Recognition of DNA/RNA bulges by antimicrobial and antitumor metallohelices

Dicobalt(ii) helices kill colon cancer cells via enantiomer-specific mechanisms; DNA damage or microtubule disruption

Highly diastereoselective self-assembly reactions give both enantiomers (Λ and Δ) of anti-parallel triple-stranded bimetallic Co(ii) and Co(iii) cationic helices, without the need for resolution; the first such reaction for Co. The complexes are water soluble and stable, even in the case of Co(ii). Studies in a range of cancer and healthy cell lines indicate high activity and selectivity, and substantial differences between enantiomers. The oxidation state has little effect, and correspondingly, Co(iii) compounds are reduced to Co(ii) e.g. by glutathione. In HCT116 colon cancer cells the Λ enantiomer induces dose-dependent G2-M arrest in the cell cycle and disrupts microtubule architectures. This Co(ii) Λ enantiomer is ca. five times more potent than the isostructural Fe(ii) compound. Since the measured cellular uptakes are similar this implies a higher affinity of the Co system for the intracellular target(s); while the two systems are isostructural they have substantially different charge distributions as shown by calculated hydrophobicity maps. In contrast to the Λ enantiomer, Δ-Co(ii) induces G1 arrest in HCT116 cells, efficiently inhibits the topoisomerase I-catalyzed relaxation of supercoiled plasmid DNA, and, unlike the isostructural Fe(ii) system, causes DNA damage. It thus seems very likely that redox chemistry plays a role in the latter.

Cavity-Containing [Fe2L3]4+ Helicates: An Examination of Host-Guest Chemistry and Cytotoxicity

Two new di(2,2′-bipyridine) ligands, 2,6-bis([2,2′-bipyridin]-5-ylethynyl)pyridine (L1) and bis(4-([2,2′-bipyridin]-5-ylethynyl)phenyl)methane (L2) were synthesized and used to generate two metallosupramolecular [Fe₂(L)₃](BF₄)₄ cylinders. The ligands and cylinders were characterized using elemental analysis, electrospray ionization mass spectrometry, UV-vis, ¹H-, ¹³C and DOSY nuclear magnetic resonance (NMR) spectroscopies. The molecular structures of the [Fe₂(L)₃](BF₄)₄ cylinders were confirmed using X-ray crystallography. Both the [Fe₂(L1)₃](BF₄)₄ and [Fe₂(L2)₃](BF₄)₄ complexes crystallized as racemic (rac) mixtures of the ΔΔ (P) and ΛΛ (M) helicates. However, ¹H NMR spectra showed that in solution the larger [Fe₂(L2)₃](BF₄)₄ was a mixture of the rac-ΔΔ/ΛΛ and meso-ΔΛ isomers. The host-guest chemistry of the helicates, which both feature a central cavity, was examined with several small drug molecules. However, none of the potential guests were found to bind within the helicates. In vitro cytotoxicity assays demonstrated that both helicates were active against four cancer cell lines. The smaller [Fe₂(L1)₃](BF₄)₄ system displayed low μM activity against the HCT116 (IC₅₀ = 7.1 ± 0.5 μM) and NCI-H460 (IC₅₀ = 4.9 ± 0.4 μM) cancer cells. While the antiproliferative effects against all the cell lines examined were less than the well-known anticancer drug cisplatin, their modes of action would be expected to be very different.

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.

Metallohelices emulate the properties of short cationic α-helical peptides

Naturally occurring peptides in many living systems perform antimicrobial and anticancer host defence roles, but their potential for clinical application is limited by low metabolic stability and relatively high costs of goods. Self-assembled helical metal complexes provide an attractive synthetic platform for non-peptidic architectures that can emulate some of the properties of short cationic α-helical peptides, with tuneable charge, shape, size and amphipathicity. Correspondingly there is a growing body of evidence demonstrating that these supramolecular architectures exhibit bioactivity that emulates that of the natural systems. We review that evidence in the context of synthetic advances in the area, driven by the potential for biomedical applications. We note some design considerations for new biologically-relevant metallohelices, and give our outlook on the future of these compounds as therapeutic peptidomimetics.

Lighting up metallohelices: from DNA binders to chemotherapy and photodynamic therapy

The design of novel agents that specifically target DNA and interrupt its normal biological processes is an attractive goal in drug design. Among the promising metallodrugs, metal-directed self-assembled metallohelices with defined three-dimensional stereochemical structures display unique structure-inherent and unprecedented noncovalent targeting abilities towards DNA, resulting in excellent anticancer or antibiotic activities. A newly burgeoning hotspot is focusing on lighting them up by embedding luminescent metal ions as the vertices. The photoactive metallohelices that combine strong interactions toward DNA targets and efficient ¹O₂ quantum yield may provide new motivation in diagnostic and photodynamic therapy (PDT) areas. This perspective focuses on research progress on metallohelices as DNA binders and chemotherapeutic agents, and highlights recent advances in fabricating luminescent examples for PDT. The relative assembly strategies are also discussed and compared. Finally, perspectives on the future development of the lit-up metallohelices are presented.

Discovery of selective, antimetastatic and anti-cancer stem cell metallohelices via post-assembly modification

Helicates and related metallofoldamers, synthesised by dynamic self-assembly, represent an area of chemical space inaccessible by traditional organic synthesis, and yet with potential for discovery of new classes of drug. Here we report that water-soluble, optically pure Fe(ii)- and even Zn(ii)-based triplex metallohelices are an excellent platform for post-assembly click reactions. By these means, the in vitro anticancer activity and most importantly the selectivity of a triplex metallohelix Fe(ii) system are dramatically improved. For one compound, a remarkable array of mechanistic and pharmacological behaviours is discovered: inhibition of Na+/K+ ATPase with potency comparable to the drug ouabain, antimetastatic properties (including inhibition of cell migration, re-adhesion and invasion), cancer stem cell targeting, and finally colonosphere inhibition competitive with the drug salinomycin.

Biomedically Relevant Self-Assembled Metallacycles and Metallacages

Diverse metal-organic complexes (MOCs), shaped as rectangles, triangles, hexagons, prisms, and cages, can be formed by coordination between metal ions (Pt, Pd, Ru, Rh, Ir, Zn, Co, and Cd) and organic ligands, with potential applications as alternatives to conventional biomedical materials for therapeutic, sensing, and imaging purposes. MOCs have been investigated as anticancer drugs in the treatment of malignant tumors in lung, cervical, breast, colon, liver, prostate, ovarian, brain, stomach, bone, skin, mouth, thyroid, and other cancers. MOCs with one, two, and three cavities have also been investigated as drug carriers and prepared for the loading and release of different drugs. In addition, MOCs can target proteins by the shape effect and recognize sugars and DNA by electrostatic interactions, as well as estradiol by host-guest interactions, etc. This Perspective mainly covers achievements in the biomedical application of MOCs. We aim to identify some key trends in the reported MOC structures in relation to their biomedical activity and potential applications.

Induction of a Four-Way Junction Structure in the DNA Palindromic Hexanucleotide 5'-d(CGTACG)-3' by a Mononuclear Platinum Complex

Four-way junctions (4WJs) are supramolecular DNA assemblies comprising four interacting DNA strands that in biology are involved in DNA-damage repair. In this study, a new mononuclear platinum(II) complex 1 was prepared that is capable of driving the crystallization of the DNA oligomer 5'-d(CGTACG)-3' specifically into a 4WJ-like motif. In the crystal structure of the 1-CGTACG adduct, the distorted-square-planar platinum complex binds to the core of the 4WJ-like motif through π-π stacking and hydrogen bonding, without forming any platinum-nitrogen coordination bonds. Our observations suggest that the specific molecular properties of the metal complex are crucially responsible for triggering the selective assembly of this peculiar DNA superstructure.

Recent Developments of Supramolecular Metal-based Structures for Applications in Cancer Therapy and Imaging

The biomedical application of discrete supramolecular metal-based structures, including supramolecular coordination complexes (SCCs), is still an emergent field of study. However, pioneering studies over the last 10 years demonstrated the potential of these supramolecular compounds as novel anticancer drugs, endowed with different mechanisms of action compared to classical small-molecules, often related to their peculiar molecular recognition properties. In addition, the robustness and modular composition of supramolecular metal-based structures allows for an incorporation of different functionalities in the same system to enable imaging in cells via different modalities, but also active tumor targeting and stimuli-responsiveness. Although most of the studies reported so far exploit these systems for therapy, supramolecular metal-based structures may also constitute ideal scaffolds to develop multimodal theranostic agents. Of note, the host-guest chemistry of 3D self-assembled supramolecular structures - within the metallacages family - can also be exploited to design novel drug delivery systems for anticancer chemotherapeutics. In this review, we aim at summarizing the pivotal concepts in this fascinating research area, starting with the main design principles and illustrating representative examples while providing a critical discussion of the state-of-the-art. A section is also included on supramolecular organometallic complexes (SOCs) whereby the (organic) linker is forming the organometallic bond to the metal node, whose biological applications are still to be explored. Certainly, the myriad of possible supramolecular metal-based structures and their almost limitless modularity and tunability suggests that the biomedical applications of such complex chemical entities will continue along this already promising path.

Exosomal packaging of trans-activation response element (TAR) RNA by HIV-1 infected cells: a pro-malignancy message delivery to cancer cells

Driven by the heterogeneous and complex nature of HIV-1 infection and tumors, the possibilities of viable cross-talk are facilitated by the intra-cellular and inter-cellular signaling regimens. There are evidences that support the clear role of exosomes containing TAR RNAs that are secreted by HIV-1 infected cells and these TAR RNA brings pro-growth and pro-survival effects upon cancer cells. Recently, the regulatory role of TAR RNAs in the intra-cellular signaling network is shown that augments cancer cells to achieve tremendous progression and malignancy. In this paper, author highlights the role of HIV-1 infected cells secreted exosomes containing TAR RNA in tumor hallmarks. Further, this paper provides future insights on new classes of cancer therapeutics centered on disrupting exosomes and TAR RNA.

Anticancer Activity and Cisplatin Binding Ability of Bis-Quinoline and Bis-Isoquinoline Derived [Pd2L4]4+ Metallosupramolecular Cages

New bis-quinoline (L q) and bis-isoquinoline-based (L iq) ligands have been synthesized, along with their respective homoleptic [Pd2(L q or L iq)4]4+ cages (C q and C iq). The ligands and cages were characterized by 1H, 13C and diffusion ordered (DOSY) NMR spectroscopies, high resolution electrospray ionization mass spectrometry (HR-ESIMS) and in the case of the bis-quinoline cage, X-ray crystallography. The crystal structure of the C q architecture showed that the [Pd2(L q)4]4+ cage formed a twisted meso isomer where the [Pd(quinoline)4]2+ units at either end of the cage architecture adopt the opposite twists (left and right handed). Conversely, Density Functional Theory (DFT) calculations on the C iq cage architecture indicated that a lantern shaped conformation, similar to what has been observed before for related [Pd2(L tripy)4]4+ systems (where L tripy = 2,6-bis(pyridin-3-ylethynyl)pyridine), was generated. The different cage conformations manifest different properties for the isomeric cages. The C iq cage is able to bind, weakly in acetonitrile, the anticancer drug cisplatin whereas the C q architecture shows no interaction with the guest under the same conditions. The kinetic robustness of the two cages in the presence of Cl- nucleophiles was also different. The C iq cage was completely decomposed into free L iq and [Pd(Cl)4]2- within 1 h. However, the C q cage was more long lived and was only fully decomposed after 7 h. The new ligands (L iq and L q) and the Pd(II) cage architectures (C iq and C q) were assessed for their cytotoxic properties against two cancerous cell lines (A549 lung cancer and MDA-MB-231 breast cancer) and one non-cancerous cell line (HDFa skin cells). It was found that L q and C q were both reasonably cytotoxic (IC50S ≈ 0.5 μM) against A549, while C iq was slightly less active (IC50 = 7.4 μM). L iq was not soluble enough to allow the IC50 to be determined against either of the two cancerous cell lines. However, none of the molecules showed any selectivity for the cancer cells, as they were all found to have similar cytotoxicities against HDFa skin cells (IC50 values ranged from 2.6 to 3.0 μM).

Increased Flexibility between Stems of Intramolecular Three-Way Junctions by the Insertion of Bulges

Intramolecular junctions are a ubiquitous structure within DNA and RNA; three-way junctions in particular have high strain around the junction because of the lack of flexibility, preventing the junctions from adopting conformations that would allow for optimal folding. In this work, we used a combination of calorimetric and spectroscopic techniques to study the unfolding of four intramolecular three-way junctions. The control three-way junction, 3H, has the sequence d(GAAATTGCGCT5GCGCGTGCT5GCACAATTTC), which has three arms of different sequences. We studied three other three-way junctions in which one (2HS1H), two (HS12HS1), and three (HS1HS1HS1) cytosine bulges were placed at the junction to allow the arms to adopt a wider range of conformations that may potentially relieve strain. Through calorimetric studies, it was concluded that bulges produce only minor effects on the enthalpic and thermal stability at physiological salt concentrations for 2HS1H and HS1HS1HS1. HS12HS1 displays the strongest effect, with the GTGC stem lacking a defined transition. In addition to unfolding thermodynamics, the differential binding of counterions, water, and protons was determined. It was found that with each bulge, there was a large increase in the binding of counterions; this correlated with a decrease in the immobilization of structural water molecules. The increase in counterion uptake upon folding likely displaces binding of structural water, which is measured by the osmotic stress method, in favor of electrostricted waters. The cytosine bulges do not affect the binding of protons; this finding indicates that the bulges are not forming base-triplet stacks. These results indicate that bulges in junctions do not affect the unfolding profile or the enthalpy of oligonucleotides but do affect the number and amount of molecules immobilized by the junction.

Antifreeze Protein Mimetic Metallohelices with Potent Ice Recrystallization Inhibition Activity

Antifreeze proteins are produced by extremophile species to control ice formation and growth, and they have potential applications in many fields. There are few examples of synthetic materials which can reproduce their potent ice recrystallization inhibition property. We report that self-assembled enantiomerically pure, amphipathic metallohelicies inhibited ice growth at just 20 μM. Structure-property relationships and calculations support the hypothesis that amphipathicity is the key motif for activity. This opens up a new field of metallo-organic antifreeze protein mimetics and provides insight into the origins of ice-growth inhibition.

Functional metallosupramolecular architectures using 1,2,3-triazole ligands: it's as easy as 1,2,3 “click”

Self-assembled metallosupramolecular architectures generated using “click” ligands have become an increasingly popular area of inorganic chemistry.

Anticancer metallohelices: nanomolar potency and high selectivity

New optically pure helicate-like architectures are extremely active against cancer cell lines, with IC₅₀ values as low as 40 nM, but nearly three orders of magnitude less active against healthy cells. There is also low toxicity to microbes and amoeba.

A range of new helicate-like architectures have been prepared via highly diastereoselective self-assembly using readily accessible starting materials. Six pairs of enantiomers [Fe₂L₃]Cl₄·nH₂O (L = various bidentate ditopic ligands NN–NN) show very good water solubility and stability. Their activity against a range of cancer cell lines in vitro is structure-dependent and gives IC₅₀ values as low as 40 nM. In an isogenic pair of HCT116 colorectal cancer cells, preferential activity was observed against cell lines that lack functional p53. Selectivity is also excellent, and against healthy human retinal pigment epithelial (ARPE19) and lung fibroblast (WI38) cells IC₅₀ values are nearly three orders of magnitude higher. Cisplatin is unselective in the same tests. The compounds also appear to have low general toxicity in a number of models: there is little if any antimicrobial activity against methicillin-resistant Staphylococcus aureus and Escherichia coli; Acanthamoeba polyphaga is unaffected at 25 μg mL^–1 (12.5 μM); Manduca sexta larvae showed clear evidence of systemic distribution of the drug, and rather than any observation of adverse effects they exhibited a significant mean weight gain vs. controls. Investigation of the mode of action revealed no significant interaction of the molecules with DNA, and stimulation of substantial cell death by apoptosis.