Enumeration of Liposomes by Multinuclear NMR and Photon Correlation Spectroscopy

Abstract

Diamagnetic dipalmitoylphosphatidylcholine (DPPC) liposomes dispersed in glucose solution as well as their paramagnetic analogs encapsulating a paramagnetic contrast agent used in magnetic resonance imaging (Gd-HPDO3A, ProHance ® ) were prepared and characterized. The vesicle diameter was assessed by photon correlation spectroscopy (PCS). 31 P NMR spectroscopy was used to measure the phospholipid content and to confirm the highly unilamellar character of the liposome membrane. For both types of liposome preparation, the internal water volume was evaluated below the phase transition temperature ( T m ) by natural abundance 17 O NMR spectroscopy in the presence of a shift reagent confined to the external compartment. For the paramagnetic vesicles, the internal water content was independently assessed by analysis of the biexponential decay of the proton transverse magnetization below T m . Knowing the unilamellarity of the vesicles ( 31 P NMR measurements), the number concentration of liposomes was assessed from the liposomal internal volume calculated from PCS data and the total internal water content obtained by 17 O NMR spectroscopy or 1 H relaxometry. The results obtained are in good agreement and validate these techniques as non invasive methods for the assessment of the number concentration of liposome in suspension.