An intelligent MRI contrast agent for tumor sensing.

An intelligent MRI contrast agent has been designed by taking advantage of a microenvironmental responsive polyion complex (PIC). The intelligent signal switching was achieved by controlling Gd ion-water interactions in the microenvironmental responsive PIC. MRI signal of the agent appeared only in tumor region, selectively, and did not appear in normal tissues, when the agent was administrated to a tumor-bearing mouse from the tail vein. The acidic microenvironment, such as lower extracellular fluid pH, higher expression of acidic proteins and polysaccharides, was suggested to cause the tumor-selective activation of the agent.