Synthesis, X-ray structure, interactions with DNA, remarkable in vivo tumor growth suppression and nephroprotective activity of cis-tetrachloro-dipivalato dirhenium(III)

DNA-Targeted Complexes of Tc and Re for Biomedical Applications

Due to their favorable chemical features, Re and Tc complexes have been widely used for the development of new therapeutic agents and imaging probes to solve problems of biomedical relevance. This review provides an update of the most relevant research efforts towards the development of novel cancer theranostic agents using Re and Tc-based compounds interacting with specific DNA structures. This includes a variety of homometallic complexes, namely those containing M(CO)3 (M=Re, Tc) moieties, that exhibit different modes of interaction with DNA, such as covalent binding, intercalation, groove binding or G-quadruplex DNA binding. Additionally, heterometallic complexes, designed to potentiate synergistic effects of different metal centers to improve DNA-targeting, cytotoxicity and fluorescence properties, are also reviewed. Particular attention is also given to 99m Tc- and 188 Re-labeled oligonucleotides that have been widely explored to develop imaging and therapeutic radiopharmaceuticals through the in vivo hybridization with a specific complementary DNA or RNA target sequence to provide useful molecular tools in precision medicine for cancer diagnosis and treatment. Finally, the need for further improvement of DNA-targeted Re and Tc-based compounds as potential therapeutic and diagnostic agents is highlighted, and future directions are discussed.

Rhenium-Based Complexes and in Vivo Testing: A Brief History

The success of metal-based anticancer therapeutics in the treatment of cancer is best exemplified by cisplatin. Currently used in 32/78 cancer regimens, metal-based drugs have a clear role in cancer therapy. Despite this, metal-based anticancer therapeutics are not without drawbacks, with issues such as toxic side effects and the development of resistance mechanisms. This has led to investigations of other metal-based drug candidates such as auranofin, a gold-based drug candidate as well as ruthenium-based candidates, NAMI-A, NKP-1339 and TLD-1433. All are currently undergoing clinical trials. Another class of complexes under study are rhenium-based; such complexes have undergone extensive in vitro testing but only nine have been reported to display antitumour in vivo activity, which is a necessary step before entering clinical trials. This review will document, chronologically, the rhenium-based drug candidates that have undergone in vivo testing and the outlook for such complexes.

X-Ray fluorescence microscopy reveals that rhenium(i) tricarbonyl isonitrile complexes remain intact in vitro

An endoplasmic reticulum stress-inducing rhenium isonitrile complex was investigated for its axial ligand stability in living cells using X-ray fluorescence microscopy.

The first examples of triply bonded dirhenium(II,II) complexes that contain bis(diphenylphosphino)methane and dithiocarbamato ligands: spectroscopic, structural, cytotoxicity and computational studies

The synthesis, structure and properties of the hitherto unreported triply bonded dirhenium(II,II) dithiocarbamato complexes have been reported.

Design of Rhenium Compounds in Targeted Anticancer Therapeutics

Background:

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

Methods:

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

Results:

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

Conclusion:

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

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.

Self-Assembled Multinuclear Complexes Incorporating 99m Tc

Multinuclear complexes or clusters are rarely investigated in medicinal inorganic chemistry although they represent structural intermediates between molecules and nanomaterials. We present in this report two strategies towards ^99m Tc-containing clusters. In a pre-assembly approach, the preformed but incomplete cluster fragment [Re₃ (μ₂ -OH)₃ (μ₃ -OH)(CO)₉ ]^- reacts with [^99m Tc(CO)₃ ]⁺ to the highly stable [^99m TcRe₃ (μ₃ -OH)₄ (CO)₁₂ ] cube. The same structure self-assembles when reacting the mononuclear Re and ^99m Tc precursors in one pot. Integrating the coordinating OH groups from Schiff bases in this concept leads straight to dinuclear, mixed-metal complexes of the type [^99m TcRe(μ₂ -O^N-R¹ )₂ (CO)₆ ] in quantitative yields. Both strategies are unprecedented and open a future path towards clusters, incorporating a ^99m Tc radiolabel while being decorated with targeting or cytotoxic moieties.

Anticancer activity of complexes of the third row transition metals, rhenium, osmium, and iridium

A summary of recent developments on the anticancer activity of complexes of rhenium, osmium, and iridium is described.

A survey of the mechanisms of action of anticancer transition metal complexes

Metal complexes have been the subject of numerous investigations in oncology but, despite the plethora of newly synthesized compounds, their precise mechanisms of action remain generally unknown or, for the best, incompletely determined. The continuous development of efficient and sensitive techniques in analytical chemistry and molecular biology gives scientists new tools to gather information on how metal complexes can be effective toward cancer. This review focuses on recent findings about the anticancer mechanism of action of metal complexes and how the ligands can be used to tune their pharmacological and physicochemical properties.

Crystal structure of bis-(ethyl-enedi-thio)-tetra-thia-fulvalenium μ2-acetato-bis-[tri-bromido-rhenate(III)] 1,1,2-tri-chloro-ethane hemisolvate

The asymmetric unit of the title salt, (C10H8S8)[Re2Br6(CH3COO)]·0.5C2H3Cl3, contains one bis-(ethyl-enedi-thio)-tetra-thia-fulvalene (ET) radical cation, one μ2-acetato-bis-[tri-bromido-rhenate(III)] anion and a 1,1,2-tri-chloro-ethane mol-ecule with half-occupancy disordered about a twofold rotation axis. The tetra-thia-fulvalene fragment adopts an almost planar configuration typical of the ET radical cation. The C atoms of both ethyl-enedi-thio fragments in the cation are disordered over two orientations with occupancy factors 0.65:0.35 and 0.77:0.23. In the anion, six Br atoms and a μ2-acetate ligand form a strongly distorted cubic O2Br6 coordination polyhedron around the Re2 dinuclear centre. In the crystal, centrosymmetrically related ET cations and Re2O2Br6 anions are linked into dimers by π-π stacking inter-actions [centroid-to-centroid distance = 3.826 (8) Å] and by pairs of additional Re⋯Br contacts [3.131 (3) Å], respectively. The dimers are further packed into a three-dimensional network by non-directional inter-ionic electrostatic forces and by C-H⋯Br and C-H⋯S hydrogen bonds. The disordered 1,1,2-tri-chloro-ethane mol-ecules occupy solvent-accessible channels along the b axis.

Crystal structure of di-μ-isobutyrato-κ(4) O:O'-bis-[cis-di-chlorido-(dimethyl sulfoxide-κS)rhenium(III)]

The title compound, [Re2(C3H7COO)2Cl4{(CH3)2SO}2], comprises binuclear complex mol-ecules [Re-Re = 2.24502 (13) Å] involving cis-oriented double carboxyl-ate bridges, four equatorial chloride ions and two weakly bonded O atoms from dimethyl sulfoxide ligands in the axial positions at the Re(III) atoms. In the crystal, mol-ecules are linked into corrugated layers parallel to (101) by very weak C-H⋯Cl and C-H⋯O hydrogen-bonding inter-actions. C-H⋯Cl hydrogen bonding provides the links between layers to consolidate a three-dimensional framework.

Determination of melting temperature and temperature melting range for DNA with multi-peak differential melting curves

Many factors that change the temperature position and interval of the DNA helix-coil transition often also alter the shape of multi-peak differential melting curves (DMCs). For DNAs with a multi-peak DMC, there is no agreement on the most useful definition for the melting temperature, Tm, and temperature melting width, ΔT, of the entire DNA transition. Changes in Tm and ΔT can reflect unstable variation of the shape of the DMC as well as alterations in DNA thermal stability and heterogeneity. Here, experiments and computer modeling for DNA multi-peak DMCs varying under different factors allowed testing of several methods of defining Tm and ΔT. Indeed, some of the methods give unreasonable "jagged" Tm and ΔT dependences on varying relative concentration of DNA chemical modifications (rb), [Na(+)], and GC content. At the same time, Tm determined as the helix-coil transition average temperature, and ΔT, which is proportional to the average absolute temperature deviation from this temperature, are suitable to characterize multi-peak DMCs. They give smoothly varying theoretical and experimental dependences of Tm and ΔT on rb, [Na(+)], and GC content. For multi-peak DMCs, Tm value determined in this way is the closest to the thermodynamic melting temperature (the helix-coil transition enthalpy/entropy ratio).

Crystal structure ofcis-bis(μ-β-alanine-κ2O:O′)bis[trichloridorhenium(III)](Re–Re) sesquihydrate

A dirhenium(III) cis-di­carboxyl­ate complex is reported, which is representative of a small class of zwitterionic ammonia­carboxyl­ato complexes involving a quadruple metal–metal bond.

The structure of the title compound, [Re₂Cl₆(C₃H₇NO₂)₂]·1.5H₂O, comprises a dinuclear complex cation [Re—Re = 2.2494 (3) Å] involving cis-oriented double carboxyl­ate bridges, four equatorial chloride ions and two weakly bonded chloride ligands in the axial positions at the two rhenium(III) atoms. In the crystal, two complex mol­ecules and two water mol­ecules constitute hydrogen-bonded dimers, while an extensive hydrogen-bonding network involving the groups of the zwitterionic ligand is important for generation of the framework. An additional partially occupied water molecule is disordered over two sets of sites about a symmetry centre with a site-occupancy ratio of 0.3:0.2.

Confocal fluorescence microscopy studies of a fluorophore-labeled dirhodium compound: visualizing metal-metal bonded molecules in lung cancer (A549) cells

The new dirhodium compound [Rh2(μ-O2CCH3)2(η(1)-O2CCH3)(phenbodipy)(H2O)3][O2CCH3] (1), which incorporates a bodipy fluorescent tag, was prepared and studied by confocal fluorescence microscopy in human lung adenocarcinoma (A549) cells. It was determined that 1 localizes mainly in lysosomes and mitochondria with no apparent nuclear localization in the 1-100 μM range. These results support the conclusion that cellular organelles rather than the nucleus can be targeted by modification of the ligands bound to the Rh2(4+) core. This is the first study of a fluorophore-labeled metal-metal bonded compound, work that opens up new venues for the study of intracellular distribution of dinuclear transition metal anticancer complexes.