X-ray Structure Analysis of Indazolium trans-[Tetrachlorobis(1H-indazole)ruthenate(III)] (KP1019) Bound to Human Serum Albumin Reveals Two Ruthenium Binding Sites and Provides Insights into the Drug Binding Mechanism

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.

Synthesis and preclinical evaluation of BOLD-100 radiolabeled with ruthenium-97 and ruthenium-103

BOLD-100 (formerly IT-139, KP1339), a well-established chemotherapeutic agent, is currently being investigated in clinical trials for the treatment of gastric, pancreatic, colorectal, and bile duct cancer. Despite numerous studies, the exact mode of action is still the subject of discussions. Radiolabeled BOLD-100 could be a powerful tool to clarify pharmacokinetic pathways of the compound and to predict therapy responses in patients using nuclear molecular imaging prior to the therapy. In this study, the radiosyntheses of carrier-added (c.a.) [97/103Ru]BOLD-100 were performed with the two ruthenium isotopes ruthenium-103 (103Ru; β-, γ) and ruthenium-97 (97Ru; EC, γ), of which in particular the latter isotope is suitable for imaging by single-photon emission computed tomography (SPECT). To identify the best tumor-to-background ratio for diagnostic imaging, biodistribution studies were performed with two different injected doses of c.a. [103Ru]BOLD-100 (3 and 30 mg kg-1) in Balb/c mice bearing CT26 allografts over a time period of 72 h. Additionally, ex vivo autoradiography of the tumors (24 h p.i.) was conducted. Our results indicate that the higher injected dose (30 mg kg-1) leads to more unspecific accumulation of the compound in non-targeted tissue, which is likely due to an overload of the albumin transport system. It was also shown that lower amounts of injected c.a. [103Ru]BOLD-100 resulted in a relatively higher tumor uptake and, therefore, a better tumor-to-background ratio, which are encouraging results for future imaging studies using c.a. [97Ru]BOLD-100.

Human Serum Albumin-Platinum(II) Agent Nanoparticles Inhibit Tumor Growth Through Multimodal Action Against the Tumor Microenvironment

To overcome the limitations of traditional platinum (Pt)-based drugs and further improve the targeting ability and therapeutic efficacy in vivo, we proposed to design a human serum albumin (HSA)-Pt agent complex nanoparticle (NP) for cancer treatment by multimodal action against the tumor microenvironment. We not only synthesized a series of Pt(II) di-2-pyridone thiosemicarbazone compounds and obtained a Pt(II) agent [Pt(Dp44mT)Cl] with significant anticancer activity but also successfully constructed a novel HSA-Pt(Dp44mT) complex nanoparticle delivery system. The structure of the HSA-Pt(Dp44mT) complex revealed that Pt(Dp44mT)Cl binds to the IIA subdomain of HSA and coordinates with His-242. The HSA-His242-Pt-Dp44mT NPs had an obvious effect on the inhibition of tumor growth, which was superior to that of Dp44mT and Pt(Dp44mT)Cl, and they had almost no toxicity. In addition, the HSA-His242-Pt-Dp44mT NPs were found to kill cancer cells by inducing apoptosis, autophagy, and inhibiting angiogenesis.

Complexes of Ruthenium(II) as Promising Dual-Active Agents against Cancer and Viral Infections

Poor responses to medical care and the failure of pharmacological treatment for many high-frequency diseases, such as cancer and viral infections, have been widely documented. In this context, numerous metal-based substances, including cisplatin, auranofin, various gold metallodrugs, and ruthenium complexes, are under study as possible anticancer and antiviral agents. The two Ru(III) and Ru(II) complexes, namely, BOLD-100 and RAPTA-C, are presently being studied in a clinical trial and preclinical studies evaluation, respectively, as anticancer agents. Interestingly, BOLD-100 has also recently demonstrated antiviral activity against SARS-CoV-2, which is the virus responsible for the COVID-19 pandemic. Over the last years, much effort has been dedicated to discovering new dual anticancer-antiviral agents. Ru-based complexes could be very suitable in this respect. Thus, this review focuses on the most recent studies regarding newly synthesized Ru(II) complexes for use as anticancer and/or antiviral agents.

Developing a Hetero-Trinuclear Erbium(III)-Copper(II) Complex Based on Apoferritin: Targeted Photoacoustic Imaging and Multimodality Therapy of Tumor

For the integration of targeted diagnosis and treatment of tumor, we innovatively designed and synthesized a single-molecule hetero-multinuclear Er(III)-Cu(II) complex (ErCu2) and then constructed an ErCu2@apoferritin (AFt) nanoparticle (NP) delivery system. ErCu2 and ErCu2@AFt NPs not only provided an evident photoacoustic imaging (PAI) signal of the tumor but also effectively inhibited tumor growth by integrating photothermal therapy, chemotherapy, and immunotherapy. ErCu2@AFt NPs improved the targeting ability and decreased the systemic toxicity of ErCu2 in vivo. Furthermore, we confirmed that ErCu2 and ErCu2@AFt NPs inhibited tumor growth by inducing apoptosis and autophagy of tumor cells and activating the immune system. The study not only provides a novel strategy to develop therapeutic metal agents but also reveals their potential for targeted accurate diagnosis and multimodality therapy of cancer.

Synthesis and Preclinical Evaluation of Radiolabeled [103Ru]BOLD-100

The first-in-class ruthenium-based chemotherapeutic agent BOLD-100 (formerly IT-139, NKP-1339, KP1339) is currently the subject of clinical evaluation for the treatment of gastric, pancreatic, colorectal and bile duct cancer. A radiolabeled version of the compound could present a helpful diagnostic tool. Thus, this study investigated the pharmacokinetics of BOLD-100 in more detail to facilitate the stratification of patients for the therapy. The synthesis of [103Ru]BOLD-100, radiolabeled with carrier added (c.a.) ruthenium-103, was established and the product was characterized by HPLC and UV/Vis spectroscopy. In order to compare the radiolabeled and non-radioactive versions of BOLD-100, both complexes were fully evaluated in vitro and in vivo. The cytotoxicity of the compounds was determined in two colon carcinoma cell lines (HCT116 and CT26) and biodistribution studies were performed in Balb/c mice bearing CT26 allografts over a time period of 72 h post injection (p.i.). We report herein preclinical cytotoxicity and pharmacokinetic data for BOLD-100, which were found to be identical to those of its radiolabeled analog [103Ru]BOLD-100.

Spectroscopic and computational study of the interaction of Pt(II) pyrrole-imine chelates with human serum albumin

Three bis(pyrrolide-imine) Pt(II) chelates were synthesised and characterized with different bridging alkyl groups, specifically 2-hydroxypropyl (1), 2,2-dimethylpropyl (2), and 1,2-(S,S)-(+)-cyclohexyl (3). Novel compounds 1 and 2 were analysed by single-crystal X-ray diffraction (space group P1̄). The asymmetric unit of 1 comprises three independent molecules linked by hydrogen bonds involving the OH groups, forming a trimeric supramolecular structure. The Pt(II) chelates were reacted with human serum albumin (HSA) to investigate how the ligand bound to the Pt(II) ion influences the compound's affinity for HSA. Fluorescence quenching data obtained for native HSA and HSA bound to site-specific probes (warfarin, subdomain IIA; ibuprofen, subdomain IIIA) indicated that the three Pt(II) chelates bind close enough (within ∼30 Å) to Trp-214 to quench its intrinsic fluorescence. The bimolecular quenching constant (kq) was 103-104 -fold higher than the maximum diffusion-controlled collision constant in water (1010 M s-1) at 310 K, while the affinity constants, Ka, ranged from ∼5 × 103 to ∼5 × 105 at 310 K, and followed the order 1 > 3 > 2. The reactions of 1 and 3 with HSA were enthalpically driven, while that for 2 was entropically driven. Macromolecular docking simulations (Glide XP) and binding site specificity assays employing site-specific probes and UV-vis CD spectroscopy indicated that 1 and 2 target Sudlow's site II in subdomain IIIA, minimally perturbing the tertiary structure of the protein. Well-resolved induced CD signals from 1 and 2 bound to HSA in subdomain IIIA were adequately simulated by hybrid QM:MM TD-DFT methods. We conclude that the structure of the bis(pyrrolide-imine) Pt(II) chelate measurably affects its uptake by HSA without detectable decomposition or demetallation. Such compounds could thus serve as metallodrug candidates capable of utilising an HSA-mediated cellular uptake pathway.

Synthesis, Characterization, DNA/HSA Interaction, and Cytotoxic Activity of a Copper(II) Thiolate Schiff Base Complex and Its Corresponding Water-Soluble Stable Sulfinato-O Complex Containing Imidazole as a Co-ligand

A Cu(II) thiolato complex [CuL(imz)] (1) (H₂L = o-HOC₆H₄C(H)=NC₆H₄SH-o) and the corresponding water-soluble stable sulfinato-O complex [CuL'(imz)] (2) (H₂L' = o-HOC₆H₄C(H)=NC₆H₄S(=O)OH) were synthesized and characterized using physicochemical techniques. Compound 2 is found to be a dimer in the solid state as characterized using single-crystal X-ray crystallography. XPS studies clearly showed the differences in the sulfur oxidation states in 1 and 2. Both compounds are found to be monomers in solution as revealed from their four-line X-band electron paramagnetic resonance spectra in CH₃CN at room temperature (RT). 1-2 were tested to assess their ability to exhibit DNA binding and cleavage activity. Spectroscopic studies and viscosity experiments suggest that 1-2 bind to CT-DNA through the intercalation mode having moderate binding affinity (K_b ∼ 10⁴ M^-1). This is further supported by molecular docking studies of complex 2 with CT-DNA. Both complexes display significant oxidative cleavage of pUC19 DNA. Complex 2 also showed hydrolytic DNA cleavage. The interaction of 1-2 with HSA revealed that they have strong ability to quench the intrinsic fluorescence of HSA by a static quenching mechanism (k_q ∼ 10¹³ M^-1 s^-1). This is further complemented by Förster resonance energy transfer studies that revealed binding distances of r = 2.85 and 2.75 nm for 1 and 2, respectively, indicating high potential for energy transfer from HSA to complex. 1-2 were capable of inducing conformational changes of HSA at secondary and tertiary levels as observed from synchronous and three-dimensional fluorescence spectroscopy. Molecular docking studies with 2 indicate that it forms strong hydrogen bonds with Gln221 and Arg222 located near the entrance of site-I of HSA. 1-2 showed potential toxicity in human cervical cancer HeLa cells, lung cancer A549 cells, and cisplatin-resistant breast cancer MDA-MB-231 cells and appeared to be most potent against HeLa cells (IC₅₀ = 2.04 μM for 1 and 1.86 μM for 2). In HeLa cells, 1-2 mediated cell cycle arrest in S and G2/M phases, which progressed into apoptosis. Apoptotic features seen from Hoechst and AO/PI staining, damaged cytoskeleton actin viewed from phalloidin staining, and increased caspase-3 activity upon treatment with 1-2 collectively suggested that they induced apoptosis in HeLa cells via caspase activation. This is further supported by western blot analysis of the protein sample extracted from HeLa cells treated with 2.

Developing a Multitargeted Anticancer Palladium(II) Agent Based on the His-242 Residue in the IIA Subdomain of Human Serum Albumin

To obtain next-generation metal drugs that can overcome the deficiencies of platinum (Pt) drugs and treat cancer more effectively, we proposed to develop a multitargeted palladium (Pd) agent to the tumor microenvironment (TME) based on the specific residue(s) of human serum albumin (HSA). To this end, we optimized a series of Pd(II) 2-benzoylpyridine thiosemicarbazone compounds to obtain a Pd agent (5b) with significant cytotoxicity. The HSA-5b complex structure revealed that 5b bound to the hydrophobic cavity in the HSA IIA subdomain and then His-242 replaced a leaving group (Cl) of 5b, coordinating with the Pd center. The in vivo results showed that the 5b/HSA-5b complex had significant capacity of inhibiting tumor growth, and HSA optimized the therapeutic behavior of 5b. In addition, we confirmed that the 5b/HSA-5b complex inhibited tumor growth through multiple actions on different components of TME: killing cancer cells, inhibiting tumor angiogenesis, and activating T cells.

Synthesis, cytotoxicity, and biomacromolecule binding: Three isomers of nitrosylruthenium complexes with bidentate bioactive molecules as co-ligands

Three isomeric nitrosylruthenium complexes [RuNO(Qn)(PZA)Cl] (P1, P2, and P3) with bioactive small molecules 8-hydroxyquinoline (Qn) and pyrazinamide (PZA) as co-ligands were synthesized, and their crystal structures were determined using X-ray diffraction technique. The cellular toxicity of the isomeric complexes was compared to understand the effects of the geometries on the biological activity of the complexes. Both the complexes and the human serum albumin (HSA) complex adducts affected the extent of proliferation of HeLa cells (IC₅₀: 0.77-1.45 μM). P2 showed prominent activity-induced cell apoptosis and arrested cell cycles at the G1 phase. The binding constants (K_b) of the complex with calf thymus DNA (CT-DNA) and HSA were quantitatively evaluated using fluorescence spectroscopy in the range of 0.17-1.56 × 10⁴ M^-1 and 0.88-3.21 × 10⁵ M^-1, respectively. The average binding site (n) number was close to 1. Moreover, the structure of HSA and the P2 complex adduct solved at the resolution of 2.48 Å revealed that one PZA-coordinated nitrosylruthenium complex bound at the subdomain I of HSA via a noncoordinative bond. HSA could serve as a potential nano-delivery system. This study provides a framework for the rational design of metal-based drugs.

Developing an Anticancer Platinum(II) Compound Based on the Uniqueness of Human Serum Albumin

To develop the next-generation Pt drug with remarkable activity and low toxicity to maximally inhibit tumor growth, we optimized a Pt(II) thiosemicarbazone compound (C4) with remarkable cytotoxicity to SK-N-MC cells and then constructed a new human serum albumin-C4 (HSA-C4) complex delivery system. The in vivo results showed that C4 and the HSA-C4 complex have remarkable therapeutic efficiency and almost no toxicity; they induced apoptosis and inhibited tumor angiogenesis. This system showed potential as a practical Pt drug. This study could pave the way for developing next-generation dual-targeted Pt drugs and achieving their targeting therapy for cancer.

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.

Synthesis, Characterization and Biological Investigations of Half-Sandwich Ruthenium(II) Complexes Containing Benzimidazole Moiety

Half-sandwich Ru(II) complexes belong to group of biologically active metallo-compounds with promising antimicrobial and anticancer activity. Herein, we report the synthesis and characterization of arene ruthenium complexes containing benzimidazole moiety, namely, [(η⁶-p-cymene)RuCl(bimCOO)] (1) and [(η⁶-p-cymene)RuCl₂(bim)] (2) (where bimCOO = benzimidazole-2-carboxylate and bim = 1-H-benzimidazole). The compounds were characterized by ¹H NMR, ¹³C NMR, IR, UV-vis and CV. Molecular structures of the complexes were determined by SC-XRD analysis, and the results indicated the presence of a pseudo-tetrahedral (piano stool) geometry. Interactions in the crystals of the Ru complexes using the Hirshfeld surface analysis were also examined. In addition, the biological studies of the complexes, such as antimicrobial assays (against planktonic and adherent microbes), cytotoxicity and lipophilicity, were performed. Antibacterial activity of the complexes was evaluated against S. aureus, E. coli, P. aeruginosa PAO1 and LES B58. Cytotoxic activity was tested against primary human fibroblasts and adenocarcinoma human alveolar basal epithelial cells. Obtained biological results show that the ruthenium compounds have bacteriostatic activity toward Pseudomonas aeruginosa PAO1 strain and are not toxic to normal cells. A molecular docking study was applied as a predictive source of information about the plausibility of examined structures binding with HSA as a transporting system.

Controversial Role of Transferrin in the Transport of Ruthenium Anticancer Drugs

Ruthenium complexes are at the forefront of developments in metal-based anticancer drugs, but many questions remain open regarding their reactivity in biological media, including the role of transferrin (Tf) in their transport and cellular uptake. A well-known anticancer drug, KP1019 ((IndH)[Ru^IIICl₄(Ind)₂], where Ind = indazole) and a reference complex, [Ru^III(nta)₂]^3- (nta = nitrilotriacetato(3-)) interacted differently with human apoTf, monoFeTf, or Fe₂Tf. These reactions were studied by biolayer interferometry (BLI) measurements of Ru-Fe-Tf binding to recombinant human transferrin receptor 1 (TfR1) in conjunction with UV-vis spectroscopy and particle size analysis. Cellular Ru uptake in human hepatoma (HepG2) cells was measured under the conditions of the BLI assays. The mode of Tf binding and cellular Ru uptake were critically dependent on the nature of Ru complex, availability of Fe(III) binding sites of Tf, and the presence of proteins that competed for metal binding, particularly serum albumin. Cellular uptake of KP1019 was not Tf-mediated and occurred mostly by passive diffusion, which may also be suitable for treatments of inoperable cancers by intratumoral injections. High cellular Ru uptake from a combination of [Ru^III(nta)₂]^3- and Fe₂Tf in the absence of significant Ru-Tf binding was likely to be due to trapping of Ru(III) species into the endosome during TfR1-mediated endocytosis of Fe₂Tf.

Developing a Copper(II) Agent Based on His-146 and His-242 Residues of Human Serum Albumin Nanoparticles: Integration To Overcome Cisplatin Resistance and Inhibit the Metastasis of Nonsmall Cell Lung Cancer

To overcome the resistance of nonsmall cell lung cancer (NSCLC) cells to cisplatin and inhibit their metastasis, we proposed to develop a Cu(II) agent based on the specific residue(s) of HSA nanoparticles (NPs) for multitargeting the tumor microenvironment (TME). To this end, we not only synthesized four Cu(II) 2-hydroxy-3-methoxybenzaldehyde thiosemicarbazone compounds (C1-C4), obtaining a Cu compound (C4) with significant cytotoxicity by studying their structure-activity relationships, but also revealed the binding mechanism of C4 to HSA through X-ray crystallography and confirmed the successful construction of a new HSA-C4 NPs delivery system. C4 and HSA-C4 NPs inhibited the A549cisR tumor growth and metastasis, and HSA NPs optimized the anticancer behavior of C4. We further confirmed the anticancer mechanism of the C4/HSA-C4 NP multitargeting TME to overcome cisplatin resistance: killing tumor cells by acting on the mtDNA and inducing apoptosis, polarizing M2-type macrophages to the M1-type, and inhibiting angiogenesis.

Crystallographic analysis of interaction between cisplatin and human serum albumin: Effect of fatty acid

Metallodrugs are important for anticancer treatments. They bind mainly to human serum albumin (HSA) in blood circulation, greatly modulating their pharmacokinetics and anticancer efficacy. Fatty acid (FA) is one of the most important endogenous ligands of HSA with tight binding to HSA and affecting its conformation. However, the effect of fatty acids on metallodrugs interaction with HSA is unknown. Here we identify the binding sites of a widely used metallodrug, cisplatin, in HSA in the presence or absence of a representative fatty acid, myristate, by X-ray crystallography. Our crystal structures indicate that the sidechain of residue Met548 becomes more exposed to solvent in the presence of fatty acid, and is the main Pt binding site together with Met329 in HSA:Myr:cisplatin ternary structure. An undoubted new Pt binding site is detected at His338 in the presence of fatty acid, and additional two sites are also identified at His146 and His440 + K436. In addition, we revealed the mechanism of cisplatin-induced HSA aggregation, which is due to the crosslinking between Met298 and His510 of two HSA molecules.

Enantioselective Cytotoxicity of Chiral Diphosphine Ruthenium(II) Complexes Against Cancer Cells

The chiral cationic complex [Ru(η¹‐OAc)(CO)((R,R)‐Skewphos)(phen)]OAc (2^R), isolated from reaction of [Ru(η¹‐OAc)(η²‐OAc)(R,R)‐Skewphos)(CO)] (1^R) with phen, reacts with NaOPiv and KSAc affording [RuX(CO)((R,R)‐Skewphos)(phen)]Y (X=Y=OPiv 3^R; X=SAc, Y=OAc 4^R). The corresponding enantiomers 2^S‐4^S have been obtained from 1^S containing (S,S)‐Skewphos. Reaction of 2^R and 2^S with (S)‐cysteine and NaPF₆ at pH=9 gives the diastereoisomers [Ru((S)‐Cys)(CO)(PP)(phen)]PF₆ (PP=(R,R)‐Skewphos 2^R‐Cys; (S,S)‐Skewphos 2^S‐Cys). The DFT energetic profile for 2^R with (S)‐cysteine in H₂O indicates that aquo and hydroxo species are involved in formation of 2^R‐Cys. The stability of the ruthenium complexes in 0.9 % w/v NaCl solution, PBS and complete DMEM medium, as well as their n‐octanol/water partition coefficient (logP), have been evaluated. The chiral complexes show high cytotoxic activity against SW1736, 8505 C, HCT‐116 and A549 cell lines with EC₅₀ values of 2.8–0.04 μM. The (R,R)‐Skewphos derivatives show higher cytotoxicity compared to their enantiomers, 4^R (EC₅₀=0.04 μM) being 14 times more cytotoxic than 4^S against the anaplastic thyroid cancer 8505 C cell line.

Highly Cytotoxic Osmium(II) Compounds and Their Ruthenium(II) Analogues Targeting Ovarian Carcinoma Cell Lines and Evading Cisplatin Resistance Mechanisms

(1) Background: Ruthenium and osmium complexes attract increasing interest as next generation anticancer drugs. Focusing on structure-activity-relationships of this class of compounds, we report on 17 different ruthenium(II) complexes and four promising osmium(II) analogues with cinnamic acid derivatives as O,S bidentate ligands. The aim of this study was to determine the anticancer activity and the ability to evade platin resistance mechanisms for these compounds. (2) Methods: Structural characterizations and stability determinations have been carried out with standard techniques, including NMR spectroscopy and X-ray crystallography. All complexes and single ligands have been tested for cytotoxic activity on two ovarian cancer cell lines (A2780, SKOV3) and their cisplatin-resistant isogenic cell cultures, a lung carcinoma cell line (A549) as well as selected compounds on three non-cancerous cell cultures in vitro. FACS analyses and histone γH2AX staining were carried out for cell cycle distribution and cell death or DNA damage analyses, respectively. (3) Results: IC50 values show promising results, specifically a high cancer selective cytotoxicity and evasion of resistance mechanisms for Ru(II) and Os(II) compounds. Histone γH2AX foci and FACS experiments validated the high cytotoxicity but revealed diminished DNA damage-inducing activity and an absence of cell cycle disturbance thus pointing to another mode of action. (4) Conclusion: Ru(II) and Os(II) compounds with O,S-bidentate ligands show high cytotoxicity without strong effects on DNA damage and cell cycle, and this seems to be the basis to circumvent resistance mechanisms and for the high cancer cell specificity.

Bioactivity and Development of Small Non-Platinum Metal-Based Chemotherapeutics

Countless expectations converge in the multidisciplinary endeavour for the search and development of effective and safe drugs in fighting cancer. Although they still embody a minority of the pharmacological agents currently in clinical use, metal-based complexes have great yet unexplored potential, which probably hides forthcoming anticancer drugs. Following the historical success of cisplatin and congeners, but also taking advantage of conventional chemotherapy limitations that emerged with applications in the clinic, the design and development of non-platinum metal-based chemotherapeutics, either as drugs or prodrugs, represents a rapidly evolving field wherein candidate compounds can be fine-tuned to access interactions with druggable biological targets. Moving in this direction, over the last few decades platinum family metals, e.g., ruthenium and palladium, have been largely proposed. Indeed, transition metals and molecular platforms where they originate are endowed with unique chemical and biological features based on, but not limited to, redox activity and coordination geometries, as well as ligand selection (including their inherent reactivity and bioactivity). Herein, current applications and progress in metal-based chemoth are reviewed. Converging on the recent literature, new attractive chemotherapeutics based on transition metals other than platinum—and their bioactivity and mechanisms of action—are examined and discussed. A special focus is committed to anticancer agents based on ruthenium, palladium, rhodium, and iridium, but also to gold derivatives, for which more experimental data are nowadays available. Next to platinum-based agents, ruthenium-based candidate drugs were the first to reach the stage of clinical evaluation in humans, opening new scenarios for the development of alternative chemotherapeutic options to treat cancer.

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.

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.

Synthesis, Characterization, Crystal Structure, DNA and HSA Interactions, and Anticancer Activity of a Mononuclear Cu(II) Complex with a Schiff Base Ligand Containing a Thiadiazoline Moiety

A mononuclear Cu(II) complex [Cu(HL)(o-phen)]·H₂O (1) [H₃L =, o-phen = 1,10-phenanthroline] was isolated from methanol, and its X-ray single-crystal structure was determined. Frozen glass X-band EPR of 1 in dimethylformamide (DMF) at LNT showed a spectrum that is characteristic of a monomeric tetragonal character with g _∥ = 2.164, g _⊥ = 2.087, A _∥ = 19.08 mT, and A _⊥ ≤ 4 mT. Electronic spectroscopic studies using calf thymus DNA (CT-DNA) showed strong binding affinity of 1 as reflected from its intrinsic binding constant (K _b) value of 2.85 × 10⁵ M^-1. Competitive behavior of 1 with ethidium bromide (EB) displayed intercalative binding of DNA (K _app = 1.3 × 10⁶ M^-1). The compound displayed significant oxidative cleavage of pUC19 DNA. The interaction between HSA and complex 1 was examined by employing fluorescence and electronic absorption spectroscopic experiments. The secondary and tertiary structures of HSA were found to be altered as suggested by three-dimensional (3D) fluorescence experiments. The affinity of 1 to bind to HSA was found to be strong as indicated from its value of the binding constant (K _a = 2.89 × 10⁵ M^-1). Intrinsic fluorescence of the protein was found to be reduced through a mechanism of static quenching as suggested from the k _q (2.01 × 10¹³ M^-1 s^-1) value, the bimolecular quenching constant. The Förster resonance energy transfer (FRET) process may also be accounted for such a high k _q value. The r value (2.85 nm) calculated from FRET theory suggested that the distance between complex 1 (acceptor) and HSA (donor) is quite close. Complex 1 primarily bound to HSA in subdomain IIA as suggested by molecular docking studies. IC₅₀ values (0.80 and 0.43 μM, respectively) obtained from the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay with HeLa and MCF7 cells suggested remarkable in vitro anticancer activity of 1. Nuclear dual staining assays revealed that cell death occurred via apoptosis in HeLa cells and reactive oxygen species (ROS) accumulation caused apoptosis induction. On treatment with a 5 μM dose of 1 in HeLa cells, the cell population significantly increased in the G2/M phase, while it was decreased in G0/G1 and S phases as compared to the control, clearly indicating G2/M phase arrest.

Ruthenium-nitrosyl complexes as NO-releasing molecules and potential anticancer drugs

The synthesis of new types of mono- and polynuclear ruthenium nitrosyl complexes is driving progress in the field of NO generation for a variety of applications. Light-induced Ru-NO bond dissociation...

A solid support-based synthetic strategy for the site-selective functionalization of peptides with organometallic half-sandwich moieties

The number of donor atoms available on peptides that can competitively coordinate to metal centers renders the site-selective generation of advanced metal-peptide conjugates in high purity a challenging venture. Herein, we present a transmetalation-based synthetic approach on solid support in which an imidazolium proligand can be used to selectively anchor a range of transition metal half-sandwich complexes onto peptides in the presence of multiple coordinative motifs. Amenable to solid support, a range of N-terminus and/or lysine conjugated metal-peptide conjugates were obtained in high purity after cleavage from the resin. The metalated peptides were evaluated for their anticancer properties against human cancer cell lines. While no cytotoxic activity was observed, this platform has the potential to i) provide a pathway to site-selective peptide labelling, ii) be explored as a biorthogonal handle and/or iii) generate a new strategy for ligand design in transition metal catalysts.

Ru(II)-Based Acetylacetonate Complexes Induce Apoptosis Selectively in Cancer Cells

The synthesis, chemical and biological characterization of seven Ru(II) polypyridyl complexes containing acetylacetonate (acac) ligands are reported. Electronic absorption spectra were determined and electrochemical potentials consistent with Ru(III/II) couples ranging from +0.60 to +0.73 V vs Ag/AgCl were measured. A series of complexes were screened against MDA-MB-231, DU-145, and MCF-10A cell lines to evaluate their cytotoxicities in cancer and normal cell lines. Although most complexes were either nontoxic or equipotent in cancer cells and normal cell lines, compound 1, [Ru(dpqy)(acac)(py)](PF₆), where dqpy is 2,6-di(quinolin-2-yl)pyridine, showed up to 2.5:1.0 selectivity for cancer as compared to normal cells, along with nanomolar EC₅₀ values in MDA-MB-231 cells. Lipophilicity, determined as the octanol/water partition coefficient, log P_o/w, ranged from -0.33 (0.06) to 1.15 (0.10) for the complexes. Although cytotoxicity was not correlated with electrochemical potentials, a moderate linear correlation between lipophilicity and toxicities was observed. Cell death mechanism studies indicated that several of the Ru-acac compounds, including 1, induce apoptosis in MDA-MB-231 cells.

Development of novel ruthenium(II)-arene complexes displaying potent anticancer effects in glioblastoma cells

Glioblastomas (GBs) are highly aggressive and malignant brain tumors, which are highly resistant to conventional multimodal treatments, leading to their abysmal prognosis. Herein, we designed two organometallic half-sandwich Ru(ii)-η6-p-cymene complexes containing Schiff bases derived from 3-aminoquinoline and 2-hydroxy-benzaldehyde (L1) and 2-hydroxy-naphthaldehyde (L2), namely [Ru(η6-p-cymene)(L1)Cl] (1) and [Ru(η6-p-cymene)(L2)Cl] (2), respectively, and studied their activity on GB cells. Both complexes were structurally characterized using single-crystal X-ray diffraction, which exhibited their half-sandwich three-legged piano-stool geometry. Furthermore, we studied their physicochemical behavior, solution speciation, aquation kinetics, and photo-substitution reactions using various spectroscopic methods. The complexes exhibited a moderate binding affinity with calf-thymus (CT)-DNA (Kb ∼ 105 M-1). The complexes effectively interacted with human serum albumin (HSA) (K ∼ 105 M-1) with preferential tryptophan binding, as determined via synchronous fluorescence studies. The in vitro studies showed their significant antiproliferative activity against an aggressive human GB cell line, LN-229 (IC50 = 22.8 μM), with moderate selectivity relative to normal mouse fibroblast L929 cells. Notably, [Ru(η6-p-cymene)(L1)Cl] (1) exhibited a higher selectivity index (S.I.) than [Ru(η6-p-cymene)(L2)Cl] (2) and cisplatin. We evaluated the clonogenic potential of the GB cells using a colony formation assay in the presence of complex 1. Excitingly, it showed ∼75% inhibition of the clonogenic potential of GB cells at the IC50 concentration. Complex 1 also effectively lowered the migratory potential of the GB cells, as assessed by the wound healing assay. The studied compound led to the apoptosis of GB cells, as evidenced by nuclear condensation, blebbing, and enhanced caspase 3/7 activity, and thus has anticipated utility in the treatment of GBs using photochemotherapy.

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.

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.

Structural and Photodynamic Studies on Nitrosylruthenium-Complexed Serum Albumin as a Delivery System for Controlled Nitric Oxide Release

How to deliver nitric oxide (NO) to a physiological target and control its release quantitatively is a key issue for biomedical applications. Here, a water-soluble nitrosylruthenium complex, [(CH₃)₄N][RuCl₃(5cqn)(NO)] (H5cqn = 5-chloro-8-quinoline), was synthesized, and its structure was confirmed with ¹H NMR and X-ray crystal diffraction. Photoinduced NO release was investigated with time-resolved Fourier transform infrared and electron paramagnetic resonance (EPR) spectroscopies. The binding constant of the [RuCl₃(5cqn)(NO)]^- complex with human serum albumin (HSA) was determined by fluorescence spectroscopy, and the binding mode was identified by X-ray crystallography of the HSA and Ru-NO complex adduct. The crystal structure reveals that two molecules of the Ru-NO complex are located in the subdomain IB, which is one of the major drug binding regions of HSA. The chemical structures of the Ru complexes were [RuCl₃(5cqn)(NO)]^- and [RuCl₃(Glycerin)NO]^-, in which the electron densities for all ligands to Ru are unambiguously identified. EPR spin-trapping data showed that photoirradiation triggered NO radical generation from the HSA complex adduct. Moreover, the near-infrared image of exogenous NO from the nitrosylruthenium complex in living cells was observed using a NO-selective fluorescent probe. This study provides a strategy to design an appropriate delivery system to transport NO and metallodrugs in vivo for potential applications.

Indazole as a Privileged Scaffold: The Derivatives and their Therapeutic Applications

BACKGROUND

Heterocyclic compounds, also called heterocycles, are a major class of organic chemical compound that plays a vital role in the metabolism of all living cells. The heterocyclic compound, indazole, has attracted more attention in recent years and is widely present in numerous commercially available drugs. Indazole-containing derivatives, representing one of the most important heterocycles in drug molecules, are endowed with a broad range of biological properties.

METHODS

A literature search was conducted in PubMed, Google Scholar and Web of Science regarding articles related to indazole and its therapeutic application.

RESULTS

The mechanism and structure-activity relationship of indazole and its derivatives were described. Based on their versatile biological activities, the compounds were divided into six groups: anti-inflammatory, antibacterial, anti-HIV, antiarrhythmic, antifungal and antitumour. At least 43 indazole-based therapeutic agents were found to be used in clinical application or clinical trials.

CONCLUSION

This review is a guide for pharmacologists who are in search of valid preclinical/clinical drug compounds where the progress of approved marketed drugs containing indazole scaffold is examined from 1966 to the present day. Future direction involves more diverse bioactive moieties with indazole scaffold and greater insights into its mechanism.

Die Wechselwirkung mit ribosomalen Proteinen begleitet die Stressinduktion des Wirkstoffkandidaten BOLD-100/KP1339 im endoplasmatischen Retikulum

Der metallhaltige Wirkstoff BOLD‐100/KP1339 zeigte bereits vielversprechende Resultate in verschiedenen In vitro‐ und In vivo‐Tumormodellen sowie in klinischen Studien. Der detaillierte Wirkmechanismus wurde jedoch noch nicht komplett aufgeklärt. Als entscheidende Wirkstoffeffekte kristallisierten sich kürzlich die Stressinduktion im endoplasmatischen Retikulum (ER) und die damit einhergehende Modulierung von HSPA5 (GRP78) heraus. Das spontane und stabile Addukt zwischen BOLD‐100 und menschlichem Serumalbumin wurde als Immobilisierungsstrategie ausgewählt, um einen chemoproteomischen Ansatz auszuführen, der die ribosomalen Proteine RPL10, RPL24 und den Transkriptionsfaktor GTF2I als potentielle Interaktoren dieser Ru(III)‐Verbindung identifizierten. Dieses Ergebnis wurde mit proteomischen und transkriptomischen Profiling‐Experimenten kombiniert, was die Interpretation einer ribosomalen Beeinträchtigung sowie der Induktion von ER‐Stress unterstützte. Die Bildung von Polyribosomen und begleitende ER‐Schwellungen in behandelten Krebszellen wurden zudem durch TEM‐Messungen bestätigt. Somit scheint eine direkte Wechselwirkung von BOLD‐100 mit ribosomalen Proteinen die ER‐Stressinduktion und die Modulierung von GRP78 in Krebszellen zu begleiten.

Interaction with Ribosomal Proteins Accompanies Stress Induction of the Anticancer Metallodrug BOLD-100/KP1339 in the Endoplasmic Reticulum

The ruthenium-based anticancer agent BOLD-100/KP1339 has shown promising results in several in vitro and in vivo tumour models as well as in early clinical trials. However, its mode of action remains to be fully elucidated. Recent evidence identified stress induction in the endoplasmic reticulum (ER) and concomitant down-modulation of HSPA5 (GRP78) as key drug effects. By exploiting the naturally formed adduct between BOLD-100 and human serum albumin as an immobilization strategy, we were able to perform target-profiling experiments that revealed the ribosomal proteins RPL10, RPL24, and the transcription factor GTF2I as potential interactors of this ruthenium(III) anticancer agent. Integrating these findings with proteomic profiling and transcriptomic experiments supported ribosomal disturbance and concomitant induction of ER stress. The formation of polyribosomes and ER swelling of treated cancer cells revealed by TEM validated this finding. Thus, the direct interaction of BOLD-100 with ribosomal proteins seems to accompany ER stress-induction and modulation of GRP78 in cancer cells.

Recent advances in protein metalation: structural studies

Recent advances in structural studies unveiling the basis of the metal compounds/protein recognition process are discussed.

Kinetically labile ruthenium(II) complexes of terpyridines and saccharin: effect of substituents on photoactivity, solvation kinetics, and photocytotoxicity

Herein, we designed six kinetically labile ruthenium(ii) complexes containing saccharin (sac) and 4'-substituted-2,2':6',2''-terpyridines (R-tpy), viz. trans-[Ru(sac)2(H2O)3(dmso-S)] (1) and [RuII(R-tpy)(sac)2(X)] [X = solvent molecule] (2-6). We intentionally kept the labile hydrolysable Ru-X bonds that were potentially activated via solvent-exchange reactions. This strategy generates a coordinative vacancy that allows further binding with potential biological targets. To gain insight into the electronic effects of ancillary ligands on Ru-X ligand-exchange kinetics or photoreactions, we have used a series of substituted terpyridines (R-tpy) and studied their solvation kinetics. The ternary complexes were also studied for their potential utility in Ru-assisted photoactivated chemotherapy (PACT) synergized with release of saccharin as a highly selective carbonic anhydrase IX (CA-IX) inhibitor, over-expressed in hypoxic tumors. The ternary complexes exhibit distorted octahedral geometry around Ru(ii) from two monodentate transoidal saccharin in the axial position, and tridentate terpyridines and labile solvent molecules at the basal plane (2-6). We studied their speciation, solvation kinetics, and photoreactivity in the presence of green LED light (λirr = 530 nm). All the complexes are relatively labile and undergo solvation in coordinating solvents (e.g. DMSO/DMF). The complexes undergo the ligand-substitution reaction, and their speciation and kinetics were studied by UV-Vis, ESI-MS, 1H-NMR, and structural analysis. We also attempted to assess the effect of various substituents on the ancillary terpyridine ligand (R-tpy) in photo-reactivity and ligand-exchange reactions. The photo-induced absorption and emission measurements suggested dissociation of the saccharin from the Ru-center supporting PACT pathways. The complexes display a significant binding affinity with CT-DNA (Kb ∼ 104-105 M-1) and bovine serum albumin (BSA) (KBSA ∼ 105 M-1). Cytotoxicity was studied in the dark and the presence of low energy UV-A light (365 nm) in cervical cancer cells (HeLa) and breast cancer cells (MCF7). Photoirradiation of the complexes induces the generation of reactive oxygen species (ROS) assessed using 1,3-diphenylisobenzofuran (DPBF) and intracellular DCFDA assays. The complexes are sufficiently internalized in cancer cells throughout the cytoplasm and nucleus and induce apoptosis as studied by staining with dual dyes using confocal microscopy.

Ruthenium Complexes as Anticancer Agents: A Brief History and Perspectives

Platinum (Pt)-based anticancer drugs such as cisplatin have been used to treat various cancers. However, they have some limitations including poor selectivity and toxicity towards normal cells and increasing chemoresistance. Therefore, there is a need for novel metallo-anticancers, which has not been met for decades. Since the initial introduction of ruthenium (Ru) polypyridyl complex, a number of attempts at structural evolution have been conducted to improve efficacy. Among them, half-sandwich Ru-arene complexes have been the most prominent as an anticancer platform. Such complexes have clearly shown superior anticancer profiles such as increased selectivity toward cancer cells and ameliorating toxicity against normal cells compared to existing Pt-based anticancers. Currently, several Ru complexes are under human clinical trials. For improvement in selectivity and toxicity associated with chemotherapy, Ru complexes as photodynamic therapy (PDT), and photoactivated chemotherapy (PACT), which can selectively activate prodrug moieties in a specific region, have also been investigated. With all these studies on these interesting entities, new metallo-anticancer drugs to at least partially replace existing Pt-based anticancers are anticipated. This review covers a brief description of Ru-based anticancer complexes and perspectives.

Cytotoxic (cis,cis-1,3,5-triaminocyclohexane)ruthenium(II)-diphosphine complexes; evidence for covalent binding and intercalation with DNA

We report cytotoxic ruthenium(ii) complexes of the general formula [RuCl(cis-tach)(diphosphine)]+ (cis-tach = cis-cis-1,3,5-triaminocyclohexane) that have been characterised by 1H, 13C and 31P{1H} NMR spectroscopy, mass spectrometry, X-ray crystallography and elemental analysis. The kinetics of aquation and stability of the active species have been studied, showing that the chlorido ligand is substituted by water at 298 K with first order rate constants of 10-2-10-3 s-1, ideal for potential clinical use as anti-tumour agents. Strong interactions with biologically relevant duplex and quadruplex DNA models correlate with the activity observed with A549, A2780 and 293T cell lines, and the degree of activity was found to be sensitive to the chelating diphosphine ligand. A label-free ptychographic cell imaging technique recorded cell death processes over 4 days. The Ru(ii) cis-tach diphosphine complexes exhibit anti-proliferative effects, in some cases outperforming cisplatin and other cytotoxic ruthenium complexes.

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.

Protein binding studies with human serum albumin, molecular docking and in vitro cytotoxicity studies using HeLa cervical carcinoma cells of Cu(ii)/Zn(ii) complexes containing a carbohydrazone ligand

The interaction of two binuclear mixed ligand Cu(ii) complexes [Cu(o-phen)LCu(OAc)] (1) and [Cu(o-phen)LCu(o-phen)](OAc) (2) (H3L = o-HOC6H4C(H)[double bond, length as m-dash]N-NH-C(OH)[double bond, length as m-dash]N-N[double bond, length as m-dash]C(H)-C6H4OH-o) and a new mononuclear Zn(ii) complex [Zn(HL)(o-phen)(H2O)](OAc)·H2O (3) (H2L = o-HOC6H4-C(H)[double bond, length as m-dash]N-NH-C([double bond, length as m-dash]O)-NH-N[double bond, length as m-dash]C(H)-C6H4OH-o, o-phen = 1,10-phenanthroline, and OAc = CH3COO-) with human serum albumin (HSA) was studied using fluorescence quenching, synchronous and 3D fluorescence measurements and UV-vis spectroscopy. 3D fluorescence studies showed that the HSA structure was altered at the secondary and tertiary levels upon binding with the complexes. This was further supported by the electronic absorption spectral studies of HSA in the absence and presence of the compounds. The average binding distance (r) between HSA and the complexes was obtained by Förster's resonance energy transfer theory. Complex 3 was structurally characterized by X-ray crystallography. Molecular docking studies indicated that all three complexes primarily bind to HSA in subdomain IIA with amino acid residues such as Arg218 and Lys199 which are located at the entrance of Sudlow's site I. The in vitro cytotoxicities of complexes 1-3 against HeLa cells showed promising anticancer activity (IC50 = 3.5, 3.9 and 16.9 μM for 1, 2 and 3, respectively). Live cell time lapse imaging for 1 was done to capture the dynamic behavior of the cells upon treatment with the complex. Cell cycle analysis by flow cytometry with HeLa cells indicated that 1 and 2 induced cell cycle arrest in the G2/M phase while 3 induced arrest in the G0/G1 phase leading to cell death. Compounds 1 and 2 but not 3 induced apoptosis through the mitochondrial pathway as suggested from the relative p53, caspase3 and bcl2 mRNA levels measured by real-time quantitative PCR analysis.

Antitumor Activity of Ruthenium(II) Terpyridine Complexes towards Colon Cancer Cells In Vitro and In Vivo

Ruthenium complexes have attracted considerable interest as potential antitumor agents. Therefore, antitumor activity and systemic toxicity of ruthenium(II) terpyridine complexes were evaluated in heterotopic mouse colon carcinoma. In the present study, cytotoxic effects of recently synthesized ruthenium(II) terpyridine complexes [Ru(Cl-tpy)(en)Cl][Cl] (en = ethylenediamine, tpy = terpyridine, Ru-1) and [Ru(Cl-tpy)(dach)Cl][Cl] (dach = 1,2-diaminocyclohexane, Ru-2) towards human and murine colon carcinoma cells were tested in vitro and in vivo and compared with oxaliplatin, the most commonly used chemotherapeutic agent against colorectal carcinoma. Ruthenium(II) complexes showed moderate cytotoxicity with IC₅₀ values ranging between 19.1 to 167.3 μM against two human, HCT116 and SW480, and one mouse colon carcinoma cell line, CT26. Both ruthenium(II) terpyridine complexes exerted a moderate apoptotic effect in colon carcinoma cells, but induced significant necrotic death. Additionally, both complexes induced cell cycle disturbances, but these effects were specific for the cell line. Further, Ru-1 significantly reduced the growth of primary heterotopic tumor in mice, similarly to oxaliplatin. Renal damage in Ru-1 treated mice was lower in comparison with oxaliplatin treated mice, as evaluated by serum levels of urea and creatinine and histological evaluation, but Ru-1 induced higher liver damage than oxaliplatin, evaluated by the serum levels of alanine aminotransferase. Additionally, the interaction of these ruthenium(II) terpyridine complexes with the tripeptide glutathione (GSH) was investigated by proton nuclear magnetic resonance (¹H NMR) spectroscopy. All reactions led to the formation of monofunctional thiolate adducts [Ru(Cl-tpy)(en)GS-S] (3) and [Ru(Cl-tpy)(dach)GS-S] (4). Our data highlight the significant cytotoxic activity of [Ru(Cl-tpy)(en)Cl][Cl] against human and mouse colon carcinoma cells, as well as in vivo antitumor activity in CT26 tumor-bearing mice similar to standard chemotherapeutic oxaliplatin, accompanied with lower nephrotoxicity in comparison with oxaliplatin.

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.

Alkyne Functionalization of a Photoactivated Ruthenium Polypyridyl Complex for Click-Enabled Serum Albumin Interaction Studies

Studying metal-protein interactions is key for understanding the fate of metallodrugs in biological systems. When a metal complex is not emissive and too weakly bound for mass spectrometry analysis, however, it may become challenging to study such interactions. In this work a synthetic procedure was developed for the alkyne functionalization of a photolabile ruthenium polypyridyl complex, [Ru(tpy)(bpy)(Hmte)](PF₆)₂, where tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′-bipyridine, and Hmte = 2-(methylthio)ethanol. In the functionalized complex [Ru(HCC-tpy)(bpy)(Hmte)](PF₆)₂, where HCC-tpy = 4′-ethynyl-2,2′:6′,2′′-terpyridine, the alkyne group can be used for bioorthogonal ligation to an azide-labeled fluorophore using copper-catalyzed “click” chemistry. We developed a gel-based click chemistry method to study the interaction between this ruthenium complex and bovine serum albumin (BSA). Our results demonstrate that visualization of the interaction between the metal complex and the protein is possible, even when this interaction is too weak to be studied by conventional means such as UV–vis spectroscopy or ESI mass spectrometry. In addition, the weak metal complex-protein interaction is controlled by visible light irradiation, i.e., the complex and the protein do not interact in the dark, but they do interact via weak van der Waals interactions after light activation of the complex, which triggers photosubstitution of the Hmte ligand.

A “clickable” and photosubstitutionally active ruthenium complex has been prepared that bears a terminal alkyne group. In the dark, the saturated coordination sphere of the complex prevents it from interacting with serum albumin. Upon photosubstitution of one ligand, the complex interacts with the protein via weak interactions that were visualized using copper-catalyzed “click” chemistry postfunctionalization with an azide fluorophore on polyacrylamide gel electrophoresis. These studies demonstrate that the metal-protein interaction is triggered by light irradiation.

Anti-adhesive action of novel ruthenium(II) chlorophenyl terpyridine complexes with a high affinity for double-stranded DNA: in vitro and in silico

Interactions of three Ru(II) chlorophenyl terpyridine complexes: [Ru(Cl-Ph-tpy)(en)Cl]Cl (1), [Ru(Cl-Ph-tpy)(dach)Cl]Cl (2) and [Ru(Cl-Ph-tpy)(bpy)Cl]Cl (3) (Cl-Ph-tpy = 4'-(4-chlorophenyl)-2,2':6',2''-terpyridine, en = 1,2-diaminoethane, dach = 1,2-diaminocyclohexane, bpy = 2,2'-bipyridine) with human serum albumin (HSA), calf thymus DNA and a double-helical oligonucleotide d(CGCGAATTCGCG)₂ (1BNA) were examined. Fluorescence emission studies were used to assess the interactions of complexes with HSA, which were of moderate strength for 1 and 2. Molecular docking allowed us to predict mostly π-π stacking and van der Waals interactions between the complexes and the protein. We suggest that the complexes bind to a novel site on HSA, which is different from its druggable sites I, II or III. We suggest a partial intercalation of complexes through the minor groove as a possible mode of interaction with double-helical DNA. Finally, when applied to normal extravillous cell line HTR8/SVneo and JAr choriocarcinoma cell line, complexes 1 and 2 exerted anti-adhesive properties at very low doses, whereas complex 3 had a negligible effect. The obtained results are completion of our studies of Ru(II) terpyridyl complexes that carry N-N ancillary ligands. We suggest a new research direction towards studying the cellular effects of Ru(II) polypyridyl compounds.

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.