapy, combined application of CO chemotherapy, photodynamic therapy (PDT), photothermal therapy (PTT), and immunotherapy, are described. Current challenges and future developments in CO-based treatment are also discussed. This review provides comprehensive information on recent advances in CO therapy and also some valuable guidance for promoting the progress of gas therapy nanomedicine.

Near-infrared light triggered in situ release of CO for enhanced therapy of glioblastoma

BACKGROUND

Photodynamic therapy (PDT) features high biocompatibility and high spatiotemporal selectivity, showing a great potential in glioblastoma (GBM) treatment. However, its application was restricted by the poor therapeutic efficacy and side effect.

RESULTS

In this study, a therapeutic nanoplatform (UCNPs@Ce6/3HBQ@CM) with combination of PDT and CO therapy was constructed, in which a photoCORM and a photosensitizer were loaded onto the surface of upconversion nanoparticles (UCNPs) functioning as photon transducer. Benefitting from NIR excitation and multicolor emission of UCNPs, the penetration depth of excitation light is enhanced and meanwhile simultaneous generation of CO and ROS in tumor site can be achieved. The as-prepared nanocomposite possessed an elevated therapeutic efficiency with the assistance of CO through influencing mitochondrial respiration and depleting ATP, accompanying with the reduced inflammatory responses. By wrapping a homologous cell membrane, the nanocomposite can target GBM and accumulate in the tumor site, affording a powerful tool for precise and efficient treatment of GBM.

CONCLUSION

This therapeutic nanoplatform UCNPs@Ce6/3HBQ@CM, which combines PDT and CO therapy enables precise and efficient treatment of refractory glioblastoma.

Multifunctional BODIPY embedded non-woven fabric for CO release and singlet oxygen generation

Materials that can simultaneously release CO and generate singlet oxygen upon visible light irradiation under ambient conditions are highly desirable for therapeutic applications. Furthermore, materials that can sequester the undesirable side products into the matrix without affecting the release of CO and singlet oxygen generation would allow them to be used for practical applications. Focussing on these aspects, we prepared two dipicolylamine appended BODIPY‑manganese(I) tricarbonyl complexes wherein the metal core was systematically tethered at 5- and 8- positions of the BODIPY core. The complexes were embedded into a polymer matrix via electrospinning and the resulting non-woven fabrics showed CO release as well as singlet oxygen generation upon irradiation. While the hybrid materials were non-toxic in dark, they were strongly photocytotoxic to c6 cancer cells when exposed to light. Rapid CO release alongside significant singlet oxygen generation, indefinite dark stability, good biocompatibility and negligible dark toxicity makes these fabrics a potent candidate for phototherapeutic applications.

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.

Examination of central nervous system by functional observation battery after massive intravenous infusion of carbon monoxide-bound and oxygen-bound hemoglobin vesicles in rats

Carbon monoxide (CO) is known as a toxic gas inducing "CO poisoning", which acutely affects the central nervous system (CNS) and which persistently affects brain functions depending on the exposure time and CO concentration. By contrast, in pathological rodent models, intravenous infusion of CO-bound hemoglobin vesicles (CO-HbV) has shown various beneficial effects such as anti-oxidative and anti-inflammatory reactions. This study assessed effects of CO-HbV infusion on CNS using a functional observation battery, sensory reflexes, grip strength, and landing foot splay measurements. The test fluids were CO-HbV and O₂-bound HbV (O₂-HbV) suspended in saline ([Hb] ​= ​10 ​g/dL), and saline alone for comparison. The rats received either 16 or 32 ​mL/kg of fluid intravenously at 1.5 ​mL/min/kg. Observations were made before infusion, and at 5 ​min, 4, 8, 24, 48 and 72 ​h after infusion. Massive doses of 16 and 32 ​mL/kg respectively corresponded to about 29 and 57% of the whole circulating blood volume (56 ​mL/kg). No toxicological effect was observed in any measurement item for any group in comparison to the control saline infusion group. Histopathological examination of hippocampal tissue at 14 days after infusion showed the number of necrotic cells to be minimal. Results obtained from rats in this experiment suggest that the massive intravenous infusion of CO-HbV yields beneficial anti-oxidative and anti-inflammatory effects without showing CO-poisoning-related symptoms of CNS damage.

Activation of Carbon Dioxide by 9-Carbene-9-borafluorene Monoanion: Carbon Monoxide Releasing Transformation of Trioxaborinanone to Luminescent Dioxaborinanone

The first structurally characterized example of a trioxaborinanone (2) is produced by the reaction of a 9-carbene-9-borafluorene monoanion and carbon dioxide. When compound 2 is heated or irradiated with UV light, carbon monoxide (CO) is released, and a luminescent dioxaborinanone (3) is formed. Notably, carbon monoxide releasing molecules (CORMs) are of interest for their ability to deliver a specific amount of CO. Due to the turn-on fluorescence observed as a result of the conversion to 3, CORM 2 serves as a means to optically observe CO loss "by eye" under thermal or photochemical conditions.

Strategy for Detecting Carbon Monoxide: Cu2+-Assisted Fluorescent Probe and Its Applications in Biological Imaging

Herein, a novel strategy was proposed for identifying carbon monoxide (CO), which plays a crucial part in living systems. For the first time, we have managed to design, synthesize, and characterize successfully this new Cu²⁺-assisted fluorescent probe (DPHP) in detecting CO. Compared with the commonly adopted Pd⁰-mediated Tsuji-Trost reaction recognition method, such a new strategy did not engage costly palladium (II) salt and generated no leaving group, indicating a satisfactory anti-interference ability. The recognition mechanism was confirmed by IR, ¹H NMR titration, HR-MS, cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and optical properties. Surprisingly, it was found that the new method achieved high selectivity and rapid identification of CO with a lower limit of detection (1.7 × 10^-8 M). More intriguingly, it could recognize endogenous and exogenous CO in HeLa cells. The cytotoxicity of this new method was so low that it allowed the detection of CO in mice and zebrafish. Basically, our results trigger a novel viewpoint of rationally designing and synthesizing advanced materials for CO detection with unique features, impelling new research in detection chemistry.

Low-dose carbon monoxide suppresses metastatic progression of disseminated cancer cells

Low-dose carbon monoxide (CO) is under investigation in clinical trials to treat non-cancerous diseases and has excellent safety profiles. Due to the early detection and cancer awareness, increasing cancer patients are diagnosed at early stages and potentially curative surgical resection can be done. However, many patients ultimately experience recurrence. Here, we evaluate the therapeutic effect of CO on cancer metastatic progression. We show that 250 ppm CO inhibits migration of multiple types of cancer cell lines including breast, pancreatic, colon, prostate, liver, and lung cancer and reduces the ability to adhere to fibronectin. We demonstrate that in mouse models, 250 ppm inhaled CO inhibits lung metastasis of breast cancer and liver metastasis of pancreatic cancer. Moreover, low-dose CO suppresses recurrence and increases survival after surgical removal of primary pancreatic cancer in mice. Mechanistically, low-dose CO blocks transcription of heme importers, leading to diminished intracellular heme levels and a heme-regulated enzyme, cytochrome P4501B1 (CYP1B1). Either supplementing heme or overexpressing CYP1B1 reverses the anti-migration effect of low-dose CO. Taken together, low-dose CO therapy inhibits cell migration, reduces adhesion to fibronectin, prevents disseminated cancer cells from expanding into gross metastases, and improves survival in pre-clinical mouse models of metastasis.

An all-in-one CO gas therapy-based hydrogel dressing with sustained insulin release, anti-oxidative stress, antibacterial, and anti-inflammatory capabilities for infected diabetic wounds

To effectively treat diabetic wounds, the development of versatile medical dressings that can long-term regulate blood glucose and highly effective anti-oxidative stress, antibacterial and anti-inflammatory are critical. Here, an all-in-one CO gas-therapy-based versatile hydrogel dressing (ICOQF) was developed via the dynamic Schiff base reaction between the amino groups on quaternized chitosan (QCS) and the aldehyde groups on benzaldehyde-terminated F108 (F108-CHO) micelles. CORM-401 (an oxidant-sensitive CO-releasing molecules) was encapsulated in the hydrophobic core of F108-CHO micelles and insulin was loaded in the three-dimensional network structure of ICOQF. The dynamic Schiff base bonds not only endowed ICOQF with good tissue adhesion, injectability and self-healing, but also gave it sustained and controllable insulin release ability. In addition, ICOQF could quickly generate CO in inflamed wound tissue by consuming reactive oxygen species. The generated CO could effectively anti-oxidative stress by activating the expression of heme oxygenase; antibacterial by inducing the rupture of bacterial cell membranes and mitochondrial dysfunction and inhibiting the synthesis of adenosine triphosphate; and anti-inflammatory by inhibiting the proliferation of activated macrophages and promoting the polarization of the M1 phenotype to the M2 phenotype. Due to these outstanding properties, ICOQF significantly promoted the healing of STZ-induced MRSA-infected diabetic wounds accompanied by good biocompatibility. This study clearly shows that ICOQF is a versatile hydrogel dressing with great application potential for the management of diabetic wounds. STATEMENT OF SIGNIFICANCE: The development of some versatile hydrogel dressings that can not only provide a prolonged and controlled insulin release property but also utilize a non-antibiotic treatment modality for highly effective antibacterial, anti-inflammatory, and anti-oxidative stress effects is vital for the successful treatment of diabetic wounds. Herein, we developed an all-in-one CO gas-therapy-based versatile hydrogel dressing (ICOQF) with sustained and controllable insulin release abilities. Moreover, ICOQF could not only quickly release CO in the inflamed wound tissue by consumption of reactive oxygen species but also utilize the generated CO to highly effectively anti-oxidative stress, antibacterial, and anti-inflammatory. ICOQF therapy substantially promoted the healing of STZ-induced MRSA-infected diabetic wounds. Overall, this work provides a multifunctional hydrogel dressing for the management of diabetic wounds.

The Advancement of Gas-Generating Nanoplatforms in Biomedical Fields: Current Frontiers and Future Perspectives

Diverse gases (NO, CO, H₂ S, H₂ , etc.) have been widely applied in the medical intervention of various diseases, including cancer, cardiovascular disease, ischemia-reperfusion injury, bacterial infection, etc., attributing to their inherent biomedical activities. Although many gases have many biomedical activities, their clinical use is still limited due to the rapid and free diffusion behavior of these gases molecules, which may cause potential side effects and/or ineffective treatment. Gas-generating nanoplatforms (GGNs) are effective strategies to address the aforementioned challenges of gas therapy by preventing gas production or release at nonspecific sites, enhancing GGNs accumulation at targeted sites, and controlling gas release in response to exogenous (UV, NIR, US, etc.) or endogenous (H₂ O₂ , GSH, pH, etc.) stimuli at the lesion site, further maintaining gas concentration within the effective range and achieving the purpose of disease treatment. This review comprehensively summarizes the advancements of "state-of-the-art" GGNs in the recent three years, with emphasis on the composition, structure, preparation process, and gas release mechanism of the nanocarriers. Furthermore, the therapeutic effects and limitations of GGNs in preclinical studies using cell/animal models are discussed. Overall, this review enlightens the further development of this field and promotes the clinical transformation of gas therapy.

Stimuli-Responsive Antibacterial Materials: Molecular Structures, Design Principles, and Biomedical Applications

Infections are regarded as the most severe complication associated with human health, which are urgent to be solved. Stimuli-responsive materials are appealing therapeutic platforms for antibacterial treatments, which provide great potential for accurate theranostics. In this review, the advantages, the response mechanisms, and the key design principles of stimuli-responsive antibacterial materials are highlighted. The biomedical applications, the current challenges, and future directions of stimuli-responsive antibacterial materials are also discussed. First, the categories of stimuli-responsive antibacterial materials are comprehensively itemized based on different sources of stimuli, including external physical environmental stimuli (e.g., temperature, light, electricity, salt, etc.) and bacterial metabolites stimuli (e.g., acid, enzyme, redox, etc.). Second, structural characteristics, design principles, and biomedical applications of the responsive materials are discussed, and the underlying interrelationships are revealed. The molecular structures and design principles are closely related to the sources of stimuli. Finally, the challenging issues of stimuli-responsive materials are proposed. This review will provide scientific guidance to promote the clinical applications of stimuli-responsive antibacterial materials.

Medical Gas Therapy for Tissue, Organ, and CNS Protection: A Systematic Review of Effects, Mechanisms, and Challenges

Gaseous molecules have been increasingly explored for therapeutic development. Here, following an analytical background introduction, a systematic review of medical gas research is presented, focusing on tissue protections, mechanisms, data tangibility, and translational challenges. The pharmacological efficacies of carbon monoxide (CO) and xenon (Xe) are further examined with emphasis on intracellular messengers associated with cytoprotection and functional improvement for the CNS, heart, retina, liver, kidneys, lungs, etc. Overall, the outcome supports the hypothesis that readily deliverable "biological gas" (CO, H₂ , H₂ S, NO, O₂ , O₃ , and N₂ O) or "noble gas" (He, Ar, and Xe) treatment may preserve cells against common pathologies by regulating oxidative, inflammatory, apoptotic, survival, and/or repair processes. Specifically, CO, in safe dosages, elicits neurorestoration via igniting sGC/cGMP/MAPK signaling and crosstalk between HO-CO, HIF-1α/VEGF, and NOS pathways. Xe rescues neurons through NMDA antagonism and PI3K/Akt/HIF-1α/ERK activation. Primary findings also reveal that the need to utilize cutting-edge molecular and genetic tactics to validate mechanistic targets and optimize outcome consistency remains urgent; the number of neurotherapeutic investigations is limited, without published results from large in vivo models. Lastly, the broad-spectrum, concurrent multimodal homeostatic actions of medical gases may represent a novel pharmaceutical approach to treating critical organ failure and neurotrauma.

Heme Oxygenase-1: An Anti-Inflammatory Effector in Cardiovascular, Lung, and Related Metabolic Disorders

The heme oxygenase (HO) enzyme system catabolizes heme to carbon monoxide (CO), ferrous iron, and biliverdin-IXα (BV), which is reduced to bilirubin-IXα (BR) by biliverdin reductase (BVR). HO activity is represented by two distinct isozymes, the inducible form, HO-1, and a constitutive form, HO-2, encoded by distinct genes (HMOX1, HMOX2, respectively). HO-1 responds to transcriptional activation in response to a wide variety of chemical and physical stimuli, including its natural substrate heme, oxidants, and phytochemical antioxidants. The expression of HO-1 is regulated by NF-E2-related factor-2 and counter-regulated by Bach-1, in a heme-sensitive manner. Additionally, HMOX1 promoter polymorphisms have been associated with human disease. The induction of HO-1 can confer protection in inflammatory conditions through removal of heme, a pro-oxidant and potential catalyst of lipid peroxidation, whereas iron released from HO activity may trigger ferritin synthesis or ferroptosis. The production of heme-derived reaction products (i.e., BV, BR) may contribute to HO-dependent cytoprotection via antioxidant and immunomodulatory effects. Additionally, BVR and BR have newly recognized roles in lipid regulation. CO may alter mitochondrial function leading to modulation of downstream signaling pathways that culminate in anti-apoptotic, anti-inflammatory, anti-proliferative and immunomodulatory effects. This review will present evidence for beneficial effects of HO-1 and its reaction products in human diseases, including cardiovascular disease (CVD), metabolic conditions, including diabetes and obesity, as well as acute and chronic diseases of the liver, kidney, or lung. Strategies targeting the HO-1 pathway, including genetic or chemical modulation of HO-1 expression, or application of BR, CO gas, or CO donor compounds show therapeutic potential in inflammatory conditions, including organ ischemia/reperfusion injury. Evidence from human studies indicate that HO-1 expression may represent a biomarker of oxidative stress in various clinical conditions, while increases in serum BR levels have been correlated inversely to risk of CVD and metabolic disease. Ongoing human clinical trials investigate the potential of CO as a therapeutic in human disease.

X-ray-Triggered CO Release Based on GdW10/MnBr(CO)5 Nanomicelles for Synergistic Radiotherapy and Gas Therapy

Carbon monoxide (CO) therapy has become a hot topic in the field of gas therapy because of its application prospect in the treatment of various diseases. Due to the high affinity for human hemoglobin, the main challenge of CO-loaded nanomedicine is the lack of selectivity and toxicity in the delivery process. Although many commercial CO-releasing molecules (CORMs) have been widely developed because of their ability to deliver CO, CORMs still have some disadvantages, including difficult on-demand controlled CO release, poor solubility, and potential toxicity, which are limiting their further application. Herein, an X-ray-triggered CO-releasing nanomicelle system (GW/MnCO@PLGA) based on GdW₁₀ nanoparticles (NPs) (GW) and MnBr(CO)₅ (MnCO) encapsulating in the poly(lactic-co-glycolic acid) (PLGA) polymer was constructed for synergistic CO radiotherapy (RT). The production of strongly oxidative superoxide anion (O₂^-•) active species can lead to cell apoptosis under the X-ray sensitization of GW. Moreover, strongly oxidative O₂^-• radicals further oxidize and compete with the Mn center, resulting in the on-demand release of CO. The radio/gas therapy synergy to enhance the efficient tumor inhibition of the nanomicelles was investigated in vivo and in vitro. Therefore, the establishment of an X-ray-triggered controlled CO release system has great application potential for further synergistic RT CO therapy in deep tumor sites.

Flavonol-Based Carbon Monoxide Delivery Molecule with Endoplasmic Reticulum, Mitochondria, And Lysosome Localization

Light-triggered carbon monoxide (CO) delivery molecules are of significant current interest for evaluating the role of CO in biology and as potential therapeutics. Herein we report the first example of a metal free CO delivery molecule that can be tracked via confocal microscopy at low micromolar concentrations in cells prior to CO release. The NEt₂-appended extended flavonol (4) localizes to the endoplasmic reticulum, mitochondria, and lysosomes. Subcellular localization of 4 results in CO-induced toxicity effects that are distinct as compared to a nonlocalized analog. Anti-inflammatory effects of 4, as measured by TNF-α suppression, occur at the nanomolar level in the absence of CO release, and are enhanced with visible-light-induced CO release. Overall, the highly trackable nature of 4 enables studies of the biological effects of both a localized flavonol and CO release at low micromolar to nanomolar concentrations.

Mechanically Triggered Carbon Monoxide Release with Turn-On Aggregation-Induced Emission

Polymers that release functional small molecules under mechanical stress potentially serve as next-generation materials for catalysis, sensing, and mechanochemical dynamic therapy. To further expand the function of mechanoresponsive materials, the discovery of chemistries capable of small molecule release are highly desirable. In this report, we detail a nonscissile bifunctional mechanophore (i.e., dual mechano-activated properties) based on a unique mechanochemical reaction involving norborn-2-en-7-one (NEO). One property is the release of carbon monoxide (CO) upon pulsed solution ultrasonication. A release efficiency of 58% is observed at high molecular weights (M_n = 158.8 kDa), equating to ∼154 molecules of CO released per chain. The second property is the bright cyan emission from the macromolecular product in its aggregated state, resulting in a turn-on fluorescence readout coincident with CO release. This report not only demonstrates a unique strategy for the release of small molecules in a nonscissile way but also guides future designs of force-responsive aggregation-induced emission (AIE) luminogens.

Recent advances in inhibiting atherosclerosis and restenosis: from pathogenic factors, therapeutic agents to nano-delivery strategies

Due to dominant atherosclerosis etiology, cardiovascular diseases (CVDs) remain the leading cause of morbidity and mortality worldwide. In clinical trials, advanced atherosclerotic plaques can be removed by angioplasty and vascular...

Electron-acceptor density adjustments for preparation conjugated polymers with NIR-II absorption and brighter NIR-II fluorescence and 1064 nm active photothermal/gas therapy

Designing conjugated polymers (CPs) with both efficient second near-infrared wavelength (NIR-II) fluorescence and NIR-II photothermal therapy performance remains a huge challenge, as the introduction of excessively strong electron donor and acceptor units significantly increase non-radiative decay. Herein, we describe an "electron acceptor density adjustment" strategy to address this problem, since a lower electron acceptor density in the conjugated polymer backbone can enhance the radiative rate constant and improve NIR-II fluorescence brightness. We used quaterthiophene (4T) with four repeated thiophene chain units and bithiophene (2 TC) modified with long alkyl side chains to reduce the electron acceptor density in the conjugated polymer backbone. The resultant 1064 nm absorption polymer, TTQ-2TC-4T displayed approximately 7.30-folds enhancement in NIR-II emission intensity compared to that of undoped TTQ-1T at the same mass concentration in toluene solution. Furthermore nanoparticles (TTQ-MnCO NPs) based on TTQ-2TC-4T and CO donors (Mn₂(CO)₁₀) were developed to realize NIR-II FI-guided 1064 nm laser-triggered NIR-II PTT/Gas synergistic therapy. The TTQ-MnCO NPs nanoparticles exhibited high photothermal conversion efficiency (η) of 44.43% at 1064 nm and high specific NIR-II fluorescence imaging of the cerebral vasculature of live mice. The in vivo results demonstrate that TTQ-MnCO NPs nanoparticles have excellent PTT/Gas synergistic therapeutic effects in MCF-7 tumor-bearing mice under 1064 nm laser irradiation. This study provides a new approach for optimizing both NIR-II fluorescence and NIR-II photothermal performance of NIR-II absorption conjugated polymers.

Photoactivatable nanogenerators of reactive species for cancer therapy

In recent years, reactive species-based cancer therapies have attracted tremendous attention due to their simplicity, controllability, and effectiveness. Herein, we overviewed the state-of-art advance for photo-controlled generation of highly reactive radical species with nanomaterials for cancer therapy. First, we summarized the most widely explored reactive species, such as singlet oxygen, superoxide radical anion (O2 ●-), nitric oxide (●NO), carbon monoxide, alkyl radicals, and their corresponding secondary reactive species generated by interaction with other biological molecules. Then, we discussed the generating mechanisms of these highly reactive species stimulated by light irradiation, followed by their anticancer effect, and the synergetic principles with other therapeutic modalities. This review might unveil the advantages of reactive species-based therapeutic methodology and encourage the pre-clinical exploration of reactive species-mediated cancer treatments.

Engineering macromolecular nanocarriers for local delivery of gaseous signaling molecules

In addition to being notorious air pollutants, nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S) have also been known as endogenous gaseous signaling molecules (GSMs). These GSMs play critical roles in maintaining the homeostasis of living organisms. Importantly, the occurrence and development of many diseases such as inflammation and cancer are highly associated with the concentration changes of GSMs. As such, GSMs could also be used as new therapeutic agents, showing great potential in the treatment of many formidable diseases. Although clinically it is possible to directly inhale GSMs, the precise control of the dose and concentration for local delivery of GSMs remains a substantial challenge. The development of gaseous signaling molecule-releasing molecules provides a great tool for the safe and convenient delivery of GSMs. In this review article, we primarily focus on the recent development of macromolecular nanocarriers for the local delivery of various GSMs. Learning from the chemistry of small molecule-based donors, the integration of these gaseous signaling molecule-releasing molecules into polymeric matrices through physical encapsulation, post-modification, or direct polymerization approach renders it possible to fabricate numerous macromolecular nanocarriers with optimized pharmacokinetics and pharmacodynamics, revealing improved therapeutic performance than the small molecule analogs. The development of GSMs represents a new means for many disease treatments with unique therapeutic outcomes.

Recent Advances on Carbon Monoxide Releasing Molecules for Antibacterial Applications

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

Mitochondria-targeted carbon monoxide delivery combined with singlet oxygen production from a single nanoplatform under 808 nm light irradiation for synergistic anticancer therapy

A multifunctional nanoplatform (1), MnCO@TPP@C-TiO2, which consists of a carrier of carbon-doped TiO2 nanoparticles with surface covalent functionalization of manganese carbonyls and a directing group of triphenylphosphine, was prepared for mitochondria-targeted carbon monoxide (CO) delivery combined with photodynamic therapy (PDT). MnCO@TPP@C-TiO2 selectively localized in the mitochondria of HeLa cells where the overexpressed-H2O2 triggered CO release resulting in mitochondrial damage. And singlet oxygen species generated upon 808 nm near infrared light irradiation further destroyed the mitochondria and induced cancer cells apoptosis. Cytotoxicity assays revealed that the nanoplatform with mitochondria-targeted CO delivery and PDT exhibited the highest lethality against cancer cells in comparison with all the other control samples tested, and it showed good dark biocompatibility with normal cells that express low H2O2 levels. This work may provide new insights into combining CO-based gas therapy with traditional PDT for efficient cancer treatment.

Sensitive and Direct Optical Monitoring of Release and Cellular Uptake of Aqueous CO from CO-Releasing Molecules

Dynamics of release and cellular uptake of aqueous CO from CO-releasing molecules (CORMs) significantly affect signaling and cell viability. So far, it has been mainly observed by IR, UV-visible, and fluorescence techniques, which suffer from poor sensitivity and slow response time. Here, we show how to directly probe the mass transfer of aqueous CO from CORMs to cells using a fluidic chamber integrated with live cells and Raman reporters of large-area Au@Pd core-shell nanoparticle assembly to emulate a physiologically relevant microenvironment. We sensitively and directly detect CO release from trace CORMs of as low as 100 nM by measuring the Raman transitions of CO via rapid chemisorption onto the surface of the Au@Pd nanoparticles. By using our method, we successfully observe the dynamics of CO release from CORM-2 despite its very short half-life. We also reveal that the initial rate of CO release from CORM-3 is dramatically decreased by tens to hundreds of times when exposed to physiologically relevant pH variations from 7.4 to 2.5, which can be attributed to the acid hydrolysis of the CO ligand. CORM-2 tends to quickly release CO regardless of pH, probably because of its rapid cleavage into two monomeric Ru complexes by the co-solvent. The decrease in the initial rate at lower temperatures is more significant for CORM-3 than for CORM-2. Finally, we observe that the cellular uptake of aqueous CO from CORM-3 by lung cancer cells is approximately 2 times higher than that of normal lung cells.