Abstract
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Objectives: Glioblastoma multiforme (GBM) is the most common type of primary malignant brain tumor. Among patients with GBM, 25-30% present a mutated form of epidermal growth factor receptor or so called EGFRvIII (1). Due to the mTOR-PI3K-driven aggressiveness of the EGFRvIII-expressing GBM (2), these patients have poor prognosis and resist to EGFR tyrosine kinase inhibitors therapy compared to those with EGFR wild-type (EGFRwt) GBM (3). GBM exhibits increased use of nutrients, including glucose and glutamine, to support its bioenergetic and biosynthetic demands (4). Given the inherent limitations of 18F-FDG PET imaging in brain tumors, we reason that PET with 11C-glutamine might have potential to differentiate the EGFR-variant GBMs in vivo. In this study, we tested this hypothesis by PET imaging with 11C-glutamine in two patient derived xenograft (PDX) models, GBM6 and GBM12, which present EGFRvIII and EGFRwt, respectively.
Methods: Patient derived GBM tumor cells (1x104) were intra-cranially injected into right frontal lobe region. Due to the differences of tumor aggressiveness between GBM6 and GBM12, we performed 11C-glutamine PET scans in 1 and 2 weeks post tumor cell injection for GBM6 and GBM12-PDX models, respectively. For the production of 11C-glutamine, the procedure was carried out by adapting its reported two-step synthetic methods on two automated radiochemistry modules (5). A 0-20 minute list-mode dynamic PET imaging was performed on each mouse bearing GBM-PDX right after intravenously tail vein injection of 11C-glutamine (10 µCi/g of body weight) and followed by a CT scan. PET images were reconstructed into 15 time frames (10 × 60 seconds, 5 × 120 seconds) and quantitatively analyzed using the scanner manufacturer’s software.
Results: Within 20 min of dynamic PET scan, 11C-glutamine showed the highest accumulation in the liver compared to other major organs due to the high expression levels of glutamine synthetase and glutaminase (6). Similar time activity curve profiles were observed in intestines and kidneys, implying that the excretion of 11C-glutamine and its metabolites might be evenly processed by the hepatobiliary and renal systems. EGFRvIII-expressing GBM6 showed significant higher uptake (3.06 ± 0.15 %ID/g; n = 3) than GBM12 (EGFRwt) (1.83 ± 0.25 %ID/g; n = 3) (p < 0.002), indicating that the former has a higher demand for glutamine than the latter. The cerebellum region was chosen as reference as it displayed a similar lower uptake level in both PDX models (1.20 ± 0.1 %ID/g vs. 1.42 ± 1.01 %ID/g; p = 0.72).
Conclusions: Our preliminary data indicates that PET imaging with 11C-glutamine can be potentially used as a noninvasive tool to differentiate the GBM EGFR variants, which might find application in glutaminase-targeted therapies. Further studies are ongoing. Acknowledgments: This work was supported by grants from the Cancer Prevention and Research Institute of Texas (RP110771 & RP170638) and the Dr. Jack Krohmer Professorship Funds. References:(1) Ju, Y.,et al., Cancer Letters, 2017, vol. 403, 224-230(2) Cancer Genome Atlas Research N. Nature,2008 vol 455, 1061-1068(3) Mellinghoff, I.K., et al., N. Engl. J. Med., 2005 vol. 353, 2012-2024(4) Wise, D.R. et al., Proc. Natl. Acad. Sci. USA., 2008 vol.105, 18782-18787(5) Gleede T, et al., Amino Acids, 2015 vol. 47, 525-33(6) Watford, M., et al., Nutrition, 2002 vol. 18, 301-303