Particle Induced X-ray Emission Imaging of Gadolinium Distribution into Xenograft U87 Human Glioblastoma after AGuIX Nanoparticles Injection

Abstract

In recent years, the use of high-Z nanoparticles (NPs) as potential tumor selective radiosensitizers has been proposed as a breakthrough in cancer radiotherapy (RT). NPs are capable of penetrating the cell and they lead to fewer adverse effects than conventional radiosensitizers [1]. Radiation sensitivity using NPs depends on cell line, irradiation energy, NPs type, size, concentration and distribution. High-Z metallic NPs (Au, Gd, Pt, Ag, Fe, etc.) have been predominantly used since they can generate shortrange photoelectrons or Auger electrons, which can enhance the therapeutic dose locally. It is of great interest to limit the diffusion of metallic NPs only to the tumor volume. So current investigations try to optimize the size of NPs and/or use chelating ligands of high affinity for tumor cells. Also the way that NPs are provided to the tumor, intravenously or by direct injection, will determine their distribution. Among those NPs, Gd-AGuIX (Activation and Guiding of Irradiation by X-ray) are 4 nm size polysiloxane GdNPs that have been proposed to be used for theragnostic as magnetic resonance imaging (MRI) contrast agents and radiotherapy sensitizers [2].The purpose of our work was to determine the quantitative distribution of gadolinium into a xenograft U87 human glioblastoma tumor, after intratumoral injection of AGuIX NPs, using PIXE (Particle Induced X-ray Emission) imaging. PIXE is based on the detection of X-ray emitted by a sample when irradiated with a MeV ion beam, usually protons. This technique offers two important specific capabilities for trace metal analysis: imaging and quantification [3]. PIXE is a multi-elemental technique that allows determining elements with atomic number higher than 11 and that is more sensitive than electron microprobe. To study biological samples PIXE can be simultaneously carried out with RBS (Rutherford Backscattering Spectrometry) a technique which provide the local sample mass. This multimodal micro-spectrometry enables expressing metal content in terms of µg of element per g of dry mass all over the scanned area [4].Female mices were grafted with human glioblastoma cells, U87, in right hind leg. 50 µL of AGuIX NPs at 100 mM were injected into the tumors and 1h after, tumors were extracted and cryofixed. Tissue sections of 50 µm thickness were deposited onto sample holders specially designed for PIXE analyses and tissue sections were freeze-dried [5]. Experiments were performed at AIFIRA facility (Applications Interdisciplinaires de Faisceaux d'Ions en Région Aquitaine) from CENBG laboratory (Centre d'Etudes Nucléaires de Bordeaux Gradignan). PIXE and RBS techniques were performed simultaneously to determine elemental content in tumor sections expressed in µg of element per g of dry mass. A proton beam was focalized at 2 µm size and 500 pA intensity. About 80 consecutive squared analyses of 750 µm side were performed to cover the all tumor slice area, of about 5 mm diameter (Fig. 1). Elemental images were reconstructed to overlay all scanned regions.