- Open Access
On and off-target effects of telomere uncapping G-quadruplex selective ligands based on pentacyclic acridinium salts
- Sara Iachettini1,
- Malcolm FG Stevens2,
- Mark Frigerio3,
- Marc G Hummersone3,
- Ian Hutchinson3,
- Thomas P Garner4,
- Mark S Searle4,
- David W Wilson5,
- Manoj Munde5,
- Rupesh Nanjunda5,
- Carmen D’Angelo1,
- Pasquale Zizza1,
- Angela Rizzo1,
- Chiara Cingolani1,
- Federica De Cicco1,
- Manuela Porru1,
- Maurizio D’Incalci6,
- Carlo Leonetti1,
- Annamaria Biroccio1Email author and
- Erica Salvati1Email author
© Iachettini et al.; licensee BioMed Central Ltd. 2013
- Received: 26 July 2013
- Accepted: 29 August 2013
- Published: 19 September 2013
Quadruplexes DNA are present in telomeric DNA as well as in several cancer-related gene promoters and hence affect gene expression and subsequent biological processes. The conformations of G4 provide selective recognition sites for small molecules and thus these structures have become important drug-design targets for cancer treatment.
The DNA G-quadruplex binding pentacyclic acridinium salt RHPS4 (1) has many pharmacological attributes of an ideal telomere-targeting agent but has undesirable off-target liabilities. Notably a cardiovascular effect was evident in a guinea pig model, manifested by a marked and sustained increase in QTcB interval. In accordance with this, significant interaction with the human recombinant β2 adrenergic receptor, and M1, M2 and M3 muscarinic receptors was observed, together with a high inhibition of the hERG tail current tested in a patch clamp assay.
Two related pentacyclic structures, the acetylamines (2) and (3), both show a modest interaction with β2 adrenergic receptor, and do not significatively inhibit the hERG tail current while demonstrating potent telomere on-target properties comparing closely with 1. Of the two isomers, the 2-acetyl-aminopentacycle (2) more closely mimics the overall biological profile of 1 and this information will be used to guide further synthetic efforts to identify novel variants of this chemotype, to maximize on-target and minimize off-target activities.
Consequently, the improvement of toxicological profile of these compounds could therefore lead to the obtainment of suitable molecules for clinical development offering new pharmacological strategies in cancer treatment.
- Telomere targeting agents
- Anti-cancer therapy
Telomeric DNA is protected and maintained at the ends of chromosomes by the action of the enzyme telomerase. Whilst the shortening of DNA telomeres during repeated cell division is a natural part of the cellular ageing mechanism, one of the hallmarks of cancer is the expression of telomerase by cancer cells which allows them to maintain telomeric length and adopt immortal characteristics[1, 2]. Telomerase requires a single-stranded DNA primer as substrate for the addition of telomeric repeats (TTAGGG), his terminal telomere G-rich single stranded tract, also called G-overhang, can fold into four-stranded G-quadruplex (G4) structures consisting of G-tetrads coordinated around a monovalent cation[4, 5]. G4 stabilization, deny access of telomerase to its substrate, representing a valid tool for telomerase targeted approach in cancer therapy. Nevertheless, for direct telomerase inhibition, a time-dependent response is observed, related to the basal length of the telomeres, due to the slow attrition of telomeres experienced after each cell division, thus limiting the efficacy of agents designed to inhibit telomerase alone[7–9]. The extremely rapid and potent cytotoxic effect triggered by G4 ligands interacting with telomeric DNA sequences (‘Telomere Targeting Agents’: TTAs) is explained by a dual mechanism of action. On one hand the inhibition of telomerase, and, on the other hand, disruption of the shelterin complex, a nulcleo-protein complex which stabilises and protects the ends of chromosomes from being recognized as double-strand breaks[7, 8]. The presence of G4 structures has been recently showed in non telomeric regions, as already hypothesized on the base of predictive studies. In particular, G4 forming regions were already found in the promoter of several cancer related genes (c-myc, bcl2, hif1, hTERT), and for some of those genes, a transcriptional inhibitory function was attributed to these structures. Consequently, G4 targeting molecules could have additional extra-telomeric features, which could improve their potential as anti-cancer agents.
3,11-Difluoro-6,8,13-trimethyl-8H-quino[4,3,2-kl]acridinium metho-sulfate 1 was prepared from 6-fluoro-1,2-dimethylquinolinium methosulfate 7 as described. 2-Acetylamino- (2) and 3-acetylamino-8,13-dimethyl-8H-quino[4,3,2-kl]-acridinium iodide (3) were prepared according to published methods.
13-Ethyl-3,11-difluoro-6,8-dimethyl-8H-quino[4,3,2-kl]acridinium trifluoromethosulfate (8)
Ethyl trifloromethosulfate (1 mL) was added to a solution of 3,11-difluoro-6,8-dimethyl-8H-quino[4,3,2-kl]acridine (6; 0.05 g, 0.15 mmol) in CHCl3 (2 mL) under nitrogen. The mixture was heated at 140°C in a sealed tube for 3 days, cooled and solvent evaporated. The residue was purified by column chromatography on silica gel (5% MeOH/DCM) to leave the salt (8) as a bright red solid (20%), mp >250°C (decomp.); IR (νmax) 1620, 1583, 1533, 1475, 1429, 1255, 1028 cm-1; 1H NMR (DMSO-d6) δ 8.58 (1H, dd, J = 10.0, 2.9 Hz), 8.43 (1H, s), 8.26 (2H, m), 8.21 (1H, dd, J = 9.4, 4.9), 8.04 (1H, m), 8.01 (1H, s), 7.78 (1H, m), 5.12 (2H, q, J = 6.8 Hz, N-CH2), 3.17 (3H, d, J = 5.1 Hz), 2.78 (3H, s, N-CH3), 1.15 (3H, t, J = 6.8 Hz, N-CH2CH 3 ); m/z 361.1 (M+).
Cardiovascular effects of anaesthetised Guinea pig
After anaesthesia with approximately 40 to 60 mg/kg (i.p.) sodium pentobarbitone, a jugular vein was cannulated for administration of the vehicle or test substance. Arterial blood pressure (systolic, diastolic and mean) was measured via a catheter inserted into the carotid artery, heart rate was derived electronically from the pressure waveform and a sample of arterial blood determined blood gases (PO2 and PCO2), O2 saturation, standard bicarbonate (HCO3), pH and base excess before the start of the experiment. Electrocardiogram (ECG) limb electrodes recorded the standard lead II configuration and QTcB interval (calculated as QTcB = QT/(√RR)). The animal was allowed to stabilise after completion of the surgical preparation for a period of at least 15 min. Then, after a further 10 min period of continuous recording of ECG and haemodynamic variables, the test substance or vehicle was administered intravenously as 3 iv infusions with each administration separated by 60 min.
For hERG study, HEK293 cells were cultured (1–7 days) in DMEM/GlutaMax-1 + 10% FBS and were plated on collagen-coated dishes (about 2×104 cells/dish). The cell was held at -80 mV. A 50-millisecond pulse to -40 mV was delivered to measure the leaking currents, which were subtracted from the tail currents online. Then the cell was depolarized to +20 mV for 2 seconds, followed by a second pulse to -40 mV for 1 second to reveal the tail currents. This paradigm was delivered once every 5 seconds to monitor the current amplitude. After the current amplitude stabilized, the test compound was delivered to the extracellular medium by a rapid solution changer perfusion system. During perfusion, the cell was repetitively stimulated with the protocol described above, and the current amplitude was continuously monitored. Data were acquired and analyzed by using pClamp (Axon Instruments), and Excel (Microsoft), and are reported as mean and individual values. The degree of inhibition (%) was obtained by measuring the tail current amplitude before and after drug superfusion (the difference current was normalized to control and multiplied by 100 to obtain the percent of inhibition). Concentration (log) response curves were fitted to a logistic equation (three parameters assuming complete block of the current at very high test compound concentrations) to generate estimates of the 50% inhibitory concentration (IC50). The concentration-response relationship of each compound was constructed from the percentage reductions of current amplitude by sequential concentrations. β2-adrenergic receptor CHO expressing cells were used for the receptor inhibition assay as described. The results are expressed as a percent of inhibition of control specific binding (100 - (measured specific binding/control specific binding) × 100)) obtained in the presence of the test compounds. The specific ligand binding to the receptors is defined as the difference between the total binding and the nonspecific binding determined in the presence of an excess of unlabelled ligand. All the in-vivo experiments were carried out at the Regina Elena Cancer Institute. All procedures involving animals and care were performed in compliance with our institutional animal care guidelines and with international directives (directive 2010/63/EU of the European parliament and of the council; Guide for the Care and Use of Laboratory Animals, United States National Research Council, 2011).
Biosensor-surface plasmon resonance (SPR) studies
Oligonucleotides 5′-biotin-d[AG3(T2AG3)3] quadruplex and 5′-biotin-CGA3T3C(CT)2GA3T3CG were purchased from Midland Certified Reagent Company (Midland, TX). Purification of DNA, preparation of solutions, collection of data, and analysis of results were conducted according to methods adopted in an earlier study.
CD spectra were recorded on an Applied Photophysics Pi-Star-180 spectrophotometer (Applied Photophysics Ltd, Surrey, UK). The optical system was configured with a 75 W Xe lamp, circular light polarizer and end-mounted photomultiplier. The instrument had previously been calibrated with (D)-camphorsulfonic acid. Temperature was regulated using a Neslab RTE-300 circulating programmable water bath (Neslab Inc). CD spectra were recorded at 298 K in a 10 mm path length cell over a wavelength range of 215–345 nm in steps of either 1 0r 2 nm, with 3 nm entrance/exit slit widths: the number of counts was set to 10,000 with adaptive sampling set to 500,000. The spectra were corrected by subtracting the spectrum of the same buffer solution of 100 mM potassium chloride and 10 mM potassium phosphate at pH 7.0. Annealing and melting profiles were recorded using a thermoelectric temperature controller (Melcor) on 4 μ M DNA samples with and without 3.5 mol.equiv. of ligands using 0.5 K temperature increments and a cooling or heating rate of 0.2 K/min over the temperature range 298-368 K.
Cells and culture conditions
BJ fibroblasts expressing hTERT (BJ-hTERT) or hTERT and SV40 early region (BJ-EHLT), were obtained as previously reported. Cells were grown in Dulbecco Modified Eagle Medium (D-MEM, Invitrogen Carlsbad, CA, USA) supplemented with 10% fetal calf serum, 2 mM L-glutamin and antibiotics.
5 × 104 cells were seeded in 60-mm Petri plates (Nunc, MasciaBrunelli, Milano, Italy) and 24 h after plating, 0.5 μM of freshly dissolved compound was added to the culture medium. Cell counts (Coulter Counter, Kontron Instruments, Milano, Italy) and viability (trypan blue dye exclusion) were determined daily, from day 2 to day 8 of culture.
Cells were fixed in 2% formaldehyde and permeabilized in 0.25% Triton X100 in PBS for 5 min at room temperature. For immunolabeling, cells were incubated with primary antibody, then washed in PBS and incubated with the secondary antibodies. The following primary antibodies were used: pAb and mAb anti-TRF1 (Abcam Ltd.; Cambridge UK); mAb (Upstate, Lake Placid, NY) and pAb anti-γH2AX (Abcam). The following secondary antibody were used: TRITC conjugated Goat anti Rabbit, FITC conjugated Goat anti Mouse (Jackson ImmunoResearch Europe Ltd., Suffolk, UK). Fluorescence signals were recorded by using a Leica DMIRE2 microscope equipped with a Leica DFC 350FX camera and elaborated by a Leica FW4000 deconvolution software (Leica, Solms, Germany). This system permits to focus single planes inside the cell generating 3D high-resolution images. For quantitative analysis of γH2AX positivity, 200 cells on triplicate slices were scored. For TIF’s analysis, in each nucleus a single plane was analyzed and at least 50 nuclei per sample were scored.
Fluorescence in situ hybridization (FISH)
For metaphase chromosome preparation cells were treated with demecolcine (Sigma, Milan, Italy) 0.1 mg/ml for 4 h and then harvested and washed in 75 mM KCl for 5 min at 37°C. After centrifugation cells were fixed in MeOH/acetic acid 3:1 overnight and then spread on slides. Hybridization with rhodamine-coupled PNA was performed as described. For each sample 20 metaphases per slice on triplicate were scored. Images of the metaphases were captured with a 100 × objective.
Chromatin immunoprecipitaion assay (ChIP)
BJ-EHLT fibroblasts were treated for 24 hrs with 0.5 μM of the compound. ChIP assay was performed as previously described. The following antibodies were used: pAb anti-TRF1 (Santa Cruz Biotechnology, Santa Cruz, Ca); mAb anti-TRF2 (Imgenex, San Diego, CA); pAb anti-POT1 (Abcam). mAb anti-β-actin (Sigma) was used as negative control of the ChIP assay.
Synthesis of quino [4,3,2-kl] acridinium salts
Efforts to prepare higher alkyl homologues of 1 were only partially successful presumably because access by larger alkylating moieties at N-13 of pentacycle 6 were impeded by hydrogen atoms at positions 1 and 12 (for numbering system see Figure 1): thus whereas the N-ethyl quaternary salt 8 (20%) could be prepared with difficulty by heating 6 and ethyl trifluoromethane sulfonate in chloroform under nitrogen at 140°C in a sealed tube for 3 days, it was not possible to prepare n-propyl or i-propyl homologues of 6 under a range of forcing conditions. The isomeric N-acetyl compounds 2 and 3 were prepared starting from the 2-aminoquinoacridine 9 or 3-chloroquinoacridine 10, respectively, in several steps according to our previously published work.
Toxicity of quinoacridinium salt 1
Initial in vivo evaluation of 1, in human tumor xenografted nude mice, did not indicate any toxicity at efficacious doses, as no toxic deaths or body weight loss was observed during or after treatment. Furthermore, histological analysis, done at the end of treatment with 1, revealed no evidence of lesions or morphological alterations in the organs and tissues examined. Nevertheless, just after 1 administration, a marked but reversible hypotension was observable, accompanied by a heart rate and cardiac output decrease in the treated compared to the control mice.
On and off target profile of pentacyclic acridinium salts 1, 2 and 3
Off-target effects: cardiac receptor inhibition
On-target effects: ligand-quadruplex interaction
hERG % inhib. (10 μM)
B2 adrenergic % inhib. (10 μM)
Surface plasmon resonancea(K x 107 M-1)
CD study thermal stabilityd
Quadruplex (Q) DNAb
Duplex (D) DNAc
86 ± 3
89 ± 3
85 ± 3
Ligand redesign to minimize off target effects
The potent hERG inhibition compromised the acceptability of 1 as a clinical candidate, despite this agent having many of the attributes of an ideal pharmaceutical. Two strategies have been adopted in an attempt to minimize the hERG interaction: (i) sterically masking the (delocalized) positive charge on the acridinium cation by increasing the size of the substituent at position 13 as in compound 8; and (ii) evaluating compounds 2 and 3 as prototypes of two series of isomeric pentacyclic acridinium salts of the same chemotype as 1.
hERG tail current inhibition was used as a marker of potential off-target liabilities. The prototypic agent 1 potently inhibited hERG by 100% at 10 μM (IC50 0.2 μM) (Table 1); inhibition of hERG was reduced to 43% at 10 μ M (IC50 3.7 μM) in the 2-acetylaminoquinoacridinium iodide 2 and to 18% by 13-ethyl homologue 8, while the least potent hERG inhibitor (IC50 18 μM) was the 3-acetylamino isomer 3, a 90-fold improvement over 1. The marked improvement of 8 over 1, was paralled by a >10-fold reduction in the on-target effect against the h-Tel DNA sequence as measured by surface plasmon resonance (see below) suggesting that increasing the size of the onium head was not a fruitful developmental approach, for these reason the compound 8 was excluded from further studies.
The interaction with β2-adrenergic receptor was determined by a binding assay of 1, 2 and 3 to the transgenic β2-adrenegic receptor expressed on the surface of CHO cells. Inhibition of receptor was reported as inhibition of control specific binding (100 - (measured specific binding/control specific binding) × 100) obtained in the presence of the test compounds. A decay of 75% and 70% of receptor inhibition is observed comparing 1 to 2 and 3 compounds respectively (Table 1). These results indicate that potential toxicities in this chemotype, as predicted by hERG and β2-adrenergic receptor interactions, can be addressed by suitable molecular modification.
On target-effects: ligand-quadruplex interactions
The ability of the three ligands to induce structure in the single stranded h-Tel sequence in aqueous solution in the absence of significant concentrations of K+ ions was also investigated. The unfolded h-Tel sequence at 298 K gives a low intensity positive band in the CD spectrum at 265 nm (Figure 4b). However, in the presence of 3.5 molar equivalents of ligand, emergence of the characteristic band at 290 nm was observed, consistent with the ligand-induced formation of the anti-parallel structures evident in the K+ buffered solution. Thus, under both sets of conditions (with and without stabilising K+ ions), evidence is adduced for ligand selectivity for the anti-parallel quadruplex structure[12, 13].
This analysis was extended to examine the effects of ligand binding on thermal stability by measuring the unfolding curves at 290 nm of the complexes formed in K+ solution, corresponding to the CD spectra shown in Figure 4a. Monitoring the thermal unfolding transition for h-Tel produces a sigmoidal unfolding curve with a transition mid-point Tm value of 72 ± 3°C (Figure 4c). All three ligands show significant effects in enhancing the stability of the quadruplex by shifting the Tm values to higher temperatures (∆Tm ~ 15-19°C compared to h-Tel without bound ligands) (Table 1).
Biological effects of quinoacridinum salts
To directly evaluate telomere damage elicited by the different ligands, the telomere status of drug-treated BJ-EHLT was analysed by a fluorescence in situ hybridization on metaphase spreads with a telomere specific fluorescent probe. The cytogenetic analysis revealed that all the compounds induced a significant increase of frequency of telomere doublets (characterized by a double telomere signal at chromosome ends) and sister telomere fusions (in which two sister chromatids telomeric signals are fused into one single spot), while other telomere aberrations (telomere losses and/or deletions) were not found. However, again telomere aberrations induced by 2 are quantitatively similar to the lead compound, while a lower effect was observed upon treatment with 3. As a result of chromosome ends fusion consequent to telomere damage, chromatin bridges are occasionally observed between daughter cells after mitosis (also called anaphase bridges). In 1 treated BJ-EHLT, anaphase bridges frequency in a cycling population was ten-fold increased. With a minor extent 2 and 3 were both able to induce anaphase bridges when administered at the same dose, closely comparing the effects of the lead compound (Figure 7e).
This response is typical of the telomere deprotection occurring during cellular senescence or upon the loss of telomeric proteins[34–40]. The ability of G-quadruplex ligands to uncap telomeres and to possess anti-tumor activity has been already described for other agents,[41–45] reinforcing the notion that these agents can act as inhibitors of a telomere-related process and therefore the rationale for the development of this class of inhibitors as anti-tumor agents must be found elsewhere other than in higher telomerase expression in cancer cells.
Taken collectively our results clearly demonstrate that compounds 2 (but less efficiently 3) rapidly disrupt telomere architecture of cells, by delocalizing the telomeric protein POT1, resulting in a potent DNA damage response characterized by the formation of several telomeric foci.
Furthermore, it is apparent that the 2-substitued quinoacridinium salt 2 more closely mimics the overall pharmaceutical profile of the prototypic compound 1 than the regioisomer 3. Our recent synthetic work has therefore focused on the 2-substituted series and our efforts to maximize on-target and minimize off-target properties will be reported separately.
Molecular modification of quinoacridinum salts 1 have shown to reduce undesired cardiotoxic effects while maintaining the on-target features as telomere targeting agents. This findings provide a strong rational for development of this class of compounds as tools for a G-quadruplex targeted anti-cancer therapy.
- Hanahan D, Weinberg RA: The hallmarks of cancer. Cell. 2000, 100: 57-70. 10.1016/S0092-8674(00)81683-9.View ArticleGoogle Scholar
- Testorelli C: Telomerase and cancer. J Exp Clin Cancer Res. 2003, 22: 165-169.Google Scholar
- Cech TR: Beginning to understand the end of the chromosome. Cell. 2004, 116: 273-279. 10.1016/S0092-8674(04)00038-8.View ArticleGoogle Scholar
- Phan AT, Kuryavyi V, Patel DJ: DNA architecture: from G to Z. Curr Opin Struct Biol. 2006, 16: 288-298. 10.1016/j.sbi.2006.05.011.View ArticleGoogle Scholar
- Huppert JL, Subramanian S: Prevalence of quadruplexes in the human genome. Nucleic Acids Res. 2005, 33: 2908-2916. 10.1093/nar/gki609.View ArticleGoogle Scholar
- Garner TP, Williams HEL, Gluszyk KI, Roe S, Oldham NJ, Stevens MF, Moses JE, Searle MS: Selectivity of small molecule ligands for parallel and anti-parallel G-quadruplex structures. Org Biomol Chem. 2009, 7: 4194-4200. 10.1039/b910505k.View ArticleGoogle Scholar
- Akiyama M, Hideshima T, Munshi NC, Anderson KC: Telomerase inhibitors as anticancer therapy. Curr Med Chem Anticancer Agents. 2002, 5: 567-575.View ArticleGoogle Scholar
- Lai XF, Shen CX, Wen Z, Qian YH, Yu CS, Wang JQ, Zhong PN, Wang HL: PinX1 regulation of telomerase activity and apoptosis in nasopharyngeal carcinoma cells. J Exp Clin Cancer Res. 2012, 31: 12-10.1186/1756-9966-31-12.View ArticleGoogle Scholar
- Yingying L, Junchao G, Dachuan J, Yanjing G, Mengbiao Y: Inhibition of telomerase activity by HDV ribozyme in cancers. J Exp Clin Cancer Res. 2011, 30: 1-10.1186/1756-9966-30-1.View ArticleGoogle Scholar
- Biffi G, Tannahill D, Mc Cafferty J, Balasubramanian S: Quantitative visualization of DNA G-quadruplex structures in human cells. Nat Chem. 2013, 5: 182-186. 10.1038/nchem.1548.View ArticleGoogle Scholar
- Cheng MK, Modi C, Cookson JC, Hutchinson I, Heald RA, McCarroll AJ, Missailidis S, Tanious F, Wilson WD, Mergny JL, Laughton CA, Stevens MF: Antitumor polycyclic acridines. 20. Search for DNA quadruplex binding selectivity in a series of 8,13-dimethylquino[4,3,2-kl]acridinium salts: telomere-targeted agents. J Med Chem. 2008, 51: 963-975. 10.1021/jm070587t.View ArticleGoogle Scholar
- Gavathiotis E, Heald RA, Stevens MFG, Searle MS: Recognition and stabilization of quadruplex DNA by a potent new telomerase inhibitor: NMR studies of the 2:1 complex of a pentacyclic methylacridinium cation with d(TTAGGGT)4. Angew Chem Int Ed. 2001, 40: 4749-4751. 10.1002/1521-3773(20011217)40:24<4749::AID-ANIE4749>3.0.CO;2-I.View ArticleGoogle Scholar
- Gavathiotis E, Heald RA, Stevens MFG, Searle MS: Drug recognition and stabilization of the parallel-stranded DNA quadruplex d(TTAGGGT)4 containing the human telomeric repeat. J Mol Biol. 2003, 334: 25-36. 10.1016/j.jmb.2003.09.018.View ArticleGoogle Scholar
- Leonetti C, Amodei S, D’Angelo C, Rizzo A, Benassi B, Antonelli A, Elli R, Stevens MF, D’Incalci M, Zupi G, Biroccio A: Biological activity of the G-quadruplex ligand RHPS4 (3,11-difluoro-6,8,13-trimethyl-8H-quino[4,3,2-kl]acridinium methosulfate) is associated with telomere capping alteration. Mol Pharmacol. 2004, 66: 1138-1146. 10.1124/mol.104.001537.View ArticleGoogle Scholar
- Salvati E, Leonetti C, Rizzo A, Scarsella M, Mottolese M, Galati R, Sperduti I, Stevens MF, D’Incalci M, Blasco M, Chiorino G, Bauwens S, Horard B, Gilson E, Stoppacciaro A, Zupi G, Biroccio A: Telomere damage induced by the G-quadruplex ligand RHPS4 has an antitumor effect. J Clin Invest. 2007, 117: 3236-3247. 10.1172/JCI32461.View ArticleGoogle Scholar
- Gowan SM, Heald R, Stevens MFG, Kelland LR: Potent inhibition of telomerase by small molecule pentacyclic acridines capable of interacting with G-quadruplexes. Mol Pharmacol. 2001, 60: 981-988.Google Scholar
- Phatak P, Cookson JC, Dai F, Smith V, Gartenhaus RB, Stevens MF, Burger AM: Telomere uncapping by the G-quadruplex ligand RHPS4 inhibits clonogenic tumour cell growth in vitro and in vivo consistent with a cancer stem cell targeting mechanism. Br J Cancer. 2007, 96: 1223-1233. 10.1038/sj.bjc.6603691.View ArticleGoogle Scholar
- Leonetti C, Scarsella M, Riggio G, Rizzo A, Salvati E, D’Incalci M, Staszewsky L, Frapolli R, Stevens MF, Stoppacciaro A, Mottolese M, Antoniani B, Gilson E, Zupi G, Biroccio A: G-quadruplex ligand RHPS4 potentiates the antitumor activity of camptothecins in preclinical models of solid tumors. Clin Cancer Res. 2008, 14 (22): 7284-7291. 10.1158/1078-0432.CCR-08-0941.View ArticleGoogle Scholar
- Salvati E, Scarsella M, Porru M, Rizzo A, Iachettini S, Tentori L, Graziani G, D’Incalci M, Stevens MF, Orlandi A, Passeri D, Gilson E, Zupi G, Leonetti C, Biroccio A: PARP1 is activated at telomeres upon G4 stabilization: possible target for telomere-based therapy. Oncogene. 2010, 29: 6280-6293. 10.1038/onc.2010.344.View ArticleGoogle Scholar
- Hutchinson I, Stevens MFG: Synthetic strategies to a telomere-targeted pentacyclic heteroaromatic salt. Org Biomol Chem. 2007, 5: 114-120. 10.1039/b613580n.View ArticleGoogle Scholar
- Ozczapowicz D, Jaroszewska-Manaj J, Ciszak E, Gdaniec M: Formation of quinoacridinium system: a novel reaction of quinaldinium salts. Tetrahedron. 1988, 44: 6645-6650. 10.1016/S0040-4020(01)90102-4.View ArticleGoogle Scholar
- Joseph SS, Lynham JA, Colledge WH, Kaumann AJ: Binding of (−)-[3H]-CGP12177 at two sites in recombinant human beta 1-adrenoceptors and interaction with beta-blockers. Naunyn Schmiedebergs Arch Pharmacol. 2004 May, 369 (5): 525-532. 10.1007/s00210-004-0884-y.View ArticleGoogle Scholar
- Lenain C, Bauwens S, Amiard S, Brunori M, Giraud-Panis MJ, Gilson E: The Apollo 50 exonuclease functions together with TRF2 to protect telomeres from DNA repair. Curr Biol. 2006, 16: 1303-1310. 10.1016/j.cub.2006.05.021.View ArticleGoogle Scholar
- Rizzo A, Salvati E, Porru M, D’Angelo C, Stevens MF, D’Incalci M, Leonetti C, Gilson E, Zupi G, Biroccio A: Stabilization of quadruplex DNA perturbs telomere replication leading to the activation of an ATR-dependent ATM signaling pathway. Nucleic Acids Res. 2009, 37: 5353-5364. 10.1093/nar/gkp582.View ArticleGoogle Scholar
- Hutchinson I, McCarroll AJ, Heald RA, Stevens MFG: Synthesis and properties of bioactive 2- and 3-amino-8-methyl-8H-quino[4,3,2-kl]acridine and 8,13-dimethyl-8H-quino[4,3,2-kl]acridinium salts. Org Biomol Chem. 2004, 2: 220-228. 10.1039/b310796p.View ArticleGoogle Scholar
- Hasenfuss G: Animal models of human cardiovascular disease, heart failure and hypertrophy. Cardiovasc Res. 1998, 39: 60-76. 10.1016/S0008-6363(98)00110-2.View ArticleGoogle Scholar
- Titus SA, Beacham D, Shahane SA, Southall N, Xia M, Huang R, Hooten E, Zhao Y, Shou L, Austin CP, Zheng W: A new homogeneous high-throughput screening assay for profiling compound activity on the human ether-a-go-go-related gene channel. Anal Biochem. 2009, 394: 30-38. 10.1016/j.ab.2009.07.003.View ArticleGoogle Scholar
- Cookson JC, Heald RA, Stevens MFG: Antitumor polycyclic acridines. 17. Synthesis and pharmaceutical profiles of pentacyclic acridinium salts designed to destabilise telomeric integrity. J Med Chem. 2005, 48: 7198-7207. 10.1021/jm058031y.View ArticleGoogle Scholar
- White EW, Tanious F, Ismail MA, Reszka AP, Neidle S, Boykin DW, Wilson WD: Structure-specific recognition of quadruplex DNA by organic cations: influence of shape, substituents and charge. Biophys Chem. 2007, 126: 140-153. 10.1016/j.bpc.2006.06.006.View ArticleGoogle Scholar
- Luu KN, Phan AT, Kuryavyi V, Lacroix L, Patel DJ: Structure of the human telomere in K + solution: an intramolecular (3 + 1) G-quadruplex Scaffold. J Am Chem Soc. 2006, 128: 9963-9970. 10.1021/ja062791w.View ArticleGoogle Scholar
- Phan AT, Luu KN, Patel DJ: Different loop arrangements of intramolecular human telomeric (3 + 1) G-quadruplexes in K + solution. Nucleic Acids Res. 2006, 34: 5715-5719. 10.1093/nar/gkl726.View ArticleGoogle Scholar
- Ambrus A, Chen D, Dai JX, Bialis T, Jones RA, Yang D: Human telomeric sequence forms a hybrid-type intramolecular G-quadruplex structure with mixed parallel/antiparallel strands in potassium solution. Nucleic Acids Res. 2006, 34: 2723-2735. 10.1093/nar/gkl348.View ArticleGoogle Scholar
- Wang Y, Patel DJ: Solution structure of the human telomeric repeat d[AG3(T2AG3)3] G-tetraplex. Structure. 1993, 1: 263-282. 10.1016/0969-2126(93)90015-9.View ArticleGoogle Scholar
- Takai H, Smogorzewska A, de Lange T: DNA damage foci at dysfunctional telomeres. Curr Biol. 2003, 13: 1549-1556. 10.1016/S0960-9822(03)00542-6.View ArticleGoogle Scholar
- van Steensel B, de Lange T: Control of telomere length by the human telomeric protein TRF1. Nature. 1997, 385: 740-743. 10.1038/385740a0.View ArticleGoogle Scholar
- van Stenseel B, Smogorzewska A, de Lange T: TRF2 protects human telomeres from end-to-end fusions. Cell. 1998, 92: 401-413. 10.1016/S0092-8674(00)80932-0.View ArticleGoogle Scholar
- Ancelin K, Brun C, Gilson E: Role of the telomeric DNA-binding protein TRF2 in the stability of human chromosome ends. Bioessays. 1998, 20: 879-883. 10.1002/(SICI)1521-1878(199811)20:11<879::AID-BIES2>3.0.CO;2-I.View ArticleGoogle Scholar
- d’Adda di Fagagna F, Reaper PM, Clay-Farrace L, Fiegler H, Carr P, Von Zglinicki T, Saretzki G, Carter NP, Jackson SP: A DNA damage checkpoint response in telomere-initiated senescence. Nature. 2003, 426: 194-198. 10.1038/nature02118.View ArticleGoogle Scholar
- Loayza D, De Lange T: POT1 as a terminal transducer of TRF1 telomere length control. Nature. 2003, 423: 1013-1018. 10.1038/nature01688.View ArticleGoogle Scholar
- Hockemeyer D, Sfeir AJ, Shay JW, Wright WE, de Lange T: POT1 protects telomeres from a transient DNA damage response and determines how human chromosomes end. EMBO J. 2005, 24: 2667-2678. 10.1038/sj.emboj.7600733.View ArticleGoogle Scholar
- Burger AM, Dai F, Schultes CM, Reszka AP, Moore MJ, Double JA, Neidle S: The G-quadruplex-interactive molecule BRACO-19 inhibits tumor growth, consistent with telomere targeting and interference with telomerase function. Cancer Res. 2005, 65: 1489-1496. 10.1158/0008-5472.CAN-04-2910.View ArticleGoogle Scholar
- Tauchi T, Shin-ya K, Sashida G, Sumi M, Okabe S, Ohyashiki JH, Ohyashiki K: Telomerase inhibition with a novel G-quadruplex interactive agent, telomestatin: in vitro and in vivo studies in acute leukemia. Oncogene. 2006, 25: 5719-5792. 10.1038/sj.onc.1209577.View ArticleGoogle Scholar
- Temime-Smaali N, Guittat L, Sidibe A, Shin-ya K, Trentesaux C, Riou JF: The G-quadruplex ligand telomestatin impairs binding of topoisomerase III alpha to G-quadruplex-forming oligonucleotides and uncaps telomeres in ALT cells. PLoS One. 2009, 4: 6919-10.1371/journal.pone.0006919.View ArticleGoogle Scholar
- Gauthier LR, Granotier C, Hoffschir F, Etienne O, Ayouaz A, Desmaze C, Mailliet P, Biard DS, Boussin FD: Rad51 and DNA-PKcs are involved in the generation of specific telomere aberrations induced by the quadruplex ligand 360A that impair mitotic cell progression and lead to cell death. Cell Mol Life Sci. 2012, 69: 629-640. 10.1007/s00018-011-0767-6.View ArticleGoogle Scholar
- Riou JF: G-quadruplex interacting agents targeting the telomeric G-overhang are more than simple telomerase inhibitors. Curr Med Chem Anticancer Agents. 2004, 4: 439-443. 10.2174/1568011043352740.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.