Mono- and Dinuclear Phosphorescent Rhenium(I) Complexes: Impact of Subcellular Localization on Anticancer Mechanisms

Design, synthesis, and antitumor mechanism investigation of iridium(III) complexes conjugated with ibuprofen

The design and synthesis of a series of metal complexes formed by non-steroidal anti-inflammatory drugs (NSAIDs) ibuprofen (IBP) and iridium(III), with the molecular formula [Ir(C^N)2bpy(4-CH2OIBP-4'-CH2OIBP)](PF6) (Ir-IBP-1, Ir-IBP-2) (C^N = 2-phenylpyridine (ppy, Ir-IBP-1), 2-(2-thienyl)pyridine (thpy, Ir-IBP-2)) was introduced in this article. Firstly, it was found that the anti-proliferative activity of these complexes was more effective than that of cisplatin. Further research showed that Ir-IBP-1 and Ir-IBP-2 can accumulate in intracellular mitochondria, thereby disrupting mitochondrial membrane potential (MMP), increasing intracellular reactive oxygen species (ROS), blocking the G2/M phase of the cell cycle, and inducing cell apoptosis. In terms of protein expression, the expression of COX-2, MMP-9, NLRP3 and Caspase-1 proteins can be downregulated, indicating their ability to anti-inflammatory and overcome immune evasion. Furthermore, Ir-IBP-1 and Ir-IBP-2 can induce immunogenic cell death (ICD) by triggering the release of cell surface calreticulin (CRT), high mobility group box 1 (HMGB1) and adenosine triphosphate (ATP). Overall, iridium(III)-IBP conjugates exhibit various anti-tumor mechanisms, including mitochondrial damage, cell cycle arrest, inflammatory suppression, and induction of ICD.

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.

Dinuclear Rhenium(I) Tricarbonyl Complexes as Anticancer Drug Candidates

Cancer is one of the deadliest diseases worldwide. Chemotherapy remains one of the most dominant forms for anticancer treatment. Despite their clinical success, the used chemotherapeutic agents are associated with severe side effect and pharmacological limitations. To overcome these drawbacks there is a need for the development of new types of chemotherapeutic agents. Herein, the chemical synthesis and biological evaluation of dinuclear rhenium(I) complexes as potential chemotherapeutic drug candidates are proposed. The metal complexes were found to be internalized by an energy dependent endocytosis pathway, primary accumulating in the mitochondria. The rhenium(I) complexes demonstrated to induce cell death against a variety of cancer cells in the micromolar range through apoptosis. The lead compound showed to eradicate a pancreatic carcinoma multicellular tumor spheroid at micromolar concentrations.

Synthesis and cellular uptake of neutral rhenium(I) morpholine complexes

Morpholine motifs have been used extensively as targeting moieties for lysosomes, primarily in fluorescence imaging agents. Traditionally these imaging agents are based on organic molecules which have several shortcomings including small Stokes shifts, short emission lifetimes, and susceptibility to photobleaching. To explore alternative lysosome targeting imaging agents we have used a rhenium based phosphorescent platform which has been previously demonstrated to have an improved Stokes shift, a long lifetime emission, and is highly photostable. Rhenium complexes containing morpholine substituted ligands were designed to accumulate in acidic compartments. Two of the three complexes prepared exhibited bright emission in cells, when incubated at low concentrations (20 μM) and were non-toxic at concentrations as high as 100 μM, making them suitable for live cell imaging. We show that the rhenium complexes are amenable to chemical modification and that the morpholine targeted derivatives can be used for live cell confocal fluorescence imaging of endosomes-lysosomes.

Cyclometalated iridium(III) complexes as anti-breast cancer and anti-metastasis agents via STAT3 inhibition

Breast cancer is the most commonly diagnosed cancer and second‑leading cause of cancer deaths in women. Signal transducer and activator of transcription 3 (STAT3) plays a critical role in promoting breast cancer cell proliferation, invasion, angiogenesis, and metastasis, and the high expression of STAT3 is related to the occurrence and poor chemotherapy sensitivity of breast cancer. Iridium(III) complexes Ir-PTS-1- 4 containing a pterostilbene-derived ligand were synthesized to inhibit the STAT3 pathway in breast cancer. Ir-PTS-4 inhibited the proliferation of breast cancer cells by suppressing the expression of phosphorylated STAT3 and STAT3-related cyclin D1, arresting cell cycle in the S-phase, inducing DNA damage and reactive oxygen species (ROS) generation, eventually leading to autophagic cell death. The cell metastasis and invasion were also inhibited after Ir-PTS-4 treatment. Besides, Ir-PTS-4 exhibited excellent anti-proliferation activity in 3D multicellular tumor spheroids, showing potential for the treatment of solid tumors. This work presents the rational design of metal-based anticancer agents to block the STAT3 pathway for simultaneously inhibiting breast cancer proliferation and metastasis.

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.

Novel mono-, bi-, tri- and tetra-nuclear copper complexes that inhibit tumor growth through apoptosis and anti-angiogenesis

To develop the next-generation metal agents for efficiently inhibiting tumor growth, a series of novel mononuclear, binuclear and trinuclear copper (Cu) thiophene-2-formaldehyde thiosemicarbazone complexes and a tetranuclear Cu 1,2,4-triazole-derived complex have been synthesized and their structure-activity relationships have been studied. The trinucleated Cu complex showed the strongest inhibitory activity against T24 cells among all the Cu complexes. Its antitumor effect in vivo was superior to that of cisplatin, with reduced side effects. Further studies on the antitumor mechanism have showed that Cu complexes not only induced apoptosis of cancer cells but also inhibited tumor angiogenesis by inhibiting the migration and invasion of vascular endothelial cells, blocking the cell cycle in the G1 phase, and inducing autophagy.

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.

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.

Metal Complexes for Cancer Sonodynamic Therapy

Sonodynamic therapy (SDT) for cancer treatment is gaining attention owing to its non-invasive property and ultrasound's (US) deep tissue penetration ability. In SDT, US activates the sonosensitizer at the target deep-seated tumors to generate reactive oxygen species (ROS), which ultimately damage tumors. However, drawbacks such as insufficient ROS production, aggregation of sonosensitizer, off-target side effects, etc., of the current organic/nanomaterial-based sonosensitizers limit the effectiveness of cancer SDT. Very recently, metal complexes with tunable physiochemical properties (such as sonostability, HOMO to LUMO energy gap, ROS generation ability, aqueous solubility, emission, etc.) have been devised as effective sonosensitizers, which could overcome the limitations of organic/nanomaterial-based sonosensitizers. This concept introduces all the reported metal-based sonosensitizers and delineates the prospects of metal complexes in cancer sonodynamic therapy. This new concept of metal-based sonosensitizer can deliver next-generation cancer drugs.

Synthesis, characterization, and in vivo evaluation of the anticancer activity of a series of 5- and 6-(halomethyl)-2,2'-bipyridine rhenium tricarbonyl complexes

We report the synthesis, characterization, and in vivo evaluation of the anticancer activity of a series of 5- and 6-(halomethyl)-2,2'-bipyridine rhenium tricarbonyl complexes. The study was promoted in order to understand if the presence and position of a reactive halomethyl substituent on the diimine ligand system of fac-[Re(CO)₃]⁺ species may be a key molecular feature for the design of active and non-toxic anticancer agents. Only compounds potentially able to undergo ligand-based alkylating reactions show significant antiproliferative activity against colorectal and pancreatic cell lines. Of the new species presented in this study, one compound (5-(chloromethyl)-2,2'-bipyridine derivative) shows significant inhibition of pancreatic tumour growth in vivo in zebrafish-Panc-1 xenografts. The complex is noticeably effective at 8 μM concentration, lower than its in vitro IC₅₀ values, being also capable of inhibiting in vivo cancer cells dissemination.

Rhenium(I)-tricarbonyl complexes with methimazole and its selenium analogue: Syntheses, characterization and cell toxicity

This study explores the effect of a thione/selone ligand on the cell toxicity (in vitro) and light activity of diimine Re(CO)₃⁺ complexes. Six rhenium(I) complexes with general formula fac-[Re(CO)₃(N,N')X]⁺ were prepared, where X = 2-mercapto-1-methylimidazole (methimazole; MMI), and 1-methylimidazole-2-selone (MSeI); N,N' = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10-phenanthroline (dmphen). Their triflate salts were characterized using single-crystal X-ray diffraction, ¹H, ¹³C and 2D NMR, UV-vis and vibrational spectroscopy. Their cytotoxic properties were tested, showing significant cytotoxicity (IC₅₀ = 8.0-55 μM) towards the human breast cancer cell line MDA-MB-231. The half-inhibitory concentration (IC₅₀) for fac-[Re(CO)₃(dmphen)(MMI)]⁺, the most toxic complex in this series (8.0 ± 0.2 μM), was comparable to that of the corresponding aqua complex fac-[Re(CO)₃(dmphen)(H₂O)]⁺ with IC₅₀ = 6.0 ± 0.1 μM. The fac-[Re(CO)₃(bpy)(MMI/MSeI)]⁺ complexes were somewhat less toxic towards the human embryonic kidney cell line HEK-293 T after 48 h of exposure. The stability of the complexes upon irradiation was monitored using UV-vis spectroscopy, with no CO released when exposed to UV-A light (λ = 365 nm).

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.

Evaluation of cytotoxic properties of two fluorescent fac-Re(CO)3 complexes bearing an N,N-bidentate benzimidazole coligand

The reaction between 1H-benzimidazol-2-ylmethyl-(N-aryl)amine derivatives (L^R) and [ReBr(CO)₅] afforded octahedral Re(i) complexes of the general formula of [ReBr(CO)₃L^R] (R = 4-Cl and 4-COOCH₃). The Re(i) complexes were screened for their potential cytotoxicity against three malignant cell lines and one normal cell line of different origins. The solvatochromic characteristics of the complexes were examined by UV/vis. spectroscopy with the aid of time-dependent density functional theory calculations. Strong autofluorescence emission can be seen in the two Re(i) complexes between 460 and 488 nm. They appeared to accumulate inside intercellular connections and surrounding cellular membranes. The substances gathered also, along the cell membrane, waiting for their entry. The mode of cell death staining and the DNA fragmentation analysis revealed that the 4-Cl complex showed increased apoptotic changes in the MCF-7, and the Caco-2 cell line, while the HepG2 cell line showed little apoptotic changes.

Post-complexation Functionalization of Cyclometalated Iridium(III) Complexes and Applications to Biomedical and Material Sciences

Cyclometalated iridium(III) (Ir(III)) complexes exhibit excellent photophysical properties that include large Stokes shift, high emission quantum yields, and microsecond-order emission lifetimes, due to low-lying metal-to-ligand charge transfer (spin-forbidden singlet-triplet (³MLCT) transition). As a result, analogs have been applied for research not only in the material sciences, such as the development of organic light-emitting diodes (OLEDs), but also for photocatalysts, bioimaging probes, and anticancer reagents. Although a variety of methods for the synthesis and the applications of functionalized cyclometalated iridium complexes have been reported, functional groups are generally introduced to the ligands prior to the complexation with Ir salts. Therefore, it is difficult to introduce thermally unstable functional groups such as peptides and sugars due to the harsh reaction conditions such as the high temperatures used in the complexation with Ir salts. In this review, the functionalization of Ir complexes after the formation of cyclometalated Ir complexes and their biological and material applications are described. These methods are referred to as "post-complexation functionalization (PCF)." In this review, applications of PCF to the design and synthesis of Ir(III) complexes that exhibit blue -red and white color emissions, luminescence pH probes, luminescent probes of cancer cells, compounds that induce cell death in cancer cells, and luminescent complexes that have long emission lifetimes are summarized.

Novel heterobimetallic Ir(III)-Re(I) complexes: design, synthesis and antitumor mechanism investigation

The reasonable design of binuclear or multinuclear metal complexes has demonstrated their potential advantages in the anticancer field. Herein, three heterobimetallic Ir(III)-Re(I) complexes, [Ir(C^N)₂LRe(CO)₃DIP](PF₆)₂ (C^N = 2-phenylpyridine (ppy, in IrRe-1), 2-(2-thienyl)pyridine (thpy, in IrRe-2) and 2-(2,4-difluorophenyl)pyridine (dfppy, in IrRe-3); L = pyridylimidazo[4,5-f][1,10]phenanthroline; DIP = 4,7-diphenyl-1,10-phenanthroline), were designed and synthesized. The heterobimetallic IrRe-1-3 complexes show pH-sensitive emission properties, which can be used for specific imaging of lysosomes. Additionally, IrRe-1-3 display higher cytotoxicity against tested tumor cell lines than the clinical chemotherapeutic drug cisplatin. Further mechanisms indicate that IrRe-1-3 can induce apoptosis and autophagy, increase intracellular reactive oxygen species (ROS), depolarize the mitochondrial membrane (MMP), block the cell cycle at the G0/G1 phase and inhibit cell migration. To the best of our knowledge, this is the first example of the synthesis of heterobimetallic Ir(III)-Re(I) complexes with superior anticancer activities and evaluation of their anticancer mechanisms.

Induction of Paraptosis by Cyclometalated Iridium Complex-Peptide Hybrids and CGP37157 via a Mitochondrial Ca2+ Overload Triggered by Membrane Fusion between Mitochondria and the Endoplasmic Reticulum

We previously reported that a cyclometalated iridium (Ir) complex-peptide hybrid (IPH) 4 functionalized with a cationic KKKGG peptide unit on the 2-phenylpyridine ligand induces paraptosis, a relatively newly found programmed cell death, in cancer cells (Jurkat cells) via the direct transport of calcium (Ca²⁺) from the endoplasmic reticulum (ER) to mitochondria. Here, we describe that CGP37157, an inhibitor of a mitochondrial sodium (Na⁺)/Ca²⁺ exchanger, induces paraptosis in Jurkat cells via intracellular pathways similar to those induced by 4. The findings allow us to suggest that the induction of paraptosis by 4 and CGP37157 is associated with membrane fusion between mitochondria and the ER, subsequent Ca²⁺ influx from the ER to mitochondria, and a decrease in the mitochondrial membrane potential (ΔΨ_m). On the contrary, celastrol, a naturally occurring triterpenoid that had been reported as a paraptosis inducer in cancer cells, negligibly induces mitochondria-ER membrane fusion. Consequently, we conclude that the paraptosis induced by 4 and CGP37157 (termed paraptosis II herein) proceeds via a signaling pathway different from that of the previously known paraptosis induced by celastrol, a process that negligibly involves membrane fusion between mitochondria and the ER (termed paraptosis I herein).

An ultrasound activated cyanine-rhenium(I) complex for sonodynamic and gas synergistic therapy

A novel cyanine-rhenium(I) tricarbonyl complex is developed as a potent sonosensitizer and sono-activatable CO-releasing agent for synergistic sonodynamic therapy and CO gas therapy of cancer. The complex induced ferroptosis as the mode of cell death.

Anticancer and Antibiotic Rhenium Tri- and Dicarbonyl Complexes: Current Research and Future Perspectives

Organometallic compounds are increasingly recognized as promising anticancer and antibiotic drug candidates. Among the transition metal ions investigated for these purposes, rhenium occupies a special role. Its tri- and dicarbonyl complexes, in particular, attract continuous attention due to their relative ease of preparation, stability and unique photophysical and luminescent properties that allow the combination of diagnostic and therapeutic purposes, thereby permitting, e.g., molecules to be tracked within cells. In this review, we discuss the anticancer and antibiotic properties of rhenium tri- and dicarbonyl complexes described in the last seven years, mainly in terms of their structural variations and in vitro efficacy. Given the abundant literature available, the focus is initially directed on tricarbonyl complexes of rhenium. Dicarbonyl species of the metal ion, which are slowly gaining momentum, are discussed in the second part in terms of future perspective for the possible developments in the field.

Electrochromic behavior of fac-tricarbonyl rhenium complexes

Tricarbonyl rhenium complex shows good electrochromic performance with a colored stage of green, rapid response and good switching stability.

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.

New approach for the synthesis of water soluble fac-[MI(CO)3]+ bis(diarylphosphino)alkylamine complexes (M = 99Tc, Re)

A novel proof-of-concept is reported to modify the water solubility and potential biological effects of a bis(diphenylphosphino)alkylamine (PNP) ligand and the corresponding metal complex, by introducing an amine group on the outer periphery of the pendant ligand arm. Thus, a tertiary butoxycarbonyl protected N'-Boc-ethylenediamine-N,N-bis(diphenylphosphino) (N'-Boc-PNP) ligand (1) was synthesized by reacting the protected ethylenediamine and chlorodiphenylphosphine in a 1 : 2 molar ratio. The corresponding fac-[Re(CO)₃(N'-Boc-PNP)Br] (1A) complex was then obtained by reacting N'-Boc-PNP (1) with (Et₄N)₂fac-[Re(CO)₃Br₃] in equimolar amounts in DCM at 50 °C. De-protection of the N'-Boc pendant amine group in 1A with TFA leads to fac-[Re(NH₃⁺-PNP)(CO)₃Br]·CF₃COO^- (1B) which is soluble in D₂O (>0.05 M). Treating 1B with saturated aqueous NaHCO₃ yields fac-[Re(NH₂-PNP)(CO)₃Br]·MeOH (1C) in near quantitative yield. Although both 1A and 1C are not soluble in D₂O, addition of TFA easily generates 1B (³¹P NMR), confirming the formation of the protonated amine. Isolation of fac-[⁹⁹Tc(CO)₃(N-Boc-PNP)(Cl)] (1D) confirmed that the rhenium and technetium (⁹⁹Tc) can be easily interchanged in this process. Reported are hence the unique rhenium series of compounds 1A, 1B and 1C and the corresponding technetium complex 1D, unequivocally characterized by single crystal XRD, as well as IR and ¹H NMR spectroscopy. Preliminary antimicrobial evaluation indicates that ligand 1 and its respective rhenium complexes (1A-1C) were not active against selected fungi (Candida albicans and Cryptococcus neoformans) and bacteria (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus aureus). These types of ligands and complexes therefore present themselves as excellent radio models for further evaluation using ¹⁸⁶Re, ¹⁸⁸Re and ^99mTc to potentially study the radiotoxicity of appropriately designed complexes.

Cyclometalated Iridium(III) Complex-Cationic Peptide Hybrids Trigger Paraptosis in Cancer Cells via an Intracellular Ca2+ Overload from the Endoplasmic Reticulum and a Decrease in Mitochondrial Membrane Potential

In our previous paper, we reported that amphiphilic Ir complex–peptide hybrids (IPHs) containing basic peptides such as KK(K)GG (K: lysine, G: glycine) (e.g., ASb-2) exhibited potent anticancer activity against Jurkat cells, with the dead cells showing a strong green emission. Our initial mechanistic studies of this cell death suggest that IPHs would bind to the calcium (Ca²⁺)–calmodulin (CaM) complex and induce an overload of intracellular Ca²⁺, resulting in the induction of non-apoptotic programmed cell death. In this work, we conduct a detailed mechanistic study of cell death induced by ASb-2, a typical example of IPHs, and describe how ASb-2 induces paraptotic programmed cell death in a manner similar to that of celastrol, a naturally occurring triterpenoid that is known to function as a paraptosis inducer in cancer cells. It is suggested that ASb-2 (50 µM) induces ER stress and decreases the mitochondrial membrane potential (ΔΨ_m), thus triggering intracellular signaling pathways and resulting in cytoplasmic vacuolization in Jurkat cells (which is a typical phenomenon of paraptosis), while the change in ΔΨ_m values is negligibly induced by celastrol and curcumin. Other experimental data imply that both ASb-2 and celastrol induce paraptotic cell death in Jurkat cells, but this induction occurs via different signaling pathways.

Synthesis of tetranuclear rhenium(I) tricarbonyl metallacycles

Re(I) tricarbonyl complexes have received much attention due to their attractive photochemical, electrochemical, and biological properties. Beyond simple mononuclear complexes, multinuclear assemblies offer greater structural diversity and properties. Despite previous reports on the preparation of di-, tri-, or tetranuclear Re(I) tricarbonyl assemblies, the synthesis of these supramolecular structures remains challenging due to overall low yields or tedious purification protocols. Herein, the facile preparation and characterization of tetranuclear Re(I) tricarbonyl metallacycles with a square geometry is reported using a tetrazole-based ligand. The synthesis of the metallacycle was optimized using different metal precursors, solvents, temperatures, and reagent concentrations. Finally, the scope of suitable tetrazole-based ligands was explored to produce several tetranuclear Re(I) tricarbonyl-based metallacycles.

Mitochondria-targeted artesunate conjugated cyclometalated iridium(iii) complexes as potent anti-HepG2 hepatocellular carcinoma agents

Hepatocellular carcinoma (HCC) poses a serious threat to people's health worldwide. Artesunate (ART), one of the classical antimalarial drugs, has recently been shown to exert significant cytotoxicity in various cancers, but its bioavailability is low. Cyclometalated iridium(iii) complexes have emerged as a promising class of anticancer therapeutic agents. Herein, through conjugation of two of them, three novel Ir(iii)-ART conjugates, [Ir(C-N)2(bpy-ART)](PF6) (bpy = 2,2'-bipyridine, C-N = 2-phenylpyridine (ppy, Ir-ART-1), 2-(2-thienyl)pyridine (thpy, Ir-ART-2), and 2-(2,4-difluorophenyl)pyridine (dfppy, Ir-ART-3)) have been synthesized, and their potential as anti-HCC agents was evaluated. We demonstrate that Ir-ART-1-3 display higher cytotoxicity against HCC cell lines than normal liver cells, and they can especially locate to mitochondria of HepG2 cells and induce a series of mitochondria-mediated apoptosis events. Moreover, Ir-ART-1-3 can regulate the cell cycle and inhibit metastasis of HepG2 cells. Finally, in vivo antitumor evaluation also demonstrates the inhibitory activity of Ir-ART-1 on tumor growth. Taken together, these Ir(iii)-ART conjugates have the potential to become drug candidates for future anti-HCC treatments.

Metal- and metalloid-based compounds to target and reverse cancer multidrug resistance

Drug resistance remains the major cause of cancer treatment failure especially at the late stage of the disease. However, based on their versatile chemistry, metal and metalloid compounds offer the possibility to design fine-tuned drugs to circumvent and even specifically target drug-resistant cancer cells. Based on the paramount importance of platinum drugs in the clinics, two main areas of drug resistance reversal strategies exist: overcoming resistance to platinum drugs as well as multidrug resistance based on ABC efflux pumps. The current review provides an overview of both aspects of drug design and discusses the open questions in the field. The areas of drug resistance covered in this article involve: 1) Altered expression of proteins involved in metal uptake, efflux or intracellular distribution, 2) Enhanced drug efflux via ABC transporters, 3) Altered metabolism in drug-resistant cancer cells, 4) Altered thiol or redox homeostasis, 5) Altered DNA damage recognition and enhanced DNA damage repair, 6) Impaired induction of apoptosis and 7) Altered interaction with the immune system. This review represents the first collection of metal (including platinum, ruthenium, iridium, gold, and copper) and metalloid drugs (e.g. arsenic and selenium) which demonstrated drug resistance reversal activity. A special focus is on compounds characterized by collateral sensitivity of ABC transporter-overexpressing cancer cells. Through this approach, we wish to draw the attention to open research questions in the field. Future investigations are warranted to obtain more insights into the mechanisms of action of the most potent compounds which target specific modalities of drug resistance.

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.

Carbazole effect on ground- and excited-state properties of rhenium(i) carbonyl complexes with extended terpy-like ligands

The ground- and excited-state properties of three novel complexes [ReCl(CO)₃(Lⁿ-κ²N)] bearing 2,2':6',2''-terpyridine, 2,6-di(thiazol-2-yl)pyridine and 2,6-di(pyrazin-2-yl)pyridine functionalized with 9-carbazole attached to the central pyridine ring of the triimine core via phenylene linkage were investigated by spectroscopic and electrochemical methods and were simulated using density functional theory (DFT) and time-dependent DFT. To get a deeper and broader understanding of structure-property relationships, the designed Re(i) carbonyl complexes were compared with previously reported analogous systems - without any groups attached to the phenyl ring and bearing pyrrolidine instead of 9-carbazole. The results indicated that attachment of the N-carbazolyl substituent to the triimine core has less influence on the nature of the triplet excited state of [ReCl(CO)₃(Lⁿ-κ²N)] than the pyrrolidine group. Additionally, the impact of the ligand structural modifications on the light emission of the Re(i) complexes under external voltage was preliminarily examined with electroluminescence spectra of diodes containing the synthesized new molecules in an active layer.

Exploring preliminary structural relationships and mitochondrial targeting of fac-[MI(CO)3]-bis(diarylphosphino)alkylamine complexes (M = 99Tc, Re)

Preliminary structural relationships in fac-[MI(CO)3]-bis(diarylphosphino)alkylamine complexes (M = 99Tc, Re), antimicrobial and mitochondrial targeting are reported.

Evaluation of cyrhetrenyl and ferrocenyl precursors as 5-lipoxygenase inhibitors – biological and computational studies

Synthesis and biological evaluation of precursors derived from ferrocene and cyrhetrene as inhibitors of enzyme 5-hLOX.

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.

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.

Recoding the Cancer Epigenome by Intervening in Metabolism and Iron Homeostasis with Mitochondria-Targeted Rhenium(I) Complexes

The development and malignancy of cancer cells are closely related to the changes of the epigenome. In this work, a mitochondria-targeted rhenium(I) complex (DFX-Re3), integrating the clinical iron chelating agent deferasirox (DFX), has been designed. By relocating iron to the mitochondria and changing the key metabolic species related to epigenetic modifications, DFX-Re3 can elevate the methylation levels of histone, DNA, and RNA. As a consequence, DFX-Re3 affects the events related to apoptosis, RNA polymerases, and T-cell receptor signaling pathways. Finally, it is shown that DFX-Re3 induces immunogenic apoptotic cell death and exhibits potent antitumor activity in vivo. This study provides a new approach for the design of novel epigenetic drugs that can recode the cancer epigenome by intervening in mitochondrial metabolism and iron homeostasis.

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.

Identification of novel potent and non-toxic anticancer, anti-angiogenic and antimetastatic rhenium complexes against colorectal carcinoma

Combination therapy targeting both tumor growth and vascularization is considered to be a cornerstone for colorectal carcinomas (CRC) treatment. However, the major obstacles of most clinical anticancer drugs are their weak selective activity towards cancer cells and inherent inner organs toxicity, accompanied with fast drug resistance development. In our effort to discover novel selective and non-toxic agents effective against CRC, we designed, synthesized and characterized a series of rhenium(I) tricarbonyl-based complexes with increased lipophilicity. Two of these novel compounds were discovered to possess remarkable anticancer, anti-angiogenic and antimetastatic activity in vivo (zebrafish-human HCT-116 xenograft model), being effective at very low doses (1-3 μM). At doses as high as 250 μM the complexes did not provoke toxicity issues encountered in clinical anticancer drugs (cardio-, hepato-, and myelotoxicity). In vivo assays showed that the two compounds exceed the anti-tumor and anti-angiogenic activity of clinical drugs cisplatin and sunitinib malate, and display a large therapeutic window.

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).

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.

Photoluminescence enhancement of Re(i) carbonyl complexes bearing D-A and D-π-A ligands

Three Re(i) carbonyl complexes [ReCl(CO)3(Ln)] bearing 2,2'-bipyridine, 2,2':6',2''-terpyridine, and 1,10-phenanthroline functionalized with diphenylamine/or triphenylamine units (L1-L3) were synthesized to explore the impact of highly electron donating units appended to the imine ligand on the thermal and optoelectronic properties of Re(i) systems. Additionally, for comparison, the ligands L1-3 and parent complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)] were investigated. The thermal stability was evaluated by differential scanning calorimetry. The ground- and excited-state electronic properties of the Re(i) complexes were studied by cyclic voltammetry and differential pulse voltammetry, absorption and emission spectroscopy, as well as using density-functional theory (DFT). The majority of the compounds form amorphous molecular materials with high glass transition temperatures above 100 °C. Compared to the unsubstituted complexes [ReCl(CO)3(bipy)], [ReCl(CO)3(phen)] and [ReCl(CO)3(terpy-κ2N)], the HOMO-LUMO gap of the corresponding Re(i) systems bearing modified imine ligands is reduced, and the decrease in the value of the ΔEH-L is mainly caused by the increase in HOMO energy level. In relation to the parent complexes, all designed Re(i) carbonyls were found to show enhanced photoluminescence, both in solution and in solid state. The investigated ligands and complexes were also preliminarily tested as luminophores in light emitting diodes with the structures ITO/PEDOT:PSS/compound/Al and ITO/PEDOT:PSS/PVK:PBD:compound/Al. The pronounced effect of the ligand chemical structure on electroluminescence ability was clearly visible.

Amphiphilic Cationic Triscyclometalated Iridium(III) Complex-Peptide Hybrids Induce Paraptosis-like Cell Death of Cancer Cells via an Intracellular Ca2+-Dependent Pathway

We report on the design and synthesis of a green-emitting iridium complex-peptide hybrid (IPH) 4, which has an electron-donating hydroxyacetic acid (glycolic acid) moiety between the Ir core and the peptide part. It was found that 4 is selectively cytotoxic against cancer cells, and the dead cells showed a green emission. Mechanistic studies of cell death indicate that 4 induces a paraptosis-like cell death through the increase in mitochondrial Ca²⁺ concentrations via direct Ca²⁺ transfer from ER to mitochondria, the loss of mitochondrial membrane potential (ΔΨ_m), and the vacuolization of cytoplasm and intracellular organelle. Although typical paraptosis and/or autophagy markers were upregulated by 4 through the mitogen-activated protein kinase (MAPK) signaling pathway, as confirmed by Western blot analysis, autophagy is not the main pathway in 4-induced cell death. The degradation of actin, which consists of a cytoskeleton, is also induced by high concentrations of Ca²⁺, as evidenced by costaining experiments using a specific probe. These results will be presented and discussed.

Antiproliferative Activities of Diimine-Based Mixed Ligand Copper(II) Complexes

A series of Cu(diimine)(X-sal)(NO₃) complexes, where the diimine is either 2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen) and X-sal is a monoanionic halogenated salicylaldehyde (X = Cl, Br, I, or H), have been synthesized and characterized by elemental analysis and X-ray crystallography. Penta-coordinate geometries copper(II) were observed for all cases. The influence of the diimine coligands and different halogen atoms on the antiproliferative activities toward human cancer cell lines have been investigated. All Cu(II) complexes were able to induce a loss of A2780 ovarian carcinoma cell viability, with phen derivatives more active than bpy derivatives. In contrast, no in vitro antiproliferative effects were observed against the HCT116 colorectal cancer cell line. These cytotoxicity differences were not due to a different intracellular concentration of the complexes determined by inductively coupled plasma atomic emission spectroscopy. A small effect of different halogen substituents on the phenolic ring was observed, with X = Cl being the most highly active toward A2780 cells among the phen derivatives, while X = Br presented the lowest IC₅₀ in A2780 cells for bpy analogs. Importantly, no reduction in normal primary fibroblasts cell viability was observed in the presence of bpy derivatives (IC₅₀ > 40 μM). Mechanistically, complex 1 seems to induce a stronger apoptotic response with a higher increase in mitochondrial membrane depolarization and an increased level of intracellular reactive oxygen species (ROS) compared to complex 3. Together, these data and the low IC₅₀ compared to cisplatin in A2780 ovarian carcinoma cell line demonstrate the potential of these bpy derivatives for further in vivo studies.