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

Overcoming Resistance of Caco-2 Cells to 5-Fluorouracil through Diruthenium Complex Encapsulation in PMMA Nanoparticles

Drug resistance, one of the main drawbacks in cancer chemotherapy, can be tackled by employing a combination of drugs that target different biological processes in the cell, enhancing the therapeutic efficacy. Herein, we report the synthesis and characterization of a new paddlewheel diruthenium complex that includes 5-fluorouracil (5-FU), a commonly used anticancer drug. This drug was functionalized with a carboxylate group to take advantage of the previously demonstrated release capacity of carboxylate ligands from the diruthenium core. The resulting hydrophobic complex, [Ru2Cl(DPhF)3(5-FUA)] (Ru-5-FUA) (DPhF = N,N'-diphenylformamidinate; 5-FUA = 5-fluorouracil-1-acetate) was subsequently entrapped in poly(methyl methacrylate) (PMMA) nanoparticles (PMMA@Ru-5-FUA) via a reprecipitation method to be transported in biological media. The optimized encapsulation procedure yielded particles with an average size of 81.2 nm, a PDI of 0.11, and a zeta potential of 29.2 mV. The cytotoxicity of the particles was tested in vitro using the human colon carcinoma cell line Caco-2. The IC50 (half maximal inhibitory concentration) of PMMA@Ru-5-FUA (6.08 μM) was just slightly lower than that found for the drug 5-FU (7.64 μM). Most importantly, while cells seemed to have developed drug resistance against 5-FU, PMMA@Ru-5-FUA showed an almost complete lethality at ∼30 μM. Conversely, an analogous diruthenium complex devoid of the 5-FU moiety, [Ru2Cl(DPhF)3(O2CCH3)] (PMMA@RuA), displayed a reduced cytotoxicity at equivalent concentrations. These findings highlight the effect of combining the anticancer properties of 5-FU with those of diruthenium species. This suggests that the distinct modes of action of the two chemical species are crucial for overcoming drug resistance.

The binding of diruthenium (II,III) and dirhodium (II,II) paddlewheel complexes at DNA/RNA nucleobases: Computational evidences of an appreciable selectivity toward the AU base pairs

Multiple medicinal strategies involve modifications of the structure of DNA or RNA, which disrupt their correct functioning. Metal complexes with medicinal effects, also known as metallodrugs, are among the agents intended specifically for the attack onto nucleosides. The diruthenium (II,III) and dirhodium (II,II) paddlewheel complexes constitute promising dual acting drugs due to their ability to release the therapeutically active bridging ligands upon their substitution by endogenous ligands. In this paper, we study the structure and the stability of the complexes formed by the diruthenium (II,III) and dirhodium (II,II) paddlewheel complexes coordinated in axial positions with the DNA/RNA nucleobases or base pairs, assuming the attainable metalation at all the accessible pyridyl nitrogens. Dirhodium complexes coordinate at the pyridyl nitrogens more strongly than the diruthenium complexes. On the other hand, we found that the diruthenium scaffold binds more selectively to nucleobase targets. Furthermore, we reveal a tighter coordination of diruthenium complex at the adenine-uracil base pair, compared to adenine-thymine, hence constituting a scarce instance of RNA-selectivity. We envision that the here reported computational outcomes may pace future experiments addressing the binding of diruthenium and dirhodium paddlewheel complexes at either single nucleobases or DNA/RNA fragments.

Diruthenium Paddlewheel Complexes Attacking Proteins: Axial versus Equatorial Coordination

Metallodrugs are an important group of medicinal agents used for the treatment of various diseases ranging from cancers to viral, bacterial, and parasitic diseases. Their distinctive features include the availability of a metal centre, redox activity, as well as the ability to multitarget. Diruthenium paddlewheel complexes are an intensely developing group of metal scaffolds, which can securely coordinate bidentate xenobiotics and transport them to target tissues, releasing them by means of substitution reactions with biomolecular nucleophiles. It is of the utmost importance to gain a complete comprehension of which chemical reactions happen with them in physiological milieu to design novel drugs based on these bimetallic scaffolds. This review presents the data obtained in experiments and calculations, which clarify the chemistry these complexes undergo once administered in the proteic environment. This study demonstrates how diruthenium paddlewheel complexes may indeed embody a new paradigm in the design of metal-based drugs of dual-action by presenting and discussing the protein metalation by these complexes.

Steric hindrance and charge influence on the cytotoxic activity and protein binding properties of diruthenium complexes

Paddlewheel diruthenium complexes are being used as metal-based drugs. It has been proposed that their charge and steric properties determine their selectivity towards proteins. Here, we explore these parameters using the first water-soluble diruthenium complex bearing two formamidinate ligands, [Ru2Cl(DPhF)2(O2CCH3)2], and two derivatives, [Ru2Cl(DPhF)(O2CCH3)3] and K2[Ru2(DPhF)(CO3)3] (DPhF- = N,N'-diphenylformamidinate), with one formamidinate. Their protein binding properties have been assessed employing hen egg white lysozyme (HEWL). The results confirm the relationship between the type of interaction (coordinate/non-coordinate bonds) and the charge of diruthenium complexes. The crystallization medium is also a key factor. In all cases, diruthenium species maintain the M-M bond and produce stable adducts. The antiproliferative properties of these diruthenium complexes have been evaluated on an eukaryotic cell-based model. Our data show a correlation between the number of the formamidinate ligands and the anticancer activity of the diruthenium derivatives against human epithelial carcinoma cells. Increased cytotoxicity may be related to increased steric hindrance and Ru25+ core electronic density. However, the effect of increasing the lipophilicity of diruthenium species by introducing a second N,N'-diphenylformamidinate must be also considered. This work illustrates a systematic approach to shed light on the relevant properties of diruthenium compounds to design metal-based metallodrugs and diruthenium metalloenzymes.

Diruthenium(II-III)-ibuprofen-loaded chitosan-based microparticles and nanoparticles systems: encapsulation, characterisation, anticancer activity of the nanoformulations against U87MG human glioma cells

The aim of this work was to investigate novel formulations containing diruthenium(II-III)-ibuprofen (RuIbp) metallodrug encapsulated into the chitosan (CT) biopolymer. Microparticles (RuIbp/CT MPs, ∼ 1 µm) were prepared by spray-drying, and RuIbp/CT-crosslinked nanoparticles (NPs) by ionic gelation (RuIbp/CT-TPP, TPP = tripolyphosphate (1), RuIbp/CT-TPP-PEG, PEG = poly(ethyleneglycol (2)) or pre-gel/polyelectrolyte complex method (RuIbp/CT-ALG, ALG = alginate (3)). Ru analysis was conducted by energy dispersive x-ray fluorescence or inductively coupled plasma atomic emission spectroscopy, and physicochemical characterisation by powder x-ray diffraction, electronic absorption and FTIR spectroscopies, electrospray ionisation mass spectrometry, thermal analysis, scanning electron, transition electron and atomic force microscopies, and dynamic light scattering. The RuIbp-loaded nanosystems exhibited encapsulation efficiency ∼ 20-37%, drug loading∼ 10-20% (w/w), hydrodynamic diameter (nm): 103.2 ± 7.9 (1), 91.7 ± 12.6 (2), 270.2 ± 58.4 (3), zeta potential (mV): +(47.7 ± 2.8) (1), +(49.2 ± 3.6) (2), -(28.2 ± 2.0) (3). Nanoformulation (1) showed the highest cytotoxicity with increased efficacy in relation to the RuIbp free metallodrug against U87MG human glioma cells.

Effect of Equatorial Ligand Substitution on the Reactivity with Proteins of Paddlewheel Diruthenium Complexes: Structural Studies

The paddlewheel [Ru₂Cl(O₂CCH₃)₄] complex was previously reported to react with the model protein hen egg white lysozyme (HEWL), forming adducts with two diruthenium moieties bound to Asp101 and Asp119 side chains upon the release of one acetate. To study the effect of the equatorial ligands on the reactivity with proteins of diruthenium compounds, X-ray structures of the adducts formed when HEWL reacts with [Ru₂Cl(D-p-FPhF)(O₂CCH₃)₃] [D-p-FPhF = N,N'-bis(4-fluorophenyl)formamidinate] under different conditions were solved. [Ru₂Cl(D-p-FPhF)(O₂CCH₃)₃] is bonded through their equatorial positions to the Asp side chains. Protein binding occurs cis or trans to D-p-FPhF. Lys or Arg side chains or even main-chain carbonyl groups can coordinate to the diruthenium core at the axial site. Data help to understand the reactivity of paddlewheel diruthenium complexes with proteins, providing useful information for the design of new artificial diruthenium-containing metalloenzymes with potential applications in the fields of catalysis, biomedicine, and biotechnology.

Kinetics of Reactions of Dirhodium and Diruthenium Paddlewheel Tetraacetate Complexes with Nucleophilic Protein Sites: Computational Insights

Recently, dirhodium and diruthenium paddlewheel complexes have drawn attention as perspective anticancer drugs. In this study, the kinetics of reaction of typical paddlewheel scaffolds Rh₂(μ-O₂CCH₃)₄(H₂O)₂, Ru₂(μ-O₂CCH₃)₄(H₂O)Cl, and [Ru₂(μ-O₂CCH₃)₄(HO)Cl]^- with protein nucleophiles were investigated by means of the density functional theory. The substitution of axial ligands─water and chloride─by the models of protein residue side chains was analyzed, revealing the binding selectivity displayed by these paddlewheel metal scaffolds. The substitution of water is under a thermodynamic control, in which, although the Arg, Cys^-, and Sec^- residues are the most favorable, their binding is expected to be scarcely selective in a biological context. On the other hand, the replacement of the axial water with a more stable hydroxo ligand induces the chloride substitution in diRu complexes, which also targets Arg, Cys^-, and Sec^-, although with a moderately higher activation barrier for any examined protein residue. Additionally, the carried out characterization of the geometrical parameters of the transition states permitted determination of the impact of an increased steric hindrance of diRh and diRu complexes on their protein site selectivity. This study corroborates the idea of the substitution of the acetate ligands with biologically active, but more hindering, carboxylate ligands, in order to yield dual acting metallodrugs. This study allows us to assume that the delivery of diRu paddlewheel complexes in their monoanionic form [Ru₂(μ-O₂CR)₄(OH)Cl]^- decorated by the bulky substituents R may constitute an approach to augment the selectivity toward anticancer targets, such as TrxR in tumor cells, although under the condition that such a selectivity is operative only in high pH conditions.

Binding of VIV O2+ , VIV OL, VIV OL2 and VV O2 L Moieties to Proteins: X-ray/Theoretical Characterization and Biological Implications

Vanadium compounds have frequently been proposed as therapeutics, but their application has been hampered by the lack of information on the different V-containing species that may form and how these interact with blood and cell proteins, and with enzymes. Herein, we report several resolved crystal structures of lysozyme with bound V^IV O²⁺ and V^IV OL²⁺ , where L=2,2'-bipyridine or 1,10-phenanthroline (phen), and of trypsin with V^IV O(picolinato)₂ and V^V O₂ (phen)⁺ moieties. Computational studies complete the refinement and shed light on the relevant role of hydrophobic interactions, hydrogen bonds, and microsolvation in stabilizating the structure. Noteworthy is that the trypsin-V^V O₂ (phen) and trypsin-V^IV O(OH)(phen) adducts correspond to similar energies, thus suggesting a possible interconversion under physiological/biological conditions. The obtained data support the relevance of hydrolysis of V^IV and V^V complexes in the several types of binding established with proteins and the formation of different adducts that might contribute to their pharmacological action, and significantly widen our knowledge of vanadium-protein interactions.

New insights into progressive ligand replacement from [Ru2Cl(O2CCH3)4]: synthetic strategies and variation in redox potentials and paramagnetic shifts

The complete series of [Ru₂Cl(Dp-FPhF)_x(O₂CCH₃)_4-x] (x = 1-4; Dp-FPhF^- = N,N'-bis(4-fluorophenyl)formamidinate) compounds, has been prepared and characterized by a multi-technique approach, including single crystal X-ray diffraction. A careful study of the different methodologies has allowed us to prepare four compounds with good yields and without an inert atmosphere or further purification. Specifically, [Ru₂Cl(Dp-FPhF)(O₂CCH₃)₃] (1) was obtained using an ultrasound-assisted (USS) method, while [Ru₂Cl(Dp-FPhF)₄] (4) was prepared by microwave assisted solvothermal synthesis (MWS). The intermediate substitution products cis-[Ru₂Cl(Dp-FPhF)₂(O₂CCH₃)₂] (2) and [Ru₂Cl(Dp-FPhF)₃(O₂CCH₃)] (3) have been prepared by conventional heating, controlling the molar ratio of the starting materials. ESI-MS and infrared spectroscopy were used to follow all the reactions and permitted a qualitative evaluation of the axial reactivity in this series. Magnetic and absorption measurements confirmed a high spin σ²π⁴δ²(π*δ*)³ electronic configuration in all cases. However, the effect of the gradual modification of the electronic density in the diruthenium core markedly affects other properties. The cyclic voltammograms of the compounds show a strong decrease in the one electron oxidation potential and an increase in the reduction potential in the series from 1 to 4. Furthermore, despite their paramagnetic nature, ¹H- and ¹⁹F-NMR spectra were recorded, and a correlation between the paramagnetic shift of the signals and the substitution degree of the diruthenium species was observed. These results provide a comprehensive guide to synthesise and understand the effects of equatorial ligand substitution on the properties of Ru₂⁵⁺ compounds.

Reactivity of a fluorine-containing dirhodium tetracarboxylate compound with proteins

Dirhodium complexes of general formula [Rh₂(O₂CR)₄]L₂ are a well-known class of bimetallic compounds that are used as efficient catalysts for a variety of reactions and have been shown to be potent antibacterial and anticancer agents. The catalytic and biological properties of these complexes largely depend on the nature of the bridging carboxylate ligands. Trifluoroacetate (tfa)-containing dirhodium compounds have been used to build artificial metalloenzymes upon reaction with peptides and have been shown to be more cytotoxic than dirhodium tetraacetate. However, there is no structural information on the interaction between these compounds and proteins. Here, cis-Rh₂(μ-O₂CCH₃)₂(μ-O₂CCF₃)₂ ([cis-Rh₂(OAc)₂(tfa)₂]) has been synthesized and its reaction with bovine pancreatic ribonuclease (RNase A) and hen egg white lysozyme (HEWL) was analyzed using a combination of different techniques, including Fluorine-19 nuclear magnetic resonance spectroscopy and macromolecular X-ray crystallography, with the aim to unveil the differences in the reactivity of tfa-containing dihrodium complexes with proteins when compared to [Rh₂(OAc)₄]. [cis-Rh₂(OAc)₂(tfa)₂] and [Rh₂(OAc)₄] bind the N atoms of His side chains of RNase A at the axial position; however the fluorine-containing compound rapidly loses its tfa ligands, while [Rh₂(OAc)₄] can retain the acetate ligands upon protein binding. The reactivity of [cis-Rh₂(OAc)₂(tfa)₂] with HEWL is slightly distinct when compared to that of [Rh₂(OAc)₄] under the same experimental conditions; however, both [cis-Rh₂(OAc)₂(tfa)₂] and [Rh₂(OAc)₄] degrade when soaked within HEWL crystals. These results provide a structural-based guide for the design of new heterogenous chiral dirhodium/peptide and dirhodium/protein adducts with application in the fields of organic synthesis and asymmetric catalysis.

Steric, Activation Method and Solvent Effects on the Structure of Paddlewheel Diruthenium Complexes

Conventional heating and solvothermal synthetic methods (with or without microwave activation) have been used to study the reaction of o-, m- and p-methoxybenzoic acid with [Ru2Cl(μ-O2CMe)4]. The tetrasubstituted series [Ru2Cl(µ-O2CC6H4-R)4], with R = o-OMe, m-OMe and p-OMe, has been prepared by the three procedures. Depending on the synthetic method and the experimental conditions, three compounds have been isolated (1a, 1b, 1c) with the o-methoxybenzoate ligand. However, with the m- and p-methoxybenzoate ligands, only the complexes 2 and 3 have been obtained, respectively. Compound 1a, with stoichiometry [Ru2Cl(µ-O2CC6H4-o-OMe)4]n, shows a polymeric structure with the chloride ions bridging the diruthenium units to form linear chains. Compounds 2 and 3, with the same stoichiometry, predictably form zig-zag chains in accordance with their insolubility and their magnetic measurements. Compound 1b, [Ru2Cl(µ-O2CC6H4-o-OMe)4(EtOH)], is a discrete molecular species with a chloride ion and one ethanol molecule occupying the axial positions of the dimetallic unit. Compound 1c is a cation-anion complex, [Ru2(µ-O2CC6H4-o-OMe)4(MeOH)2][Ru2Cl2(µ-O2CC6H4-o-OMe)4]. The cationic complex has two solvent molecules at the axial positions whereas the anionic complex has two chloride ligands at these positions. Complexes have been characterized by elemental analyses, mass spectrometry and IR and UV-vis-NIR spectroscopies. A magnetic study of complexes 1a, 1b, 2 and 3 have also been carried out. The crystal structure of compounds 1b and 1c have been solved by single X-ray crystal methods.

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...

Ultrasound-assisted synthesis of water-soluble monosubstituted diruthenium compounds

The elusive monosubstituted diruthenium complexes [Ru₂Cl(DAniF)(O₂CMe)₃] (1), [Ru₂Cl(DPhF)(O₂CMe)₃] (2), [Ru₂Cl(D-p-CNPhF)(O₂CMe)₃] (3), [Ru₂Cl(D-o-TolF)(O₂CMe)₃] (4), [Ru₂Cl(D-m-TolF)(O₂CMe)₃] (5), [Ru₂Cl(D-p-TolF)(O₂CMe)₃] (6) and [Ru₂Cl(p-TolA)(O₂CMe)₃] (7) have been synthesized using for the first time ultrasound-assisted synthesis to carry out a substitution reaction in metal-metal bonded dinuclear compounds (DAniF^- = N,N'-bis(4-anisyl)formamidinate; DPhF^- = N,N'-diphenylformamidinate; D-p-CNPhF^- = N,N'-bis(4-cyanophenyl)formamidinate; D-o/m/p-TolF^- = N,N'-bis(2/3/4-tolyl)formamidinate; p-TolA^- = N-4-tolylamidate). This is a simpler and greener method than the tedious procedures described in the literature, and it has permitted to obtain water-soluble complexes with good yields in a short period of time. A synthetic study has been implemented to find the best experimental conditions to prepare compounds 1-7. Two different types of ligands, formamidinate and amidate, have been used to check the generality of the method for the preparation of monosubstituted complexes. Five new compounds (2-6) have been obtained using a formamidinate ligand, the synthesis of the previously described compound 1 has been improved, and an unprecedented monoamidate complex has been achieved (7). The crystal structures of compounds 3 and 7 have been solved by single crystal X-ray diffraction. These compounds show the typical paddlewheel structure with three acetate ligands and one formamidinate (3) or amidate (7) bridging ligand at the equatorial positions. The axial positions are occupied by the chloride ligand giving rise to one-dimensional polymer structures that were previously unknown for monosubstituted compounds.

A novel μ-oxo-diruthenium(III,III)-ibuprofen-(4-aminopyridine) chloride derived from the diruthenium(II,III)-ibuprofen paddlewheel metallodrug shows anticancer properties

Diruthenium(II,III) metal-metal multiply bonded paddlewheel complexes bearing non-steroidal anti-inflammatory drugs are promising anticancer metallodrugs. The [Ru₂(Ibp)₄Cl] (Ibp, ibuprofenate anion from HIbp ibuprofen drug), free or encapsulated, shows anticancer activity against glioblastoma (in vitro, in vivo), and against human breast and prostate cancer cells. Herein we report the interaction of [Ru₂(Ibp)₄Cl] and of [Ru₂(Ac)₄(H₂O)₂]PF₆ (Ac, acetate) with the 4-aminopyridine (4Apy) drug. The N-ligand was capable of cleaving the paddlewheel unit with oxidation of Ru₂(II,III) to Ru₂(III,III)O μ-oxo core in the ibuprofen complex while the acetate complex underwent axial substitution of water by 4Apy. Carefully designed synthetic and chromatographic methods succeeded in giving the novel [Ru₂O(Ibp)₂(4Apy)₆]Cl₂ metallodrug, the first diruthenium(III,III) μ-oxo having chloride as counterion. Characterization was performed by elemental analysis, mass spectrometry, thermogravimetric analysis, electronic absorption and vibrational spectroscopies, molar conductivity and cyclic voltammetry. Kinetic studies for the μ-oxo complex (in 50:50 v/v ethanol:water) suggested an aquation/complexation equilibrium in consecutive step reactions with the exchange of the two 4Apy trans to the μ-oxo bridge by water (aquation) and the back coordination of 4Apy in excess of the N-ligand (complexation). Trypan blue assays for the novel compound showed time- and dose- dependent antiproliferative effects (at 5-50 μmol L^-1) and cytotoxicity (> 20 μmol L^-1), and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) assays gave IC₅₀ value of 7.6 ± 1.5 μmol L^-1 (at 48 h, 1-20 μmol L^-1) against U87MG human glioblastoma cells (aggressive brain glioma cancer) pointing the metallodrug as potential candidate for novel therapies in gliomas.

Two mixed valence diruthenium(II,III) isomeric complexes show different anticancer properties

In this paper it is demonstrated that the nature of the ligands of two Ru2(ii,iii) paddlewheel complexes dramatically affects the overall anticancer properties in cells. Herein, the complex [Ru2(EB776)4Cl] was found to be more active against a glioblastoma model with respect to its isomer [Ru2(EB106)4Cl]. These different effects depend on the steric hindrance, on the allowed conformations of the complexes and on the presence of hydrophilic regions in [Ru2(EB776)4Cl], which overall lead to a lower "steric protection".

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.

Unusual Structural Features in the Adduct of Dirhodium Tetraacetate with Lysozyme

The structures of the adducts formed upon reaction of the cytotoxic paddlewheel dirhodium complex [Rh₂(μ-O₂CCH₃)₄] with the model protein hen egg white lysozyme (HEWL) under different experimental conditions are reported. Results indicate that [Rh₂(μ-O₂CCH₃)₄] extensively reacts with HEWL:it in part breaks down, at variance with what happens in reactions with other proteins. A Rh center coordinates the side chains of Arg14 and His15. Dimeric Rh–Rh units with Rh–Rh distances between 2.3 and 2.5 Å are bound to the side chains of Asp18, Asp101, Asn93, and Lys96, while a dirhodium unit with a Rh–Rh distance of 3.2–3.4 Å binds the C-terminal carboxylate and the side chain of Lys13 at the interface between two symmetry-related molecules. An additional monometallic fragment binds the side chain of Lys33. These data, which are supported by replicated structural determinations, shed light on the reactivity of dirhodium tetracarboxylates with proteins, providing useful information for the design of new Rh-containing biomaterials with an array of potential applications in the field of catalysis or of medicinal chemistry and valuable insight into the mechanism of action of these potential anticancer agents.

A mixed-valence diruthenium(II,III) complex endowed with high stability: from experimental evidence to theoretical interpretation

We herein report the synthesis and multi-technique characterization of [Ru2Cl((2-phenylindol-3-yl)glyoxyl-l-leucine-l-phenylalanine)4], a novel diruthenium(ii,iii) complex obtained by reacting [Ru2(μ-O2CCH3)4Cl] with a dual indolylglyoxylyl dipeptide anticancer agent. We soon realised that the compound is very stable under several different conditions including aqueous buffers or organic solvents. It is also completely unreactive toward proteins. The high stability is also suggested by cellular experiments in a glioblastoma cell line. Indeed, while the parent ligand exerts high cytotoxic effects in the low μM range, the complex is completely non-cytotoxic against the same line, most probably because of the lack of ligand release. To investigate the reasons for such high stability, we carried out DFT calculations that are fully consistent with the experimental findings. The results highlight that the stability of [Ru2Cl((2-phenylindol-3-yl)glyoxyl-l-leucine-l-phenylalanine)4] relies on the nature of the ligand, including its steric hindrance that prevents the reaction of any nucleophilic group with the Ru2 core. Ligand displacement is the key step to allow reactivity with the biological targets of metal-based prodrugs. Accordingly, we discuss the implications of some important aspects that should be considered when active molecules are chosen as ligands for the synthesis of paddle-wheel-like complexes with medicinal applications.

Protein interactions of dirhodium tetraacetate: a structural study

The interactions between the cytotoxic paddlewheel dirhodium complex [Rh2(μ-O2CCH3)4] and the model protein bovine pancreatic ribonuclease (RNase A) were investigated by high-resolution mass spectrometry and X-ray crystallography. The results indicate that [Rh2(μ-O2CCH3)4] extensively reacts with RNase A. The metal compound binds the protein via coordination of the imidazole ring of a His side chain to one of its axial sites, while the dirhodium center and the acetato ligands remain unmodified. Data provide valuable information for the design of artificial dirhodium-containing metalloenzymes.

pH- and Time-Dependent Release of Phytohormones from Diruthenium Complexes

The controlled release of functionally active compounds is important in a variety of applications. Here, we have synthesized, characterized, and studied the magnetic properties of three novel metal-metal-bonded tris(formamidinato) Ru₂⁵⁺ complexes. We have used different auxin-related hormones, indole-3-acetate (IAA), 2,4-dichlorophenoxyacetate (2,4-D), and 1-naphthaleneacetate (NAA), to generate [Ru₂Cl(μ-DPhF)₃(μ-IAA)] (RuIAA), [Ru₂Cl(μ-DPhF)₃(μ-2,4-D)] (Ru2,4-D), and [Ru₂Cl(μ-DPhF)₃(μ-NAA)] (RuNAA) (DPhF = N,N'-diphenylformamidinate). The crystal structures of RuIAA, RuIAA·THF, Ru2,4-D·CH₂Cl₂, and RuNAA·0.5THF have been determined by single-crystal X-ray diffraction. To assess the releasing capacity of the bound hormone, we have employed a biological assay that relied on Arabidopsis thaliana plants expressing an auxin reporter gene and we demonstrate that the release of the phytohormones from RuIAA, Ru2,4-D, and RuNAA is pH- and time-dependent. These studies serve as a proof of concept showing the potential of these types of compounds as biological molecule carriers.

Synthesis of terpolymer-lipid encapsulated diruthenium(II,III)-anti-inflammatory metallodrug nanoparticles to enhance activity against glioblastoma cancer cells

Novel formulations of diruthenium(II,III)-NSAID (NSAID, non-steroidal anti-inflammatory drug) metallodrugs encapsulated into biocompatible terpolymer-lipid nanoparticles (TPLNs) to target glioblastoma cancer were developed. The nanoformulations of Ibuprofenate (RuIbp) and Naproxenate (RuNpx) metallodrugs were synthesized and characterized. The procedure rationally designed to avoid structural changes on the coordination sphere of the [Ru₂(NSAID)₄]⁺ paddlewheel unit succeeded in giving colloidally stable and nearly spherical shaped loaded [Ru₂(NSAID)₄]-TPLNs with appropriate parameters (~90% loading efficiency; drug loading around 10%; particle size ~130 nm; zeta potential around - 40 mV). The maintenance of the [Ru₂(NSAID)₄]⁺ framework was confirmed by spectroscopy and mass spectrometry. The encapsulation enhanced antiproliferative effects in U87MG cells for both metallodrugs. The RuIbp-TPLNs showed efficacy also against the cisplatin chemoresistant T98G cancer cells. Lack of significant effects for the loaded-Ibuprofen-TPLNs (HIbp-TPLNs) on both types of cells supports the key role of the dimetal core in the anticancer activity of the [Ru₂(NSAID)₄]⁺ metallodrugs. The high cell viability (>70%) found for both types of cells suggests activity associated mainly to antiproliferative effects. The blank-TPLNs internalized into U87MG cell cytoplasm mostly at the first 6 h, by energy-dependent mechanism. The cell uptake of the RuIbp-TPLNs occurred during the first 24 h and it was enhanced in relation to the non-encapsulated metallodrug. The development of these novel metallodrug-loaded TPLN nanoformulations, which exhibit colloidal stability suitable for intravenous injection and enhanced drug cellular uptake, expands the perspective for diruthenium(II,III)-NSAID metallodrugs targeting brain glioblastoma cancer.

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.

Coordination capacity of cytosine, adenine and derivatives towards open-paddlewheel diruthenium compounds

[Ru₂Cl₂(DPhF)₃] (DPhF = diphenylformamidinate) links preferentially to the junctions of RNA (ribonucleic acid) structures, although the bonding mode is not known. In order to clarify this question the reactions between [Ru₂Cl₂(DPhF)₃] and cytosine (Hcyto), cytidine (Hcyti), cytidine 2',3'-cyclic monophosphate sodium salt (NacCMP), adenine (Hade), adenosine (Haden) and adenosine 3',5'-cyclic monophosphate (HcAMP) have been carried out. In the resultant complexes, cyto (cytosinate), cyti (cytidinate), cCMP (cytidine 2',3'-cyclic monophosphate monoanion), ade (adeninate), aden (adenosinate) and cAMP (deprotonated adenosine 3',5'-cyclic monophosphate) are bonded to the diruthenium unit as N,N'-bridging ligands, as confirmed by the solution of the crystal structures of [RuCl(DPhF)₃(cyto)] and [RuCl(DPhF)₃(ade)] by X-ray diffraction. The axial positions of the diruthenium species are still available for additional interactions with other residues that could explain its preference towards RNA junctions.

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.

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.

Diruthenium(ii,iii) metallodrugs of ibuprofen and naproxen encapsulated in intravenously injectable polymer-lipid nanoparticles exhibit enhanced activity against breast and prostate cancer cells

A unique class of diruthenium(ii,iii) metallodrugs containing non-steroidal anti-inflammatory drug (NSAID), Ru₂(NSAID), have been reported to show anticancer activity in glioma models in vitro and in vivo. This work reports the encapsulation of the lead metallodrug of ibuprofen (HIbp), [Ru₂(Ibp)₄Cl] or RuIbp, and also of the new analogue of naproxen (HNpx), [Ru₂(Npx)₄Cl] or RuNpx, in novel intravenously (i.v.) injectable solid polymer-lipid nanoparticles (SPLNs). A rationally selected composition of lipids/polymers rendered nearly spherical Ru₂(NSAID)-SPLNs with a mean size of 120 nm and zeta potential of about -20 mV. The Ru₂(NSAID)-SPLNs are characterized by spectroscopic techniques and the composition in terms of ruthenium-drug species is analyzed by mass spectrometry. The metallodrug-loaded nanoparticles showed high drug loading (17-18%) with ∼100% drug loading efficiency, and good colloidal stability in serum at body temperature. Fluorescence-labeled SPLNs were taken up by the cancer cells in a time- and energy-dependent manner as analyzed by confocal microscopy and fluorescence spectrometry. The Ru₂(NSAID)-SPLNs showed enhanced cytotoxicity (IC₅₀ at 60-100 μmol L^-1 ) in relation to the corresponding Ru₂(NSAID) metallodrugs in breast (EMT6 and MDA-MB-231) and prostate (DU145) cancer cells in vitro. The cell viability of both metallodrug nanoformulations is also compared with those of the parent NSAIDs, HIbp and HNpx, and their corresponding NSAID-SPLNs. In vivo and ex vivo fluorescence imaging revealed good biodistribution and high tumor accumulation of fluorescence-labeled SPLNs following i.v. injection in an orthotopic breast tumor model. The enhanced anticancer activity of the metallodrug-loaded SPLNs in these cell lines can be associated with the advantages of the nanoformulations, assigned mainly to the stability of the colloidal nanoparticles suitable for i.v. injection and enhanced cellular uptake. The findings of this work encourage future in vivo efficacy studies to further exploit the potential of the novel Ru₂(NSAID)-SPLN nanoformulations for clinical application.

ESI-MS studies of the reactions of novel platinum(II) complexes containing O,O'-chelated acetylacetonate and sulfur ligands with selected model proteins

A group of mixed-ligand Pt(II) complexes bearing acetylacetonate and sulphur ligands were recently developed in the University of Lecce as a new class of prospective anticancer agents that manifested promising pharma-cological properties in preliminary in vitro and in vivo tests. Though modelled on the basis of cisplatin, these Pt(II) complexes turned out to exhibit a profoundly distinct mode of action as they were found to act mainly on non-genomic targets rather than on DNA. Accordingly, we have explored here their reactions with two representative model proteins through an established ESI-MS procedure with the aim to describe their general interaction mechanism with protein targets. A pronounced reactivity with the tested proteins was indeed documented; the nature of the resulting metallodrug-protein interactions could be characterised in depth in the various cases. Preferential binding to protein targets compared to DNA is supported by independent ICP-OES measurements. The implications of these findings are discussed.

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.

Axially-modified paddlewheel diruthenium(II,III)-ibuprofenato metallodrugs and the influence of the structural modification on U87MG and A172 human glioma cell proliferation, apoptosis, mitosis and migration

The metallodrug chloridotetrakis(ibuprofenato)diruthenium(II,III) ([Ru₂(Ibp)₄Cl] or RuIbpCl (1), Ibp=carboxylate anion derived from the non-steroidal anti-inflammatory drug ibuprofen) has shown promising results in vitro and in vivo, which point to its potential as an inhibitor of glioma tumour growth in vivo. In order to get insight into the influence of structural changes on the biological response of the metallodrug, the [Ru₂(Ibp)₄] metal-metal multiply bonded paddlewheel unit was modified for the axial ligand. Two new analogues, [Ru₂(Ibp)₄(CF₃SO₃)] (2) and [Ru₂(Ibp)₄(EtOH)₂]PF₆ (3), were synthesized and fully characterized by elemental analysis, ESI-MS, vibrational (FTIR, Raman) and electronic (UV/VIS/NIR) spectroscopy, magnetic susceptibility, molar conductivity measurements, and thermal analysis. RuIbpCl was re-prepared and complementary characterization to previous work was performed. The three axially-modified RuIbp metallodrugs were compared for their effects on U87MG and A172 human glioma cell proliferation, apoptosis, mitosis, and cell migration in vitro. The results provide evidence that the chloride ligand in RuIbpCl may play key role in the mode of action of the metallodrug, since the best results for antiproliferative activity were found for (1) in both types of human glioma cells. All the metallodrugs, (1), (2) and (3), were uptaken by the cells, and were shown to cause increase on number of apoptotic cells and decrease on number of mitotic cells. Additionally, the RuIbp metallodrugs were capable of inhibiting cell migration process in both human glioma cell lines. These data are extremely promising as drugs which can inhibit both cell proliferation/mitosis and inhibit cell migration could target two major chemotherapeutic targets in high grade gliomas.

Effect of temperature on the interaction of cisplatin with the model protein hen egg white lysozyme

The products of the reaction between cisplatin (CDDP) and the model protein hen egg white lysozyme (HEWL) at 20, 37 and 55 °C in pure water were studied by UV-Vis absorption spectroscopy, intrinsic fluorescence and circular dichroism, dynamic and electrophoretic light scattering and inductively coupled plasma mass spectrometry. X-ray structures were also solved for the adducts formed at 20 and 55 °C. Data demonstrate that high temperature facilitates the formation of CDDP-HEWL adducts, where Pt atoms bind ND1 atom of His15 or NE2 atom of His15 and NH1 atom of Arg14. Our study suggests that high human body temperature (fever) could increase the rate of drug binding to proteins thus enhancing possible toxic side effects related to CDDP administration.

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.

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.