A polymeric micelle MRI contrast agent with changeable relaxivity

Polymeric micelles were formed from cationic polymers (polyallylamine or protamine) and anionic block copolymers (poly(ethylene glycol)-b-poly(aspartic acid) derivative) that bound Gd ions providing high contrasts in Magnetic Resonance Imaging (MRI) by shortening the T1 longitudinal relaxation time of protons of water. The Gd-binding block copolymer alone showed high relaxivity (T1-shortening ability) values from 10 to 11 mol− 1 s− 1, while the polymeric micelles exhibited low relaxivity values from 2.1 to 3.6 mol− 1 s− 1. These findings point to the feasibility of a novel MRI contrast agent that selectively provides high contrasts at solid tumor sites owing to a dissociation of the micelle structures, while selective delivery to the tumor sites is achieved in the polymeric micelle form.