Gd-DTPA-loaded polymer–metal complex micelles with high relaxivity for MR cancer imaging

Nanodevices for magnetic resonance imaging of cancer were self-assembled to core–shell micellar structures by metal complex formation of K2PtCl6 with diethylenetriaminepentaacetic acid gadolinium (III) dihydrogen (Gd-DTPA), a T1-contrast agent, and poly(ethylene glycol)-b-poly{N-[N′-(2-aminoethyl)-2-aminoethyl]aspartamide} (PEG-b-PAsp(DET)) copolymer in aqueous solution. Gd-DTPA-loaded polymeric micelles (Gd-DTPA/m) showed a hydrodynamic diameter of 45 nm and a core size of 22 nm. Confining Gd-DTPA inside the core of the micelles increased the relaxivity of Gd-DTPA more than 13 times (48 mm−1 s−1). In physiological conditions Gd-DTPA/m sustainedly released Gd-DTPA, while the Pt(IV) complexes remain bound to the polymer. Gd-DTPA/m extended the circulation time in plasma and augmented the tumor accumulation of Gd-DTPA leading to successful contrast enhancement of solid tumors. μ-Synchrotron radiation-X-ray fluorescence results confirmed that Gd-DTPA was delivered to the tumor site by the micelles. Our study provides a facile strategy for incorporating contrast agents, dyes and bioactive molecules into nanodevices for developing safe and efficient drug carriers for clinical application.