Effect of Notch inhibition on radiation in an explant model of glioblastoma multiforme

e22080 Background: Glioblastoma Multiforme (GBM) is highly radioresistant, possibly due to a subpopulation of Brain Tumor “Stem-like‘ Cells (BTSC) capable of repopulating the tumor. The Notch signaling pathway is essential for maintaining normal neural stem cells. We asked whether Notch inhibition can overcome radioresistance by allowing BTSCs to exit their stem cell state. Methods: To maintain the BTSCs in their original niche we used an organotypic culture method (explants). GBM explants are cultured on a semiporous membrane in an air-medium interface. They are treated with the Notch inhibitor DAPT, radiation or a combination thereof. Proliferation and neurosphere forming capacity are assessed. Results: The explant model faithfully maintained the cytoarchitecture of the tumor (preservation of blood vessels and pericytes) and its high proliferation rate (%ki67+ cells: 14.3% -17.1%). The effect of DAPT or radiation treatment alone differed among tumors (from no change to a two-fold and nine-fold decrease post DAPT or radiation respectively). However, in all tumors tested the combination of DAPT and radiation dramatically decreased the proliferation rate compared to either treatment alone (%ki67 + cells: 0.19%-2.24%). Interestingly the rate of neurosphere formation was highest following radiation alone (threefold increase). Neurosphere formation after DAPT treatment differed amongst the tumors from a decrease (fourfold) to an increase (twofold). The combination treatment resulted in a modest increase over control (twofold). Conclusions: Our data demonstrates that inhibition of proliferation does not correlate with the status of neurosphere forming ability, arguably a surrogate for cancer stem cell function. Combining radiation with notch inhibition has a profound effect on GBM proliferation, most likely due to selectively inhibiting the BTSC repopulating ability. Ongoing studies will determine the impact of this approach on tumor growth and progression in in vivo tumor models. No significant financial relationships to disclose.