Non-invasive detection of apoptosis using magnetic resonance imaging and a targeted contrast agent

Magnet-Resonanz-Tomographie: Möglichkeiten der molekularen Bildgebung

Molecular imaging is "the in-vivo characterization and measurement of biological processes at the cellular and molecular level" and allows the imaging of molecular abnormalities associated with diseases long before morphological changes can be detected. At present, the use of magnetic resonance imaging (MRI) for molecular and cellular imaging is rapidly increasing. MRI is a very attractive candidate, since current MRI protocols already provide anatomic, functional, and biochemical information of excellent image quality and with high spatial resolution. Combining this high spatial resolution/high contrast imaging modality with specific MRI contrast imaging agents for molecular imaging is currently the focus of research in many laboratories worldwide. This paper summarizes the rationale for molecular MRI imaging and describes the basic features of modern molecular imaging strategies with MRI. Finally, a special focus is given to the growing field of applications, e.g., stem cell imaging, imaging of apoptosis, plaques, and other biological targets of interest.

Molecular imaging using magnetic resonance: new tools for the development of tumour therapy

Molecular imaging - the exploitation of specific molecules as the source of image contrast - promises new insights into disease processes in the laboratory and since the imaging modalities employed are applicable clinically, can be used to translate this knowledge into new diagnostics and treatments in the clinic. This brief review focuses on the use of MR-based molecular imaging techniques for developing tumour therapy. As examples, methods for detecting drug-induced tumour cell apoptosis; the response of tumours and their susceptibilities to an antivascular drug; early signs of tumour immune rejection and methods for detecting immune cell infiltration of tumours are described.

Iron oxide MR contrast agents for molecular and cellular imaging

Molecular and cellular MR imaging is a rapidly growing field that aims to visualize targeted macromolecules or cells in living organisms. In order to provide a different signal intensity of the target, gadolinium-based MR contrast agents can be employed although they suffer from an inherent high threshold of detectability. Superparamagnetic iron oxide (SPIO) particles can be detected at micromolar concentrations of iron, and offer sufficient sensitivity for T2(*)-weighted imaging. Over the past two decades, biocompatible particles have been linked to specific ligands for molecular imaging. However, due to their relatively large size and clearance by the reticuloendothelial system (RES), widespread biomedical molecular applications have yet to be implemented and few studies have been reproduced between different laboratories. SPIO-based cellular imaging, on the other hand, has now become an established technique to label and detect the cells of interest. Imaging of macrophage activity was the initial and still is the most significant application, in particular for tumor staging of the liver and lymph nodes, with several products either approved or in clinical trials. The ability to now also label non-phagocytic cells in culture using derivatized particles, followed by transplantation or transfusion in living organisms, has led to an active research interest to monitor the cellular biodistribution in vivo including cell migration and trafficking. While most of these studies to date have been mere of the 'proof-of-principle' type, further exploitation of this technique will be aimed at obtaining a deeper insight into the dynamics of in vivo cell biology, including lymphocyte trafficking, and at monitoring therapies that are based on the use of stem cells and progenitors.

Molecular imaging in oncology

Cancer is a genetic disease that manifests in loss of normal cellular homeostatic mechanisms. The biology and therapeutic modulation of neoplasia occurs at the molecular level. An understanding of these molecular processes is therefore required to develop novel prognostic and early biomarkers of response. In addition to clinical applications, increased impetus for the development of such technologies has been catalysed by pharmaceutical companies investing in the development of molecular therapies. The discipline of molecular imaging therefore aims to image these important molecular processes in vivo. Molecular processes, however, operate at short length scales and concentrations typically beyond the resolution of clinical imaging. Solving these issues will be a challenge to imaging research. The successful implementations of molecular imaging in man will only be realised by the close co-operation amongst molecular biologists, chemists and the imaging scientists.

Real-time imaging of single nerve cell apoptosis in retinal neurodegeneration

Apoptotic nerve cell death is implicated in the pathogenesis of several devastating neurodegenerative conditions, including glaucoma and Alzheimer's and Parkinson's diseases. We have devised a noninvasive real-time imaging technique using confocal laser-scanning ophthalmoscopy to visualize single nerve cell apoptosis in vivo, which allows longitudinal study of disease processes that has not previously been possible. Our method utilizes the unique optical properties of the eye, which allow direct microscopic observation of nerve cells in the retina. We have been able to image changes occurring in nerve cell apoptosis over hours, days, and months and show that effects depend on the magnitude of the initial apoptotic inducer in several models of neurodegenerative disease in rat and primate. This technology enables the direct observation of single nerve cell apoptosis in experimental neurodegeneration, providing the opportunity for detailed investigation of fundamental disease mechanisms and the evaluation of interventions with potential clinical applications, together with the possibility of taking this method through to patients.

Mouse MRI: Concepts and Applications in Physiology

The purpose of this review is to provide an introduction to the rapidly expanding field of mouse magnetic resonance imaging (MRI). It is by no means meant to be all-inclusive but rather to provide a brief introduction to the basics of MRI theory, provide some insight into the basic experiments that can be performed in mice by using MRI, and bring to light some factors to consider when planning a mouse MRI experiment.

Molecular magnetic resonance imaging with targeted contrast agents

Magnetic resonance imaging (MRI) produces high-resolution three-dimensional maps delineating morphological features of the specimen. Differential contrast in soft tissues depends on endogenous differences in water content, relaxation times, and/or diffusion characteristics of the tissue of interest. The specificity of MRI can be further increased by exogenous contrast agents (CA) such as gadolinium chelates, which have been successfully used for imaging of hemodynamic parameters including blood perfusion and vascular permeability. Development of targeted MR CA directed to specific molecular entities could dramatically expand the range of MR applications by combining the noninvasiveness and high spatial resolution of MRI with specific localization of molecular targets. However, due to the intrinsically low sensitivity of MRI (in comparison with nuclear imaging), high local concentrations of the CA at the target site are required to generate detectable MR contrast. To meet these requirements, the MR targeted CA should recognize targeted cells with high affinity and specificity. They should also be characterized by high relaxivity, which for a wide variety of CA depends on the number of contrast-generating groups per single molecule of the agent. We will review different designs and applications of targeted MR CA and will discuss feasibility of these approaches for in vivo MRI.

Apoptosis-detecting radioligands: current state of the art and future perspectives

This review provides a critical and thorough overview of the radiopharmaceutical development and in vivo evaluation of all apoptosis-detecting radioligands that have emerged so far, along with their possible applications in nuclear medicine. The following SPECT and PET radioligands are discussed: all forms of halogenated Annexin V (i.e. (123)I-labelled, (124)I-labelled, (125)I-labelled, (18)F-labelled), (99m)Tc/(94m)Tc-labelled Annexin V derivatives using different chelators and co-ligands (i.e. BTAP, Hynic, iminothiolane, MAG(3), EDDA, EC, tricarbonyl, SDH) or direct (99m)Tc-labelling, (99m)Tc-labelled Annexin V mutants and (99m)Tc/(18)F-radiopeptide constructs (i.e. AFIM molecules), (111)In-DTPA-PEG-Annexin V, (11)C-Annexin V and (64)Cu-, (67)Ga- and (68)Ga-DOTA-Annexin V. In addition, the potential role and clinical relevance of anti-PS monoclonal antibodies and other alternative apoptosis markers are reviewed, including: anti-Annexin V monoclonal antibodies, radiolabelled caspase inhibitors and substrates and mitochondrial membrane permeability targeting radioligands. Nevertheless, major emphasis is placed on the group of Annexin V-based radioligands, in particular (99m)Tc-Hynic-Annexin V, since this molecule is by far the most extensively investigated and best-characterised apoptosis marker at present. Furthermore, the newly emerging imaging modalities for in vivo detection of programmed cell death, such as MRI, MRS, optical, bioluminescent and ultrasound imaging, are briefly described. Finally, some future perspectives are presented with the aim of promoting the development of potential new strategies in pursuit of the ideal cell death-detecting radioligand.

Apoptosis in cancer--implications for therapy

Resistance towards apoptosis is a key factor for the survival of a malignant cell. Cancer results if there is too little apoptosis and cells grow faster and live longer than normal cells. In addition, defects in apoptosis signaling contribute to drug resistance of tumor cells. Thus, one of the main goals for oncologic treatment is to overcome resistance of tumor cells towards apoptosis. The exciting challenge in oncology is to translate the growing knowledge of apoptotic pathways into clinical applications. In this review we address the role of apoptosis signaling in tumorigenesis and drug resistance of tumor cells and discuss therapeutic approaches interfering with apoptosis pathways.

Seeing Within

Molecular imaging is a rapidly evolving discipline with the goal of developing tools to display and quantify molecular and cellular targets in vivo. The heart of this field is based on the rational design and screening of targeted and activatable imaging reporter agents to sense fundamental processes of biology. Parallel advances in small animal imaging systems and in agent synthesis have allowed molecular imaging applications to extend into the in vivo arena. These advances have permitted, for example, in vivo sensing of inflammation, apoptosis, cell trafficking, and gene expression. In this review, we first review core principles of molecular imaging with an emphasis on smart, activatable agent technology. We then discuss applications of state-of-the-art molecular probes to interrogate important aspects of cardiovascular biology, with a focus on atherosclerosis, thrombosis, and heart failure. In the ensuing years, we anticipate that fundamental aspects of cardiovascular biology will be detectable in vivo, and that promising molecular imaging agents will be translated into the clinical arena to guide diagnosis and therapy of human cardiovascular illness.

Magnetic resonance imaging of activated proliferating rhesus macaque T cells labeled with superparamagnetic monocrystalline iron oxide nanoparticles

Imaging of adoptively transferred cells in vivo by magnetic resonance imaging (MRI) could provide important information on disease-related patterns of lymphocyte homing in nonhuman primate models of AIDS. As a preliminary study to assess the feasibility of visualizing activated rhesus T cells by MRI, anti-CD3/CD28-expanded CD4+ T lymphocytes were labeled in vitro with monocrystalline iron oxide nanoparticles (MION). Intracellular incorporation of MION was determined by transmission electron microscopy (TEM) and inductively coupled plasma mass spectrography (ICP-MS). Pretreatment with colchicine did not affect MION labeling, suggesting that cellular uptake of MION occurred by adsorptive pinocytosis or receptor-mediated endocytosis. TEM analysis revealed that MION were intracellularly compartmentalized exclusively in the cytoplasm and did not cause any measurable physiologic effects on T-cell function, including viability, proliferation, synthesis of select cytokines (interleukin [IL]-2, IL-4, IL-6, IL-10, tumor necrosis factor-alpha, and interferon-gamma), activation antigens (CD25 and CD69), adhesion molecules (alpha4beta7 and CD49d), and susceptibility to in vitro infection with simian immunodeficiency virus mac239. A sensitivity of 0.05% (1 MION-labeled T cell in 2000 unlabeled cells) could be achieved using T2-weighted gradient echo imaging. Furthermore, under these experimental conditions, the MRI signal did not decrease in proliferating MION-labeled CD4+ T cells over a period of 120 hours. These results indicate that intracellular labeling with MION can be a useful technique for noninvasively monitoring trafficking patterns of adoptively transferred leukocyte subsets in real-time by MRI in nonhuman primate models of AIDS.

Recommendations of the National Heart, Lung, and Blood Institute Nanotechnology Working Group

Recent rapid advances in nanotechnology and nanoscience offer a wealth of new opportunities for diagnosis and therapy of cardiovascular, pulmonary, and hematologic diseases and sleep disorders. To review the challenges and opportunities offered by these nascent fields, the National Heart, Lung, and Blood Institute convened a Working Group on Nanotechnology. Working Group participants discussed the various aspects of nanotechnology and its applications to heart, lung, blood, and sleep (HLBS) diseases. This report summarizes their discussions according to scientific opportunities, perceived needs and barriers, specific disease examples, and recommendations on facilitating research in the field. An overarching recommendation of the Working Group was to focus on translational applications of nanotechnology to solve clinical problems. The Working Group recommended the creation of multidisciplinary research centers capable of developing applications of nanotechnology and nanoscience to HLBS research and medicine. Centers would also disseminate technology, materials, and resources and train new investigators. Individual investigators outside these centers should be encouraged to conduct research on the application of nanotechnology to biological and clinical problems. Pilot programs and developmental research are needed to attract new investigators and to stimulate creative, high-impact research. Finally, encouragement of small businesses to develop nanotechnology-based approaches to clinical problems was considered important.

The year(s) of the contrast agent - micro-MRI in the new millennium

The beginning of the new millennium has been a dynamic time for the field of magnetic resonance imaging (MRI). Exciting recent advances have been made at all levels of imaging, ranging from the visualization of single cells to rodents, birds and the human brain. Many of these techniques employ contrast agents to visualize the movement or activity of cells or organs in vivo; examples of this include the observation of stem cell migration, the tracking of labeled T cells, and the visualization of the events of gastrulation in developing Xenopus embryos. Other advances include improved techniques for elucidating white matter tracts in brain by either monitoring the diffusion of water along the tracts or tracing active neuronal tracts in vivo with Mn(2+) ions. Imaging of the immune system presents two dramatically different challenges: imaging most if not all of the body to follow cell trafficking, and imaging at cellular resolution to follow key intercellular and intracellular events.

Combination of transfection agents and magnetic resonance contrast agents for cellular imaging: relationship between relaxivities, electrostatic forces, and chemical composition

The purpose of this study was to investigate the changes in electrostatic and magnetic resonance (MR) properties observed when MR contrast agents (CAs) (Feridex, MION-46L, or G5-dendrimer-DOTA-Gd) are combined with transfection agents (TAs) under various conditions for use as a CA-TA complex basis for cellular labeling and MRI. CAs were incubated with various classes of TAs for 0-48 hr in solutions of varying concentrations and pH values. NMR relaxation rates (1/T(1), 1/T(2)), MRI and zeta potential (ZP) of CA-TA solutions were measured. TAs decreased the 1/T(1) and 1/T(2) of G5-DOTA-Gd, Feridex, and MION-46L by 0-95%. Altering the pH of G5-DOTA-Gd-TA decreased the T(1)-weighted signal intensity (SI) on MRI from 0 to 78%. Measured ZP values for G5-DOTA-Gd, Feridex, and MION-46L were -51, -41, and -2.0 mV, respectively. The TA LV had a negative ZP, while the other TAs had ZPs ranging from +20 to +65 mV. The alteration of the ZP and NMR relaxivities of the MR CAs, Feridex, MION-46L, and G5-DOTA-Gd by TAs has been demonstrated. These results enhance our understanding of the relationship between electrostatic and MR properties.

Integration of vascular biology and magnetic resonance imaging in the understanding of atherothrombosis and acute coronary syndromes

The interaction between the vulnerable atherosclerotic plaque prone to disruption and thrombus formation is the cornerstone of acute coronary syndrome (ACS). Although distinct from one another, the atherosclerotic and thrombotic processes appear to be interdependent, hence the term atherothrombosis. Inflammation is a crucial common pathophysiological mechanism. Overall, the association of plaque vulnerability and ACS has been well documented. Given the multifactorial origin of atherothrombosis the best preventive approach should be aggressive management of all the risk factors. New interventions should be directed toward decreasing vulnerability of the lesions thereby decreasing the risk of ACS.

Imaging of vascular gene therapy

Gene therapy is an exciting frontier in medicine today. Many genes have been shown to be useful for treatment of various vascular diseases, including chronic cardiac and limb ischemia syndromes, vasculoproliferative disorder, hypercholesterolemia, atherosclerosis, thrombosis, and hypertension. Precise delivery of genes into target vessels, efficient transfer of genes into vascular cells of the target, and prompt assessment of gene expression over time are three challenging tasks for successful vascular gene therapy. Thus, in vivo imaging methods that can be used to monitor gene delivery and localize gene expression are needed. Modern imaging techniques provide an opportunity to monitor and direct vascular gene therapy. Radiologists play a key role not only in developing and mastering endovascular genetic interventions but also in assessing the success of vascular gene therapy and directing further refinement of vascular gene therapy technology. This article provides an overview of the current status of imaging of vascular gene therapy.

Early detection of tumour immune-rejection using magnetic resonance imaging

Dynamic contrast agent-enhanced magnetic resonance imaging measurements of the perfusion of an immunogenic murine tumour showed that immune rejection was preceded by an increase in the apparent vascular volume of the tumour. This increase in vascularity, which has been observed previously in other tumours undergoing immune rejection, was confirmed by histological analysis of tumour sections obtained postmortem. Magnetic resonance imaging measurements similar to this could be used in the clinic to monitor the early responses of tumours to immunotherapy, before there is any change in tumour growth rate or volume.

In vivo monitoring of apoptosis

The biochemical and physiological processes involved in apoptosis were described from the perspective of detection by standard, clinical, noninvasive imaging modalities. The difficulties of monitoring apoptosis in vivo were discussed. Magnetic resonance imaging (MRI) approaches used to study apoptosis were surveyed. The cell shrinkage associated with apoptosis can be detected due to changes in tissue water T(2) and T(1)rho relaxation times and apparent diffusion coefficient (ADC). Magnetic resonance spectroscopy (MRS) approaches used to study apoptosis in vivo have largely centered on the formation of cytoplasmic lipid bodies, detected by 1H MRS, and metabolic/bioenergetic changes detected by 31P and 13C MRS. The most successful approach to in vivo mapping of apoptosis uses the high specific binding of annexin V or synaptotagmin I to phosphatidylserine (PS) that appears on the extracellular plasma membrane of cells during apoptosis. Technetium-99m (99mTc)-radiolabeling of the annexin V and superparamagnetic iron oxide (SPIO) labeling of the C2 domain of synaptotagmin I allow good in vivo apoptosis detection by gamma camera imaging and MRI, respectively.

Vascular MRI in the diagnosis and therapy of the high risk atherosclerotic plaque

Disruption of a high risk plaque is known as the primary cause of cardiovascular events. Characterization of arterial wall components has become an essential adjunct in the identification of patients with plaques prone to rupture. Magnetic Resonance Imaging (MRI) has been revealed as one of the noninvasive tools possibly capable of identifying and characterizing high risk atherosclerotic plaque. MRI may facilitate diagnosis, and guide and serially monitor interventional and pharmacological treatment of atherosclerotic disease. In addition, it permits the simultaneous assessment of the anatomy, morphology, and hemodynamics for the study of flow-induced atherogenesis. It possibly will identify asymptomatic patients with subclinical atherosclerosis. This has potential significance for the improvement of strategies in primary and secondary prevention.

Techniques: Visualizing apoptosis using nuclear magnetic resonance

Apoptosis plays a key role in tumour biology, and the induction of apoptosis forms a cornerstone of most anticancer therapies. New developments in nuclear magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) have taken these techniques far beyond their original roles as the workhorses of structural and pharmaceutical chemistry and clinical imaging to the detection of previously inaccessible and unrecognized biological phenomena in living cells and tissues undergoing apoptosis. These new MR techniques can be used in the development of new drugs and in the improved detection of treatment responses in the clinic.

Stem cells for myocardial regeneration

Stem cells are being investigated for their potential use in regenerative medicine. A series of remarkable studies suggested that adult stem cells undergo novel patterns of development by a process referred to as transdifferentiation or plasticity. These observations fueled an exciting period of discovery and high expectations followed by controversy that emerged from data suggesting cell-cell fusion as an alternate interpretation for transdifferentiation. However, data supporting stem cell plasticity are extensive and cannot be easily dismissed. Myocardial regeneration is perhaps the most widely studied and debated example of stem cell plasticity. Early reports from animal and clinical investigations disagree on the extent of myocardial renewal in adults, but evidence indicates that cardiomyocytes are generated in what was previously considered a postmitotic organ. On the basis of postmortem microscopic analysis, it is proposed that renewal is achieved by stem cells that infiltrate normal and infarcted myocardium. To further understand the role of stem cells in regeneration, it is incumbent on us to develop instrumentation and technologies to monitor myocardial repair over time in large animal models. This may be achieved by tracking labeled stem cells as they migrate into myocardial infarctions. In addition, we must begin to identify the environmental cues that are needed for stem cell trafficking and we must define the genetic and cellular mechanisms that initiate transdifferentiation. Only then will we be able to regulate this process and begin to realize the full potential of stem cells in regenerative medicine.

Chemotherapy: targeting the mitochondrial cell death pathway

One of the mechanisms by which chemotherapeutics destroy cancer cells is by inducing apoptosis. Apoptosis can be activated through several different signalling pathways, but these all appear to converge at a single event - mitochondrial membrane permeabilization (MMP). This 'point-of-no-return' in the cell death program is a complex process that is regulated by the composition of the mitochondrial membrane and pre-mitochondrial signal-transduction events. MMP is subject to a complex regulation, and local alterations in the composition of mitochondrial membranes, as well as alterations in pre-mitochondrial signal-transducing events, can determine chemotherapy resistance in cancer cells. Detecting MMP might thus be useful for detecting chemotherapy responses in vivo. Several cytotoxic drugs induce MMP by a direct action on mitochondria. This type of agents can enforce death in cells in which upstream signals normally leading to apoptosis have been disabled. Cytotoxic components acting on mitochondria can specifically target proteins from the Bcl-2 family, the peripheral benzodiazepin receptor, or the adenine nucleotide translocase, and/or act by virtue of their physicochemical properties as steroid analogues, cationic ampholytes, redox-active compounds or photosensitizers. Some compounds acting on mitochondria can overcome the cytoprotective effect of Bcl-2-like proteins. Several agents which are already used in anti-cancer chemotherapy can induce MMP, and new drugs specifically designed to target mitochondria are being developed.

Phosphatidylserine binding of class B scavenger receptor type I, a phagocytosis receptor of testicular sertoli cells

Testicular Sertoli cells phagocytose apoptotic spermatogenic cells in a manner depending on the membrane phospholipid phosphatidylserine (PS) expressed at the surface of the latter cell type. Our previous studies have indicated that class B scavenger receptor type I (SR-BI) is responsible for the PS-mediated phagocytosis by Sertoli cells. We examined here whether SR-BI binds directly to PS. A cell line acquired the ability to bind to PS-exposing apoptotic cells and to incorporate PS-containing liposomes when it was forced to express SR-BI. Furthermore, the extracellular domain of rat SR-BI fused with human Fc (SRBIecd-Fc) bound to PS with a dissociation equilibrium constant of 2.4 x 10(-7) m in a cell-free solid-phase assay, whereas other phospholipids including phosphatidylethanolamine, phosphatidylinositol, and phosphatidylcholine were poor binding targets. The binding activity was enhanced when CaCl(2) was included in the assay or when SRBIecd-Fc was pre-treated with N-glycanase. A portion of the extracellular domain spanning amino acid positions 33 and 191 (numbered with respect to the amino terminus) fused with Fc (SRBI33-191-Fc) showed activity and phospholipid specificity equivalent to those of SRBIecd-Fc. Finally, SRBI33-191-Fc bound to the surface of apoptotic cells with externalized PS, and the injection of SRBI33-191-Fc into the seminiferous tubules of live mice increased the number of apoptotic spermatogenic cells. These results allowed us to conclude that SR-BI is a phagocytosis-inducing PS receptor of Sertoli cells.

Annexin V-CLIO: a nanoparticle for detecting apoptosis by MRI

Annexin V, which recognizes the phosphatidylserine of apoptotic cells, was conjugated to crosslinked iron oxide (CLIO) nanoparticles, a functionalized superparamagnetic preparation developed for target-specific magnetic resonance imaging (MRI). The resulting nanoparticle had an average of 2.7 annexin V proteins linked per CLIO nanoparticle through disulfide bonds. Using camptothecin to induce apoptosis, a mixture of Jurkat T cells (69% healthy and 31% apoptotic) was incubated with annexin V-CLIO and was applied to magnetic columns. The result was an almost complete removal of the apoptotic cells (> 99%). In a phantom MRI experiment, untreated control cells (12% apoptotic cells, 88% healthy cells) and camptothecin-treated cells (65% apoptotic cells, 35% healthy cells) were incubated with either annexin V-CLIO (1.0, 0.5, and 0.1 microgram Fe/mL) or with unlabeled CLIO. A significant signal decrease of camptothecin-treated cells relative to untreated cells was observed even at the lowest concentration tested. Unmodified CLIO failed to cause a significant signal change of apoptotic cells. Hence, annexin V-CLIO allowed the identification of cell suspensions containing apoptotic cells by MRI even at very low concentrations of magnetic substrate. Conjugation of annexin V to CLIO affords a strategy for the development of a MRI imaging probe for detecting apoptosis.