TSPO PET as a pharmacodynamic biomarker to image a rat model of diffuse intrinsic pontine human glioma and assess ketogenic diet efficacy

Abstract

Diffuse intrinsic pontine gliomas (DIPG) are highly infiltrative paediatric gliomas. It remains the first cause of death linked to cerebral tumor in children. There is an urgent need to find new therapeutic strategies to increase the survival of these patients. In this context, specific pharmacodynamic (PD) biomarkers are needed to accurately assess therapeutic response and ease therapies’ validation. Molecular imaging (MI), especially positron emission tomography (PET) offers the possibility to quantify biologic processes taking place at cellular and subcellular levels. MI can define the real extent of tumors and treatment response on highly infiltrative tumor such as DIPG. 15% of DIPG's tumors present a dysregulated PIK3 pathway which promotes tumor growth. Ketogenic diet (KD), a metabolic therapy, consisting in a high intake of fat and a low intake of carbohydrates can increase the vulnerability of tumors cells. It could act as a tumor suppressor via the inhibition of PI3 K pathway. Here, we investigated the interest of using 18F-DPA-714 (i.e. ligand of the translocator protein of 18 kDa, TSPO) as a PD PET ligand to evaluate the extent of DIPG tumors. We explored via TSPO PET, the benefits of ketogenic diet (4:1) on human DIPG cells (HSJD-DIPG-007, PI3KCA-mutated, established at the Sant Joan de Déu Hospital (Barcelona, Spain)). In vivo study was performed on nude rats orthotopically grafted. KD was introduced one week after the tumor implementation in relevant groups. Ketosis was followed by dosage of beta-hydroxybutyrate and glucose blood concentrations. Four groups were imaged at 5 weeks post-implementation: sham with and without KD ( n = 4, for each condition), grafted with and without KD ( n = 8, for each condition). Brains of the animals were collected after imaging sessions to correlate 18F-DPA-714 PET signal with immunofluorescence biomarkers distribution. TSPO is highly expressed on this cell line, that opens up an opportunity for PET imaging. Ketosis was successfully induced in the rat groups receiving KD (corroborated by an increased brain uptake of 18F-FDG in treated groups). An increased 18F-DPA-714 uptake is observed in the pons and ex vivo immunofluorescence revealed a strong correlation between TSPO signaling and DIPG cells presence. We are the first team to prove that TSPO can be a potent biomarker to follow DIPG progression. Further analyses are ongoing to statute about the KD efficacy.