A Photochemical Precursor for Carbon Monoxide Release in Aerated Aqueous Media

Insights into the photoactivatable CO releasing properties of dicarbonyl Ru(II) complex with 8-amino quinoline ligand: Experimental and theoretical studies

Reaction between the polymeric [RuCl2(CO)2]n and the N,N-bidentate ligand, 8-amino-quinoline (Quin), in methanol, afforded the photoactivated CO releasing molecule with the formula of trans-(Cl,Cl)-[RuCl2(CO)2Quin]. In the presence of biomolecules or in solvents with varying polarity and coordinating abilities, the solvatochromic characteristics and dark stability were investigated. A new board band emerged in the visible spectrum during the illumination, and its position varies according to the type of solvent used, indicating the role of the solvent in controlling the nature of the CO-depleted species. Spectral methods were used in combination with density functional theory simulations to get insight into the local minimum structure and the electronic properties of the Ru(II) complex. The results of the myoglobin assay showed that within the first two hours of illumination, one of the two CO molecules was released. The cytotoxic properties of the Ru(II)-based complex were investigated against normal mice bone marrow stromal cells and malignant human acute monocytic leukaemia cells.

Metal-based carbon monoxide releasing molecules with promising cytotoxic properties

Carbon monoxide, the "silent killer" gas, is increasingly recognised as an important signalling molecule in human physiology, which has beneficial biological properties. A particular way of achieving controlled CO administration is based on the use of biocompatible molecules that only release CO when triggered by internal or external factors. These approaches include the development of pharmacologically effective prodrugs known as CO releasing molecules (CORMs), which can supply biological systems with CO in well-regulated doses. An overview of transition metal-based CORMs with cytotoxic properties is here reported. The mechanisms at the basis of the biological activities of these molecules and their potential therapeutical applications with respect to their stability and CO releasing properties have been discussed. The activation of metal-based CORMs is determined by the type of metal and by the nature and features of the auxiliary ligands, which affect the metal core electronic density and therefore the prodrug resistance towards oxidation and CO release ability. A major role in regulating the cytotoxic properties of these CORMs is played by CO and/or CO-depleted species. However, several mysteries concerning the cytotoxicity of CORMs remain as intriguing questions for scientists.

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.

In vitro cytotoxicity of Mn(I) and Ru(II) carbonyls with a diphenyl pyridyl phosphine coligand towards leukaemia

Human acute monocytic leukaemia cells were tested under both dark and light conditions for their susceptibility to Mn(I) and Ru(II) carbonyl complexes with a diphenyl pyridyl phosphine coligand. The Ru(II) complex (IC₅₀ = 7.13 ± 0.8 μM) displayed higher outstanding potency against leukaemia than the Mn(I) analogue (54.58 ± 4.1 μM) in the dark and both complexes were completely harmless to healthy mouse bone marrow cells.

Spectroscopical and Molecular Studies of Four Manganese(I) PhotoCORMs with Bioinspired Ligands Containing Non-Coordinated Phenol Groups

Carbonyl compounds are widely explored in medicinal inorganic chemistry and have drawn attention due to their signaling functions in homeostasis. Carbon-monoxide-releasing molecules (CORMs) were developed with the purpose of keeping the CO inactive until its release in the intracellular environment, considering its biological relevance. However, for therapeutic applications, the mechanisms of photorelease and which electronic and structural variations influence its rates must be fully understood. In this work, four ligands containing a pyridine, a secondary amine, and a phenolic group with different substituents were used to prepare new Mn(I) carbonyl compounds. Structural and physicochemical characterization of these complexes was carried out and confirmed the proposed structures. X-ray diffractometry structures obtained for the four organometallic compounds revealed that the substituents in the phenolic ring promote only negligible distortions in their geometry. Furthermore, UV-Vis and IR kinetics showed the direct dependence of the electron-withdrawing or donating ability of the substituent group, indicating an influence of the phenol ring on the CO release mechanism. These differences in properties were also supported by theoretical studies at the DFT, TD-DFT, and bonding situation analyses (EDA-NOCV). Two methods were used to determine the CO release constants (k_CO,old and k_CO,new), where Mn-HbpaBr (1) had the greatest k_CO by both methods (K_co,old = 2.36 × 10^-3 s^-1 and k_CO,new = 2.37 × 10^-3 s^-1). Carbon monoxide release was also evaluated using the myoglobin assay, indicating the release of 1.248 to 1.827 carbon monoxides upon light irradiation.

Water-Soluble Carbon Monoxide-Releasing Molecules (CORMs)

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

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.

Role of Carbon Monoxide in Oxidative Stress-Induced Senescence in Human Bronchial Epithelium

Prolonged or excessive stimulation from inhaled toxins may cause oxidative stress and DNA damage that can lead to stress-induced senescence in epithelial cells, which can contribute to several airway diseases. Mounting evidence has shown carbon monoxide (CO) confers cytoprotective effects. We investigated the effects of CO on oxidative stress-induced senescence in human airway epithelium and elucidated the underlying molecular mechanisms. Here, CO pretreatment reduced H₂O₂-mediated increases in total reactive oxygen species (ROS) production and mitochondrial superoxide in a human bronchial epithelial cell line (BEAS-2B). H₂O₂ treatment triggered a premature senescence-like phenotype with enlarged and flattened cell morphology accompanied by increased SA-β-gal activity, cell cycle arrest in G0/G1, reduced cell viability, and increased transcription of senescence-associated secretory phenotype (SASP) genes. Additionally, exposure to H₂O₂ increased protein levels of cellular senescence markers (p53 and p21), reduced Sirtuin 3 (SIRT3) and manganese superoxide dismutase (MnSOD) levels, and increased p53 K382 acetylation. These H₂O₂-mediated effects were attenuated by pretreatment with a CO-containing solution. SIRT3 silencing induced mitochondrial superoxide production and triggered a senescence-like phenotype, whereas overexpression decreased mitochondrial superoxide production and alleviated the senescence-like phenotype. Air-liquid interface (ALI) culture of primary human bronchial cells, which becomes a fully differentiated pseudostratified mucociliary epithelium, was used as a model. We found that apical and basolateral exposure to H₂O₂ induced a vacuolated structure that impaired the integrity of ALI cultures, increased goblet cell numbers, decreased SCGB1A1+ club cell numbers, increased p21 protein levels, and increased SASP gene transcription, consistent with our observations in BEAS-2B cells. These effects were attenuated in the apical presence of a CO-containing solution. In summary, we revealed that CO has a pivotal role in epithelial senescence by regulating ROS production via the SIRT3/MnSOD/p53/p21 pathway. This may have important implications in the prevention and treatment of age-associated respiratory pathologies.

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.

Carbon monoxide and a change of heart

The relationship between carbon monoxide and the heart has been extensively studied in both clinical and preclinical settings. The Food and Drug Administration (FDA) is keenly focused on the ill effects of carbon monoxide on the heart when presented with proposals for clinical trials to evaluate efficacy of this gasotransmitter in a various disease settings. This review provides an overview of the rationale that examines the actions of the FDA when considering clinical testing of CO, and contrast that with the continued accumulation of data that clearly show not only that CO can be used safely, but is potently cardioprotective in clinically relevant small and large animal models. Data emerging from Phase I and Phase II clinical trials argues against CO being dangerous to the heart and thus it needs to be redefined and evaluated as any other substance being proposed for use in humans. More than twenty years ago, the belief that CO could be used as a salutary molecule was ridiculed by experts in physiology and medicine. Like all agents designed for use in humans, careful pharmacology and safety are paramount, but continuing to hinder progress based on long-standing dogma in the absence of data is improper. Now, CO is being tested in multiple clinical trials using innovative delivery methods and has proven to be safe. The hope, based on compelling preclinical data, is that it will continue to be evaluated and ultimately approved as an effective therapeutic.

Carbon Monoxide Therapy Using Hybrid Carbon Monoxide-Releasing/Nrf2-Inducing Molecules through a Neuroprotective Lens

Carbon monoxide (CO) has long been known for its toxicity. However, in recent decades, new applications for CO as a therapeutic compound have been proposed, and multiple forms of CO therapy have since been developed and studied. Previous research has found that CO has a role as a gasotransmitter and promotes anti-inflammatory and antioxidant effects, making it an avenue of interest for medicine. Such effects are possible because of the Nrf2/HO1 pathway, which has become a target for therapy development because its activation also leads to CO release. Currently, different forms of treatment involving CO include inhaled CO (iCO), carbon monoxide-releasing molecules (CORMs), and hybrid carbon monoxide-releasing molecules (HYCOs). In this article, we review the progression of CO studies to develop possible therapies, the possible mechanisms involved in the effects of CO, and the current forms of therapy using CO.

Phototriggered cytotoxic properties of tricarbonyl manganese(I) complexes bearing α-diimine ligands towards HepG2

Reaction between bromo tricarbonyl manganese(I) and N,N'-bis(phenyl)-1,4-diaza-1,3-butadiene ligands, bearing different electron-donating and electron-withdrawing groups R = OCH₃, Cl, and NO₂ in the ortho- and para-positions on the phenyl substituent, afforded [MnBr(CO)₃(N-N)] complexes. The influence of the character and position of the substituent on the dark stability and carbon monoxide releasing kinetics was systematically investigated and correlated with the data of the time-dependent density functional theory calculations. The combined UV/Vis and IR data clearly revealed that the aerated solutions of [MnBr(CO)₃(N-N)] in either coordinating or noncoordinating solvents are dark stable and the fluctuations observed during the incubation period especially in the case of the nitro derivatives may be attributed to the exchange of the axial bromo ligand with the coordinating solvent molecules. The free ligands and nitro complexes were non-cytotoxic to HepG2 cells under both the dark and illumination conditions. In the dark, Mn(I) compounds, incorporating o-OCH₃ and o-Cl, exhibited excellent cytotoxicity with IC₅₀ values of 18.1 and 11.8 μM, while their para-substituted analogues were inactive in the dark and active upon the irradiation at 365 nm with IC₅₀ values of 5.7 and 6.7 μM, respectively.

Methionine-based carbon monoxide releasing polymer for the prevention of biofilm formation

A new water-soluble methionine-based CO releasing polymer shows slow and spontaneous release of CO with sustained-release kinetics, preventing biofilm formation against Pseudomonas aeruginosa.

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

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

Luminescent PhotoCORMs: Enabling/Disabling CO Delivery upon Blue Light Irradiation

The new luminescent carbonyl compounds [Mn(Oxa-H)(CO)₃Br] (1) and [Mn(Oxa-NMe₂)(CO)₃Br] (2) were synthesized and fully characterized. Complexes 1 and 2 showed CO release under blue light (λ₄₅₃). Spectroscopic techniques and TD-DFT and SOC-TD-DFT calculations indicated that 1 and 2 release the Oxa-H and Oxa-NMe₂ coligands in addition to the carbonyl ligands, increasing the luminescence during photoinduction.

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

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

Photo-activated CO-release in the amino tungsten Fischer carbene complex, [(CO)5WC(NC4H8)Me], picosecond time resolved infrared spectroscopy, time-dependent density functional theory, and an antimicrobial study

Picosecond time-resolved infrared spectroscopy was used to probe the photo-induced early state dynamics preceding CO loss in the Fischer carbene complex, [(CO)₅WC(NC₄H₈)CH₃]. Time-dependent density functional theory calculations were employed to help in understanding the photochemical and photophysical processes leading to CO-loss. Electrochemical initiated CO release was quantified using gas chromatography. The potential of [(CO)₅WC(NC₄H₈)CH₃], as an antimicrobial agent under irradiation conditions was studied using a Staphylococcus aureus strain.

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

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

The monoiron anionfac-[Fe(CO)3I3]−and its organic aminium salts: their preparation, CO-release, and cytotoxicity

The anionfac-[Fe(CO)₃I₃]⁻undergoes rapid decomposition to release CO and involve iodine radical. The CO-release can be tuned by its cations. The radical causes severe cytotoxicity which may endow the anion a great potential as an anticancer drug.

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

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

Green Light-Responsive CO-Releasing Polymeric Materials Derived from Ring-Opening Metathesis Polymerization

Carbon monoxide (CO) is an important biological gasotransmitter in living cells. Precise spatial and temporal control over release of CO is a major requirement for clinical application. To date, the most reported carbon monoxide releasing materials use expensive fabrication methods and require harmful and poorly designed tissue-penetrating UV irradiation to initiate the CO release precisely at infected sites. Herein, we report the first example of utilizing a green light-responsive CO-releasing polymer P synthesized via ring-opening metathesis polymerization. Both monomer M and polymer P were very stable under dark conditions and CO release was effectively triggered using minimal power and low energy wavelength irradiation (550 nm, ≤28 mW). Time-dependent density functional theory (TD-DFT) calculations were carried out to simulate the electronic transition and insight into the nature of the excitations for both L and M. TD-DFT calculations indicate that the absorption peak of M is mainly due to the excitation of the seventh singlet excited state, S7. Furthermore, stretchable materials using polytetrafluoroethylene (PTFE) strips based on P were fabricated to afford P-PTFE, which can be used as a simple, inexpensive, and portable CO storage bandage. Insignificant cytotoxicity as well as cell permeability was found for M and P against human embryonic kidney cells.

Single-Pot Self-Assembly of Heteroleptic Mn(I)-Based Aminoquinonato-Bridged Ester/Amide-Functionalized Dinuclear Metallastirrups: Potential Anticancer and Visible-Light-Triggered CORMs

Multicomponent self-assembly of Mn₂(CO)₁₀, a bis-chelating aminoquinonato (ON∩ON) bridge (L), and an ester/amide-functionalized flexible neutral ditopic linker (L') has resulted into the formation of M₂LL'-type manganese(I)-based dinuclear metallastirrups of general formula [{(CO)₃Mn(μ-η⁴-L)Mn(CO)₃}(μ-L')] (1-10). Compounds 1-10 were accomplished via orthogonal bonding of the aminoquinone ligand (2,5-bis(n-butylamino)-1,4-benzoquinone/2,5-bis(phenethylamino)-1,4-benzoquinone) and ditopic pyridyl ligand to manganese carbonyl. The resultant metallastirrups were characterized using elemental analyses and IR, UV-vis, ¹H NMR, and electrospray ionization-mass spectroscopic techniques. The molecular structure of 6 was confirmed by single-crystal X-ray diffraction methods. Furthermore, molecular recognition capabilities of 1, 5, 7, and 9 were evaluated with aromatic compounds containing hydroxy/amine functionalities. Anticancer activities of compounds 1-3, 5-7, 9, and 10 were investigated against three cancer cell lines, that is, lung (A549), colon (HCT-15), and cervical (HeLa) as well as on normal cells (HEK 293). Compound 9 showed a broad-spectrum inhibition toward these cancer cells upon exposure to visible light. The myoglobin assay was performed using UV-vis absorption spectroscopy to investigate the visible-light-triggered CO release from 5 and 9 that could be related to their ability to effectively inhibit cancer cells. In addition, morphological studies confirmed the induction of autophagy due to the treatment of cancer cells using compound 9.

Gas Therapy: An Emerging “Green” Strategy for Anticancer Therapeutics

As an emerging area, gas therapy has attracted more and more attention in treating many diseases including cancer. The fabrication of stimuli‐responsive delivery systems with on‐demand release behavior is very promising for precision gas therapy, which can obtain optimal therapeutic performance without gas poisoning risks. In this review, the authors introduce the recent progress in the preparation of different kinds of gas carriers for efficient delivery of gaseous molecules (NO, H2S, CO, O2). Particularly, in order to achieve targeted accumulation of gaseous molecules in tumor tissues, gaseous molecules–integrated nanoparticles were constructed. Most importantly, by combination of gas therapy with other therapeutic modalities such as chemotherapy, photodynamic therapy (PDT), and radiotherapy, various multifunctional nanocarriers have been designed for synergistic cancer therapy. Especially, the recent developments of multifunctional gas‐carrying nanocarriers for synergistic cancer therapy are discussed in detail.

Use of gasotransmitters for the controlled release of polymer-based nitric oxide carriers in medical applications

Nitric Oxide (NO) is a small molecule gasotransmitter synthesized by nitric oxide synthase in almost all types of mammalian cells. NO is synthesized by NO synthase by conversion of l-arginine to l-citrulline in the human body. NO then stimulates soluble guanylate cyclase, from which various physiological functions are mediated in a concentration-dependent manner. High concentrations of NO induce apoptosis or antibacterial responses whereas low NO circulation leads to angiogenesis. The bidirectional effect of NO has attracted considerable attention, and efforts to deliver NO in a controlled manner, especially through polymeric carriers, has been the topic of much research. This naturally produced signaling molecule has stood out as a potentially more potent therapeutic agent compared to exogenously synthesized drugs. In this review, we will focus on past efforts of using the controlled release of NO via polymer-based materials to derive specific therapeutic results. We have also added studies and our future suggestions on co-delivery methods with other gasotransmitters as a step towards developing multifunctional carriers.

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

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