Positron Emission Tomography/Magnetic Resonance Imaging of Glioblastoma Using a Functionalized Gadofullerene Nanoparticle

Abstract

Water-soluble gadofullerene nanomaterials have been extensively investigated as magnetic resonance imaging (MRI) contrast agents, radical scavengers, sensitizers for photodynamic therapy, and inherent antineoplastic agents. Most recently, an alanine-modified gadofullerene nanoparticle (Gd@C₈₂-Ala) with excellent anticancer activity has been reported; however, the absolute tumor uptake of Gd@C₈₂-Ala is still far from being satisfactory, and its dynamic pharmacokinetics and long-term metabolic behaviors remain to be elucidated. Herein, Gd@C₈₂-Ala was chemically modified with eight-arm polyethylene glycol amine to improve its biocompatibility and provide the active sites for the attachment of a tumor-homing ligand (cRGD) and positron emission tomography (PET) isotopes (i.e., ⁶⁴Cu or ⁸⁹Zr). The physical and chemical properties (e.g., size, surface functionalization condition, radiochemical stability, etc.) of functionalized Gd@C₈₂-Ala were properly characterized. Also, its glioblastoma cell targeting capacity was evaluated in vitro by flow cytometry, confocal fluorescence microscopy, and dynamic cellular interaction assays. Because of the presence of gadolinium ions, the gadofullerene conjugates can act simultaneously as T₁* MRI contrast agents and PET probes. Thus, the pharmacokinetic behavior of functionalized Gd@C₈₂-Ala was investigated by PET/MRI, which combines the merits of high resolution and excellent sensitivity. The functionalized Gd@C₈₂-Ala-PEG-cRGD-NOTA-⁶⁴Cu (NOTA stands for 1,4,7-triazacyclononane-triacetic acid) demonstrated much higher accumulation in U87-MG tumor than its counterpart without cRGD attachment from in vivo PET observation, consistent with observation at the cellular level. In addition, Gd@C₈₂-Ala-PEG-Df-⁸⁹Zr (Df stands for desferrioxamine) was employed to investigate the metabolic behavior of gadofullerene conjugates in vivo for up to 30 days. It was estimated that nearly 70% of Gd@C₈₂-Ala-PEG-Df-⁸⁹Zr was excreted from the test subjects primarily through renal pathways within 24 h. With proper surface engineering, functionalized Gd@C₈₂-Ala nanoparticles can show an improved accumulation in glioblastoma. Pharmacokinetic studies also confirmed the safety of this nanoplatform, which can be used as an image-guidable therapeutic agent for glioblastoma.

Keywords: gadofullerene; magnetic resonance imaging; metabolic study; positron emission tomography; tumor targeting.