Photochemically activated carbon monoxide release for biological targets. Toward developing air-stable photoCORMs labilized by visible light

Axial Ligand Effects on the Mechanism of Ru-CO Bond Photodissociation and Photophysical Properties of Ru(II)-Salen PhotoCORMs/Theranostics: A Density Functional Theory Study

Density functional theory (DFT) calculations were employed to study a series of complexes of general formula [Ru(salen)(X)(CO)]0/-1 (X = Cl-, F-, SCN-, DMSO, Phosphabenzene, Phosphole, TPH, CN-, N3-, NO3-, CNH-, NHC, P(OH)3, PF3, PH3). The effect of ligands X on the Ru-CO bond was quantified by the trans-philicity, Δσ13C NMR parameter. The potential of Δσ13C to be used as a probe of the CO photodissociation by Ru(II) transition metal complexes is established upon comparing it with other trans-effect parameters. An excellent linear correlation is found between the energy barrier for the Ru-CO photodissociation and the Δσ13C parameter, paving the way for studying photoCORMs with the 13C NMR method. The strongest trans-effect on the Ru-CO bond in the [Ru(salen)(X)(CO)]0/-1 complexes are found when X = CNH-, NHC, and P(OH)3, while the weakest for X = Cl-, NO3- and DMSO trans-axial ligands. The Ru-CO bonding properties were scrutinized using Natural Bond Orbital (NBO), Natural Energy Decomposition Analysis (NEDA) and Natural Orbital of Chemical Valence (NOCV) methods. The nature of the Ru-CO bond is composite, i.e., electrostatic, covalent and charge transfer. Both donation and backdonation between CO ligand and Ru metal centre equally stabilize the Ru(II) complexes. Ru-CO photodissociation proceeds via a 3MC triplet excited state, exhibiting a conical intersection with the T13MLCT excited state. Calculations show that these complexes show bands within visible while they are expected to be red emitters. Therefore, the [Ru(salen)(X)(CO)]0/-1 complexes under study could potentially be used for dual action, photoCORMs and theranostics compounds.

Visible-light-induced CO-releasing properties and cytotoxicity of a Ru(II) carbonyl complex containing 2-(pyridin-2-yl)-quinoxaline

The photo-induced CO-releasing properties of the dark-stable complex [RuCl2(CO)2L] (L = 2-(pyridin-2-yl)quinoxaline) were investigated under 468 nm light exposure in the presence and absence of biomolecules such as histidine, calf thymus DNA and hen egg white lysozyme. The CO release kinetics were consistent regardless of the presence of these biomolecules, suggesting that they did not influence the CO release mechanism. The quinoxaline ligand demonstrated exceptional cytotoxicity against human acute monocytic leukemia cells (THP-1), with evidence of potential DNA damage ascertained by comet assay, while it remained non-toxic to normal kidney epithelial cells derived from African green monkey (Vero) cell lines. In contrast, upon light activation, the Ru(II) complex showed no toxicity against THP-1 cells but was detrimental to Vero cells. In human colorectal carcinoma (HCT-116) cells, the ligand and the Ru(II) complex produced ROS under light and dark conditions. However, HCT-116 cells retained their ability to consume oxygen and produce ATP following CO treatment, suggesting that the ROS levels were insufficient to cause significant cellular damage. Morphological features of apoptosis, including apoptotic bodies, chromatin condensation, cell shrinkage, and membrane leakage, were observed in the presence of both the ligand and its complex, irrespective of light exposure.

Sonodynamic Cancer Therapy by Mn(I)-tricarbonyl Complexes via Ultrasound-triggered CO Release and ROS Generation

A novel ferrocene conjugated Mn(I)-tricarbonyl complex viz [Mn(Fc-tpy)(CO)3Br] (Mn2) where, Fc-tpy=4'-ferrocenyl-2,2':6',2''-terpyridine was synthesized and fully characterized along with its non-ferrocene analog [Mn(Ph-tpy)(CO)3Br] Ph-tpy=4'-phenyl-2,2':6',2''-terpyridine (Mn1) for ultrasound (US) activated anticancer applications. The X-ray structure of Mn2 confirmed its distorted octahedral geometry. Mn1 and Mn2, for the first time, showed US-triggered release of CO and ROS generation (1O2 and ⋅OH) in an aqueous solution from any Mn(I)-tricarbonyl complexes, indicating its potential for synergetic CO gas therapy and sonodynamic therapy. The above-mentioned in-solution chemistry was successfully translated into in vitro cellular models. These complexes showed unprecedented US-triggered toxicity against T-cell lymphoma and human breast cancer cells (IC50 for Mn2<1 μM) while were minimally toxic without US or against normal spleen and human embryonic kidney cells. Mn2 was ca. 12 fold more anticancer active than Mn1, indicating that the ferrocene conjugation augmented the US sensitivity. The apoptotic sonotoxicity of Mn2 was due to US-promoted mitochondrial depolarization via ROS generation and CO release. The apoptosis was triggered by caspase 3 activation. This is the first report of Mn(I)-tricarbonyl-based sonosensitizers for cancer SDT. Overall, this study, for the first time, establishes the effectiveness of 3d metal carbonyls in SDT.

Tumor-Targeted CO Nanodelivery System Design and Therapy for Hepatocellular Carcinoma

In recent years, carbon monoxide (CO) has garnered increased attention as a novel green therapy for hepatocellular carcinoma (HCC) treatment. However, the CO donor is still limited in clinical application due to its lack of targeted ability and unstable release rate. Here, self-assembled amphiphilic nanomicelles glucose-polyethylene glycol (PEG)-lipoic acid (LA)-Fe2(CO)6 (Glu-Fe2(CO)6) are first designed as a CO donor and synthesized via a chemical method, combining glucose with Fe2(CO)6 through PEG-LA. Some advantages of this tumor-targeted Glu-Fe2(CO)6 delivery system include (I) good water-solubility, (II) the glutathione responsive CO slow release, (III) the active tumor-targeted ability of glucose as targeted ligands, and (IV) outstanding efficacy of antitumor and safety of CO therapy of HCC both in vitro and in vivo. These findings suggest that Glu-Fe2(CO)6 nanomicelles hold promise for enhancing antitumor therapeutic capabilities, presenting a novel tumor-targeted delivery strategy in gas therapy for HCC treatment.

Zirconium-Based Metal-Organic Frameworks with Free Hydroxy Groups for Enhanced Perfluorooctanoic Acid Uptake in Water

Perfluorooctanoic acid (PFOA) is a highly recalcitrant organic pollutant, and its bioaccumulation severely endangers human health. While various methods are developed for PFOA removal, the targeted design of adsorbents with high efficiency and reusability remains largely unexplored. Here the rational design and synthesis of two novel zirconium-based metal‒organic frameworks (MOFs) bearing free ortho-hydroxy sites, namely noninterpenetrated PCN-1001 and twofold interpenetrated PCN-1002, are presented. Single crystal analysis of the pure ligand reveals that intramolecular hydrogen bonding plays a pivotal role in directing the formation of MOFs with free hydroxy groups. Furthermore, the transformation from PCN-1001 to PCN-1002 is realized. Compared to PCN-1001, PCN-1002 displays higher chemical stability due to interpenetration, thereby demonstrating an exceptional PFOA adsorption capacity of up to 632 mg g-1 (1.53 mmol g-1), which is comparable to the reported record values. Moreover, PCN-1002 shows rapid kinetics, high selectivity, and long-life cycles in PFOA removal tests. Solid-state nuclear magnetic resonance results and density functional theory calculations reveal that multiple hydrogen bonds between the free ortho-hydroxy sites and PFOA, along with Lewis acid-base interaction, work collaboratively to enhance PFOA adsorption.

Photoinduced behaviour of N,N-bidentate manganese(I) and rhenium(I) tricarbonyl complexes for singlet oxygen generation and CO release

The combined action of singlet oxygen (1O2) and photoinduced carbon monoxide (CO) released by tricarbonyl metal complexes is a promising synergic treatment against multi-resistant bacterial infections. In this work, we explore the use of a polydentate ligand (bpm = 2,2-bipyrimidine) that offers the opportunity to accommodate two metal centers exhibiting both singlet oxygen generation and carbon monoxide releasing properties in a single molecule. A series of monometallic ([(bpm)M(CO)3Br]; M = Mn, Re) and homo or hetero bimetallic ([Br(CO)3M(bpm)M'(CO)3Br]; M = Mn, Re) compounds were synthesized in moderate to good yields by modulating the metal precursor or the stoichiometry, also the syn:anti isomers ratio for the bimetallic complexes was dependent on the experimental conditions used. DFT modelling shows the anti-isomer is more stable than the syn-isomer by less than 8 kJ mol-1, which is consistent with those experimentally observed in terms of majority product and the effect of experimental conditions over the anti-syn ratio. The HOMO-LUMO gap is lower for the mono and bimetallic rhenium(I) compounds compared to the values for the manganese(I) analogues, while the heterometallic complex shows intermediate values for the anti-isomer. The photophysical characterization shows typical absorption and emission bands with MLCT character. In addition, CO-release and 1O2 generation quantum yields were evaluated for the monometallic Mnbpm and Rebpm homologues and compared with values obtained for the homo- and hetero-bimetallic complexes. Interestingly the replacement of a Mn(CO)3Br moiety in MnbpmMn by a Re(CO)3Br one makes the heterometallic MnbpmRe molecule a molecular oxygen sensitizer and partially retaining its carbon monoxide releasing ability.

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

Recently reported manganese(I) complexes with chelating arylisocyanide ligands exhibit luminescent metal-to-ligand charge-transfer (MLCT) excited states, similar to ruthenium(II) polypyridine complexes with the same d6 valence electron configuration used for many different applications in photophysics and photochemistry. However, chelating arylisocyanide ligands require substantial synthetic effort, and therefore it seemed attractive to explore the possibility of using more readily accessible monodentate arylisocyanides instead. Here, we synthesized the new Mn(I) complex [Mn(CNdippPhOMe2)6]PF6 with the known ligand CNdippPhOMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide. This complex was investigated by NMR spectroscopy, single-crystal structure analysis, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) measurements, IR spectroscopy supported by density functional theory (DFT) calculations, cyclic voltammetry, and time-resolved as well as steady-state UV-vis absorption spectroscopy. The key finding is that the new Mn(I) complex is nonluminescent and instead undergoes arylisocyanide ligand loss during continuous visible laser irradiation into ligand-centered and charge-transfer absorption bands, presumably owed to the population of dissociative d-d excited states. Thus, it seems that chelating bi- or tridentate binding motifs are essential for obtaining emissive MLCT excited states in manganese(I) arylisocyanides. Our work contributes to understanding the basic properties of photoactive first-row transition metal complexes and could help advance the search for alternatives to precious metal-based luminophores, photocatalysts, and sensors.

Phototherapeutic anticancer strategies with first-row transition metal complexes: a critical review

Photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) are therapeutic techniques based on a photosensitizer (PS) and light. These techniques allow the spatial and temporal control of the activation of drugs with light. Transition metal complexes are attractive compounds as photoactivatable prodrugs since their excited states can be appropriately designed by subtle modifications of the ligands, the metal centre, or the oxidation state. However, most metal-based PSs contain heavy metals such as Ru, Os, Ir, Pt or Au, which are expensive and non-earth-abundant, contrary to first-row transition metals. In this context, the exploration of the photochemical properties of complexes based on first-row transition metals appears to be extremely promising. This did encourage several groups to develop promising PSs based on these metals. This review presents up-to-date state-of-the-art information on first-row-transition metal complexes, from titanium to zinc in regard to their application as PSs for phototherapeutic applications.

Ultrafast photochemistry of a molybdenum carbonyl-nitrosyl complex with a triazacyclononane coligand

Transition metal complexes capable of releasing small molecules such as carbon monoxide and nitric oxide upon photoactivation are versatile tools in various fields of chemistry and biology. In this work, we report on the ultrafast photochemistry of [Mo(CO)₂(NO)(iPr₃tacn)]PF₆ (iPr₃tacn = 1,4,7-triisopropyl-1,4,7-triazacyclononane), which was characterized under continuous illumination and with femtosecond UV-pump/UV-probe and UV-pump/MIR-probe spectroscopy, as well as with stationary calculations. The experimental and theoretical results demonstrate that while the photodissociation of one of the two CO ligands upon UV excitation can be inferred both on an ultrafast timescale as well as under exposure times of several minutes, no evidence of NO release is observed under the same conditions. The binding mode of the diatomic ligands is impacted by the electronic excitation, and transient intermediates are observed on a timescale of tens of picoseconds before CO is released from the coordination sphere. Furthermore, based on calculated potential energy scans, we suggest that photolysis of NO could be possible after a subsequent excitation of an electronically excited state with a second laser pulse, or by accessing low-lying excited states that otherwise cannot be directly excited by light.

Light-activated cytotoxicity of dicarbonyl Ru(II) complexes with a benzimidazole coligand towards breast cancer

Reaction between [RuCl2(CO)2]n and 1H-benzimidazol-2-ylmethyl-(N-phenyl)amine ligands (LR) functionalized with various electron-donating and electron-withdrawing substituents on the phenyl ring (R = H, 4-CH3, 4-Cl, 4-COOCH3, and 3-COOCH3) afforded the dark-stable photoactivatable carbon monoxide prodrugs of the general formula [RuCl2(CO)2LR]. Release of the CO molecules from the Ru(II) compounds was examined by monitoring the electronic and IR spectra upon illumination at 365 nm. A noticeable decrease in the intensities of the two characteristic ν(CO) modes for Ru(CO)II2 species, and the growth of two new bands for the mono-carbonyl species and free CO, were the main features of the photolysis profiles. The cytotoxicity of the complexes towards breast cancer (MCF-7) cells was assessed with and without illumination at 365 nm. All the complexes except that with a 4-COOCH3 group (IC50 = 45.08 ± 3.5 μM) are nontoxic under dark conditions. Upon illumination, all the compounds acquired cytotoxicity in the following order: H > 4-COOCH3 > 4-CH3 > 4-Cl > 3-COOCH3. Investigation of the cytotoxicity of the CO-depleted fragments showed that the light-induced cytotoxicity can be attributed to the liberated CO and CO-depleted metal fragments, including the liberated benzimidazole ligands.

Photochemistry of Rhenium(i) Diimine Tricarbonyl Complexes in Biological Applications

Luminescent rhenium complexes continue to be the focus of growing scientific interest for catalytic, diagnostic and therapeutic applications, with emphasis on the development of their photophysical and photochemical properties. In this short review, we explore such properties with a focus on the biological applications of the molecules. We discuss the importance of the ligand choice to the contribution and their involvement towards the most significant electronic transitions of the metal species and what strategies are used to exploit the potential of the molecules in medicinal applications. We begin by detailing the photophysics of the molecules; we then describe the three most common photoreactions of rhenium complexes as photosensitizers in H₂ production, photocatalysts in CO₂ reduction and photochemical ligand substitution. In the last part, we describe their applications as luminescent cellular probes and how photochemical ligand substitution is utilized in the development of photoactive carbon monoxide-releasing molecules as anticancer and antimicrobial agents.

Photochemistry of P,N-bidentate rhenium(i) tricarbonyl complexes: reactive species generation and potential application for antibacterial photodynamic therapy

In this work, we describe the photoisomerization of facial rhenium(i) tricarbonyl complexes bearing P,N-bidentate pyridyl/phosphine ligands with different chelating rings and anions: RePNBr, RePNTfO, and RePNNBr, which are triggered under irradiation at 365 nm in solutions. The apparent photodegradation rate constants (k app) depend on the coordinating ability of the solvent, being lowest in acetonitrile. The k app value increases as the temperature rises, suggesting a reactive IL excited state thermally populated from the MLCT excited state involved. Using the Eyring equation, positive activation enthalpies (ΔH ≠) accompanied by high negative values for the activation entropy (ΔS ≠) were obtained. These results suggest whatever the P,N-ligand or anion, the reaction proceeds through a strongly solvated or a compact transition state, which is compatible with an associative mechanism for the photoisomerization. A 100-fold decrease in the log10 CFU value is observed for E. coli and S. aureus in irradiated solutions of the compounds, which follows the same tendency as their singlet oxygen generation quantum yield: RePNBr > RePNTfO > RePNNBr, while no antibacterial activity is observed in the darkness. This result indicates that the generation of singlet oxygen plays a key role in the antibacterial capacity of these complexes.

Photo-uncaging a Ru(II) intercalator via photodecomposition of a bridged Mn(I) photoCORM

A Ru(ii) intercalating complex capped with a Mn(i) photoCORM allows for a new mode of DNA intercalator delivery. The steric bulk of the Mn(i) photoCORM inhibits intercalation in the dark, and visible light irradiation (470 nm) dissociates the photoCORM, allowing for DNA intercalation of the Ru(ii) complex.

Photodissociation of the Product from a Transition-Metal Center Allows the Catalytic Cycle to Proceed: The Rhodium(I)-Catalyzed [2+2+1] Carbonylative Cycloaddition of Diynes

We describe the effective rhodium(I)-catalyzed [2+2+1] carbonylative cycloaddition of diynes, yielding cyclopentadienes (CPDs), under photoirradiation. The catalysis involves the promotion of the photodissociation of the product CPD, with the simultaneous production of an essential vacant coordination site on the rhodium for an unreacted substrate. The combined use of cationic [Rh(cod)₂]BF₄ as a catalyst and photoirradiation was also found to give various CPDs in high yields (≤96%).

Enhanced luminescence intensity of near-infrared-sensitized upconversion nanoparticles via Ca2+ doping for a nitric oxide release platform

Light-induced NO release based on exogenous NO donors has attracted substantial attention in clinical applications; the induction light source usually converts near-infrared light to blue or ultraviolet light. However, the low efficiency of near-infrared light-assisted chemical light energy conversion remains a challenge, especially for NaYF4:Yb3+/Tm3+ photoconverting near-infrared light to ultraviolet (UV) and blue light. In this paper, a luminescence-enhanced strategy is reported by doping Ca2+ into NaYF4:Yb3+/Tm3+ and coating it with NaGdF4 through a two-step solvothermal method. Then, UCNPs modified with methyl-β-cyclodextrin (M-β-CD) are loaded on a ruthenium nitrosyl complex [(3)Ru(NO)(Cl)] as nitric oxide release-molecules (NORMs). X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) data demonstrated that Ca2+ was successfully doped into NaYF4:Yb3+/Tm3+ nanoparticles as the core, and a pure hexagonal phase, NaYF4, was obtained from the doping of Ca2+. TEM revealed that the crystallinity was significantly improved after Ca2+ doping, and the core-shell structure was successfully synthesized, with NaGdF4 directionally grown on the NaYF4:Ca/Yb/Tm core. Fluorescence tests showed that, especially in the ultraviolet and blue light excitation wavelength regions, the UC emission intensity of the Ca-doped NaYF4:Yb3+/Tm3+@NaGdF4 core-shell UCNPs increased by 302.95 times vs. NaYF4:Yb3+/Tm3+ UCNPs. Finally, the release of NO was tested by the Griess method. Under 980 nm irradiation, the cell viability distinctly decreased with increasing UCNPs@M-β-CD-NORMs concentration. This study shows that NORM release of NO is triggered by enhanced up-converted UV and blue light, which can be used for the development of UV photo-sensitive drugs.

Solid-phase synthesis of peptide Mn(i)-carbonyl bioconjugates and their CO release upon visible light activation

A one-pot synthetic route has been developed for the assembly of peptide Mn(i)-carbonyl bioconjugates. It allows the installation of a variety of chelating agents at the late stage, and after just one purification step the TAT-MnCO complexes can be obtained. The resulting bioconjugates showed different and tunable CO releasing kinetics upon visible light activation.

A Dinuclear Persulfide-Bridged Ruthenium Compound is a Hypoxia-Selective Hydrogen Sulfide (H2 S) Donor

Hydrogen sulfide (H2 S) is a gaseous molecule that has received attention for its role in biological processes and therapeutic potential in diseases, such as ischemic reperfusion injury. Despite its clinical relevance, delivery of H2 S to biological systems is hampered by its toxicity at high concentrations. Herein, we report the first metal-based H2 S donor that delivers this gas selectively to hypoxic cells. We further show that H2 S release from this compound protects H9c2 rat cardiomyoblasts from an in vitro model of ischemic reperfusion injury. These results validate the utility of redox-activated metal complexes as hypoxia-selective H2 S-releasing agents for use as tools to study the role of this gaseous molecule in complex biological systems.

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

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

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

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

Light-Activated Carbon Monoxide Prodrugs Based on Bipyridyl Dicarbonyl Ruthenium(II) Complexes

Two photoactivatable dicarbonyl ruthenium(II) complexes based on an amide-functionalised bipyridine scaffold (4-position) equipped with an alkyne functionality or a green-fluorescent BODIPY (boron-dipyrromethene) dye have been prepared and used to investigate their light-induced decarbonylation. UV/Vis, FTIR and 13 C NMR spectroscopies as well as gas chromatography and multivariate curve resolution alternating least-squares analysis (MCR-ALS) were used to elucidate the mechanism of the decarbonylation process. Release of the first CO molecule occurs very quickly, while release of the second CO molecule proceeds more slowly. In vitro studies using two cell lines A431 (human squamous carcinoma) and HEK293 (human embryonic kidney cells) have been carried out in order to characterise the anti-proliferative and anti-apoptotic activities. The BODIPY-labelled compound allows for monitoring the cellular uptake, showing fast internalisation kinetics and accumulation at the endoplasmic reticulum and mitochondria.

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

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

Smart Hydrogels - Synthetic Stimuli-Responsive Antitumor Drug Release Systems

Among modern drug formulations, stimuli-responsive hydrogels also called "smart hydrogels" deserve a special attention. The basic feature of this system is the ability to change their mechanical properties, swelling ability, hydrophilicity, bioactive molecules permeability, etc., influenced by various stimuli, such as temperature, pH, electromagnetic radiation, magnetic field and biological factors. Therefore, stimuli-responsive matrices can be potentially used in tissue engineering, cell cultures and technology of innovative drug delivery systems (DDSs), releasing the active substances under the control of internal or external stimuli. Moreover, smart hydrogels can be used as injectable DDSs, due to gel-sol transition connected with in situ cross-linking process. Innovative smart hydrogel DDSs can be utilized as matrices for targeted therapy, which enhances the effectiveness of tumor chemotherapy and subsequently limits systemic toxicity. External stimulus sensitivity allows remote control over the drug release profile and gel formation. On the other hand, internal factors provide drg accumulation in tumor tissue and reduce the concentration of active drug form in healthy tissue. In this report, we summarise the basic knowledge and chemical strategies for the synthetic smart hydrogel DDSs applied in antitumor therapy.

A multistage assembly/disassembly strategy for tumor-targeted CO delivery

CO gas molecule not only could selectively kill cancer cells but also exhibits limited anticancer efficacy because of the lack of active tumor-targeted accumulation capability. In this work, a multistage assembly/disassembly strategy is developed to construct a new intelligent nanomedicine by encapsulating a mitochondria-targeted and intramitochondrial microenvironment-responsive prodrug (FeCO-TPP) within mesoporous silica nanoparticle that is further coated with hyaluronic acid by step-by-step electrostatic assembly, realizing tumor tissue-cell-mitochondria-targeted multistage delivery and controlled release of CO in a step-by-step disassembly way. Multistage targeted delivery and controlled release of CO involve (i) the passive tumor tissue-targeted nanomedicine delivery, (ii) the active tumor cell-targeted nanomedicine delivery, (iii) the acid-responsive prodrug release, (iv) the mitochondria-targeted prodrug delivery, and (v) the ROS-responsive CO release. The developed nanomedicine has effectively augmented the efficacy and safety of CO therapy of cancer both in vitro and in vivo. The proposed multistage assembly/disassembly strategy opens a new window for targeted CO therapy.

Ferracyclic carbonyl complexes as anti-inflammatory agents

Reaction of Fe(CO)₄Br₂ with 2-aminopyridine and 2-aminonapthalene yields ferracyclic iron(ii) complexes bearing two CO ligands. These release CO in the light, but suppress inflammation only in the dark.

Diminished viability of human ovarian cancer cells by antigen-specific delivery of carbon monoxide with a family of photoactivatable antibody-photoCORM conjugates

Antibodies conjugated to a photoactive transition metal carbonyl complex afford antigen-directed delivery of cytotoxic carbon monoxide to ovarian cancer cells.

Carbon monoxide (CO)-releasing antibody conjugates were synthesized utilizing a photoactivatable CO-releasing molecule (photoCORM) and mouse monoclonal antibodies linked by a biotin-streptavidin system. Different monoclonal antibodies raised against different surface-expressed antigens that are implicated in ovarian cancer afforded a family of antibody-photoCORM conjugates (Ab-photoCORMs). In an immunosorbent/cell viability assay, Ab-photoCORMs accumulated onto ovarian cancer cells expressing the target antigens, delivering cytotoxic doses of CO in vitro. The results described here provide the first example of an “immunoCORM”, a proof-of-the-concept antibody-drug conjugate that delivers a gaseous molecule as a warhead to ovarian cancer.

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.

Synthesis, Structural Characterization and Photodecarbonylation Study of a Dicarbonyl Ruthenium(II)-Bisquinoline Complex

A photoactivatable ruthenium(II) carbonyl complex mer,cis-[Ru(II)Cl(BisQ)(CO)2]PF6 2 was prepared using a tridentate bisquinoline ligand (BisQ=(2,6-diquinolin-2-yl)pyridin). Compound 2 was thoroughly characterized by standard analytical methods and single crystal X-ray diffraction. The crystal structure of the complex cation reveals a distorted octahedral geometry. The decarbonylation upon exposure to 350 and 420 nm light was monitored by UV/VIS absorbance and Fourier transform infrared spectroscopies in acetonitrile and 1 % (v/v) DMSO in water, respectively. The kinetic of the photodecarbonylation has been elucidated by multivariate curve resolution alternating least-squares analysis. The stepwise decarbonylation follows a serial mechanism. The first decarbonylation occurs very quickly whereas the second decarbonylation step proceeds more slowly. Moreover, the second rate constant is lower in 1 % (v/v) DMSO in water than in acetonitrile. In comparison to 350 nm irradiation, exposure to 420 nm light in acetonitrile results in a lower second rate constant.