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

Superparamagnetic iron oxide nanoparticles (SPIONs): development, surface modification and applications in chemotherapy

At present, nanoparticles are used for various biomedical applications where they facilitate laboratory diagnostics and therapeutics. More specifically for drug delivery purposes, the use of nanoparticles is attracting increasing attention due to their unique capabilities and their negligible side effects not only in cancer therapy but also in the treatment of other ailments. Among all types of nanoparticles, biocompatible superparamagnetic iron oxide nanoparticles (SPIONs) with proper surface architecture and conjugated targeting ligands/proteins have attracted a great deal of attention for drug delivery applications. This review covers recent advances in the development of SPIONs together with their possibilities and limitations from fabrication to application in drug delivery. In addition, the state-of-the-art synthetic routes and surface modification of desired SPIONs for drug delivery purposes are described.

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

Imaging the life and death of tumors in living subjects: Preclinical PET imaging of proliferation and apoptosis

Cancer is characterized by deregulation of cell proliferation and altered cell death apoptosis, which constitutes, in almost all instances, the minimal common platform upon which all neoplastic evolution occurs. The most implicit and clinically attractive anticancer strategies, therefore, consist of eliminating tumor cells by preventing their expansion and ultimately inducing cell death apoptosis. In this context, the non-invasive molecular assessment of tumor cell proliferation and apoptosis status using PET imaging constitutes a major strategy in preclinical studies to assess the efficacy of new anticancer therapeutics using small animal PET imaging, and in clinical settings for the monitoring of treatment responses in patients. For this purpose, a variety of PET tracers targeting specific molecular entities allowing the non-invasive measurement of biological processes, including cell proliferation and apoptosis, are under development for use in preclinical studies and clinical trials to non-invasively image in vivo the lifeline of tumors.

Reaction-diffusion systems in intracellular molecular transport and control

Chemical reactions make cells work only if the participating chemicals are delivered to desired locations in a timely and precise fashion. Most research to date has focused on active-transport mechanisms, although passive diffusion is often equally rapid and energetically less costly. Capitalizing on these advantages, cells have developed sophisticated reaction-diffusion (RD) systems that control a wide range of cellular functions-from chemotaxis and cell division, through signaling cascades and oscillations, to cell motility. These apparently diverse systems share many common features and are "wired" according to "generic" motifs such as nonlinear kinetics, autocatalysis, and feedback loops. Understanding the operation of these complex (bio)chemical systems requires the analysis of pertinent transport-kinetic equations or, at least on a qualitative level, of the characteristic times of the constituent subprocesses. Therefore, in reviewing the manifestations of cellular RD, we also describe basic theory of reaction-diffusion phenomena.

In vivo intravital endoscopic confocal fluorescence microscopy of normal and acutely injured rat lungs

We present a new lung imaging technique based on endoscopic confocal fluorescence microscopy (ECFM), which is a new method that is able to provide cellular and structural assessment of living tissue using a small confocal probe in direct contact with the visceral pleura. To observe distal airspace structure and cellular condition in normal and injured lungs (hyperoxic and bleomycin challenged), we used fluorescent-specific marker contrast and ECFM. Alveolar space ECFM with spectral analyses were performed at 488-nm excitation using FITC-labeled markers or naturally fluorescent dyes. The normal lung was compared with the sick lung, where our in vivo imaging experiments correlated well with results obtained with corresponding ex vivo conventional assays. Four main elements pertaining to the acute lung injury/acute respiratory distress syndrome (ALI/ARDS) pathophysiology and established early key events were specifically studied: alveolar epithelial membrane phenotype, lung cell apoptosis, neutrophil recruitment, and edema. ECFM allowed visualization of (i) fine-tuned ultrastructural lectin (RCA-1) and sialoglycoprotein (RTI40) epithelial cell membrane expression, (ii) YO-PRO-1-related DNA linking of lung cell apoptosis, (iii) PKH2 green fluorescent cell linker-labeled neutrophil tracking in lung microcirculatory network and airspaces, (iv) FITC-dextran plasma contrast and extravasation with edema formation. ECFM provides reliable results to corresponding ex vivo fluorescent methods. ECFM, using the minimally invasive Five-1(R) optical instrument and specific fluorescent markers, is able to provide real-time potentially useful imaging of live unfixed normal and injured lung tissue with promising developments for improving bedside diagnostic and decision-making therapeutic strategy in patients with ALI.

Mouse models in neurological disorders: applications of non-invasive imaging

Neuroimaging techniques represent powerful tools to assess disease-specific cellular, biochemical and molecular processes non-invasively in vivo. Besides providing precise anatomical localisation and quantification, the most exciting advantage of non-invasive imaging techniques is the opportunity to investigate the spatial and temporal dynamics of disease-specific functional and molecular events longitudinally in intact living organisms, so called molecular imaging (MI). Combining neuroimaging technologies with in vivo models of neurological disorders provides unique opportunities to understand the aetiology and pathophysiology of human neurological disorders. In this way, neuroimaging in mouse models of neurological disorders not only can be used for phenotyping specific diseases and monitoring disease progression but also plays an essential role in the development and evaluation of disease-specific treatment approaches. In this way MI is a key technology in translational research, helping to design improved disease models as well as experimental treatment protocols that may afterwards be implemented into clinical routine. The most widely used imaging modalities in animal models to assess in vivo anatomical, functional and molecular events are positron emission tomography (PET), magnetic resonance imaging (MRI) and optical imaging (OI). Here, we review the application of neuroimaging in mouse models of neurodegeneration (Parkinson's disease, PD, and Alzheimer's disease, AD) and brain cancer (glioma).

Phosphatidylserine targeting for diagnosis and treatment of human diseases

Cells are able to execute apoptosis by activating series of specific biochemical reactions. One of the most prominent characteristics of cell death is the externalization of phosphatidylserine (PS), which in healthy cells resides predominantly in the inner leaflet of the plasma membrane. These features have made PS-externalization a well-explored phenomenon to image cell death for diagnostic purposes. In addition, it was demonstrated that under certain conditions viable cells express PS at their surface such as endothelial cells of tumor blood vessels, stressed tumor cells and hypoxic cardiomyocytes. Hence, PS has become a potential target for therapeutic strategies aiming at Targeted Drug Delivery. In this review we highlight the biomarker PS and various PS-binding compounds that have been employed to target PS for diagnostic purposes. We emphasize the 35 kD human protein annexin A5, that has been developed as a Molecular Imaging agent to measure cell death in vitro, and non-invasively in vivo in animal models and in patients with cardiovascular diseases and cancer. Recently focus has shifted from diagnostic towards therapeutic applications employing annexin A5 in strategies to deliver drugs to cells that express PS at their surface.

Renal inflammation: targeted iron oxide nanoparticles for molecular MR imaging in mice

PURPOSE

To determine the feasibility of T2-weighted magnetic resonance (MR) imaging in the noninvasive quantification of renal inflammation by using superparamagnetic iron oxide (SPIO) nanoparticles targeted to tissue-bound C3 activation fragments in a mouse model of lupus nephritis.

MATERIALS AND METHODS

All animal procedures were approved by the University of Colorado-Denver animal care and use committee. SPIO nanoparticles were encapsulated by using amine-functionalized phospholipids. A recombinant protein containing the C3d-binding region of complement receptor type 2 (CR2) was then conjugated to the surface of the SPIO nanoparticle. Five MRL/lpr mice (a model of lupus nephritis) and six C57BL/6 wild-type mice were assessed with T2-weighted MR imaging at baseline and after SPIO injection. The same five MRL/lpr mice and three C57BL/6 mice also underwent MR imaging after injection of CR2-targeted SPIO. A series of T2-weighted pulses with 16 echo times was used to enable precise T2 mapping and calculation of T2 relaxation times in the cortex and outer and inner medulla of the kidneys, as well as in the spleen, muscle, and fat. The effects of treatment and animal genotype on T2 relaxation times were analyzed with repeated-measures analysis of variance.

RESULTS

At baseline, the T2-weighted signal intensity in the kidneys of MRL/lpr mice was higher than that in the kidneys of wild-type mice. Injection of untargeted SPIO did not alter the T2-weighted signal in the kidneys in either strain of mice. Injection of CR2-targeted SPIO in MRL/lpr mice, however, caused a significant accumulation of targeted iron oxide with a subsequent decrease in T2 relaxation times in the cortex and outer and inner medulla of the kidneys. No changes in T2 relaxation time were observed in the wild-type mice after injection of targeted SPIO.

CONCLUSION

Injection of CR2-conjugated SPIO caused a significant reduction in T2-weighted MR imaging signal and T2 relaxation time in nephritic kidneys.

Apoptosis- and necrosis-induced changes in light attenuation measured by optical coherence tomography

Optical coherence tomography (OCT) was used to determine optical properties of pelleted human fibroblasts in which necrosis or apoptosis had been induced. We analysed the OCT data, including both the scattering properties of the medium and the axial point spread function of the OCT system. The optical attenuation coefficient in necrotic cells decreased from 2.2 ± 0.3 mm⁻¹ to 1.3 ± 0.6 mm⁻¹, whereas, in the apoptotic cells, an increase to 6.4 ± 1.7 mm⁻¹ was observed. The results from cultured cells, as presented in this study, indicate the ability of OCT to detect and differentiate between viable, apoptotic, and necrotic cells, based on their attenuation coefficient. This functional supplement to high-resolution OCT imaging can be of great clinical benefit, enabling on-line monitoring of tissues, e.g. for feedback in cancer treatment.

In situ time-resolved XAFS study on the formation mechanism of Cu nanoparticles using poly(N-vinyl-2-pyrrolidone) as a capping agent

The formation mechanism of copper nanoparticles (NPs) using poly(N-vinyl-2-pyrrolidone) (PVP) as a capping agent was investigated by measurements of X-ray diffraction (XRD), transmission electron microscopy (TEM), in situ time-resolved X-ray adsorption fine structure (XAFS) analysis, in situ UV-vis spectroscopy, and an indicator method. XAFS analyses, in combination with TEM observations and the indicator method, revealed that the stable intermediates such as Cu(OH)(2) and Cu(+)-PVP intermediate were formed during an induction period of nucleation of Cu NPs, which play a critical role in the Cu NP formation. Our results suggest that the PVP capping agent is important not only to protect NPs from overgrowth and aggregation but also to control the reaction kinetics of NP formation.

Design and fabrication of magnetic nanoparticles for targeted drug delivery and imaging

Magnetic nanoparticles (MNPs) represent a class of non-invasive imaging agents that have been developed for magnetic resonance (MR) imaging. These MNPs have traditionally been used for disease imaging via passive targeting, but recent advances have opened the door to cellular-specific targeting, drug delivery, and multi-modal imaging by these nanoparticles. As more elaborate MNPs are envisioned, adherence to proper design criteria (e.g. size, coating, molecular functionalization) becomes even more essential. This review summarizes the design parameters that affect MNP performance in vivo, including the physicochemical properties and nanoparticle surface modifications, such as MNP coating and targeting ligand functionalizations that can enhance MNP management of biological barriers. A careful review of the chemistries used to modify the surfaces of MNPs is also given, with attention paid to optimizing the activity of bound ligands while maintaining favorable physicochemical properties.

A simple and highly sensitive method for magnetic nanoparticle quantitation using 1H-NMR spectroscopy

Iron oxide superparamagnetic nanoparticles (SPIONs) have drawn significant attention because of their potential impact on medical diagnosis and therapy. However, the difficulty of achieving reliable and standardized quantification of these nanoparticles has limited the uniform study of nanoparticle systems. Current measurement techniques have limited sensitivity, and are sophisticated and subject to individual instrumental settings. Here, a characterization method using proton nuclear magnetic resonance ((1)H-NMR) spectroscopy is presented that can quantify SPIONs regardless of surface modification. In addition to routine quantification of SPIONs during nanoparticle development, the method can also be used with in vitro nanoparticle assays and potentially with tissue samples for biodistribution studies. Specifically, measurement of water relaxivity shifts (R(1) or R(2)) of dissolved SPION samples is correlated with nanoparticle concentration. Unmodified and dextran- and poly(ethylene glycol)-coated SPIONs gave linear correlations between SPION concentration and R(1) and R(2) relaxivities over five orders of magnitude, to below 10 ppb iron. Quantification of SPION concentration was also demonstrated in the presence of RAW 264.7 macrophage cells. A linear correlation between the SPION concentration and relaxivities was observed to <10 ng Fe/mL. This method is a rapid and inexpensive approach for quantitation of SPIONs and exhibits a number of advantages over many of the current methods for quantitative SPION analysis.

Efficient site-specific radiolabeling of a modified C2A domain of synaptotagmin I with [99mTc(CO)3]+: a new radiopharmaceutical for imaging cell death

We describe the design and synthesis of a new Tc-99m labeled bioconjugate for cell-death imaging, based on C2A, the phosphatidylserine (PS)-binding domain of rat synaptotagmin I. Since several lysine residues in this protein are critical for PS binding, we engineered a new protein, C2AcH, to include the C-terminal sequence CKLAAALEHHHHHH, incorporating a free cysteine (for site-specific covalent modification) and a hexahistidine tag (for site-specific radiolabeling with [99mTc(CO)3(OH2)3]+). We also engineered a second derivative, C2Ac, in which the C-terminal sequence included only the C-terminal cysteine. These proteins were characterized by electrospray mass spectrometry, SDS/PAGE, and size exclusion chromatography and radiolabeled with [99mTc(CO)3(OH2)3]+. Conjugates of the proteins with the rhenium analogue [Re(CO)3(OH2)3]+ were also synthesized. Site-specific labeling was confirmed by performing a tryptic digest of rhenium tricarbonyl-labeled C2AcH, and only peptides containing the His-tag contained the [Re(CO)3]+. The labeled proteins were tested for binding to red blood cells (RBC) with exposed PS in a calcium dependent manner. Labeling 100 microg of C2AcH with [99mTc(CO)3(OH2)3]+ at 37 degrees C for 30 min gave a radiochemical yield of > 96%. However, C2AcH that had first been conjugated with fluorescein maleimide or iodoacetamide via the Cys residue gave only 50% and 83% radiochemical yield, respectively, after incubation for 30 min at 37 degrees C. Serum stability results indicated that >95% of radiolabeled C2AcH remained stable for at least 18 h at 37 degrees C. Site-specifically labeled C2AcH exhibited calcium-dependent binding to the PS on the RBC, whereas a nonspecifically modified derivative, C2AcH-B, in which lysines had been modified with benzyloxycarbonyloxy, did not. We conclude that (i) the combination of Cys and a His-tag greatly enhances the rate and efficiency of labeling with [99mTc(CO)3(OH2)3]+ compared to either the His-tag or the Cys alone, and this sequence deserves further evaluation as a radiolabeling tag; (ii) non-site-specific modification of C2A via lysine residues impairs target binding affinity; (iii) 99mTc-C2AcH has excellent radiolabeling, stability and PS binding characteristics and warrants in vivo evaluation as a cell-death imaging agent.

Recent developments and new perspectives on imaging of atherosclerotic plaque: role of anatomical, cellular and molecular MRI part III

Atherosclerotic plaque disruption accounts for the major part of cardiovascular mortality and the risk of disruption appears to depend on plaque composition. Carotid plaques in patients, scheduled for endarterectomy, have been successfully characterised with MRI. MRI has the advantage of combining information about morphology and function. Unfortunately, the tortuosity and size of the coronary arteries, and the respiratory and cardiac motion hinder the in vivo characterisation of human coronary plaque. In addition to plaque composition several molecular markers of the different processes involved in atherosclerosis, such as integrins, matrix metalloproteinases and fibrin seem to correlate with risk of plaque rupture and clinical outcome. These molecular markers can be targeted with antibodies coupled to carriers, which are loaded with gadolinium for detection (molecular MRI). Several cellular/molecular MRI studies in animal models and some in human patients have been conducted with varying levels of success. The advent of clinical high field magnets, the development of contrast agent carriers with high relaxivity and the development of relatively new MR contrast techniques appear to be promising in the field of plaque imaging. Future MRI studies will have to focus on the molecular target of the atherosclerotic process, which has the highest prognostic value with regard to acute coronary syndromes and on the most suitable contrast agent to visualize that target.

Current applications of nanotechnology for magnetic resonance imaging of apoptosis

Apoptosis, or programmed cell death, is a morphologically and biochemically distinct form of cell death, which together with proliferation plays an important role in tissue development and homeostasis. Insufficient apoptosis is important in the pathology of various disorders such as cancer and autoimmune diseases, whereas a high apoptotic activity is associated with myocardial infarction, neurodegenerative diseases, and advanced atherosclerotic lesions. Consequently, apoptosis is recognized as an important therapeutic target, which should be either suppressed, e.g., during an ischemic cardiac infarction, or promoted, e.g., in the treatment of cancerous lesions. Imaging tools to address location, amount, and time course of apoptotic activity non-invasively in vivo are therefore of great clinical use in the evaluation of such therapies. This chapter reviews current literature and new developments in the application of nanoparticles for non-invasive apoptosis imaging. Focus is on functionalized nanoparticle contrast agents for MR imaging and bimodal nanoparticle agents that combine magnetic and fluorescent properties.

Porous hollow Fe(3)O(4) nanoparticles for targeted delivery and controlled release of cisplatin

We report a new approach to cisplatin storage and release using porous hollow nanoparticles (PHNPs) of Fe(3)O(4). We prepared the PHNPs by controlled oxidation of Fe NPs at 250 degrees C followed by acid etching. The opening pores ( approximately 2-4 nm) facilitated the cisplatin diffusion into the cavity of the hollow structure. The porous shell was stable in neutral or basic physiological conditions, and cisplatin escape from the cavity through the same pores was a diffusion-controlled slow process with t(1/2) = 16 h. However, in low pH (<6) conditions, the pores were subject to acidic etching, resulting in wider pore gaps and faster release of cisplatin with t(1/2) < 4 h. Once coupled with Herceptin to the surface, the cisplatin-loaded hollow NPs could target to breast cancer SK-BR-3 cells with IC(50) reaching 2.9 muM, much lower than 6.8 muM needed for free cisplatin. Our model experiments indicate that the low pH-responsive PHNPs of Fe(3)O(4) can be exploited as a cisplatin delivery vehicle for target-specific therapeutic applications.

High-resolution ocular imaging: combining advanced optics and microtechnology

Recent developments in imaging technologies offer great potential for the assessment of retinal ganglion cell disorders, with particular relevance to glaucoma. In particular, advances in this field have allowed unprecedented in vivo access to the retinal layers, using many different properties of light to differentiate cellular structures. This article is a summary of currently available and investigational advanced, high-resolution imaging technologies and their potential applications to glaucoma. It represents the topics of discussion at the annual Optic Nerve Rescue and Restoration Think Tank, sponsored by The Glaucoma Foundation, entitled "High Resolution Imaging of the Eye: Advanced Optics, Microtechnology and Nanotechnology" and held in New York, New York, September 28-29, 2007.

Production of hyperpolarized [1,4-13C2]malate from [1,4-13C2]fumarate is a marker of cell necrosis and treatment response in tumors

Dynamic nuclear polarization of (13)C-labeled cell substrates has been shown to massively increase their sensitivity to detection in NMR experiments. The sensitivity gain is sufficiently large that if these polarized molecules are injected intravenously, their spatial distribution and subsequent conversion into other cell metabolites can be imaged. We have used this method to image the conversion of fumarate to malate in a murine lymphoma tumor in vivo after i.v. injection of hyperpolarized [1,4-(13)C(2)]fumarate. In isolated lymphoma cells, the rate of labeled malate production was unaffected by coadministration of succinate, which competes with fumarate for transport into the cell. There was, however, a correlation with the percentage of cells that had lost plasma membrane integrity, suggesting that the production of labeled malate from fumarate is a sensitive marker of cellular necrosis. Twenty-four hours after treating implanted lymphoma tumors with etoposide, at which point there were significant levels of tumor cell necrosis, there was a 2.4-fold increase in hyperpolarized [1,4-(13)C(2)]malate production compared with the untreated tumors. Therefore, the formation of hyperpolarized (13)C-labeled malate from [1,4-(13)C(2)]fumarate appears to be a sensitive marker of tumor cell death in vivo and could be used to detect the early response of tumors to treatment. Given that fumarate is an endogenous molecule, this technique has the potential to be used clinically.

Making stem cells infarct avid

A major factor limiting the engraftment of transplanted stem cells after myocardial infarction is the low rate of retention in the infarcted site. Our long-term objective is to improve engraftment by enabling stem cells to recognize and bind infarcted tissue. To this end, we proposed to modify the surface of embryonic stem cells (ESCs) with the C2A domain of synaptotagmin I; this allows the engineered stem cells to bind to dead and dying cardiac cells by recognizing phosphatidylserine (PS). The latter is a molecular marker for apoptotic and necrotic cells. The C2A domain of synaptotagmin I, which binds PS with high affinity and specificity, was attached to the surface of mouse ESCs using the biotin-avidin coupling mechanism. Binding of C2A-ESCs to dead and dying cardiomyocytes was tested in vitro. After the surface modification, cellular physiology was examined for viability, pluripotency, and differentiation potential. C2A covalently attached to the ESC surface at an average of about 1 million C2A molecules per cell under mild conjugation reaction conditions. C2A-ESCs avidly bound to dying, but not viable, cardiomyocytes in culture. The normal physiology of C2A-modified ESCs was maintained. The binding of C2A-ESCs to moribund cardiomyocytes demonstrates that the retention of transplanted cells may be improved by conferring these cells with the ability to bind infarcted tissue. Once established, this novel approach may be applicable to other types of transplanted therapeutic cells.

Fabrication of Mn-ferrite nanoparticles from MnO colloids

The reaction mechanism for conversion of MnO nanoparticles to Mn-ferrite nanoparticles was studied, which involved sequential consumption of MnO and the growth of ferrite. The method could be applied to other ferrite nanoparticles including cobalt ferrite.

Detection of apoptotic cell death in vitro in the presence of Gd-DTPA-BMA

Due to variability in patient response to cancer therapy, there is a growing interest in monitoring patient progress during treatment. Apoptotic cell death is one early marker of tumor response to treatment. Using known extracellular concentrations of gadolinium diethylenetriamine pentaacetic acid bismethylamide (Gd-DTPA-BMA) to vary the exchange regime, T(1) and T(2) relaxation data for acute myeloid leukemia (AML) cell samples were obtained and then analyzed using a two-pool model of relaxation with exchange. Leukemia cells treated with cisplatin to induce apoptosis exhibited a statistically significant (P < 0.05) decrease in intracellular longitudinal relaxation time, T(1I), from 1030 ms to 940 ms, a decrease (P < 0.001) in the intracellular water fraction, M(0I), from 0.86 to 0.68 and a statistically significant increase (P < 0.01) in transmembrane water exchange rate, k(IE), from 1.4 s(-1) to 6.8 s(-1). The changes in MR parameters correlated with quantitative histology, such as cellular cross-sectional area and average nuclear area measurements. The results of this study emphasize the importance of accounting for water exchange in dynamic contrast-enhanced MRI (DCE-MRI) studies, particularly those that examine tumor response to therapies in which apoptotic changes occur.

Temporal and dose dependence of T2 and ADC at 9.4 T in a mouse model following single fraction radiation therapy

The purpose of this study is to use magnetic resonance imaging to monitor the response of human glioma tumor xenografts to single fraction radiation therapy. Mice were divided into four treatment groups (n = 6 per group) that received 50, 200, 400, or 800 cGy of 200 kVp x rays. A fifth group (n = 6) received no radiation dose and served as the control. Quantitative maps of the treated tumor tissue were produced of water apparent diffusion coefficient (ADC) and transverse relaxation time (T2). Mice were imaged before and at multiple time points after treatment. There was a statistically significant difference in tumor growth relative to that of the control for all treatment groups. Only the highest dose group showed T2 values that were significantly different at all measured time points after treatment. In this group, there was an 8.3% increase in T2 relative to controls 2 days after treatment, but when measured 14 days after treatment, mean tumor T2 had dropped to 10.1% below the initial value. ADC showed statistically significant differences from the control at all dose points. A radiation dose dependence was observed. In the highest dose group, the fractional increases in ADC were higher than those observed for T2. ADC was sensitive to radiation-induced changes in lower dose groups that did not have significant T2 change. At all doses, elevation of mean tumor ADC preceded deviations in tumor growth from the control. These observations support the potential application of ADC as a time and dose sensitive marker of tumor response to radiation therapy.

Molecular MRI detects low levels of cardiomyocyte apoptosis in a transgenic model of chronic heart failure

BACKGROUND

The ability to image cardiomyocyte (CM) apoptosis in heart failure could facilitate more accurate diagnostics and optimize targeted therapeutics. We thus aimed to develop a platform to image CM apoptosis quantitatively and specifically in heart failure in vivo. The myocardium in heart failure, however, is characterized by very low levels of CM apoptosis and normal vascular permeability, factors thought to preclude the use of molecular MRI.

METHODS AND RESULTS

Female mice with overexpression of Gaq were studied. Two weeks postpartum, these mice develop a cardiomyopathy characterized by low levels of CM apoptosis and minimal myocardial necrosis or inflammation. The mice were injected with the annexin-labeled nanoparticle (AnxCLIO-Cy5.5) or a control probe (CLIO-Cy5.5) and imaged in vivo at 9.4 T. Uptake of AnxCLIO-Cy5.5 occurred in isolated clusters, frequently in the subendocardium. Myocardial T2* was significantly lower (7.6+/-1.5 versus 16.8+/-2.7 ms, P<0.05) in the mice injected with AnxCLIO-Cy5.5 versus CLIO-Cy5.5, consistent with the uptake of AnxCLIO-Cy5.5 by apoptotic CMs. A strong correlation (r(2)=0.86, P<0.05) was seen between in vivo T2* (AnxCLIO-Cy5.5 uptake) and myocardial caspase-3 activity.

CONCLUSIONS

The ability of molecular MRI to image sparsely expressed targets in the myocardium is demonstrated in this study. Moreover, a novel platform for high-resolution and specific imaging of CM apoptosis in heart failure is established. In addition to providing novel insights into the pathogenesis of CM apoptosis, the developed platform could facilitate the development of novel antiapoptotic therapies in heart failure.

Easy route to functionalize iron oxide nanoparticles via long-term stable thiol groups

The functionalization of superparamagnetic iron oxide nanoparticles (SPIOs) by meso-2,3-dimercaptosuccinic acid (DMSA) was investigated. Under ambient conditions, the thiol groups from DMSA are not stable and do not allow a direct functionalization without storage in stringent conditions or a chemical regeneration of free thiols. In this study, we have developed a protocol based on poly(ethylene glycol) (PEG) grafting of SPIO prior to DMSA anchoring. We have observed that PEG helps to increase the stability of thiol groups under ambient conditions. The thiol functionalized SPIOs were stable under physiological pH and ionic strength as determined by Ellman's essay and allowed us to graft a thiol reactive fluorescent dye: tetramethylrhodamine-5-maleimide (TMRM).

Tracking the migration of cardially delivered therapeutic stem cells in vivo: state of the art

Cell-based therapy is a promising, novel therapeutic strategy for cardiovascular disease. The rapid transition of this approach from the benchside to clinical trials has left a gap in the understanding of the mechanisms of cell therapy. Monitoring of cell homing and the fate of cardially delivered stem cells is fundamental for clarification of the myocardial regenerative process. Noninvasive imaging techniques allow an in vivo evaluation of the survival, migration and differentiation of implanted stem cells over time, and by this means, can help to answer unresolved questions. The most promising in vivo tracking methods involve the direct, nonspecific labeling of cells including MRI, radionuclide imaging and the use of reporter-gene imaging. This review summarizes the most important results of animal and human studies in which the fate and biodistribution of cardially delivered stem cells are assessed through different in vivo tracking methods.

Characterization of image heterogeneity using 2D Minkowski functionals increases the sensitivity of detection of a targeted MRI contrast agent

A targeted Gd(3+)-based contrast agent has been developed that detects tumor cell death by binding to the phosphatidylserine (PS) exposed on the plasma membrane of dying cells. Although this agent has been used to detect tumor cell death in vivo, the differences in signal intensity between treated and untreated tumors was relatively small. As cell death is often spatially heterogeneous within tumors, we investigated whether an image analysis technique that parameterizes heterogeneity could be used to increase the sensitivity of detection of this targeted contrast agent. Two-dimensional (2D) Minkowski functionals (MFs) provided an automated and reliable method for parameterization of image heterogeneity, which does not require prior assumptions about the number of regions or features in the image, and were shown to increase the sensitivity of detection of the contrast agent as compared to simple signal intensity analysis.

Molecular imaging: a primer for interventionalists and imagers

The characterization of human diseases by their underlying molecular and genomic aberrations has been the hallmark of molecular medicine. From this, molecular imaging has emerged as a potentially revolutionary discipline that aims to visually characterize normal and pathologic processes at the cellular and molecular levels within the milieu of living organisms. Molecular imaging holds promise to provide earlier and more precise disease diagnosis, improved disease characterization, and timely assessment of therapeutic response. This primer is intended to provide a broad overview of molecular imaging with specific focus on future clinical applications relevant to interventional radiology.

Quantitative measurement of multifunctional quantum dot binding to cellular targets using flow cytometry

Semiconductor nanocrystals such as quantum dots (QDs) are a potentially powerful resource in the fields of flow cytometry and fluorescence microscopy. QD size and fluorescence characteristics offer attractive features for use in targeted delivery systems and detection by flow cytometry. While quantitative measurements of a variety of fluorescent molecules are routinely performed, fluorophores for which no calibration standards exist, such as QDs, pose a problem for quantitation in flow cytometry. Our goal was to develop a targeted nanoparticle delivery platform as well as a corresponding method to accurately and quantitatively assess the performance of this system. We synthesized surface-modified QD probes targeted to cellular surface receptors and measured the MFI of the resulting cell-probe conjugates by flow cytometry. MFI was converted to mean equivalent R-PE intensity (MEPE) using standard calibration microspheres. Known concentrations of both R-PE and QD probes were measured by fluorometry to relate R-PE and QD fluorescence. Fluorometry results were then used to translate MEPE measurements to the number of bound QD probes. The targeted probes exhibited superior binding characteristics over unmodified and untargeted particles. This binding interaction was shown to be specific and mediated by the NGR targeting peptide tethered to the QD surface. The calibration method developed to assess this system proved successful at converting raw fluorescence data to quantitative probe binding values. We demonstrate the synthesis and performance of a highly modular nanoparticle system capable of targeted binding and fluorescent imaging. The calibration method implemented to quantify the performance of this system represents a potentially powerful tool to utilize truly quantitative flow cytometry measurements with an array of fluorescent molecules, including QDs.