Peptides and Gd complexes containing colloidal assemblies as tumor-specific contrast agents in MRI: physicochemical characterization.

The aggregation behavior of an amphiphilic supramol. system, with potential application as a tumor-specific magnetic resonance imaging contrast agent, has been studied in detail by dynamic light scattering, small-angle neutron scattering and cryotransmission electron microscopy. The system was constituted of mixed aggregates formed by an anionic unimer contg. the DTPAGlu, a chelating agent for the paramagnetic Gd3+ ion, and an uncharged unimer contg. the bioactive peptide CCK8, capable of directing the assembly toward tumor cells. Mixed aggregates formed by both unimers, and in the case of the DTPAGlu unimer with the chelating agent as free base or as Gd3+ complex, have been investigated. A no. of interesting features of the aggregation behavior were revealed: at physiol. pH, micelles and bilayer structures were present, whereas upon decreasing soln. pH or increasing ionic strength, the formation of bilayer structures was favored. On the basis of the above observations, the aggregating mechanism has been elucidated by considering the screening effect on intra- and interaggregate electrostatic repulsions.