Mixed Micelles Composed of Peptides and Gadolinium Complexes as Tumor-Specific Contrast Agents in MRI: A SANS Study Gaetano Mangiapia, ² Antonella Accardo, § Fabrizio Lo Celso, ‡ Diego Tesauro, § Giancarlo Morelli, § Aurel Radulescu, | and Luigi Paduano* ,² Dipartimento di Chimica, UniVersita ` degli Studi di Napoli “Federico II”, Via Cinthia, 80126 Napoli, Italy, Dipartimento di Chimica Biologica, Centro InteruniVersitario per la Ricerca sui Peptidi BioattiVi (CIRPeB), UniVersita ` degli Studi di Napoli “Federico II”, Via Mezzocannone 6, 80134 Napoli, Italy, Dipartimento di Chimica Fisica, UniVersita ` degli Studi di Palermo, Viale delle Scienze, Parco d’Orleans II, 90128 Palermo, Italy, and Institute for Solid State Research, Research Center Ju ¨lich GmbH, Ju ¨lich D52425, Germany ReceiVed: April 7, 2004; In Final Form: July 21, 2004 A detailed structural investigation on mixed micelle aggregates as target-specific contrast agents for magnetic resonance imaging technique has been carried out by way of small angle neutron scattering measurements. These mixed micelles are formed by two new amphiphilic molecules formed by a bioactive peptide and a claw moiety. The first molecule, C 18 H 37 CONH(AdOO) x -G-CCK8 (C18LxCCK8, x ) 2, 5), contains an 18- carbon-atom alkylic chain bound to the C-terminal of the cholecystokinin octapeptide amide (CCK 26-33 or CCK8) and is able to bind to the overexpressed CCK receptor of some tumor cells. The second molecule, C 18 H 37 CONHLys(DTPAGlu)CONH 2 5- (C18DTPAGlu) or its gadolinium complex [C 18 H 37 CONHLys(DTPAGlu)- CONH 2 Gd] 2- , C18DTPAGlu(Gd), contains the same 18-carbon-atom alkylic chain bound, through a lysine residue, to the DTPAGlu chelating agent. Small angle neutron scattering measurements have been performed on ternary systems at different total concentrations and at various ratios of the two molecules. The effect of the concentration on the aggregation number as well as on the shape of the micelle has been investigated. Furthermore in order to optimize the exposure of the peptide on the micelle surface, C18LxCCK8 having the spacer L of different length has been used. The pure binary systems of the synthesized molecules are also presented. Introduction Magnetic resonance imaging (MRI) is a very promising noninvasive medical diagnostic procedure giving high-quality images of the inside of the human body. MRI is based on the same principles of nuclear magnetic resonance (NMR) and measures the characteristics of hydrogen nuclei of water modified by chemical environment. To enhance the quality of the images, a reporter compound (contrast agent) has to be accumulated in the area of interest. 1,2 The contrast activity is generally obtained by a paramagnetic atom that increases the relaxivity of the surrounding water molecules, i.e., produces an alteration of the water protons’ relaxation time that it easily detected through the MRI technique. This effect is enhanced if the paramagnetic atom is complexed in a large size molecule that has a slow rotation. Due to the very low sensitivity, MRI needs a relatively high concentration (∼10 -4 mol/L) of contrast agents such as paramagnetic Gd(III) complexes. To reach the required local concentration of the contrast agent, many nonspecific supramolecular systems have been developed such as liposomes 3 and other microparticulates, 4 micelles, 5 dendri- mers, 6 linear polymers, 7,8 proteins, 9 or peptides, 10 all of these derivatized with the metal complex of interest. Among these carriers, due to their easily controlled properties and good pharmacological characteristics, micelle aggregates are very promising contrast agents. 11,12 Currently the research in this field is devoted to develop new contrast agents with improved properties such as high contrast activity (high relaxivity) and high specificity toward target cells or organs. In fact, high relaxivity allows reducing the concentra- tion of the reporter compound in the area of interest, while the target specificity allows addressing the reporter compound only in a restricted area, such as membrane receptors overexpressed by cancer cells. Therefore, in both cases, the total amount of the contrast agent to be injected for diagnostic analysis could be reduced. With these aims we have recently synthesized new amphi- philic molecules containing a chelating agent, DTPAGlu, able to complex the Gd(III) ion or a bioactive peptide that shows high affinity for the cholecystokinin receptors that are overex- pressed by the cells in many tumors 13-15 (see Figure 1 for the structure of the molecules). These molecules have been as- sembled in supramolecular micellar aggregates that fulfill the above properties. In fact the supramolecular aggregation of the C18DTPAGlu(Gd) complexes in the mixed micelle gives rise to a slow rotation and, consequently, increases the proton relaxivity with respect to that shown by monomeric Gd(III)- chelate complexes, 12 while the bioactive peptide on the external * Corresponding author. Tel: +39 081 674229. Fax: +39 081 674090. E-mail: luigi.paduano@unina.it. ² Dipartimento di Chimica, Universita ` degli Studi di Napoli “Federico II”. § Dipartimento di Chimica Biologica, Universita ` degli Studi di Napoli “Federico II”. ‡ Universita ` degli Studi di Palermo. | Institute for Solid State Research. 17611 J. Phys. Chem. B 2004, 108, 17611-17617 10.1021/jp048466h CCC: $27.50 © 2004 American Chemical Society Published on Web 10/14/2004