Brain Tumor Imaging and Delivery of Sub-5 nm Magnetic Iron Oxide Nanoparticles in an Orthotopic Murine Model of Glioblastoma

Magnetic iron oxide nanoparticles (IONP) are promising theranostic platforms for cancer imaging and treatment However, delivery of nanoparticles with conventional sizes to intracranial tumors is challenging due to the physiological and biological barriers in the central nervous system. Since advanced glioblastoma (GBM) is highly angiogenic with leaky tumor blood vessels, sub-5 nm ultrafine IONP (uIONP) may preferentially extravasate from the tumor vasculature to exert blood-brain-barrier/blood-tumor-barrier crossing and intratumoral accumulation. Herein, we demonstrate that the renally clearable sub-5 nm oligosaccharide-coated uIONP can efficiently reach intracranial brain tumors in the orthotopic mouse model of GBM after intravenous (i.v.) injection, enabling the visualization of tumors by T-1-weighted magnetic resonance imaging (MRI). Delivery and intratumoral distribution of uIONP are primarily promoted by the enhanced permeability and retention (EPR) effect due to the sub-5 nm core size. uIONP exhibited time-dependent brain tumor uptake with bright T-1-enhanced MRI contrast gradually peaking at 40-60 min after injection. uIONP were compartmentalized in tumor blood vessels at early time points (similar to 20 min), before extravasating and subsequently reaching different areas of tumors (similar to 60 mM) but not in the normal brain tissues. The size advantage of sub-5 nm uIONP in delivery and intratumoral distribution was examined by the comparison between co-injected uIONP and IONP (10 nm core size) labeled with different fluorescent dyes, using MRI, near-infrared (NIR) imaging, fluorescence microscopy, and immunohistochemistry analysis. NIR dye NIR830-labeled uIONP exhibited a sixfold higher penetration and retention in the intracranial tumor than the fluorescein isothiocyanate-labeled IONP with a 10-nm core diameter at 1 h after co-injection. Together, our results demonstrate the sub-5 nm size advantage in delivery of renally clearable nanotheranostics, such as uIONP, for molecular imaging and treatment of brain tumors.