Towards advanced paramagnetic nanoassemblies of highly ordered interior nanostructures as potential MRI contrast agents

This paper reports on the synthesis, physicochemical characterisation and relaxation properties of a novel paramagnetic amphiphile, Gd–DTPA–Tris–trioleate (Gd–DTPA–Tris–TO), designed with the aim of obtaining paramagnetic nanoassemblies of highly ordered internal nanostructures made solely from the paramagnetic amphiphiles. The synthesis involved conjugating a Gd(III) chelated diethylenetriaminepentaacetic acid to three unsaturated acyl chains (oleates) via a tris(hydroxymethyl)aminomethane molecule. Bulk lyotropic mesophases of the amphiphile were found to possess hexagonal nanostructures in excess water as characterised by small angle X-ray scattering (SAXS). Upon dispersion of the bulk mesophases in an aqueous solution containing a steric stabiliser, stable nanoassemblies were obtained, which were confirmed to be liposomal when imaged by cryogenic-transmission electron microscopy (cryo-TEM) and analysed by synchrotron SAXS. Relaxation properties were determined by the analyses of the variable temperature 1H nuclear magnetic resonance dispersion (NMRD) profiles using the classical inner and outer sphere theories, and a ‘model-free’ Lipari and Szabo approach. The relaxivities of the paramagnetic nanoassemblies were found to be higher than for Magnevist® (Gd–DTPA), and comparable to previously reported nanoassemblies with paramagnetic amphiphiles. The Gd–DTPA–Tris–TO nanoassemblies were found to be slowly reorientating and their water exchange rate comparable to Omniscan™ (Gd–DTPA–bis(methylamide) or Gd–DTPA–BMA). The ability of these nanoassemblies to deliver large payloads of Gd(III) ions along with slow molecular reorientation and high relaxivity makes them suitable candidates to be explored as advanced MRI contrast agents.