P13.16 Metastatic potential of systemic glioblastoma stem cell lines in vivo

BACKGROUND Despite aggressive tumor behavior, extracranial metastases rarely develop in glioblastoma (GBM) patients. Two potential explanations have been suggested: 1) The blood-brain-barrier functions as a physical barrier that prevents the dissemination of GBM cells out of the central nervous system (CNS) or 2) that extracranial metastasis do occur, but the patients die before extracranial metastases manifest themselves. The first theory has been questioned based on the fact that circulating tumor cells (CTC) were found in blood samples of GBM patients without systemic metastases. To date it has not been proven if CTCs are able to reenter the brain and to what extent they are able to form systemic extracranial metastatic lesions. Therefore, the current study aimed at analyzing the dissemination patterns and the underlying mechanisms associated with the ability of GBM CTCs to form extracranial metastases. MATERIAL AND METHODS Five highly characterized human GBM stem cell (GSC) lines (P3, BG5, BG7, GG6, GG16), displaying GBM CNV patterns, were intracranially implanted in a first cohort, then transduced with a lentiviral Firefly Luciferase-eGFP vector and injected into the left cardiac ventricle of NOD/SCID mice in a second cohort. Mice were observed closely and tumor burden was assessed using in vivo as well as ex vivo bioluminescence imaging, MRI and PET. Mice were euthanized when the objective endpoint criteria (tumor burden) was met, then organs were harvested and fixed for further analysis. RESULTS First, a detailed characterization of the GSC line invasion patterns were assessed when grown as orthotopic xenografts in vivo dividing them into three categories: 1) Highly invasive without apparent angiogenesis (BG5) 2) Invasive with perivascular infiltration and angiogenesis (P3, BG7 and GG16) and 3) Angiogenic and highly circumscribed (GG6). Following intracardial injection, (7 out of 8) P3 animals developed extracranial and intracranial tumors with a distinctive pattern. Brain, adrenal gland, ovary and liver were amongst the organs most susceptible for tumor growth in the P3 group. For the BG5 and BG7 cell lines, no metastases were observed whereas only 1 animal out of 10 developed metastases in both groups GG16 and GG6. CONCLUSION Only one out of 5 GSC lines exhibited a strong metastatic potential when injected into the left cardiac ventricle. Compared to other tumors which exhibit a strong metastatic potential from the circulation, GSC lines do only to a very limited extent show this potential reflecting observations made in the clinic.