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

Bovine serum albumin uptake and polypeptide disaggregation studies of hypoglycemic ruthenium(II) uracil Schiff-base complexes

Our prior studies have illustrated that the uracil ruthenium(II) diimino complex, [Ru(H3ucp)Cl(PPh3)] (1) (H4ucp = 2,6-bis-((6-amino-1,3-dimethyluracilimino)methylene)pyridine) displayed high hypoglycemic effects in diet-induced diabetic rats. To rationalize the anti-diabetic effects of 1, three new derivatives have been prepared, cis-[Ru(bpy)2(urdp)]Cl2 (2) (urdp = 2,6-bis-((uracilimino)methylene)pyridine), trans-[RuCl2(PPh3)(urdp)] (3), and cis-[Ru(bpy)2(H4ucp)](PF6)2 (4). Various physicochemical techniques were utilized to characterize the structures of the novel ruthenium compounds. Prior to biomolecular interactions or in vitro studies, the stabilities of 1-4 were monitored in anhydrous DMSO, aqueous phosphate buffer containing 2% DMSO, and dichloromethane (DCM) via UV-Vis spectrophotometry. Time-dependent stability studies showed ligand exchange between DMSO nucleophiles and chloride co-ligands of 1 and 3, which was suppressed in the presence of an excess amount of chloride ions. In addition, the metal complexes 1 and 3 are stable in both DCM and an aqueous phosphate buffer containing 2% DMSO. In the case of compounds 2 and 4 with no chloride co-ligands within their coordination spheres, high stability in aqueous phosphate buffer containing 2% DMSO was observed. Fluorescence emission titrations of the individual ruthenium compounds with bovine serum albumin (BSA) showed that the metal compounds interact non-discriminately within the protein's hydrophobic cavities as moderate to strong binders. The metal complexes were capable of disintegrating mature amylin amyloid fibrils. In vivo glucose metabolism studies in liver (Chang) cell lines confirmed enhanced glucose metabolism as evidenced by the increased glucose utilization and glycogen synthesis in liver cell lines in the presence of complexes 2-4.

Investigating the Interaction of an Anticancer Nucleolipidic Ru(III) Complex with Human Serum Proteins: A Spectroscopic Study

Ruthenium(III) complexes are very promising candidates as metal-based anticancer drugs, and several studies have supported the likely role of human serum proteins in the transport and selective delivery of Ru(III)-based compounds to tumor cells. Herein, the anticancer nanosystem composed of an amphiphilic nucleolipid incorporating a Ru(III) complex, which we named DoHuRu, embedded into the biocompatible cationic lipid DOTAP, was investigated as to its interaction with two human serum proteins thought to be involved in the mechanism of action of Ru(III)-based anticancer drugs, i.e., human serum albumin (HSA) and human transferrin (hTf). This nanosystem was studied in comparison with the simple Ru(III) complex named AziRu, a low molecular weight metal complex previously designed as an analogue of NAMI-A, decorated with the same ruthenium ligands as DoHuRu but devoid of the nucleolipid scaffold and not inserted in liposomal formulations. For this study, different spectroscopic techniques, i.e., Fluorescence Spectroscopy and Circular Dichroism (CD), were exploited, showing that DoHuRu/DOTAP liposomes can interact with both serum proteins without affecting their secondary structures.

Metal-Based Complexes as Potential Anti-cancer Agents

Metal based therapy is no new in biomedical research. In early days the biggest limitation was the inequality among therapeutical and toxicological dosages. Ever since, Barnett Rosenberg discovered cisplatin, a new era has begun to treat cancer with metal complexes. Platinum complexes such as oxaliplatin, cisplatin, and carboplatin, seem to be the foundation of metal/s-based components to challenge malignancies. With an advancement in the biomolemoecular mechanism, researchers have started developing non-classical platinum-based complexes, where a different mechanistic approach of the complexes is observed towards the biomolecular target. Till date, larger number of metal/s-based complexes was synthesized by overhauling the present structures chemically by substituting the ligand or preparing the whole novel component with improved cytotoxic and safety profiles. Howsoever, due to elevated accentuation upon the therapeutic importance of metal/s-based components, a couple of those agents are at present on clinical trials and several other are in anticipating regulatory endorsement to enter the trial. This literature highlights the detailed heterometallic multinuclear components, primarily focusing on platinum, ruthenium, gold and remarks on possible stability, synergism, mechanistic studies and structure activity relationships.

A Dual-Pronged Approach: A Ruthenium(III) Complex That Modulates Amyloid-β Aggregation and Disrupts Its Formed Aggregates

Alzheimer's disease (AD) is a devastating neurological disorder for which soluble oligomers of the peptide amyloid-β (Aβ) are now recognized as the neurotoxic species. Metal-based therapeutics are uniquely suited to target Aβ, with ruthenium-based (Ru) complexes emerging as propitious candidates. Recently, azole-based Ru(III) complexes were observed to modulate the aggregation of Aβ in solution, where the inclusion of a primary amine proximal to the ligand coordination site improved the activity of the complexes. To advance these structure-activity relationships, a series of oxazole-based Ru complexes were prepared and evaluated for their ability to modulate Aβ aggregation. From these studies, a lead candidate, Oc, emerged that had superior activity relative to its azole predecessors in modulating the aggregation of soluble Aβ and diminishing its cytotoxicity. Further evaluation of Oc demonstrated its ability to disrupt formed Aβ aggregates, resulting in smaller amorphous species. Because altering both sides of the aggregation equilibrium for Aβ has not been previously suggested for metal-based complexes for AD, this work represents an exciting new avenue for improved therapeutic success.

Importance of Hydrogen Bonding: Structure-Activity Relationships of Ruthenium(III) Complexes with Pyridine-Based Ligands for Alzheimer's Disease Therapy

Alzheimer's disease (AD) is the most common form of dementia, where one of the pathological hallmarks of AD is extracellular protein deposits, the primary component of which is the peptide amyloid-β (Aβ). Recently, the soluble form of Aβ has been recognized as the primary neurotoxic species, making it an important target for therapeutic development. Metal-based drugs are promising candidates to target Aβ, as the interactions with the peptide can be tuned by ligand design. In the current study, 11 ruthenium complexes containing pyridine-based ligands were prepared, where the functional groups at the para position on the coordinated pyridine ligand were varied to determine structure-activity relationships. Overall, the complexes with terminal primary amines had the greatest impact on modulating the aggregation of Aβ and diminishing its cytotoxicity. These results identify the importance of specific intermolecular interactions and are critical in the advancement of metal-based drugs for AD therapy.

A new bis-pyrazolylpyridine ruthenium(III) complex as a potential anticancer drug: in vitro and in vivo activity in murine colon cancer

We synthesized and characterized the ruthenium(iii) pincer-type complex [RuCl3(H2Lt-Bu] (H2Lt-Bu = 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine, 1) by elemental analysis, IR and UV-Vis spectroscopy, and the mass spectrometry (MS) method ESI Q-TOF. For comparison reasons, we also studied ruthenium(iii) terpyridine complexes of the general formula [Ru(N-N-N)Cl3], where N-N-N = 4'-chloro-terpyridine (Cl-tpy; 2) or 4'-chlorophenyl-terpyridine (Cl-Ph-tpy; 3). A kinetic study of the substitution reactions of 1-3 with biomolecules showed that the rate constants depend on the properties of the spectator ligand and the nature of the entering nucleophile. The DNA/HSA binding study showed that in comparison to complex 1 (bis-pyrazolylpyridine), the other two (2 and 3) terpyridine complexes had a slightly better binding affinity to calf thymus DNA (CT DNA), while in the case of human serum albumin (HSA), complex 1 exhibited the strongest quenching ability. We demonstrated that 1 possesses significant in vitro cytotoxic activity against mouse colon carcinoma CT26 cells and in vivo antitumor activity in murine heterotopic colon carcinoma. Complex 1 induced G0/G1 cell cycle arrest and apoptotic death in CT26 cells. Additionally, 1 showed antiproliferative activity, as evaluated by the detection of the expression levels of the Ki67 protein. Furthermore, the in vivo results showed that 1 reduced primary tumour growth and the number and growth of lung and liver metastases, significantly prolonging the treated mice's survival rate. This study highlighted that 1 does not show hepato- and nephrotoxicity. Our data demonstrated the considerable antitumor activity of the ruthenium(iii) pincer complex against CT26 tumour cells and implicated further investigations of its role as a potential chemotherapeutic agent for colon carcinoma.

Modulation of Diverse Procoagulant Venom Activities by Combinations of Platinoid Compounds

Procoagulant snake venoms have been inhibited by the ruthenium containing compounds CORM-2 and RuCl₃ separately, presumably by interacting with critical histidine or other sulfur-containing amino acids on key venom enzymes. However, combinations of these and other platinoid containing compounds could potentially increase, decrease or not affect the procoagulant enzyme function of venom. Thus, the purpose of this investigation was to determine if formulations of platinoid compounds could inhibit venom procoagulant activity and if the formulated compounds interacted to enhance inhibition. Using a human plasma coagulation kinetic model to assess venom activity, six diverse venoms were exposed to various combinations and concentrations of CORM-2, CORM-3, RuCl₃ and carboplatin (a platinum containing compound), with changes in venom activity determined with thrombelastography. The combinations of CORM-2 or CORM-3 with RuCl₃ were found to enhance inhibition significantly, but not in all venoms nor to the same extent. In sharp contrast, carboplatin-antagonized CORM-2 mediated the inhibition of venom activity. These preliminary results support the concept that platinoid compounds may inhibit venom enzymatic activity at the same or different molecular sites and may antagonize inhibition at the same or different sites. Further investigation is warranted to determine if platinoid formulations may serve as potential antivenoms.

Insights into metalloproteins and metallodrugs from electron paramagnetic resonance spectroscopy

Metal ions play an important role in diverse biological processes, and much of the basic knowledge derived from studying native bioinorganic systems are applied in the synthesis of new molecules with the aim of diagnosing and treating diseases. At first glance, metalloproteins and metallodrugs are very different systems, but metal ion coordination, redox chemistry and substrate binding play essential roles in advancing both of these research fields. In this article, we discuss recent metalloprotein and metallodrug studies where electron paramagnetic resonance spectroscopy served as a major tool to gain a better understanding of metal-based structures and their function.

Ruthenium(III) complexes with imidazole ligands that modulate the aggregation of the amyloid-β peptide via hydrophobic interactions

Alzheimer's disease (AD) is the most common form of dementia, characterized by extracellular protein deposits, comprised primarily of the peptide amyloid-beta (Aβ), are a pathological indicator of the disease. Commonly known as Aβ plaques, these deposits contain a relatively high concentration of metals, making metallotherapeutics uniquely suited to target soluble Aβ, thereby limiting its aggregation and cytotoxicity. Ruthenium-based complexes are promising candidates for advancement, as the complex PMRU20 (2-aminothiazolium [trans-RuCl₄(2-aminothiazole)₂]) and several thiazole-based derivatives were found to prevent the aggregation of Aβ, with hydrogen-bonding functional groups improving their performance. Further investigation into the impact of the heteroatom in the azole ring on the activity of Ru complexes was achieved through the synthesis and evaluation of a small set of imidazole-based compounds. The ability of the complexes to prevent the aggregation of Aβ was determined where the same sample was subjected to analysis by three complementary methods: ThT fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). It was found that hydrophobic interactions, along with hydrogen-bonding via the imidazole nitrogen heteroatom, promoted interactions with the Aβ peptide, thereby limiting its aggregation. Furthermore, it was found that having rapid and sequential exchange proved detrimental as it resulted in a decreased association with Aβ. These results highlight important considerations between a balance of intermolecular interactions and ligand exchange kinetics in the design of further therapeutic candidates.

Unveiling the Role of Hydrogen Bonding and g-Tensor in the Interaction of Ru-Bis-DMSO with Amino Acid Residue and Human Serum Albumin

Density functional theory calculations have been carried out to observe the role of hydrogen bonding in hydrolysis and the coordination mechanism of three amino acid residues (histidine, cysteine, and alanine) with Ru-bis-DMSO complex via which the complex tends to interact with the HSA protein receptor. The interaction mechanism shows that ruthenium complexes prefer to bind protein receptor through cysteine and histidine residues rather than through alanine, which has been confirmed by DFT evaluated H-bonding and g-tensor analysis. The number of H-bonds plays a major role in stabilizing the intermediates and transition states involved in the Ru-bis-DMSO and amino acid residue interactions. Our theoretical g-tensor values are in good agreement with the available experimental results. Further QM/MM calculation on the Ru-bis-DMSO-HSA adducts reveals that the adduct is more stable when Ru gets coordinated with histidine imidazole rather than cysteine. These investigations helped us in understanding the type of amino acid residue responsible for binding the metal complex Ru-bis-DMSO with the carrier protein HSA.

Ruthenium(iii) complexes containing thiazole-based ligands that modulate amyloid-β aggregation

Alzheimer's Disease (AD) is a devastating neurodegenerative disorder where one of the commonly observed pathological hallmarks is extracellular deposits of the peptide amyloid-β (Aβ). These deposits contain a high concentration of metals and initially presented a promising target for therapy; however it has become increasingly evident that the soluble form of the peptide is neurotoxic, not the amyloidogenic species. Metal-based therapeutics are uniquely suited to target soluble Aβ and have shown considerable promise to prevent the aggregation and induced cytotoxicity of the peptide in vitro. Herein, we have prepared a small series of derivatives of two promising Ru(iii) complexes NAMI-A (imidazolium [trans-RuCl₄(1H-imidazole)(dimethyl sulfoxide-S)]) and PMRU20 (2-aminothiazolium [trans-RuCl₄(2-aminothiazole)₂]), to determine structure-activity relationships (SAR) for Ru(iii) therapeutics for AD. Using the three complementary methods of Thioflavin T fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM), it was determined that the symmetry around the metal center did not significantly impact the activity of the complexes, but rather the attached thiazole ligand(s) mitigated Aβ aggregation. Across both families of Ru(iii) complexes the determined SAR for the functional groups on the thiazole ligands to modulate Aβ aggregation were NH₂ > CH₃ > H. These results highlight the importance of secondary interactions between the metallotherapeutic and the Aβ peptide where hydrogen-bonding has the greatest impact on modulating Aβ aggregation.

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.

Understanding the Coordination Modes of [Cu(acac)2(imidazole)n=1,2] Adducts by EPR, ENDOR, HYSCORE, and DFT Analysis

The interaction of imidazole with a [Cu(acac)₂] complex was studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), hyperfine sublevel correlation spectroscopy (HYSCORE), and density functional theory (DFT). At low Im ratios (Cu:Im 1:10), a 5-coordinate [Cu(acac)₂Im_n=1] monoadduct is formed in frozen solution with the spin Hamiltonian parameters g₁ = 2.063, g₂ = 2.063, g₃ = 2.307, A₁ = 26, A₂ = 15, and A₃ = 472 MHz with Im coordinating along the axial direction. At higher Im concentrations (Cu:Im 1:50), a 6-coordinate [Cu(acac)₂Im_n=2] bis-adduct is formed with the spin Hamiltonian parameters g₁ = 2.059, g₂ = 2.059, g₃ = 2.288, A₁ = 30, A₂ = 30, and A₃ = 498 MHz with a poorly resolved ¹⁴N superhyperfine pattern. The isotropic EPR spectra revealed a distribution of species ([Cu(acac)₂], [Cu(acac)₂Im_n=1], and [Cu(acac)₂Im_n=2]) at Cu:Im ratios of 1:0, 1:10, and 1:50. The superhyperfine pattern originates from two strongly coordinating N³ imino nitrogens of the Im ring. Angular selective ¹⁴N ENDOR analysis revealed the ^NA tensor of [34.8, 43.5, 34.0] MHz, with e²qQ/h = 2.2 MHz and η = 0.2 for N³. The hyperfine and quadrupole values for the remote N¹ amine nitrogens (from HYSCORE) were found to be [1.5, 1.4, 2.5] MHz with e²qQ/h = 1.4 MHz and η = 0.9. ¹H ENDOR also revealed three sets of ^HA tensors corresponding to the nearly equivalent H²/H⁴ protons in addition to the H⁵ and H¹ protons of the Im ring. The spin Hamiltonian parameters for the geometry optimized structures of [Cu(acac)₂Im_n=2], including cis-mixed plane, trans-axial, and trans-equatorial, were calculated. The best agreement between theory and experiment indicated the preferred coordination is trans-equatorial [Cu(acac)₂Im_n=2]. A number of other Im derivatives were also investigated. 4(5)-methyl-imidazole forms a [Cu(acac)₂(Im-3)_n=2] trans-equatorial adduct, whereas the bulkier 2-methyl-imidazole (Im-2) and benzimidazole (Im-4) form the [Cu(acac)₂(Im-2,4)_n=1] monoadduct only. Our data therefore show that subtle changes in the substrate structure lead to controllable changes in coordination behavior, which could in turn lead to rational design of complexes for use in catalysis, imaging, and medicine.

Metal complexes in cancer therapy - an update from drug design perspective

In the past, metal-based compounds were widely used in the treatment of disease conditions, but the lack of clear distinction between the therapeutic and toxic doses was a major challenge. With the discovery of cisplatin by Barnett Rosenberg in 1960, a milestone in the history of metal-based compounds used in the treatment of cancers was witnessed. This forms the foundation for the modern era of the metal-based anticancer drugs. Platinum drugs, such as cisplatin, carboplatin and oxaliplatin, are the mainstay of the metal-based compounds in the treatment of cancer, but the delay in the therapeutic accomplishment of other metal-based compounds hampered the progress of research in this field. Recently, however, there has been an upsurge of activities relying on the structural information, aimed at improving and developing other forms of metal-based compounds and nonclassical platinum complexes whose mechanism of action is distinct from known drugs such as cisplatin. In line with this, many more metal-based compounds have been synthesized by redesigning the existing chemical structure through ligand substitution or building the entire new compound with enhanced safety and cytotoxic profile. However, because of increased emphasis on the clinical relevance of metal-based complexes, a few of these drugs are currently on clinical trial and many more are awaiting ethical approval to join the trial. In this review, we seek to give an overview of previous reviews on the cytotoxic effect of metal-based complexes while focusing more on newly designed metal-based complexes and their cytotoxic effect on the cancer cell lines, as well as on new approach to metal-based drug design and molecular target in cancer therapy. We are optimistic that the concept of selective targeting remains the hope of the future in developing therapeutics that would selectively target cancer cells and leave healthy cells unharmed.

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.

An iridium(iii)-based irreversible protein-protein interaction inhibitor of BRD4 as a potent anticancer agent

Bromodomain-containing protein 4 (BRD4) has recently emerged as an attractive epigenetic target for anticancer therapy. In this study, an iridium(iii) complex is reported as the first metal-based, irreversible inhibitor of BRD4. Complex 1a is able to antagonize the BRD4-acetylated histone protein-protein interaction (PPI) in vitro, and to bind BRD4 and down-regulate c-myc oncogenic expression in cellulo. Chromatin immunoprecipitation (ChIP) analysis revealed that 1a could modulate the interaction between BRD4 and chromatin in melanoma cells, particular at the MYC promoter. Finally, the complex showed potent activity against melanoma xenografts in an in vivo mouse model. To our knowledge, this is the first report of a Group 9 metal complex inhibiting the PPI of a member of the bromodomain and extraterminal domain (BET) family. We envision that complex 1a may serve as a useful scaffold for the development of more potent epigenetic agents against cancers such as melanoma.