Gadolinium-Encapsulating Iron Oxide Nanoprobe as Activatable NMR/MRI Contrast Agent

Abstract

Herein we report a novel gadolinium-encapsulating iron oxide nanoparticle-based activatable NMR/MRI nanoprobe. In our design, Gd-DTPA is encapsulated within the poly(acrylic acid) (PAA) polymer coating of a superparamagnetic iron oxide nanoparticle (IO-PAA), yielding a composite magnetic nanoprobe (IO-PAA-Gd-DTPA) with quenched longitudinal spin–lattice magnetic relaxation (T₁). Upon release of the Gd-DTPA complex from the nanoprobe’s polymeric coating in acidic media, an increase in the T₁ relaxation rate (1/T₁) of the composite magnetic nanoprobe was observed, indicating a dequenching of the nanoprobe with a corresponding increase in the T₁-weighted MRI signal. When a folate-conjugated nanoprobe was incubated in HeLa cells, a cancer cell line overexpressing folate receptors, an increase in the 1/T₁ signal was observed. This result suggests that, upon receptor-mediated internalization, the composite magnetic nanoprobe degraded within the cell’s lysosome acidic (pH 5.0) environment, resulting in an intracellular release of Gd-DTPA complex with subsequent T₁ activation. In addition, when an anticancer drug (Taxol) was coencapsulated with the Gd-DTPA within the folate receptor targeting composite magnetic nanoprobe, the T₁ activation of the probe coincided with the rate of drug release and corresponding cytotoxic effect in cell culture studies. Taken together, these results suggest that our activatable T₁ nanoagent could be of great importance for the detection of acidic tumors and assessment of drug targeting and release by MRI.