Marc Port

Magnetic iron oxide nanoparticles: synthesis, stabilization, vectorization, physicochemical characterizations, and biological applications.

1. Introduction 20642. Synthesis of Magnetic Nanoparticles 20662.1. Classical Synthesis by Coprecipitation 20662.2. Reactions in Constrained Environments 20682.3. Hydrothermal and High-TemperatureReactions20692.4. Sol-Gel Reactions 20702.5. Polyol Methods 20712.6. Flow Injection Syntheses 20712.7. Electrochemical Methods 20712.8. Aerosol/Vapor Methods 20712.9. Sonolysis 20723. Stabilization of Magnetic Particles 20723.1. Monomeric Stabilizers 20723.1.1. Carboxylates 20733.1.2. Phosphates 20733.2. Inorganic Materials 20733.2.1. Silica 20733.2.2. Gold 20743.3. Polymer Stabilizers 20743.3.1. Dextran 20743.3.2. Polyethylene Glycol (PEG) 20753.3.3. Polyvinyl Alcohol (PVA) 20753.3.4. Alginate 20753.3.5. Chitosan 20753.3.6. Other Polymers 20753.4. Other Strategies for Stabilization 20764. Methods of Vectorization of the Particles 20765. Structural and Physicochemical Characterization 20785.1. Size, Polydispersity, Shape, and SurfaceCharacterization20795.2. Structure of Ferro- or FerrimagneticNanoparticles20805.2.1. Ferro- and Ferrimagnetic Nanoparticles 20805.3. Use of Nanoparticles as Contrast Agents forMRI20825.3.1. High Anisotropy Model 20845.3.2. Small Crystal and Low Anisotropy EnergyLimit20855.3.3. Practical Interests of Magnetic NuclearRelaxation for the Characterization ofSuperparamagnetic Colloid20855.3.4. Relaxation of Agglomerated Systems 20856. Applications 20866.1. MRI: Cellular Labeling, Molecular Imaging(Inflammation, Apoptose, etc.)20866.2.

Clinical and biological consequences of transmetallation induced by contrast agents for magnetic resonance imaging: a review

Gadolinium‐based contrast agents (CAs) are widely used to enhance the contrast of images in magnetic resonance imaging procedures. Two categories of gadolinium chelates exist: the macrocyclic molecules where Gd3+ is caged in the pre‐organized cavity of the ligand and the linear molecules. Gadolinium chelates differ in their thermodynamic stability constants and in their kinetic stability. In general, macrocyclic chelates such as Gd‐DOTA or Gd‐HP‐DO3A are more stable than linear molecules. Even among linear agents, differences can be found. There is increasing evidence that transmetallation can be found in vivo, in the case of certain CAs (especially linear chelates), with body cations such as zinc, calcium or iron. Furthermore, analytical interference with colorimetric determination of calcium has been clinically evidenced with two linear chelates, Gd‐DTPA‐BMA and Gd‐DTPA‐BMEA. Clinical cases of spurious hypocalcaemia have been reported with these molecules. Such interference with some colorimetric assays for calcium is clinically relevant in that it can lead to unnecessary and potentially harmful treatment for hypocalcaemia.

Evaluation of Matrix Metalloproteinases in Atherosclerosis Using a Novel Noninvasive Imaging Approach

Objective—Despite great advances in our knowledge, atherosclerosis continues to kill more people than any other disease in the Western world. This is because our means of identifying truly vulnerable patients is limited. Prediction of atherosclerotic plaque rupture may be addressed by MRI of activated matrix metalloproteinases (MMPs), a family of enzymes that have been implicated in the vulnerability of plaques prone to rupture. This study evaluated the ability of the novel gadolinium-based MRI contrast agent P947 to target MMPs in atherosclerotic plaques. Methods and Results—The affinity of P947 toward activated MMPs was demonstrated in vitro. The affinity and specificity of P947 toward matrix metalloproteinase (MMP)-rich plaques was evaluated both in vivo using ApoE−/− mice and ex vivo in hyperlipidemic rabbits. Gadolinium content quantification and MRI showed a preferential accumulation of P947 in atherosclerotic lesions compared with the nontargeted reference compound, Gd-DOTA. The ex vivo assay on rabbit plaques revealed a higher uptake of P947. Moreover, using human carotid artery endarterectomy specimens, P947 facilitated discrimination between histologically defined MMP-rich and MMP-poor plaques. An in vivo MRI investigation in mice revealed that P947 greatly improved the ability to visualize and delineate atherosclerotic plaques. Conclusions—P947 may be a useful tool for the detection and characterization of the MMP-rich atherosclerotic plaques.

Effect of Nanoparticle and Aggregate Size on the Relaxometric Properties of MR Contrast Agents Based on High Quality Magnetite Nanoparticles

Colloidal dispersions of monodispersed and high-crystalline magnetite nanoparticles have been used to establish a relationship between magnetic properties and magnetic resonance (MR) relaxometric parameters in vitro. Magnetite nanoparticles with diameters between 4 and 14 nm were synthesized by thermal decomposition of Fe(acac)3 in different organic solvents and transformed to hydrophilic by changing oleic acid for dimercaptosuccinic acid (DMSA). A final treatment in alkaline water was critical to make the suspension stable at pH 7 with xi-potential values of -45 mV and hydrodynamic sizes as low as 50 nm. Samples showed superparamagnetic behavior at room temperature, which is an important parameter for biomedical applications. Susceptibility increased with both particle and aggregate size, and for particles larger than 9 nm, the aggregate size was the key factor controlling the susceptibility. Relaxivity values followed the same trend as the suspension susceptibilities, indicating that the aggregate size is an important factor above a certain particle size governing the proton relaxation times. The highest relaxivity value, r2=317 s(-1) mM(-1), much higher than those for commercial contrast agents with similar hydrodynamic size, was obtained for a suspension consisting of 9 nm particles and 70 nm of hydrodynamic size, and it was assigned to the higher particle crystallinity in comparison to particles prepared by coprecipitation. Therefore, it can be concluded that in addition to the sample crystallinity, both particle size and aggregate size should be considered in order to explain the magnetic and relaxivity values of a suspension.

P792: a rapid clearance blood pool agent for magnetic resonance imaging: preliminary results

An original MRI contrast agent, called P792, is described. P792 is a gadolinium macrocyclic compound based on a Gd-DOTA structure substituted by hydrophilic arms. The chemical structure of P792 has been optimized in order to provide (1) a high r1 relaxivity in the clinical field for MRI: 29 mM−1 x s−1 at 60 MHz. (2) a high biocompatibility profile and (3) a high molecular volume: the apparent hydrodynamic volume of P792 is 125 times greater than that of Gd-DOTA. As a result of this high molecular volume, P792 presents an unusual pharmacokinetic profile, as it is a Rapid Clearance Blood Pool Agent (RCBPA) characterized by limited diffusion across the normal endothelium. The original pharmacokinetic properties of this RCBPA are expected to be well suited to MR coronary angiography, angiography, perfusion imaging (stress and rest), and permeability imaging (detection of ischemia and tumor grading). Further experimental imaging studies are ongoing to define the clinical value of this compound.

Role of thermodynamic and kinetic parameters in gadolinium chelate stability.

In recent years there has been a renewed interest in the physicochemical properties of gadolinium chelates (GC). The aim of this review is to discuss the physicochemical properties of marketed GC with regard to possible biological consequences. GC can be classified according to three key molecular features: 1) the nature of the chelating moiety: either macrocyclic molecules in which Gd3+ is caged in the preorganized cavity of the ligand, or linear, open‐chain molecules; 2) ionicity: the ionicity of the molecule varies from neutral to tri‐anionic agents; and 3) the presence or absence of an aromatic lipophilic moiety, which has a profound impact on the biodistribution of the GC. These parameters can also explain why GC differ considerably with regard to their thermodynamic stability constants and kinetic stability, as demonstrated by numerous studies. The concept of thermodynamic and kinetic stability is critically discussed, as it remains somewhat controversial, especially in predicting the amount of free gadolinium that may result from decomplexation of chelates in physiologic or pathologic situations. This review examines the possibility that the high kinetic stability provided by the macrocyclic structure combined with a high thermodynamic stability (reinforced by ionicity for macrocyclic chelates) can minimize the amount of free Gd3+ released in the body. J. Magn. Reson. Imaging 2009;30:1249–1258.

Peptidic Targeting of Phosphatidylserine for the MRI Detection of Apoptosis in Atherosclerotic Plaques

Molecular and cellular imaging of atherosclerosis has garnered more interest at the beginning of the 21st century, with aims to image in vivo biological properties of plaque lesions. Apoptosis seems an attractive target for the diagnosis of vulnerable atherosclerotic plaques prone to a thrombotic event. The aim of the present work was to screen for apoptosis peptide binders by phage display with the final purpose to detect apoptotic cells in atherosclerotic plaques by magnetic resonance imaging (MRI). A phosphatidylserine-specific peptide identified by phage display was thus used to design an MRI contrast agent (CA), which was evaluated as a potential in vivo reporter of apoptotic cells. A library of linear 6-mer random peptides was screened in vitro against immobilized phosphatidylserine. Phage DNA was isolated and sequenced, and the affinity of peptides for phosphatidylserine was evaluated by enzyme-linked immunosorbent assay. The phosphatidylserine-specific peptide and its scrambled homologue were attached to a linker and conjugated to DTPA-isothiocyanate. The products were purified by dialysis and by column chromatography and complexed with gadolinium chloride. After their evaluation using apoptotic cells and a mouse model of liver apoptosis, the phosphatidylserine-targeted CA was used to image atherosclerotic lesions on ApoE(-/-) transgenic mice. Apoptotic cells were detected on liver and aorta specimens by the immunostaining of phosphatidylserine and of active caspase-3. Sequencing of the phage genome highlighted nine different peptides. Their alignment with amino acid sequences of relevant proteins revealed a frequent homology with Ca2+ channels, reminiscent of the function of annexins. Alignment with molecules involved in apoptosis provides a direct correlation between peptide selection and utility. The in vivo MRI studies performed at 4.7 T provide proof of concept that apoptosis-related pathologies could be diagnosed by MRI with a low molecular weight paramagnetic agent. The new CA could have real potential in the diagnosis and therapy monitoring of atherosclerotic disease and of other apoptosis-associated pathologies, such as cancer, ischemia, chronic inflammation, autoimmune disorders, transplant rejection, neurodegenerative disorders, and diabetes mellitus. The phage display-derived peptide could also play a potential therapeutic role through anticoagulant activity by mimicking the role of annexin V, the endogenous ligand of phosphatidylserine.

Comparison of different types of blood pool agents (P792, MS325, USPIO) in a rabbit MR angiography-like protocol.

Rationale and objectivesThe objective of this study is to determine the influence of the pharmacokinetic behaviors of different classes of blood pool agents (BPA) on a rabbit experimental model that mimics a magnetic resonance angiographic protocol. BPA were as follows: P792, a macromolecular agent (RCBPA), USPIO, an ultrasmall superparamagnetic iron oxide particle agent (SCBPA), and MS-325, a small gadolinium chelate that expresses intravascular behavior by reversible albumin binding. MethodsThe 2 main phases of early distribution following contrast agent injection, that is, the bolus phase and the steady-state phase, are investigated by measuring Gd or Fe blood concentrations in the first 5 minutes postinjection. T1 relaxation times and r1 relaxivity were calculated at each time point of blood sampling. Furthermore, in the case of MS-325, the concentrations of the free and bound forms were calculated, according to the measured concentrations and the apparent r1 relaxivities. ResultsInjected under similar conditions, the 3 BPA have, during the bolus phase, a comparable profile to Gd-DOTA. Signal enhancement was maximum during this short bolus phase, as were the T1 relaxation times under 30 ms for all agents.At 1 minute postinjection, P792 (r1 = 39 seconds−1.mmol/L−1, 20 MHz) demonstrated the same pharmacokinetic behavior as USPIO (r1 = 33 seconds−1.mmol/L−1, 20 MHz): C1 minute/C0 values were 91 ± 6% and 92 ± 12%, respectively.Immediately after the injection at clinical dose, 74% of MS-325 was in free form, resulting in an apparent r1 relaxivity of only 13 seconds−1.mmol/L−1 (20 MHz); 1 minute postinjection, the C1 minute/C0 value of 61 ± 4% was the lowest as compared with P792 and USPIO and the bound form represented 75% of the MS-325 molecules. ConclusionsThe BPA P792 and USPIO have favorable properties that result from their intravascular retention and their lack of extravasation, allowing optimal contrast between the vessel and the adjacent tissue for several minutes postinjection. Combining a rapid body clearance and a marked T1 effect, P792 presents optimal blood pool characteristics for angiographic applications. During the bolus phase, MS-325 is mainly in free form, which presents the disadvantage of increasing the tissue signal background, due to extravasation of the free form.

Hydrogels incorporating GdDOTA: towards highly efficient dual T1/T2 MRI contrast agents.

Do not tumble dry: Gadolinium-DOTA encapsulated into polysaccharide nanoparticles (GdDOTA NPs) exhibited high relaxivity (r(1) =101.7 s(-1) mM(-1) per Gd(3+) ion at 37 °C and 20 MHz). This high relaxation rate is due to efficient Gd loading, reduced tumbling of the Gd complex, and the hydrogel nature of the nanoparticles. The efficacy of the nanoparticles as a T(1)/T(2) dual-mode contrast agent was studied in C6 cells.

Physicochemical and biological evaluation of P792, a rapid-clearance blood-pool agent for magnetic resonance imaging.

RATIONALE AND OBJECTIVES To summarize the physicochemical characterization, pharmacokinetic behavior, and biological evaluation of P792, a new monogadolinated MRI blood-pool agent. METHODS The molecular modeling of P792 was described. The r1 relaxivity properties of P792 were measured in water and 4% human serum albumin at different magnetic fields (20, 40, 60 MHz). The stability of the gadolinium complex was assessed. The pharmacokinetic and biodistribution profiles were studied in rabbits. Renal tolerance in dehydrated rats undergoing selective intrarenal injection was evaluated. Hemodynamic safety in rats and in vitro histamine and leukotriene B4 release were also tested. RESULTS The mean diameter of P792 is 50.5 A and the r1 relaxivity of this monogadolinium contrast agent is 29 L x mmol(-1) x s(-1) at 60 MHz. The stability of the gadolinium complex in transmetallation is excellent. The pharmacokinetic and biodistribution profiles are consistent with that of a rapid-clearance blood-pool agent: P792 is mainly excreted by glomerular filtration, and its diffusion across normal endothelium is limited. Renal and hemodynamic safety is comparable to that of the nonspecific agent gadolinium-tetraazacyclododecane tetraacetic acid. No histamine or leukotriene B4 release was found in RBL-2H3 isolated mastocytes. CONCLUSIONS The relaxivity of P792 at clinical field is very high for a monogadolinium complex without protein binding. The pharmacokinetic and biodistribution profiles are consistent with those of a rapid-clearance blood-pool agent. Its initial safety profile is satisfactory. Experimental and clinical studies are underway to confirm the potential of P792 in MRI.