Innovative magnetic resonance imaging diagnostic agents based on paramagnetic Gd(III) complexes

New opportunities for RGD-engineered metal nanoparticles in cancer

The advent of nanotechnology has opened new possibilities for bioimaging. Metal nanoparticles (such as gold, silver, iron, copper, etc.) hold tremendous potential and offer enormous opportunities for imaging and diagnostics due to their broad optical characteristics, ease of manufacturing technique, and simple surface modification. The arginine-glycine-aspartate (RGD) peptide is a three-amino acid sequence that seems to have a considerably greater ability to adhere to integrin adhesion molecules that exclusively express on tumour cells. RGD peptides act as the efficient tailoring ligand with a variety of benefits including non-toxicity, greater precision, rapid clearance, etc. This review focuses on the possibility of non-invasive cancer imaging using metal nanoparticles with RGD assistance.

Complementarity of molecular and elemental mass spectrometric imaging of Gadovist™ in mouse tissues

Drug biodistribution analyses can be considered a key issue in pharmaceutical discovery and development. Here, mass spectrometric imaging can be employed as a powerful tool to investigate distributions of drug compounds in biologically and medically relevant tissue sections. Both matrix-assisted laser desorption ionization-mass spectrometric imaging as molecular method and laser ablation inductively coupled plasma-mass spectrometric imaging as elemental detection method were applied to determine drug distributions in tissue thin sections. Several mouse organs including the heart, kidney, liver, and brain were analyzed with regard to distribution of Gadovist^™, a gadolinium-based contrast agent already approved for clinical investigation. This work demonstrated the successful detection and localization of Gadovist^™ in several organs. Furthermore, the results gave evidence that gadolinium-based contrast agents in general can be well analyzed by mass spectrometric imaging methods. In conclusion, the combined application of molecular and elemental mass spectrometry could complement each other and thus confirm analytical results or provide additional information.

Specific lipase-responsive polymer-coated gadolinium nanoparticles for MR imaging of early acute pancreatitis

Currently, available methods for diagnosis of acute pancreatitis (AP) are mainly dependent on serum enzyme analysis and imaging techniques that are too low in sensitivity and specificity to accurately and promptly diagnose AP. The lack of early diagnostic tools highlights the need to search for a highly effective and specific diagnostic method. In this study, we synthesized a conditionally activated, gadolinium-containing, nanoparticle-based MRI nanoprobe as a diagnostic tool for the early identification of AP. Gadolinium diethylenetriaminepentaacetic fatty acid (Gd-DTPA-FA) nanoparticles were synthesized by conjugation of DTPA-FA ligand and gadolinium acetate. Gd-DTPA-FA exhibited low cytotoxicity and excellent biocompatibility when characterized in vitro and in vivo studies. L-arginine induced a gradual increase in the intensity of the T1-weighted MRI signal from 1 h to 36 h in AP rat models. The increase in signal intensity was most significant at 1 h, 6 h and 12 h. These results suggest that the Gd-DTPA-FA as an MRI contrast agent is highly efficient and specific to detect early AP.

MRI-guided neutron capture therapy by use of a dual gadolinium/boron agent targeted at tumour cells through upregulated low-density lipoprotein transporters

The upregulation of low-density lipoprotein (LDL) transporters in tumour cells has been exploited to deliver a sufficient amount of gadolinium/boron/ligand (Gd/B/L) probes for neutron capture therapy, a binary chemio-radiotherapy for cancer treatment. The Gd/B/L probe consists of a carborane unit (ten B atoms) bearing an aliphatic chain on one side (to bind LDL particles), and a Gd(III)/1,4,7,10-tetraazacyclododecane monoamide complex on the other (for detection by magnetic resonance imaging (MRI)). Up to 190 Gd/B/L probes were loaded per LDL particle. The uptake from tumour cells was initially assessed on cell cultures of human hepatoma (HepG2), murine melanoma (B16), and human glioblastoma (U87). The MRI assessment of the amount of Gd/B/L taken up by tumour cells was validated by inductively coupled plasma-mass-spectrometric measurements of the Gd and B content. Measurements were undertaken in vivo on mice bearing tumours in which B16 tumour cells were inoculated at the base of the neck. From the acquisition of magnetic resonance images, it was established that after 4-6 hours from the administration of the Gd/B/L-LDL particles (0.1 and 1 mmol kg(-1) of Gd and (10)B, respectively) the amount of boron taken up in the tumour region is above the threshold required for successful NCT treatment. After neutron irradiation, tumour growth was followed for 20 days by MRI. The group of treated mice showed markedly lower tumour growth with respect to the control group.

RGD-targeted paramagnetic liposomes for early detection of tumor: in vitro and in vivo studies

Magnetic resonance molecular imaging has emerged as a potential approach for tumor diagnosis in the last few decades. This approach consists of the delivery of MR contrast agents to the tumor by specific targeted carriers. For this purpose, a lipopeptide was constructed by using a cyclic RGD peptide headgroup coupled to palmitic acid anchors via a KGG tripeptide spacer. Targeted paramagnetic liposomes were then prepared by the incorporation of RGD-coupled-lipopeptides into lipid bilayers for specific bounding to tumor. In vitro, study demonstrated that RGD-targeted liposomes exhibited a better binding affinity to targeted cells than non-targeted liposomes. MR imaging of mice bearing A549 tumors with the RGD-targeted paramagnetic liposomes also resulted in a greater signal enhancement of tumor compared to non-targeted liposomes and pure contrast agents groups. In addition, biodistribution study also showed specific tumor targeting of RGD-targeted paramagnetic liposomes in vivo. Therefore, RGD-targeted paramagnetic liposomes prepared in the present study may be a more promising method for early tumor diagnosis.

Intracellular bimodal nanoparticles based on quantum dots for high-field MRI at 21.1 T

Multimodal, biocompatible contrast agents for high magnetic field applications represent a new class of nanomaterials with significant potential for tracking of fluorescence and MR in vitro and vivo. Optimized for high-field MR applications-including biomedical imaging at 21.1 T, the highest magnetic field available for MRI-these nanoparticles capitalize on the improved performance of chelated Dy(3+) with increasing magnetic field coupled to a noncytotoxic Indium Phosphide/Zinc Sulfide (InP/ZnS) quantum dot that provides fluorescence detection, MR responsiveness, and payload delivery. By surface modifying the quantum dot with a cell-penetrating peptide sequence coupled to an MR contrast agent, the bimodal nanomaterial functions as a self-transfecting high-field MR/optical contrast agent for nonspecific intracellular labeling. Fluorescent images confirm sequestration in perinuclear vesicles of labeled cells, with no apparent cytotoxicity. These techniques can be extended to impart cell selectivity or act as a delivery vehicle for genetic or pharmaceutical interventions.

Colloidal amphiphile self-assembly particles composed of gadolinium oleate and myverol: evaluation as contrast agents for magnetic resonance imaging

Gadolinium oleate has been added at various concentrations to a Myverol inverse bicontinuous cubic phase forming system, and the potential of these systems as magnetic resonance imaging (MRI) contrast agents has been investigated. Differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and cryo-transmission electron microscopy (cryo-TEM) measurements on the Gd oleate/Myverol systems indicate that Gd oleate is at least partially incorporated within the cubic phase of Myverol. However, at Gd oleate concentrations greater than 1 wt %, partial phase separation of the system may occur with the formation of a Gd-oleate-rich lamellar phase as well as the cubic phase. Bulk Gd oleate/Myverol mixtures can be dispersed into stable colloidal dispersions. SAXS and cryo-TEM measurements on these dispersions indicate that the presence of Gd oleate in the Myverol system prevents the formation of cubosomes from the bulk cubic phase. Instead, the dispersion consists of putative Gd-oleate-rich nonswelling lamellar nanoparticles as well as colloidal particles lacking ordered internal structure. In vitro studies on these dispersions demonstrated that the relaxivity of select Gd oleate/Myverol systems is much higher than that of pure Gd oleate, exemplifying the promise of this system type for magnetic resonance imaging. The highest water proton relaxivities (r(1) = 34.2 mM(-1) s(-1) and r(2) = 27.3 mM(-1) s(-1) at 20 MHz and room temperature) were obtained at a Gd oleate loading concentration of 1 wt %, with a subsequent decrease in relaxivity with increasing Gd oleate concentration. These maximum relaxivities compare favorably with the relaxivities for the commercial contrast agent, Magnevist (r(1) = 4.91 mM(-1) s(-1) and r(2) = 6.26 mM(-1) s(-1) at 20 MHz and room temperature).

Molecular and magnetic resonance imaging: The value of immunoliposomes

Molecular imaging has the potential to transform the field of diagnostic imaging through enabling far more detailed investigation and characterisation of disease processes than is currently possible. Magnetic resonance imaging (MRI) is capable of three-dimensional non-invasive imaging of opaque tissues at near cellular resolution. Among the imaging techniques available today, MRI has, perhaps, the greatest potential to exploit the possibilities that molecular imaging presents. Nanoparticles are the focus of intense research, due to a wide variety of potential applications in the biomedical, optical, and electronic fields. In this article we examine the progress made in the development of nanoparticles as targeted contrast agents for molecular magnetic resonance imaging. In particular, we will examine the potential of antibody-targeted liposomes (immunoliposomes) as vehicles for delivering MRI contrast agents to cellular biomarkers, thus enabling visualisation of structures and processes at the molecular level. We will address some of the challenges that must be faced by researchers in this field before the progress made in the laboratory can be translated into improved clinical diagnostics and therapeutics.

Gadolinium-Loaded Polychelating Polymer-Containing Cancer Cell-Specific Immunoliposomes

Liposome loading with Gd via the membrane-incorporated polychelating amphiphilic polymers (PAPs) significantly increases the Gd content and relaxivity (T1 parameter) of PEGylated liposomes, which can be used as contrast agents for magnetic resonance imaging (MRI). Here, we demonstrate that such Gd-containing liposomes can be additionally modified with the monoclonal anticancer antibody 2C5 (mAb 2C5) possessing the nucleosome(NS)-restricted specificity via the PEG spacer. Liposome-bound antibody preserves its specific activity (ELISA) and such Gd-loaded PEGylated 2C5-immunoliposomes specifically recognize various cancer cells in vitro and target an increased amount of Gd to their surface compared to antibody-free Gd-liposomes or Gd-liposomes modified with tumor nonspecific antibody. Gd-loaded cancer cell-targeted immunoliposomes may represent promising agents for enhanced tumor MRI.

Detection and quantification of magnetically labeled cells by cellular MRI

Labeling cells with superparamagnetic iron oxide (SPIO) nanoparticles, paramagnetic contrast agent (gadolinium) or perfluorocarbons allows for the possibility of tracking single or clusters of labeled cells within target tissues following either direct implantation or intravenous injection. This review summarizes the practical issues regarding detection and quantification of magnetically labeled cells with various MRI contrast agents with a focus on SPIO nanoparticles.

In vivo molecular imaging of vascular stress

Noninvasive in vivo imaging is an emerging specialty in experimental radiology aiming at developing hardware and appropriate contrast agents to visualize the molecular basis and pathophysiological processes of many pathological conditions, including atherosclerosis. The list of potentially useful tracers and targets for in vivo molecular imaging in the cascade of early atherosclerotic events has been narrowed down to some very promising endothelial factors, i.e., cell adhesion molecules, macrophages, apoptosis, lipoproteins, heat shock proteins, and others. In this review, we will update on the progress of recent developments in the field of noninvasive molecular imaging in experimental atherosclerosis.

Magnetic resonance molecular imaging contrast agents and their application in atherosclerosis

Heart disease is the most prevalent cause of mortality in the Western world and is most frequently caused by rupture of lesions in the arteries, which are formed by atherosclerosis. Atherosclerosis is a progressive disease, and therefore, there is a strong motivation to be able to image the stages of this disease in vivo. The pathogenesis of this disease is now well established, and a number of markers such as macrophages, vascular adhesion molecules, fibrin, and the alphanubeta3-integrin have been identified that are of particular interest for imaging. Furthermore, the differentiation between the stable and unstable plaque with imaging is a central goal of the field. Contrast can be generated in magnetic resonance imaging through the application of several types of agents such as T1, T2, chemical exchange saturation transfer or 19F-based imaging agents. Subsequent to the discussion of the above topics, we will describe some examples of molecular imaging agents that successfully detect specific markers in atherosclerotic plaques that are of interest in several stages of this disease.

Magnetic and fluorescent nanoparticles for multimodality imaging

The development of nanoparticulate contrast agents is providing an increasing contribution to the field of diagnostic and molecular imaging. Such agents provide several advantages over traditional compounds. First, they may contain a high payload of the contrast-generating material, which greatly improves their detectability. Second, multiple properties may be easily integrated within one nanoparticle to allow its detection with several imaging techniques or to include therapeutic qualities. Finally, the surface of such nanoparticles may be modified to improve circulation half-lives or to attach targeting groups. Magnetic resonance imaging and optical techniques are highly complementary imaging methods. Combining these techniques would therefore have significant advantages and may be realized through the use of nanoparticulate contrast agents. This review gives a survey of the different types of fluorescent and magnetic nanoparticles that have been employed for both magnetic resonance and optical imaging studies.

Evaluation of Gd(III)DTPA-terminated poly(propylene imine) dendrimers as contrast agents for MR imaging

Different generations of Gd(III)DTPA-terminated poly(propylene imine) dendrimers {G1 [n = 4 Gd(III) ions per molecule], G3 (n = 16) and G5 (n = 64)} and reference Gd(III)DTPA complex [G0 (n = 1)] were characterized in terms of (i) longitudinal (r1) and transverse (r2) relaxivities in mouse blood plasma, (ii) concentration detection limits in vitro and (iii) in vivo contrast-enhanced MR imaging (CE-MRI) in mice at 1.5 T. Serial and dynamic CE-MRI were performed to monitor the distribution of MRI contrast agent in the heart, arteries, renal system, liver, spleen, bladder and tumor periphery. The relaxivities increased non-linearly with molecular weight (for G0 ionic r1 = 8.1 mM(-1) s(-1) and ionic r2 = 8.6 mM(-1) s(-1) to G5 19.3 and 25.0, respectively). The minimal detectable dendrimer concentration was more than two orders of magnitude lower for G5 (8.1 x 10(-8) M) than for G0 (3.1 x 10(-5) M). Sub-millimeter-sized blood vessels were well visualized with serial CE-MRI with each contrast agent. Dynamic CE-MRI showed timely renal clearance for all contrast agents, but a stronger and a prolonged blood signal enhancement for the higher generations of the dendritic contrast agent. Moreover, G0 and G1 showed a rapid tumor wash-in and wash-out, whereas G3 and G5 displayed a more gradual and prolonged tumor wash-in. In conclusion, both G0 and dendritic contrast agents G1, G3 and G5 are well suited for non-tissue-specific MRI of sub-millimeter-sized blood vessels and evaluating tumor microcirculatory characteristics in mice. Higher generations of dendritic contrast agents display lower concentration detection limits, which suggests their future use for molecular imaging.

Quantum dots with a paramagnetic coating as a bimodal molecular imaging probe

MRI detectable and targeted quantum dots were developed. To that aim, quantum dots were coated with paramagnetic and pegylated lipids, which resulted in a relaxivity, r(1), of nearly 2000 mM(-1)s(-1) per quantum dot. The quantum dots were functionalized by covalently linking alphavbeta3-specific RGD peptides, and the specificity was assessed and confirmed on cultured endothelial cells. The bimodal character, the high relaxivity, and the specificity of this nanoparticulate probe make it an excellent contrast agent for molecular imaging purposes.

Magnetic Resonance Imaging Contrast Agents in the Study of Development

The use of magnetic resonance imaging (MRI) as a research tool in the study of development has increased in recent years due in part to improvements in spatial resolution, new imaging agents, and increased availability of MRI scanners. This chapter describes how contrast agent-enhanced MRI can be used to study development. In addition, we highlight some novel applications of contrast agent-enhanced MRI in biology that may be useful as tools for the study of development.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.