Sense and Release: A Thiol-Responsive Flavonol-Based Photonically Driven Carbon Monoxide-Releasing Molecule That Operates via a Multiple-Input AND Logic Gate

Sonoinduced Tumor Therapy and Metastasis Inhibition by a Ruthenium Complex with Dual Action: Superoxide Anion Sensitization and Ligand Fracture

Photoresponsive ruthenium(II) complexes have recently emerged as a promising tool for synergistic photodynamic therapy and chemotherapy in oncology, as well as for antimicrobial applications. However, the limited penetration power of photons prevents the treatment of deep-seated lesions. In this study, we introduce a sonoresponsive ruthenium complex capable of generating superoxide anion (O2•-) via type I process and initiating a ligand fracture process upon ultrasound triggering. Attaching hydroxyflavone (HF) as an "electron reservoir" to the octahedral-polypyridyl-ruthenium complex resulted in decreased highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps and triplet-state metal to ligand charge transfer (3MLCT) state energy (0.89 eV). This modification enhanced the generation of O2•- under therapeutic ultrasound irradiation at a frequency of 1 MHz. The produced O2•- rapidly induced an intramolecular cascade reaction and HF ligand fracture. As a proof-of-concept, we engineered the Ru complex into a metallopolymer platform (PolyRuHF), which could be activated by low-power ultrasound (1.5 W cm-2, 1.0 MHz, 50% duty cycle) within a centimeter range of tissue. This activation led to O2•- generation and the release of cytotoxic ruthenium complexes. Consequently, PolyRuHF induced cellular apoptosis and ferroptosis by causing mitochondrial dysfunction and excessive toxic lipid peroxidation. Furthermore, PolyRuHF effectively inhibited subcutaneous and orthotopic breast tumors and prevented lung metastasis by downregulating metastasis-related proteins in mice. This study introduces the first sonoresponsive ruthenium complex for sonodynamic therapy/sonoactivated chemotherapy, offering new avenues for deep tumor treatment.

Biomimetic engineering of a neuroinflammation-targeted MOF nanozyme scaffolded with photo-trigger released CO for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is one of the most fatal and irreversible neurodegenerative diseases, which causes a huge emotional and financial burden on families and society. Despite the progress made with recent clinical use of inhibitors of acetylcholinesterase and amyloid-β (Aβ) antibodies, the curative effects of AD treatment remain unsatisfactory, which is probably due to the complexity of pathogenesis and the multiplicity of therapeutic targets. Thus, modulating complex pathological networks could be an alternative approach to treat AD. Here, a neutrophil membrane-coated MOF nanozyme (denoted as Neu-MOF/Fla) is biomimetically engineered to disturb the malignant Aβ deposition-inflammation cycle and ameliorate the pathological network for effective AD treatment. Neu-MOF/Fla could recognize the pathological inflammatory signals of AD, and deliver the photo-triggered anti-inflammatory CO and MOF based hydrolytic nanozymes to the lesion area of the brain in a spontaneous manner. Based on the in vitro and in vivo studies, Neu-MOF/Fla significantly suppresses neuroinflammation, mitigates the Aβ burden, beneficially modulates the pro-inflammatory microglial phenotypes and improves the cognitive defects of AD mice models. Our work presents a good example for developing biomimetic multifunctional nanotherapeutics against AD by means of amelioration of multiple symptoms and improvement of cognitive defects.

Synthesis and Characterization of Guanidinylated CO-Releasing Micelles Based on Biodegradable Polycarbonate

As one of the gaseous signals in living cells, carbon monoxide (CO) not only participates in many biological activities but also serves as a therapeutic agent for the treatment of diseases. However, the limited applicability of CO in gas therapy emerges from the inconvenience of direct administration of CO. Here we reported the construction of guanidinylated CO-releasing micelles, which are composed of poly(trimethylene carbonate) (PTMC)-based CO donors. The in vitro studies demonstrated that micelles in the presence of light irradiation can induce cancer death, whereas no obvious toxicity to normal cells was observed. Moreover, the functionalization of guanidine groups imparts improved cellular uptake efficiency to micelles owing to the specific interactions with the surface of cells, which synergistically increase the anticancer capacity of the system. The guanidine-functionalized CO-releasing micelles provide a new strategy for the construction of CO-releasing nanocarriers, which are expected to find applications in gas therapeutics.

Enhancing Control Over Nitric Oxide Photorelease via a Molecular Keypad Lock

Based on Boolean logic, molecular keypad locks secure molecular information, typically with an optical output. Here we investigate a rare example of a molecular keypad lock with a chemical output. To this end, the light-activated release of biologically important nitric oxide from a ruthenium complex is studied, using proton concentration and photon flux as inputs. In a pH-dependent equilibrium, a nitritoruthenium(II) complex is turned into a nitrosylruthenium(II) complex, which releases nitric oxide under irradiation with visible light. The precise prediction of the output nitric oxide concentration as function of the pH and photon flux is achieved with an artificial intelligence approach, namely the adaptive neuro-fuzzy inference system. In this manner an exceptionally high level of control over the output concentration is obtained. Moreover, the provided concept to lock a chemical output as well as the output prediction may be applied to other (photo)release schemes.

Natural flavonols as probes for direct determination of borax: From conventional fluorescence analysis to paper-based smartphone sensing

Borax is strictly regulated in the food processing and pharmaceutical industry due to its physiological toxicity, and the development of a direct analytical method is essential for effectively monitoring the borax abuse. In this work, the fluorescence properties of flavonoids, including flavones, isoflavones and flavonols, were systematically investigated from aqueous to borax solutions, and it was found that the weak intrinsic fluorescence of flavonols could be pervasively sensitized by borax. A natural flavonol, morin, was subsequently chosen as a representative probe to develop a turn-on fluorescence sensing method for borax analysis, which achieved a linear response spanning four orders of magnitude with a detection limit of 1.07 μM (0.22 μg mL-1 in terms of Na2B4O7 content). Furthermore, a smartphone-assisted paper-based test device was designed and constructed by 3D printing technology. Using morin-impregnated test strips as the carrier, the borax could be visually detected by the RGB signals of the captured images, with a detection limit of 0.13 mM (27.05 μg mL-1 for Na2B4O7). Combining ion exchange treatment for food samples and sodium periodate oxidation for drug samples, the developed methods were successfully applied for the direct analysis of borax in various products with the recoveries of 86.9-106.3% for traditional fluorescence analysis and 82.7-108.8% for smartphone-assisted fluorescence sensing. The fluorescence property of the morin-borax system was studied using time-dependent density functional theory, and the sensing mechanism was discussed in conjunction with experimental research.

Fluorescent small molecule donors

Small molecule donors (SMDs) play subtle roles in the signaling mechanism and disease treatments. While many excellent SMDs have been developed, dosage control, targeted delivery, spatiotemporal feedback, as well as the efficiency evaluation of small molecules are still key challenges. Accordingly, fluorescent small molecule donors (FSMDs) have emerged to meet these challenges. FSMDs enable controllable release and non-invasive real-time monitoring, providing significant advantages for drug development and clinical diagnosis. Integration of FSMDs with chemotherapeutic, photodynamic or photothermal properties can take full advantage of each mode to enhance therapeutic efficacy. Given the remarkable properties and the thriving development of FSMDs, we believe a review is needed to summarize the design, triggering strategies and tracking mechanisms of FSMDs. With this review, we compiled FSMDs for most small molecules (nitric oxide, carbon monoxide, hydrogen sulfide, sulfur dioxide, reactive oxygen species and formaldehyde), and discuss recent progress concerning their molecular design, structural classification, mechanisms of generation, triggered release, structure-activity relationships, and the fluorescence response mechanism. Firstly, from the large number of fluorescent small molecular donors available, we have organized the common structures for producing different types of small molecules, providing a general strategy for the development of FSMDs. Secondly, we have classified FSMDs in terms of the respective donor types and fluorophore structures. Thirdly, we discuss the mechanisms and factors associated with the controlled release of small molecules and the regulation of the fluorescence responses, from which universal guidelines for optical properties and structure rearrangement were established, mainly involving light-controlled, enzyme-activated, reactive oxygen species-triggered, biothiol-triggered, single-electron reduction, click chemistry, and other triggering mechanisms. Fourthly, representative applications of FSMDs for trackable release, and evaluation monitoring, as well as for visible in vivo treatment are outlined, to illustrate the potential of FSMDs in drug screening and precision medicine. Finally, we discuss the opportunities and remaining challenges for the development of FSMDs for practical and clinical applications, which we anticipate will stimulate the attention of researchers in the diverse fields of chemistry, pharmacology, chemical biology and clinical chemistry. With this review, we hope to impart new understanding thereby enabling the rapid development of the next generation of FSMDs.

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.

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.

Visible-Light-Activated Carbon Monoxide Release from Porphyrin-Flavonol Hybrids

We report on porphyrin-flavonol hybrids consisting of a porphyrin antenna and four covalently bound 3-hydroxyflavone (flavonol) groups, which act as highly efficient photoactivatable carbon monoxide (CO)-releasing molecules (photoCORMs). These bichromophoric systems enable activation of the UV-absorbing flavonol chromophore by visible light up to 650 nm and offer precise spatial and temporal control of CO administration. The physicochemical properties of the porphyrin antenna system can also be tuned by inserting a metal cation. Our computational study revealed that the process occurs via endergonic triplet-triplet energy transfer from porphyrin to flavonol and may become feasible thanks to flavonol energy stabilization upon intramolecular proton transfer. This mechanism was also indirectly supported by steady-state and transient absorption spectroscopy techniques. Additionally, the porphyrin-flavonol hybrids were found to be biologically benign. With four flavonol CO donors attached to a single porphyrin chromophore, high CO release yields, excellent uncaging cross sections, low toxicity, and CO therapeutic properties, these photoCORMs offer exceptional potential for their further development and future biological and medical applications.

Strategies for gaseous neuromodulator release in chemical neuroscience: Experimental approaches and translational validation

Gasotransmitters are a group of short-lived gaseous signaling molecules displaying diverse biological functions depending upon their localized concentration. Nitric oxide (NO), hydrogen sulfide (H2S), and carbon monoxide (CO) are three important examples of endogenously produced gasotransmitters that play a crucial role in human neurophysiology and pathogenesis. Alterations in their optimal physiological concentrations can lead to various severe pathophysiological consequences, including neurological disorders. Exogenous administration of gasotransmitters has emerged as a prominent therapeutic approach for treating such neurological diseases. However, their gaseous nature and short half-life limit their therapeutic delivery. Therefore, developing synthetic gasotransmitter-releasing strategies having control over the release and duration of these gaseous molecules has become imperative. However, the complex chemistry of synthesis and the challenges of specific quantified delivery of these gases, make their therapeutic application a challenging task. This review article provides a focused overview of emerging strategies for delivering gasotransmitters in a controlled and sustained manner to re-establish neurophysiological homeostasis.

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.

Rational Design of Esterase-Insensitive Fluorogenic Probes for In Vivo Imaging

Small-molecule fluorogenic probes are indispensable tools for performing research in biomedical fields and chemical biology. Although numerous cleavable fluorogenic probes have been developed to investigate various bioanalytes, few of them meet the baseline requirements for in vivo biosensing for disease diagnosis due to their insufficient specificity resulted from the remarkable esterase interferences. To address this critical issue, we developed a general approach called fragment-based fluorogenic probe discovery (FBFPD) to design esterase-insensitive probes for in vitro and in vivo applications. With the designed esterase-insensitive fluorogenic probe, we successfully achieved light-up in vivo imaging and quantitative analysis of cysteine. This strategy was further extended to design highly specific fluorogenic probes for other representative targets, sulfites, and chymotrypsin. The present study expands the bioanalytical toolboxes available and offers a promising platform to develop esterase-insensitive cleavable fluorogenic probes for in vivo biosensing and bioimaging for the early diagnosis of diseases.

Endoplasmic Reticulum-Targeted Carbon Monoxide Photoreleaser for Drug-Induced Hepatotoxicity Remediation

The alleviation of drug-induced liver injury has been a long-term public health concern. Growing evidence suggests that endoplasmic reticulum (ER) stress plays a critical role in the pathogenesis of drug-induced hepatotoxicity. Therefore, the inhibition of ER stress has gradually become one of the important pathways to alleviate drug-induced liver injury. In this work, we developed an ER-targeted photoreleaser, ERC, for controllable carbon monoxide (CO) release with a near-infrared light trigger. By employing peroxynitrite (ONOO^-) as an imaging biomarker of hepatotoxicity, the remediating effect of CO was mapped upon drug acetaminophen (APAP) challenge. The direct and visual evidence of suppressing oxidative and nitrosative stress by CO was obtained both in living cells and in mice. Additionally, the ER stress inhibiting the effect of CO was verified during drug-induced hepatotoxicity. This work demonstrated that CO may be employed as a potent potential antidote for APAP-related oxidative and nitrative stress remediation.

Strategies toward Metal-Free Carbon Monoxide Prodrugs: An Update

Carbon monoxide is an important gasotransmitter in mammals, with pleiotropic therapeutic potential against a wide range of human diseases. However, clinical translation of CO is severely hampered by the lack of a reliable CO delivery form. The development of metal-free CO prodrugs is the key to resolving such delivery issues. Over the past three years, some new exciting progress has been made in this field. In this review, we highlight these advances and discuss related issues.

A simple lysosome-targeted fluorescent probe based on flavonoid for detection of cysteine in living cells

Cysteine (Cys) is one of the most important biothiols that plays a crucial role in many physiological and pathological processes, and therefore it is of great importance to detect and analyze Cys in subcellular environments, such as in lysosomes. However, only a few fluorescent probes were reported to be capable of detecting Cys in lysosomes selectively. In this wok, we designed and developed a simple, accessible flavone-based fluorescent probe LFA for detecting Cys in lysosomes. Morpholine was employed as the targeting unit for lysosome, and acrylate group was chosen as the Cys-response unit. The probe was easily prepared by a two-step procedure and displayed large Stokes shift, high sensitivity, turn-on response toward Cys over homocysteine (Hcy), glutathione (GSH), and other amino acids. With low cytotoxicity and good cell permeability, the probe could be successfully applied for fluorescence imaging of Cys in living cells. Furthermore, colocalization experiment revealed that lysosomal-targetable ability of LFA was significant. These results indicated that such simple fluorescent probe could provide a promising tool for detection of lysosomal Cys in living biological systems.

External stimuli-responsive gasotransmitter prodrugs: Chemistry and spatiotemporal release

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

Chemiexcitation-Triggered Prodrug Activation for Targeted Carbon Monoxide Delivery

Photolysis-based prodrug strategy can address some critical drug delivery issues, which otherwise are very challenging to tackle with traditional prodrug strategy. However, the need for external light irradiation significantly hampers its in vivo application due to the poor light accessibility of deep tissue. Herein, we propose a new strategy of chemiexcitation-triggered prodrug activation, wherein a photoresponsive prodrug is excited for drug payload release by chemiexcitation instead of photoirradiation. As such, the bond-cleavage power of photolysis can be employed to address some critical drug delivery issues while obviating the need for external light irradiation. We have established the proof of concept by the successful development of a chemiexcitation responsive carbon monoxide delivery platform, which exhibited specific CO release at the tumor site and pronounced tumor suppression effects. We anticipate that such a concept of chemiexcitation-triggered prodrug activation can be leveraged for the targeted delivery of other small molecule-based drug payloads.

Photo-Responsive Micelles with Controllable and Co-Release of Carbon Monoxide, Formaldehyde and Doxorubicin

Endogenous gases have attracted much attention due to their potent applications in disease therapies. The combined therapy, including gaseous molecules and other medicines that can create synergistic effects, is a new way for future treatment. However, due to the gaseous state, gas utilization in medical service is still limited. To pave the way for future usage, in this work, an amphiphilic block copolymer containing nitrobenzyl ether, 3-hydroxyflavone (3-HF) derivatives and ether linker was constructed. The nitrobenzyl ether group endows the polymer with a photo-responsive character. Upon light illumination, 3-HF derivatives can be triggered for carbon monoxide (CO) release. The ether linker can also be released emitting formaldehyde (FA). The self-assembly induced micelle can encompass medicine, e.g., doxorubicin (DOX), into it and a controlled release of DOX can be realized upon light illumination. As far as we know, there is no report on the combination donor of CO and DOX and this is the first attempt on the co-release of CO, FA and DOX.

One- and Two-Photon Uncaging of Carbon Monoxide (CO) with Real-Time Monitoring: On-Demand Carbazole-Based Dual CO-Releasing Platform to Test over Single and Combinatorial Approaches for the Efficient Regression of Orthotopic Murine Melanoma In Vivo

Herein, we report three new metal-free, photochemically active single, dual, and combinatorial CORMs (photoCORMs) based on a carbazole-fused 1,3-dioxol-2-one moiety which released one equivalent of CO, two equivalent of CO, and a combination of one equivalent of each CO and anticancer drug upon one- and two-photon excitation, respectively. The photoCORMs exhibited good cellular uptake and real-time monitoring ability of CO uncaging by a color change approach in cancerous B16F10 cells. Interestingly, the cytotoxicity assay on B16F10 cells indicated that the dual photoCORM has increased anticancer activity over the single and combinatorial photoCORMs upon irradiation. Our results also showed that CO could accelerate the effectiveness of the well-known anticancer drug (chlorambucil). Finally, the in vivo evaluation of the dual photoCORM on an established murine melanoma tumor (C57BL/6J mouse model) manifested a significant regression of tumor volume and led to significant improvement (>50%) in the overall survivability.

Specific ultralow level chemo-recognition using Graphene-fluorophore supramolecular assembly: Fine-tuning the fluorophore framework

The non-covalent interactions between graphene and aromatic fluorophores have generated highly sensitive fluorimetric turn-on sensors for various significant analytes. Herein, the supramolecular interaction between reduced graphene oxide and 7-Hydroxy-4-Methyl-8-Amino Coumarin is made use of for tracing Cu²⁺ at sub-zeptomole level with excellent selectivity among a collection of nineteen metal ions. The system enables quantification of the analyte in a commendably wide range, from micromolar to zeptomolar, a feature that almost all-optical sensors lack. Handy solid-state sensor strip fabricated using the above-mentioned supramolecular combination enabled visual recognition of Cu²⁺ions at the molecular level. Based on the chemo recognition ability of the fluorophore, multiple Boolean logic devices operating at the molecular level are proposed. By screening pertinent coumarin derivatives, it is demonstrated that the selectivity and sensitivity of the sensors of this sort are decided by the number of π- interaction centers of the fluorophores and the strength by which they interact with graphene, respectively, which will enable identification and modification of proper fluorophores for ultra-trace detection of contaminants of environmental relevance from aqueous solutions.

Mechanistic studies of visible light-induced CO release from a 3-hydroxybenzo[g]quinolone

Organic compounds that can be triggered using light to release CO in biological environments are of significant current interest to probe the role of CO in biology and as potential therapeutics. We recently reported that a 3-hydroxybenzo[g]quinolone (5) can be used as a CO delivery molecule to produce anticancer and potent anti-inflammatory effects. Herein we report mechanistic studies of the visible light-induced CO release reaction of this compound. In wet CH₃CN under aerobic conditions, 5 releases 0.90(2) equivalents of CO upon illumination with visible light (419 nm) to give a single depside product. Performing the same reaction under an ¹⁸O₂ atmosphere results in quantitative incorporation of two labeled oxygen atoms in the depside product. Monitoring via¹H NMR and UV-vis during the illumination of 5 in CH₃CN using 419 nm light revealed the substoichiometric formation of a diketone (6) in the reaction mixture. H₂O₂ formation was detected in the same reaction mixtures. DFT studies indicate that upon light absorption an efficient pathway exists for the formation of a triplet excited state species (5b) that can undergo reaction with ³O₂ resulting in CO release. DFT investigations also provide insight into diketone (6) and H₂O₂ formation and subsequent reactivity. The presence of water and exposure to visible light play an important role in lowering activation barriers in the reaction between 6 and H₂O₂ to give CO. Overall, two reaction pathways have been identified for CO release from a 3-hydroxybenzo[g]quinolone.

Illumination of a 3-hydroxybenzo[g]quinolone with visible light results in CO release via two different reaction pathways.

Indole-substituted flavonol-based cysteine fluorescence sensing and subsequent precisely controlled linear CO liberation

This study describes the first indole-substituted flavonol-based fluorescent probe to effectively sense and image Cys in vivo, as a precursor of photoCORM, actuated by Cys, triggered by visible-light, release precisely controlled linear CO under O2.

Cysteine ratiometric fluorescence sensing reaction actuated B-ring naphthalene-substituted flavonol-based PhotoCORM: precisely controlled linear CO liberation

This study describes the first B-ring-naphthalene-substituted flavonol-based ratiometric fluorescent probe to efficiently detect and image endo/exo-genous Cys both in vivo, and subsequent Cys-driven, visible-light triggered linear CO delivery under O2.

Orchestrating Nitric Oxide and Carbon Monoxide Signaling Molecules for Synergistic Treatment of MRSA Infections

The local delivery of gaseous signaling molecules (GSMs) has shown promising therapeutic potential. However, although GSMs have a subtle interplay in physiological and pathological conditions, the co-delivery of different GSMs for therapeutic purposes remains unexplored. Herein, we covalently graft a nitric oxide (NO)-releasing N-nitrosamine moiety onto the carbon monoxide (CO)-releasing 3-hydroxyflavone (3-HF) antenna, resulting in the first NO/CO-releasing donor. Under visible light irradiation, photo-mediated co-release of NO and CO reveals a superior antimicrobial effect toward Gram-positive bacteria with a combination index of 0.053. The synergy of NO and CO hyperpolarizes and permeabilizes bacterial membranes, which, however, shows negligible hemolysis and no evident toxicity toward normal mammalian cells. Moreover, the co-release of NO and CO can efficiently treat MRSA infection in a murine skin wound model, showing a better therapeutic capacity than vancomycin.

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

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

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.

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

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

Low Dose Soft X-Ray Remotely Triggered Lanthanide Nanovaccine for Deep Tissue CO Gas Release and Activation of Systemic Anti-Tumor Immunoresponse

Gas-based therapy has emerged as a new green therapy strategy for anti-tumor treatment. However, the therapeutic efficacy is still restricted by the deep tissue controlled release, poor lymphocytic infiltration, and inherent immunosuppressive tumor microenvironment (TME). Herein, a new type of nanovaccine is designed by integrating low dose soft X-ray-triggered CO releasing lanthanide scintillator nanoparticles (ScNPs: NaLuF4 :Gd,Tb@NaLuF4 ) with photo-responsive CO releasing moiety (PhotoCORM) for synergistic CO gas/immuno-therapy of tumors. The designed nanovaccine presents significantly boosted radioluminescence and enables deep tissue CO generation at unprecedented tissue depths of 5 cm under soft X-ray irradiation. Intriguingly, CO as a superior immunogenic cell death (ICD) inducer further reverses the deep tissue immunosuppressive TME and concurrently activates adaptive anti-tumor immunity through efficient reactive oxygen species (ROS) generation. More importantly, the designed nanovaccine presents efficient growth inhibition of both local and distant tumors via a soft X-ray activated systemic anti-tumor immunoresponse. This work provides a new strategy of designing anti-tumor nanovaccines for synergistic deep tissue gas-therapy and remote soft X-ray photoactivation of the immune response.

Multifunctional Carbon Monoxide Prodrug-Loaded Nanoplatforms for Effective Photoacoustic Imaging-Guided Photothermal/Gas Synergistic Therapy

Multifunctional cancer treatments based on gas therapy combined with other cancer treatments have gained tremendous attention and hold great promise in biomedical applications. In this study, a carbon monoxide-releasing nanoplatform combined with near-infrared (NIR) laser-triggered photothermal therapy (PTT) was constructed. The nanoplatform was composed of manganese pentacarbonyl bromide (MnCO)-loaded g-carbon nitride/polypyrrole (CNPpy) nanomaterials (MnCO@CNPpy). MnCO can be triggered to produce CO under H₂O₂ conditions. Upon exogenous NIR light stimulation and tumor microenvironment-overexpressed H₂O₂, MnCO@CNPpy exhibited excellent CO generation performance and photothermal effect. The generation of CO induced intracellular oxidative stress and caused cell apoptosis. Additionally, photoacoustic (PA) imaging was performed to track the delivery and accumulation of the nanomaterial in tumor sites because of the great photothermal conversion of CNPpy. The presented MnCO@CNPpy nanoplatform displayed desirable PTT and CO therapy in the inhibition of tumor growth and may provide a promising strategy for multifunctional antitumor synergistic treatments.

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

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

Tumor Microenvironment-"AND" Near-Infrared Light-Activated Coordination Polymer Nanoprodrug for On-Demand CO-Sensitized Synergistic Cancer Therapy

Carbon monoxide (CO) as an emerging treatment holds great promise for inducing the apoptosis of cancer cells. Here coordination assembled strategy is first reported for synthesis of Cu(II)-flavone coordination polymer (NCu-FleCP) CO nanoprodrug that is stable in normal physiological conditions, and yet readily reduces to small size prodrug complex and releases CO on demand under glutathione (GSH) and near infrared (NIR) light. Specifically, after uptaking by cancer cells, local GSH attacked coordination bond within NCu-FleCP, resulting in the release of Cu(I) and free Fle. The CC bond of Fle is cleavage under NIR light to release CO for gas therapy, and Cu(I) reacts with local H₂ O₂ through Fenton like reaction to generate hydroxyl radicals (^• OH) for chemodynamic therapy. Detailed in vitro and in vivo experiments demonstrate that the CO prodrug system in generating a sufficient quantity of CO and ^• OH offers remarkable destructive effects against cancer cells without causing toxicity to surrounding normal tissues. The study provides a solid foundation to develop smart coordination polymer CO prodrugs with on-demand CO release, enhanced permeability and retention effect, and biodegradability for multimodal synergistic therapy.

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

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

CO as a therapeutic agent: discovery and delivery forms

Carbon monoxide (CO) as one of the three important endogenously produced signaling molecules, termed as "gasotransmitter," has emerged as a promising therapeutic agent for treating various inflammation and cellular-stress related diseases. In this review, we discussed CO's evolution from a well-recognized toxic gas to a signaling molecule, and the effort to develop different approaches to deliver it for therapeutic application. We also summarize recently reported chemistry towards different CO delivery forms.

Visible light-controlled carbon monoxide delivery combined with the inhibitory activity of histone deacetylases from a manganese complex for an enhanced antitumor therapy

Multifunctional drugs with synergistic effects have been widely developed to enhance the treatment efficiency of various diseases, such as malignant tumors. Herein, a novel bifunctional manganese(I)-based prodrug [MnBr(CO)₃(APIPB)] (APIPB = N-(2-aminophen-yl)-4-(1H-imidazo[4,5-f] [1, 10] phenanthrolin-2-yl)benzamide) with inhibitory histone deacetylase (HDAC) activity and light-controlled carbon monoxide (CO) delivery was successfully designed and synthesized. [MnBr(CO)₃(APIPB)] readily released CO under visible light irradiation (λ > 400 nm) through which the amount of released CO could be controlled by manipulating light power density and illumination time. In the absence of light irradiation, the cytotoxic effect of [MnBr(CO)₃(APIPB)] on cancer cells was greater than that of the commercially available HDAC inhibitor MS-275. Consequently, with a combination of CO delivery and HDAC inhibitory activity, [MnBr(CO)₃(APIPB)] showed a remarkably enhanced antitumor effect on HeLa cells (IC₅₀ = 3.2 μM) under visible light irradiation. Therefore, this approach shows potential for the development of medicinal metal complexes for combined antitumor therapies.

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.

Role of Carbon Monoxide in Host-Gut Microbiome Communication

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

Development of Triggerable, Trackable, and Targetable Carbon Monoxide Releasing Molecules

Carbon monoxide (CO) is a gaseous signaling molecule produced in humans via the breakdown of heme in an O2-dependent reaction catalyzed by heme oxygenase enzymes. A long-lived species relative to other signaling molecules (e.g., NO, H2S), CO exerts its physiological effects via binding to low-valent transition metal centers in proteins and enzymes. Studies involving the administration of low doses of CO have shown its potential as a therapeutic agent to produce vasodilation, anti-inflammatory, antiapoptotic, and anticancer effects. In pursuit of developing tools to define better the role and therapeutic potential of CO, carbon monoxide releasing molecules (CORMs) were developed. To date, the vast majority of reported CORMs have been metal carbonyl complexes, with the most well-known being Ru2Cl4(CO)6 (CORM-2), Ru(CO)3Cl(glycinate) (CORM-3), and Mn(CO)4(S2CNMe(CH2CO2H)) (CORM-401). These complexes have been used to probe the effects of CO in hundreds of cell- and animal-based experiments. However, through recent investigations, it has become evident that these reagents exhibit complicated reactivity in biological environments. The interpretation of the effects produced by some of these complexes is obscured by protein binding, such that their formulation is not clear, and by CO leakage and potential redox activity. An additional weakness with regard to CORM-2 and CORM-3 is that these compounds cannot be tracked via fluorescence. Therefore, it is unclear where or when CO release occurs, which confounds the interpretation of experiments using these molecules. To address these weaknesses, our research team has pioneered the development of metal-free CORMs based on structurally tunable extended flavonol or quinolone scaffolds. In addition to being highly controlled, with CO release only occurring upon triggering with visible light (photoCORMs), these CO donors are trackable via fluorescence prior to CO release in cellular environments and can be targeted to specific cellular locations.In the Account, we highlight the development and application of a series of structurally related flavonol photoCORMs that (1) sense characteristics of cellular environments prior to CO release; (2) enable evaluation of the influence of cytosolic versus mitochondrial-localized CO release on cellular bioenergetics; (3) probe the cytotoxicity and anti-inflammatory effects of intracellular versus extracellular CO delivery; and (4) demonstrate that albumin delivery of a photoCORM enables potent anticancer and anti-inflammatory effects. A key advantage of using triggered CO release compounds in these investigations is the ability to examine the effects of the molecular delivery vehicle in the absence and presence of localized CO release, thus providing insight into the independent contributions of CO. Overall, flavonol-based CO delivery molecules offer opportunities for triggerable, trackable, and targetable CO delivery that are unprecedented in terms of previously reported CORMs and, thus, offer significant potential for applications in biological systems.