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Peptides and Gd Complexes Containing Colloidal Assemblies as Tumor-Specific Contrast Agents in MRI: Physicochemical Characterization

Mauro Vaccaro ^*  , Antonella Accardo , Gerardino D'Errico ^* , Karin Schillén , Aurel Radulescu ^¶, Diego Tesauro , Giancarlo Morelli  and Luigi Paduano ^* 

^* Department of Chemistry, University of Naples "Federico II", Naples, Italy; CSGI (Consorzio per lo Sviluppo dei Sistemi a Grande Interfase), Florence, Italy; Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, Lund, Sweden; Department of Biological Sciences, CIRPeB University of Naples "Federico II", and the National Research Council Institute of Bioimaging and Biostructure (IBB CNR), Naples, Italy; and ^¶ Jülich Centre for Neutron Science (JCNS), Forschungszentrum Jülich GmbH, Jülich, Germany

Correspondence: Address reprint requests to Luigi Paduano, Dept. of Chemistry, University of Naples "Federico II", Via Cynthia, 80126 Naples, Italy. E-mail: luigi.paduano{at}unina.it.

The aggregation behavior of an amphiphilic supramolecular 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 containing the DTPAGlu, a chelating agent for the paramagnetic Gd³⁺ ion, and an uncharged unimer containing 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 number of interesting features of the aggregation behavior were revealed: at physiological pH, micelles and bilayer structures were present, whereas upon decreasing solution 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.