Hyphenated Methods for the Quantification of Free and Chelated Gadolinium Species in Nanoemulsion-based MRI Contrast Agent Formulations
Danielle Cleveland, Stephen E. Long, Lane C. Sander, Karen E. Murphy, Ryan J. Case, and Catherine Rimmer
Analytical Chemistry Division, NIST, 100 Bureau Drive, Gaithersburg, MD 20899
Nuclear magnetic resonance imaging (NMRI), referred to as magnetic resonance imaging (MRI) in the health-care community, is a prominent clinical diagnostic tool for in-vivo, noninvasive tissue characterization. The MRI technique is based on the spatial detection of 1H NMR signals of water protons following the application of an external magnetic field. Paramagnetic contrast agents, like the gadolinium ion, can be used to generate a signal enhancement to improve differentiation between adjacent tissues. Unfortunately, uncomplexed gadolinium is known to have toxic effects in-vivo, so gadolinium-based contrast agents generally consist of a gadolinium-ligand complex to improve acute tolerance. Several gadolinium chelates have been approved by the FDA and are commercially available. For example, the commercial formulation Magnevist consists of gadolinium chelated with diethylenetriamine pentaacetic acid (DTPA).
One existing challenge during the formulation of novel MRI contrast agents is the possibility of the presence of uncomplexed gadolinium in the final product. Therefore, it is crucial to develop methods that accurately quantify the amounts of complexed and uncomplexed gadolinium species in MRI contrast agent formulations. Ideally, these methods should be robust and use equipment routinely found in the clinical setting. This paper describes two methods for the simultaneous, quantitative determination of free and complexed gadolinium species in the presence of emulsifiers, surfactants, biomarkers, and oils in a proprietary nanoemulsion-based MRI contrast agent formulation. In this formulation, the chelator diethylenetriamine pentaacetic acid-phosphatidylethanolamine (DTPA-PE) was used as the chelating ligand for gadolinium. The first method coupled reversed-phase chromatography (RP-HPLC) with fluorescence (FL) detection. The chelating agent EDTA was added to the aqueous mobile phase to bind any free gadolinium and form the fluorescent Gd-EDTA species. Additionally, the output from the fluorescence detector was coupled to an inductively coupled plasma mass spectrometer for analysis by online isotope dilution (ID-ICP-MS). A second method coupled size exclusion chromatography (SEC) to online ID-ICP-MS to quantify free, bound, and total gadolinium. The results of the RP-HPLC-FL-ID-ICP-MS experiments were then compared to the results for SEC-ID-ICP-MS. Finally, the total gadolinium in the formulation was quantified by ID-ICP-MS.
Author: Danielle Cleveland
Mentor: Stephen E. Long
Division: Analytical Chemistry
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