Structure-based design and evaluation of naphthalene diimide G-quadruplex ligands as telomere targeting agents in pancreatic cancer cells

Development of a multi-targeted chemotherapeutic approach based on G-quadruplex stabilisation and carbonic anhydrase inhibition

A novel class of compounds designed to hit two anti-tumour targets, G-quadruplex structures and human carbonic anhydrases (hCAs) IX and XII is proposed. The induction/stabilisation of G-quadruplex structures by small molecules has emerged as an anticancer strategy, disrupting telomere maintenance and reducing oncogene expression. hCAs IX and XII are well-established anti-tumour targets, upregulated in many hypoxic tumours and contributing to metastasis. The ligands reported feature a berberine G-quadruplex stabiliser scaffold connected to a moiety inhibiting hCAs IX and XII. In vitro experiments showed that our compounds selectively stabilise G-quadruplex structures and inhibit hCAs IX and XII. The crystal structure of a telomeric G-quadruplex in complex with one of these ligands was obtained, shedding light on the ligand/target interaction mode. The most promising ligands showed significant cytotoxicity against CA IX-positive HeLa cancer cells in hypoxia, and the ability to stabilise G-quadruplexes within tumour cells.

Structural descriptions of ligand interactions to RNA quadruplexes folded from the non-coding region of pseudorabies virus

To rationalise the binding of specific ligands to RNA-quadruplex we investigated several naphthalene diimide ligands that interact with the non-coding region of Pseudorabies virus (PRV). Herein we report on the x-ray structure of the naphthalene diimide ND11 with an RNA G-quadruplex putative forming sequence from rPRV. Consistent with previously observed rPRV sequence it assembles into a bimolecular RNA G-quadruplex consisting of a pair of two tetrads stacked 3' to 5'. We observe that ND11 interacts by binding on both the externally available 5' and 3' quartets. The CUC (loop 1) is structurally altered to enhance the 5' mode of interaction. These loop residues are shifted significantly to generate a new ligand binding pocket whereas the terminal A14 residue is lifted away from the RNA G-quadruplex tetrad plane to be restacked above the bound ND11 ligand NDI core. CD analysis of this family of NDI ligands shows consistency in the spectra between the different ligands in the presence of the rPRV RNA G-quadruplex motif, reflecting a common folded topology and mode of ligand interaction. FRET melt assay confirms the strong stabilising properties of the tetrasubstituted NDI compounds and the contributions length of the substituted groups have on melt temperatures.

Design and synthesis of dual functional NBD-fluorophore-incorporated naphthalene diimide derivatives as G-quadruplex ligands

Nitrobenzoxadiazole (NBD)-incorporated naphthalene diimide derivatives were designed and synthesized as candidates of antitumor agents with cytotoxicity against human pancreatic cancer cell MIA PaCa-2. Among these, compounds 1NND and 3NND exhibited fluorescent "turn-off" property toward human telomeric G-quadruplex (G4), which allows the direct measurement of dissociation constant (Kd) of ligands against G4 by fluorescence titration method. Notably, the compound 1NND not only exhibited great cytotoxic activity against MIA PaCa-2 with a half maximal inhibitory concentration (IC50) of 77.9 nM, but also exhibited high affinity against G4 with Kd of 1.72 μM. Furthermore, the target binding properties were investigated by circular dichroism (CD) spectra and further studied by molecular docking methods.

Understanding, diagnosing, and treating pancreatic cancer from the perspective of telomeres and telomerase

Telomerase is associated with cellular aging, and its presence limits cellular lifespan. Telomerase by preventing telomere shortening can extend the number of cell divisions for cancer cells. In adult pancreatic cells, telomeres gradually shorten, while in precancerous lesions of cancer, telomeres in cells are usually significantly shortened. At this time, telomerase is still in an inactive state, and it is not until before and after the onset of cancer that telomerase is reactivated, causing cancer cells to proliferate. Methylation of the telomerase reverse transcriptase (TERT) promoter and regulation of telomerase by lactate dehydrogenase B (LDHB) is the mechanism of telomerase reactivation in pancreatic cancer. Understanding the role of telomeres and telomerase in pancreatic cancer will help to diagnose and initiate targeted therapy as early as possible. This article reviews the role of telomeres and telomerase as biomarkers in the development of pancreatic cancer and the progress of research on telomeres and telomerase as targets for therapeutic intervention.

Pharmacological properties of extracts and prenylated isoflavonoids from the fruits of Osage orange (Maclura pomifera (Raf.) C.K.Schneid.)

Osage orange trees (Maclura pomifera (Raf.) C.K.Schneid.) are distributed worldwide, particularly in south-east states of the USA. They produce large quantities of strong yellow fruits, bigger than oranges, but these fruits are inedible, with an acid milky juice which is little consumed by birds and insects. Extracts prepared from Osage orange fruits (hedge apple) have revealed a range of pharmacological properties of interest in human and veterinary medicine. In addition, Osage orange extracts can be used in agriculture and aquaculture, and as dyeing agent for the textile industry. Extracts contain potent antioxidant compounds, notably the isoflavonoids pomiferin and auriculasin, together with other terpenoids and flavonoids. The structural characteristics and pharmacological properties of the major prenylated isoflavones isolated from M. pomifera are discussed here, with a focus on the two phenolic compounds osajin and warangalone, and the two catechol analogues pomiferin and auriculasin. The mechanisms at the origin of their potent antioxidant and anti-inflammatory effects are presented, notably inhibition of xanthine oxidase, phosphodiesterase 5A and kinases such as RKS2 and kRAS. Osajin and auriculasin display marked anticancer properties, owing to their ability to inhibit tumor cell proliferation, migration and tumor angiogenesis. Different molecular mechanisms are discussed, including osajin‑copper complexation and binding to quadruplex DNA. An overview of the mechanism of action of the prenylated isoflavones from Osage orange is presented, with the objective to promote their knowledge and to raise opportunities to better exploit the fruits of Osage orange, abundant but largely neglected at present.

A Phenotypic Approach to the Discovery of Potent G-Quadruplex Targeted Drugs

G-quadruplex (G4) sequences, which can fold into higher-order G4 structures, are abundant in the human genome and are over-represented in the promoter regions of many genes involved in human cancer initiation, progression, and metastasis. They are plausible targets for G4-binding small molecules, which would, in the case of promoter G4s, result in the transcriptional downregulation of these genes. However, structural information is currently available on only a very small number of G4s and their ligand complexes. This limitation, coupled with the currently restricted information on the G4-containing genes involved in most complex human cancers, has led to the development of a phenotypic-led approach to G4 ligand drug discovery. This approach was illustrated by the discovery of several generations of tri- and tetra-substituted naphthalene diimide (ND) ligands that were found to show potent growth inhibition in pancreatic cancer cell lines and are active in in vivo models for this hard-to-treat disease. The cycles of discovery have culminated in a highly potent tetra-substituted ND derivative, QN-302, which is currently being evaluated in a Phase 1 clinical trial. The major genes whose expression has been down-regulated by QN-302 are presented here: all contain G4 propensity and have been found to be up-regulated in human pancreatic cancer. Some of these genes are also upregulated in other human cancers, supporting the hypothesis that QN-302 is a pan-G4 drug of potential utility beyond pancreatic cancer.

Comprehensive analysis of intramolecular G-quadruplex structures: furthering the understanding of their formalism

G-quadruplexes (G4) are helical structures found in guanine-rich DNA or RNA sequences. Generally, their formalism is based on a few dozen structures, which can produce some inconsistencies or incompleteness. Using the website ASC-G4, we analyzed the structures of 333 intramolecular G4s, of all types, which allowed us to clarify some key concepts and present new information. To each of the eight distinguishable topologies corresponds a groove-width signature and a predominant glycosidic configuration (gc) pattern governed by the directions of the strands. The relative orientations of the stacking guanines within the strands, which we quantified and related to their vertical gc successions, determine the twist and tilt of the helices. The latter impact the minimum groove widths, which represent the space available for lateral ligand binding. The G4 four helices have similar twists, even when these twists are irregular, meaning that they have various angles along the strands. Despite its importance, the vertical gc succession has no strict one-to-one relationship with the topology, which explains the discrepancy between some topologies and their corresponding circular dichroism spectra. This study allowed us to introduce the new concept of platypus G4s, which are structures with properties corresponding to several topologies.

G-quadruplex ligands in cancer therapy: Progress, challenges, and clinical perspectives

Guanine-rich sequences can form G-quadruplexes (G4) in living cells, making these structures promising anti-cancer targets. Compounds able to recognize these structures have been investigated as potential anticancer drugs; however, no G4 binder has yet been approved in the clinic. Here, we describe G4 ligands structure-activity relationships, in vivo effects as well as clinical trials. Addressing G4 ligand characteristics, targeting challenges, and structure-activity relationships, this review provides insights into the development of potent and selective G4-targeting molecules for therapeutic applications.

Structure-activity relationships for the G-quadruplex-targeting experimental drug QN-302 and two analogues probed with comparative transcriptome profiling and molecular modeling

The tetrasubstituted naphthalene diimide compound QN-302 binds to G-quadruplex (G4) DNA structures. It shows high potency in pancreatic ductal adenocarcinoma (PDAC) cells and inhibits the transcription of cancer-related genes in these cells and in PDAC animal models. It is currently in Phase 1a clinical evaluation as an anticancer drug. A study of structure-activity relationships of QN-302 and two related analogues (CM03 and SOP1247) is reported here. These have been probed using comparisons of transcriptional profiles from whole-genome RNA-seq analyses, together with molecular modelling and molecular dynamics simulations. Compounds CM03 and SOP1247 differ by the presence of a methoxy substituent in the latter: these two compounds have closely similar transcriptional profiles. Whereas QN-302 (with an additional benzyl-pyrrolidine group), although also showing down-regulatory effects in the same cancer-related pathways, has effects on distinct genes, for example in the hedgehog pathway. This distinctive pattern of genes affected by QN-302 is hypothesized to contribute to its superior potency compared to CM03 and SOP1247. Its enhanced ability to stabilize G4 structures has been attributed to its benzyl-pyrrolidine substituent fitting into and filling most of the space in a G4 groove compared to the hydrogen atom in CM03 or the methoxy group substituent in SOP1247.

G-quadruplex structural dynamics at MAPK12 promoter dictates transcriptional switch to determine stemness in breast cancer

P38γ (MAPK12) is predominantly expressed in triple negative breast cancer cells (TNBC) and induces stem cell (CSC) expansion resulting in decreased survival of the patients due to metastasis. Abundance of G-rich sequences at MAPK12 promoter implied the functional probability to reverse tumorigenesis, though the formation of G-Quadruplex (G4) structures at MAPK12 promoter is elusive. Here, we identified two evolutionary consensus adjacent G4 motifs upstream of the MAPK12 promoter, forming parallel G4 structures. They exist in an equilibria between G4 and duplex, regulated by the binding turnover of Sp1 and Nucleolin that bind to these G4 motifs and regulate MAPK12 transcriptional homeostasis. To underscore the gene-regulatory functions of G4 motifs, we employed CRISPR-Cas9 system to eliminate G4s from TNBC cells and synthesized a naphthalene diimide (NDI) derivative (TGS24) which shows high-affinity binding to MAPK12-G4 and inhibits MAPK12 transcription. Deletion of G4 motifs and NDI compound interfere with the recruitment of the transcription factors, inhibiting MAPK12 expression in cancer cells. The molecular basis of NDI-induced G4 transcriptional regulation was analysed by RNA-seq analyses, which revealed that MAPK12-G4 inhibits oncogenic RAS transformation and trans-activation of NANOG. MAPK12-G4 also reduces CD44High/CD24Low population in TNBC cells and downregulates internal stem cell markers, arresting the stemness properties of cancer cells.

QN-302 demonstrates opposing effects between i-motif and G-quadruplex DNA structures in the promoter of the S100P gene

GC-rich sequences can fold into G-quadruplexes and i-motifs and are known to control gene expression in many organisms. The potent G-quadruplex experimental anticancer drug QN-302 down-regulates a number of cancer-related genes, in particular S100P. Here we show this ligand has strong opposing effects with i-motif DNA structures and is one of the most potent i-motif destabilising agents reported to date. QN-302 down-regulates the expression of numerous cancer-related genes by pan-quadruplex targeting. QN-302 exhibits exceptional combined synergistic effects compared to many other G-quadruplex and i-motif interacting compounds. This work further emphasises the importance of considering G-quadruplex and i-motif DNA structures as one dynamic system.

Small Molecules against Metastatic Tumors: Concrete Perspectives and Shattered Dreams

Metastasis is the main cause of anti-cancer therapy failure, leading to unfavorable prognosis for patients. The true challenge to increase cancer patient life expectancy by making cancer a chronic disease with periodic but manageable relapses relies on the development of efficient therapeutic strategies specifically directed against key targets in the metastatic process. Traditional chemotherapy with classical alkylating agents, microtubule inhibitors, and antimetabolites has demonstrated its limited efficacy against metastatic cells due to their capacity to select chemo-resistant cell populations that undergo epithelial-to-mesenchymal transition (EMT), thus promoting the colonization of distant sites that, in turn, sustain the initial metastatic process. This scenario has prompted efforts aimed at discovering a wide variety of small molecules and biologics as potential anti-metastatic drugs directed against more specific targets known to be involved in the various stages of metastasis. In this short review, we give an overview of the most recent advances related to important families of antimetastatic small molecules: intracellular tyrosine kinase inhibitors, cyclin-dependent kinase inhibitors, KRAS inhibitors, and integrin antagonists. Although the majority of these small molecules are not yet approved and not available in the drug market, any information related to their stage of development could represent a precious and valuable tool to identify new targets in the endless fight against metastasis.

Application of G-quadruplex targets in gastrointestinal cancers: Advancements, challenges and prospects

Genomic instability and inflammation are considered to be two enabling characteristics that support cancer development and progression. G-quadruplex structure is a key element that contributes to genomic instability and inflammation. G-quadruplexes were once regarded as simply an obstacle that can block the transcription of oncogenes. A ligand targeting G-quadruplexes was found to have anticancer activity, making G-quadruplexes potential anticancer targets. However, further investigation has revealed that G-quadruplexes are widely distributed throughout the human genome and have many functions, such as regulating DNA replication, DNA repair, transcription, translation, epigenetics, and inflammatory response. G-quadruplexes play double regulatory roles in transcription and translation. In this review, we focus on G-quadruplexes as novel targets for the treatment of gastrointestinal cancers. We summarize the application basis of G-quadruplexes in gastrointestinal cancers, including their distribution sites, structural characteristics, and physiological functions. We describe the current status of applications for the treatment of esophageal cancer, pancreatic cancer, hepatocellular carcinoma, gastric cancer, colorectal cancer, and gastrointestinal stromal tumors, as well as the associated challenges. Finally, we review the prospective clinical applications of G-quadruplex targets, providing references for targeted treatment strategies in gastrointestinal cancers.

Polyamine-Drug Conjugates: Do They Boost Drug Activity?

Over the past two decades, the strategy of conjugating polyamine tails with bioactive molecules such as anticancer and antimicrobial agents, as well as antioxidant and neuroprotective scaffolds, has been widely exploited to enhance their pharmacological profile. Polyamine transport is elevated in many pathological conditions, suggesting that the polyamine portion could improve cellular and subcellular uptake of the conjugate via the polyamine transporter system. In this review, we have presented a glimpse on the polyamine conjugate scenario, classified by therapeutic area, of the last decade with the aim of highlighting achievements and fostering future developments.

Probing juxtaposed G-quadruplex and hairpin motifs using a responsive nucleoside probe: a unique scaffold for chemotherapy

Paucity of efficient probes and small molecule ligands that can distinguish different G-quadruplex (GQ) topologies poses challenges not only in understanding their basic structure but also in targeting an individual GQ form from others. Alternatively, G-rich sequences that harbour unique chimeric structural motifs (e.g., GQ-duplex or GQ-hairpin junctions) are perceived as new therapeutic hotspots. In this context, the epidermal growth factor receptor (EGFR) gene, implicated in many cancers, contains a 30 nucleotide G-rich segment in the promoter region, which adopts in vitro two unique architectures each composed of a GQ topology (parallel and hybrid-type) juxtaposed with a hairpin domain. Here, we report the use of a novel dual-app probe, C5-trifluoromethyl benzofuran-modified 2'-deoxyuridine (TFBF-dU), in the systematic analysis of EGFR GQs and their interaction with small molecules by fluorescence and 19F NMR techniques. Notably, distinct fluorescence and 19F NMR signals exhibited by the probe enabled the quantification of the relative population of random, parallel and hybrid-type GQ structures under different conditions, which could not be obtained by conventional CD and 1H NMR techniques. Using the fluorescence component, we quantified ligand binding properties of GQs, whereas the 19F label enabled the assessment of ligand-induced changes in GQ dynamics. Studies also revealed that mutations in the hairpin domain affected GQ formation and stability, which was further functionally verified in polymerase stop assay. We anticipate that these findings and useful properties of the nucleoside probe could be utilized in designing and evaluating binders that jointly target both GQ and hairpin domains for enhanced selectivity and druggability.

cRGD-Functionalized Silk Fibroin Nanoparticles: A Strategy for Cancer Treatment with a Potent Unselective Naphthalene Diimide Derivative

Developing drug delivery systems to target cytotoxic drugs directly into tumor cells is still a compelling need with regard to reducing side effects and improving the efficacy of cancer chemotherapy. In this work, silk fibroin nanoparticles (SFNs) have been designed to load a previously described cytotoxic compound (NDI-1) that disrupts the cell cycle by specifically interacting with non-canonical secondary structures of DNA. SFNs were then functionalized on their surface with cyclic pentapeptides incorporating the Arg-Gly-Asp sequence (cRGDs) to provide active targeting toward glioma cell lines that abundantly express ανβ3 and ανβ5 integrin receptors. Cytotoxicity and selective targeting were assessed by in vitro tests on human glioma cell lines U373 (highly-expressing integrin subunits) and D384 cell lines (low-expressing integrin subunits in comparison to U373). SFNs were of nanometric size (d50 less than 100 nm), round shaped with a smooth surface, and with a negative surface charge; overall, these characteristics made them very likely to be taken up by cells. The active NDI-1 was loaded into SFNs with high encapsulation efficiency and was not released before the internalization and degradation by cells. Functionalization with cRGDs provided selectivity in cell uptake and thus cytotoxicity, with a significantly higher cytotoxic effect of NDI-1 delivered by cRGD-SFNs on U373 cells than on D384 cells. This manuscript provides an in vitro proof-of-concept of cRGD-silk fibroin nanoparticles’ active site-specific targeting of tumors, paving the way for further in vivo efficacy tests.

The Potent G-Quadruplex-Binding Compound QN-302 Downregulates S100P Gene Expression in Cells and in an In Vivo Model of Pancreatic Cancer

The naphthalene diimide compound QN-302, designed to bind to G-quadruplex DNA sequences within the promoter regions of cancer-related genes, has high anti-proliferative activity in pancreatic cancer cell lines and anti-tumor activity in several experimental models for the disease. We show here that QN-302 also causes downregulation of the expression of the S100P gene and the S100P protein in cells and in vivo. This protein is well established as being involved in key proliferation and motility pathways in several human cancers and has been identified as a potential biomarker in pancreatic cancer. The S100P gene contains 60 putative quadruplex-forming sequences, one of which is in the promoter region, 48 nucleotides upstream from the transcription start site. We report biophysical and molecular modeling studies showing that this sequence forms a highly stable G-quadruplex in vitro, which is further stabilized by QN-302. We also report transcriptome analyses showing that S100P expression is highly upregulated in tissues from human pancreatic cancer tumors, compared to normal pancreas material. The extent of upregulation is dependent on the degree of differentiation of tumor cells, with the most poorly differentiated, from more advanced disease, having the highest level of S100P expression. The experimental drug QN-302 is currently in pre-IND development (as of Q1 2023), and its ability to downregulate S100P protein expression supports a role for this protein as a marker of therapeutic response in pancreatic cancer. These results are also consistent with the hypothesis that the S100P promoter G-quadruplex is a potential therapeutic target in pancreatic cancer at the transcriptional level for QN-302.

Modulating the G-Quadruplex and Duplex DNA Binding by Controlling the Charge of Fluorescent Molecules

Two fluorescent and non-toxic spirobifluorene molecules bearing either positive (Spiro-NMe3) or negative (Spiro-SO3) charged moieties attached to the same aromatic structure have been investigated as binders for DNA. The novel Spiro-NMe3 containing four alkylammonium substituents interacts with G-quadruplex (G4) DNA structures and shows preference for G4s over duplex by means of FRET melting and fluorescence experiments. The interaction is governed by the charged substituents of the ligands as deduced from the lower binding of the sulfonate analogue (Spiro-SO3). On the contrary, Spiro-SO3 exhibits higher binding affinity to duplex DNA structure than to G4. Both molecules show a moderate quenching of the fluorescence upon DNA binding. The confocal microscopy evaluation shows the internalization of both molecules in HeLa cells and their lysosomal accumulation.

The Intertwined Role of 8-oxodG and G4 in Transcription Regulation

The guanine base in nucleic acids is, among the other bases, the most susceptible to being converted into 8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) when exposed to reactive oxygen species. In double-helix DNA, 8-oxodG can pair with adenine; hence, it may cause a G > T (C > A) mutation; it is frequently referred to as a form of DNA damage and promptly corrected by DNA repair mechanisms. Moreover, 8-oxodG has recently been redefined as an epigenetic factor that impacts transcriptional regulatory elements and other epigenetic modifications. It has been proposed that 8-oxodG exerts epigenetic control through interplay with the G-quadruplex (G4), a non-canonical DNA structure, in transcription regulatory regions. In this review, we focused on the epigenetic roles of 8-oxodG and the G4 and explored their interplay at the genomic level.

Revisiting recent unusual drug-DNA complex structures: Implications for cancer and neurological disease diagnostics and therapeutics

DNA plays a crucial role in various biological processes such as protein production, replication, recombination etc. by adopting different conformations. Targeting these conformations by small molecules is not only important for disease therapy, but also improves our understanding of the mechanisms of disease development. In this review, we provide an overview of some of the most recent ligand-DNA complexes that have diagnostic and therapeutic applications in neurological diseases caused by abnormal repeat expansions and in cancer associated with mismatches. In addition, we have discussed important implications of ligands targeting higher-order structures, such as four-way junctions, G-quadruplexes and triplexes for drug discovery and DNA nanotechnology. We provide an overview of the results and perspectives of such structural studies on ligand-DNA interactions.

Insights into the Small Molecule Targeting of Biologically Relevant G-Quadruplexes: An Overview of NMR and Crystal Structures

G-quadruplexes turned out to be important targets for the development of novel targeted anticancer/antiviral therapies. More than 3000 G-quadruplex small-molecule ligands have been described, with most of them exerting anticancer/antiviral activity by inducing telomeric damage and/or altering oncogene or viral gene expression in cancer cells and viruses, respectively. For some ligands, in-depth NMR and/or crystallographic studies were performed, providing detailed knowledge on their interactions with diverse G-quadruplex targets. Here, the PDB-deposited NMR and crystal structures of the complexes between telomeric, oncogenic or viral G-quadruplexes and small-molecule ligands, of both organic and metal-organic nature, have been summarized and described based on the G-quadruplex target, from telomeric DNA and RNA G-quadruplexes to DNA oncogenic G-quadruplexes, and finally to RNA viral G-quadruplexes. An overview of the structural details of these complexes is here provided to guide the design of novel ligands targeting more efficiently and selectively cancer- and virus-related G-quadruplex structures.

Phen-DC3 Induces Refolding of Human Telomeric DNA into a Chair-Type Antiparallel G-Quadruplex through Ligand Intercalation

Human telomeric G-quadruplex DNA structures are attractive anticancer drug targets, but the target's polymorphism complicates the drug design: different ligands prefer different folds, and very few complexes have been solved at high resolution. Here we report that Phen-DC3 , one of the most prominent G-quadruplex ligands in terms of high binding affinity and selectivity, causes dTAGGG(TTAGGG)3 to completely change its fold in KCl solution from a hybrid-1 to an antiparallel chair-type structure, wherein the ligand intercalates between a two-quartet unit and a pseudo-quartet, thereby ejecting one potassium ion. This unprecedented high-resolution NMR structure shows for the first time a true ligand intercalation into an intramolecular G-quadruplex.

Design, synthesis and bioactivity of novel naphthalimide-benzotriazole conjugates against A549 cells via targeting BCL2 G-quadruplex and inducing autophagy

AIMS

In this study, a series of novel naphthalimide-benzotriazole conjugates (1a-3c) based on 1, 8-naphthalimide as a core skeleton, aiming at G-quadruplexes, were designed and synthesized, and their anti-cancer activity and mechanism were studied.

MATERIALS AND METHODS

Using the CCK-8 assay, FRET melting, EMSA, CD, and molecular docking, intracellular assays, western blotting, immunofluorescence, and flow cytometry.

KEY FINDINGS

By the CCK-8 assay, it was found that the compound, 2-(3-(piperazin-1-yl)propyl)-6-(1H-benzo [d][1,2,3]triazol-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (3a), has better activity against A549 cells. Through extracellular assays, including FRET melting, EMSA, CD, and molecular docking, results showed that 3a selectively interacted with BCL2 G-quadruplex(es). Further studies by intracellular assays, including western blotting, immunofluorescence, flow cytometry, etc., verified that 3a mediated the death of A549 cells by two pathways: inhibition of the expression of the BCL2 gene, causing tumor cell apoptosis, and promotion of genetic instability, causing autophagy. This study suggests that the type of compounds, in particular, 3a, may be a potential molecule to explore for BCL2 G-quadruplex-targeted drugs against lung cancer.

SIGNIFICANCE

Our findings demonstrate that compound 3a as a BCL2 G-quadruplex ligand induces DNA damage, autophagy, and apoptosis in A549 cells. This study provides us with a type of lead compound as an anti-tumor drug.

Targeting the RNA G-Quadruplex and Protein Interactome for Antiviral Therapy

In recent years, G-quadruplexes (G4s), types of noncanonical four-stranded nucleic acid structures, have been identified in many viruses that threaten human health, such as HIV and Epstein-Barr virus. In this context, G4 ligands were designed to target the G4 structures, among which some have shown promising antiviral effects. In this Perspective, we first summarize the diversified roles of RNA G4s in different viruses. Next, we introduce small-molecule ligands developed as G4 modulators and highlight their applications in antiviral studies. In addition to G4s, we comprehensively review the medical intervention of G4-interacting proteins from both the virus (N protein, viral-encoded helicases, severe acute respiratory syndrome-unique domain, and Epstein-Barr nuclear antigen 1) and the host (heterogeneous nuclear ribonucleoproteins, RNA helicases, zinc-finger cellular nucelic acid-binding protein, and nucleolin) by inhibitors as an alternative way to disturb the normal functions of G4s. Finally, we discuss the challenges and opportunities in G4-based antiviral therapy.

More than forty years of nucleic acid structural science

As scientists who have worked with Stephen Neidle over many years and stages of his career, we present our perspective of his contributions to nucleic acid structural science. We trace some of the highlights of his research on nucleic acid drug interactions and the unique insights about the importance of hydration.

Ruthenium Polypyridyl Complex Bound to a Unimolecular Chair-Form G-Quadruplex

The DNA G-quadruplex is known for forming a range of topologies and for the observed lability of the assembly, consistent with its transient formation in live cells. The stabilization of a particular topology by a small molecule is of great importance for therapeutic applications. Here, we show that the ruthenium complex Λ-[Ru(phen)₂(qdppz)]²⁺ displays enantiospecific G-quadruplex binding. It crystallized in 1:1 stoichiometry with a modified human telomeric G-quadruplex sequence, GGGTTAGGGTTAGGGTTTGGG (htel21T₁₈), in an antiparallel chair topology, the first structurally characterized example of ligand binding to this topology. The lambda complex is bound in an intercalation cavity created by a terminal G-quartet and the central narrow lateral loop formed by T₁₀–T₁₁–A₁₂. The two remaining wide lateral loops are linked through a third K⁺ ion at the other end of the G-quartet stack, which also coordinates three thymine residues. In a comparative ligand-binding study, we showed, using a Klenow fragment assay, that this complex is the strongest observed inhibitor of replication, both using the native human telomeric sequence and the modified sequence used in this work.

Major Achievements in the Design of Quadruplex-Interactive Small Molecules

Organic small molecules that can recognize and bind to G-quadruplex and i-Motif nucleic acids have great potential as selective drugs or as tools in drug target discovery programs, or even in the development of nanodevices for medical diagnosis. Hundreds of quadruplex-interactive small molecules have been reported, and the challenges in their design vary with the intended application. Herein, we survey the major achievements on the therapeutic potential of such quadruplex ligands, their mode of binding, effects upon interaction with quadruplexes, and consider the opportunities and challenges for their exploitation in drug discovery.

Rational design of small-molecules to recognize G-quadruplexes of c-MYC promoter and telomere and the evaluation of their in vivo antitumor activity against breast cancer

DNA G4-structures from human c-MYC promoter and telomere are considered as important drug targets; however, the developing of small-molecule-based fluorescent binding ligands that are highly selective in targeting these G4-structures over other types of nucleic acids is challenging. We herein report a new approach of designing small molecules based on a non-selective thiazole orange scaffold to provide two-directional and multi-site interactions with flanking residues and loops of the G4-motif for better selectivity. The ligands are designed to establish multi-site interactions in the G4-binding pocket. This structural feature may render the molecules higher selectivity toward c-MYC G4s than other structures. The ligand–G4 interaction studied with ¹H NMR may suggest a stacking interaction with the terminal G-tetrad. Moreover, the intracellular co-localization study with BG4 and cellular competition experiments with BRACO-19 may suggest that the binding targets of the ligands in cells are most probably G4-structures. Furthermore, the ligands that either preferentially bind to c-MYC promoter or telomeric G4s are able to downregulate markedly the c-MYC and hTERT gene expression in MCF-7 cells, and induce senescence and DNA damage to cancer cells. The in vivo antitumor activity of the ligands in MCF-7 tumor-bearing mice is also demonstrated.

Structured Waters Mediate Small Molecule Binding to G-Quadruplex Nucleic Acids

The role of G-quadruplexes in human cancers is increasingly well-defined. Accordingly, G-quadruplexes can be suitable drug targets and many small molecules have been identified to date as G-quadruplex binders, some using computer-based design methods and co-crystal structures. The role of bound water molecules in the crystal structures of G-quadruplex-small molecule complexes has been analyzed in this study, focusing on the water arrangements in several G-quadruplex ligand complexes. One is the complex between the tetrasubstituted naphthalene diimide compound MM41 and a human intramolecular telomeric DNA G-quadruplex, and the others are in substituted acridine bimolecular G-quadruplex complexes. Bridging water molecules form most of the hydrogen-bond contacts between ligands and DNA in the parallel G-quadruplex structures examined here. Clusters of structured water molecules play essential roles in mediating between ligand side chain groups/chromophore core and G-quadruplex. These clusters tend to be conserved between complex and native G-quadruplex structures, suggesting that they more generally serve as platforms for ligand binding, and should be taken into account in docking and in silico studies.

Insights into the free energy landscape and salt-controlled mechanism of the conformational conversions between human telomeric G-quadruplex structures

G-quadruplexes have become attractive drug targets in cancer therapy. However, due to the polymorphism of G-quadruplex structures, it is difficult to experimentally verify the relevant structures of multiple intermediates and transition states in dynamic equilibrium. Hence, understanding the mechanism by which structural conversions of G-quadruplexes occur is still challenging. We conducted targeted molecular dynamics simulation with umbrella sampling to investigate how salt affects the conformational conversion of human telomeric G-quadruplex. Our results explore a unique view into the structures and energy barrier of the intermediates and transition states in the interconversion process. The pathway of G-quadruplex conformational interconversion was mapped out by a free energy landscape, consisting of branched parallel pathways with multiple energy basins. We propose a salt-controlled mechanism that as the salt concentration increases, the conformational conversion mechanism switches from multi-pathway folding to sequential folding pathways. The hybrid-I and hybrid-II structures are intermediates in the basket-propeller transformation. In high-salt solutions, the conformational conversion upon K⁺ binding is more feasible than upon Na⁺ binding. The free energy barrier for conformational conversions ranges from 1.6 to 4.6 kcal/mol. Our work will be beneficial in developing anticancer agents.

Triazolyl Dibenzo[a,c]phenazines Stabilize Telomeric G-quadruplex and Inhibit Telomerase

We herein report the synthesis and biophysical evaluation of triazolyl dibenzo[a,c]phenazine derivatives as a novel class of G-quadruplex ligands. The aromatic core facilitates π-π interaction and the flexible, protonatable side chains interact with the phosphate backbone of DNA via electrostatic interactions. Förster resonance energy transfer (FRET) melting assay and isothermal titration calorimetry (ITC) studies suggest that these ligands show binding preference for the hTELO G-quadruplex over G-quadruplexes found in the promoter region of various oncogenes and duplex DNA. The in vitro telomeric repeat amplification protocol (Q-TRAP) assay reveals that these ligands reduce telomerase activity in cancer cells.

Two coexisting pseudo-mirror heteromolecular telomeric G-quadruplexes in opposite loop progressions differentially recognized by a low equivalent of Thioflavin T

The final 3′-terminal residue of the telomeric DNA G-overhang is inherently less precise. Here, we describe how alteration of the last 3′-terminal base affects the mutual recognition between two different G-rich oligomers of human telomeric DNA in the formation of heteromolecular G-quadruplexes (hetero-GQs). Associations between three- and single-repeat fragments of human telomeric DNA, target d(GGGTTAGGGTTAGGG) and probe d(TAGGGT), in Na⁺ solution yield two coexisting forms of (3 + 1) hybrid hetero-GQs: the kinetically favourable LLP-form (left loop progression) and the thermodynamically controlled RLP-form (right loop progression). However, only the adoption of a single LLP-form has been previously reported between the same probe d(TAGGGT) and a target variant d(GGGTTAGGGTTAGGGT) having one extra 3′-end thymine. Moreover, the flanking base alterations of short G-rich probe variants also significantly affect the loop progressions of hetero-GQs. Although seemingly two pseudo-mirror counter partners, the RLP-form exhibits a preference over the LLP-form to be recognized by a low equivalent of fluorescence dye thioflavin T (ThT). To a greater extent, ThT preferentially binds to RLP hetero-GQ than with the corresponding telomeric DNA duplex context or several other representative unimolecular GQs.

The Binding Pocket at the Interface of Multimeric Telomere G-quadruplexes: Myth or Reality?

Human telomeric DNA with hundreds of repeats of the 5'-TTAGGG-3' motif plays a crucial role in several biological processes. It folds into G-quadruplex (G4) structures and features a pocket at the interface of two contiguous G4 blocks. Up to now no structural NMR and crystallographic data are available for ligands interacting with contiguous G4s. Naphthalene diimide monomers and dyads were investigated as ligands of a dimeric G4 of human telomeric DNA comparing the results with those of the model monomeric G4. Time-resolved fluorescence, circular dichroism, isothermal titration calorimetry and molecular modeling were used to elucidate binding features. Ligand fluorescence lifetime and induced circular dichroism unveiled occupancy of the binding site at the interface. Thermodynamic parameters confirmed the hypothesis as they remarkably change for the dyad complexes of the monomeric and dimeric telomeric G4. The bi-functional ligand structure of the dyads is a fundamental requisite for binding at the G4 interface as only the dyads engage in complexes with 1 : 1 stoichiometry, lodging in the pocket at the interface and establishing multiple interactions with the DNA skeleton. In the absence of NMR and crystallographic data, our study affords important proofs of binding at the interface pocket and clues on the role played by the ligand structure.

G-Quadruplexes and Their Ligands: Biophysical Methods to Unravel G-Quadruplex/Ligand Interactions

Progress in the design of G-quadruplex (G4) binding ligands relies on the availability of approaches that assess the binding mode and nature of the interactions between G4 forming sequences and their putative ligands. The experimental approaches used to characterize G4/ligand interactions can be categorized into structure-based methods (circular dichroism (CD), nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography), affinity and apparent affinity-based methods (surface plasmon resonance (SPR), isothermal titration calorimetry (ITC) and mass spectrometry (MS)), and high-throughput methods (fluorescence resonance energy transfer (FRET)-melting, G4-fluorescent intercalator displacement assay (G4-FID), affinity chromatography and microarrays. Each method has unique advantages and drawbacks, which makes it essential to select the ideal strategies for the biological question being addressed. The structural- and affinity and apparent affinity-based methods are in several cases complex and/or time-consuming and can be combined with fast and cheap high-throughput approaches to improve the design and development of new potential G4 ligands. In recent years, the joint use of these techniques permitted the discovery of a huge number of G4 ligands investigated for diagnostic and therapeutic purposes. Overall, this review article highlights in detail the most commonly used approaches to characterize the G4/ligand interactions, as well as the applications and types of information that can be obtained from the use of each technique.

G-quadruplex targeting chemical nucleases as a nonperturbative tool for analysis of cellular G-quadruplex DNA

G-quadruplex structures are associated with various biological activities, while in vivo evidence is essential to confirm the formation of G-quadruplexes inside cells. Most conventional agents that recognize G-quadruplex, including antibodies and small-molecule G-quadruplex ligands, either stabilize the G-quadruplex or prevent G-quadruplex unfolding by helicase, thereby artificially increasing the G-quadruplex levels in cells. Unambiguous study of G-quadruplexes at natural cellular levels requires agents that do not enhance the stability of G-quadruplex. Herein, we report the first example of nonperturbative chemical nucleases that do not influence the stability of G-quadruplex telomeric DNA but can selectively cleave G-quadruplex DNA over duplex DNA. These chemical nucleases can be readily taken up by cells and promote selective cleavage of telomeric DNA with low levels of nonselective DNA cleavage of other regions of the genome. The cleavage of G-quadruplex telomeric DNA by nonperturbative chemical nucleases confirms the formation of G-quadruplex telomeric DNA in live cells.

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Novel chemical nucleases exhibit no effect on G-quadruplex telomeric DNA stability

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Selective nucleases cleave G-quadruplex DNA over duplex DNA

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Cleavage of G-quadruplex telomeric DNA motifs confirms their existence in cells

Unraveling the binding characteristics of small ligands to telomeric DNA by pressure modulation

Recently, non-canonical DNA structures, such as G-quadruplexes (GQs), were found to be highly pressure sensitive, suggesting that pressure modulation studies can provide additional mechanistic details of such biomolecular systems. Using FRET and CD spectroscopy as well as binding equilibrium measurements, we investigated the effect of pressure on the binding reaction of the ligand ThT to the quadruplex 22AG in solutions containing different ionic species and a crowding agent mimicking the intracellular milieu. Pressure modulation helped us to identify the different conformational substates adopted by the quadruplex at the different solution conditions and to determine the volumetric changes during complex formation and the conformational transitions involved. The magnitudes of the binding volumes are a hallmark of packing defects and hydrational changes upon ligand binding. The conformational substates of the GQ as well as the binding strength and the stoichiometry of complex formation depend strongly on the solution conditions as well as on pressure. High hydrostatic pressure can also impact GQs inside living cells and thus affect expression of genetic information in deep sea organisms. We show that sub-kbar pressures do not only affect the conformational dynamics and structures of GQs, but also their ligand binding reactions.

Disentangling the Structure-Activity Relationships of Naphthalene Diimides as Anticancer G-Quadruplex-Targeting Drugs

In the context of developing efficient anticancer therapies aimed at eradicating any sort of tumors, G-quadruplexes represent excellent targets. Small molecules able to interact with G-quadruplexes can interfere with cell pathways specific of tumors and common to all cancers. Naphthalene diimides (NDIs) are among the most promising, putative anticancer G-quadruplex-targeting drugs, due to their ability to simultaneously target multiple G-quadruplexes and their strong, selective in vitro and in vivo anticancer activity. Here, all the available biophysical, biological, and structural data concerning NDIs targeting G-quadruplexes were systematically analyzed. Structure–activity correlations were obtained by analyzing biophysical data of their interactions with G-quadruplex targets and control duplex structures, in parallel to biological data concerning the antiproliferative activity of NDIs on cancer and normal cells. In addition, NDI binding modes to G-quadruplexes were discussed in consideration of the structures and properties of NDIs by in-depth analysis of the available structural models of G-quadruplex/NDI complexes.

DNA G-quadruplexes for native mass spectrometry in potassium: a database of validated structures in electrospray-compatible conditions

G-quadruplex DNA structures have become attractive drug targets, and native mass spectrometry can provide detailed characterization of drug binding stoichiometry and affinity, potentially at high throughput. However, the G-quadruplex DNA polymorphism poses problems for interpreting ligand screening assays. In order to establish standardized MS-based screening assays, we studied 28 sequences with documented NMR structures in (usually ∼100 mM) potassium, and report here their circular dichroism (CD), melting temperature (Tm), NMR spectra and electrospray mass spectra in 1 mM KCl/100 mM trimethylammonium acetate. Based on these results, we make a short-list of sequences that adopt the same structure in the MS assay as reported by NMR, and provide recommendations on using them for MS-based assays. We also built an R-based open-source application to build and consult a database, wherein further sequences can be incorporated in the future. The application handles automatically most of the data processing, and allows generating custom figures and reports. The database is included in the g4dbr package (https://github.com/EricLarG4/g4dbr) and can be explored online (https://ericlarg4.github.io/G4_database.html).

G-quadruplexes: a promising target for cancer therapy

DNA and RNA can fold into a variety of alternative conformations. In recent years, a particular nucleic acid structure was discussed to play a role in malignant transformation and cancer development. This structure is called a G-quadruplex (G4). G4 structure formation can drive genome instability by creating mutations, deletions and stimulating recombination events. The importance of G4 structures in the characterization of malignant cells was currently demonstrated in breast cancer samples. In this analysis a correlation between G4 structure formation and an increased intratumor heterogeneity was identified. This suggests that G4 structures might allow breast cancer stratification and supports the identification of new personalized treatment options. Because of the stability of G4 structures and their presence within most human oncogenic promoters and at telomeres, G4 structures are currently tested as a therapeutic target to downregulate transcription or to block telomere elongation in cancer cells. To date, different chemical molecules (G4 ligands) have been developed that aim to target G4 structures. In this review we discuss and compare G4 function and relevance for therapeutic approaches and their impact on cancer development for three cancer entities, which differ significantly in their amount and type of mutations: pancreatic cancer, leukemia and malignant melanoma. G4 structures might present a promising new strategy to individually target tumor cells and could support personalized treatment approaches in the future.

Cellular senescence and tumor promotion: Role of the Unfolded Protein Response

Senescence is a cellular state which can be viewed as a stress response phenotype implicated in various physiological and pathological processes, including cancer. Therefore, it is of fundamental importance to understand why and how a cell acquires and maintains a senescent phenotype. Direct evidence has pointed to the homeostasis of the endoplasmic reticulum whose control appears strikingly affected during senescence. The endoplasmic reticulum is one of the sensing organelles that transduce signals between different pathways in order to adapt a functional proteome upon intrinsic or extrinsic challenges. One of these signaling pathways is the Unfolded Protein Response (UPR), which has been shown to be activated during senescence. Its exact contribution to senescence onset, maintenance, and escape, however, is still poorly understood. In this article, we review the mechanisms through which the UPR contributes to the appearance and maintenance of characteristic senescent features. We also discuss whether the perturbation of the endoplasmic reticulum proteostasis or accumulation of misfolded proteins could be possible causes of senescence, and-as a consequence-to what extent the UPR components could be considered as therapeutic targets allowing for the elimination of senescent cells or altering their secretome to prevent neoplastic transformation.

Recent Developments in Small-Molecule Ligands of Medicinal Relevance for Harnessing the Anticancer Potential of G-Quadruplexes

G-quadruplexes, a family of tetraplex helical nucleic acid topologies, have emerged in recent years as novel targets, with untapped potential for anticancer research. Their potential stems from the fact that G-quadruplexes occur in functionally-important regions of the human genome, such as the telomere tandem sequences, several proto-oncogene promoters, other regulatory regions and sequences of DNA (e.g., rDNA), as well as in mRNAs encoding for proteins with roles in tumorigenesis. Modulation of G-quadruplexes, via interaction with high-affinity ligands, leads to their stabilization, with numerous observed anticancer effects. Despite the fact that only a few lead compounds for G-quadruplex modulation have progressed to clinical trials so far, recent advancements in the field now create conditions that foster further development of drug candidates. This review highlights biological processes through which G-quadruplexes can exert their anticancer effects and describes, via selected case studies, progress of the last few years on the development of efficient and drug-like G-quadruplex-targeted ligands, intended to harness the anticancer potential offered by G-quadruplexes. The review finally provides a critical discussion of perceived challenges and limitations that have previously hampered the progression of G-quadruplex-targeted lead compounds to clinical trials, concluding with an optimistic future outlook.

On the interaction of an anticancer trisubstituted naphthalene diimide with G-quadruplexes of different topologies: a structural insight

Naphthalene diimides showed significant anticancer activity in animal models, with therapeutic potential related to their ability to strongly interact with G-quadruplexes. Recently, a trifunctionalized naphthalene diimide, named NDI-5, was identified as the best analogue of a mini-library of novel naphthalene diimides for its high G-quadruplex binding affinity along with marked, selective anticancer activity, emerging as promising candidate drug for in vivo studies. Here we used NMR, dynamic light scattering, circular dichroism and fluorescence analyses to investigate the interactions of NDI-5 with G-quadruplexes featuring either parallel or hybrid topology. Interplay of different binding modes of NDI-5 to G-quadruplexes was observed for both parallel and hybrid topologies, with end-stacking always operative as the predominant binding event. While NDI-5 primarily targets the 5'-end quartet of the hybrid G-quadruplex model (m-tel24), the binding to a parallel G-quadruplex model (M2) occurs seemingly simultaneously at the 5'- and 3'-end quartets. With parallel G-quadruplex M2, NDI-5 formed stable complexes with 1:3 DNA:ligand binding stoichiometry. Conversely, when interacting with hybrid G-quadruplex m-tel24, NDI-5 showed multiple binding poses on a single G-quadruplex unit and/or formed different complexes comprising two or more G-quadruplex units. NDI-5 produced stabilizing effects on both G-quadruplexes, forming complexes with dissociation constants in the nM range.

Going Platinum to the Tune of a Remarkable Guanine Quadruplex Binder: Solution- and Solid-State Investigations

Guanine quadruplex recognition has gained increasing attention, inspired by the growing awareness of the key roles played by these non-canonical nucleic acid architectures in cellular regulatory processes. We report here the solution and solid-state studies of a novel planar platinum(II) complex that is easily assembled from a simple ligand, and exhibits notable binding affinity for guanine quadruplex structures, while maintaining good selectivity for guanine quadruplex over duplex structures. A crystal structure of this ligand complexed with a telomeric quadruplex confirms double end-capping, with dimerization at the 5' interface.

A G-Quadruplex-Binding Small Molecule and the HDAC Inhibitor SAHA (Vorinostat) Act Synergistically in Gemcitabine-Sensitive and Resistant Pancreatic Cancer Cells

The stabilisation of G-quadruplexes (G4s) by small-molecule compounds is an effective approach for causing cell growth arrest, followed by cell death. Some of these compounds are currently being developed for the treatment of human cancers. We have previously developed a substituted naphthalene diimide G4-binding molecule (CM03) with selective potency for pancreatic cancer cells, including gemcitabine-resistant cells. We report here that CM03 and the histone deacetylase (HDAC) inhibitor SAHA (suberanilohydroxamic acid) have synergistic effects at concentrations close to and below their individual GI₅₀ values, in both gemcitabine-sensitive and resistant pancreatic cancer cell lines. Immunoblot analysis showed elevated levels of γ-H2AX and cleaved PARP proteins upon drug combination treatment, indicating increased levels of DNA damage (double-strand break events: DSBs) and apoptosis induction, respectively. We propose that the mechanism of synergy involves SAHA relaxing condensed chromatin, resulting in higher levels of G4 formation. In turn, CM03 can stabilise a greater number of G4s, leading to the downregulation of more G4-containing genes as well as a higher incidence of DSBs due to torsional strain on DNA and chromatin structure.

Topological conversion of human telomeric G-quadruplexes from hybrid to parallel form induced by naphthalene diimide ligands

G-quadruplexes (GQs) have become promising anti-cancer therapeutic targets, which are formed by the folding of a guanine-rich repeat DNA/RNA sequence at human telomeres or oncogene promoters. Polymorphism has been observed for the folding topologies of intramolecular GQs. Here we report the topological conversion of human telomeric GQ induced by naphthalene diimide (NDI) ligands in K⁺ solution. The ligands selectively induce metastable hybrid-type GQs to highly stable parallel-type GQ at physiological temperature (37 °C) in dilute aqueous solutions and under crowding conditions that mimic cellular bioenvironment. According to spectroscopic analyses, the topological conversion is speculated to undergo stepwise unfolding of hybrid-type GQ through intermediate states to parallel-type GQ. The results will prompt further studies on the designs of ligands with GQ conformation regulation functions and nanotechnological systems based on nucleic acids with dynamic regulation of GQ conformation.