Exploring the coordination chemistry of ruthenium complexes with lysozymes: structural and in-solution studies

This study presents a comprehensive structural analysis of the adducts formed upon the reaction of two Ru(III) complexes [HIsq][trans-RuIIICl4(dmso)(Isq)] (1) and [H2Ind][trans-RuIIICl4(dmso)(HInd)] (2) (where HInd-indazole, Isq-isoquinoline, analogs of NAMI-A) and two Ru(II) complexes, cis-[RuCl2(dmso)4] (c) and trans-[RuCl2(dmso)4] (t), with hen-egg white lysozyme (HEWL). Additionally, the crystal structure of an adduct of human lysozyme (HL) with ruthenium complex, [H2Ind][trans-RuCl4(dmso)(HInd)] was solved. X-ray crystallographic data analysis revealed that all studied Ru complexes, regardless of coordination surroundings and metal center charge, coordinate to the same amino acids (His15, Arg14, and Asp101) of HEWL, losing most of their original ligands. In the case of the 2-HL adduct, two distinct metalation sites: (i) Arg107, Arg113 and (ii) Gln127, Gln129, were identified. Crystallographic data were supported by studies of the interaction of 1 and 2 with HEWL in an aqueous solution. Hydrolytic stability studies revealed that both complexes 1 and 2 liberate the N-heterocyclic ligand under crystallization-like conditions (pH 4.5) as well as under physiological pH conditions, and this process is not significantly affected by the presence of HEWL. A comparative examination of nine crystal structures of Ru complexes with lysozyme, obtained through soaking and co-crystallization experiments, together with in-solution studies of the interaction between 1 and 2 with HEWL, indicates that the hydrolytic release of the N-heterocyclic ligand is one of the critical factors in the interaction between Ru complexes and lysozyme. This understanding is crucial in shedding light on the tendency of Ru complexes to target diverse metalation sites during the formation and in the final forms of the adducts with proteins.

Advantageous Reactivity of Unstable Metal Complexes: Potential Applications of Metal-Based Anticancer Drugs for Intratumoral Injections

Injections of highly cytotoxic or immunomodulating drugs directly into the inoperable tumor is a procedure that is increasingly applied in the clinic and uses established Pt-based drugs. It is advantageous for less stable anticancer metal complexes that fail administration by the standard intravenous route. Such hydrophobic metal-containing complexes are rapidly taken up into cancer cells and cause cell death, while the release of their relatively non-toxic decomposition products into the blood has low systemic toxicity and, in some cases, may even be beneficial. This concept was recently proposed for V(V) complexes with hydrophobic organic ligands, but it can potentially be applied to other metal complexes, such as Ti(IV), Ga(III) and Ru(III) complexes, some of which were previously unsuccessful in human clinical trials when administered via intravenous injections. The potential beneficial effects include antidiabetic, neuroprotective and tissue-regenerating activities for V(V/IV); antimicrobial activities for Ga(III); and antimetastatic and potentially immunogenic activities for Ru(III). Utilizing organic ligands with limited stability under biological conditions, such as Schiff bases, further enhances the tuning of the reactivities of the metal complexes under the conditions of intratumoral injections. However, nanocarrier formulations are likely to be required for the delivery of unstable metal complexes into the tumor.

Digging into protein metalation differences triggered by fluorine containing-dirhodium tetracarboxylate analogues

Catalytic and biological properties of dirhodium tetracarboxylates ([Rh2(μ-O2CR)4L2], L=axial ligand, R=CH3-, CH3CH2-, etc) largely depend on the nature of the bridging carboxylate equatorial μ-O2CR ligands, which can be easily exchanged...

Anthracenyl Functionalization of Half-Sandwich Carbene Complexes: In Vitro Anticancer Activity and Reactions with Biomolecules

N-Heterocyclic carbene (NHC) ligands are widely investigated in medicinal inorganic chemistry. Here, we report the preparation and characterization of a series of half-sandwich [M(L)(NHC)Cl₂] (M = Ru, Os, Rh, Ir; L = cym/Cp*) complexes with a N-flanking anthracenyl moiety attached to imidazole- and benzimidazole-derived NHC ligands. The anticancer activity of the complexes was investigated in cell culture studies where, in comparison to a Rh derivative with an all-carbon-donor-atom-based ligand (5a), they were found to be cytotoxic with IC₅₀ values in the low micromolar range. The Ru derivative 1a was chosen as a representative for stability studies as well as for biomolecule interaction experiments. It underwent partial chlorido/aqua ligand exchange in DMSO-d₆/D₂O to rapidly form an equilibrium in aqueous media. The reactions of 1a with biomolecules proceeded quickly and resulted in the formation of adducts with amino acids, DNA, and protein. Hen egg white lysozyme crystals were soaked with 1a, and the crystallographic analysis revealed an interaction with an l-aspartic acid residue (Asp119), resulting in the cleavage of the p-cymene ligand but the retention of the NHC moiety. Cell morphology studies for the Rh analog 3a suggested that the cytotoxicity is exerted via mechanisms different from that of cisplatin.

Insights into the Protein Ruthenation Mechanism by Antimetastatic Metallodrugs: High-Resolution X-ray Structures of the Adduct Formed between Hen Egg-White Lysozyme and at Various Time Points

The pharmacological profile of medicinally relevant Ru(III) coordination compounds has been ascribed to their interactions with proteins, as several studies have provided evidence that DNA is not the primary target. In this regard, numerous spectroscopic and crystallographic studies have indicated that the Ru(III) ligands play an important role in determining the metal binding site, acting as the recognition element in the early stages of the protein-complex formation. Herein, we present a series of near-atomic-resolution X-ray crystal structures of the adducts formed between the antimetastatic metallodrug imidazolium trans-[tetrachlorido(S-dimethyl sufoxide)(1H-imidazole)ruthenate(III)] (NAMI-A) and hen egg-white lysozyme (HEWL). These structures elucidate a series of binding events starting from the noncovalent interaction of intact NAMI-A ions with HEWL (1.5 h), followed by the stepwise exchange of all Ru ligands except for 1H-imidazole (26 h) to the final "ruthenated" protein comprising one aquated Ru ion coordinated to histidine-15 of HEWL (98 h). Our structural data clearly support a two-step mechanism of protein ruthenation, illustrating the ligand-mediated recognition step of the process.

Role of the ancillary ligand in controlling the lysozyme affinity and electronic properties of terpyridine fac-Re(CO)3 complexes

The lysozyme binding affinity and the electronic properties of [ReX(CO)3(terpy-κ2N1,N2)] (X = Br- and triazolateCOOCH2CH3,CF3) were reported. The triazolate complex was prepared in a [3 + 2] cycloaddition click reaction. The bromo compound reacted with lysozyme affording adducts with Re(CO)3+ fragments, while the triazolate compound persisted. A red shift of the MLCT band of the triazolate compound in progressively less polar solvents may be due to the negative solvatochromism.

A Combined Spectroscopic and Protein Crystallography Study Reveals Protein Interactions of RhI(NHC) Complexes at the Molecular Level

While most Rh-N-heterocyclic carbene (NHC) complexes currently investigated in anticancer research contain a Rh(III) metal center, an increasing amount of research is focusing on the cytotoxic activity and mode of action of square-planar [RhCl(COD)(NHC)] (where COD = 1,5-cyclooctadiene) which contains a Rh(I) center. The enzyme thioredoxin reductase (TrxR) and the protein albumin have been proposed as potential targets, but the molecular processes taking place upon protein interaction remain elusive. Herein, we report the preparation of peptide-conjugated and its nonconjugated parent [RhCl(COD)(NHC)] complexes, an in-depth investigation of both their stability in solution, and a crystallographic study of protein interaction. The organorhodium compounds showed a rapid loss of the COD ligand and slow loss of the NHC ligand in aqueous solution. These ligand exchange reactions were reflected in studies on the interaction with hen egg white lysozyme (HEWL) as a model protein in single-crystal X-ray crystallographic investigations. Upon treatment of HEWL with an amino acid functionalized [RhCl(COD)(NHC)] complex, two distinct rhodium adducts were found initially after 7 d of incubation at His15 and after 4 weeks also at Lys33. In both cases, the COD and chlorido ligands had been substituted with aqua and/or hydroxido ligands. While the histidine (His) adduct also indicated a loss of the NHC ligand, the lysine (Lys) adduct retained the NHC core derived from the amino acid l-histidine. In either case, an octahedral coordination environment of the metal center indicates oxidation to Rh(III). This investigation gives the first insight on the interaction of Rh(I)(NHC) complexes and proteins at the molecular level.

Modification of Aβ Peptide Aggregation via Covalent Binding of a Series of Ru(III) Complexes

Alzheimer's disease (AD) is the most common form of dementia, leading to loss of cognition, and eventually death. The disease is characterized by the formation of extracellular aggregates of the amyloid-beta (Aβ) peptide and neurofibrillary tangles of tau protein inside cells, and oxidative stress. In this study, we investigate a series of Ru(III) complexes (Ru-N) derived from NAMI-A in which the imidazole ligand has been substituted for pyridine derivatives, as potential therapeutics for AD. The ability of the Ru-N series to bind to Aβ was evaluated by NMR and ESI-MS, and their influence on the Aβ peptide aggregation process was investigated via electrophoresis gel/western blot, TEM, turbidity, and Bradford assays. The complexes were shown to bind covalently to the Aβ peptide, likely via a His residue. Upon binding, the complexes promote the formation of soluble high molecular weight aggregates, in comparison to peptide precipitation for peptide alone. In addition, TEM analysis supports both amorphous and fibrillar aggregate morphology for Ru-N treatments, while only large amorphous aggregates are observed for peptide alone. Overall, our results show that the Ru-N complexes modulate Aβ peptide aggregation, however, the change in the size of the pyridine ligand does not substantially alter the Aβ aggregation process.

Anticancer Ruthenium(III) Complexes and Ru(III)-Containing Nanoformulations: An Update on the Mechanism of Action and Biological Activity

The great advances in the studies on metal complexes for the treatment of different cancer forms, starting from the pioneering works on platinum derivatives, have fostered an increasingly growing interest in their properties and biomedical applications. Among the various metal-containing drugs investigated thus far, ruthenium(III) complexes have emerged for their selective cytotoxic activity in vitro and promising anticancer properties in vivo, also leading to a few candidates in advanced clinical trials. Aiming at addressing the solubility, stability and cellular uptake issues of low molecular weight Ru(III)-based compounds, some research groups have proposed the development of suitable drug delivery systems (e.g., taking advantage of nanoparticles, liposomes, etc.) able to enhance their activity compared to the naked drugs. This review highlights the unique role of Ru(III) complexes in the current panorama of anticancer agents, with particular emphasis on Ru-containing nanoformulations based on the incorporation of the Ru(III) complexes into suitable nanocarriers in order to enhance their bioavailability and pharmacokinetic properties. Preclinical evaluation of these nanoaggregates is discussed with a special focus on the investigation of their mechanism of action at a molecular level, highlighting their pharmacological potential in tumour disease models and value for biomedical applications.

NAMI-A and KP1019/1339, Two Iconic Ruthenium Anticancer Drug Candidates Face-to-Face: A Case Story in Medicinal Inorganic Chemistry

NAMI-A ((ImH)[trans-RuCl₄(dmso-S)(Im)], Im = imidazole) and KP1019/1339 (KP1019 = (IndH)[trans-RuCl₄(Ind)₂], Ind = indazole; KP1339 = Na[trans-RuCl₄(Ind)₂]) are two structurally related ruthenium(III) coordination compounds that have attracted a lot of attention in the medicinal inorganic chemistry scientific community as promising anticancer drug candidates. This has led to a considerable amount of studies on their respective chemico-biological features and to the eventual admission of both to clinical trials. The encouraging pharmacological performances qualified KP1019 mainly as a cytotoxic agent for the treatment of platinum-resistant colorectal cancers, whereas the non-cytotoxic NAMI-A has gained the reputation of being a very effective antimetastatic drug. A critical and strictly comparative analysis of the studies conducted so far on NAMI-A and KP1019 allows us to define the state of the art of these experimental ruthenium drugs in terms of the respective pharmacological profiles and potential clinical applications, and to gain some insight into the inherent molecular mechanisms. Despite their evident structural relatedness, deeply distinct biological and pharmacological profiles do emerge. Overall, these two iconic ruthenium complexes form an exemplary and unique case in the field of medicinal inorganic chemistry.

Ferritin-based anticancer metallodrug delivery: Crystallographic, analytical and cytotoxicity studies

The encapsulation of anticancer metal-based drugs within a protein nanocage represents a valuable strategy to improve the efficacy and selectivity of these compounds towards cancer cells. The preparation, characterization of the in vitro cytotoxicity and X-ray structures of several ferritin-metallodrug nanocomposites (mainly containing platinum-, ruthenium- and gold-based anticancer agents) are here reviewed. The molecular mechanisms of action of these Ft-metallodrug adducts are discussed and future directions in the field are outlined.

Reaction with Proteins of a Five-Coordinate Platinum(II) Compound

Stable five-coordinate Pt(II) complexes have been highlighted as a promising and original platform for the development of new cytotoxic drugs. Their interaction with proteins has been scarcely studied. Here, the reactivity of the five-coordinate Pt(II) compound [Pt(I)(Me) (dmphen)(olefin)] (Me = methyl, dmphen = 2,9-dimethyl-1,10-phenanthroline, olefin = dimethylfumarate) with the model proteins hen egg white lysozyme (HEWL) and bovine pancreatic ribonuclease (RNase A) has been investigated by X-ray crystallography and electrospray ionization mass spectrometry. The X-ray structures of the adducts of RNase A and HEWL with [Pt(I)(Me)(dmphen)(olefin)] are not of very high quality, but overall data indicate that, upon reaction with RNase A, the compound coordinates the side chain of His105 upon releasing the iodide ligand, but retains the pentacoordination. On the contrary, upon reaction with HEWL, the trigonal bi-pyramidal Pt geometry is lost, the iodide and the olefin ligands are released, and the metal center coordinates the side chain of His15 probably adopting a nearly square-planar geometry. This work underlines the importance of the combined use of crystallographic and mass spectrometry techniques to characterize, in detail, the protein–metallodrug recognition process. Our findings also suggest that five-coordinate Pt(II) complexes can act either retaining their uncommon structure or functioning as prodrugs, i.e., releasing square-planar platinum complexes as bioactive species.

On the pH-Modulated Ru-Based Prodrug Activation Mechanism

The Ru^III-based prodrug AziRu efficiently binds to proteins, but the mechanism of its release is still disputed. Herein, in order to test the hypothesis of a reduction-mediated Ru release from proteins, a Raman-assisted crystallographic study on AziRu binding to a model protein (hen egg white lysozyme), in two different oxidation states, Ru^II and Ru^III, was carried out. Our results indicate Ru reduction, but the Ru release upon reduction is dependent on the reducing agent. To better understand this process, a pH-dependent, spectroelectrochemical surface-enhanced Raman scattering (SERS) study was performed also on AziRu-functionalized Au electrodes as a surrogate and simplest model system of Ru^II- and Ru^III-based drugs. This SERS study provided a p K_a of 6.0 ± 0.4 for aquated AziRu in the Ru^III state, which falls in the watershed range of pH values separating most cancer environments from their physiological counterparts. These experiments also indicate a dramatic shift of the redox potential E₀ by >600 mV of aquated AziRu toward more positive potentials upon acidification, suggesting a selective AziRu reduction in cancer lumen but not in healthy ones. It is expected that the nature of the ligands (e.g., pyridine vs imidazole, present in well-known Ru^III complex NAMI-A) will modulate the p K_a and E₀, without affecting the underlying reaction mechanism.

Preparation, structure, cytotoxicity and mechanism of action of ferritin-Pt(II) terpyridine compound nanocomposites

AIM

A Pt(II)-terpyridine compound, bearing two piperidine substituents at positions 2 and 2' of the terpyridine ligand (1), is highly cytotoxic and shows a mechanism of action distinct from cisplatin. 1 has been incorporated within the ferritin nanocage (AFt).

MATERIALS & METHODS

Spectroscopic and crystallographic data of the Pt(II)-AFt nanocomposite have been collected and in vitro anticancer activity has been explored using cancer cells.

RESULTS

Pt(II)-containing fragments bind His49, His114 and His132. Pt(II)-AFt nanocomposite is less cytotoxic than 1, but it is more toxic than cisplatin at high concentrations. The Pt(II)-AFt nanocomposite triggers necrosis in cancer cells, as free 1 does.

CONCLUSION

Pt(II)-AFt nanocomposites are promising vehicles to deliver Pt-based drugs to cancer cells.

The NAMI A - human ferritin system: a biophysical characterization

The reaction of the antimetastatic ruthenium(iii) drug NAMI A with human H-chain ferritin (HuHf) was investigated through a variety of biophysical methods. We observed that the addition of HuHf to NAMI A solutions significantly increases the rate of spontaneous NAMI A hydrolysis suggesting the occurrence of a direct metallodrug-protein interaction. The resulting hydrolyzed Ru species binds the protein mostly forming a relatively tight 1 : 1 ruthenium/ferritin (subunit) adduct that was then separated and characterized. Notably, this adduct shows a characteristic CD spectrum in the visible region, which is diagnostic of the existence of at least one protein bound ruthenium center. The crystal structure of this NAMI A/HuHf adduct was subsequently solved at 1.58 Å resolution; clear evidence is given for the selective binding of a single Ru ion to His105 of each subunit with concomitant release of all other original Ru ligands in agreement with previous observations. We also noted that NAMI A produces a partial inhibition of HuHf ferroxidase activity. The implications of the above results are discussed.

Synthesis and Characterization of trans-Dichlorotetrakis(imidazole)cobalt(III) Chloride: A New Cobalt(III) Coordination Complex with Potential Prodrug Properties

Numerous therapies for the treatment of cancer have been explored with increasing evidence that the use of metal-containing compounds could prove advantageous as anticancer therapeutics. Previous works on Ru(III) complexes suggest that structurally similar Co(III) complexes may provide good alternative, low-cost, effective prodrugs. Herein, a new complex, trans-[Co(imidazole)₄Cl₂]Cl (2), has been synthesized in high yields utilizing ligand exchange under refluxing conditions. The structure of 2 has been characterized by elemental analysis, ¹H and ¹³C·NMR, ESI-MS, CV, and UV-Vis. The ability of 2 to become reduced in the presence of ascorbic acid was probed demonstrating the likely reduction of the Co(III) metal center to Co(II). In addition, preliminary cell line testing on 2 shows a lack of cytotoxicity.

Principles and methods used to grow and optimize crystals of protein-metallodrug adducts, to determine metal binding sites and to assign metal ligands

The characterization of the interactions between biological macromolecules (proteins and nucleic acids) and metal-based drugs is a fundamental prerequisite for understanding their mechanisms of action. X-ray crystallography enables the structural analysis of such complexes with atomic level detail. However, this approach requires the preparation of highly diffracting single crystals, the measurement of diffraction patterns and the structural analysis and interpretation of macromolecule-metal interactions from electron density maps. In this review, we describe principles and methods used to grow and optimize crystals of protein-metallodrug adducts, to determine metal binding sites and to assign and validate metal ligands. Examples from the literature and experience in our own laboratory are provided and key challenges are described, notably crystallization and molecular model refinement against the X-ray diffraction data.

Protein metalation by metal-based drugs: X-ray crystallography and mass spectrometry studies

The combined use of X-ray crystallography and mass spectrometry represents a valuable strategy to investigate and characterize protein metalation induced by anticancer metal-based drugs. Here, we summarize a series of significant results recently obtained in our laboratories upon the examination of the structures of several adducts of proteins with representative metallodrugs (mostly containing ruthenium, gold and platinum). The general mechanisms of protein metalation that emerge from a careful comparative analysis of these structures are illustrated and their mechanistic implications are discussed. Possible directions for future work in the field are delineated.

Structure of in cell protein crystals containing organometallic complexes

The molecular structures of in cell protein crystals containing organometallic Pd(allyl) complexes were determined by performing microfocus X-ray diffraction experiments. The coordination sites in a polyhedrin mutant with deletion of selected amino acid residues located at the interface of the polyhedrin trimer are dramatically altered compared to those of the wild-type composite.

Exploring the interactions between model proteins and Pd(ii) or Pt(ii) compounds bearing charged N,N-pyridylbenzimidazole bidentate ligands by X-ray crystallography

X-ray structure of the adducts formed between lysozyme and Pd(ii) and Pt(ii) compounds bearing N,N-pyridylbenzimidazole derivatives with an alkylated sulfonate or phosphonium side chain are reported.

A case of extensive protein platination: the reaction of lysozyme with a Pt(ii)–terpyridine complex

The interaction between a Pt(ii)–terpyridine cytotoxic compound and the model protein lysozyme has been investigated by X-ray crystallography and electrospray mass spectrometry under different experimental conditions. The compound shows a high reactivity with the model protein.

Calculation of two-dimensional scattering patterns for oriented systems

A versatile procedure to calculate two-dimensional scattering patterns of oriented systems is presented. The systems are represented by a set of dummy atoms with different scattering length densities, which allows the construction of very complex shapes either for single particles or for sets of particles. By the use of oriented pair distance distribution functions it is possible to perform a fast calculation of the scattering intensity from the oriented system in a given direction in the scattering vector (q) space and generate the two-dimensional scattering pattern on a givenqplane. Several examples of the calculations are presented, demonstrating the method and its applicability. The presented results open new possibilities for the analysis of scattering patters obtained from oriented systems.

Mitochondria Targeting with Luminescent Rhenium(I) Complexes

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

Protein modification via alkyne hydrosilylation using a substoichiometric amount of ruthenium(ii) catalyst

Transition metal catalysis has emerged as a powerful strategy to expand synthetic flexibility of protein modification. Herein, we report a cationic Ru(ii) system that enables the first example of alkyne hydrosilylation between dimethylarylsilanes and O-propargyl-functionalized proteins using a substoichiometric amount or low-loading of Ru(ii) catalyst to achieve the first C-Si bond formation on full-length substrates. The reaction proceeds under physiological conditions at a rate comparable to other widely used bioorthogonal reactions. Moreover, the resultant gem-disubstituted vinylsilane linkage can be further elaborated through thiol-ene coupling or fluoride-induced protodesilylation, demonstrating its utility in further rounds of targeted modifications.

Ruthenium(II) piano stool coordination compounds with aminomethylphosphanes: Synthesis, characterisation and preliminary biological study in vitro

Reaction of {[Ru(η⁶-p-cymene)Cl]₂(μ-Cl)₂} (1) with aminomethylphosphane derived from morpholine (P{CH₂N(CH₂CH₂)₂O}₃ (A), PPh₂{CH₂N(CH₂CH₂)₂O} (B)) or piperazine (P{CH₂N(CH₂CH₂)₂NCH₂CH₃}₃ (C), PPh₂{CH₂N(CH₂CH₂)₂NCH₂CH₃} (D)) results in four new piano stool ruthenium(II) coordination compounds: [Ru(η⁶-p-cymene)Cl₂(A)] (2A), [Ru(η⁶-p-cymene)Cl₂(B)] (2B), [Ru(η⁶-p-cymene)Cl₂(C)] (2C) and [Ru(η⁶-p-cymene)Cl₂(D)] (2D). Every complex was fully characterized using spectroscopic methods (¹H, ¹³C{¹H}, ³¹P{¹H} NMR and ESI-MS), elemental analysis, X-ray single crystal diffraction and DFT calculations. Preliminary studies of in vitro cytotoxicity on the A549 (human lung adenocarcinoma) and MCF7 (human breast adenocarcinoma) cell lines revealed 2A-2D activity in the same order of magnitude as in the case of cisplatin. Additionally, the study confirmed the ability of 2A-2D to interact with DNA helix and transferrin.

Ru-Based CO releasing molecules with azole ligands: interaction with proteins and the CO release mechanism disclosed by X-ray crystallography

Structural data on the adducts formed upon reaction of Ru-based CO releasing molecules containing azole ligands with model proteins are reported.

Impact of cyclometalated ruthenium(II) complexes on lactate dehydrogenase activity and cytotoxicity in gastric and colon cancer cells

Lactate dehydrogenase (LDH) is a redox enzyme often overexpressed in cancer cells allowing their survival in stressful metabolic tumor environment. Ruthenium(II) complexes have been shown to impact on the activity of purified horseradish peroxidase and glucose oxidase but the physiological relevance remains unclear. In this study we investigated how ruthenium complexes impact on the activity of LDH in vitro and in cancer cells and performed a comparative study using polypyridine ruthenium(II) complex [Ru(bpy)₃]²⁺ (1) and its structurally related cyclometalated 2-phenylpyridinato counterpart [Ru(phpy)(bpy)₂]⁺ (2) (bpy=2,2'-bipyridine, phpyH=2-phenylpyridine). We show that the cytotoxicity in gastric and colon cancer cells induced by 2 is significantly higher compared to 1. The kinetic inhibition mechanisms on purified LDH and the corresponding inhibition constants K_i or i_0.5 values were calculated. Though complexes 1 and 2 are structurally very similar (one Ru-C bond in 2 replaces one Ru-N bond in 1), their inhibition modes are different. Cyclometalated complex 2 behaves exclusively as a non-competitive inhibitor of LDH from rabbit muscle (LDH_rm), strongly suggesting that 2 does not interact with LDH in the vicinities of either lactate/pyruvate or NAD⁺/NADH binding sites. Sites of interaction of 1 and 2 with LDH_rm were revealed theoretically through computational molecular docking. Inhibition of LDH activity by 2 was confirmed in cancer cells. Altogether, these results revealed an inhibition of LDH activity by ruthenium complex through a direct interaction structurally tuned by a Ru-C bond.

Interaction of anticancer Ru(III) complexes with single stranded and duplex DNA model systems

The interaction of the anticancer Ru(iii) complex AziRu - in comparison with its analogue NAMI-A, currently in advanced clinical trials as an antimetastatic agent - with DNA model systems, both single stranded and duplex oligonucleotides, was investigated using a combined approach, including absorption UV-vis spectroscopy, circular dichroism (CD) and electrospray mass spectrometry (ESI-MS) techniques. UV-vis absorption spectra of the Ru complexes were recorded at different times in a pseudo-physiological solution, to monitor the ligand exchange processes in the absence and in the presence of the examined oligonucleotides. CD experiments provided information on the overall conformational changes of the DNA model systems induced by these metal complexes. UV- and CD-monitored thermal denaturation studies were performed to analyse the effects of AziRu and NAMI-A on the stability of the duplex structures. ESI-MS experiments, carried out on the oligonucleotide/metal complex mixtures under investigation, allowed us to detect the formation of stable adducts between the guanine-containing oligomers and the ruthenium complexes. These data unambiguously demonstrate that both AziRu and NAMI-A can interact with the DNA model systems. Although very similar in their structures, the two metal compounds manifest a markedly different reactivity with the examined sequences, respectively, with either a naked Ru(3+) ion or a Ru(Im)(3+) (Im = imidazole) fragment being incorporated into the oligonucleotide structure via stable linkages.

Unusual mode of protein binding by a cytotoxic π-arene ruthenium(ii) piano-stool compound containing an O,S-chelating ligand

A new π-arene Ru(ii) piano-stool compound, showing significant cytotoxic activityin vitro, was synthesized. The X-ray structure of this compound and that of its complex with RNase A were determined.

Induction of Cytotoxicity in Pyridine Analogues of the Anti-metastatic Ru(III) Complex NAMI-A by Ferrocene Functionalization

A series of novel ferrocene (Fc) functionalized Ru(III) complexes was synthesized and characterized. These compounds are derivatives of the anti-metastatic Ru(III) complex imidazolium [trans-RuCl4(1H-imidazole) (DMSO-S)] (NAMI-A) and are derived from its pyridine analogue (NAMI-Pyr), with direct coupling of Fc to pyridine at the 4 or 3 positions, or at the 4 position via a two-carbon linker, which is either unsaturated (vinyl) or saturated (ethyl). Electron paramagnetic resonance (EPR) and UV-vis spectroscopic studies of the ligand exchange processes of the compounds in phosphate buffered saline (PBS) report similar solution behavior to NAMI-Pyr. However, the complex with Fc substitution at the 3 position of the coordinated pyridine shows greater solution stability, through resistance to the formation of oligomeric species. Further EPR studies of the complexes with human serum albumin (hsA) indicate that the Fc groups enhance noncoordinate interactions with the protein and help to inhibit the formation of protein-coordinated species, suggesting the potential for enhanced bioavailability. Cyclic voltammetry measurements demonstrate that the Fc groups modestly reduce the reduction potential of the Ru(III) center as compared to NAMI-Pyr, while the reduction potentials of the Fc moieties of the four compounds vary by 217 mV, with the longer linkers giving significantly lower values of E1/2. EPR spectra of the compounds with 2-carbon linkers show the formation of a high-spin Fe(III) species (S = 5/2) in PBS with a distinctive signal at g = 4.3, demonstrating oxidation of the Fe(II) ferrocene center and likely reflecting degradation products. Density functional theory calculations and paramagnetic (1)H NMR describe delocalization of spin density onto the ligands and indicate that the vinyl linker could be a potential pathway for electron transfer between the Ru and Fe centers. In the case of the ethyl linker, electron transfer is suggested to occur via an indirect mechanism enabled by the greater flexibility of the ligand. In vitro assays with the SW480 cell line reveal cytotoxicity induced by the ruthenium ferrocenylpyridine complexes that is at least an order of magnitude higher than the unfunctionalized complex, NAMI-Pyr. Furthermore, migration studies with LNCaP cells reveal that Fc functionalization does not reduce the ability of the compounds to inhibit cell motility. Overall, these studies demonstrate that NAMI-A-type compounds can be functionalized with redox-active ligands to produce both cytotoxic and anti-metastatic activity.

Platinum(II) Complexes with O,S Bidentate Ligands: Biophysical Characterization, Antiproliferative Activity, and Crystallographic Evidence of Protein Binding

We recently characterized a series of novel platinum(II) compounds bearing a conserved O,S binding moiety as a bifunctional ligand and evaluated their solution behavior and antiproliferative properties in vitro against a representative cancer cell line. On the whole, those platinum compounds showed an appreciable stability in mixed dimethyl sulfoxide-aqueous buffers and promising in vitro cytotoxic effects; yet they manifested a rather limited solubility in aqueous media making them poorly suitable for further pharmaceutical development. To overcome this drawback, four new derivatives of this series were prepared and characterized based on a careful choice of substituents on the O,S bidentate ligand. The solubility and stability profile of these novel compounds in a reference buffer was determined, as well as the ligands' log P(o/w) value (P(o/w) = n-octanol-water partition coefficient) as an indirect measure for the complexes' lipophilicity. The antiproliferative properties were comparatively evaluated in a panel of three cancer cell lines. The protein binding properties of the four platinum compounds were assessed using the model protein hen egg white lysozyme (HEWL), and the molecular structures of two relevant HEWL-metallodrug adducts were solved. Overall, it is shown that a proper choice of the substituents leads to a higher solubility and enables a selective fine-tuning of the antiproliferative properties. The implications of these results are thoroughly discussed.

Interactions of carboplatin and oxaliplatin with proteins: Insights from X-ray structures and mass spectrometry studies of their ribonuclease A adducts

Oxaliplatin and carboplatin are two platinum(II) drugs in widespread clinical use for the treatment of various types of cancers; yet, structural information on their interactions with proteins is scarce. Here, the X-ray structures of the adducts formed upon reaction of carboplatin and oxaliplatin with bovine pancreatic ribonuclease (RNase A) are reported and compared with results obtained for the structure of the RNase A-cisplatin adduct derived from isomorphous crystals, under the same experimental conditions. Additional details on the binding mode of these metallodrugs toward RNase A are provided by electrospray ionization mass spectrometry (ESI MS) measurements, thus offering insight on the occurring metal-protein interactions. Notably, while carboplatin and cisplatin mainly bind the side chain of Met29, oxaliplatin also binds the side chains of Asp14, of catalytically important His119 and, to a lesser extent, of His105. On the basis of the available data, a likely mechanism for oxaliplatin hydrolysis and binding to the protein is proposed. These results are potentially useful for a better understanding of the biological chemistry, toxicity and side effects of this important class of antitumor agents.

Albumin binding and ligand-exchange processes of the Ru(III) anticancer agent NAMI-A and its bis-DMSO analogue determined by ENDOR spectroscopy

The ruthenium anticancer compound NAMI-A, imidazolium [trans-RuCl4(1H-imidazole)(DMSO-S)], is currently undergoing advanced clinical evaluation. As with other Ru(iii) chemotherapeutic candidates, interactions with human serum albumin (HSA) have been identified as a key component of the speciation of NAMI-A following intravenous administration. To characterize coordination to HSA, we have performed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopic analysis of deuterium-labelled isotopologues of both NAMI-A and its bis-DMSO analogue, [(DMSO)2H][trans-RuCl4(DMSO-S)2] (Ru-bis-DMSO). Samples were prepared using phosphate buffered saline, in the presence of HSA, and with the individual amino acids histidine, cysteine, and alanine. Analysis of (1)H ENDOR spectra shows characteristic hyperfine interactions from DMSO, water, and imidazole ligands. Furthermore, coordination of imidazole ligands was confirmed from diagnostic (14)N ENDOR signals. Combined with the EPR data from the complexes following incubation in the presence of histidine, the ENDOR data demonstrate that both complexes bind to HSA via histidine imidazoles. Furthermore, the protein-bound species are shown to have water ligands and, in the case of Ru-bis-DMSO, one species has a remaining coordinated DMSO.

Interactions of gold-based drugs with proteins: the structure and stability of the adduct formed in the reaction between lysozyme and the cytotoxic gold(III) compound Auoxo3

The structure and stability of the adduct formed in the reaction between Auoxo3, a dinuclear gold(iii) compound, and the model protein hen egg white lysozyme (HEWL) are investigated by X-ray crystallography, UV-Vis absorption spectroscopy and circular dichroism (CD). It is found that Auoxo3 breaks down completely, undergoes reduction and produces reactive gold(i) species able to bind the protein and form stable derivatives. The behaviour of Auoxo3 is compared with that of two analogous gold(iii) complexes previously studied: a few significant differences are highlighted. The general implications of these new results for the mode of action of cytotoxic gold complexes are discussed.

Protein Recognition of Gold-Based Drugs: 3D Structure of the Complex Formed When Lysozyme Reacts with Aubipy(c.)

The structure of the adduct formed in the reaction between Aubipy(c), a cytotoxic organogold(III) compound, and the model protein hen egg white lysozyme (HEWL) has been solved by X-ray crystallography. It emerges that Aubipy(c), after interaction with HEWL, undergoes reduction of the gold(III) center followed by detaching of the cyclometalated ligand; the resulting naked gold(I) ion is found bound to the protein at Gln121. A direct comparison between the present structure and those previously solved for the lysozyme adducts with other gold(III) compounds demonstrates that coordinated ligands play a key role in the protein-metallodrug recognition process. Structural data support the view that gold(III)-based antitumor prodrugs are activated through metal reduction.

Interactions between anticancer trans-platinum compounds and proteins: crystal structures and ESI-MS spectra of two protein adducts of trans-(dimethylamino)(methylamino)dichloridoplatinum(II)

The adducts formed between trans-(dimethylamino)(methylamino)dichloridoplatinum(II), [t-PtCl2(dma)(ma)], and two model proteins, i.e., hen egg white lysozyme and bovine pancreatic ribonuclease, were independently characterized by X-ray crystallography and electrospray ionization mass spectrometry. In these adducts, the Pt(II) center, upon chloride release, coordinates either to histidine or aspartic acid residues while both alkylamino ligands remain bound to the metal. Comparison with the cisplatin derivatives of the same proteins highlights for [t-PtCl2(dma)(ma)] a kind of biomolecular metalation remarkably different from that of cisplatin.

Unusual structural features in the lysozyme derivative of the tetrakis(acetato)chloridodiruthenium(II,III) complex

The reaction between the paddle-wheel tetrakis(acetato)chloridodiruthenium(II,III) complex, [Ru2(μ-O2CCH3)4Cl] and hen egg-white lysozyme (HEWL) was investigated through ESI-MS and UV/Vis spectroscopy and the formation of a stable metal-protein adduct was unambiguously demonstrated. Remarkably, the diruthenium core is conserved in the adduct while two of the four acetate ligands are released. The crystal structure of this diruthenium-protein derivative was subsequently solved through X-ray diffraction analysis to 2.1 Å resolution. The structural data are in agreement with the solution results. It was found that HEWL binds two diruthenium moieties, at Asp101 and Asp119, respectively, with the concomitant release of two acetate ligands from each diruthenium center.