Supramolecular Aggregates of Amphiphilic Gadolinium Complexes as Blood Pool MRI/MRA Contrast Agents: Physicochemical Characterization.

The authors present the development of a new potential blood pool contrast agent for magnetic resonance imaging applications (MRA/MRI) based on gadolinium complexes contg. amphiphilic supramol. aggregates. A novel amphiphilic unimer, contg. the DTPAGlu chelating agent covalently bound to two C18 alkylic chains, was synthesized. DTPAGlu is a known chelating agent for a wide no. of ions such as the paramagnetic metal ion Gd3+ used as contrast agent in MRA/MRI. The wide aggregation behavior of this surfactant, as free base or as gadolinium complex, was studied and compared by dynamic light scattering, small-angle neutron scattering and cryogenic TEM techniques. Near neutral pH in both cases, the dominant aggregates are micelles. The high neg. actual charge of the surfactant headgroup causes a strong headgroups repulsion, promoting the formation of large and high curvature aggregates. By decreasing pH and less markedly increasing the ionic strength, the authors observe a micelle-to-vesicle transition driven by a decreased electrostatic repulsion. A straightforward switch between different aggregation states can be particularly useful in the development of pH-responsive MRA/MRI contrast agents.