Improved paramagnetic chelate for molecular imaging with MRI

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Abstract

The relaxivity and transmetallation of two lipophilic paramagnetic chelates incorporated onto perfluorocarbon nanoparticles, i.e., gadolinium-methoxy-tetraazacyclododecane-tetraacetic acid phosphatidylethanolamine (Gd-MeO-DOTA-PE) and gadolinium-methoxy-tetraazacyclododecane-tetraacetic acid triglycine phosphatidylethanolamine (Gd-MeO-DOTA-triglycine-PE (Gd-MeO-DOTA-triglycine-PE)), were compared to a prototypic gadolinium-diethylene-triamine-pentaacetic acid bis-oleate (Gd-DTPA-BOA) paramagnetic formulation. Nanoparticles with MeO-DOTA-based chelates demonstrated higher relaxivity (40% higher for Gd-MeO-DOTA-PE and 55% higher for Gd-MeO-DOTA-triglycine-PE) and less transmetallation than the original Gd-DTPA-BOA-based agent.

Introduction

Molecular imaging is emerging as a sensitive and specific method for the detection and localization of the biochemical signatures of disease. Magnetic resonance imaging (MRI) offers several advantages over other clinical modalities for molecular imaging, including high resolution, non-invasiveness, high anatomical contrast and lack of ionizing radiation. The partial volume dilution effect, which is inherent in MRI, has often led to the failure of targeted contrast agents in vivo [1]. Initial attempts of targeted imaging with MRI focused on coupling gadolinium atoms directly to antibodies or proteins [2], [3], but these approaches delivered insufficient paramagnetic material to effectively decrease local relaxation times, and provided inadequate MR signal enhancement on T1-weighted images at typical clinical field strengths. Later, ligand-directed macromolecular constructs, such as liposomes [4], nanoparticles [5], [6], [7], dendrimers [8], polymers [9], were used to increase gadolinium ion payload at each binding site, which generated in some instances acceptable levels of targeted MRI contrast. The two principal factors for the development of successful targeted paramagnetic contrast agents are the number of gadolinium ions delivered to each binding site, i.e. the paramagnetic payload, and the relaxivity influence provided by each gadolinium [5].

We have previously reported and characterized a paramagnetic lipid-encapsulated perfluorocarbon nanoparticle contrast agent for the sensitive and specific detection and localization of fibrin [5], [10] and molecular signatures of angiogenesis [6], [7]. These prototype nanoparticles (250 nominal diameter) incorporate a lipophilic chelate, i.e., gadolinium-diethylene-triamine-pentaacetic acid bis-oleate (Gd-DTPA-BOA, metal oriented outward) into the surfactant layer. These relatively large paramagnetic constructs have high relaxivity per gadolinium atom (ion-based relaxivity) due to the slower tumbling rate of the particle in comparison with free gadolinium-diethylene-triamine-pentaacetic acid (Gd-DTPA) [11] and support high surface payloads of metal, typically in excess of 50,000 gadolinium ions per 250 nm particle, providing extremely high relaxivity per particle (particle-based relaxivity). Since biochemical epitopes of interest are often present in nano- or picomolar concentrations, particulate relaxivities around 1,000,000 s−1 mM−1 are required to achieve acceptable contrast-to-noise at typical clinical field strength (1.5 T) [12].

While Gd-DTPA-BOA nanoparticles have been used effectively to image thrombi [5] and angiogenesis [6], [7] in vivo, increases in particulate relaxivity will improve detection of nascent disease even at lower scanning resolution and will reduce the incidence of false-negative studies. In clinical practice, low-resolution detection will be required to interrogate wide regions of interest. Once detected in lower resolution scans, high-resolution molecular imaging can be pursued for diagnosis and quantification.

Unlike the blood-pool paramagnetic chelates, site-specific MR contrast agents circulate longer and accumulate at the target site over time, where they provide persistent signal for several hours. The requisite extended biological half-life of these agents necessitates stable gadolinium chelate complexes to minimize toxicity potential through transmetallation [13], [14].

In the present report, we compare the relaxivity and transmetallation potential of gadolinium-methoxy-tetraazacyclododecane-tetraacetic acid (Gd-MeO-DOTA)-based nanoparticles with our previously reported Gd-DTPA-BOA prototype agent.

Section snippets

Nanoparticle formulation

Nanoparticle contrast agents were produced with methods developed in our laboratory [5], [10]. Emulsions comprised 20% (v/v) of perfluorooctylbromide (PFOB; Minnesota Manufacturing and Mining), 2% (w/v) safflower oil, 2% (w/v) of a surfactant commixture, 1.7% (w/v) glycerin and water representing the balance. The surfactant commixture included 58 mol% lecithin (Avanti Polar Lipids), 10 mol% cholesterol (Sigma Chemical Co.), 2 mol% biotinylated dipalmitoyl-phosphatidylethanolamine (Avanti Polar

Results

All nanoparticle formulations were very similar with respect to their physical and chemical properties. The nominal diameters were very similar, ranging from 190 to 210 nm, with nearly identical size distributions. All formulations contained very similar concentrations of gadolinium, and therefore each formulation had a similar number of Gd3+-complexes per particle (49,000–50,000).

The ionic r1 relaxivity (Table 1) of the prototype Gd-DTPA-BOA nanoparticles was very high (21.3 s·mM−1) as reported

Discussion

We have utilized relaxivity and transmetallation measurements to evaluate two new paramagnetic nanoparticle formulations for MR molecular imaging. The Gd-MeO-DOTA-PE and Gd-MeO-DOTA-triglycine-PE nanoparticles were compared to our Gd-DTPA-BOA prototype formulation in terms of particle size, metal stability and relaxivity. In general, the increased relaxivity and decreased transmetallation characteristics of the new MeO-DOTA-PE-based nanoparticle formulations were favorable enhancements over the

Acknowledgements

We acknowledge grant support from the National Institutes of Health (HL-42950, HL-59865 and NO1-CO-07121), the American Heart Association, the Barnes-Jewish Hospital Research Foundation and Philips Medical Systems.

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