MRI of Breast Tumor Initiating Cells Using the Extra Domain-B of Fibronectin Targeting Nanoparticles

The identification of breast tumor initiating cells (BTICs) is important for the diagnosis and therapy of breast cancers. This study was undertaken to evaluate whether the extra domain-B of fibronectin (EDB-FN) could be used as a new biomarker for BTICs and whether EDB-FN targeting superparamagnetic iron oxide nanoparticles (SPIONs) could be used as a magnetic resonance imaging (MRI) contrast agent for BTIC imaging in vitro and in vivo. BTICs (NDY-1) exhibited high EDB-FN expression, whereas non-BTICs (MCF-7, BT-474, SUM-225, MDA-MB-231) did not exhibit EDB-FN expression. Furthermore, Cy3.3-labeled EDB-FN specific peptides (APT(EDB)) showed preferential binding to the targeted NDY-1 cells. To construct an EDB-FN targeted imaging probe, APT(EDB) was covalently attached to a thermally cross-linked SPION (TCL-SPION) to yield APT(EDB)-TCL-SPION. In the in vitro MRI of cell phantoms, selective binding of APT(EDB)-TCL-SPION to NDY-1 cells was evident, but little binding was observed in MCF-7 cells. After the intravenous injection of APT(EDB)-TCL-SPION into the NDY-1 mouse tumor xenograft model, a significant decrease in the signal within the tumor was observed in the T-2*-weighted images; however, there was only a marginal change in the signal of non-targeting SPIONs such as APT(scramble)-TCL-SPION or TCL-SPION. Taken together, we report for the first time that EDB-FN was abundantly expressed in BTICs and may therefore be useful as a new biomarker for identifying BTICs. Our study also suggests that APT(EDB)-TCL-SPION could be used as an MRI contrast agent for BTIC imaging.