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

Recent Development of Transition Metal Complexes as Chemotherapeutic Hypoxia Activated Prodrug (HAP)

Hypoxia is a state characterized by low concentration of Oxygen. Hypoxic state is often found in the central region of solid tumors. Hypoxia is associated with abnormal neovascularization resulted in poor blood flow in tissues and increased proliferation of tumor cells, imbalance between O2 supply and O2 consumption in tumor cells, high concentration of proton and strong reducibility. And, these abnormalities enhance the survival potency of the hypoxic tumours and increase the resistance towards chemotherapy and radiotherapy. One of the approach for treating hypoxic region of tumour is to use reducing environment of hypoxic tumours for reducing a molecule (hypoxia activated prodrug, HAP) and as a result the active drug will be released in hypoxic region in a controlled manner from the prodrug and kill the hypoxic tumour. Co(III) and Pt(IV) complexes with monodentate active drug molecule in the axial position can be reduced to Co(II) and Pt(II) moieties and as a result, the axial ligands (active drug) could come out from the metal center and could show its anticancer activity. In this review we have highlighted the research articles where transition metal-based complexes are used as chemotherapeutic hypoxia activated prodrug molecules which are reported in last 5 years.

2-Aryl-1H-imidazo[4,5-f][1,10]phenanthroline-Based Binuclear Ru(II)/Ir(III)/Re(I) Complexes as Mitochondria Targeting Cancer Stem Cell Therapeutic Agents

A series of novel Ru(II)/Ir(III)/Re(I)-based organometallic complexes [Ru2L1, Ru2L2, Ir2L1, Ir2L2, Re2L1, and Re2L2] have been synthesized to assess their potency and selectivity against multiple cancer cells A549, HCT-116, and HCT-116 colon CSCs. The cytotoxic screening of the synthesized complexes has revealed that complex Ru2L1 and Ir2L2 are two proficient complexes among all, but Ru2L1 is the most potent complex. A significant binding constant value was observed for DNA and BSA in all complexes. Significant lipophilic properties allow them to penetrate cancer cell membranes, and substantial quantum yield (ϕf) values support bioimaging potential. Again, these complexes are particular for mitochondrial localization and produce a profuse amount of ROS to damage the mitochondrial DNA and then G1 phase cell-cycle arrest. Protein expression analysis unveiled that pro-apoptotic Bax protein overexpressed in Ru2L1-treated cells, whereas antiapoptotic Bcl-2 protein was expressed twofold in Ir2L2-treated cells, which correlated with autophagy reticence.

Novel Re(I) Complexes as Potential Selective Theranostic Agents in Cancer Cells and In Vivo in Caenorhabditis elegans Tumoral Strains

A series of rhenium(I) complexes of the type fac-[Re(CO)3(N^N)L]0/+, Re1-Re9, was synthesized, where N^N = benzimidazole-derived bidentate ligand with an ester functionality and L = chloride or pyridine-type ligand. The new compounds demonstrated potent activity toward ovarian A2780 cancer cells. The most active complexes, Re7-Re9, incorporating 4-NMe2py, exhibited remarkable activity in 3D HeLa spheroids. The emission in the red region of Re9, which contains an electron-deficient benzothiazole moiety, allowed its operability as a bioimaging tool for in vitro and in vivo visualization. Re9 effectivity was tested in two different C. elegans tumoral strains, JK1466 and MT2124, to broaden the oncogenic pathways studied. The results showed that Re9 was able to reduce the tumor growth in both strains by increasing the ROS production inside the cells. Moreover, the selectivity of the compound toward cancerous cells was remarkable as it did not affect neither the development nor the progeny of the nematodes.

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.

Organometallic Ru(II), Rh(III) and Re(I) complexes of sterane-based bidentate ligands: synthesis, solution speciation, interaction with biomolecules and anticancer activity

In this study, we present the synthesis, characterization and in vitro cytotoxicity of six organometallic [Ru(II)(η6-p-cymene)(N,N)Cl]Cl, [Rh(III)(η5-C5Me5)(N,N)Cl]Cl and [Re(I)(CO)3(N,N)Cl] complexes, in which the (N,N) ligands are sterane-based 2,2'-bipyridine derivatives (4-Me-bpy-St-OH, 4-Ph-bpy-St-OH). The solution chemical behavior of the ligands and the complexes was explored by UV-visible spectrophotometry and 1H NMR spectroscopy. The ligands and their Re(I) complexes are neutral at pH = 7.40; this contributes to their highly lipophilic character (log D7.40 > +3). The Ru(II) and Rh(III) half-sandwich complexes are much more hydrophilic, and this property is greatly affected by the actual chloride ion content of the medium. The half-sandwich Ru and Rh complexes are highly stable in 30% (v/v) DMSO/water (<5% dissociation at pH = 7.40); this is further increased in water. The Rh(III)(η5-C5Me5) complexes were characterized by higher water/chloride exchange and pKa constants compared to their Ru(II)(η6-p-cymene) counterparts. The Re(I)(CO)3 complexes are also stable in solution over a wide pH range (2-12) without the release of the bidentate ligand; only the chlorido co-ligand can be replaced with OH- at higher pH values. A comprehensive discussion of the binding affinity of the half-sandwich Ru(II) and Rh(III) complexes toward human serum albumin and calf-thymus DNA is also provided. The Ru(II)(η6-p-cymene) complexes interact with human serum albumin via intermolecular forces, while for the Rh(III)(η5-C5Me5) complexes the coordinative binding mode is suggested as well. They are also able to interact with calf-thymus DNA, most likely via the coordination of the guanine nitrogen. The Ru(II)(η6-p-cymene) complexes were found to be the most promising among the tested compounds as they exhibited moderate-to-strong cytotoxic activity (IC50 = 3-11 μM) in LNCaP as well as in PC3 prostate cells in an androgen receptor-independent manner. They were also significantly cytotoxic in breast and colon adenocarcinoma cancer cell lines and showed good selectivity for cancer cells.

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.

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.

Rhenium(I) Tricarbonyl Complexes of 1,10-Phenanthroline Derivatives with Unexpectedly High Cytotoxicity

Eight rhenium(I) tricarbonyl aqua complexes with the general formula fac-[Re(CO)3(N,N'-bid)(H2O)][NO3] (1-8), where N,N'-bid is (2,6-dimethoxypyridyl)imidazo[4,5-f]1,10-phenanthroline (L1), (indole)imidazo[4,5-f]1,10-phenanthroline (L2), (5-methoxyindole)-imidazo[4,5-f]1,10-phenanthroline (L3), (biphenyl)imidazo[4,5-f]1,10-phenanthroline (L4), (fluorene)imidazo[4,5-f]1,10-phenanthroline (L5), (benzo[b]thiophene)imidazo[4,5-f]1,10-phenanthroline (L6), (5-bromothiazole)imidazo[4,5-f]1,10-phenanthroline (L7), and (4,5-dimethylthiophene)imidazo[4,5-f]1,10-phenanthroline (L8), were synthesized and characterized using 1H and 13C{1H} NMR, FT-IR, UV/Vis absorption spectroscopy, and ESI-mass spectrometry, and their purity was confirmed by elemental analysis. The stability of the complexes in aqueous buffer solution (pH 7.4) was confirmed by UV/Vis spectroscopy. The cytotoxicity of the complexes (1-8) was then evaluated on prostate cancer cells (PC3), showing a low nanomolar to low micromolar in vitro cytotoxicity. Worthy of note, three of the Re(I) tricarbonyl complexes showed very low (IC50 = 30-50 nM) cytotoxic activity against PC3 cells and up to 26-fold selectivity over normal human retinal pigment epithelial-1 (RPE-1) cells. The cytotoxicity of both complexes 3 and 6 was lowered under hypoxic conditions in PC3 cells. However, the compounds were still 10 times more active than cisplatin in these conditions. Additional biological experiments were then performed on the most selective complexes (complexes 3 and 6). Cell fractioning experiments followed by ICP-MS studies revealed that 3 and 6 accumulate mostly in the mitochondria and nucleus, respectively. Despite the respective mitochondrial and nuclear localization of 3 and 6, 3 did not trigger the apoptosis pathways for cell killing, whereas 6 can trigger apoptosis but not as a major pathway. Complex 3 induced a paraptosis pathway for cell killing while 6 did not induce any of our other tested pathways, namely, necrosis, paraptosis, and autophagy. Both complexes 3 and 6 were found to be involved in mitochondrial dysfunction and downregulated the ATP production of PC3 cells. To the best of our knowledge, this report presents some of the most cytotoxic Re(I) carbonyl complexes with exceptionally low nanomolar cytotoxic activity toward prostate cancer cells, demonstrating further the future viability of utilizing rhenium in the fight against cancer.

Metal-Based Complexes in Cancer

Metal-based drugs have attracted growing interest in biomedicine [...].

Luminescent 11-{Naphthalen-1-yl}dipyrido[3,2-a:2',3'-c]phenazine-Based Ru(II)/Ir(III)/Re(I) Complexes for HCT-116 Colorectal Cancer Stem Cell Therapy

Due to a number of unpleasant considerations, marketed drugs have steadily lost their importance in the treatment of cancer. In order to find a viable cancer cell diagnostic agent, we therefore focused on metal complexes that displayed target adequacy, permeability to cancer cells, high standard water solubility, cytoselectivity, and luminescent behavior. In this aspect, luminescent 11-{naphthalen-1-yl} dipyrido [3,2-a:2',3'-c] phenazine based Ru(II)/Ir(III)/Re(I) complexes have been prepared for HCT-116 colorectal cancer stem cell therapy. Our study successfully established the possible cytotoxicity of IrL complex at different doses on HCT-116 colorectal cancer stem cells (CRCSCs). Additionally, an immunochemistry analysis of the complex IrL showed that the molecule was subcellularly localized in the nucleus and other regions of the cytoplasm, where it caused nuclear DNA damage and mitochondrial dysfunction. The level of BAX and Bcl-2 was further quantified by qRT-PCR. The expression of proapoptotic BAX showed increased expression in the complex IrL-treated cell compared to the control, indicating the potential of complex IrL for apoptotic induction. Upon further validation, complex IrL was developed as an inhibitor of autophagy for the eradication of cancer stem cells.

Rhenium(I) Complexes with Neutral Monodentate Coligands and Monoanionic 2-(1,2,4-Triazol-5-yl)pyridine-Based Chelators as Bidentate Luminophores with Tunable Color and Photosensitized Generation of 1O2: An Integrated Case Study Involving Photophysics and Theory

In this work, we describe the synthesis as well as structural, photophysical, and theoretical investigation of a new coordination chemical concept involving rhenium(I) complexes bearing monoanionic 1,2,4-triazolylpyridine-based bidentate chromophores. The X-ray diffractometric analysis of single crystals revealed particular packing features: the trifluoromethylated exemplar displayed two kinds of arrangements of the coordination centers, where the bidentate ligands at the edges of the unit cell are staggered parallel to each other, whereas those inside show antiparallel stacking with respect to the external ligands. On the other hand, the complexes bearing an adamantyl substituent yield a linear arrangement, where the bulky moiety of one luminophore points to the pyridine center of the adjacent ligand of the neighboring complex while including methanol molecules hydrogen-bonded to the triazolato unit. We observed that the photophysical properties of the complexes (photoexcited-state lifetimes, photoluminescence maxima and quantum yields) can be adjusted by tuning of the substitution pattern at the bidentate luminophore as well as by variation of the monodentate coligand. The photoluminescence spectra and photoexcited-state lifetimes of the crystalline phases were measured by phosphorescence lifetime micro(spectro)scopy. Interestingly, the vibrationally resolved emission spectra of the crystals closely resemble those of diluted frozen glassy matrixes at 77 K, in contrast with the broad bands observed in amorphous solids and in fluid solutions, where the charge-transfer character is enhanced. While the photoluminescence quantum yields (Φ_L) reach up to 15%, the complexes are able to attain up to 55% efficiency regarding the photosensitization of ¹O₂ (Φ_Δ), depending on the combination of luminophore and coligand. Theoretical calculations showed that the photoexcited triplet (T₁) state has a metal-ligand-to-ligand charge-transfer character, where promotion to the excited electronic configuration shortens the Re(I)-N bond involving the bidentate triazolylpyridine while stretching the three fac-CO-Re(I) bonds as well as the linkage to the axial monodentate coligand. The calculated vertical (E^vl) and 0-0 (E^(0-0)) radiative transition energies are in very good agreement with the experimental values (E_exp^lum).

Synthesis, Characterization and Antitumor Mechanism Investigation of Heterometallic Ru(Ⅱ)-Re(Ⅰ) Complexes

The development of heteronuclear metal complexes as potent anticancer agents has received increasing attention in recent years. In this study, two new heteronuclear Ru(Ⅱ)-Re(Ⅰ) metal complexes, [Ru(bpy)₂LRe(CO)₃(DIP)](PF₆)₃ and [Ru(phen)₂LRe(CO)₃(DIP)](PF₆)₃ [RuRe-1 and RuRe-2, L = 2-(4-pyridinyl)imidazolio[4,5-f][1,10]phenanthroline, bpy = 2,2′-bipyridine, DIP = 4,7-diphenyl-1,10-phenanthroline, phen = 1,10-phenanthroline], were synthesized and characterized. Cytotoxicity assay shows that RuRe-1 and RuRe-2 exhibit higher anticancer activity than cisplatin, and exist certain selectivity toward human cancer cells over normal cells. The anticancer mechanistic studies reveal that RuRe-1 and RuRe-2 can induce apoptosis through the regulation of cell cycle, depolarization of mitochondrial membrane potential (MMP), elevation of intracellular reactive oxygen species (ROS), and caspase cascade. Moreover, RuRe-1 and RuRe-2 can effectively inhibit cell migration and colony formation. Taken together, heteronuclear Ru(Ⅱ)-Re(Ⅰ) metal complexes possess the prospect of developing new anticancer agents with high efficacy.

Relationship between the oxidative status and the tumor growth in transplanted triple-negative 4T1 breast tumor mice after oral administration of rhenium(I)-diselenoether

BACKGROUND

Selective inhibitory effects of rhenium(I)-diselenoether (Re-diSe) were observed in cultured breast malignant cells. They were attributed to a decrease in Reactive Oxygen Species (ROS) production. A concomitant decrease in the production of Transforming Growth Factor-beta (TGFβ1), Insulin Growth Factor 1 (IGF1), and Vascular Endothelial Growth Factor A (VEGFA) by the malignant cells was also observed.

AIM

The study aimed to investigate the anti-tumor effects of Re-diSe on mice bearing 4T1 breast tumors, an experimental model of triple-negative breast cancer, and correlate them with several biomarkers.

MATERIAL AND METHODS

4T1 mammary breast cancer cells were orthotopically inoculated into syngenic BALB/c Jack mice. Different doses of Re-diSe (1, 10, and 60 mg/kg) were administered orally for 23 consecutive days to assess the efficacy and toxicity. The oxidative status was evaluated by assaying Advanced Oxidative Protein Products (AOPP), and by the dinitrophenylhydrazone (DNPH) test in plasma of healthy mice, non-treated tumor-bearing mice (controls), treated tumor-bearing mice, and tumors in all tumor-bearing mice. Tumor necrosis factor (TNFα), VEGFA, VEGFB, TGFβ1, Interferon, and selenoprotein P (selenoP) were selected as biomarkers.

RESULTS

Doses of 1 and 10 mg/kg did not affect the tumor weights. There was a significant increase in the tumor weights in mice treated with the maximum dose of 60 mg/kg, concomitantly with a significant decrease in AOPP, TNFα, and TGFβ1 in the tumors. SelenoP concentrations increased in the plasma but not in the tumors.

CONCLUSION

We did not confirm the anti-tumor activity of the Re-diSe compound in this experiment. However, the transplantation of the tumor cells did not induce an expected pro-oxidative status without any increase of the oxidative biomarkers in the plasma of controls compared to healthy mice. This condition could be essential to evaluate the effect of an antioxidant drug. The choice of the experimental model will be primordial to assess the effects of the Re-diSe compound in further studies.

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.

V, Kar B, Das U, Paira P. Target-specific mononuclear and binuclear rhenium(i) tricarbonyl complexes as upcoming anticancer drugs

Metal complexes have gradually been attracting interest from researchers worldwide as potential cancer therapeutics. Driven by the many side effects of the popular platinum-based anticancer drug cisplatin, the tireless endeavours of researchers have afforded strategies for the design of appropriate metal complexes with minimal side effects compared to cisplatin and its congeners to limit the unrestricted propagation of cancer. In this regard, transition metal complexes, especially rhenium-based complexes are being identified and highlighted as promising cancer theranostics, which are endowed with the ability to detect and annihilate cancer cells in the body. This is attributed the amazing photophysical properties of rhenium complexes together with their ability to selectively attack different organelles in cancer cells. Therefore, this review presents the properties of different rhenium-based complexes to highlight their recent advances as anticancer agents based on their cytotoxicity results.

In this review, rhenium-based complexes are highlighted as promising cancer theranostics, which are endowed with the ability to detect and annihilate cancer cells in the body.

Origin of Increased Reactivity in Rhenium-Mediated Cycloadditions of Tetrazines

Pyridyl tetrazines coordinated to metals like rhenium have been shown to be more reactive in [4 + 2] cycloadditions than their uncomplexed counterparts. Using density functional theory calculations, we found a more favorable interaction energy caused by stronger orbital interactions as the origin of this increased reactivity. Additionally, the high regioselectivity is due to a greater degree of charge stabilization in the transition state, leading to the major product.

Ru(ii), Ir(iii), Re(i) and Rh(iii) based complexes as next generation anticancer metallopharmaceuticals

Several anticancer drugs such as cisplatin, and its analogues, epirubicin, and doxorubicin are well known for their anticancer activity but the therapeutic value of these drugs comes with certain side effects and they cannot distinguish between normal and cancer cells. Thus, a major challenge for researchers around the world is to develop an anticancer drug with the least toxicity and more target specificity. With the successful reporting of NAMI-A and KP1019, a new path has emerged in the anticancer field. Recently, several Ru(ii) complexes have been reported for their anticancer activity due to their enhanced cellular uptake and selectivity towards cancer cells. Apart from the Ru(ii) complexes, a large amount of research has been carried out with Ir(iii), Re(i), and Rh(iii) based complexes, which exhibited promising anticancer activity. The present review reports various Ru(ii), Ir(iii), Re(i), and Rh(iii) based complexes for their anticancer activity based on their cytotoxicity profiles, biological targets and mechanism of action.

Phosphorescent rhenium(I) complexes conjugated with artesunate: Mitochondrial targeting and apoptosis-ferroptosis dual induction

Cell death is essential for cancer, which can be induced through multiple mechanisms. Ferroptosis, a newly emerging form of non-apoptotic cell death, involves the generation of iron-dependent reactive oxygen species (ROS). In this study, we designed and synthesized two artesunate (ART) conjugated phosphorescent rhenium(I) complexes (Re(I)-ART conjugates), [Re(N^N)(CO)₃(PyCH₂OART)](PF₆) (Re-ART-1 and Re-ART-2) (Py = pyridine, N^N = 1,10-phenanthroline (phen, in Re-ART-1) and 4,7-diphenyl-1,10-phenanthroline (DIP, in Re-ART-2)) that can specifically locate in the mitochondria of human cervical carcinoma (HeLa). Mechanism studies show that Re-ART-1 and Re-ART-2 exhibit high cytotoxicity against cancer cells lines and can induce both apoptosis and ferroptosis in HeLa cells through mitochondrial damage, caspase cascade, glutathione (GSH) depletion, glutathione peroxidase 4 (GPX4) inactivation and lipid peroxidation accumulation. As a result, this work presents the rational design of Re(I)-ART conjugates as a promising strategy to induce both apoptosis and ferroptosis and improve therapeutic efficiency of cancer treatment.

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.

ReI Tricarbonyl Complexes as Coordinate Covalent Inhibitors for the SARS-CoV-2 Main Cysteine Protease

Since its outbreak, the severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) has impacted the quality of life and cost hundreds-of-thousands of lives worldwide. Based on its global spread and mortality, there is an urgent need for novel treatments which can combat this disease. To date, the 3-chymotrypsin-like protease (3CLpro ), which is also known as the main protease, is considered among the most important pharmacological targets. The vast majority of investigated 3CLpro inhibitors are organic, non-covalent binders. Herein, the use of inorganic, coordinate covalent binders is proposed that can attenuate the activity of the protease. ReI tricarbonyl complexes were identified that demonstrate coordinate covalent enzymatic inhibition of 3CLpro . Preliminary studies indicate the selective inhibition of 3CLpro over several human proteases. This study presents the first example of metal complexes as inhibitors for the 3CLpro cysteine protease.

Building C(sp3 ) Molecular Complexity on 2,2'-Bipyridine and 1,10-Phenanthroline in Rhenium Tricarbonyl Complexes

The reactions of [Re(N-N)(CO)₃ (PMe₃ )]OTf (N-N=2,2'-bipyridine, bipy; 1,10-phenanthroline, phen) compounds with tBuLi and with LiHBEt₃ have been explored. Addition to the N-N chelate took place with different site-selectivity depending on both chelate and nucleophile. Thus, with tBuLi, an unprecedented addition to C5 of bipy, a regiochemistry not accessible for free bipy, was obtained, whereas coordinated phen underwent tBuLi addition to C2 and C4. Remarkably, when LiHBEt₃ reacted with [Re(bipy)(CO)₃ (PMe₃ )]OTf, hydride addition to the 4 and 6 positions of bipy triggered an intermolecular cyclodimerization of two dearomatized pyridyl rings. In contrast, hydride addition to the phen analog resulted in partial reduction of one pyridine ring. The resulting neutral Re^I products showed a varied reactivity with HOTf and with MeOTf to yield cationic complexes. These strategies rendered access to Re^I complexes containing bipy- and phen-derived chelates with several C(sp³ ) centers.

Rhenium N-heterocyclic carbene complexes block growth of aggressive cancers by inhibiting FGFR- and SRC-mediated signalling

BACKGROUND

Platinum-based anticancer drugs have been at the frontline of cancer therapy for the last 40 years, and are used in more than half of all treatments for different cancer types. However, they are not universally effective, and patients often suffer severe side effects because of their lack of cellular selectivity. There is therefore a compelling need to investigate the anticancer activity of alternative metal complexes. Here we describe the potential anticancer activity of rhenium-based complexes with preclinical efficacy in different types of solid malignancies.

METHODS

Kinase profile assay of rhenium complexes. Toxicology studies using zebrafish. Analysis of the growth of pancreatic cancer cell line-derived xenografts generated in zebrafish and in mice upon exposure to rhenium compounds.

RESULTS

We describe rhenium complexes which block cancer proliferation in vitro by inhibiting the signalling cascade induced by FGFR and Src. Initially, we tested the toxicity of rhenium complexes in vivo using a zebrafish model and identified one compound that displays anticancer activity with low toxicity even in the high micromolar range. Notably, the rhenium complex has anticancer activity in very aggressive cancers such as pancreatic ductal adenocarcinoma and neuroblastoma. We demonstrate the potential efficacy of this complex via a significant reduction in cancer growth in mouse xenografts.

CONCLUSIONS

Our findings provide a basis for the development of rhenium-based chemotherapy agents with enhanced selectivity and limited side effects compared to standard platinum-based drugs.

Tetrazine metallation boosts rate and regioselectivity of inverse electron demand Diels-Alder (iEDDA) addition of dienophiles

Reported herein is the coordination of rhenium complexes to tetrazine ligand in [ReCl(CO)₃(TzPy)] [1] (TzPy = 3-(2-pyridyl)-1,2,4,5-tetrazine) and the rates of addition of different dienophiles to the tetrazine. Tetrazine coordiation lowers the ΔS^‡ contribution to ΔG^‡ for iEDDA addition.

Recent Emergence of Rhenium(I) Tricarbonyl Complexes as Photosensitisers for Cancer Therapy

Photodynamic therapy (PDT) is emerging as a significant complementary or alternative approach for cancer treatment. PDT drugs act as photosensitisers, which upon using appropriate wavelength light and in the presence of molecular oxygen, can lead to cell death. Herein, we reviewed the general characteristics of the different generation of photosensitisers. We also outlined the emergence of rhenium (Re) and more specifically, Re(I) tricarbonyl complexes as a new generation of metal-based photosensitisers for photodynamic therapy that are of great interest in multidisciplinary research. The photophysical properties and structures of Re(I) complexes discussed in this review are summarised to determine basic features and similarities among the structures that are important for their phototoxic activity and future investigations. We further examined the in vitro and in vivo efficacies of the Re(I) complexes that have been synthesised for anticancer purposes. We also discussed Re(I) complexes in conjunction with the advancement of two-photon PDT, drug combination study, nanomedicine, and photothermal therapy to overcome the limitation of such complexes, which generally absorb short wavelengths.

Influence of the Nucleophilic Ligand on the Reactivity of Carbonyl Rhenium(I) Complexes towards Methyl Propiolate: A Computational Chemistry Perspective

A comparative theoretical study on the reactivity of the complexes [ReY(CO)₃(bipy)] (Y = NH₂, NHMe, NHpTol, OH, OMe, OPh, PH₂, PHMe, PMe₂, PHPh, PPh₂, PMePh, SH, SMe, SPh; bipy = 2,2′-bipyridine) towards methyl propiolate was carried out to analyze the influence of both the heteroatom (N, O, P, S) and the alkyl and/or aryl substituents of the Y ligand on the nature of the product obtained. The methyl substituent tends to accelerate the reactions. However, an aromatic ring bonded to N and O makes the reaction more difficult, whereas its linkage to P and S favour it. On the whole, ligands with O and S heteroatoms seem to disfavour these processes more than ligands with N and P heteroatoms, respectively. Phosphido and thiolato ligands tend to yield a coupling product with the bipy ligand, which is not the general case for hydroxo, alcoxo or amido ligands. When the Y ligand has an O/N and an H atom the most likely product is the one containing a coupling with the carbonyl ligand, which is not always obtained when Y contains P/S. Only for OMe and OPh, the product resulting from formal insertion into the Re-Y bond is the preferred.

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.

Exploring the In Vivo and In Vitro Anticancer Activity of Rhenium Isonitrile Complexes

The established platinum-based drugs form covalent DNA adducts to elicit their cytotoxic response. Although they are widely employed, these agents cause toxic side-effects and are susceptible to cancer-resistance mechanisms. To overcome these limitations, alternative metal complexes containing the rhenium(I) tricarbonyl core have been explored as anticancer agents. Based on a previous study ( Chem. Eur. J. 2019, 25, 9206), a series of highly active tricarbonyl rhenium isonitrile polypyridyl (TRIP) complexes of the general formula fac-[Re(CO)3(NN)(ICN)]+, where NN is a chelating diimine and ICN is an isonitrile ligand, that induce endoplasmic reticulum (ER) stress via activation of the unfolded protein response (UPR) pathway are investigated. A total of 11 of these TRIP complexes were synthesized, modifying both the equatorial polypyridyl and axial isonitrile ligands. Complexes with more electron-donating equatorial ligands were found to have greater anticancer activity, whereas the axial ICN ligands had a smaller effect on their overall potency. All 11 TRIP derivatives trigger a similar phenotype that is characterized by their abilities to induce ER stress and activate the UPR. Lastly, we explored the in vivo efficacy of one of the most potent complexes, fac-[Re(CO)3(dmphen)(ptolICN)]+ (TRIP-1a), where dmphen = 2,9-dimethyl-1,10-phenanthroline and ptolICN = para-tolyl isonitrile, in mice. The 99mTc congener of TRIP-1a was synthesized, and its biodistribution in BALB/c mice was investigated in comparison to the parent Re complex. The results illustrate that both complexes have similar biodistribution patterns, suggesting that 99mTc analogues of these TRIP complexes can be used as diagnostic partner agents. The in vivo antitumor activity of TRIP-1a was then investigated in NSG mice bearing A2780 ovarian cancer xenografts. When administered at a dose of 20 mg/kg twice weekly, this complex was able to inhibit tumor growth and prolong mouse survival by 150% compared to the vehicle control cohort.

Breast Cancer Chemotherapeutic Options: A General Overview on the Preclinical Validation of a Multi-Target Ruthenium(III) Complex Lodged in Nucleolipid Nanosystems

In this review we have showcased the preclinical development of original amphiphilic nanomaterials designed for ruthenium-based anticancer treatments, to be placed within the current metallodrugs approach leading over the past decade to advanced multitarget agents endowed with limited toxicity and resistance. This strategy could allow for new options for breast cancer (BC) interventions, including the triple-negative subtype (TNBC) with poor therapeutic alternatives. BC is currently the second most widespread cancer and the primary cause of cancer death in women. Hence, the availability of novel chemotherapeutic weapons is a basic requirement to fight BC subtypes. Anticancer drugs based on ruthenium are among the most explored and advanced next-generation metallotherapeutics, with NAMI-A and KP1019 as two iconic ruthenium complexes having undergone clinical trials. In addition, many nanomaterial Ru complexes have been recently conceived and developed into anticancer drugs demonstrating attractive properties. In this field, we focused on the evaluation of a Ru(III) complex-named AziRu-incorporated into a suite of both zwitterionic and cationic nucleolipid nanosystems, which proved to be very effective for the in vivo targeting of breast cancer cells (BBC). Mechanisms of action have been widely explored in the context of preclinical evaluations in vitro, highlighting a multitarget action on cell death pathways which are typically deregulated in neoplasms onset and progression. Moreover, being AziRu inspired by the well-known NAMI-A complex, information on non-nanostructured Ru-based anticancer agents have been included in a precise manner.

Dinuclear phosphorescent rhenium(i) complexes as potential anticancer and photodynamic therapy agents

Two dinuclear organometallic Re(i) complexes increase intracellular ROS levels, causing lysosomal dysfunction and cell apoptosis.

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.