Peptide derivatized lamellar aggregates as target-specific MRI contrast agents.

The relaxivity behavior and the structural characterization of new supramol. aggregates (bilayer structures and micelles) obtained by combining two different amphiphilic monomers are reported. One monomer, (C18)2DTPAGlu-Gd, contains a very stable gadolinium complex, and the other, DSPE-PEG2000-CCK8, contains the bioactive CCK8 peptide. Samples that contained only DSPE-PEG2000-CCK8, or up to 50% (C18)2DTPAGlu-Gd, aggregated as double-layer structures (lamellar aggregates) with a thickness of .apprx.80-100 .ANG., as evaluated by SANS measurement and Cryo-TEM imaging. A transition to micelle formation was obsd. when the amt. of (C18)2DTPAGlu-Gd in the aggregate was increased. These were rod-like micelles .apprx.40 .ANG. in radius and > 200 .ANG. in length. The proton relaxivities for both lamellar aggregates and rod-like micelles were the same (17.2 mM-1s-1), although the values were the results of different combinations of local and global contributions. The in vitro target selectivity of aggregates that contained the CCK-8 peptide was demonstrated by using nuclear medicine techniques.