A phospholipid spin label used as a liposome-associated MRI contrast agent

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Abstract
Given current clinical use of phospholipid bilayer structures (liposomes/vesicles) as non-toxic drug delivery vehicles, we have addressed the possibility of employing the phospholipids themselves as MRI contrast agents. To this end we have synthesized phosphatidylcholine with a nitroxide spin label replacing one methyl residue of the choline headgroup. This material was mixed with natural phosphatidylcholine in mole ratios from 150 to 1:1 and used to prepare sonicated unilamellar vesicles in saline. Expected structural features of these vesicles were verified by freeze-fracture electron microscopy. Proton T1 values of saline were readily decreased to less than 0.3 s by such preparations, yielding a net relaxivity of 0.6 M−1 s−1. The approach seems to be a realistic way of firmly associating a contrast agent of minimal toxicity with ordinary liposomes/vesicles in a manner that is not subject to leakage. © 1987 Academic Press, Inc.