Metal-carbonyl units for vibrational and luminescence imaging: towards multimodality

Re(I)[2-aryl-1H-imidazo[4,5-f][1,10]phenanthroline] tricarbonyl chloride complexes for selective cancer therapy via a potential DNA damage mechanism

Recently, achieving selective cancer therapy with trifling side effects has been a great challenge in the eradication of cancer. Thus, to amplify the cytoselective approach of complexes, herein, we developed a series of Re(I)[2-aryl-1H-imidazo[4,5-f][1,10]phenanthroline] tricarbonyl chloride complexes and screened their potency against HeLa and MCF-7 cell lines together with the evaluation of their toxicity towards a normal kidney cell line (HEK-293). On meticulous investigation, complex [ReI(CO)3Cl(K2-N,N-(2c))] (3c) was found to be the most potent anticancer entity among other complexes. Complex 3c also showed competency to induce apoptosis in MCF-7 cells through G2/M phase cell-cycle arrest in association with the generation of ample reactive oxygen species (ROS), eventually leading to DNA intercalation and internucleosomal cleavage. The order of the cytotoxicity of these complexes depended on their lipophilic character and the electron-withdrawing halogen substitution at the para-position of the phenyl ring in the imidazophenanthroline ligand.

Rhenium fac-Tricarbonyl Bisimine Chalcogenide Complexes: Synthesis, Photophysical Studies, and Confocal and Time-Resolved Cell Microscopy

We describe the preparation, characterization, and imaging studies of rhenium carbonyl complexes with a pyta (4-(2-pyridyl)-1,2,3-triazole) or tapy (1-(2-pyridyl)-1,2,3-triazole)-based heteroaromatic N∧N ligand and thiolate or selenoate X ligand. The stability and photophysical properties of the selenolate complexes are compared with parent chloride complexes and previously described analogues with benzenethiolate ligands. Two complexes were imaged in A549 cells upon excitation at 405 nm. Colocalization studies suggest a lysosomal accumulation, while one parent chloride complex was described to localize at the Golgi apparatus. Preliminary fluorescence lifetime measurements and imaging demonstrate potential for application in time-resolved microscopy techniques due to the long and variable lifetimes observed in cellular environments, including an increase in lifetime between the solution and solid state many times larger than previously reported.

Necrosis-Inducing High-Valent Oxo-Rhenium(V) Complexes with Potent Antitumor Activity: Synthesis, Aquation Chemistry, Cisplatin Cross-Resistance Profile, and Mechanism of Action

Chemotherapy with the cytotoxic platinum (Pt) drugs cisplatin, carboplatin, and oxaliplatin is the mainstay of anticancer therapy in the clinic. The antitumor activity of Pt drugs originates from their ability to induce apoptosis via covalent adduct formation with nuclear DNA. While the phenomenal clinical success is highly encouraging, resistance and adverse toxic side effects limit the wider applicability of Pt drugs. To circumvent these limitations, we embarked on an effort to explore the antitumor potential of a new class of oxo-rhenium(V) complexes of the type [(N∧N)(EG)Re(O)Cl] (where EG = ethylene glycolate and N∧N = bipyridine, Bpy (1); phenanthroline, Phen (2); 3,4,7,8-tetramethyl-phenanthroline, Me4Phen (3)). Investigation of speciation chemistry in aqueous media revealed the formation of [(N∧N)Re(O)(OH)3] as the biologically active species. Complex 3 was found to be the most potent among the three, with IC50 values ranging from 0.1 to 0.4 μM against a panel of cancer cells, which is 5-70-fold lower when compared with cisplatin. The higher potency of 3 is attributed to its higher lipophilicity, which enhanced cellular uptake. Importantly, complex 3 efficiently overcomes cisplatin resistance in ovarian, lung, and prostate cancer cells. In addition to reporting the aquation chemistry and identifying the active species in aqueous media, we performed in-depth in vitro mechanistic studies, which revealed that complex 3 preferentially accumulates in mitochondria, depletes mitochondrial membrane potential, and upregulates intracellular reactive oxygen species (ROS), leading to ER stress-mediated necrosis-mediated cancer cell death.

Towards multimodal cellular imaging: optical and X-ray fluorescence

Imaging techniques permit the study of the molecular interactions that underlie health and disease. Each imaging technique collects unique chemical information about the cellular environment. Multimodal imaging, using a single probe that can be detected by multiple imaging modalities, can maximise the information extracted from a single cellular sample by combining the results of different imaging techniques. Of particular interest in biological imaging is the combination of the specificity and sensitivity of optical fluorescence microscopy (OFM) with the quantitative and element-specific nature of X-ray fluorescence microscopy (XFM). Together, these techniques give a greater understanding of how native elements or therapeutics affect the cellular environment. This review focuses on recent studies where both techniques were used in conjunction to study cellular systems, demonstrating the breadth of biological models to which this combination of techniques can be applied and the potential for these techniques to unlock untapped knowledge of disease states.

Recent Development of Rhenium-Based Materials in the Application of Diagnosis and Tumor Therapy

Rhenium (Re) is widely used in the diagnosis and treatment of cancer due to its unique physical and chemical properties. Re has more valence electrons in its outer shell, allowing it to exist in a variety of oxidation states and to form different geometric configurations with many different ligands. The luminescence properties, lipophilicity, and cytotoxicity of complexes can be adjusted by changing the ligand of Re. This article mainly reviews the development of radionuclide 188Re in radiotherapy and some innovative applications of Re as well as the different therapeutic approaches and imaging techniques used in cancer therapy. In addition, the current application and future challenges and opportunities of Re are also discussed.

Replacement of the phosphodiester backbone between canonical nucleosides with a dirhenium carbonyl "click" linker-a new class of luminescent organometallic dinucleoside phosphate mimics

The first-in-class luminescent dinucleoside phosphate analogs with a [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] "click" linker as a replacement for the natural phosphate group are reported together with the synthesis of luminescent adenosine and thymidine derivatives having the [Re₂(μ-Cl)₂(CO)₆(μ-pyridazine)] entity attached to positions 5' and 3', respectively. These compounds were synthesized by applying inverse-electron-demand Diels-Alder and copper(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition reactions in three or four steps. The obtained compounds exhibited orange emission (λ_PL ≈ 600 nm, Φ_PL ≈ 0.10, and τ = 0.33-0.61 μs) and no toxicity (except for one nucleoside) to human HeLa cervical epithelioid and Ishikawa endometrial adenocarcinoma cancer cells in vitro. Furthermore, the compounds' ability to inhibit the growth of Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacterial strains was moderate and only observed at a high concentration of 100 μM. Confocal microscopy imaging revealed that the "dirhenium carbonyl" dinucleosides and nucleosides localized mainly in the membranous structures of HeLa cells and uniformly inside S. aureus and E. coli bacterial cells. An interesting finding was that some of the tested compounds were also found in the nuclei of HeLa cells.

Challenges and opportunities in the development of metal-based anticancer theranostic agents

Around 10 million fatalities were recorded worldwide in 2020 due to cancer and statistical projections estimate the number to increase by 60% in 2040. With such a substantial rise in the global cancer burden, the disease will continue to impose a huge socio-economic burden on society. Currently, the most widely used clinical treatment modality is cytotoxic chemotherapy using platinum drugs which is used to treat variety of cancers. Despite its clinical success, critical challenges like resistance, off-target side effects and cancer variability often reduce its overall therapeutic efficiency. These challenges require faster diagnosis, simultaneous therapy and a more personalized approach toward cancer management. To this end, small-molecule ‘theranostic’ agents have presented a viable solution combining diagnosis and therapy into a single platform. In this review, we present a summary of recent efforts in the design and optimization of metal-based small-molecule ‘theranostic’ anticancer agents. Importantly, we highlight the advantages of a theranostic candidate over the purely therapeutic or diagnostic agent in terms of evaluation of its biological properties.

Anticancer Properties of Rhenium(I) Tricarbonyl Complexes of N-Heterocyclic Carbene Ligands

A family of eight rhenium(I) tricarbonyl complexes bearing pyridyl-imidazolylidene or bis-imidazolylidene ligand in combination with a series of N-acetyl amino acids ligands (glycine, isoleucine, proline) and acetate have been synthesised...

Functionalizing Collagen with Vessel-Penetrating Two-Photon Phosphorescence Probes: A New In Vivo Strategy to Map Oxygen Concentration in Tumor Microenvironment and Tissue Ischemia

The encapsulation and/or surface modification can stabilize and protect the phosphorescence bio-probes but impede their intravenous delivery across biological barriers. Here, a new class of biocompatible rhenium (ReI ) diimine carbonyl complexes is developed, which can efficaciously permeate normal vessel walls and then functionalize the extravascular collagen matrixes as in situ oxygen sensor. Without protective agents, ReI -diimine complex already exhibits excellent emission yield (34%, λem   = 583 nm) and large two-photon absorption cross-sections (σ2   = 300 GM @ 800 nm) in water (pH 7.4). After extravasation, remarkably, the collagen-bound probes further enhanced their excitation efficiency by increasing the deoxygenated lifetime from 4.0 to 7.5 µs, paving a way to visualize tumor hypoxia and tissue ischemia in vivo. The post-extravasation functionalization of extracellular matrixes demonstrates a new methodology for biomaterial-empowered phosphorescence sensing and imaging.

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 Bis-Chelating O N O ^ / N N ^ Ligand for the Synthesis of Heterobimetallic Platinum(II)/Rhenium(I) Complexes: Tools for the Optimization of a New Class of Platinum(II) Anticancer Agents

The two independent and N N ^ coordination sites of a newly synthesized bis[2-(hydroxyphenyl)-1,2,4-triazole] platform have been exploited to prepare four monometallic neutral ()Pt^II complexes carrying DMSO, pyridine, triphenylphosphine, or N-heterocyclic carbene as the fourth ligand. Then, the second N N ^ coordination site was used to introduce an IR-active rhenium tricarbonyl entity, affording the four corresponding heterobimetallic neutral Pt^II /Re^I complexes, as well as a cationic Pt^II /Re^I derivative. X-ray crystallographic studies showed that distortion of the organic platform occurred to accommodate the coordination geometry of both metal centers. No ligand exchange or transchelation occurred upon incubation of the Pt^II complexes in aqueous environment or in the presence of Fe^III , respectively. The antiproliferative activity of the ligand and complexes was first screened on the triple-negative breast cancer cell line MDA-MB-231. Then, the IC₅₀ values of the most active candidates were determined on a wider panel of human cancer cells (MDA-MB-231, MCF-7, and A2780), as well as on a nontumorigenic cell line (MCF-10A). Low micromolar activities were reached for the complexes carrying a DMSO ligand, making them the first examples of highly active, but hydrolytically stable, Pt^II complexes. Finally, the characteristic mid-IR signature of the {Re(CO)₃ } fragment in the Pt/Re heterobimetallic complexes was used to quantify their uptake in breast cancer cells.

Towards better understanding of photophysical properties of rhenium(I) tricarbonyl complexes with terpy-like ligands

Series of Re(I) carbonyls complexes were designed and synthesized to explore the impact of the triimine skeleton and number of methoxy groups attached to aryl substituents on their optoelectronic and thermal properties. The chemical structures of the prepared complexes were confirmed by ¹H and ¹³C NMR spectroscopy, HR-MS, elemental anlsysis, and X-ray measurements. DSC measuremtns showed that they melted in the range of 198-325 °C. Some of them form stable molecular glasses with high glass transition temperatures (158-173 °C). Experimentally obtained optical properties were supported by DFT calculations. The UV-Vis spectra display a series of overlapping absorption bands in the range 200-350 nm, and much weaker broad band in the visible spectral region, due to intraligand and charge transfer transitions, respectively. All synthesized complexes were emissive in solution and in solid state as powder. Moreover, when applied in diodes, some of them exhibited ability for emission of light under external voltage with maximum of electroluminescence band located at 591-630 nm.

Functionalized Pt(II) and Ir(III) NIR Emitters and Their Covalent Conjugates with Polymer-Based Nanocarriers

Two NIR-emitting platinum [Pt(N^N^C)(phosphine)] and iridium [Ir(N^C)2(N^N)]⁺ complexes containing reactive succinimide groups were synthesized and characterized with spectroscopic methods (N^N^C, 1-phenyl-3-(pyridin-2-yl)benzo[4,5]imidazo[1,2-a]pyrazine, N^C, 6-(2-benzothienyl)phenanthridine, phosphine-3-(diphenylphosphaneyl)propanoic acid N-hydroxysuccinimide ether, and N^N, 4-oxo-4-((1-(pyridin-2-yl)-1H-1,2,3-triazol-4-yl)methoxy)butanoic acid N-hydroxysuccinimide ether). Their photophysics were carefully studied and analyzed using time-dependent density functional theory calculations. These complexes were used to prepare luminescent micro- and nanoparticles with the "core-shell" morphology, where the core consisted of biodegradable polymers of different hydrophobicity, namely, poly(d,l-lactic acid), poly(ε-caprolactone), and poly(ω-pentadecalactone), whereas the shell was formed by covalent conjugation with poly(l-lysine) covalently labeled with the platinum and iridium emitters. The surface of the species was further modified with heparin to reverse their charge from positive to negative values. The microparticles' size determined with dynamic laser scanning varies considerably from 720 to 1480 nm, but the nanoparticles' diameter falls in a rather narrow range, 210-230 nm. The species with a poly(l-lysine) shell display a high positive (>30 mV) zeta-potential that makes them essentially stable in aqueous media. Inversion of the surface charge to a negative value with the heparin cover did not deteriorate the species' stability. The iridium- and platinum-containing particles displayed emissions the spectral patterns of which were essentially similar to those of unconjugated complexes, which indicate retention of the chromophore nature upon binding to the polymer and further immobilization onto polyester micro- and nanoparticles for drug delivery. The obtained particles were tested to determine their ability to penetrate into different cells types: cancer cells, stem cells, and fibroblasts. It was found that all types of particles could effectively penetrate into all cells types under investigation. Nanoparticles were shown to penetrate into the cells more effectively than microparticles. However, positively charged nanoparticles covered with poly(l-lysine) seem to interact with negatively charged proteins in the medium and enter the inner part of the cells less effectively than nanoparticles covered with poly(l-lysine)/heparin. In the case of microparticles, the species with positive zeta-potentials were more readily up-taken by the cells than those with negative values.

Monitoring the Kinetics of the Cellular Uptake of a Metal Carbonyl Conjugated with a Lipidic Moiety in Living Cells Using Synchrotron Infrared Spectromicroscopy

Presented here is the exploitation of synchrotron infrared spectromicroscopy to evaluate the feasibility of monitoring the cellular uptake of rhenium-tris-carbonyl-tagged (Re(CO)₃) lipophilic chains in living cells. To this aim, an in-house thermostated microfluidic device was used to limit water absorption while keeping cells alive. Indeed, cells showed a high survival rate in the microfluidic device over the course of the experiment, proving the short-term biocompatibility of the device. We recorded spectra of single, living, fully hydrated breast cancer MDA-MB231 cells and could follow the penetration of the rhenium complexes for up to 2 h. Despite the strong variations observed in the uptake kinetics between individual cells, the Re(CO)₃ complex was traced inside the cells at low concentration and shown to enter them on the hour time scale by active transport.

Intracellular location matters: rationalization of the anti-inflammatory activity of a manganese(ii) superoxide dismutase mimic complex

The study of Mn-based superoxide dismutase mimic conjugated with a multimodal Re-probe in a cellular model of oxidative stress revealed that its bioactivity is associated with its accumulation at the mitochondria.

In Vivo Anticancer Activity of a Rhenium(I) Tricarbonyl Complex

The rhenium(I) complex fac-[Re(CO)₃(2,9-dimethyl-1,10-phenanthroline)(OH₂)]⁺ (1) was previously shown to exhibit potent in vitro anticancer activity in a manner distinct from conventional platinum-based drugs (J. Am. Chem. Soc. 2017, 139, 14302-14314). In this study, we report further efforts to explore its aqueous speciation and antitumor activity. The cellular uptake of 1 was measured in A2780 and cisplatin-resistant A2780CP70 ovarian cancer cells by inductively coupled plasma mass spectrometry, revealing similar uptake efficiency in both cell lines. High accumulation in the mitochondria was observed, contradicting prior fluorescence microscopy studies. The luminescence of 1 is highly dependent on pH and coordination environment, making fluorescence microscopy somewhat unreliable for determining compound localization. The in vivo anticancer activity of 1 was evaluated in mice bearing patient-derived ovarian cancer tumor xenografts. These studies conclusively show that 1 is capable of inhibiting tumor growth, providing further credibility for the use of these compounds as anticancer agents.

Mitochondria-Accumulating Rhenium(I) Tricarbonyl Complexes Induce Cell Death via Irreversible Oxidative Stress and Glutathione Metabolism Disturbance

Mitochondria play a critical role in tumorigenesis. Targeting mitochondria and disturbing related events have been emerging as a promising way for chemotherapy. In this work, two binuclear rhenium(I) tricarbonyl complexes of the general formula [Re₂(CO)₆(dip)₂L](PF₆)₂ (dip = 4,7-diphenyl-1,10-phenanthroline; L = 4,4'-azopyridine (ReN) or 4,4'-dithiodipyridine (ReS)) were synthesized and characterized. ReN and ReS can react with glutathione (GSH). They exhibit good in vitro anticancer activity against cancer cell lines screened. Besides, they can target mitochondria, cause oxidative stress, and disturb GSH metabolism. Both ReN and ReS can induce necroptosis and caspase-dependent apoptosis simultaneously. We also demonstrate that ReN and ReS can inhibit tumor growth in nude mice bearing carcinoma xenografts. Our study shows the potential of Re(I) complexes as chemotherapeutic agents to kill cancer cells via a mitochondria-to-cellular redox strategy.

Combinatorial Synthesis to Identify a Potent, Necrosis-Inducing Rhenium Anticancer Agent

Combinatorial synthesis can be applied for developing a library of compounds that can be rapidly screened for biological activity. Here, we report the application of microwave-assisted combinatorial chemistry for the synthesis of 80 rhenium(I) tricarbonyl complexes bearing diimine ligands. This library was evaluated for anticancer activity in three different cancer cell lines, enabling the identification of three lead compounds with cancer cell growth-inhibitory activities of less than 10 μM. These three lead structures, Re-9B, Re-9C, and Re-9D, were synthesized independently and fully characterized by NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray crystallography. The most potent of these three complexes, Re-9D, was further explored to understand its mechanism of action. Complex Re-9D is equally effective in both wild-type and cisplatin-resistant A2780 ovarian cancer cells, indicating that it circumvents cisplatin resistance. This compound was also shown to possess promising activity against ovarian cancer tumor spheroids. Additionally, flow cytometry showed that Re-9D does not induce cell cycle arrest or flipping of phosphatidylserine to the outer cell membrane. Analysis of the morphological changes of cancer cells treated with Re-9D revealed that this compound gives rise to rapid plasma membrane rupture. Collectively, these data suggest that Re-9D induces necrosis in cancer cells. To assess the in vivo biodistribution and stability of this compound, a radioactive ^99mTc analogue of Re-9D, ^99mTc-9D(H₂O), was synthesized and administered to naı̈ve BALB/c mice. Results of these studies indicate that ^99mTc-9D(H₂O) exhibits high metabolic stability and a distinct biodistribution profile. This research demonstrates that combinatorial synthesis is an effective approach for the development of new rhenium anticancer agents with advantageous biological properties.

Ferrocifens labelled with an infrared rhenium tricarbonyl tag: synthesis, antiproliferative activity, quantification and nano IR mapping in cancer cells

Antiproliferative activities of several members of the ferrocifen family, both in vitro and in vivo, are well documented although their precise location in cancer cells has not yet been elucidated. However, two different infrared imaging techniques have been used to map the non-cytotoxic cyrhetrenyl analogue of ferrociphenol in a single cell. This observation prompted us to tag two ferrocifens with a cyrhetrenyl unit [CpRe(CO)3; Cp = η5-cyclopentadienyl] by grafting it, via an ester bond, either to one of the phenols (4, 5) or to the hydroxypropyl chain (6). Complexes 4-6 retained a high cytotoxicity on breast cancer cells (MDA-MB-231) with IC50 values in the range 0.32-2.5 μM. Transmission IR spectroscopy was used to quantify the amount of cyrhetrenyl tag present in cells incubated with 5 or 6. The results show that after a 1-hour incubation of cells at 37 °C, complexes 5 and 6 are mainly present within cells while only a limited percentage, quantified by ICP-OES, remained in the incubation medium. AFM-IR spectroscopy, a technique coupling infrared irradiation with near-field AFM detection, was used to map the cyrhetrenyl unit in a single MDA-MB-231 cell, incubated at 37 °C for 1 hour with 10 μM of 6. The results show that signal distribution of the characteristic band of the Re(CO)3 entity at 1950 cm-1 matched those of amide and phosphate, thus indicating a location of the complex mainly in the cell nucleus.

Importance of Metal-Ion Exchange for the Biological Activity of Coordination Complexes of the Biomimetic Ligand N4Py

Metal coordination complexes can display interesting biological activity, as illustrated by the bleomycins (BLMs), a family of natural antibiotics that when coordinated to a redox-active metal ion, show antitumor activity. Yet, which metal ion is required for the activity in cells is still subject to debate. In this study, we described how different metal ions affect the intracellular behavior and activity of the synthetic BLM-mimic N, N-bis(2-pyridylmethyl)- N-bis(2-pyridyl)methylamine (N4Py). Our study shows that a mixture of iron(II), copper(II), and zinc(II) complexes can be generated when N4Py is added to cell cultures but that the metal ion can also be exchanged by other metal ions present in cells. Moreover, the combination of chemical data, together with the performed biological experiments, shows that the active complex causing oxidative damage to cells is the FeII-N4Py complex and not per se the metal complex that was initially added to the cell culture medium. Finally, it is proposed that the high activity observed upon the addition of the free N4Py ligand is the result of a combination of scavenging of biologically relevant metals and oxidative damage caused by the iron(II) complex.

Chromophore-Functionalized Phenanthro-diimine Ligands and Their Re(I) Complexes

A series of diimine ligands has been designed on the basis of 2-pyridyl-1 H-phenanthro[9,10- d]imidazole (L1, L2). Coupling the basic motif of L1 with anthracene-containing fragments affords the bichromophore compounds L3-L5, of which L4 and L5 adopt a donor-acceptor architecture. The latter allows intramolecular charge transfer with intense absorption bands in the visible spectrum (lowest λabs 464 nm (ε = 1.2 × 104 M-1 cm-1) and 490 nm (ε = 5.2 × 104 M-1 cm-1) in CH2Cl2 for L4 and L5, respectively). L1-L5 show strong fluorescence in a fluid medium (Φem = 22-92%, λem 370-602 nm in CH2Cl2); discernible emission solvatochromism is observed for L4 and L5. In addition, the presence of pyridyl (L1-L5) and dimethylaminophenyl (L5) groups enables reversible alteration of their optical properties by means of protonation. Ligands L1-L5 were used to synthesize the corresponding [Re(CO)3X(diimine)] (X = Cl, 1-5; X = CN, 1-CN) complexes. 1 and 2 exhibit unusual dual emission of singlet and triplet parentage, which originate from independently populated 1ππ* and 3MLCT excited states. In contrast to the majority of the reported Re(I) carbonyl luminophores, complexes 3-5 display moderately intense ligand-based fluorescence from an anthracene-containing secondary chromophore and complete quenching of emission from the 3MLCT state presumably due to the triplet-triplet energy transfer (3MLCT → 3ILCT).

Graftable SCoMPIs enable the labeling and X-ray fluorescence imaging of proteins

Bio-imaging techniques alternative to fluorescence microscopy are gaining increasing interest as complementary tools to visualize and analyze biological systems. Among them, X-ray fluorescence microspectroscopy provides information on the local content and distribution of heavy elements (Z ≥ 14) in cells or biological samples. In this context, similar tools to those developed for fluorescence microscopy are desired, including chemical probes or tags. In this work, we study rhenium complexes as a convenient and sensitive probe for X-ray fluorescence microspectroscopy. We demonstrate their ability to label and sense exogenously incubated or endogenous proteins inside cells.

Labeling of Hyaluronic Acids with a Rhenium-tricarbonyl Tag and Percutaneous Penetration Studied by Multimodal Imaging

Hyaluronic acids were labeled with a rhenium-tricarbonyl used as single core multimodal probe for imaging and their penetration into human skin biopsies was studied using IR microscopy and fluorescence imaging (labeled SCoMPI). The penetration was shown to be dependent on the molecular weight of the molecule and limited to the upper layer of the skin.

Acetoxymethyl Concept for Intracellular Administration of Carbon Monoxide with Mn(CO)3 -Based PhotoCORMs

Targeted administration of carbon monoxide with CO releasing molecules (CORMs) inside of cells proved to be very challenging. Consequently, there are only very few reports on intracellular uptake of CORMs requiring high extracellular CORM loading because an equilibrium between extra- and intracellular concentrations can be assumed. Here we present a strategy for a targeted intracellular administration of manganese(I)-based CORMs that are altered inside of cells to trap these complexes. Thereafter, carbon monoxide can be liberated by irradiation (photoCORMs). To achieve this innovative task, acetoxymethyl (AM) groups are attached at the periphery of the hydrophobic manganese(I) carbonyl complexes to not influence the CO release behavior. Inside of cells these AM substituents are cleaved by esterases yielding hydrophilic manganese(I) carbonyl compounds which are captured inside of cells. This objective is realized by using the bidentate bases 4-(acetoxymethoxycarbonyl)phenyl-bis(3,5-dimethylpyrazolyl)methane (1) and 4-(acetoxymethoxy)phenyl-bis(3,5-dimethylpyrazolyl)methane (4) at facial (OC)₃ MnBr fragments yielding CORM-AM1 (2) and CORM-AM2 (5), respectively. Besides synthesis, crystal structures and spectroscopic properties we present targeted administration and intracellular accumulation of these AM-containing CORMs.

The unsuspected influence of the pyridyl-triazole ligand isomerism upon the electronic properties of tricarbonyl rhenium complexes: an experimental and theoretical insight

How can the intimate nature of the triazole moiety govern the geometry and luminescence properties of a rhenium complex?

Rhenium tricarbonyl complexes with arenethiolate axial ligands

Pyta and Tapy-based [Re(N^N)(CO)₃X] complexes with para-substituted benzenethiolates as axial ligand are reported along with their electrochemical and photophysical properties.

In Vitro Anticancer Activity and in Vivo Biodistribution of Rhenium(I) Tricarbonyl Aqua Complexes

Seven rhenium(I) complexes of the general formula fac-[Re(CO)3(NN)(OH2)]+ where NN = 2,2'-bipyridine (8), 4,4'-dimethyl-2,2'-bipyridine (9), 4,4'-dimethoxy-2,2'-bipyridine (10), dimethyl 2,2'-bipyridine-4,4'-dicarboxylate (11), 1,10-phenanthroline (12), 2,9-dimethyl-1,10-phenanthroline (13), or 4,7-diphenyl-1,10-phenanthroline (14), were synthesized and characterized by 1H NMR spectroscopy, IR spectroscopy, mass spectrometry, and X-ray crystallography. With the exception of 11, all complexes exhibited 50% growth inhibitory concentration (IC50) values that were less than 20 μM in HeLa cells, indicating that these compounds represent a new potential class of anticancer agents. Complexes 9, 10, and 13 were as effective in cisplatin-resistant cells as wild-type cells, signifying that they circumvent cisplatin resistance. The mechanism of action of the most potent complex, 13, was explored further by leveraging its intrinsic luminescence properties to determine its intracellular localization. These studies indicated that 13 induces cytoplasmic vacuolization that is lysosomal in nature. Additional in vitro assays indicated that 13 induces cell death without causing an increase in intracellular reactive oxygen species or depolarization of the mitochondrial membrane potential. Further studies revealed that the mode of cell death does not fall into one of the canonical categories such as apoptosis, necrosis, paraptosis, and autophagy, suggesting that a novel mode of action may be operative for this class of rhenium compounds. The in vivo biodistribution and metabolism of complex 13 and its 99mTc analogue 13* were also evaluated in naı̈ve mice. Complexes 13 and 13* exhibited comparable biodistribution profiles with both hepatic and renal excretion. High-performance liquid chromatography inductively coupled plasma mass-spectrometry (HPLC-ICP-MS) analysis of mouse blood plasma and urine postadministration showed considerable metabolic stability of 13, rendering this potent complex suitable for in vivo applications. These studies have shown the biological properties of this class of compounds and demonstrated their potential as promising theranostic anticancer agents that can circumvent cisplatin resistance.

Linking ReI and PtII Chromophores with Aminopyridines: A Simple Route to Achieve a Complicated Photophysical Behavior

The bifunctional aminopyridine ligands H₂ N-(CH₂ )_n -4-C₅ H₄ N (n=0, L1; 1, L2; 2, L3) have been utilized for the preparation of the rhenium complexes [Re(phen)(CO)₃ (L1-L3)]⁺ (1-3; phen=phenanthroline). Complexes 2 and 3 with NH₂ -coordinated L2 and L3, respectively, were coupled with cycloplatinated motifs {Pt(ppy)Cl} and {Pt(dpyb)}⁺ (ppy=2-phenylpyridine, dpyb=dipyridylbenzene) to give the bimetallic species [Re(phen)(CO)₃ (μ-L2/L3)Pt(ppy)Cl]⁺ (4, 6) and [Re(phen)(CO)₃ (μ-L2/L3)Pt(dpyb)]²⁺ (5, 7). In solution, complexes 4 and 6 show ³ MLCT {Re}-based emission at 298 K, which changes to the ³ IL(ppy) state at 77 K. The photophysical properties of compounds 5 and 7 display a pronounced concentration dependence, presumably due to the formation of bimolecular aggregates. Analysis of the spectroscopic data, combined with TD-DFT simulations, suggest that unconventional heteroleptic {Re(phen)}⋅⋅⋅{Pt(dpyb)} π-π stacking operates as the driving force for ground-state association. The latter, together with intra- and intermolecular energy-transfer processes, determines the appearance of multiple emission bands and results in nonlinear relaxation kinetics of the excited states.

Mitochondria Targeting with Luminescent Rhenium(I) Complexes

Two new neutral fac-[Re(CO)₃(phen)L] compounds (1,2), with phen = 1,10-phenanthroline and L = O₂C(CH₂)₅CH₃ or O₂C(CH₂)₄C≡CH, were synthetized in one-pot procedures from fac-[Re(CO)₃(phen)Cl] and the corresponding carboxylic acids, and were fully characterized by IR and UV-Vis absorption spectroscopy, ¹H- and ¹³C-NMR, mass spectrometry and X-ray crystallography. The compounds, which display orange luminescence, were used as probes for living cancer HeLa cell staining. Confocal microscopy revealed accumulation of both dyes in mitochondria. To investigate the mechanism of mitochondrial staining, a new non-emissive compound, fac-[Re(CO)₃(phen)L], with L = O₂C(CH₂)₃((C₅H₅)Fe(C₅H₄), i.e., containing a ferrocenyl moiety, was synthetized and characterized (3). 3 shows the same mitochondrial accumulation pattern as 1 and 2. Emission of 3 can only be possible when ferrocene-containing ligand dissociates from the metal center to produce a species containing the luminescent fac-[Re(CO)₃(phen)]⁺ core. The release of ligands from the Re center was verified in vitro through the conjugation with model proteins. These findings suggest that the mitochondria accumulation of compounds 1-3 is due to the formation of luminescent fac-[Re(CO)₃(phen)]⁺ products, which react with cellular matrix molecules giving secondary products and are uptaken into the negatively charged mitochondrial membranes. Thus, reported compounds feature a rare dissociation-driven mechanism of action with great potential for biological applications.

Recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents

This Perspective summarizes recent advances in the biological applications of luminescent rhenium(i) tricarbonyl polypyridine complexes.