Trackable Metallodrugs Combining Luminescent Re(I) and Bioactive Au(I) Fragments

A noble nexus: a phosphino-phen ligand for tethering precious metals

Controlled formation of mixed-metal bimetallics was achieved via two derivatised 1,10-phenanthroline ligands bearing an imino- or amino-phosphine appendage at the 5-position. Selective coordination of the phen group to the [Re(CO)3Cl] core was achieved enabling precise construction of bimetallic complexes with a second rhenium centre or with gold. The mixed Ru/Au complex was similarly obtained with the imino-phosphine but access to the heterobimetallic iridium systems required prior formation of the P-bound gold complexes subsequent to the introduction of the [Ir(Ppy)2]+ fragment. The Re/Pd, Re/Pt, Ir/Pd and Ir/Pt compounds were prepared from the combination of κ-N'',P-Pd(Pt)Cl2 and the appropriate rhenium or iridium precursors. Spectroscopic and theoretical analyses have been employed to investigate the structural and electronic impact of the second metal.

Welcoming Neighbour or Inhospitable Host? Selective Second Metal Binding in 5- and 6-Phospha-Substituted Bpy Ligands

The controlled formation of mixed-metal bimetallics was realised through use of a fac-[Re(CO)3(N,N'-bpy-P)Cl] complex bearing an exogenous 2,4,6-trioxa-1,3,5,7-tetramethyl-8-phosphaadamantane donor at the 5-position of the bpy. The introduction of gold, silver, and rhodium with appropriate secondary ligands was readily achieved from established starting materials. Restricted rotation about the C(bpy)-P bond was observed in several of the bimetallic complexes and correlated with the relative steric bulk of the second metal moiety. Related chemistry with the 6-substituted derivative proved more limited in scope with only the bimetallic Re/Au complex being isolated.

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.

Synthesis and luminescent properties of hetero-bimetallic and hetero-trimetallic Ru(II)/Au(I) or Ir(III)/Au(I) complexes

A series of Ru(II) and Ir(III) based photoluminescent complexes were synthesised that incorporate an ancillary 2,2'-bipyridine ligand adorned with either one or two pendant N-methyl imidazolium groups. These complexes have been fully characterised by an array of spectroscopic and analytical techniques. One Ir(III) example was unequivocally structurally characterised in the solid state using single crystal X-ray diffraction confirming the proposed formulation and coordination sphere. These complexes were then transformed into their heterometallic, Au(I)-containing, analogues in two steps to yield either bi- or trimetallic complexes that integrate {Au(PPh3)}+ units. X-ray diffraction was used to corroborate the solid state structure of the hetero bimetallic complex, based upon a Ru(II)-Au(I) species. The heterometallic complexes all displayed red photoluminescent features (λem = 616-629 nm) that were consistent with the parent Ru(II) or Ir(III) lumophores in each case. The modulation of the emission from the Ru(II)-Au(I) complexes was much more strongly evident than for the Ir(III)-Au(I) analogues, which is ascribed to the inherent differences in the specific triplet excited state character of the emitting states within each heterometallic species.

Synthesis, Structure, Biological Activity, and Luminescence Properties of a "Butterfly"-Type Silver Cluster with 3-Benzyl-4-phenyl-1,2,4-triazol-5-thiol

A new silver(I) cluster [Ag8L4(Py)(Pype)]·4Py·11H2O (I) with 3-benzyl-4-phenyl-1,2,4-triazol-5-thiol (L) was synthesized via the direct reaction of AgNO3 and L in MeOH, followed by recrystallization from a pyridine-piperidine mixture. The compound I was isolated in a monocrystal form and its crystal structure was determined via single crystal X-ray diffraction. The complex forms a "butterfly" cluster with triazol-5-thioles. The purity of the silver complex and its stability in the solution was confirmed via NMR analysis. Excitation and emission of the free ligand and its silver complex were studied at room temperature for solid samples. The in vitro biological activity of the free ligand and its complex was studied in relation to the non-pathogenic Mycolicibacterium smegmatis strain. Complexation of the free ligand with silver increases the biological activity of the former by almost twenty times. For the newly obtained silver cluster, a bactericidal effect was established.

Rational design of co-ordination compounds in combination of bipyridine type of ligands and group 7 metal (M = Mn, Re) for photoCORM: a DFT study

CONTEXT

A large number of manganese and rhenium tricarbonyl complexes are known in literature along with various applications in different fields. CO-releasing molecules (CORMs) got recent research attention because CO can act as a prodrug for different diseases. CORMs offer the promising prospect of a safe and controllable amount of CO release. In this research work, we have explored the electronic properties of compounds such as bipyridine-related [Mn(CO)3] and [Re(CO)3] and we have compared the electronic properties of both manganese and rhenium tricarbonyl complexes in the light of carbon monoxide releasing tendency. The chosen Mn and Re metals have enough possibility to vary or play with ligands and design a new and novel CORM molecule. In this context, we have taken a range of 4,4'-disubstituted 2,2' bipyridyl ligands (Rbpy, where R = NH2, tBu, OCH3, H, CF3, CN, NO2) to investigate CO's liberation ability to identify and study such molecules. The calculated absorbance of designed complexes (1-14) shows visible/near-IR region (350-850 nm). The HOMO-LUMO energy gap of 7 (ΔE=2.40 eV) complex and for complex 14 (ΔE=2.28 eV) which is lesser in all complexes but the MLCT percentage is greater in Mn tricarbonyl complexes in comparison to Re tricarbonyl complexes. The calculated results of the FMO approach revealed that complex 7 and 14 have the lowest energy gap which is also in good agreement with DOSs and TDM results. The theoretically calculated results revealed that the both Mn and Re tricarbonyl complexes have a tendency for labialization of CO, but Mn tricarbonyl complexes are more prone to CO release because they have higher MLCT percentage.

METHODS

In this research work, we have performed density functional theory (DFT) calculations to explore the physical properties of compounds such as bipyridine-related [Mn(CO)3] and [Re(CO)3] and we have compared the physical properties of both manganese and rhenium tricarbonyl complexes in the light of carbon monoxide releasing tendency. DFT-based calculations were performed by using B3LYP/LANL2DZ basis set followed by acetonitrile solvent using the conductor-like polarizable continuum model (CPCM) for different calculations. Various geometrical calculations were performed using the Gaussian16 suite of programs and the output results obtained from Gaussian16 were visualized using GaussView 5.0.16. The same level of theory was used for various calculations, including frontier molecular orbital (FMO) analysis, metal to ligand charge transfer (MLCT), density of state (DOS) calculations, and transition density of matrix (TDM) calculations. For specific calculations, GaussSum 2.2 software package was used to calculate the density of states, and the Multiwfn 3.8 program was used to analyze the transition density matrix, which is presented using heat maps for both electrons and holes.

Rh(III) and Ru(II) complexes with phosphanyl-alkylamines: inhibition of DNA synthesis induced by anticancer Rh complex

Aim: This investigation was designed to synthesize half-sandwich Rh(III) and Ru(II) complexes and study their antiproliferative activity in human cancer cell lines. Materials & methods: Nine compounds were prepared and tested by various assays for their anticancer activity and mechanism of action. Results: Hit Rh(III) complex 6 showed low-micromolar potency in cisplatin-sensitive (A2780) and -resistant (A2780cis) ovarian carcinoma cell lines, promising selectivity toward these cancer cells over normal lung fibroblasts and an unprecedented mechanism of action in the treated cells. DNA synthesis was decreased and CDKN1A expression was upregulated, but p21 expression was not induced. Conclusion: Rh complex 6 showed high antiproliferative activity, which is induced through a p21-independent mechanism of action.

Luminescent Heterobimetallic PtII-AuI Complexes Bearing N-Heterocyclic Carbenes (NHCs) as Potent Anticancer Agents

This study aims to probe into new series of heterobimetallic PtII-AuI complexes with a general formula of [Pt(p-MeC6H4)(dfppy)(μ-dppm)Au(NHC)]OTf, NHC = IPr, 2; IMes, 3; dfppy = 2-(2,4-difluorophenyl)pyridinate; dppm = 1,1-bis(diphenylphosphino)methane, which are the resultant of the reaction between [Pt(p-MeC6H4)(dfppy)(κ1-dppm)], 1, with [AuCl(NHC)], NHC = IPr, B; IMes, C, in the presence of [Ag(OTf)]. In the heterobimetallic complexes, the dppm ligand is settled between both metals as an unsymmetrical bridging ligand. Several techniques are employed to characterize the resulting compounds. Moreover, the photophysical properties of the complexes are investigated by means of UV-vis and photoluminescence spectroscopy. Furthermore, the experimental study is enriched by ab initio calculations (density functional theory (DFT) and time-dependent DFT (TD-DFT)) to assess the role of Pt and Au moieties in the observed optical properties. It is revealed that 1-3 is luminescent in the solid state and solution at different temperatures. In addition, the achieved results indicate the emissive properties of 1-3 are originated from a mixed 3IL/3MLCT excited state with major contribution of intraligand charge transfer (dfppy). A comparative study is conducted into the cytotoxic activities of starting materials and 1-3 against different human cancer cell lines such as the pancreas (MIA-PaCa2), breast (MDA-MB-231), cervix (HeLa), and noncancerous breast epithelial cell line (MCF-10A). The achieved results suggest the heterobimetallic PtII-AuI species as optimal compounds that signify the existence of cooperative and synergistic effects in their structures. The complex 3 is considered as the most cytotoxic compound with the maximum selectivity index in our screened complex series. Moreover, it is disclosed that 3 effectively causes cell death by inducing apoptosis in MIA-PaCa2 cells. Furthermore, the finding results by fluorescent cell microscopy manifest cytoplasmic staining of 3 rather than nucleus.

Next Generation Gold Drugs and Probes: Chemistry and Biomedical Applications

The gold drugs, gold sodium thiomalate (Myocrisin), aurothioglucose (Solganal), and the orally administered auranofin (Ridaura), are utilized in modern medicine for the treatment of inflammatory arthritis including rheumatoid and juvenile arthritis; however, new gold agents have been slow to enter the clinic. Repurposing of auranofin in different disease indications such as cancer, parasitic, and microbial infections in the clinic has provided impetus for the development of new gold complexes for biomedical applications based on unique mechanistic insights differentiated from auranofin. Various chemical methods for the preparation of physiologically stable gold complexes and associated mechanisms have been explored in biomedicine such as therapeutics or chemical probes. In this Review, we discuss the chemistry of next generation gold drugs, which encompasses oxidation states, geometry, ligands, coordination, and organometallic compounds for infectious diseases, cancer, inflammation, and as tools for chemical biology via gold-protein interactions. We will focus on the development of gold agents in biomedicine within the past decade. The Review provides readers with an accessible overview of the utility, development, and mechanism of action of gold-based small molecules to establish context and basis for the thriving resurgence of gold in medicine.

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.

Non-destructive molecular FTIR spectromicroscopy for real time assessment of redox metallodrugs

Recent emergence of FTIR spectromicroscopy (micro-FTIR) as a dynamic spectroscopy for imaging to study biological chemistry has opened new possibilities for investigating in situ drug release, redox chemistry effects on biological molecules, DNA and drug interactions, membrane dynamics, and redox reactions with proteins at the single cell level. Micro-FTIR applied to metallodrugs has been playing an important role since the last decade because of its great potential to achieve more robust and controlled pharmacological effects against several diseases, including cancer. An important aspect in the development of these drugs is to understand their cellular properties, such as uptake, accumulation, activity, and toxicity. In this review, we present the potential application of micro-FTIR and its importance for studying metal-based drugs, highlighting the perspectives of chemistry of living cells. We also emphasise bioimaging, which is of high importance to localize the cellular processes, for a proper understanding of the mechanism of action.

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.

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.

Base-assisted synthesis of 4-pyridinate gold(I) metallaligands: a study of their use in self-assembly reactions

The synthesis of di- and tritopic gold(i) metallaligands of the type [(Au4-py)2(μ2-diphosphane)] (diphosphane = bis(diphenylphosphanyl)isopropane or dppip (1), 1,2-bis(diphenylphosphanyl)ethane or dppe (2), 1,3-bis(diphenylphosphanyl)propane or dppp (3) and 1,4-bis(diphenylphosphanyl)butane or dppb (4)) and [(Au4-py)3(μ3-triphosphane)] (triphosphane = 1,1,1-tris(diphenylphosphanylmethyl)ethane or triphos (5) and 1,3,5-tris(diphenylphosphanyl)benzene or triphosph (6)) from [(AuCl)2(μ2-diphosphane)] or [(AuCl)3(μ3-triphosphane)] and 4-pyridylboronic acid in the presence of Cs2CO3 has been conducted. Interestingly, when [(AuCl)2(μ2-dppm)] (dppm = bis(diphenylphosphanyl)methane) was used as a starting material, the cyclic tetranuclear gold(i) compound [(Au4-py)2(CH)2{μ2-Au(PPh2)2}2] (I) was obtained instead. All the products have been characterized by IR and multinuclear NMR spectroscopy, mass spectrometry and elemental analysis and in the case of 1, 3, 5 and I by X-ray crystallography, which showed the presence of aurophilic interactions in all of them. The obtained metallaligands have been used as building blocks in self-assembly reactions with cis-blocked palladium or platinum acceptor moieties producing [2 + 2] metallamacrocycles or trigonal bipyramidal (TBP) [2 + 3] metallacages in good yields. The photophysical properties of both the metallaligands and the corresponding assemblies have been investigated.

Near-infrared emitting fluorescent homobimetallic gold(I) complexes displaying promising in vitro and in vivo therapeutic properties

Three near-infrared (NIR-I) optical theranostic systems were synthesized, characterized and studied in vitro and in vivo. These original homo-bimetallic gold(I)-based aza-BODIPY complexes proved to be trackable through near-infrared optical imaging in cells and in mice. They display anti-proliferative properties in micromolar range against human and murine cancer cell lines (4T1, MDA-MB-231, CT26, and SW480). Moreover, the injection of the most promising theranostic agent in CT26 tumor-bearing BALB/c mice induced a significant anti-cancer activity.

2-Azabutadiene complexes of rhenium(I): S,N-chelated species with photophysical properties heavily governed by the ligand hidden traits

The reaction of [Re(CO)3(THF)(μ-Br)]2 or [Re(CO)5X] (X = Cl, Br, I) with the diaryl-2-azabutadienes [(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2] containing two thioether arms at the 4,4-position forms the luminescent S,N-chelate complexes fac-[(OC)3ReX{(RS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CAr2}] (1a-h). The halide abstraction by silver triflate converts [(OC)3ReCl{(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1c) to [(OC)3Re(OS([double bond, length as m-dash]O)2CF3){(PhS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2}] (1j) bearing a covalently bound triflate ligand. The cyclic voltammograms reveal reversible S^N ligand-centred reduction and irreversible oxidation waves for all complexes. The crystal structures of nine octahedral complexes have been determined along with that of (NaphtylS)2C[double bond, length as m-dash]C(H)-N[double bond, length as m-dash]CPh2 (L6). A rich system of weak non-covalent intermolecular secondary interactions through CHX(Cl, Br)Re, CHO, COπ(Ph), CHπCO, CHO and CHS contacts has been evidenced. The photophysical properties have been investigated by steady-state and time-resolved absorption (fs transient absorption, fs-TAS) and emission (ns-TCSPC and ps-Streak camera) spectroscopy in 2-MeTHF solution at 298 and 77 K. The emission bands are composed of either singlet (450 < λmax < 535 nm) and/or triplet emissions (at 77 K only, λmax < 640 nm, or appearing as a tail at λ > 600 nm), which decay in a multiexponential manner for the fluorescence (short ps (i.e. <IRF) < τF < 1.9 ns at 298 and 77 K) and monoexponentially for the phosphorescence (4.0 < τP < 7.0 ns at 77 K). The fs-TAS data reveal the presence of 2 to 4 transient species decaying in four narrow time windows (generally 125-165 fs, 370-685 fs, 3-6 ps, 30-45 ps). The complexity of these kinetics was explained by studying the photophysical behaviour of ligand L6. Its behaviour is the same as the complexes thus indicating that the ligand dictates the kinetic traits of the Re-species, except for the triplet emission as L6 is not phosphorescent. The triplet lifetime (4.0 < τP < 7.0 ns) is considered very short but not unprecedented. Furthermore, the nature of the lowest energy excited states of these chelate compounds and L6 has been addressed using DFT and TDDFT calculations and been assigned to metal-to-ligand (MLCT) and/or intraligand charge-transfer (ILCT).

Theranostics Through the Synergistic Cooperation of Heterometallic Complexes

Heterometallic drugs are emerging as a great alternative to conventional metallodrugs. Careful selection of different metallic fragments makes possible to enhance not only the therapeutic potential by a synergistic effect, but also to incorpore key features like traceability. Drugs that integrate traceability and therapy in one system are known as theranostic agents. In cancer research, theranostic agents are becoming increasingly important. They deliver crucial information regarding their biological interplay that can ultimately be used for optimization. The well-established therapeutic potential of Pt^II -, Ru^II - and Au^I -based drugs combined with the outstanding optical properties of d⁶ transition metal complexes grant the delivery of traceable metallodrugs. These species can be easily fine-tuned through modification of their respective ligands to provide a new generation of drugs.

Anthracene-Based Fluorophore and Its Re(I) Complexes: Investigation of Electrical Properties and Schottky Diode Behavior

A novel fluorophore (HL) [1-((E)-(quinolin-8-ylimino)methyl)anthracen-2-ol] using a suitably designed anthrol and quinoline derivative was synthesized and well characterized. Then, two Re(I) complexes with the fac-[Re(CO)₃] ⁺ moiety were prepared with the ligand under different reaction conditions. Both the complexes [Re(L)(CO)₃] (1) and [Re(HL)(CO)₃Cl] (2) absorbed in the visible region. Steady-state fluorescence measurements and time-correlated single-photon count experiments were performed to elucidate the nature of the excited state. The ground- and excited-state geometries were theoretically investigated using density functional theory (DFT) calculations. The electrical properties of the ligand and the complexes have been explored with the help of a sandwich-structured thin-film device of an Al/sample/indium tin oxide (ITO) configuration at room temperature. The thermionic emission (TE) theory was adopted for the extraction of Schottky diode parameters such as ideality factor, barrier height, and series resistance. Further, the space-charge-limited current (SCLC) theory was employed for a better understanding of the charge transport phenomenon.

Novel ureido-dihydropyridine scaffolds as theranostic agents

In this work, the synthesis of interesting urea derivatives 5 based on 1,4-dihydropyridines 3 is described for the first time. Considering that both families exhibit potential as drugs to treat various diseases, their activity as anticancer agents has been evaluated in HeLa (cervix), Jurkat (leukaemia) and A549 (lung) cancer cell lines as well as on healthy mice in vivo. In general, whereas 1,4-dihydropyridines show a moderate cytotoxic activity, their urea analogues cause an extraordinary increase in their antiproliferative activity, specially towards HeLa cells. Because of the chiral nature of these compounds, enantiomerically enriched samples were also tested, showing different cytotoxic activity than the racemic mixture. Although the reason is not clear, it could be caused by a complex amalgam of physical and chemical contributions. The studied compounds also exhibit luminescent properties, which allow performing a biodistribution study in cancer cells. They have emission maxima between 420 and 471 nm, being the urea derivatives in general red shifted. Emission quenching was observed for those compounds containing a nitro group (3e,f and 5e,f). Fluorescence microscopy showed that 1,4-dihydropyridines 3a and 3g localised in the lysosomes, in contrast to the urea derivatives 5h that accumulated in the cell membrane. This different distribution could be key to explain the differences found in the cytotoxic activity and in the mechanism of action. Interestingly, a preliminary in vivo study regarding the acute toxicity of some of these compounds on healthy mice has been conducted, using a concentration up to 7200 times higher than the corresponding IC₅₀ value. No downgrade in the welfare of the tested mice was observed, which could support their use in preclinical tumour models.

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

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

Luminescent Bimetallic IrIII /AuI Peptide Bioconjugates as Potential Theranostic Agents

Diverse iridium peptide bioconjugates and the corresponding iridium/gold bimetallic complexes have been synthesized starting from a cyclometallated carboxylic acid substituted IrIII complex [Ir(ppy)2 (Phen-5-COO)] by solid phase peptide synthesis (SPPS). The selected peptide sequences were an enkephalin derivative Tyr-Gly-Gly-Phe-Leu together with the propargyl-substituted species Tyr-Gly-Pgl-Phe-Leu to allow gold coordination (Pgl: propyrgyl-glycine, HC≡C-Gly), and a specific short peptide, Ala-Cys-Ala-Phen, containing a cysteine residue. Introduction of the gold center has been achieved via a click reaction with the alkynyl group leading to an organometallic Au-C(triazole) species, or by direct coordination to the sulfur atom of the cysteine. The photophysical properties of these species revealed predominantly an emission originating from the Ir complex, using mixed metal-to-ligand and ligand-to-ligand charge transfer excited states of triplet multiplicity. The formation of the peptide bioconjugates caused a systematic redshift of the emission profiles. Lysosomal accumulation was observed for all the complexes, in contrast to the expected mitochondrial accumulation triggered by the gold complexes. Only the cysteine-containing Ir/Au bioconjugate displayed cytotoxic activity. The absence of activity may be related to the lack of endosomal/lysosomal escape for the cationic peptide conjugates. Interestingly, the different coordination sphere of the gold atom may play a crucial role, as the Au-S(cysteine) bond may be more readily cleaved in a biological environment than the Au-C(triazole) bond, and thus the Au fragment could be released from or trapped in the lysosomes, respectively. This work represents a starting point in the development of bimetallic peptide bioconjugates as theranostics and in the knowledge of factors that contribute to anti-proliferative activity.

Ultrasound-assisted multicomponent synthesis of 4H-pyrans in water and DNA binding studies

A simple approach to synthesize new highly substituted 4H-pyran derivatives is described. Efficient Et3N acts as a readily accessible catalyst of this process performed in pure water and with only a 20 mol% of catalyst loading. The extremely simple operational methodology, short reaction times, clean procedure and excellent product yields render this new approach extremely appealing for the synthesis of 4H-pyrans, as potentially biological scaffolds. Additionally, DNA interaction analysis reveals that 4H-pyran derivatives behave preferably as minor groove binders over major groove or intercalators. Therefore, this is one of the scarce examples where pyrans have resulted to be interesting DNA binders with high binding constants (Kb ranges from 1.53 × 104 M-1 to 2.05 × 106 M-1).

Luminescent Re(I)/Au(I) Species As Selective Anticancer Agents for HeLa Cells

A series of neutral and cationic heterotrimetallic complexes of the type fac-[Re(CO)₃(bipy(CC)₂-(AuL)₂)X]ⁿ, where bipy(CC)₂ is 4,4'-alkynyl-2,2'-bipyridine; L is either triphenylphosphine (PPh₃), [1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene] (IPr), or tert-butyl isocyanide (CN^tBu); and X is a chloride (n = 0) or acetonitrile (n = 1), were synthesized and characterized together with their Re(I) precursors, i.e., fac-[Re(CO)₃(bipy(CC)₂)X]ⁿ. X-ray diffraction of complexes 1, 3, and 6 corroborated the expected octahedral and linear distribution of the ligands along the Re(I) and Au(I) centers, respectively. Luminescent studies showed that all the complexes displayed a broad emission band centered between 565 and 680 nm, corresponding to a ³MLCT from the Re(I) to the diimine derivative. The presence of the gold fragment coordinated to the diimine ligand shifted in all cases the emission maxima toward higher energies. Such an emission difference could be potentially used for assessing the precise moment of interaction of the probe with the biological target if the gold fragment is implicated. Antiproliferative studies in cancer cells, A549 (lung cancer) and HeLa (cervix cancer), showed a generalized selectivity toward HeLa cells for those heterotrimetallic species incubated at longer times (72 vs 24 h). ICP-MS spectrometry revealed the greater cell internalization of cationic vs neutral species. Preliminary fluorescence microscopy experiments showed a different behavior of the complexes in HeLa and A549 cell lines. Whereas the complexes in A549 were randomly distributed in the outside of the cell, those incubated with HeLa cells were located close to the cellular membrane, suggesting some type of interaction, and possibly explaining their cellular selectivity when it comes to the antiproliferative activity displayed in the different cell lines.

Synthesis of mono/dinuclear rhenium(i) tricarbonyl substituted with 4-mercaptopyridine related ligands: spectral and theoretical evidence of thiolate/thione interconversion

κ¹-S complexes show solvent-mediated tautomerism. The S–S bridge cleavage in κ¹-N derivatives is attributed to the presence of a proton source.

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.

Ruthenium(ii) and palladium(ii) homo- and heterobimetallic complexes: synthesis, crystal structures, theoretical calculations and biological studies

Four Ru-Pd heterobimetallic complexes, each one in two different coordination modes (NNSS and NS) having metals connected by a binucleating dialkyldithiooxamidate [N(R)SC-CS(R)N] [R = methyl, ethyl, n-butyl and isopropyl], were prepared by reacting the monochelate [(triⁿpropyl-phosphine)ClPd(HR₂C₂N₂S₂κ-S,S-Pd)] with [(η⁶-p-cymene)RuCl₂]₂. Furthermore, two palladium homobimetallic complexes having two (triⁿpropyl-phosphine)ClPd moieties joined by a diethyldithiooxamidate in both κ-N,S Pd, κ-N',S' Pd' and κ-N,N' Pd, κ-S,S' Pd' coordination modes were synthesized. For both kinds of complexes, homo- and heterobimetallic, at room temperature and in chloroform solution, the NNSS coordination mode (kinetic compounds) turns out to be unstable and therefore the resulting complexes rearrange into a thermodynamically more stable form (NS coordination mode). The crystal structures of [(triⁿpropyl-phosphine)ClPd]₂[μ-(ethyl)₂-DTO κ-N,S Pd, κ-N',S' Pd'] (2) and [(η⁶-p-cymene)ClRu][μ-(methyl)₂-DTO κ-N,S Ru, κ-N,S Pd] [(triⁿpropyl-phosphine)ClPd] (1c) were determined by solid state X-ray crystallography. Moreover, the higher stability of the thermodynamic species in the heterobimetallic complexes (Ru-Pd) was evaluated by means of computational studies in accordance with the maximum hardness principle. All stable NS complexes (i.e.1c-4c, 2 and the previously reported homobimetallic Ru complex 3) were tested against two leukemia cell lines, namely the drug-sensitive CCRF-CEM cell line and its multidrug-resistant sub-cell line CEM/ADR5000 showing anti-proliferative activity in the low micromolar range (∼1-5 μM) and micromolar range (∼10-25 μM), respectively. In addition, these complexes efficaciously block at least two out of the three proteolytic activities of the tumor target 20S proteasome, with heterobimetallic complex 3c and homobimetallic complex 3 possessing the best inhibitory profile.

Bioactive and luminescent indole and isatin based gold(i) derivatives

Combination of bioactive indole and isatin derivatives with Au(i) affords highly cytotoxic metallic species even for cisplatin resistant leukemia cells (Jurkat-shBak).

Bimetallic titanocene-gold phosphane complexes inhibit invasion, metastasis, and angiogenesis-associated signaling molecules in renal cancer

Following promising recent in vitro and in vivo studies of the anticancer efficacies of heterometallic titanocene-gold chemotherapeutic candidates against renal cancer, we report here on the synthesis, characterization, stability studies and biological evaluation of a new titanocene complex containing a gold-triethylphosphane fragment [(η-C₅H₅)₂TiMe(μ-mba)Au(PEt₃)] (4) Titanofin. The compound is more stable in physiological fluid than those previously reported, and it is highly cytotoxic against a line of human clear cell renal carcinoma. We describe here preliminary mechanistic data for this compound and previously reported [(η-C₅H₅)₂TiMe(μ-mba)Au(PPh₃)] (2) Titanocref which displayed remarkable activity in an in vivo mouse model. Mechanistic studies were carried out in the human clear cell renal carcinoma Caki-1 line for the bimetallic compounds [(η-C₅H₅)₂TiMe(μ-mba)Au(PR₃)] (PR₃ = PPh₃2 Titanocref and PEt₃4 Titanofin), the two monometallic gold derivatives [Au(Hmba)(PR₃)] (PR₃ = PPh₃1 cref; PEt₃3 fin), titanocene dichloride and Auranofin as controls. These studies indicate that bimetallic compounds Titanocref (2) and Titanofin (4) are more cytotoxic than gold monometallic derivatives (1 and 3) and significantly more cytotoxic than titanocene dichloride while being quite selective. Titanocref (2) and Titanofin (4) inhibit migration, invasion, and angiogenic assembly along with molecular markers associated with these processes such as prometastatic IL(s), MMP(s), TNF-α, and proangiogenic VEGF, FGF-basic. The bimetallic compounds also strongly inhibit the mitochondrial protein TrxR often overexpressed in cancer cells evading apoptosis and also inhibit FOXC2, PECAM-1, and HIF-1α whose overexpression is linked to resistance to genotoxic chemotherapy. In summary, bimetallic titanocene-gold phosphane complexes (Titanocref 2 and Titanofin 4) are very promising candidates for further preclinical evaluations for the treatment of renal cancer.

Near-unity thermally activated delayed fluorescence efficiency in three- and four-coordinate Au(i) complexes with diphosphine ligands

The synthesis and photoluminescence properties of three-coordinate Au(i) complexes with rigid diphosphine ligands LMe {1,2-bis[bis(2-methylphenyl)phosphino]benzene}, LEt {1,2-bis[bis(2-ethylphenyl)phosphino]benzene}, and LiPr {1,2-bis[bis(2-isopropylphenyl)phosphino]benzene} are investigated. The LMe and LEt ligands afford two types of complexes: dinuclear complexes [μ-LMe(AuCl)2] (1d) and [μ-LEt(AuCl)2] (2d) with an Au(i)-Au(i) bond and mononuclear three-coordinate Au(i) complexes LMeAuCl (1) and LEtAuCl (2). On the other hand, the bulkiest ligand, LiPr, affords three-coordinate Au(i) complexes, LiPrAuCl (3) and LiPrAuI (4), but no dinuclear complexes. X-ray analysis suggests that both 3 and 4 possess a highly distorted trigonal planar geometry. Moreover, luminescence data reveal that at room temperature, 3 and 4 exhibit yellow-green thermally activated delayed fluorescence in the crystalline state with maximum emission wavelengths at 558 and 549 nm, respectively. The emission yields are close to unity. Quantum chemical calculations suggest that the emission of 4 originates from the (σ + X) → π* excited state that possesses strong intraligand charge-transfer character. The luminescent properties of four-coordinate Au(i) complex (5) possessing a tetrahedral geometry are discussed on the basis of the emission spectra and decay times measured in a temperature range of 309-77 K.

A heterometallic ruthenium-gold complex displays antiproliferative, antimigratory, and antiangiogenic properties and inhibits metastasis and angiogenesis-associated proteases in renal cancer

Heterobimetallic compounds are designed to harness chemotherapeutic traits of distinct metal species into a single molecule. The ruthenium-gold (Ru-Au) family of compounds based on Au-N-heterocyclic carbene (NHC) fragments [Cl2(p-cymene)Ru(μ-dppm)Au(NHC)]ClO4 was conceived to combine the known antiproliferative and cytotoxic properties of Au-NHC-based compounds and the antimigratory, antimetastatic, and antiangiogenic characteristic of specific Ru-based compounds. Following recent studies of the anticancer efficacies of these Ru-Au-NHC complexes with promising potential as chemotherapeutics against colorectal, and renal cancers in vitro, we report here on the mechanism of a selected compound, [Cl2(p-cymene)Ru(μ-dppm)Au(IMes)]ClO4 (RANCE-1, 1). The studies were carried out in vitro using a human clear cell renal carcinoma cell line (Caki-1). These studies indicate that bimetallic compound RANCE-1 (1) is significantly more cytotoxic than the Ru (2) or Au (3) monometallic derivatives. RANCE-1 significantly inhibits migration, invasion, and angiogenesis, which are essential for metastasis. RANCE-1 was found to disturb pericellular proteolysis by inhibiting cathepsins, and the metalloproteases MMP and ADAM which play key roles in the etiopathogenesis of cancer. RANCE-1 also inhibits the mitochondrial protein TrxR that is often overexpressed in cancer cells and facilitates apoptosis evasion. We found that while auranofin perturbed migration and invasion to similar degrees as RANCE-1 (1) in Caki-1 renal cancer cells, RANCE-1 (1) inhibited antiangiogenic formation and VEGF expression. We found that auranofin and RANCE-1 (1) have distinct proteolytic profiles. In summary, RANCE-1 constitutes a very promising candidate for further preclinical evaluations in renal cancer.

Heterobimetallic Complexes for Theranostic Applications

The design of more efficient anticancer drugs requires a deeper understanding of their biodistribution and mechanism of action. Cell imaging agents could help to gain insight into biological processes and, consequently, the best strategy for attaining suitable scaffolds in which both biological and imaging properties are maximized. A new concept arises in this field that is the combination of two metal fragments as collaborative partners to provide the precise emissive properties to visualize the cell as well as the optimum cytotoxic activity to build more potent and selective chemotherapeutic agents.

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.