Entrapment of Gadolinium-DTPA in Liposomes: Characterization of Vesicles by P-31 NMR Spectroscopy

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Abstract
The use of paramagnetic ion chelates to enhance contrast between pathologic and surrounding parenchyma is extensively documented in the magnetic resonance imaging (MRI) literature. Liposomes can be used to increase chelate concentration in the pathologic area, thereby enhancing the efficiency of paramagnetic compounds as contrast agents. Liposomes (50 + 20 nm) were prepared by sonicating a solution of dipalmitoylphosphatidylcholine and cholesterol containing 16.5 mM Gadolinium-DTPA (Gd-DTPA) in pharmaceutical formulation (Schering Laboratories, France) and 25 mM inorganic phosphate (Pi). The solutions were dialyzed against 0.9% NaCl before analysis by phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy. Spectra of liposomes displayed a sharp resonance ascribed to Pi and a broad signal arising from the phosphate groups of the phospholipid bilayer. The content of Gd-DTPA in liposomes was directly estimated, based on specific modifications of the longitudinal relaxation rate of intraliposomal Pi. Entrapment ratio was estimated by P-31 NMR spectroscopy and atomic absorption spectroscopy to represent 2.5% to 5% of the initial Gd-DTPA content in the solution. This work illustrates the usefulness of NMR spectroscopy in the characterization of liposomes to be used for MRI applications.