The Interaction of CORM‐2 with Block Copolymers Containing Poly(4‐vinylpyridine): Macromolecular Scaffolds for Carbon Monoxide Delivery in Biological Systems

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.

Red light-triggered release of ROS and carbon monoxide for combinational antibacterial application

The abuse of antibiotics has led to the emergence of a wide range of drug-resistant bacteria. To address the challenge of drug-resistant bacterial infections and related infectious diseases, several effective antibacterial strategies have been developed. To achieve enhanced therapeutic effects, combinational treatment approaches should be employed. With this in mind, a metal-organic framework (MOF) based nanoreactor with integrated photodynamic therapy (PDT) and gas therapy which can release reactive oxygen species (ROS) and carbon monoxide (CO) under red light irradiation has been developed. The release of ROS and CO under red light irradiation exerts a preferential antibacterial effect on Gram-positive/Gram-negative bacteria. The bactericidal effects of ROS and CO on Staphylococcus aureus (S. aureus) and methicillin-resistant S. aureus (MRSA) are better than ROS only, showing a combinational antibacterial effect. Furthermore, the fluorescence emission properties of porphyrin moieties can be leveraged for real-time tracking and imaging of the nanoreactors. The simple preparation procedures of this material further enhance its potential as a versatile and effective antibacterial candidate, thereby presenting a new strategy for PDT and gas combinational treatment.

Recent advance in self-assembled polymeric nanomedicines for gaseous signaling molecule delivery

Gaseous signaling molecules such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2 S) have recently been recognized as essential signal mediators that regulate diverse physiological and pathological processes in the human body. With the evolution of gaseous signaling molecule biology, their therapeutic applications have attracted growing attention. One of the challenges in translational research of gaseous signaling molecules is the lack of efficient and safe delivery systems. To tackle this issue, researchers developed a library of gas donors, which are low molecular weight compounds that can release gaseous signaling molecules upon decomposition under physiological conditions. Despite the significant efforts to control gaseous signaling molecule release from gas donors, the therapeutic potential of gaseous signaling molecules cannot be fully explored due to their unfavorable pharmacokinetics and toxic side effects. Recently, the use of nanoparticle-based gas donors, especially self-assembled polymeric gas donors, have emerged as a promising approach. In this review, we describe the development of conventional small gas donors and the challenges in their therapeutic applications. We then illustrate the concepts and critical aspects for designing self-assembled polymeric gas donors and discuss the advantages of this approach in gasotransmistter delivery. We also highlight recent efforts to develop the delivery systems for those molecules based on self-assembled polymeric nanostructures. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Partially Reduced MIL-100(Fe) as a CO Carrier for Sustained CO Release and Regulation of Macrophage Phenotypic Polarization

Carbon monoxide (CO) is a bioactive molecule with high potential as it shows promising efficacy for regulating inflammation. Materials capable of storing and delivering CO are of great potential therapeutic value. Although CO-releasing molecules (CORMs) have been developed to deliver CO, the short CO duration of minutes to 2 h confines their practical use. In this study, partially reduced MIL-100(Fe) as a new CO-releasing nanoMOF was developed and used for sustained CO release and macrophage (MA) phenotypic polarization regulation. MIL-100(Fe) was synthesized and mildly annealed in vacuum for partial reduction. When the annealing temperature was lower than 250 °C, less Fe(II) present in MIL-100(Fe) and the subsequent CO adsorption and desorption profiles displayed typical features of physisorption. While it was annealed at 250 °C, it showed about 20% of Fe(III) was reduced, which resulted in chemisorption of CO due to the high coordination affinity of Fe(II) to CO. The loading amount of CO was increased, and the CO release was prolonged for about 24 h. Furthermore, the CO release from this nanoMOF could alter the lipopolysaccharide (LPS)-induced macrophage from M1 to the alternative M2 phenotype and promoted the growth of endothelial cells (ECs) by paracrine regulation of MA. It can be envisioned as a promising CO-releasing solid for biomedical application.

Controlled therapeutic delivery of CO from carbon monoxide-releasing molecules (CORMs)

Carbon monoxide (CO) has been regarded as a "silent killer" for its toxicity toward biological systems. However, a low concentration of endogenously produced CO has shown a number of therapeutic benefits such as anti-inflammatory, anti-proliferative, anti-apoptosis, and cytoprotective activities. Carbon monoxide-releasing molecules (CORMs) have been developed as alternatives to direct CO inhalation, which requires a specialized setting for strict dose control. CORMs are efficient CO donors, with central transition metals (such as ruthenium, iron, cobalt, and manganese) surrounded by CO as a ligand. CORMs can stably store and subsequently release their CO payload in the presence of certain triggers including solvent, light, temperature, and ligand substitution. However, CORMs require appropriate delivery strategies to improve short CO release half-life and target specificity. Herein, we highlighted the therapeutic potential of inhalation and CORMs-delivered CO. The applications of conjugate and nanocarrier systems for controlling CO release and improving therapeutic efficacy of CORMs are also described in detail. The review concludes with some of the hurdles that limit clinical translation of CORMs. Keeping in mind the tremendous potential and growing interest in CORMs, this review would be helpful for designing controlled CO release systems for clinical applications.

Photoactive carbon monoxide-releasing coordination polymer particles

We report the synthesis of photoactive carbon monoxide-releasing coordination polymer particles through the assembly of Mn(I) carbonyl complexes with bis(imidazole) ligands. The use of Mn(I) carbonyl complexes as metallic nodes in the coordination network avoids the potential for aggregation-induced self-quenching, favouring their use in the solid state.

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.

Ultrasound responsive carbon monoxide releasing micelle

Carbon monoxide (CO), an endogenously produced gasotransmitter, has shown various therapeutic effects in previous studies. In this work, we developed an ultrasound responsive micelle for localized CO delivery. The micelle is composed of a pluronic shell and a core of a CO releasing molecule, CORM-2. The mechanism is based on the ultrasound response of pluronics, and the reaction between CORM-2 and certain biomolecules, e.g. cysteine. The latter allows CO release without significantly breaking the micelles. In a 3.5 mM cysteine solution, the micelles released low level of CO, indicating effective encapsulation of CORM-2. Treatment with a low intensity, non-focused ultrasound led to four times as much CO as the sample without ultrasonication, which is close to that of unencapsulated CORM-2. Significantly reduced proliferation of prostate cancer cells (PC-3) was observed 24 h after the PC-3 cells were treated with the CORM-2 micelles followed by ultrasound activation.

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.

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.

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

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

Poly(butyl cyanoacrylate) nanoparticle containing an organic photoCORM

A new organic photoCORM encapsulated in a poly(butyl cyanoacrylate) nanoparticle showed nearly quantitative CO release under visible light and low cytotoxicity.

Dual-Gated Microparticles for Switchable Antibody Release

We pioneer the design of dual-gated microparticles, both responsive to changes in temperature and pH, for stimuli-responsive chromatography targeted at the efficient separation of antibodies. Dual-gated microspheres were synthesized by introducing RAFT-based thiol-terminal block copolymers of poly(N-isopropylacrylamide-b-4-vinylpyridine) (P(NIPAM-b-4VP, 4800 ≤ M_n/Da ≤ 10 000, featuring block length ratios of 29:7, 29:15, and 29:30, respectively) by thiol-epoxy driven ligation to the surface of poly(glycidyl methacrylate) (PGMA) microparticles (10-12 μm), whereby the 4-vinylpyridine units within the lateral chain enable protein binding. The switchable protein release abilities of the resulting microparticle resins are demonstrated by adsorption of immunoglobulins at 40 °C and pH 8 and their release at 5 °C or pH 3, respectively. We demonstrate that P(NIPAM₂₉-b-4VP₃₀)-grafted PGMA particles show a maximum adsorption capacity for immunoglobulins of 18.9 mg mL^-1 settled resin at 40 °C/pH 8, whereas the adsorption capacity decreased to 7.5 mg mL^-1 settled resin at 5 °C while retaining the pH value, allowing the unloading of the chromatographic column by a facile temperature switch. Critically, regeneration of the dual-gated microspheres became possible by lowering the pH to 3.

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

Carbon monoxide (CO) is attracting increasing attention because of its role as a gasotransmitter with cytoprotective and homeostatic properties. Carbon monoxide releasing molecules (CORMs) are spatially and temporally controlled CO releasers that exhibit superior and more effective pharmaceutical traits than gaseous CO because of their chemistry and structure. Experimental and preclinical research in animal models has shown the therapeutic potential of inhaled CO and CORMs, and the biological effects of CO and CORMs have also been observed in preclinical trials via the genetic modulation of heme oxygenase-1 (HO-1). In this review, we describe the pharmaceutical use of CO and CORMs, methods of detecting CO release, and developments in CORM design and synthesis. Many valuable clinical CORMs formulated using macromolecules and nanomaterials are also described.

Eradication of HT-29 colorectal adenocarcinoma cells by controlled photorelease of CO from a CO-releasing polymer (photoCORP-1) triggered by visible light through an optical fiber-based device

The gaseous signaling molecule carbon monoxide (CO) has recently been recognized for its wide range of physiological activity as well as its antineoplastic properties. However, site-specific delivery of this noxious gas presents a major challenge in hospital settings. In this work, a visible light-sensitive CO-releasing molecule (photoCORM) derived from manganese(I) and 2-(quinolyl)benzothiazole (qbt) namely, [Mn(CO)₃(qbt)(4-vpy)](CF₃SO₃) (1), has been co-polymerized within a gas-permeable HEMA/EGDMA hydrogel. The resulting photoactive CO-releasing polymer (photoCORP-1) incorporates 1 such that neither the carbonyl complex nor its photoproduct(s) exits the polymer at any time. The material can be triggered to photorelease CO remotely by low-power broadband visible light (<1mWcm^-2) with the aid of fiber optics technology. The CO photorelease rates of photoCORP-1 (determined by spectrophotometry) can be modulated by both the concentration of 1 in the hydrogel and the intensity of the light. A CO-delivery device has been assembled to deliver CO to a suspension of human colorectal adenocarcinoma cells (HT-29) under the control of visible light and the extent of CO-induced apoptotic death of the cancer cells has been determined via Annexin V/Propidium iodide stain and flow cytometry. This photoactive CO-releasing polymer could find use in delivering controlled doses of CO to cellular targets such as malignant tissues in remote parts of the body.

Synthesis of Air-Stable Cyclopentadienyl Fe(CO)2 (Fp) Polymers by a Host-Guest Interaction of Cyclodextrin with Air-Sensitive Fp Pendant Groups

Host-guest chemistry is used to address the challenge of the synthesis of air-stable polymers containing air-sensitive metal complexes. The complexation of the CpFe(CO)₂ (Fp) pendent group with cyclodextrin (CD) molecules created air-stable poly(Fp-methylstyrene) P(CD/FpMSt). This CD complexation resulted in dimerization of the adjacent Fp groups, which was characterized by NMR, FTIR, and cyclic voltammetry (CV) analyses. P(CD/FpMSt) was soluble in DMSO and remained stable even the solution was exposed to air for months. The host-guest chemistry accounted for the improved stability, because the Fp groups decomposed upon removal of the CD molecules using competing guest molecules. The CD-complexed polymer showed light-trigged properties, including CO release and antimicrobial activity. Host-guest chemistry of air-sensitive organometallic complexes is therefore a promising technique that can be used to broaden the scope of metal-containing polymers (MCPs) with processable novel functions.

Light responsive metal-organic frameworks as controllable CO-releasing cell culture substrates

A new carbon monoxide (CO)-releasing material has been developed by embedding a manganese carbonyl complex, MnBr(bpydc)(CO)3 (bpydc = 5,5'-dicarboxylate-2,2'-bipyridine) into a highly robust Zr(iv)-based metal-organic framework (MOF). Efficient and controllable CO-release was achieved under exposure to low intensity visible light. Size-controllable nanocrystals of the photoactive MOF were obtained and their CO-releasing properties were correlated with their crystal sizes. The photoactive crystals were processed into cellular substrates with a biocompatible polymer matrix, and the light-induced delivery of CO and its subsequent cellular uptake were monitored using a fluorescent CO-probe. The results discussed here demonstrate a new opportunity to use MOFs as macromolecular scaffolds towards CO-releasing materials and the advantage of MOFs for high CO payloads, which is essential in future therapeutic applications.

Core–shell materials bearing iron(ii) carbonyl units and their CO-release via an upconversion process

A core–shell nanoplatform was constructed with upconversion nanomaterials onto which iron carbonyl units were chemically loaded. The materials with excellent biocompatibility release CO upon irradiation with a NIR laser.

Polymer–protein hybrid scaffolds as carriers for CORM-3: platforms for the delivery of carbon monoxide (CO)

The carbon monoxide releasing molecule, CORM-3, was grafted onto a polymer–protein conjugate thereby improving its half-life and release characteristics.

CO-releasing molecule (CORM) conjugate systems

To try to advance CORMs toward medical applications, they are covalently bound to peptides, polymers, nanoparticles, dendrimers, and protein cages or are incorporated into non-wovens, tablets, or metal–organic frameworks.