Organelle-targeting metal complexes: From molecular design to bio-applications

Supramolecular Chemistry: Host-Guest Molecular Complexes

In recent times, researchers have emphasized practical approaches for capturing coordinated and selective guest entrap. The physisorbed nanoporous supramolecular complexes have been widely used to restrain various guest species on compact supporting surfaces. The host-guest (HG) interactions in two-dimensional (2D) permeable porous linkages are growing expeditiously due to their future applications in biocatalysis, separation technology, or nanoscale patterning. The different crystal-like nanoporous network has been acquired to enclose and trap guest molecules of various dimensions and contours. The host centers have been lumped together via noncovalent interactions (such as hydrogen bonds, van der Waals (vdW) interactions, or coordinate bonds). In this review article, we enlighten and elucidate recent progress in HG chemistry, explored via scanning tunneling microscopy (STM). We summarize the synthesis, design, and characterization of typical HG structural design examined on various substrates, under ambient surroundings at the liquid-solid (LS) interface, or during ultrahigh vacuum (UHV). We emphasize isoreticular complexes, vibrant HG coordination, or hosts functional cavities responsive to the applied stimulus. Finally, we critically discuss the significant challenges in advancing this developing electrochemical field.

Photoresponsive metallopolymer nanoparticles for cancer theranostics

Over the past decades, transition metal complexes have been successfully used in anticancer phototherapies. They have shown promising properties in many different areas including photo-induced ligand exchange or release, rich excited state behavior, and versatile biochemical properties. When encorporated into polymeric frameworks and become part of nanostructures, photoresponsive metallopolymer nanoparticles (MPNs) show enhanced water solubility, extended blood circulation and increased tumor-specific accumulation, which greatly improves the tumor therapeutic effects compared to low-molecule-weight metal complexes. In this review, we aim to present the recent development of photoresponsive MPNs as therapeutic nanomedicines. This review will summarize four major areas separately, namely platinum-containing polymers, zinc-containing polymers, iridium-containing polymers and ruthenium-containing polymers. Representative MPNs of each type are discussed in terms of their design strategies, fabrication methods, and working mechanisms. Current challenges and future perspectives in this field are also highlighted.

Mitochondria-Targeting and Reversible Near-Infrared Emissive Iridium(III) Probe for in vivo ONOO-/GSH Redox Cycles Monitoring

Peroxynitrite (ONOO^-) and glutathione (GSH), two unique reactive species, play an essential regulating role in the oxidation and antioxidation in the living body and are closely associated with various physiological and pathological processes, like cancer, cardiovascular disorders, diabetes, inflammation, Alzheimer's disease, and hepatotoxicity. Thus, it is crucial to study mitochondria ONOO^-/GSH redox cycles by an effective molecular tool. In this work, a mitochondria-targeting and redox-reversible near-infrared (NIR) phosphorescent iridium complex, Ir-diol, has been synthesized and used for the detection and imaging of a cellular redox state by visualizing endogenous ONOO^-/GSH content. Ir-diol shows excellent photophysical properties, including NIR emission (the maximum emissive wavelength for 704 nm, approximately) and high phosphorescent quantum yield (Φ = 0.136) and exhibits high sensitivity and selectivity toward ONOO^-/GSH redox cycles in aqueous solution and living cells. Therefore, these features, combined with low cytotoxicity and excellent cell permeability, enable probe Ir-diol to monitor the changes of the intracellular ONOO^-/GSH level induced by drug both in vitro and in vivo.

Bioorthogonal control of the phosphorescence and singlet oxygen photosensitisation properties of iridium(III) tetrazine complexes

In this work, we demonstrate bioorthogonal control of the phosphorescence and singlet oxygen photosensitisation properties of new iridium(iii) tetrazine complexes by different reaction partners; the system was exploited for organelle-specific staining and modulated photocytotoxic activity applications.

The effect of sodium thiosulfate on cytotoxicity of a diimine Re(I) tricarbonyl complex

Recently, diimine Re(i) tricarbonyl complexes have attracted great interest due to their promising cytotoxic effects. Here, we compare the cytotoxicity and cellular uptake of two Re(i) compounds fac-[(Re(CO)3(bpy)(H2O)](CF3SO3) (1) and Na(fac-[(Re(CO)3(bpy)(S2O3)])·H2O (bpy = 2,2'-bipyridine) (2). The Re-thiosulfate complex in 2 was characterized in two solvated crystal structures {Na(fac-[Re(CO)3(bpy)(S2O3)])·1.75H2O·C2H5OH}4 (2 + 0.75H2O + C2H5OH)4 and (fac-[Re(CO)3(bpy)(H2O)]) (fac-[Re(CO)3(bpy)(S2O3)])·4H2O (3). The cytotoxicity of 1 and 2 was tested in the MDA-MB-231 breast cancer cell line and compared with that of cisplatin. The cellular localization of the Re(i) complexes was investigated using synchrotron-based X-ray fluorescence microscopy (XFM). The results show that replacement of the aqua ligand with thiosulfate renders the complex less toxic most likely by distrupting its cellular entry. Therefore, thiosulfate could potentially have a similar chemoprotective effect against diimine fac-Re(CO)3 complexes as it has against cisplatin.

cis-Locked Ru(II)-DMSO Precursors for the Microwave-Assisted Synthesis of Bis-Heteroleptic Polypyridyl Compounds

We describe a synthetic strategy for the preparation of bis-heteroleptic polypyridyl Ru(II) complexes of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands) from well-defined Ru(II) precursors. For this purpose, a series of six neutral, anionic, and cationic cis-locked Ru(II)-DMSO complexes (2–7) of the general formula [Y] fac-[RuX(DMSO–S)₃(O–O)]ⁿ (where O–O is a symmetrical chelating anion: oxalate (ox), malonate (mal), acetylacetonate (acac); X = DMSO–O or Cl^–; n = −1/0/+1 depending on the nature and charge of X and O–O; when present, Y = K⁺ or PF₆^–) were efficiently prepared from the well-known cis-[RuCl₂(DMSO)₄] (1). When treated with diimine chelating ligands (L1 = bpy, phen, dpphen), the compounds 2–7 afforded the target [Ru(L1)₂(O–O)]^0/+ complex together with the undesired (and unexpected) [Ru(L1)₃]²⁺ species. Nevertheless, we found that the formation of [Ru(L1)₃]²⁺can be minimized by carefully adjusting the reaction conditions: in particular, high selectivity toward [Ru(L1)₂(O–O)]^0/+ and almost complete conversion of the precursor was obtained within minutes, also on a 100–200 mg scale, when the reactions were performed in absolute ethanol at 150 °C in a microwave reactor. Depending on the nature of L1 and concentration, with the oxalate and malonate precursors, the neutral product [Ru(L1)₂(O–O)] can precipitate spontaneously from the final mixture, in pure form and acceptable-to-good yields. When spontaneous precipitation of the disubstituted product does not occur, purification from [Ru(L1)₃]²⁺ can be rather easily accomplished by column chromatography or solvent extraction. By comparison, under the same conditions, compound 1 is much less selective, thus demonstrating that locking the geometry of the precursor through the introduction of O–O in the coordination sphere of Ru is a valid strategic approach. By virtue of its proton-sensitive nature, facile and quantitative replacement of O–O in [Ru(L1)₂(O–O)]^0/+ by L2, selectively affording [Ru(L1)₂(L2)]²⁺, was accomplished in refluxing ethanol in the presence of a slight excess of trifluoroacetic acid or HPF₆.

cis-Locked Ru(II)-DMSO complexes bearing a symmetrical chelating anion, such as [K] fac-[RuCl(DMSO−S)₃(η²-mal)] (2), are suitable precursors for the two-step selective preparation of bis-heteroleptic polypyridyl compounds of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands).

A focus on the biological targets for coinage metal-NHCs as potential anticancer complexes

Metal complexes of N-heterocyclic carbene (NHC) ligands are the object of increasing attention for therapeutic purposes. Among the different metal centres, interest on Au-based compounds started with the application as anti-arthritis drugs. On the other hand, Ag(I) antimicrobial properties have been known for a long time. For Au(I)/Au(III)-NHC and Ag(I)-NHC anti-tumour and anti-proliferative properties have been quite recently demonstrated. In addition to these and as for Group 11, copper is a much less investigated metal centre, but a few papers underline its pharmacological potential. This review wants to focus on the different biological targets for these metal-based compounds. It is divided into chapters which are respectively devoted on: i) mitochondria and thiol oxidoreductase systems; ii) other relevant enzymes; iii) nucleic acids. Examples of representative coinage NHCs for each of the targets are provided together with significant references on recent advances on the topic. Moreover, a final comment summarises the aspects enlightened by each chapter and provides some hints to better understand the metal-NHCs mechanistic behaviour based on structure-activity relationships.

A Cyclodextrin-Hosted Ir(III) Complex for Ratiometric Mapping of Tumor Hypoxia In Vivo

Hypoxia is considered as a key microenvironmental feature of solid tumors. Luminescent transition metal complexes particularly those based on iridium and ruthenium have shown remarkable potentials for constructing sensitive oxygen-sensing probes due to their unique oxygen quenching pathway. However, the low aqueous solubility of these complexes largely retards their sensing applications in biological media. Moreover, it remains difficult so far to use the existing complexes typically possessing only one luminescent domain to quantitatively detect the intratumoral hypoxia degree. Herein, an Ir(III) complex showing red emissions is designed and synthesized, and innovatively encapsulated within a hydrophobic pocket of Cyanine7-modified cyclodextrin. The Ir(III) complex enables the oxygen detection, while the cyclodextrin is used not only for improving the water solubility and suppressing the luminescence quenching effect of the surrounding aqueous media, but also for carrying Cyanine7 to establish a ratiometric oxygen fluorescence probe. 2D nuclear magnetic resonance is carried out to confirm the host-guest structure. The oxygen-responsive ability of the resulting ratiometric probe is evaluated through in vitro cell and multicellular experiments. Further animal studies about tumor oxygen level mapping demonstrate that the probe can be successfully used for quantitatively visualizing tumor hypoxia in vivo.

In-vitro and In-vivo Photocatalytic Cancer Therapy with Biocompatible Iridium(III) Photocatalysts

Photocatalytic anticancer profile of a Ir^III photocatalyst (Ir3) with strong light absorption, high turnover frequency, and excellent biocompatibility is reported. Ir3 showed selective photo-cytotoxicity against cisplatin- and sorafenib-resistant cell lines while remaining dormant to normal cell lines in the dark. Ir3 exhibited excellent photo-catalytic oxidation of cellular co-enzyme, the reduced nicotinamide adenine dinucleotide phosphate (NADPH), and amino acids via a single electron transfer mechanism. The photo-induced intracellular redox imbalance and change in mitochondrial membrane potential resulted in necrosis and apoptosis of cancer cells. Importantly, Ir3 exhibited high biocompatibility and photo-catalytic anticancer efficiency as evident from in vivo zebrafish and mouse cancer models. To the best of our knowledge, Ir3 is the first Ir^III based photocatalyst with such a high biocompatibility and photocatalytic anticancer therapeutic effect.

Photorelease of a metal-binding pharmacophore from a Ru(II) polypyridine complex

The adoption of compounds that target metalloenzymes comprises a relatively low (<5%) percentage of all FDA approved therapeutics. Metalloenzyme inhibitors typically coordinate to the active site metal ions and therefore contain ligands with charged or highly polar functional groups. While these groups may generate highly water-soluble compounds, this functionalization can also limit their pharmacological properties. To overcome this drawback, drug candidates can be formulated as prodrugs. While a variety of protecting groups have been developed, increasing efforts have been devoted towards the use of caging groups that can be removed upon exposure to light to provide spatial and temporal control over the treatment. Among these, the application of Ru(ii) polypyridine complexes is receiving increased attention based on their attractive biological and photophysical properties. Herein, a conjugate consisting of a metalloenzyme inhibitor and a Ru(ii) polypyridine complex as a photo-cage is presented. The conjugate was designed using density functional theory calculations and docking studies. The conjugate is stable in an aqueous solution, but irradiation of the complex with 450 nm light releases the inhibitor within several minutes. As a model system, the biochemical properties were investigated against the endonucleolytic active site of the influenza virus. While showing no inhibition in the dark in an in vitro assay, the conjugate generated inhibition upon light exposure at 450 nm, demonstrating the ability to liberate the metalloenzyme inhibitor. The presented inhibitor-Ru(ii) polypyridine conjugate is an example of computationally-guided drug design for light-activated drug release and may help reveal new avenues for the prodrugging of metalloenzyme inhibitors.

Disruption of the Microtubule Network and Inhibition of VEGFR2 Phosphorylation by Cytotoxic N,O-Coordinated Pt(II) and Ru(II) Complexes of Trimethoxy Aniline-Based Schiff Bases

Platinum-based complexes are one of the most successful chemotherapeutic agents having a significant ground in cancer chemotherapy despite their side effects. During the past few decades, Ru(II) complexes have been emerging as efficient alternatives owing to their promising activities against platinum-resistant cancer. The pathway of action, lipophilicity, and cytotoxicity of a Pt or Ru complex may be tuned by varying the attached ligands, the coordination mode, and the leaving group. In this work, we report a family of Pt(II) and Ru(II) complexes (1-5) of three N,O and N,N donor-based trimethoxyanilines containing Schiff bases with the general formula [Pt^II(L)(DMSO)Cl], [Ru^II(L)(p-cymene)Cl], [Ru^II(L)(p-cymene)Cl]⁺, and [Pt^II(L)Cl₂]. All of the complexes are characterized by different analytical techniques. ¹H NMR and electrospray ionization mass spectrometry (ESI-MS) data suggest that the N,O-coordinated Pt(II) complexes undergo slower aquation compared to the Ru(II) analogues. The change of the coordination mode to N,N causes the Ru complexes to be more inert to aquation. The N,O-coordinating complexes show superiority over N,N-coordinating complexes by displaying excellent in vitro antiproliferative activity against different aggressive cancer cells, viz., triple-negative human metastatic breast adenocarcinoma MDA-MB-231, human pancreatic carcinoma MIA PaCa-2, and hepatocellular carcinoma Hep G2. In vitro cytotoxicity studies suggest that Pt(II) complexes are more effective than their corresponding Ru(II) analogues, and the most cytotoxic complex 3 is 10-15 times more toxic than the clinical drugs cisplatin and oxaliplatin against MDA-MB-231 cells. Cellular studies show that all of the N,O-coordinated complexes (1-3) initiate disruption of the microtubule network in MDA-MB-231 cells in a dose-dependent manner within 6 h of incubation and finally lead to the arrest of the cell cycle in the G2/M phase and render apoptotic cell death. The disruption of the microtubule network affects the agility of the cytoskeleton rendering inhibition of tyrosine phosphorylation of vascular endothelial growth factor receptor 2 (VEGFR2), a key step in angiogenesis. Complexes 1 and 2 inhibit VEGFR2 phosphorylation in a dose-dependent fashion. Among the Pt(II) and Ru(II) complexes, the former displays higher cytotoxicity, a stronger effect on the cytoskeleton, better VEGFR2 inhibition, and strong interaction with the model nucleobase 9-ethylguanine (9-EtG).

An ER-Targeting Iridium(III) Complex That Induces Immunogenic Cell Death in Non-Small-Cell Lung Cancer

Immunogenic cell death (ICD) is a vital component of therapeutically induced anti-tumor immunity. An iridium(III) complex (Ir1), containing an N,N-bis(2-chloroethyl)-azane derivate, as an endoplasmic reticulum-localized ICD inducer for non-small cell lung cancer (NSCLC) is reported. The characteristic discharge of damage-associated molecular patterns (DAMPs), that is, cell surface exposure of calreticulin (CRT), extracellular exclusion of high mobility group box 1 (HMGB1), and ATP, were generated by Ir1 in A549 lung cancer cells, accompanied by an increase in endoplasmic reticulum stress and reactive oxygen species (ROS). The vaccination of immunocompetent mice with Ir1-treated dying cells elicited an antitumor CD8⁺ T cell response and Foxp3⁺ T cell depletion, which eventually resulted in long-acting anti-tumor immunity by the activation of ICD in lung cancer cells. Ir1 is the first Ir-based complex that is capable of developing an immunomodulatory response by immunogenic cell death.

Rhenium carbonyl complexes bearing methylated triphenylphosphonium cations as antibody-free mitochondria trackers for X-ray fluorescence imaging

A convenient rhenium-based multimodal mitochondrial-targeted probe compatible with Synchrotron Radiation X-ray Fluorescence nano-imaging.

A mitochondrial-targeting iridium(iii) complex for H2O2-responsive and oxidative stress amplified two-photon photodynamic therapy

The mitochondrial-targeting iridium(iii) complex Ir–B(OH)2 is activated in the presence of endogenous H2O2 to release the two-photon photosensitizer Ir–OH and the GSH scavenger methylquinone for amplified two-photon photodynamic therapy.

Multinuclear 2-(Quinolin-2-yl)quinoxaline-Coordinated Iridium(III) Complexes Tethered by Carbazole Derivatives: Synthesis and Photophysics

Five mono/di/trinuclear iridium(III) complexes (1-5) bearing the carbazole-derivative-tethered 2-(quinolin-2-yl)quinoxaline (quqo) diimine (N^N) ligand were synthesized and characterized. The photophysical properties of these complexes and their corresponding diimine ligands were systematically studied via UV-vis absorption, emission, and transient absorption (TA) spectroscopy and simulated by time-dependent density functional theory. All complexes possessed strong well-resolved absorption bands at <400 nm that have predominant ligand-based ¹π,π* transitions and broad structureless charge-transfer (¹CT) absorption bands at 400-700 nm. The energies or intensities of these ¹CT bands varied pronouncedly when the number of tethered Ir(quqo)(piq)₂⁺ (piq refers to 1-phenylisoquinoline) units, π conjugation of the carbazole derivative linker, or attachment positions on the carbazole linker were altered. All complexes were emissive at room temperature, with 1-3 showing near-IR (NIR) ³MLCT (metal-to-ligand charge-transfer)/³LLCT (ligand-to-ligand charge-transfer) emission at ∼710 nm and 4 and 5 exhibiting red or NIR ³ILCT (intraligand charge-transfer)/³LMCT (ligand-to-metal charge-transfer) emission in CH₂Cl₂. In CH₃CN, 1-3 displayed an additional emission band at ca. 590 nm (³ILCT/³LMCT/³MLCT/³π,π* in nature) in addition to the 710 nm band. The different natures of the emitting states of 1-3 versus those of 4 and 5 also gave rise to different spectral features in their triplet TA spectra. It appears that the parentage and characteristics of the lowest triplet excited states in these complexes are mainly impacted by the π systems of the bridging carbazole derivatives and essentially no interactions among the Ir(quqo)(piq)₂⁺ units. In addition, all of the diimine ligands tethered by the carbazole derivatives displayed a dramatic solvatochromic effect in their emission due to the predominant intramolecular charge-transfer nature of their emitting states. Aggregation-enhanced emission was also observed from the mixed CH₂Cl₂/ethyl acetate or CH₂Cl₂/hexane solutions of these ligands.

Cytochrome c light-up graphene oxide nanosensor for the targeted self-monitoring of mitochondria-mediated tumor cell death

Targeting mitochondria-mediated apoptosis has emerged as a promising strategy for tumor therapy. However, technologies used to treat tumors that enable the direct visualization of mitochondria-mediated apoptosis in living cells have not been developed to date. Cytochrome c (Cyt c) translocation from mitochondria is a central mediating event in cell apoptosis. In this study, we developed a multifunctional nanosensor that can monitor the real-time translocation of Cyt c from mitochondria in living cells to evaluate the antitumor effect of dihydroartemisinin (DHA). A fluorophore-tagged DNA aptamer is loaded on a graphene oxide (GO)-based nanovehicle, and the cytosolic release of Cyt c causes the dissociation of the aptamer from the GO nanovehicle and triggers the emission of a red fluorescence signal. Furthermore, DHA linked with a coumarin derivative is loaded on GO as a mitochondria-targeting ligand to improve its antitumor activity. This DHA prodrug also emits a green fluorescence signal when delivered to mitochondria. This nanosensor provides a convenient mechanism to monitor mitochondrial targeting by drugs and mitochondria-induced therapeutic efficacy, which may be possible to diagnose the drug efficacy to optimize the treatment for patients with cancer.

Smart Design of Nanomaterials for Mitochondria-Targeted Nanotherapeutics

Mitochondria are the powerhouse of cells. They are vital organelles that maintain cellular function and metabolism. Dysfunction of mitochondria results in various diseases with a great diversity of clinical appearances. In the past, strategies have been developed for fabricating subcellular-targeting drug-delivery nanocarriers, enabling cellular internalization and subsequent organelle localization. Of late, innovative strategies have emerged for the smart design of multifunctional nanocarriers. Hierarchical targeting enables nanocarriers to evade and overcome various barriers encountered upon in vivo administration to reach the organelle with good bioavailability. Stimuli-responsive nanocarriers allow controlled release of therapeutics to occur at the desired target site. Synergistic therapy can be achieved using a combination of approaches such as chemotherapy, gene and phototherapy. In this Review, we survey the field for recent developments and strategies used in the smart design of nanocarriers for mitochondria-targeted therapeutics. Existing challenges and unexplored therapeutic opportunities are also highlighted and discussed to inspire the next generation of mitochondrial-targeting nanotherapeutics.

Bifunctional Peptide-Conjugated Gold Nanoparticles for Precise and Efficient Nucleus-Targeting Bioimaging in Live Cells

Real-time in situ imaging of organelles is increasingly important in modern biomedical analysis and diseases diagnosis. To realize this goal, organelle-targeting nanoparticles as one of the most commonly used technologies in subcellular sensing and imaging has attracted a lot of interest. The biocompatibility, specificity, and binding efficiency are especially critical for efficient organelle-targeting bioimaging. Gold nanoparticles (AuNPs) fabricated with bifunctional peptides constructed with both Au-binding affinity and nucleus-targeting ability were designed and examined for efficient nucleus-targeting bioimaging. Such a design is expected to achieve an oriented assembling of peptides by the medium of the Au-binding peptides specifically assembled on the surface of AuNPs, with the nucleus-targeting end open for accessibility. The bifunctional peptides showed strong binding affinity toward AuNPs and led to a binding capability ∼1.5 times higher than that of the bare nucleus-targeting peptides, ensuring good surface coverage of the nanoparticles for enhanced nucleus-targeting ability. Such fabricated AuNPs demonstrated over 90% cell viability after incubation for 24 h with HepG2 cells, which were highly biocompatible. Precise and efficient bioimaging of the nucleus was achieved for HepG2 cells by using the fabricated AuNPs as observed with a confocal laser scanning microscope, a dark-field/fluorescence microscope, and a transmission electron microscope. The high surface coverage and oriented binding pattern appeared to be a promising strategy for construction of organelle-targeting agencies.

Smart Microenvironment-Responsive Organocopper(II) Supramolecular Polymers to Regulate the Stability and Anticancer Efficacy by Different Substituents

The search for chemotherapeutic drugs with a high efficiency and low toxicity continues to be a challenge in tumor treatment for scientists. Organometallic supramolecular polymers are an attractive option to achieve this goal, not only due to the fact that they possess both advantages of metal complexes and nanostructures but also because they are usually sensitive to pH. Here, we report the design and synthesis of a series novel smart microenvironment-responsive organocopper(II) supramolecular polymers with various substituted ligands to regulate their stability and anticancer efficacy. The investigation of the possible mechanisms revealed that the organocopper(II) polymers enter cancer cells through endocytosis and then induce apoptosis of cancer cells. Furthermore, the in vivo anticancer efficacy study demonstrated that these organocopper(II) polymers inhibited the tumor growth effectively without damage to the major organs. Overall, the organocopper(II) supramolecular polymers present a promising pathway to achieve high-efficiency and low-toxicity chemotherapy.

Bis-cyclometallated Ir(III) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands; influence of substituents and cyclometallating ligands on response to changes in pH

Bis-cyclometallated Ir(iii) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands have been synthesised. Their absorption is almost unchanged with changes in pH however the emission intensities vary by a factor of up to three and the complexes have emission pKas in the range 8.0 to 10.0. Substituents on the pyrazole have only a minor effect on the emission pKa. Surprisingly the complexes with phenylpyrazole cyclometallated ligands 3aL1-3 showed an intensity decrease with increasing pH (switch off) whilst the corresponding phenylpyridine ones 3cL1-3 showed an increase in emission intensity with increasing pH. Putting electron-withdrawing CF3 substituents on the cyclometallating phenyls reduced the pKa of the complexes to 6.8-7.8, thereby extending the useful pKa range; however, in general it tended to reduce the magnitude of the change in emission intensity. Surprisingly the CF3-substituted complexes also showed a complete reversal in the direction of the intensity change when compared to their respective unsubstituted congeners.

pH-Sensitive Near-IR Emitting Dinuclear Ruthenium Complex for Recognition, Two-Photon Luminescent Imaging, and Subcellular Localization of Cancer Cells

A dinuclear Ru(II) complex of [(bpy)₂Ru(Hdip)Ru(H₂bip)](ClO₄)₄ {bpy is 2,2'-bipyridine, Hdip is 2-(2,6-di(pyridin-2-yl)-pyridin-4-yl)-1H-imidazo[4,5-f]-[1,10]phenanthroline, and H₂bip is 2,6-bis(imidazole-2-yl)-pyridine} was synthesized and characterized by elemental analysis, mass spectrometry, and ¹H NMR spectroscopy. Spectrophotometric pH titrations in aqueous buffer and in vitro cell experiments indicated the response ability of the complex to pH fluctuations in the physiological pH range (6.0-8.0). The complex was found to be capable of differentiating live HeLa cells from healthy HEK293 cells by selectively accumulating in lysosomes of the HeLa cells. The low cytotoxicity (IC₅₀ > 100 μM), a large Stokes shift (∼200 nm), strong near-IR emission at ∼700 nm, a relatively long excited state lifetime, high photostability, and solubility make this complex considerably promising in real-time tracking and visualization of lysosomes in live cells. More interestingly, the tumor cell-specific two-photon luminescent imaging properties also endow this Ru complex with potential for applications in high-resolution tumor imaging and luminescence-guided tumor resection.

Dual-targeted photothermal agents for enhanced cancer therapy

Photothermal therapy, in which light is converted into heat and triggers local hyperthermia to ablate tumors, presents an inherently specific and noninvasive treatment for tumor tissues. In this area, the development of efficient photothermal agents (PTAs) has always been a central topic. Although many efforts have been made on the investigation of novel molecular architectures and photothermal materials over the past decades, PTAs can cause severe damage to normal tissues because of the poor tumor aggregate ability and high irradiation density. Recently, dual-targeted photothermal agents (DTPTAs) provide an attractive strategy to overcome these problems and enhance cancer therapy. DTPTAs are functionalized with two classes of targeting units, including tumor environment targeting sites, tumor targeting sites and organelle targeting sites. In this perspective, typical targeted ligands and representative examples of photothermal therapeutic agents with dual-targeted properties are systematically summarized and recent advances using DTPTAs in tumor therapy are highlighted.

Synthesis, Photophysics, and Reverse Saturable Absorption of trans-Bis-cyclometalated Iridium(III) Complexes (C^N^C)Ir(R-tpy)+ (tpy = 2,2':6',2″-Terpyridine) with Broadband Excited-State Absorption

Extending the bandwidth of triplet excited-state absorption in transition-metal complexes is appealing for developing broadband reverse saturable absorbers. Targeting this goal, five bis-terdentate iridium(III) complexes (Ir1-Ir5) bearing trans-bis-cyclometalating (C^N^C) and 4'-R-2,2':6',2″-terpyridine (4'-R-tpy) ligands were synthesized. The effects of the structural variation in cyclometalating ligands and substituents at the tpy ligand on the photophysics of these complexes have been systematically explored using spectroscopic methods (i.e., UV-vis absorption, emission, and transient absorption spectroscopy) and time-dependent density functional theory (TDDFT) calculations. All complexes exhibited intensely structured ¹π,π* absorption bands at <400 nm and broad charge transfer (¹CT)/¹π,π* transitions at 400-600 nm. Ligand structural variations exerted a very small effect on the energies of the ¹CT/¹π,π* transitions; however, they had a significant effect on the molar extinction coefficients of these absorption bands. All complexes emitted featureless deep red phosphorescence in solutions at room temperature and gave broad-band and strong triplet excited-state absorption ranging from the visible to the near-infrared (NIR) spectral regions, with both originating from the ³π,π*/³CT states. Although alteration of the ligand structures influenced the emission energies slightly, these changes significantly affected the emission lifetimes and quantum yields, transient absorption spectral features, and the triplet excited-state quantum yields of the complexes. Except for Ir3, the other four complexes all manifested reverse saturable absorption (RSA) upon nanosecond laser pulse excitation at 532 nm, with the decreasing trend of RSA following Ir2 ≈ Ir4 > Ir1 > Ir5 > Ir3. The RSA trend corresponded well with the strength of the excited-state and ground-state absorption differences (ΔOD) at 532 nm for these complexes.

Synthesis, characterization and anticancer mechanism studies of fluorinated cyclometalated ruthenium(ii) complexes

The drug-resistance of cancer cells has become a major obstacle to the development of clinical drugs for chemotherapy. In order to overcome cisplatin-resistance, seven cyclometalated ruthenium(ii) complexes were synthesized with a varying degree of fluorine substitution, for use as anticancer agents. A cytotoxicity assay testified that the complexes possessed a more cytotoxic effect than cisplatin towards the cisplatin-resistant cell line A549R. The number of fluorine atoms regulated the lipophilicity of the complexes, but the relationship was not linear. Ru1 containing one fluorine atom had the highest lipophilicity and the best therapeutic effect. The complexes enter cells through an energy-dependent pathway and then localize in the nuclei and mitochondria. The complexes induced nuclear dysfunction by the inhibition of DNA replication as well as mitochondrial dysfunction by the loss of membrane potential. The damage to these vital organelles leads to cell apoptosis via the caspase 3/7 pathway. Our results indicated that the modulation of the number of fluorine atoms in therapeutic agents can have a profound effect and Ru1 is a complex with a high potential as a drug for the treatment of cisplatin-resistant cancer.

A HCBP1 peptide conjugated ruthenium complex for targeted therapy of hepatoma

An HCBP1 peptide-ruthenium conjugate (Ru-β-Ala-FQHPSFI) as a potential candidate for targeted therapy of hepatoma was synthesized. Ru-β-Ala-FQHPSFI shows drastically enhanced cytotoxicity and high selectivity for hepatoma cells versus noncancer liver cells. Raman imaging shows that this peptide-based drug can be taken up well by the hepatoma cells compared with the bare ruthenium complex (Ru) and the opposite sequence peptide-ruthenium conjugate (Ru-β-Ala-IFSPHQF). This study presents a new strategy for the construction of tumor-targeting metal-based anticancer therapeutics.

The Self-Assembly of a Cyclometalated Palladium Photosensitizer into Protein-Stabilized Nanorods Triggers Drug Uptake In Vitro and In Vivo

Enhanced passive diffusion is usually considered to be the primary cause of the enhanced cellular uptake of cyclometalated drugs because cyclometalation lowers the charge of a metal complex and increases its lipophilicity. However, in this work, monocationic cyclometalated palladium complexes [1]OAc (N^N^C^N) and [2]OAc (N^N^N^C) were found to self-assemble, in aqueous solutions, into soluble supramolecular nanorods, while their tetrapyridyl bicationic analogue [3](OAc)₂ (N^N^N^N) dissolved as isolated molecules. These nanorods formed via metallophilic Pd···Pd interaction and π–π stacking and were stabilized in the cell medium by serum proteins, in the absence of which the nanorods precipitated. In cell cultures, these protein-stabilized self-assembled nanorods were responsible for the improved cellular uptake of the cyclometalated compounds, which took place via endocytosis (i.e., an active uptake pathway). In addition to triggering self-assembly, cyclometalation in [1]OAc also led to dramatically enhanced photodynamic properties under blue light irradiation. These combined penetration and photodynamic properties were observed in multicellular tumor spheroids and in a mice tumor xenograft, demonstrating that protein-stabilized nanoaggregation of cyclometalated drugs such as [1]OAc also allows efficient cellular uptake in 3D tumor models. Overall, serum proteins appear to be a major element in drug design because they strongly influence the size and bioavailability of supramolecular drug aggregates and hence their efficacy in vitro and in vivo.

Zinc-Containing Metalloenzymes: Inhibition by Metal-Based Anticancer Agents

DNA is considered to be the primary target of platinum-based anticancer drugs which have gained great success in clinics, but DNA-targeted anticancer drugs cause serious side-effects and easily acquired drug resistance. This has stimulated the search for novel therapeutic targets. In the past few years, substantial research has demonstrated that zinc-containing metalloenzymes play a vital role in the occurrence and development of cancer, and they have been identified as alternative targets for metal-based anticancer agents. Metal complexes themselves have also exhibited a lot of appealing features for enzyme inhibition, such as: (i) the facile construction of 3D structures that can increase the enzyme-binding selectivity and affinity; (ii) the intriguing photophysical and photochemical properties, and redox activities of metal complexes can offer possibilities to design enzyme inhibitors with multiple modes of action. In this review, we discuss recent examples of zinc-containing metalloenzyme inhibition of metal-based anticancer agents, especially three zinc-containing metalloenzymes overexpressed in tumors, including histone deacetylases (HDACs), carbonic anhydrases (CAs), and matrix metalloproteinases (MMPs).

Influence of the introduction of a triphenylphosphine group on the anticancer activity of a copper complex

Aiming at obtaining new copper complexes with good cytotoxicity against cancer cells, triphenylphosphine (TPP) was introduced to obtain insight into the influence of the co-ligands. In this paper, two copper complexes, Cu(2-pbmq)(CH₃OH)Br₂ (1) and [Cu(2-pbmq)(TPP)Br]₂ (2) were designed, synthesized, and characterized by X-ray crystallography, 2-((2-(pyrazin-2-yl)-1H-benzo[d]imidazol-1-yl)methyl))quinolone (2-pbmq), to investigate the influence of the TPP group on the anticancer activity of the metal complex. Although the presence of the TPP group diminished the intensity of the interaction properties of the complex with DNA, the in vitro anticancer activity and cellular uptake of the TPP-containing complex were markedly superior to those of its TPP-lacking counterpart. Detailed studies on the more potently cytotoxic complex 2 revealed that it accumulated in nucleus, arrested the cell cycle at the G0-G1 phase, causing mitochondrial dysfunction, involving the potential simultaneous mitochondrial membrane collapse, cellular ATP level depletion, and Ca²⁺ leakage, eventually inducing cell apoptosis. In summary, the introduction of a TPP group enhances the biological activity and cytotoxicity of the complex.

A mitochondria-targeting magnetothermogenic nanozyme for magnet-induced synergistic cancer therapy

Magnetic hyperthermia therapy (MHT) and chemodynamic therapy (CDT) are non-invasive in situ treatments without depth limitations and with minimum adverse effects on surrounding healthy tissue. We herein report a mitochondria-targeting magnetothermogenic nanozyme (Ir@MnFe2O4 NPs) for highly efficient cancer therapy. An iridium(III) complex (Ir) acts as a mitochondria-targeting agent on the surface of MnFe2O4 NPs. On exposure to an alternating magnetic field (AMF), the Ir@MnFe2O4 NPs induce a localized increase in temperature causing mitochondrial damage (MHT effect). Meanwhile glutathione (GSH) reduces Fe(III) to Fe(II) on the NPs surface, which in turn catalyzes the conversion of H2O2 to cytotoxic •OH (CDT effect). The depletion of GSH (a •OH scavenger) increases CDT efficacy, while the localized increase in temperature increases the rate of conversion of both Fe(III) to Fe(II) and H2O2 to •OH further enhancing the CDT effect. In addition, the disruption of cellular redox homeostasis due to CDT, leads to greater sensitivity of the cell towards MHT. This nanoplatform integrates these excellent therapeutic properties, with two-photon microscopy (TPM) (demonstrated in vitro) and magnetic resonance imaging (MRI) (demonstrated in vivo) to enable the precise and effective treatment of cancer.

Responsive ruthenium complex probe for phosphorescence and time-gated luminescence detection of bisulfite

Sensitive and selective quantification of specific analytes is of great significance in analytical and environmental sciences, as well as in the food industry. Herein, we report the design, synthesis, characterization, and application of a responsive ruthenium(ii) complex probe, Ru-azo, for phosphorescence and time-gated luminescence (TGL) detection of bisulfite, an important additive in the food industry. Upon a specific nucleophilic addition reaction between bisulfite and the azo group of Ru-azo, a new ruthenium(ii) complex, Ru-SO3, was obtained, which resulted in a remarkable increase in phosphorescence intensity, allowing the bisulfite detection to be achieved. In addition, long-lived emissions of Ru-azo (τ = 258 ns) and Ru-SO3 (τ = 261 ns) also enabled the TGL detection of bisulfite in autofluorescence-rich food samples. Through theoretical computations, the photoinduced electron transfer (PET) process within the ruthenium(ii) complex was validated, which unveiled the rationality of the luminescence "off-on" response of Ru-azo to bisulfite. The probe showed advantages of good water solubility, and high sensitivity, selectivity and accuracy for responding to bisulfite, facilitating its application in phosphorescence and TGL detection of bisulfite in aqueous and food samples.

Mitochondria-targeted phosphorescent cyclometalated iridium(III) complexes: synthesis, characterization, and anticancer properties

Cyclometalated iridium(III) complexes represent a promising approach to developing new anticancer metallodrugs. In this work, three phosphorescent cyclometalated iridium(III) complexes Ir1-Ir3 have been explored as mitochondria-targeted anticancer agents. All three complexes display higher antiproliferative activity than cisplatin against the cancer cells screened, and with the IC₅₀ values ranging from 0.23 to 5.6 μM. Colocalization studies showed that these complexes are mainly localized in the mitochondria. Mechanism studies show that these complexes exert their anticancer efficacy through initiating a series of events related to mitochondrial dysfunction, including depolarization of mitochondrial membrane potential (MMP), elevation of intracellular reactive oxygen species (ROS) levels, and induction of apoptosis. Mitochondria-targted cyclometalated iridium complexes induce apoptosis through depolarized mitochondria, elevation of intracellular ROS and activated caspase.