CO Releasing Properties and Cytoprotective Effect of cis-trans- [ReII(CO)2Br2L2]n Complexes

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.

Carbon monoxide as a cellular protective agent in a swine model of cardiac arrest protocol

Out-of-hospital cardiac arrest (OHCA) affects over 360,000 adults in the United States each year with a 50-80% mortality prior to reaching medical care. Despite aggressive supportive care and targeted temperature management (TTM), half of adults do not live to hospital discharge and nearly one-third of survivors have significant neurologic injury. The current treatment approach following cardiac arrest resuscitation consists primarily of supportive care and possible TTM. While these current treatments are commonly used, mortality remains high, and survivors often develop lasting neurologic and cardiac sequela well after resuscitation. Hence, there is a critical need for further therapeutic development of adjunctive therapies. While select therapeutics have been experimentally investigated, one promising agent that has shown benefit is CO. While CO has traditionally been thought of as a cellular poison, there is both experimental and clinical evidence that demonstrate benefit and safety in ischemia with lower doses related to improved cardiac/neurologic outcomes. While CO is well known for its poisonous effects, CO is a generated physiologically in cells through the breakdown of heme oxygenase (HO) enzymes and has potent antioxidant and anti-inflammatory activities. While CO has been studied in myocardial infarction itself, the role of CO in cardiac arrest and post-arrest care as a therapeutic is less defined. Currently, the standard of care for post-arrest patients consists primarily of supportive care and TTM. Despite current standard of care, the neurological prognosis following cardiac arrest and return of spontaneous circulation (ROSC) remains poor with patients often left with severe disability due to brain injury primarily affecting the cortex and hippocampus. Thus, investigations of novel therapies to mitigate post-arrest injury are clearly warranted. The primary objective of this proposed study is to combine our expertise in swine models of CO and cardiac arrest for future investigations on the cellular protective effects of low dose CO. We will combine our innovative multi-modal diagnostic platform to assess cerebral metabolism and changes in mitochondrial function in swine that undergo cardiac arrest with therapeutic application of CO.

Gas Therapy: Generating, Delivery, and Biomedical Applications

Oxygen (O2 ), nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2 S), and hydrogen (H2 ) with direct effects, and carbon dioxide (CO2 ) with complementary effects on the condition of various diseases are known as therapeutic gases. The targeted delivery and in situ generation of these therapeutic gases with controllable release at the site of disease has attracted attention to avoid the risk of gas poisoning and improve their performance in treating various diseases such as cancer therapy, cardiovascular therapy, bone tissue engineering, and wound healing. Stimuli-responsive gas-generating sources and delivery systems based on biomaterials that enable on-demand and controllable release are promising approaches for precise gas therapy. This work highlights current advances in the design and development of new approaches and systems to generate and deliver therapeutic gases at the site of disease with on-demand release behavior. The performance of the delivered gases in various biomedical applications is then discussed.

Rational design of co-ordination compounds in combination of bipyridine type of ligands and group 7 metal (M = Mn, Re) for photoCORM: a DFT study

CONTEXT

A large number of manganese and rhenium tricarbonyl complexes are known in literature along with various applications in different fields. CO-releasing molecules (CORMs) got recent research attention because CO can act as a prodrug for different diseases. CORMs offer the promising prospect of a safe and controllable amount of CO release. In this research work, we have explored the electronic properties of compounds such as bipyridine-related [Mn(CO)3] and [Re(CO)3] and we have compared the electronic properties of both manganese and rhenium tricarbonyl complexes in the light of carbon monoxide releasing tendency. The chosen Mn and Re metals have enough possibility to vary or play with ligands and design a new and novel CORM molecule. In this context, we have taken a range of 4,4'-disubstituted 2,2' bipyridyl ligands (Rbpy, where R = NH2, tBu, OCH3, H, CF3, CN, NO2) to investigate CO's liberation ability to identify and study such molecules. The calculated absorbance of designed complexes (1-14) shows visible/near-IR region (350-850 nm). The HOMO-LUMO energy gap of 7 (ΔE=2.40 eV) complex and for complex 14 (ΔE=2.28 eV) which is lesser in all complexes but the MLCT percentage is greater in Mn tricarbonyl complexes in comparison to Re tricarbonyl complexes. The calculated results of the FMO approach revealed that complex 7 and 14 have the lowest energy gap which is also in good agreement with DOSs and TDM results. The theoretically calculated results revealed that the both Mn and Re tricarbonyl complexes have a tendency for labialization of CO, but Mn tricarbonyl complexes are more prone to CO release because they have higher MLCT percentage.

METHODS

In this research work, we have performed density functional theory (DFT) calculations to explore the physical properties of compounds such as bipyridine-related [Mn(CO)3] and [Re(CO)3] and we have compared the physical properties of both manganese and rhenium tricarbonyl complexes in the light of carbon monoxide releasing tendency. DFT-based calculations were performed by using B3LYP/LANL2DZ basis set followed by acetonitrile solvent using the conductor-like polarizable continuum model (CPCM) for different calculations. Various geometrical calculations were performed using the Gaussian16 suite of programs and the output results obtained from Gaussian16 were visualized using GaussView 5.0.16. The same level of theory was used for various calculations, including frontier molecular orbital (FMO) analysis, metal to ligand charge transfer (MLCT), density of state (DOS) calculations, and transition density of matrix (TDM) calculations. For specific calculations, GaussSum 2.2 software package was used to calculate the density of states, and the Multiwfn 3.8 program was used to analyze the transition density matrix, which is presented using heat maps for both electrons and holes.

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.

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.

Anticancer and Antibiotic Rhenium Tri- and Dicarbonyl Complexes: Current Research and Future Perspectives

Organometallic compounds are increasingly recognized as promising anticancer and antibiotic drug candidates. Among the transition metal ions investigated for these purposes, rhenium occupies a special role. Its tri- and dicarbonyl complexes, in particular, attract continuous attention due to their relative ease of preparation, stability and unique photophysical and luminescent properties that allow the combination of diagnostic and therapeutic purposes, thereby permitting, e.g., molecules to be tracked within cells. In this review, we discuss the anticancer and antibiotic properties of rhenium tri- and dicarbonyl complexes described in the last seven years, mainly in terms of their structural variations and in vitro efficacy. Given the abundant literature available, the focus is initially directed on tricarbonyl complexes of rhenium. Dicarbonyl species of the metal ion, which are slowly gaining momentum, are discussed in the second part in terms of future perspective for the possible developments in the field.

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.

Efficient Direct Nitrosylation of α-Diimine Rhenium Tricarbonyl Complexes to Structurally Nearly Identical Higher Charge Congeners Activable towards Photo-CO Release

The reaction of rhenium α-diimine (N-N) tricarbonyl complexes with nitrosonium tetrafluoroborate yields the corresponding dicarbonyl-nitrosyl [Re(CO)2(NO)(N-N)X]+ species (where X = halide). The complexes, accessible in a single step in good yield, are structurally nearly identical higher charge congeners of the tricarbonyl molecules. Substitution chemistry aimed at the realization of equivalent dicationic species (intended for applications as potential antimicrobial agents), revealed that the reactivity of metal ion in [Re(CO)2(NO)(N-N)X]+ is that of a hard Re acid, probably due to the stronger π-acceptor properties of NO+ as compared to those of CO. The metal ion thus shows great affinity for π-basic ligands, which are consequently difficult to replace by, e.g., σ-donor or weak π-acids like pyridine. Attempts of direct nitrosylation of α-diimine fac-[Re(CO)3]+ complexes bearing π-basic OR-type ligands gave the [Re(CO)2(NO)(N-N)(BF4)][BF4] salt as the only product in good yield, featuring a stable Re-FBF3 bond. The solid state crystal structure of nearly all molecules presented could be elucidated. A fundamental consequence of the chemistry of [Re(CO)2(NO)(N-N)X]+ complexes, it that the same can be photo-activated towards CO release and represent an entirely new class of photoCORMs.

Aerobically stable and substitutionally labile α-diimine rhenium dicarbonyl complexes

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes are described. The molecules are prepared in high yield from the cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br₂]⁻ anion (2, where ^tBu₂bpy is 4,4′-di-tert-butyl-2,2′-bipyridine), which can be isolated from the one electron reduction of the corresponding 17-electron complex (1). Compound 2 is stable in the solid state, but in solution it is oxidized by molecular oxygen back to 1. Replacement of a single bromide of 2 by σ-donor monodentate ligands (Ls) yields stable neutral 18-electron cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO)₂(^tBu₂bpy)Br(L)] species can be further reacted with Ls to prepare stable cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes in good yield. Ligand substitution of Re(i) complexes proceeds via pentacoordinate intermediates capable of Berry pseudorotation. In addition to the cis–cis–trans-complexes, cis–cis–cis- (all cis) isomers are also formed. In particular, cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes establish an equilibrium with all cis isomers in solution. The solid state crystal structure of nearly all molecules presented could be elucidated. The molecules adopt a slightly distorted octahedral geometry. In comparison to similar fac-[Re(CO)₃]⁺complexes, Re(i) diacarbonyl species are characterized by a bend (ca. 7°) of the axial ligands towards the α-diimine unit. [Re(CO)₂(^tBu₂bpy)Br₂]⁻ and [Re(CO)₂(^tBu₂bpy)Br(L)] complexes may be considered as synthons for the preparation of a variety of new stable diamagnetic dicarbonyl rhenium cis-[Re(CO)₂]⁺ complexes, offering a convenient entry in the chemistry of the core.

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes offer a convenient entry in the chemistry of the cis-[Re(CO)₂]⁺ core.

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.

Diversification of quinoline-triazole scaffolds with CORMs: Synthesis, in vitro and in silico biological evaluation against Plasmodium falciparum

A discrete series of tricarbonyl manganese and rhenium complexes conjugated to a quinoline-triazole hybrid scaffold were synthesised and their inhibitory activities evaluated against Plasmodium falciparum. In general, the complexes show moderate activity with improved inhibitory activities for the photoactivatable manganese(I) tricarbonyl complexes in the malaria parasite. All complexes are active in the dark against the NF54 CQS (chloroquine-sensitive) and K1 MDR (multidrug-resistant) strains of Plasmodium falciparum, with IC₅₀ values in the low micromolar range. Of significance, the complexes retain their activity in the MDR strain with resistance indices ranging between 1.1 and 2.1. The Mn(I) analogues display photodissociation of all three CO ligands upon irradiation at 365 nm. More importantly, the complexes show increased antimalarial activity in vitro upon photoactivation, something not observed by the clinically used reference drug, chloroquine. As a purported mechanism of action, the compounds were evaluated as β-haematin inhibitors. To further understand the interactions of the complexes, in silico hemozoin docking simulations were performed, attesting to the fact that CO-release could be vital for blocking the hemozoin formation pathway. These results show that this strategy may be a valuable, novel route to design antimalarial agents with higher efficacy.

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.

Syntheses and Characterization of a Pair of Isomers of Heteroleptic Bis(Bidentate) Ruthenium(II) Complexes with Two Different Monodentate Ligands

Two isomers of heteroleptic bis(bidentate) ruthenium(II) complexes with dimethyl sulfoxide (dmso) and chloride ligands, trans(Cl,N_bpy )- and trans(Cl,N_Hdpa )-[Ru(bpy)Cl(dmso-S)(Hdpa)]⁺ (bpy: 2,2'-bipyridine; Hdpa: di-2-pyridylamine), are synthesized. This is the first report on the selective synthesis of a pair of isomers of cis-[Ru(L)(L')XY]ⁿ⁺ (L≠L': bidentate ligands; X≠Y: monodentate ligands). The structures of the ruthenium(II) complexes are clarified by means of X-ray crystallography, and the signals in the ¹ H NMR spectra are assigned based on ¹ H-¹ H COSY spectra. The colors of the two isomers are clearly different in both the solid state and solution: the trans(Cl,N_bpy ) isomer has a deep red color, whereas the trans(Cl,N_Hdpa ) isomer is yellow. Although both complexes have intense absorption bands at λ≈440-450 nm, only the trans(Cl,N_bpy ) isomer has a shoulder band at λ≈550 nm. DFT calculations indicate that the LUMOs of both isomers are the π* orbitals in the bpy ligand, and that the LUMO level of the trans(Cl,N_bpy ) isomer is lower than that of the trans(Cl,N_Hdpa ) isomer due to the trans effect of the Cl ligand; thus resulting in the appearance of the shoulder band. The HOMO levels are almost the same in both isomers. The energy levels are experimentally supported by cyclic voltammograms, in which these isomers have different reduction potentials and similar oxidation potentials.

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.

Design of Rhenium Compounds in Targeted Anticancer Therapeutics

Background:

Many rhenium (Re) complexes with potential anticancer properties have been synthesized in the recent years with the aim to overcome the clinical limitations of platinum agents. Re(I) tricarbonyl complexes are the most common but Re compounds with higher oxidation states have also been investigated, as well as hetero-metallic complexes and Re-loaded self-assembling devices. Many of these compounds display promising cytotoxic and phototoxic properties against malignant cells but all Re compounds are still at the stage of preclinical studies.

Methods:

The present review focused on the rhenium based cancer drugs that were in preclinical and clinical trials were examined critically. The detailed targeted interactions and experimental evidences of Re compounds reported by the patentable and non-patentable research findings used to write this review.

Results:

In the present review, we described the most recent and promising rhenium compounds focusing on their potential mechanism of action including, phototoxicity, DNA binding, mitochondrial effects, oxidative stress regulation or enzyme inhibition. Many ligands have been described that modulating the lipophilicity, the luminescent properties, the cellular uptake, the biodistribution, and the cytotoxicity, the pharmacological and toxicological profile.

Conclusion:

Re-based anticancer drugs can also be used in targeted therapies by coupling to a variety of biologically relevant targeting molecules. On the other hand, combination with conventional cytotoxic molecules, such as doxorubicin, allowed to take into profit the targeting properties of Re for example toward mitochondria. Through the example of the diseleno-Re complex, we showed that the main target could be the oxidative status, with a down-stream regulation of signaling pathways, and further on selective cell death of cancer cells versus normal cells.

2,2'-Dipyridylamines: more than just sister members of the bipyridine family. Applications and achievements in homogeneous catalysis and photoluminescent materials

2,2'-Dipyridylamines (dpa) and related compounds belong to the family of polydentate nitrogen ligands. More than a century has passed since their first report but new complexes and applications have been emerging in recent years owing to the versatility of dpa-based architectures. This review aims to present and highlight the main achievements attained with dpa-containing metal complexes in the domains of homogeneous catalysis and luminescent materials.

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.

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.

Esterase-Sensitive and pH-Controlled Carbon Monoxide Prodrugs for Treating Systemic Inflammation

A bottleneck for developing CO-based therapeutics is the lack of a safe and controllable delivery form. Herein, we describe efforts toward organic CO prodrugs with dual-responsive endogenous triggers. One representative CO prodrug showed significant anti-inflammatory effects both in vitro and in a LPS-simulated systemic inflammation model. These results firmly establish such CO prodrugs as either research tools or candidate compounds for the treatment of systemic inflammation or inflammation related organ injuries.

A series of Mn(i) photo-activated carbon monoxide-releasing molecules with benzimidazole coligands: synthesis, structural characterization, CO releasing properties and biological activity evaluation

Five Mn(i) photo-activated carbon monoxide-releasing molecules were synthesized by reactions of MnBr(CO)₅ with L1–L4, and characterized via single crystal X-ray diffraction, ¹H-NMR, ¹³C-NMR, IR, UV-vis and fluorescence spectroscopy.

Phosphine oxide-based tricarbonylrhenium(i) complexes from phosphine/phosphine oxide and dihydroxybenzoquinones

Neutral phosphine oxide (P[double bond, length as m-dash]O) donor-based organometallic complexes [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (1), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-DHBQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (2), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PCy3}] (3), [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-THQ}{Re(CO)3O[double bond, length as m-dash]PPh3}] (4), [{Re(CO)3O[double bond, length as m-dash]PCy3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PCy3}] (5), and [{Re(CO)3O[double bond, length as m-dash]PPh3}{μ-CA}{Re(CO)3O[double bond, length as m-dash]PPh3}] (6) were assembled from phosphine/phosphine oxide, a dihydroxybenzoquinone donor and Re2(CO)10via a one-pot solvothermal approach. The soft phosphine donor was transformed into a hard phosphine oxide donor during the formation of 1, 3, 4, 5, and 6. The complexes 1-6 were air and moisture stable and were soluble in polar organic solvents. The complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopic methods. The molecular structures of 1, 2, 4, and 6 were analyzed by single-crystal X-ray diffraction analysis. The UV-Visible absorption studies indicated that 1-6 in THF display strong visible light absorption in the range of ∼350-700 nm.

Affinity-Enhanced Luminescent Re(I)- and Ru(II)-Based Inhibitors of the Cysteine Protease Cathepsin L

Two new Re(I)- and Ru(II)-based inhibitors were synthesized with the formulas [Re(phen)(CO)₃(1)](OTf) (7; phen = 1,10-phenanthroline, OTf = trifluoromethanesulfonate) and [Ru(bpy)₂(2)](Cl)₂ (8; bpy = 2,2'-bipyridine), where 1 and 2 are the analogues of CLIK-148, an epoxysuccinyl-based cysteine cathepsin L inhibitor (CTSL). Compounds 7 and 8 were characterized using various spectroscopic techniques and elemental analysis; 7 and 8 both show exceptionally long excited state lifetimes. Re(I)-based complex 7 inhibits CTSL in the low nanomolar range, affording a greater than 16-fold enhancement of potency relative to the free inhibitor 1 with a second-order rate constant of 211000 ± 42000 M^-1 s^-1. Irreversible ligation of 7 to papain, a model of CTSL, was analyzed with mass spectroscopy, and the major peak shown at 24283 au corresponds to that of papain-1-Re(CO)₃(phen). Compound 7 was well tolerated by DU-145 prostate cancer cells, with toxicity evident only at high concentrations. Treatment of DU-145 cells with 7 followed by imaging via confocal microscopy showed substantial intracellular fluorescence that can be blocked by the known CTSL inhibitor CLIK-148, consistent with the ability of 7 to label CTSL in living cells. Overall this study reveals that a Re(I) complex can be attached to an enzyme inhibitor and enhance potency and selectivity for a medicinally important target, while at the same time allowing new avenues for tracking and quantification due to long excited state lifetimes and non-native element composition.

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.

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.

Light-responsive paper strips as CO-releasing material with a colourimetric response

Carbon monoxide (CO) is known for its multifaceted role in human physiology, and molecules that release CO in a controlled way have been proposed as therapeutic drugs. In this work, a light-responsive CO-releasing molecule (CORM-Dabsyl) showed a strong colourimetric response upon photochemical CO-release, owing to the tight conjugation of a Mn(i) tricarbonyl centre to a dabsyl chromophoric ligand (L). Whereas the complex was very stable in the dark in nitrogen-purged aqueous media, CO-release was effectively triggered using 405 nm irradiation. CORM-Dabsyl, L and the inactive product iCORM-Dabsyl have been investigated by DFT and TD-DFT calculations. Only mild toxicity of CORM-Dabsyl was observed against LX-2 and HepaRG® human cell lines (IC50 ∼ 30 μM). Finally, to develop a CO storage and release material that is readily applicable to therapeutic situations, CORM-Dabsyl was loaded on low-cost and easily disposable paper strips, from which the light triggered CO-release was conveniently visible with the naked eye.

Organic CO Prodrugs: Structure-CO-Release Rate Relationship Studies

Carbon monoxide (CO) is an endogenously produced gasotransmitter in mammals, and may have signaling roles in bacteria as well. It has many recognized therapeutic effects. A significant challenge in this field is the development of pharmaceutically acceptable forms of CO delivery with controllable and tunable release rates. Herein, the structure-release rate studies of the first class of organic CO prodrugs that release CO in aqueous solution at neutral pH is described.