Gold(III) Complexes of Pyridyl- and Isoquinolylamido Ligands: Structural, Spectroscopic, and Biological Studies of a New Class of Dual Topoisomerase I and II Inhibitors

Chiral Au(III) chelates exhibit unique NCI-60 cytotoxicity profiles and interactions with human serum albumin

Au(III) bis(pyrrolide-imine) chelates are emerging as a class of versatile, efficacious metallodrug candidates. Here, we synthesised two enantiopure chiral ligands H2L1 and H2L2 (tetradentate cyclohexane-1,2-diamine-bridged bis(pyrrole-imine) derivatives). Metallation of the ligands with Au(III) afforded the chiral cationic complexes AuL1 and AuL2. The in vitro cytotoxicities of AuL1 and AuL2 determined in the NCI-60 single-dose drug screen were 56.5% and 89.1%, respectively. AuL1 was subsequently selected for a five-dose NCI-60 screen, attaining GI50, IC50, and LC50 values of 4.7, 9.3 and 39.8 μM, respectively. Hierarchical cluster analysis of the NCI-60 data indicated that the profile for AuL1 was similar to that of vinblastine sulfate, a microtubule-targeting vinca alkaloid. Reactions of AuL1 with glutathione (GSH) in vitro confirmed its susceptibility to reduction, Au(III) → Au(I), by intracellular thiols. Because human serum albumin (HSA) is responsible for transporting clinically deployed and investigational drugs, we studied the uptake of AuL1 and AuL2 by HSA to delineate how chirality impacts their protein-binding affinity. Steady-state fluorescence quenching data acquired on the native protein and data from site-specific probes showed that the compounds bind at sites close enough to Trp-214 (subdomain IIA) of HSA to quench the fluorophore. The bimolecular quenching rate constants, Kq, were ca. 102 times higher than the maximum diffusion-controlled collision constant of a biomolecule in water (1010 M-1 s-1), confirming that static fluorescence quenching was the dominant mechanism. The Stern-Volmer constants, KSV, were ∼104 M-1 at 37 °C, while the affinity constants, Ka (37 °C), measured ∼2.1 × 104 M-1 (AuL1) and ∼1.2 × 104 M-1 (AuL2) for enthalpy-driven ligand uptake targeting Sudlow's site I. Although far- and near-UV CD spectroscopy indicated that both complexes minimally perturb the secondary and tertiary structure of HSA, substantial shifts in the CD spectra were recorded for both protein-bound ligands. This study highlights the role of chirality in determining the cytotoxicity profiles and protein binding behaviour of enantiomeric Au(III) chelates.

Next Generation Gold Drugs and Probes: Chemistry and Biomedical Applications

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

The design and discovery of topoisomerase I inhibitors as anticancer therapies

INTRODUCTION

Cancer has been identified as one of the leading causes of death worldwide. The biological target of some anticancer agents is topoisomerase I, an enzyme involved in the relaxation of supercoiled DNA. The synthesis of new compounds with antiproliferative effect and behaving as topoisomerase I inhibitors has become an active field of research. Depending on their mechanism of inhibition, they can be classified as catalytic inhibitors or poisons.

AREAS COVERED

This review article summarizes the state of the art for the development of selective topoisomerase I inhibitors. Collected compounds showed inhibition of the enzyme, highlighting those approved for clinical use, the combination therapies developed, as well as related drawbacks and future focus.

EXPERT OPINION

Research related to topoisomerase I inhibitors in cancer therapy started with camptothecin (CPT). This compound was first selected as a good anticancer agent and then topoisomerase I was identified as its therapeutic target. Derivatives of CPT irinotecan, topotecan, and belotecan are the only clinically approved inhibitors. Currently, their limitations are being addressed by different stretegies. Future studies should focus not only on developing other active molecules but also on improving the bioavailability and pharmacokinetics of potent synthetic derivatives.

Gold (III) Derivatives in Colon Cancer Treatment

Cancer is one of the leading causes of morbidity and mortality worldwide. Colorectal cancer (CRC) is the third most frequently diagnosed cancer in men and the second in women. Standard patterns of antitumor therapy, including cisplatin, are ineffective due to their lack of specificity for tumor cells, development of drug resistance, and severe side effects. For this reason, new methods and strategies for CRC treatment are urgently needed. Current research includes novel platinum (Pt)- and other metal-based drugs such as gold (Au), silver (Ag), iridium (Ir), or ruthenium (Ru). Au(III) compounds are promising drug candidates for CRC treatment due to their structural similarity to Pt(II). Their advantage is their relatively good solubility in water, but their disadvantage is an unsatisfactory stability under physiological conditions. Due to these limitations, work is still underway to improve the formula of Au(III) complexes by combining with various types of ligands capable of stabilizing the Au(III) cation and preventing its reduction under physiological conditions. This review summarizes the achievements in the field of stable Au(III) complexes with potential cytotoxic activity restricted to cancer cells. Moreover, it has been shown that not nucleic acids but various protein structures such as thioredoxin reductase (TrxR) mediate the antitumor effects of Au derivatives. The state of the art of the in vivo studies so far conducted is also described.

Cancer molecular biology and strategies for the design of cytotoxic gold(I) and gold(III) complexes: a tutorial review

This tutorial review highlights key principles underpinning the design of selected metallodrugs to target specific biological macromolecules (DNA and proteins). The review commences with a descriptive overview of the eukaryotic cell cycle and the molecular biology of cancer, particularly apoptosis, which is provided as a necessary foundation for the discovery, design, and targeting of metal-based anticancer agents. Drugs which target DNA have been highlighted and clinically approved metallodrugs discussed. A brief history of the development of mainly gold-based metallodrugs is presented prior to addressing ligand systems for stabilizing and adding functionality to bio-active gold(I) and gold(III) complexes, particularly in the burgeoning field of anticancer metallodrugs. Concepts such as multi-modal and selective cytotoxic agents are covered where necessary for selected compounds. The emerging role of carbenes as the ligand system of choice to achieve these goals for gold-based metallodrug candidates is highlighted prior to closing the review with comments on some future directions that this research field might follow. The latter section ultimately emphasizes the importance of understanding the fate of metal complexes in cells to garner key mechanistic insights.

Synthesis and Biological Activity of Ferrocenyl and Ruthenocenyl Analogues of Etoposide: Discovery of a Novel Dual Inhibitor of Topoisomerase II Activity and Tubulin Polymerization

Two series of the ferrocenyl and ruthenocenyl analogues of etoposide bearing 1,2,3-triazolyl or aminoalkyl linker were synthesized and evaluated for their cytotoxic properties, influence on the cell cycle, ability to induce tubulin polymerization, and inhibition of topoisomerase II activity. We found that the replacement of the etoposide carbohydrate moiety with a metallocenyl group led to organometallic conjugates exhibiting differentiated antiproliferative activity. Biological studies demonstrated that two ferrocenylalkylamino conjugates were notably more active than etoposide, with submicromolar or low-micromolar IC₅₀ values towards SW620, etoposide-resistant SW620E, and methotrexate-resistant SW620M cancer cell lines. Moreover, the simplest ferrocenylmethylamino conjugate exerted dual inhibitory action against tubulin polymerization and topoisomerase II activity while other studied compounds affected only topoisomerase II activity.

A "Golden Age" for the discovery of new antileishmanial agents: Current status of leishmanicidal gold complexes and prospective targets beyond the trypanothione system

Leishmaniasis is one of the most neglected diseases worldwide and is considered a serious public health issue. The current therapeutic options have several disadvantages that make the search for new therapeutics urgent. Gold compounds are emerging as promising candidates based on encouraging in vitro and limited in vivo results for several Au^I and Au^III complexes. The antiparasitic mechanisms of these molecules remain only partially understood. However, a few studies have proposed the trypanothione redox system as a target, similar to the mammalian thioredoxin system, pointed out as the main target for several gold compounds with significant antitumor activity. In this review, we present the current status of the investigation and design of gold compounds directed at treating leishmaniasis. In addition, we explore potential targets in Leishmania parasites beyond the trypanothione system, taking into account previous studies and structure modulation performed for gold-based compounds.

Gold Derivatives Development as Prospective Anticancer Drugs for Breast Cancer Treatment

Commonly used anticancer drugs are cisplatin and other platinum-based drugs. However, the use of these drugs in chemotherapy causes numerous side effects and the onset of frequent drug resistance phenomena. This review summarizes the most recent results on the gold derivatives used for their significant inhibitory effects on the in vitro proliferation of breast cancer cell models and for the consequences deriving from morphological changes in the same cells. In particular, the study discusses the antitumor activity of gold nanoparticles, gold (I) and (III) compounds, gold complexes and carbene-based gold complexes, compared with cisplatin. The results of screening studies of cytotoxicity and antitumor activity for the gold derivatives show that the death of cancer cells can occur intrinsically by apoptosis. Recent research has shown that gold (III) compounds with square planar geometries, such as that of cisplatin, can intercalate the DNA and provide novel anticancer agents. The gold derivatives described can make an important contribution to expanding the knowledge of medicinal bioorganometallic chemistry and broadening the range of anticancer agents available, offering improved characteristics, such as increased activity and/or selectivity, and paving the way for further discoveries and applications.

Is the Way to Fight Cancer Paved with Gold? Metal-Based Carbene Complexes with Multiple and Fascinating Biological Features

Herein, we report the synthesis and the multiple anti-tumor properties of new gold and silver carbene complexes. The chemical modifications, grounded on our previous studies, led us to identify a good lead complex, gold-based, whose biological features are very exciting and promising in the anti-cancer research and could be further developed. Indeed, the bis-[4,5-dichloro-(N-methyl-N’(2-hydroxy-2-phenyl)ethyl-imidazole-2-ylidene)gold(I)]⁺[dichloro-gold]⁻ (AuL7) complex possesses the ability to interfere with at least three important and different intracellular targets, namely the human topoisomerases I and II and tubulin, which are able to modulate metabolic processes not directly correlated each other. We proved that the modifications of the ligands structure in AuL7, with respect to another already published complex, i.e., bis-[4,5-dichloro-(N-methyl-N’(cyclopentane-2ol)-imidazole-2-ylidine)gold(I)]⁺[dichloro-gold]⁻ (AuL4), produce a different behavior toward tubulin-polymerization process, since AuL7 is a tubulin-polymerization inhibitor and AuL4 a stabilizer, with the final same result of hampering the tumor growth. Taken together, our outcomes designate AuL7 as a promising compound for the development of multi-targeted anti-cancer therapies.

Synthesis, characterization and anticancer activity in vitro evaluation of novel dicyanoaurate (I)-based complexes

Molecular structures containing gold, such as auranofin, have been extensively studied in the diagnosis and treatment of many diseases, including cancer treatment. The pharmacological properties of the newly synthesized unique gold-ligand structures have been reported for different cancer cell lines. However, findings on bishydeten-metal salt complexes with gold are rare. In this work, the synthesis of five novel cyanide-bridged coordination compounds having the closed formulae [Ni(bishydeten)][Au(CN)₂]₂ (1), [Cu(bishydeten)][Au(CN)₂]₂ (2), [Zn(bishydeten)₂Au₃(CN)₄][Au₂(CN)₃] (3), [Cd(bishydeten)_0,5]₂[Au(CN)₂]₄.2H₂O (4), and [Cd(bishydeten)₂][Au(CN)₂]₂ (5) (where bisyhdeten = N,N-bis(2-hydroxyethyl)ethylene diamine), and their characterization by elemental, infrared, ESI-MS, X-ray (for 2) and thermic measurement methods were performed. Complexes 1 and 3 are thermally more stable than the other three complexes. For these, pharmacological adequacies were also tested. The nucleic acid and protein binding affinities of the Au (I) compounds were also estimated by spectroscopic and electrophoretic techniques. Au (I) complexes were identified as strong chemotherapeutic with mild cytotoxicity, and they demonstrated a dose-dependent inhibition on the growth of cancer cells with IC50 at 0.11 to 0.47 μM. Investigation of mechanisms of action on cells revealed that Au (I) compounds managed to inhibit cell migration and led to a decrease in cytoskeletal proteins such as CK7 and CK20. However, Au (I) compounds failed to inhibit DNA topoisomerase I. Overall, and we suggest that potent antiproliferative activity, mild cytotoxicity, good solubility, and micromolar dosage of Au (I) compounds containing bisyhdeten-metal derivatives render them the potential focus of further studies as chemotherapeutic agents.

Novel phosphine sulphide gold(i) complexes: topoisomerase I inhibitors and antiproliferative agents

Gold(i) increases the cytotoxicity of phosphine sulfide quinolines against cancer cell lines, while heterocycles maintain the TopI inhibitory activity.

From coins to cancer therapy: Gold, silver and copper complexes targeting human topoisomerases

Cancer is a complex issue and, even though the prevention basics and therapy have been implemented, it is still the second leading death cause worldwide. With the hope to discover new powerful and safer molecules to fight cancer, many researchers focused their attention on metal-based compounds, starting from the most famous and successfully employed anticancer drug, i.e. cisplatin. The current article aims to report the most recent discoveries about the use of gold, silver and copper complexes as antitumor agents, highlighting their influences on important enzymes, namely human topoisomerases. The latter are fundamental for the cell life and, if overexpressed, strongly implicated in cancer onset and progression. The identification of lead complexes targeting human topoisomerases and gifted with the appropriate chemical and pharmacological properties represents a fecund starting point to obtain new and more effective anticancer molecules.

Linked Picolinamide Nickel Complexes as Redox Carriers for Nonaqueous Flow Batteries

The use of nickel complexes utilizing non-innocent ligands based on picolinamide to function as redox carriers in flow batteries was explored. The picolinamide moiety was linked together with -CH₂ CH₂ - (bpen), -CH₂ CH₂ CH₂ - (bppn), and -C₆ H₄ - (bpb) moieties, resulting in two, three, and four quasi-reversible waves, respectively, for the nickel complexes and >3 V difference between the outermost positive and negative waves. The redox events were theoretically modelled for each complex, showing excellent agreement (<0.3 V difference) between the experimental and modelled potentials. Bulk cycling of the most soluble complex, Ni(bppn), indicated only one of the three waves was reversible. Therefore, Ni(bppn) has the ability to act as a negative charge redox carrier in flow cells.

Synthesis and cytotoxic characteristics displayed by a series of Ag(i)-, Au(i)- and Au(iii)-complexes supported by a common N-heterocyclic carbene

The design, synthesis and anticancer properties of a series of Ag(i), Au(i) and Au(iii)–NHC complexes supported by pyridyl[1,2-a]{2-acetylylphenylimidazol}-3-ylidene are described.

Five novel dicyanidoaurate(i)-based complexes exhibiting significant biological activities: synthesis, characterization and three crystal structures

Herein the anticancer, antibacterial, and antifungal effects of the five novel dicyanidoaurate(i)-based complexes have been reported.

Fluorescein hydrazones as novel nonintercalative topoisomerase catalytic inhibitors with low DNA toxicity

Fluorescein hydrazones (3a-3l) were synthesized in three steps with 86-91% overall yields. Topo I- and IIα-mediated relaxation and cell viability assay were evaluated. 3d inhibited 47% Topo I (camptothecin, 34%) and 20% Topo II (etoposide 24%) at 20 μM. 3l inhibited 61% Topo II (etoposide 24%) at 20 μM. 3d and 3l were further evaluated to determine their mode of action with diverse methods of kDNA decatenation, DNA-Topo cleavage complex, comet, DNA intercalating/unwinding, and Topo IIα-mediated ATP hydrolysis assays. 3d functioned as a nonintercalative dual inhibitor against the catalytic activities of Topo I and Topo IIα. 3l acted as a Topo IIα specific nonintercalative catalytic inhibitor. 3d activated apoptotic proteins as it increased the level of cleaved capase-3 and cleaved PARP in a dose- and time-dependent manner. The dose- and time-dependent increase of G1 phase population was observed by treatment of 3d along with the increase of p27(kip1) and the decrease of cyclin D1 expression.

Gold(III) complexes in medicinal chemistry

A number of gold(III) compounds has been designed with the objective of overcoming the disadvantages associated with the platinum-based drugs for cancer treatment. Compounds of a remarkable structural manifold show significant antiproliferative effects in vitro against a number of cancer cells, including cisplatin resistant ones. The target of most of them is, unlike that of cisplatin, not the DNA. Although the mechanisms of action displayed by the gold compounds in biological media are still under investigation, many studies show evidence that the cellular targets are mitochondria-based. Recent advances in gold(III) medicinal chemistry also recommend such compounds for other pharmacological applications such as the treatment of viral or parasitic diseases. The radioactive isotopes 198Au and 199Au present potential in radiotherapy.

Synthesis of Stable Gold(III) Pincer Complexes with Anionic Heteroatom Donors

A series of gold(III) complexes supported by pyridine-based bis(amidate), bis(carboxylate), and bis(iminothiolate) substituents is reported. These compounds represent rare examples of pincer-ligated gold(III) centers with multiple anionic heteroaom donors. Reactivity and electrochemical studies demonstrate the stability of these compounds and the marked difference in reduction potentials with varying ligand scaffolds.

Gold(III) macrocycles: nucleotide-specific unconventional catalytic inhibitors of human topoisomerase I

Topoisomerase IB (Top1) is a key eukaryotic nuclear enzyme that regulates the topology of DNA during replication and gene transcription. Anticancer drugs that block Top1 are either well-characterized interfacial poisons or lesser-known catalytic inhibitor compounds. Here we describe a new class of cytotoxic redox-stable cationic Au³⁺ macrocycles which, through hierarchical cluster analysis of cytotoxicity data for the lead compound, 3, were identified as either poisons or inhibitors of Top1. Two pivotal enzyme inhibition assays prove that the compounds are true catalytic inhibitors of Top1. Inhibition of human topoisomerase IIα (Top2α) by 3 was 2 orders of magnitude weaker than its inhibition of Top1, confirming that 3 is a type I-specific catalytic inhibitor. Importantly, Au³⁺ is essential for both DNA intercalation and enzyme inhibition. Macromolecular simulations show that 3 intercalates directly at the 5′-TA-3′ dinucleotide sequence targeted by Top1 via crucial electrostatic interactions, which include π–π stacking and an Au···O contact involving a thymine carbonyl group, resolving the ambiguity of conventional (drug binds protein) vs unconventional (drug binds substrate) catalytic inhibition of the enzyme. Surface plasmon resonance studies confirm the molecular mechanism of action elucidated by the simulations.

Antiglioma activity of GoPI-sugar, a novel gold(I)-phosphole inhibitor: chemical synthesis, mechanistic studies, and effectiveness in vivo

Glioblastoma, an aggressive brain tumor, has a poor prognosis and a high risk of recurrence. An improved chemotherapeutic approach is required to complement radiation therapy. Gold(I) complexes bearing phosphole ligands are promising agents in the treatment of cancer and disturb the redox balance and proliferation of cancer cells by inhibiting disulfide reductases. Here, we report on the antitumor properties of the gold(I) complex 1-phenyl-bis(2-pyridyl)phosphole gold chloride thio-β-d-glucose tetraacetate (GoPI-sugar), which exhibits antiproliferative effects on human (NCH82, NCH89) and rat (C6) glioma cell lines. Compared to carmustine (BCNU), an established nitrosourea compound for the treatment of glioblastomas that inhibits the proliferation of these glioma cell lines with an IC50 of 430μM, GoPI-sugar is more effective by two orders of magnitude. Moreover, GoPI-sugar inhibits malignant glioma growth in vivo in a C6 glioma rat model and significantly reduces tumor volume while being well tolerated. Both the gold(I) chloro- and thiosugar-substituted phospholes interact with DNA albeit more weakly for the latter. Furthermore, GoPI-sugar irreversibly and potently inhibits thioredoxin reductase (IC50 4.3nM) and human glutathione reductase (IC50 88.5nM). However, treatment with GoPI-sugar did not significantly alter redox parameters in the brain tissue of treated animals. This might be due to compensatory upregulation of redox-related enzymes but might also indicate that the antiproliferative effects of GoPI-sugar in vivo are rather based on DNA interaction and inhibition of topoisomerase I than on the disturbance of redox equilibrium. Since GoPI-sugar is highly effective against glioblastomas and well tolerated, it represents a most promising lead for drug development. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.