Molecular Imaging Agents Research Articles

A Targeting Microbubble for Ultrasound Molecular Imaging.

RationaleMicrobubbles conjugated with targeting ligands are used as contrast agents for ultrasound molecular imaging. However, they often contain immunogenic (strept)avidin, which impedes application in humans. Although targeting bubbles not employing the biotin-(strept)avidin conjugation chemistry have been explored, only a few reached the stage of ultrasound imaging in vivo, none were reported/evaluated to show all three of the following properties desired for clinical applications: (i) low degree of non-specific bubble retention in more than one non-reticuloendothelial tissue; (ii) effective for real-time imaging; and (iii) effective for acoustic quantification of molecular targets to a high degree of quantification. Furthermore, disclosures of the compositions and methodologies enabling reproduction of the bubbles are often withheld.ObjectiveTo develop and evaluate a targeting microbubble based on maleimide-thiol conjugation chemistry for ultrasound molecular imaging.Methods and ResultsMicrobubbles with a previously unreported generic (non-targeting components) composition were grafted with anti-E-selectin F(ab’)2 using maleimide-thiol conjugation, to produce E-selectin targeting microbubbles. The resulting targeting bubbles showed high specificity to E-selectin in vitro and in vivo. Non-specific bubble retention was minimal in at least three non-reticuloendothelial tissues with inflammation (mouse heart, kidneys, cremaster). The bubbles were effective for real-time ultrasound imaging of E-selectin expression in the inflamed mouse heart and kidneys, using a clinical ultrasound scanner. The acoustic signal intensity of the targeted bubbles retained in the heart correlated strongly with the level of E-selectin expression (|r|≥0.8), demonstrating a high degree of non-invasive molecular quantification.ConclusionsTargeting microbubbles for ultrasound molecular imaging, based on maleimide-thiol conjugation chemistry and the generic composition described, may possess properties (i)–(iii) desired for clinical applications.

A Lipopeptide-Based αvβ3 Integrin-Targeted Ultrasound Contrast Agent for Molecular Imaging of Tumor Angiogenesis

The design and fabrication of targeted ultrasound contrast agents are key factors in the success of ultrasound molecular imaging applications. Here, we introduce a transformable αvβ3 integrin-targeted microbubble (MB) by incorporation of iRGD-lipopeptides into the MB membrane for non-invasive ultrasound imaging of tumor angiogenesis. First, the iRGD-lipopeptides were synthesized by conjugating iRGD peptides to distearoylphosphatidylethanolamine–polyethylene glycol 2000–maleimide. The resulting iRGD-lipopeptides were used for fabrication of the iRGD-carrying αvβ3 integrin-targeted MBs (iRGD-MBs). The binding specificity of iRGD-MBs for endothelial cells was found to be significantly stronger than that of control MBs (p < 0.01) under in vitro static and dynamic conditions. The binding of iRGD-MBs on the endothelial cells was competed off by pre-incubation with the anti-αv or anti-β3 antibody (p < 0.01). Ultrasound images taken of mice bearing 4T1 breast tumors after intravenous injections of iRGD-MBs or control MBs revealed strong contrast enhancement within the tumors from iRGD-MBs but not from the control MBs; the mean acoustic signal intensity was 10.71 ± 2.75 intensity units for iRGD-MBs versus 1.13 ± 0.18 intensity units for the control MBs (p < 0.01). The presence of αvβ3 integrin was confirmed by immunofluorescence staining. These data indicate that iRGD-MBs can be used as an ultrasound imaging probe for the non-invasive molecular imaging of tumor angiogenesis, and may have further implications for ultrasound image-guided tumor targeting drug delivery.

Recent Advances in Targeted, Self-Assembling Nanoparticles to Address Vascular Damage Due to Atherosclerosis.

Self-assembling nanoparticles functionalized with targeting moieties have significant potential for atherosclerosis nanomedicine. While self-assembly allows the easy construction (and degradation) of nanoparticles with therapeutic or diagnostic functionality, or both, the targeting agent can direct them to a specific molecular marker within a given stage of the disease. Therefore, supramolecular nanoparticles have been investigated in the last decade as molecular imaging agents or explored as nanocarriers that can decrease the systemic toxicity of drugs by producing accumulation predominantly in specific tissues of interest. In this Progress Report, the pathogenesis of atherosclerosis and the damage caused to vascular tissue are described, as well as the current diagnostic and treatment options. An overview of targeted strategies using self-assembling nanoparticles is provided, including liposomes, high density lipoproteins, protein cages, micelles, proticles, and perfluorocarbon nanoparticles. Finally, an overview is given of current challenges, limitations, and future applications for personalized medicine in the context of atherosclerosis of self-assembling nanoparticles.

WE-D-210-02: Design and Development of Ultrasound Contrast Agents for Molecular Imaging

The utilization of ultrasound contrast agents offers significant promise in enhancing the diagnostic capabilities of ultrasound imaging as well as offering new, noninvasive approaches for therapeutic intervention. The presentations in this session will provide an overview of current and future clinical diagnostic applications of ultrasound contrast for patient management, recent advances in the design and development of ultrasound contrast agents for molecular imaging, as well as the development of novel microbubble agents for theragnostic applications. Learning Objectives: 1. Develop an understanding of the current and future applications of ultrasound contrast agents for clinical application and preclinical research. 2. Develop a general understanding of the underlying technologies associated with improving the diagnostic capabilities of contrast– enhanced ultrasound imaging as well as potential noninvasive approaches for therapeutic intervention. Dr. Rychak is an employee of Targeson, Inc; NIH RO1 NS047603 and NIH RO1 HL074002, Holland: NIH RO1 NS047603 and NIH RO1 HL074002

WE-D-210-01: Use of Contrast Enhanced Ultrasound in the Care of Liver Patients

The utilization of ultrasound contrast agents offers significant promise in enhancing the diagnostic capabilities of ultrasound imaging as well as offering new, noninvasive approaches for therapeutic intervention. The presentations in this session will provide an overview of current and future clinical diagnostic applications of ultrasound contrast for patient management, recent advances in the design and development of ultrasound contrast agents for molecular imaging, as well as the development of novel microbubble agents for theragnostic applications. Learning Objectives: 1. Develop an understanding of the current and future applications of ultrasound contrast agents for clinical application and preclinical research. 2. Develop a general understanding of the underlying technologies associated with improving the diagnostic capabilities of contrast– enhanced ultrasound imaging as well as potential noninvasive approaches for therapeutic intervention. Dr. Rychak is an employee of Targeson, Inc; NIH RO1 NS047603 and NIH RO1 HL074002, Holland: NIH RO1 NS047603 and NIH RO1 HL074002

WE-D-210-03: Microbubble Pumps: Â Ultrasound Theragnostic Agents

The utilization of ultrasound contrast agents offers significant promise in enhancing the diagnostic capabilities of ultrasound imaging as well as offering new, noninvasive approaches for therapeutic intervention. The presentations in this session will provide an overview of current and future clinical diagnostic applications of ultrasound contrast for patient management, recent advances in the design and development of ultrasound contrast agents for molecular imaging, as well as the development of novel microbubble agents for theragnostic applications. Learning Objectives: 1. Develop an understanding of the current and future applications of ultrasound contrast agents for clinical application and preclinical research. 2. Develop a general understanding of the underlying technologies associated with improving the diagnostic capabilities of contrast– enhanced ultrasound imaging as well as potential noninvasive approaches for therapeutic intervention. Dr. Rychak is an employee of Targeson, Inc; NIH RO1 NS047603 and NIH RO1 HL074002, Holland: NIH RO1 NS047603 and NIH RO1 HL074002

WE-D-210-00: Advances in Ultrasound Contrast Agents for Diagnosis &amp; Therapy

The utilization of ultrasound contrast agents offers significant promise in enhancing the diagnostic capabilities of ultrasound imaging as well as offering new, noninvasive approaches for therapeutic intervention. The presentations in this session will provide an overview of current and future clinical diagnostic applications of ultrasound contrast for patient management, recent advances in the design and development of ultrasound contrast agents for molecular imaging, as well as the development of novel microbubble agents for theragnostic applications. Learning Objectives: 1. Develop an understanding of the current and future applications of ultrasound contrast agents for clinical application and preclinical research. 2. Develop a general understanding of the underlying technologies associated with improving the diagnostic capabilities of contrast– enhanced ultrasound imaging as well as potential noninvasive approaches for therapeutic intervention. Dr. Rychak is an employee of Targeson, Inc; NIH RO1 NS047603 and NIH RO1 HL074002, Holland: NIH RO1 NS047603 and NIH RO1 HL074002

Two Novel Nanosized Radiolabeled Analogues of Somatostatin for Neuroendocrine Tumor Imaging.

The somatostatin receptors (SR), which are overexpressed in a majority of neuroendocrine tumors, are targets for radiopeptide-based imaging using for example the 99mTc-Tyr3-Octreotide peptide. Dendrimers are hyperbranched polymeric structures. The nanoscopic size and near-monodisperse nature properties give polyamidoamine (PAMAM) dendrimers an edge over linear polymers in the context of drug delivery. Gold nanoparticles (AuNPs) conjugated to peptides produces stable multimeric systems with target-specific molecular recognition. The aim of this research was to prepare two nanosized multimeric systems for neuroendocrine tumor imaging, 99mTc-PAMAM-Tyr3-Octreotide and 99mTc-AuNP-Tyr-Octreotide, and to compare their in vitro uptake in SR-positive AR42J cancer cells as well as their biodistribution profile in athymic mice bearing AR42J tumors. [Tyr3, Lys(Boc)5]-Octreotide was conjugated to the carboxylate groups of the PAMAM dendrimer (G3.5) with further Boc deprotection using TFA. 99mTc labeling was carried out by a direct method. 99mTc-Tyr3-Octreotide was conjugated to AuNPs (20 nm) by spontaneous reaction with the thiol group of cysteine. Radiochemical purity (RP) was determined by size-exclusion HPLC and ITLC-SG analyses. In vitro binding studies were carried out in AR42J cancer cells. Biodistribution studies were accomplished in athymic mice with AR42J-induced tumors with blocked and unblocked receptors. Elemental analysis demonstrated that 26 Tyr3-Octreotide molecules were successfully conjugated to one molecule of PAMAM. RP for both nanosized conjugates was > 94% and showed recognition for SR in AR42J cells. The tissue distribution of radioactivity 2 h after 99mTc-PAMAM-Tyr3-Octreotide administration in mice showed specific tumor uptake (4.12 ± 0.57% of injected dose/g) and high accumulation in the pancreas (15.08 ± 3.11% of injected dose/g) which expresses SR. No significant difference in the tumor uptake was found between 99mTc-PAMAM-Tyr3-Octreotide and 99mTc-AuNP-Tyr3-Octreotide. However, the dendrimer-peptide conjugate showed a significant renal excretion. Both radiopharmaceuticals demonstrated properties suitable for use as target-specific agents for molecular imaging of tumors that overexpressed SR.

Optoacoustic imaging enabled biodistribution study of cationic polymeric biodegradable nanoparticles.

Nanosized contrast agents for molecular imaging have attracted widespread interest for diagnostic applications with high resolution in medicine. However, many solid nanoparticles exhibit a great potential to induce toxicity, hindering their use for clinical applications. On the other hand, near-infrared (NIR) dyes have also been used for extensive biological applications, but show some limitations due to their poor aqueous stability, tendency to aggregation and rapid elimination from the body. An alternative proposed in this work to overcome these limitations is the use of NIR dye-loaded nanoparticles. Here we introduce nanoparticles constructed with poly(D,L-lactide-co-glycolic acid) (PLGA), a biodegradable and biocompatible polymer widely used for biomedical applications, attached to the polycation polyethyleneimine (PEI) to obtain positively charged nanoparticles. The in vivo biodistribution of the cationic PEI-PLGA nanoparticles was investigated after administration through three different routes (intravenous, intraperitoneal and subcutaneous) using multispectral optoacoustic tomography (MSOT). The prepared nanoparticles exhibited good colloidal stability and adequate optical properties for optoacoustic imaging. The in vivo biodistribution assays indicated a strong accumulation of the particles in the liver and spleen, and retention in these organs for at least 24 h. Therefore, these nanoparticles could find promising applications in MSOT due to a sharp and characteristic optoacoustic spectrum and high optoacoustic signal generation, and become a promising building block for theranostic strategies.

Synthesis, characterization and radiolabeling of carborane-functionalized tetrazines for use in inverse electron demand Diels–Alder ligation reactions

Carborane-tetrazine derivatives were developed that can be used to enable boron clusters to bind specific targets in vivo using pretargeting strategies and bioorthogonal inverse electron demand Diels–Alder chemistry. Specifically, 1,12-dicarba-closo-dodecaborate-1-(4-(1,2,4,5-tetrazin-3-yl)phenyl)methanamide and 1,2-dicarba-closo-dodecaborate-1-(4-(1,2,4,5-tetrazin-3-yl)phenyl)methanamide were synthesized in 49% and 78% yield respectively, and X-ray structures of both compounds determined. The ortho-carborane derivative was converted to the corresponding nido-cluster under mild conditions, and the product successfully radiolabelled with 125I and 123I. The carborane-tetrazines react rapidly and in quantitative yields with (E)-cyclooct-4-enol (TCO) via a [4 + 2] inverse-electron demand Diels–Alder type reaction, where the second order rate constant in acetonitrile for the iodinated cluster was 199 ± 26 M−1s−1. The labelled tetrazines were stable in solution for extended periods and they were shown to bind in vitro to H520 cells, labelled with a TCO-modified antibody. Imaging studies on the 123I-labeled carborane-tetrazine were performed in healthy mice, and demonstrated minimal loss of iodine in vivo and high uptake in the liver and gall bladder. The reported compounds offer an alternative means of developing targeted boron neutron capture therapy (BNCT) and boron-based molecular imaging agents.

Multiple Bacteriophage Selection Strategies for Improved Affinity of a Peptide Targeting ERBB2.

Due to the heterogeneity of ERBB2-expression between tumors and over the course of treatment, a non-invasive molecular imaging agent is needed to accurately detect overall ERBB2 status. Peptides are a highly advantageous platform for molecular imaging, since they have excellent tumor penetration and rapid pharmacokinetics. One limitation of peptides however, is their traditionally low target affinity, and consequently, tumor uptake. The peptide KCCYSL was previously selected from a bacteriophage (phage) display library to bind ERBB2 and did so with moderate affinity of 295 nM. In order to enhance tumor uptake and clinical utility of the peptide, a novel phage microlibrary was created by flanking the parent sequence with random amino acids, followed by reselection using parallel strategies for high affinity and specific ERBB2 binding in an attempt to affinity maturate the peptide. One limitation of traditional phage display selections is difficulty in releasing the highest affinity phages from the target by incubation of acidic buffer. In an attempt to recover high affinity second-generation peptides from the ERBB2 microlibrary, two elution strategies, sonication and target elution, were undertaken. Sonication resulted in an approximately 50-fold enhancement in recovered phage per round of selection in comparison to target elution. Despite the differences in elution efficiency, the affinities of phage-displayed peptides selected from either strategy were relatively similar. Although both selections yielded peptides with significantly improved affinity in comparison to KCCYSL, the improvements were modest, most likely because the parental peptide binding cannot be improved by additional amino acids.

99mTc-Tilmanocept: A Novel Molecular Agent for Lymphatic Mapping and Sentinel Lymph Node Localization.

Preoperative lymphatic mapping in conjunction with intraoperative γ-probe detection is widely used for sentinel node localization in melanoma, breast cancer, and other malignancies. (99m)Tc-radiocolloids have been the standard radiotracers used for sentinel node mapping. (99m)Tc-tilmanocept is a receptor-binding molecular imaging agent approved by the U.S. Food and Drug Administration for lymphatic mapping and lymph node localization in breast cancer, melanoma, clinically node-negative squamous cell carcinoma of the oral cavity, and other solid tumors. It has several advantages over conventional radiocolloids, including rapid injection site clearance, high sentinel node extraction, and low distal node accumulation, which can lead to efficient resource use.

Challenges of Pancreatic Cancer.

The development of novel molecular cancer imaging agents has considerably advanced in recent years. Numerous cancer imaging agents have demonstrated remarkable potential for aiding the diagnosis, staging, and treatment planning at the preclinical stage, which in turn has led to a number of agents being approved for human trials. Pancreatic ductal adenocarcinoma is currently the most deadly common carcinoma with an overall 5-year survival rate of about 6%. As detection technologies progress, the need for molecular imaging tools that will allow the diagnosis at an early stage will be crucial to improving patient outcomes. In this review, we will highlight agents that illuminate various cell populations that comprise the tumor: epithelial, endothelial, and stromal tumor cells.

Exploring structural requirements of imaging agents against Aβ plaques in Alzheimer's disease: a QSAR approach.

Exploring molecular imaging agents against the beta amyloid (Aβ) plaques for an early detection of Alzheimer's disease (AD) is one of the emerging research areas in medicinal chemistry. In the present in-silico study, a congeneric series of 44 imaging agents, including 17 positron emission tomography (PET) and 27 single photon emission computed tomography (SPECT) imaging agents, was utilized to understand the structural features required for having essential binding affinity against Aβ plaques. Here, 2D-quantitative structure-activity relationship (2D-QSAR) and group-based QSAR (G-QSAR) models have been developed using genetic function approximation (GFA) and validated using various statistical metrics. Both the models showed satisfactory performance signifying the reliability and robustness of the developed QSAR models. The vital information gained from both the QSAR models will be useful in developing new PET and SPECT imaging agents and also in predicting their binding affinity against Aβ plaques. The results of this study would be important in view of the widespread clinical applicability of the SPECT imaging agents, especially in the developing countries. In this study, we have also designed some imaging agents based on the information provided by the models. Some of these designed compounds were predicted to be similar to or more active than the most active imaging agents present in the original dataset.

In Vitro Evaluation of Gd(3+)-Anionic Linear Globular Dendrimer-Monoclonal Antibody: Potential Magnetic Resonance Imaging Contrast Agents for Prostate Cancer Cell Imaging.

Early stage prostate cancer diagnosis is of high global interest. Magnetic resonance imaging (MRI) is a non-invasive modality for early cancer diagnosis, in particular for prostate cancer detection. The research aim is to synthesize a nanodendrimer and its conjugate with C595 monoclonal antibody (mAb C595), against prostate cancer, followed by its chelating with Gd(3+). Anti-MUC-1 mAb C595 was conjugated to an anionic linear globular dendrimer (ALGDG2). The polyethylene glycol core and citric acid shell were synthesized followed by loading with Gd(3+) to make novel contrast agents for functional MRI. The in vitro behavior and MRI parameters of the nanoconjugate were investigated performing several studies such as cell toxicity and TNF-alpha evaluations. The investigation of magnetic resonance imaging parameters indicated how well nanoconjugate performs in (1)H-NMR and (17)O-NMR in vitro. Results showed a potential specific MRI activity by improving the swelling responses cell binding. The MTT (2-(4,5-dimethyl-2-thiazolyl)-3,5-diphenyl-2H-tetrazolium bromide) assay demonstrated that this contrast agent had significant cytotoxicity on prostate cancer cells. These results showed that Gd(3+)-ALGDG2-C595 is a potential prostate molecular imaging agent and could be considered as an ideal functional nanoprobe. Additionally, further investigations by clinical trials are in the pipeline.

Preclinical Validation of the Heparin-Reactive Peptide p5+14 as a Molecular Imaging Agent for Visceral Amyloidosis.

Amyloid is a complex pathologic matrix comprised principally of paracrystalline protein fibrils and heparan sulfate proteoglycans. Systemic amyloid diseases are rare, thus, routine diagnosis is often challenging. The glycosaminoglycans ubiquitously present in amyloid deposits are biochemically and electrochemically distinct from those found in the healthy tissues due to the high degree of sulfation. We have exploited this unique property and evaluated heparin-reactive peptides, such as p5+14, as novel agents for specifically targeting and imaging amyloid. Herein, we demonstrate that radiolabeled p5+14 effectively bound murine AA amyloid in vivo by using molecular imaging. Biotinylated peptide also reacted with the major forms of human amyloid in tissue sections as evidenced immunohistochemically. Furthermore, we have demonstrated that the peptide also binds synthetic amyloid fibrils that lack glycosaminoglycans implying that the dense anionic motif present on heparin is mimicked by the amyloid protein fibril itself. These biochemical and functional data support the translation of radiolabeled peptide p5+14 for the clinical imaging of amyloid in patients.

Evolution of DNA aptamers through in vitro metastatic-cell-based systematic evolution of ligands by exponential enrichment for metastatic cancer recognition and imaging.

Metastasis, the capability of tumor cells to spread and grow at distant sites, is the primary factor in cancer mortality. Because metastasis in sentinel lymph nodes suggests the original spread of tumors from a primary site, the detection of lymph node involvement with cancer serves as an important prognostic and treatment parameter. Here we have developed a panel of DNA aptamers specifically binding to colon cancer cell SW620 derived from metastatic site lymph node, with high affinity after 14 rounds of selection by the cell-SELEX (systematic evolution of ligands by exponential enrichment) method. The binding affinities of selected aptamers were evaluated by flow cytometry. Aptamer XL-33 with the best binding affinity (0.7 nM) and its truncated sequence XL-33-1 with 45 nt showed excellent selectivity for recognizing target cell SW620. The binding entity of the selected aptamer has been preliminarily determined as a membrane protein on the cell surface. Tissue imaging results showed that XL-33-1 was highly specific to the metastatic tumor tissue or lymph node tissue with corresponding cancer metastasis and displayed an 81.7% detection rate against colon cancer tissue with metastasis in regional lymph nodes. These results suggest that XL-33-1 has great potential to become a molecular imaging agent for early detection of lymph node tissue with colon cancer metastasis. More importantly, this study clearly demonstrates that DNA ligands selectively recognizing metastatic cancer cells can be readily generated by metastatic-cell-based SELEX for potential applications in metastatic cancer diagnosis and treatment.

Ultrasound molecular imaging of tumor angiogenesis with a neuropilin-1-targeted microbubble

Ultrasound molecular imaging has great potential to impact early disease diagnosis, evaluation of disease progression and the development of target-specific therapy. In this paper, two neuropilin-1 (NRP) targeted peptides, CRPPR and ATWLPPR, were conjugated onto the surface of lipid microbubbles (MBs) to evaluate molecular imaging of tumor angiogenesis in a breast cancer model. Development of a molecular imaging agent using CRPPR has particular importance due to the previously demonstrated internalizing capability of this and similar ligands. In vitro, CRPPR MBs bound to an NRP-expressing cell line 2.6 and 15.6 times more than ATWLPPR MBs and non-targeted (NT) MBs, respectively, and the binding was inhibited by pretreating the cells with an NRP antibody. In vivo, the backscattered intensity within the tumor, relative to nearby vasculature, increased over time during the ∼6 min circulation of the CRPPR-targeted contrast agents providing high contrast images of angiogenic tumors. Approximately 67% of the initial signal from CRPPR MBs remained bound after the majority of circulating MBs had cleared (8 min), 8 and 4.5 times greater than ATWLPPR and NT MBs, respectively. Finally, at 7–21 days after the first injection, we found that CRPPR MBs cleared faster from circulation and tumor accumulation was reduced likely due to a complement-mediated recognition of the targeted microbubble and a decrease in angiogenic vasculature, respectively. In summary, we find that CRPPR MBs specifically bind to NRP-expressing cells and provide an effective new agent for molecular imaging of angiogenesis.

Residualization Rates of Near-Infrared Dyes for the Rational Design of Molecular Imaging Agents.

Near-infrared (NIR) fluorescence imaging is widely used for tracking antibodies and biomolecules in vivo. Clinical and preclinical applications include intraoperative imaging, tracking therapeutics, and fluorescent labeling as a surrogate for subsequent radiolabeling. Despite their extensive use, one of the fundamental properties of NIR dyes, the residualization rate within cells following internalization, has not been systematically studied. This rate is required for the rational design of probes and proper interpretation of in vivo results. In this brief report, we measure the cellular residualization rate of eight commonly used dyes encompassing three core structures (cyanine, boron-dipyrromethene (BODIPY), and oxazine/thiazine/carbopyronin). We identify residualizing (half-life >24h) and non-residualizing (half-life <24h) dyes in both the far-red (~650-680nm) and near-infrared (~740-800nm) regions. This data will allow researchers to independently and rationally select the wavelength and residualizing nature of dyes for molecular imaging agent design.

Selective nuclei accumulation of ruthenium(II) complex enantiomers that target G-quadruplex DNA

Different enantiomers exhibit large differences in their biological activity and/or toxicity, but they rarely involve the relationship of the agents for molecular and cellular imaging with the chiral structure of ruthenium complexes. Here, we report that an enantiomer of a polypyridyl ruthenium complex can selectively accumulate in the nucleus of HepG2 cells. Confocal laser scanning microscopy studies show that this phenomenon occurs via a non-endocytotic, but temperature-dependent, mechanism of cellular uptake in HepG2 cells. DNA oligonucleotides with repetitive tracts of guanine bases that can form G-quadruplex structures have aroused interest as therapeutic agents and as targets for anticancer drug design. Various biophysical techniques show that the Λ-enantiomer of ruthenium complexes can selectively stabilize human telomeric G-quadruplex DNA and has a strong preference for G-quadruplex over duplex DNA. Judged from the NMR results, we speculate that at higher 4:1 ligand/G-quadruplex stoichiometry, complex Λ–Ru is likely to bind with each groove of the tetraplex in a dimeric form or intercalate with the G-tetrad in the 3′ terminal face and coexist with other modes. The molecular modeling analysis is in agreement with the NMR titrations performed in this investigation indicating that ruthenium complexes are actually characterized by a mixed binding mode. The results provide many opportunities for the development of novel agents for living cell-related studies.