Improvement of MRI probes to allow efficient detection of gene expression

Recently, it has been demonstrated that magnetic resonance imaging (MRI) ut ilizing monocrystalline iron oxide nanoparticles (MIONs) targeted to an eng ineered transferrin receptor enables imaging of gene expression. However, t he relatively high doses of iron oxides used indicated the need for improve d MR imaging probes to monitor changes in gene expression in vivo. Using al ternative conjugation chemistries to link targeting ligands and iron oxide nanoparticles, we present the development and characterization as well as i mproved receptor binding and MRI detection of a novel imaging probe. Iron o xide nanoparticles with a cross-linked dextran coat were conjugated to tran sferrin (Tf) through the linker molecule N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP) to yield Tf-S-S-CLIO. The characteristics of this conjuga te were evaluated in comparison to Tf-MION and Tf-CLIO generated by oxidati ve activation of the dextran-coat with subsequent reduction of Schiffs base . SPDP conjugation allowed approximately a 4-fold increase in the number of Tf molecules attached per iron oxide nanoparticle and resulted in a more t han 10-fold improvement of binding and uptake by cells. This translated int o an imaging probe that was 16 times better for imaging gene expression in a cellular MRI assay. This novel probe for MRI may substantially increase t he sensitivity for the detection of endogenous or genetically induced trans ferrin receptor expression in small numbers of cells and may significantly reduce the imaging dose from over 100 mg/kg to doses of iron oxides that ar e currently used in clinical imaging.