Interaction of dinuclear Co(III) cylinders with higher-order DNA structures

Alternative DNA structures play critical roles in fundamental biological processes linked to human diseases. Thus, targeting and stabilizing these structures by specific ligands could affect the progression of cancer and other diseases. Here, we describe, using methods of molecular biophysics, the interactions of two oxidatively locked [Co2L3]6+ cylinders, rac-2 and meso-1, with diverse alternative DNA structures, such as junctions, G quadruplexes, and bulges. This study was motivated by earlier results demonstrating that both Co(III) cylinders exhibit potent and selective activity against cancer cells, accumulate in the nucleus of cancer cells, and prove to be efficient DNA binders. The results show that the bigger cylinder rac-2 stabilizes all DNA structures, while the smaller cylinder meso-1 stabilizes just the Y-shaped three-way junctions. Collectively, the results of this study suggest that the stabilization of alternative DNA structures by Co(III) cylinders investigated in this work might contribute to the mechanism of their biological activity.

Photoactivatable, mitochondria targeting dppz iridium(III) complex selectively interacts and damages mitochondrial DNA in cancer cells

Cyclometalated Ir(III) complex [Ir(L)2(dppz)]PF6 (where L = 1-methyl-2-(thiophen-2-yl)-1H-benzo[d]imidazole and dppz = dipyrido [3,2-a:2',3'-c]phenazine) (Ir1) is potent anticancer agent whose potency can be significantly increased by irradiation with blue light. Structural features of the cyclometalated Ir(III) complex Ir1 investigated in this work, particularly the presence of dppz ligand possessing an extended planar area, suggest that this complex could interact with DNA. Here, we have shown that Ir1 accumulates predominantly in mitochondria of cancer cells where effectively and selectively binds mitochondrial (mt)DNA. Additionally, the results demonstrated that Ir1 effectively suppresses transcription of mitochondria-encoded genes, especially after irradiation, which may further affect mitochondrial (and thus also cellular) functions. The observation that Ir1 binds selectively to mtDNA implies that the mechanism of its biological activity in cancer cells may also be connected with its interaction and damage to mtDNA. Further investigations revealed that Ir1 tightly binds DNA in a cell-free environment, with sequence preference for GC over AT base pairs. Although the dppz ligand itself or as a ligand in structurally similar DNA-intercalating Ru polypyridine complexes based on dppz ligand intercalates into DNA, the DNA binding mode of Ir1 comprises surprisingly a groove binding rather than an intercalation. Also interestingly, after irradiation with visible (blue) light, Ir1 was capable of cleaving DNA, likely due to the production of superoxide anion radical. The results of this study show that mtDNA damage by Ir1 plays a significant role in its mechanism of antitumor efficacy. In addition, the results of this work are consistent with the hypothesis and support the view that targeting the mitochondrial genome is an effective strategy for anticancer (photo)therapy and that the class of photoactivatable dipyridophenazine Ir(III) compounds may represent prospective substances suitable for further testing.

Novel 2-(5-Arylthiophen-2-yl)-benzoazole Cyclometalated Iridium(III) dppz Complexes Exhibit Selective Phototoxicity in Cancer Cells by Lysosomal Damage and Oncosis

A second-generation series of biscyclometalated 2-(5-aryl-thienyl)-benzimidazole and -benzothiazole Ir(III) dppz complexes [Ir(C^N)2(dppz)]+, Ir1-Ir4, were rationally designed and synthesized, where the aryl group attached to the thienyl ring was p-CF3C6H4 or p-Me2NC6H4. These new Ir(III) complexes were assessed as photosensitizers to explore the structure-activity correlations for their potential use in biocompatible anticancer photodynamic therapy. When irradiated with blue light, the complexes exhibited high selective potency across several cancer cell lines predisposed to photodynamic therapy; the benzothiazole derivatives (Ir1 and Ir2) were the best performers, Ir2 being also activatable with green or red light. Notably, when irradiated, the complexes induced leakage of lysosomal content into the cytoplasm of HeLa cancer cells and induced oncosis-like cell death. The capability of the new Ir complexes to photoinduce cell death in 3D HeLa spheroids has also been demonstrated. The investigated Ir complexes can also catalytically photo-oxidate NADH and photogenerate 1O2 and/or •OH in cell-free media.

Asymmetric triplex metallohelices stabilise DNA G-quadruplexes in promoter oncogene sequences and efficiently reduce their expression in cancer cells

Some metallo-supramolecular helical assemblies with size, shape, charge and amphipathic architectures similar to short cationic α-helical peptides have been shown to target and stabilise DNA G-quadruplexes (G4s) in vitro and downregulate the expression of G4-regulated genes in human cells. To expand the library of metallohelical structures that can act as efficient DNA G4 binders and downregulate genes containing G4-forming sequences in their promoter regions, we investigated the interaction of the two enantiomeric pairs of asymmetric Fe(II) triplex metallohelices with a series of five different DNA G4s formed by the human telomeric sequence (hTelo) and in the promoter regions of c-MYC, c-KIT, and k-RAS oncogenes. The metallohelices display preferential binding to G4s over duplex DNA in all investigated G4-forming sequences and induced arrest of DNA polymerase on template strands containing G4-forming sequences. Moreover, the investigated metallohelices suppressed the expression of c-MYC and k-RAS genes at mRNA and protein levels in HCT116 human cancer cells, as revealed by RT-qPCR analysis and western blotting.

N-Indolyl diiron vinyliminium complexes exhibit antiproliferative effects in cancer cells associated with disruption of mitochondrial homeostasis, ROS scavenging, and antioxidant activity

The indole scaffold has been established as a key organic moiety for developing new drugs; on the other hand, a range of diiron bis-cyclopentadienyl complexes have recently emerged for their promising anticancer potential. Here, we report the synthesis of novel diiron complexes with an indole-functionalized vinyliminium ligand (2-5) and an indole-lacking analogue for comparative purposes (6), which were characterized by analytical and spectroscopic techniques. Complexes 2-6 are substantially stable in DMSO‑d6 and DMEM-d solutions at 37 °C (8% average degradation after 48 h) and display a balanced hydrophilic/lipophilic behaviour (LogPow values in the range -0.32 to 0.47), associated with appreciable water solubility. The complexes display selective antiproliferative potency towards several cancer cells in monolayer cultures, mainly in the low micromolar range, with reduced toxicity towards noncancerous epithelial cells. Thus, the cytotoxicity of the complexes is comparable to or better than clinically used metallopharmaceutical cisplatin. Comparing the antiproliferative activity obtained for complexes containing different ligands, we confirmed the importance of the indolyl group in the mechanism of antiproliferative activity of these complexes. Cell-based mechanistic studies suggest that the investigated diiron vinyliminium complexes (DVCs) show cytostatic rather than cytotoxic effects and subsequently induce a population of cells to undergo apoptosis. Furthermore, the molecular mechanism of action involves interactions with mitochondrial DNA and proteins, the reactive oxygen species (ROS)-scavenging properties and antioxidant activity of these complexes in cancer cells. This study highlights the importance of DVCs to their cancer cell activity and reinforces their prospective therapeutic potential as anticancer agents.

Multitargeting Prodrugs that Release Oxaliplatin, Doxorubicin and Gemcitabine are Potent Inhibitors of Tumor Growth and Effective Inducers of Immunogenic Cell Death

A multitargeting prodrug (2) that releases gemcitabine, oxaliplatin, and doxorubicin in their active form in cancer cells is a potent cytotoxic agent with nM IC50s ; it is highly selective to cancer cells with mean selectivity indices to human (136) and murine (320) cancer cells. It effectively induces release of DAMPs (CALR, ATP & HMGB1) in CT26 cells facilitating more efficient phagocytosis by J774 macrophages than the FDA drugs or their co-administration. The viability of CT26 cells co-cultured with J774 macrophages and treated with 2 was reduced by 32 % compared to the non-treated cells, suggesting a synergistic antiproliferative effect between the chemical and immune reactions. 2 inhibited in vivo tumor growth in two murine models (LLC and CT26) better than the FDA drugs or their co-administration with significantly lower body weight loss. Mice inoculated with CT26 cells treated with 2 showed slightly better tumor free survival than doxorubicin.

The mechanism of antiproliferative activity of the oxaliplatin pyrophosphate derivative involves its binding to nuclear DNA in cancer cells

(1R,2R-diaminocyclohexane)(dihydropyrophosphato) platinum(II), also abbreviated as RRD2, belongs to a class of potent antitumor platinum cytostatics called phosphaplatins. Curiously, several published studies have suggested significant mechanistic differences between phosphaplatins and conventional platinum antitumor drugs. Controversial findings have been published regarding the role of RRD2 binding to DNA in the mechanism of its antiproliferative activity in cancer cells. This prompted us to perform detailed studies to confirm or rule out the role of RRD2 binding to DNA in its antiproliferative effect in cancer cells. Here, we show that RRD2 exhibits excellent antiproliferative activity in various cancer cell lines, with IC50 values in the low micromolar or submicromolar range. Moreover, the results of this study demonstrate that DNA lesions caused by RRD2 contribute to killing cancer cells treated with this phosphaplatin derivative. Additionally, our data indicate that RRD2 accumulates in cancer cells but to a lesser extent than cisplatin. On the other hand, the efficiency of cisplatin and RRD2, after they accumulate in cancer cells, in binding to nuclear DNA is similar. Our results also show that RRD2 in the medium, in which the cells were cultured before RRD2 accumulated inside the cells, remained intact. This result is consistent with the view that RRD2 is activated by releasing free pyrophosphate only in the environment of cancer cells, thereby allowing RRD2 to bind to nuclear DNA.

Metallohelices stabilize DNA three-way junctions and induce DNA damage in cancer cells

DNA three-way junctions (3WJ) represent one of the simplest supramolecular DNA structures arising as intermediates in homologous recombination in the absence of replication. They are also formed transiently during DNA replication. Here we examine the ability of Fe(II)-based metallohelices to act as DNA 3WJ binders and induce DNA damage in cells. We investigated the interaction of eight pairs of enantiomerically pure Fe(II) metallohelices with four different DNA junctions using biophysical and molecular biology methods. The results show that the metallohelices stabilize all types of tested DNA junctions, with the highest selectivity for the Y-shaped 3WJ and minimal selectivity for the 4WJ. The potential of the best stabilizer of DNA junctions and, at the same time, the most selective 3WJ binder investigated in this work to induce DNA damage was determined in human colon cancer HCT116 cells. These metallohelices proved to be efficient in killing cancer cells and triggering DNA damage that could yield therapeutic benefits.

Ir(III) Compounds Containing a Terdentate Ligand Are Potent Inhibitors of Proliferation and Effective Antimetastatic Agents in Aggressive Triple-Negative Breast Cancer Cells

Herein, we report a series of new octahedral iridium(III) complexes Ir1-Ir9 of the type [Ir(N^N^N)(C^N)Cl]PF₆ (N^N^N = 4'-(p-tolyl)-2,2':6',2″-terpyridine; C^N = deprotonated 2-arylbenzimidazole backbone) to introduce new metal-based compounds for effective inhibition of metastatic processes in triple-negative breast cancer (TNBC). The results show that the structural modifications within the C^N scaffold strongly impact the antimetastatic properties of these complexes in TNBC cells. Furthermore, testing the antimetastatic effects of the investigated Ir complexes revealed that the highest antimetastatic activity in TNBC cells is exhibited by complex Ir1. This result was in contrast to the effects of the clinically used drug doxorubicin used in conventional chemotherapy of TNBC, which conversely promoted metastatic properties of TNBC cells. Thus, the latter result suggests that doxorubicin chemotherapy may increase the risk of metastasis of breast cancer cells, so the search for new drugs to treat breast cancer that would show better antitumor effects than doxorubicin is justified.

Palladium and Platinum Complexes of the Antimetabolite Fludarabine with Vastly Enhanced Selectivity for Tumour over Non-Malignant Cells

The purine derivative fludarabine is part of frontline therapy for chronic lymphocytic leukaemia (CLL). It has shown positive effects on solid tumours such as melanoma, breast, and colon carcinoma in clinical phase I studies. As the treatment of CLL cells with combinations of fludarabine and metal complexes of antitumoural natural products, e.g., illudin M ferrocene, has led to synergistically enhanced apoptosis, in this research study different complexes of fludarabine itself. Four complexes bearing a trans-[Br(PPh₃)₂]Pt/Pd fragment attached to atom C-8 via formal η¹-sigma or η²-carbene bonds were synthesised in two or three steps without protecting polar groups on the arabinose or adenine. The platinum complexes were more cytotoxic than their palladium analogues, with low single-digit micromolar IC₅₀ values against cells of various solid tumour entities, including cisplatin-resistant ones and certain B-cell lymphoma and CLL, presumably due to the ten-fold higher cellular uptake of the platinum complexes. However, the palladium complexes interacted more readily with isolated Calf thymus DNA. Interestingly, the platinum complexes showed vastly greater selectivity for cancer over non-malignant cells when compared with fludarabine.

Platinum(IV) Derivatives of [Pt(1S,2S-diaminocyclohexane)(5,6-dimethyl-1,10-phenanthroline)] with Diclofenac Ligands in the Axial Positions: A New Class of Potent Multi-action Agents Exhibiting Selectivity to Cancer Cells

The platinum(II) complex [Pt(1S,2S-diaminocyclohexane)(5,6-dimethyl-1,10-phenanthroline)]²⁺ (Pt^II56MeSS, 1) exhibits high potency across numerous cancer cell lines acting by a multimodal mechanism. However, 1 also displays side toxicity and in vivo activity; all details of its mechanism of action are not entirely clear. Here, we describe the synthesis and biological properties of new platinum(IV) prodrugs that combine 1 with one or two axially coordinated molecules of diclofenac (DCF), a non-steroidal anti-inflammatory cancer-selective drug. The results suggest that these Pt(IV) complexes exhibit mechanisms of action typical for Pt(II) complex 1 and DCF, simultaneously. The presence of DCF ligand(s) in the Pt(IV) complexes promotes the antiproliferative activity and selectivity of 1 by inhibiting lactate transporters, resulting in blockage of the glycolytic process and impairment of mitochondrial potential. Additionally, the investigated Pt(IV) complexes selectively induce cell death in cancer cells, and the Pt(IV) complexes containing DCF ligands induce hallmarks of immunogenic cell death in cancer cells.

Triplex metallohelices have enantiomer-dependent mechanisms of action in colon cancer cells

Self-assembled enantiomers of an asymmetric di-iron metallohelix differ in their antiproliferative activities against HCT116 colon cancer cells such that the compound with Λ-helicity at the metals becomes more potent than the Δ compound with increasing exposure time. From concentration- and temperature-dependent ⁵⁷Fe isotopic labelling studies of cellular accumulation we postulate that while the more potent Λ enantiomer undergoes carrier-mediated efflux, for Δ the process is principally equilibrative. Cell fractionation studies demonstrate that both enantiomers localise in a similar fashion; compound is observed mostly within the cytoskeleton and/or genomic DNA, with significant amounts also found in the nucleus and membrane, but with negligible concentration in the cytosol. Cell cycle analyses using flow cytometry reveal that the Δ enantiomer induces mild arrest in the G₁ phase, while Λ causes a very large dose-dependent increase in the G₂/M population at a concentration significantly below the relevant IC₅₀. Correspondingly, G₂-M checkpoint failure as a result of Λ-metallohelix binding to DNA is shown to be feasible by linear dichroism studies, which indicate, in contrast to the Δ compound, a quite specific mode of binding, probably in the major groove. Further, spindle assembly checkpoint (SAC) failure, which could also be responsible for the observed G₂/M arrest, is established as a feasible mechanism for the Λ helix via drug combination (synergy) studies and the discovery of tubulin and actin inhibition. Here, while the Λ compound stabilizes F-actin and induces a distinct change in tubulin architecture of HCT116 cells, Δ promotes depolymerization and more subtle changes in microtubule and actin networks.

Cyclometalated Benzimidazole Osmium(II) Complexes with Antiproliferative Activity in Cancer Cells Disrupt Calcium Homeostasis

We present the synthesis and characterization of six new heteroleptic osmium(II) complexes of the type [Os(C^N)(N^N)₂]OTf (N^N = 2,2'-bipyridine and dipyrido[3,2-d:2',3'-f]quinoxaline; C^N = deprotonated methyl 1-butyl-2aryl-benzimidazolecarboxylate) with varying substituents in the R3 position of the phenyl ring of the cyclometalating C^N ligand. The new compounds are highly kinetically inert and absorb a full-wavelength range of visible light. An investigation of the antiproliferative activity of the new compounds has been performed using a panel of human cancer and noncancerous 2D cell monolayer cultures under dark conditions and green light irradiation. The results demonstrate that the new Os(II) complexes are markedly more potent than conventional cisplatin. The promising antiproliferative activity of selected Os(II) complexes was also confirmed using 3D multicellular tumor spheroids, which have the characteristics of solid tumors and can mimic the tumor tissue microenvironment. The mechanism of antiproliferative action of complexes has also been investigated and revealed that the investigated Os(II) complexes activate the endoplasmic reticulum stress pathway in cancer cells and disrupt calcium homeostasis.

Ruthenium(II)-Tris-pyrazolylmethane Complexes Inhibit Cancer Cell Growth by Disrupting Mitochondrial Calcium Homeostasis

While ruthenium arene complexes have been widely investigated for their medicinal potential, studies on homologous compounds containing a tridentate tris(1-pyrazolyl)methane ligand are almost absent in the literature. Ruthenium(II) complex 1 was obtained by a modified reported procedure; then, the reactions with a series of organic molecules (L) in boiling alcohol afforded novel complexes 2–9 in 77–99% yields. Products 2–9 were fully structurally characterized. They are appreciably soluble in water, where they undergo partial chloride/water exchange. The antiproliferative activity was determined using a panel of human cancer cell lines and a noncancerous one, evidencing promising potency of 1, 7, and 8 and significant selectivity toward cancer cells. The tested compounds effectively accumulate in cancer cells, and mitochondria represent a significant target of biological action. Most notably, data provide convincing evidence that the mechanism of biological action is mediated by the inhibiting of mitochondrial calcium intake.

Is antitumor Pt(IV) complex containing two axial lonidamine ligands a true dual- or multi-action prodrug?

This work studied the mechanism of action of a Pt(IV) complex 2 bearing two axial lonidamine ligands, which are selective inhibitors of aerobic glycolysis. The presence of two lonidamine ligands in 2 compared to the parent Pt(II) complex increased its antiproliferative activity, cellular accumulation, and changed its cell cycle profile and mechanism of cell death. In 3D cell culture, 2 showed exceptional antiproliferative activity with IC50 values as low as 1.6 μM in MCF7 cells. The study on the influence of the lonidamine ligands in the Pt complex on glycolysis showed only low potency of ligands to affect metabolic processes in cancer cells, making the investigated complex, not a dual- or multi-action prodrug. However, the Pt(IV) prodrug effectively delivers the cytotoxic Pt(II) complex into cancer cells.

Dipyridophenazine iridium(III) complex as a phototoxic cancer stem cell selective, mitochondria targeting agent

In this work, the mechanism underlying the anticancer activity of a photoactivatable Ir(III) compound of the type [Ir(CˆN)₂(dppz)][PF₆] where CˆN = 1-methyl-2-(2'-thienyl)benzimidazole (complex 1) was investigated. Complex 1 photoactivated by visible light shows potent activity against highly aggressive and poorly treatable Rhabdomyosarcoma (RD) cells, the most frequent soft tissue sarcomas of children. This remarkable activity of 1 was observed not only in RD cells cultured in 2D monolayers but, more importantly, also in 3D spheroids, which resemble in many aspects solid tumors and serve as a promising model to mimic the in vivo situation. Importantly, photoactivated 1 kills not only differentiated RD cells but also even more effectively cancer stem cells (CSCs) of RD. One of the factors responsible for the activity of irradiated 1 in RD CSCs is its ability to produce ROS in these cells more effectively than in differentiated RD cells. Moreover, photoactivated 1 caused in RD differentiated cells and CSCs a significant decrease of mitochondrial membrane potential and promotes opening mitochondrial permeability transition pores in these cells, a mechanism that has never been demonstrated for any other metal-based anticancer complex. The results of this work give evidence that 1 has a potential for further evaluation using in vivo models as a promising chemotherapeutic agent for photodynamic therapy of hardly treatable human Rhabdomyosarcoma, particularly for its activity in both stem and differentiated cancer cells.

Metallohelix vectors for efficient gene delivery via cationic DNA nanoparticles

The design of efficient and safe gene delivery vehicles remains a major challenge for the application of gene therapy. Of the many reported gene delivery systems, metal complexes with high affinity for nucleic acids are emerging as an attractive option. We have discovered that certain metallohelices-optically pure, self-assembling triple-stranded arrays of fully encapsulated Fe-act as nonviral DNA delivery vectors capable of mediating efficient gene transfection. They induce formation of globular DNA particles which protect the DNA from degradation by various restriction endonucleases, are of suitable size and electrostatic potential for efficient membrane transport and are successfully processed by cells. The activity is highly structure-dependent-compact and shorter metallohelix enantiomers are far less efficient than less compact and longer enantiomers.

Pt(ii) complex containing the 1R,2R enantiomer of trans-1,2-diamino-4-cyclohexene ligand effectively and selectively inhibits the viability of aggressive pancreatic adenocarcinoma cells and alters their lipid metabolism

New compounds structurally derived from oxaliplatin exhibit high potency in malignant pancreatic adenocarcinoma cells. Their mechanism of antiproliferative action in pancreatic cancer cells involves inhibition of de novo lipid synthesis.

Dinuclear nickel(ii) supramolecular helicates down-regulate gene expression in human cells by stabilizing DNA G-quadruplexes formed in the promoter regions

Dinuclear nickel(ii) supramolecular helicates selectively stabilize DNA G-quadruplexes and suppress G-quadruplex-regulated genes.

A [Pt(cis-1,3-diaminocycloalkane)Cl2] analog exhibits hallmarks typical of immunogenic cell death inducers in model cancer cells

The platinum drugs belong to prevailing chemotherapeutics used in the treatment of cancer. At present, however, the search for new anticancer metal-based drugs that operate by the mechanisms distinct from those of the conventional chemotherapeutics is very active. Furthermore, it has been demonstrated that cytotoxic chemotherapy and immunotherapy may exert a highly synergistic anticancer activity. Thus, the development of antitumor platinum and other metal-based drugs that exhibit cytostatic effects and concurrently elicit immunogenic cell death (ICD) has shown promise for cancer treatment. Notably, conventional platinum drug oxaliplatin ([Pt(1R,2R-DACH)(oxalate)], DACH = diaminocyclohexane) is a well-known agent that displays both cytostatic and immune responses. Moreover, it was also demonstrated that even minor derivatization of the unleaving cycloalkyl moiety in oxaliplatin might have a pronounced effect on its immunomodulatory activity. Here, we investigated how replacing the 1R,2R- diaminocyclohexane ring by 1,3-diaminocycloalkane (alkane = butane, pentane, or hexane) affects the ability to evoke secretion of damage-associated molecular patterns characteristic of ICD in model murine colorectal carcinoma cell line CT26. The results indicate that among the investigated [Pt(cis-1,3-diaminocycloalkane)Cl₂] complexes, the complex containing the cyclobutyl moiety exhibits the hallmarks typical of ICD inducers. Thus, [Pt(cis-1,3-diaminocyclobutane)Cl₂] may expand the spectrum of anticancer chemotherapeutics capable of inducing ICD in cancer cells and might be of interest for further (pre)clinical development.

Processing and Bypass of a Site-Specific DNA Adduct of the Cytotoxic Platinum-Acridinylthiourea Conjugate by Polymerases Involved in DNA Repair: Biochemical and Thermodynamic Aspects

DNA-dependent DNA and RNA polymerases are important modulators of biological functions such as replication, transcription, recombination, or repair. In this work performed in cell-free media, we studied the ability of selected DNA polymerases to overcome a monofunctional adduct of the cytotoxic/antitumor platinum–acridinylthiourea conjugate [PtCl(en)(L)](NO₃)₂ (en = ethane-1,2-diamine, L = 1-[2-(acridin-9-ylamino)ethyl]-1,3-dimethylthiourea) (ACR) in its favored 5′-CG sequence. We focused on how a single site-specific ACR adduct with intercalation potency affects the processivity and fidelity of DNA-dependent DNA polymerases involved in translesion synthesis (TLS) and repair. The ability of the G(N7) hybrid ACR adduct formed in the 5′-TCGT sequence of a 24-mer DNA template to inhibit the synthesis of a complementary DNA strand by the exonuclease-deficient Klenow fragment of DNA polymerase I (KF^exo−) and human polymerases eta, kappa, and iota was supplemented by thermodynamic analysis of the polymerization process. Thermodynamic parameters of a simulated translesion synthesis across the ACR adduct were obtained by using microscale thermophoresis (MST). Our results show a strong inhibitory effect of an ACR adduct on enzymatic TLS: there was only small synthesis of a full-length product (less than 10%) except polymerase eta (~20%). Polymerase eta was able to most efficiently bypass the ACR hybrid adduct. Incorporation of a correct dCMP opposite the modified G residue is preferred by all the four polymerases tested. On the other hand, the frequency of misinsertions increased. The relative efficiency of misinsertions is higher than that of matched cytidine monophosphate but still lower than for the nonmodified control duplex. Thermodynamic inspection of the simulated TLS revealed a significant stabilization of successively extended primer/template duplexes containing an ACR adduct. Moreover, no significant decrease of dissociation enthalpy change behind the position of the modification can contribute to the enzymatic TLS observed with the DNA-dependent, repair-involved polymerases. This TLS could lead to a higher tolerance of cancer cells to the ACR conjugate compared to its enhanced analog, where thiourea is replaced by an amidine group: [PtCl(en)(L)](NO₃)₂ (complex AMD, en = ethane-1,2-diamine, L = N-[2-(acridin-9-ylamino)ethyl]-N-methylpropionamidine).

Are Pt(IV) Prodrugs That Release Combretastatin A4 True Multi-action Prodrugs?

"Multi-action" Pt(IV) derivatives of cisplatin with combretastatin A4 (CA4) bioactive ligands that are conjugated to Pt(IV) by carbonate are unique because the ligand (IC₅₀ < 10 nM) is dramatically 1000-folds more cytotoxic than cisplatin in vitro. The Pt(IV)-CA4 prodrugs were as cytotoxic as CA4 itself, indicating that the platinum moiety probably plays an insignificant role in triggering cytotoxicity, suggesting that the Pt(IV)-CA4 complexes act as prodrugs for CA4 rather than as true multi-action prodrugs. In vivo tests (Lewis lung carcinoma) show that ctc-[Pt(NH₃)₂(PhB)(CA4)Cl₂] inhibited tumor growth by 93% compared to CA4 (67%), cisplatin (84%), and 1:1:1 cisplatin/CA4/PhB (85%) while displaying <5% body weight loss compared to cisplatin (20%) or CA4 (10%). In this case, and perhaps with other extremely potent bioactive ligands, platinum(IV) acts merely as a self-immolative carrier triggered by reduction in the cancer cell with only a minor contribution to cytotoxicity.

On the Biology of Werner's Complex

Werner's Complex, as a cationic coordination complex (CCC), has hitherto unappreciated biological properties derived from its binding affinity to highly anionic biomolecules such as glycosaminoglycans (GAGs) and nucleic acids. Competitive inhibitor and spectroscopic assays confirm the high affinity to GAGs heparin, heparan sulfate (HS), and its pentasaccharide mimetic Fondaparinux (FPX). Functional consequences of this affinity include inhibition of FPX cleavage by bacterial heparinase and mammalian heparanase enzymes with inhibition of cellular invasion and migration. Werner's Complex is a very efficient condensing agent for DNA and tRNA. In proof-of-principle for translational implications, it is demonstrated to display antiviral activity against human cytomegalovirus (HCMV) at micromolar concentrations with promising selectivity. Exploitation of non-covalent hydrogen-bonding and electrostatic interactions has motivated the unprecedented discovery of these properties, opening new avenues of research for this iconic compound.

DNA-Intercalative Platinum Anticancer Complexes Photoactivated by Visible Light

Photoactivatable agents offer the prospect of highly selective cancer therapy with low side effects and novel mechanisms of action that can combat current drug resistance. 1,8-Naphthalimides with their extended π system can behave as light-harvesting groups, fluorescent probes and DNA intercalators. We conjugated N-(carboxymethyl)-1,8-naphthalimide (gly-R-Nap) with an R substituent on the naphthyl group to photoactive diazido PtIV complexes to form t,t,t-[Pt(py)2 (N3 )2 (OH)(gly-R-Nap)], R=H (1), 3-NO2 (2) or 4-NMe2 (3). They show enhanced photo-oxidation, cellular accumulation and promising photo-cytotoxicity in human A2780 ovarian, A549 lung and PC3 prostate cancer cells with visible light activation, and low dark cytotoxicity. Complexes 1 and 2 exhibit pre-intercalation into DNA, resulting in enhanced photo-induced DNA crosslinking. Complex 3 has a red-shifted absorption band at 450 nm, allowing photoactivation and photo-cytotoxicity with green light.

FeII Metallohelices Stabilize DNA G-Quadruplexes and Downregulate the Expression of G-Quadruplex-Regulated Oncogenes

DNA G-quadruplexes (G4s) have been identified within the promoter regions of many proto-oncogenes. Thus, G4s represent attractive targets for cancer therapy, and the design and development of new drugs as G4 binders is a very active field of medicinal chemistry. Here, molecular biophysics and biology methods were employed to investigate the interaction of chiral metallohelices with a series of four DNA G4s (hTelo, c-myc, c-kit1, c-kit2) that are formed by the human telomeric sequence (hTelo) and in the promoter regions of c-MYC and c-KIT proto-oncogenes. We show that the investigated water-compatible, optically pure metallohelices, which are made by self-assembly of simple nonpeptidic organic components around Fe^II ions and exhibit bioactivity emulating the natural systems, bind with high affinity to G4 DNA and much lower affinity to duplex DNA. Notably, both enantiomers of a metallohelix containing a m-xylenyl bridge (5 b) were found to effectively inhibit primer elongation catalyzed by Taq DNA polymerase by stabilizing G4 structures formed in the template strands containing c-myc and c-kit2 G4-forming sequences. Moreover, both enantiomers of 5 b downregulated the expression of c-MYC and c-KIT oncogenes in human embryonic kidney cells at mRNA and protein levels. As metallohelices also bind alternative nucleic acid structures, they hold promise as potential multitargeted drugs.

Unexpected solution behaviour of ester-functionalized half-sandwich Ru(II) and Ir(III) complexes

Complexes [Ru(η6-pcym)(bpydca)Cl]PF6 (Rudca) and [Ir(η5-Cp*)(bpydca)Cl]PF6 (Irdca) were developed as model compounds for the investigation of multi-targeted ester-functionalized half-sandwich ruthenium(ii) and iridium(iii) complexes; pcym = 1-methyl-4-(propan-2-yl)benzene (p-cymene), bpydca = 2,2'-bipyridine-4,4'-diyldimethanediyl bis(dichloroacetate), Cp* = pentamethylcyclopentadienyl. Aiming to understand the in-solution behaviour of these first-in-class complexes containing the pyruvate dehydrogenase kinase inhibitor dichloroacetate (dca) as the terminal bioactive substituent, several experiments were performed under aqueous conditions for Rudca and Irdca, as well as for compounds [Ru(η6-pcym)(bpyOH)Cl]PF6 (RuOH) and [Ir(η5-Cp*)(bpyOH)Cl]PF6 (IrOH), and acetyl analogues [Ru(η6-pcym)(bpyac)Cl]PF6 (Ruac) and [Ir(η5-Cp*)(bpyac)Cl]PF6 (Irac) bearing a different (biologically inactive) terminal substituent; bpyOH = 2,2'-bipyridine-4,4'-diyldimethanol, bpyac = 2,2'-bipyridine-4,4'-diyldimethanediyl diacetate. The experiments were also conducted in the presence of porcine liver esterase (PLE). All the six complexes were characterized by relevant techniques (e.g., NMR and mass spectrometry), including a single-crystal X-ray analysis of complexes Rudca, Ruac, RuOH and IrOH. Although designed as model compounds, Rudca, Irdca, RuOH and IrOH were also screened for their antiproliferative activity in four human cancer cell lines (HCT116 colon carcinoma, MDA-MB-231 and MCF-7 breast adenocarcinomas, DU145 prostate carcinoma), where the tested complexes did not show any effect (IC50 > 100 μM).

Hetero-Bis-Conjugation of Bioactive Molecules to Half-Sandwich Ruthenium(II) and Iridium(III) Complexes Provides Synergic Effects in Cancer Cell Cytotoxicity

Four bipyridine-type ligands variably derivatized with two bioactive groups (taken from ethacrynic acid, flurbiprofen, biotin, and benzylpenicillin) were prepared via sequential esterification steps from commercial 2,2'-bipyridine-4,4'-dicarboxylic acid and subsequently coordinated to ruthenium(II) p-cymene and iridium(III) pentamethylcyclopentadienyl scaffolds. The resulting complexes were isolated as nitrate salts in high yields and fully characterized by analytical and spectroscopic methods. NMR and MS studies in aqueous solution and in cell culture medium highlighted a substantial stability of ligand coordination and a slow release of the bioactive fragments in the latter case. The complexes were assessed for their antiproliferative activity on four cancer cell lines, showing cytotoxicity to the low micromolar level (equipotent with cisplatin). Additional biological experiments revealed a multimodal mechanism of action of the investigated compounds, involving DNA metalation and enzyme inhibition. Synergic effects provided by specific combinations of metal and bioactive fragments were identified, pointing toward an optimal ethacrynic acid/flurbiprofen combination for both Ru(II) and Ir(III) complexes.

A Cyclometalated IrIII Complex Conjugated to a Coumarin Derivative Is a Potent Photodynamic Agent against Prostate Differentiated and Tumorigenic Cancer Stem Cells

A cyclometalated Ir^III complex conjugated to a far-red-emitting coumarin, Ir^III -COUPY (3), was recently shown as a very promising photosensitizer suitable for photodynamic therapy of cancer. Therefore, the primary goal of this work was to deepen knowledge on the mechanism of its photoactivated antitumor action so that this information could be used to propose a new class of compounds as drug candidates for curing very hardly treatable human tumors, such as androgen resistant prostatic tumors of metastatic origin. Conventional anticancer chemotherapies exhibit several disadvantages, such as limited efficiency to target cancer stem cells (CSCs), which are considered the main reason for chemotherapy resistance, relapse, and metastasis. Herein, we show, using DU145 tumor cells, taken as the model of hormone-refractory and aggressive prostate cancer cells resistant to conventional antineoplastic drugs, that the photoactivated conjugate 3 very efficiently eliminates both prostate bulk (differentiated) and prostate hardly treatable CSCs simultaneously and with a similar efficiency. Notably, the very low toxicity of Ir^III -COUPY conjugate in the prostate DU145 cells in the dark and its pronounced selectivity for tumor cells compared with noncancerous cells could result in low side effects and reduced damage of healthy cells during the photoactivated therapy by this agent. Moreover, the experiments performed with the 3D spheroids formed from DU145 CSCs showed that conjugate 3 can penetrate the inner layers of tumor spheres, which might markedly increase its therapeutic effect. Also interestingly, this conjugate induces apoptotic cell death in prostate cancer DU145 cells associated with calcium signaling flux in these cells and autophagy. To the best of our knowledge, this is the first study demonstrating that a photoactivatable metal-based compound is an efficient agent capable of killing even hardly treatable CSCs.

Triazole-based, optically-pure metallosupramolecules; highly potent and selective anticancer compounds

Functionalised triazole aldehydes are used in the highly selective self-assembly of water-compatible, optically pure, low symmetry Fe(ii)- and Zn(ii)-based metallohelices. Sub-micromolar antiproliferative activity is observed against various cancerous cell lines, accompanied by excellent selectivity versus non-cancerous cells and potential for synergistic combinatorial therapy with cisplatin.

Antitumor substitution-inert polynuclear platinum complexes stabilize G-quadruplex DNA and suppress G-quadruplex-mediated gene expression

Anticancer substitution-inert polynuclear platinum(ii) complexes (SI-PPCs) effectively stabilize DNA G-quadruplexes (G4) and terminate DNA polymerization on templates containing G4-forming sequences.

A photoactivated Ir(iii) complex targets cancer stem cells and induces secretion of damage-associated molecular patterns in melanoma cells characteristic of immunogenic cell death

The new iridium complex selectively targets cancer stem cells and has potential to induce immunogenic cell death in cancer cells.

Platinum(IV)-Estramustine Multiaction Prodrugs Are Effective Antiproliferative Agents against Prostate Cancer Cells

Herein, we describe the synthesis, characterization, and biological properties of Pt(IV) derivatives of cisplatin with estramustine at the first axial position, which is known to disrupt the microtubule assembly and act as an androgen antagonist, and varying the second axial position using an innocent ligand (acetate or hydroxyl) to prepare dual-action and triple-action prodrugs with known inhibitors of histone deacetylase, cyclooxygenase, and pyruvate dehydrogenase kinase. We demonstrate superior antiproliferative activity at submicromolar concentrations of the prodrugs against a panel of cancer cell lines, particularly against prostate cancer cell lines. The results obtained in this study exemplify the complex mode of action of "multiaction" Pt(IV) prodrugs. Interestingly, changing the second axial ligand in the Pt-estramustine complex has a significant effect on the mode of action, suggesting that all three components of the Pt(IV) prodrugs (platinum moiety and axial ligands) contribute to the killing of cells and not just one dominant component.

Thermodynamic Insights by Microscale Thermophoresis into Translesion DNA Synthesis Catalyzed by DNA Polymerases Across a Lesion of Antitumor Platinum-Acridine Complex

Translesion synthesis (TLS) through DNA adducts of antitumor platinum complexes has been an interesting aspect of DNA synthesis in cells treated with these metal-based drugs because of its correlation to drug sensitivity. We utilized model systems employing a DNA lesion derived from a site-specific monofunctional adduct formed by antitumor [PtCl(en)(L)](NO₃)₂ (complex AMD, en = ethane-1,2-diamine, L = N-[2-(acridin-9-ylamino)ethyl]-N-methylpropionamidine) at a unique G residue. The catalytic efficiency of TLS DNA polymerases, which differ in their processivity and fidelity for the insertion of correct dCTP, with respect to the other incorrect nucleotides, opposite the adduct of AMD, was investigated. For a deeper understanding of the factors that control the bypass of the site-specific adducts of AMD catalyzed by DNA polymerases, we also used microscale thermophoresis (MST) to measure the thermodynamic changes associated with TLS across a single, site-specific adduct formed in DNA by AMD. The relative catalytic efficiency of the investigated DNA polymerases for the insertion of correct dCTP, with respect to the other incorrect nucleotides, opposite the AMD adduct, was reduced. Nevertheless, incorporation of the correct C opposite the G modified by AMD of the template strand was promoted by an increasing thermodynamic stability of the resulting duplex. The reduced relative efficiency of the investigated DNA polymerases may be a consequence of the DNA intercalation of the acridine moiety of AMD and the size of the adduct. The products of the bypass of this monofunctional lesion produced by AMD and DNA polymerases also resulted from the misincorporation of dNTPs opposite the platinated G residues. The MST analysis suggested that thermodynamic factors may contribute to the forces that governed enhanced incorporation of the incorrect dNTPs by DNA polymerases.

Substitution-Inert Polynuclear Platinum Complexes Inhibit Reverse Transcription Preferentially in RNA Triplex-Forming Templates

RNA triplexes are significant tertiary structure motifs that are found in many functional RNAs. Hence, small molecules capable of recognition, binding, and stabilization of the triple-helical RNA structures are emerging as attractive potential molecular biology tools and therapeutic agents. Here, we utilize methods of molecular biology and biophysics to study the interactions of a series of antitumor substitution-inert polynuclear platinum complexes (SI-PPCs) with triple-helical RNA structures. We show that SI-PPCs recognize and stabilize RNA triplexes and inhibit reverse transcription preferentially in the RNA template prone to the triplex formation. These so far unexplored properties of SI-PPCs suggest that the targeting of triple-stranded regions in RNA might contribute to the biological effects of SI-PPCs.

A Multi-action PtIV Conjugate with Oleate and Cinnamate Ligands Targets Human Epithelial Growth Factor Receptor HER2 in Aggressive Breast Cancer Cells

HER2-positive breast cancer is an aggressive subtype that typically responds poorly to standard chemotherapy. To design an anticancer drug selective for HER2-expressing breast cancer, a Pt^IV prodrug with axial oleate and cinnamate ligands was synthesized. We demonstrate its superior antiproliferative activity in monolayer and 3D spheroid models; the antiproliferative efficiency increases gradually with increasing expression of HER2. The results also suggest that the released Pt^II compound inhibits the proliferation of cancer cells by a DNA-damage-mediated mechanism. Simultaneously, the released oleic and cinnamic acid can effectively inhibit HER2 expression. To our knowledge, this is the first platinum-based complex inhibiting HER2 expression that does not contain protein or peptide. Moreover, this Pt^IV prodrug is capable of overcoming the resistance of cancer stem cells (CSCs), inducing death in both CSCs and differentiated cancer cells. Thus, the results substantiate our design strategy and demonstrate the potential of this approach for the development of new, therapeutically relevant compounds.

Stabilization of human telomeric RNA G-quadruplex by the water-compatible optically pure and biologically-active metallohelices

RNA G-quadruplexes have been suggested to play key roles in fundamental biological processes and are linked to human diseases. Thus, they also represent good potential therapeutic targets. Here, we describe, using the methods of molecular biophysics, interactions of a series of biologically-active supramolecular cationic metallohelices with human telomeric RNA G-quadruplex. We demonstrate that the investigated metallohelices bind with a high affinity to human telomeric RNA G-quadruplex and that their binding selectivity considerably differs depending on the dimensions and overall shape of the metallohelices. Additionally, the investigated metallohelices inhibit DNA synthesis on the RNA template containing four repeats of the human telomeric sequence by stabilizing the RNA G-quadruplex structure. Collectively, the results of this study suggest that stabilization of RNA sequences capable of G-quadruplex formation by metallohelices investigated in this work might contribute to the mechanism of their biological activity.

Synthesis, antiproliferative activity in cancer cells and DNA interaction studies of [Pt(cis-1,3-diaminocycloalkane)Cl2] analogs

The search for more effective platinum anticancer drugs has led to the design, synthesis, and preclinical testing of hundreds of new platinum complexes. This search resulted in the recognition and subsequent FDA approval of the third-generation Pt(II) anticancer drug, [Pt(1,2-diaminocyclohexane)(oxalate)], oxaliplatin, as an effective agent in treating colorectal and gastrointestinal cancers. Another promising example of the class of anticancer platinum(II) complexes incorporating the Pt(1,n-diaminocycloalkane) moiety is kiteplatin ([Pt(cis-1,4-DACH)Cl₂], DACH = diaminocyclohexane). We report here our progress in evaluating the role of the cycloalkyl moiety in these complexes focusing on the synthesis, characterization, evaluation of the antiproliferative activity in tumor cells and studies of the mechanism of action of new [Pt(cis-1,3-diaminocycloalkane)Cl₂] complexes wherein the cis-1,3-diaminocycloalkane group contains the cyclobutyl, cyclopentyl, and cyclohexyl moieties. We demonstrate that [Pt(cis-1,3-DACH)Cl₂] destroys cancer cells with greater efficacy than the other two investigated 1,3-diamminocycloalkane derivatives, or cisplatin. Moreover, the investigated [Pt(cis-1,3-diaminocycloalkane)Cl₂] complexes show selectivity toward tumor cells relative to non-tumorigenic normal cells. We also performed several mechanistic studies in cell-free media focused on understanding some early steps in the mechanism of antitumor activity of bifunctional platinum(II) complexes. Our data indicate that reactivities of the investigated [Pt(cis-1,3-diaminocycloalkane)Cl₂] complexes and cisplatin with glutathione and DNA binding do not correlate with antiproliferative activity of these platinum(II) complexes in cancer cells. In contrast, we show that the higher antiproliferative activity in cancer cells of [Pt(cis-1,3-DACH)Cl₂] originates from its highest hydrophobicity and most efficient cellular uptake.

Glycoconjugated Metallohelices have Improved Nuclear Delivery and Suppress Tumour Growth In Vivo

Monosaccharides are added to the hydrophilic face of a self-assembled asymmetric FeII metallohelix, using CuAAC chemistry. The sixteen resulting architectures are water-stable and optically pure, and exhibit improved antiproliferative selectivity against colon cancer cells (HCT116 p53+/+ ) with respect to the non-cancerous ARPE-19 cell line. While the most selective compound is a glucose-appended enantiomer, its cellular entry is not mainly glucose transporter-mediated. Glucose conjugation nevertheless increases nuclear delivery ca 2.5-fold, and a non-destructive interaction with DNA is indicated. Addition of the glucose units affects the binding orientation of the metallohelix to naked DNA, but does not substantially alter the overall affinity. In a mouse model, the glucose conjugated compound was far better tolerated, and tumour growth delays for the parent compound (2.6 d) were improved to 4.3 d; performance as good as cisplatin but with the advantage of no weight loss in the subjects.

Oxidation of cis-Diamminediacetato PtII with Hydrogen Peroxide Can Give Rise to Two Isomeric PtIV Products

The oxidation of cis-[Pt(NH₃ )₂ (OAc)₂ ] with H₂ O₂ yields a mixture of two isomers: ctc-[Pt(NH₃ )₂ (OH)₂ (OAc)₂ ] and ctc-[Pt(NH₃ )₂ (OH)(OAc)(OH)(OAc)]. Following modification with 4-phenylbutyric (PhB) anhydride, two isomers were separated and characterized; the symmetric ctc-[Pt(NH₃ )₂ (PhB)₂ (OAc)₂ ] (1) and the nonsymmetric ctc-[Pt(NH₃ )₂ (PhB)(OAc)(PhB)(OAc)] (2). They differ in their log P values and despite having similar cellular uptake and similar DNA platination levels, the symmetric ctc-[Pt(NH₃ )₂ (OH)₂ (OAc)₂ ] is more than 4-fold more potent than the nonsymmetric isomer in a panel of 4 cancer cell lines.