Osmotically shrunken LIPOCEST agents: an innovative class of magnetic resonance imaging contrast media based on chemical exchange saturation transfer.

The peculiar properties of osmotically shrunken liposomes acting as magnetic resonance imaging-chemical exchange saturation transfer (MRI-CEST) contrast agents have been investigated. Attention has been primarily devoted to assessing the contribution arising from encapsulated and incorporated paramagnetic lanthanide(III)-based shift reagents in determining the chemical shift of the intraliposomal water protons, which is a relevant factor for generating the CEST contrast. It is demonstrated that a highly shifted resonance for the encapsulated water can be attained by increasing the percentage of the amphiphilic shift reagent incorporated in the liposome bilayer. It is also demonstrated that the shift contribution arising from the bulk magnetic susceptibility can be optimized through the modulation of the osmotic shrinkage. In terms of sensitivity, it is shown that the saturation transfer efficiency can be significantly improved by increasing the size of the vesicle, thus allowing a high number of exchangeable protons to be saturated. In addition, the role played by the intensity of the saturating radiofrequency field has also been highlighted.

Multimodal liposomes for SPECT/MR imaging as a tool for in situ relaxivity measurements.

One of the major challenges of MR imaging is the quantification of local concentrations of contrast agents. Cellular uptake strongly influences different parameters such as the water exchange rate and the pool of water protons, and results in alteration of the contrast agent's relaxivity, therefore making it difficult to determine contrast agent concentrations based on the MR signal only. Here, we propose a multimodal radiolabeled paramagnetic liposomal contrast agent that allows simultaneous imaging with SPECT and MRI. As SPECT-based quantification allows determination of the gadolinium concentration, the MRI signal can be deconvoluted to get an understanding of the cellular location of the contrast agent. The cell experiments indicated a reduction of the relaxivity from 2.7 ± 0.1 m m(-1) s(-1) to a net relaxivity of 1.7 ± 0.3 m m(-1) s(-1) upon cellular uptake for RGD targeted liposomes by means of the contrast agent concentration as determined by SPECT. This is not observed for nontargeted liposomes that serve as controls. We show that receptor targeted liposomes in comparison to nontargeted liposomes are taken up into cells faster and into subcellular structures of different sizes. We suggest that the presented multimodal contrast agent provides a functional readout of its response to the biological environment and is furthermore applicable in in vivo measurements. As this approach can be extended to several MRI-based contrast mechanisms, we foresee a broader use of multimodal SPECT/MRI nanoparticles to serve as in vivo sensors in biological or medical research.

The binding of CNA35 contrast agents to collagen fibrils.

CryoTEM demonstrates that a CNA35-bearing liposomal MRI contrast agent selectively binds to poorly assembled collagen type I as opposed to well-assembled collagen fibrils, whereas monomeric CNA35 binds to all forms of collagen. It is shown that upon conjugation to liposomes and micelles CNA35 loses its ability to dissociate ordered collagen fibrils and thereby to create its own binding sites.

Surface modification of PLGA nanospheres with Gd-DTPA and Gd-DOTA for high-relaxivity MRI contrast agents.

The preparation of particulate contrast agents for magnetic resonance imaging (MRI) based on biodegradable poly(D,L-lactide-co-glycolide) (PLGA) nanocarriers is reported. By spacer-aided covalent surface-grafting of the prominent chelating ligands diethylenetriaminepentaacetic acid (DTPA) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), respectively, up to 236 µg gadolinium per mg PLGA can be immobilized in a stable manner. Due to the localisation at the particle surface, water protons may effectively interact with the gadolinium chelates and the modified particles exhibit high proton relaxivities as confirmed by T1 relaxivities of up to 17.5 mm(-1)s(-1) (25 °C, 1.41 T) in case of Gd-DOTA-functionalized carriers and also supported by NMRD profiles. The obtained values compare favorably with marketed low-molecular weight contrast agents and thus suggest suitability for in vivo use.

Characterization and in vitro and in vivo testing of CB2-receptor- and NGAL-targeted paramagnetic micelles for molecular MRI of vulnerable atherosclerotic plaque.

PURPOSE: Atherosclerotic plaque macrophages express the peripheral cannabinoid receptor (CB2-R) and promote fibrous cap degradation by secretion of neutrophil gelatinase-associated lipocalin 2 (NGAL). In this study, we report the preparation, characterization, and in vitro and in vivo testing of double-labeled (MR and fluorescent) CB2-R- and NGAL-targeted micelles. PROCEDURES/RESULTS: Specific CB2-R agonists or antibodies directed to 24p3 (mouse homolog of NGAL) were incorporated into di-oleoyl-polyethylene glycol-phosphatidylethanolamine 1000 (DOPE-PEG1000) micelles or di-stearoyl-polyethylene glycol-phosphatidylethanolamine 2000 (DSPE-PEG2000) micelles. The hydrodynamic diameter, determined by dynamic light scattering, was 16.5 and 19.0 nm for CB2-R-targeted DOPE-PEG1000 and DSPE-PEG2000 micelles, respectively, and 23.0 nm for Ab-conjugated DSPE-PEG2000 micelles. In vitro and in vivo MRI and fluorescence microscopy showed specific binding of CB2-R-targeted and 24p3-targeted micelles to in vitro systems and to aortic plaque in apoE(-/-)/eNOS(-/-) mice, respectively. CONCLUSIONS: CB2-R- and NGAL-targeted micelles show promise as tools for in vivo characterization of vulnerable plaque.

A high relaxivity Gd(III)DOTA-DSPE-based liposomal contrast agent for magnetic resonance imaging.

The field of molecular imaging aims to visualize and quantify (patho)physiological processes at the cellular and molecular level. Sensitive and site-targeted contrast agents are employed to visualize molecular constituents of processes of interest. The principal aim of this study was to develop a magnetic resonance imaging (MRI) detectable liposome with high relaxivity and stability. To this end, Gd(III)DOTA-DSPE was synthesized and incorporated in a liposomal formulation. The resulting liposomes were extensively characterized in vitro in terms of contrast agent efficiency and structural properties. The liposomes were shown to have a high longitudinal relaxivity, which is crucial for the detection of low concentration molecular markers in molecular imaging studies. We also demonstrated that Gd(III)DOTA-DSPE exhibits no detectable transmetallation upon incubation with Zn(II). This is important as it significantly contributes to the biocompatibility of the contrast agent. The present liposome preparation will serve as versatile and well characterized platform for molecular imaging and targeted drug delivery studies.