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

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 T 1 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.