Abstract 1308A: Novel antiinvasive and antiangiogenic mechanisms of mTOR inhibition in glioblastoma
Authors: Pfenning Philipp-Niclas , Weiler Markus , Thiepold Anna-Luisa , Jestaedt Leonie , Berger Benjamin , Combs Stephanie , Gronych Jan , Dittmann Laura , Bendszus Martin , Wick Wolfgang 
Published: February 2011

Abstract
Two key properties of glioblastomas are vascular proliferation and invasion of tumor cells into the surrounding tissues, both limiting the efficacy of surgery and radiochemotherapy treatment regimens. Inhibition of mammalian target of rapamycin (mTOR) by Temsirolimus (CCI-779) only has modest single compound activity in recurrent glioma. Given the fact that postoperative radiochemotherapy is the standard of care in the first-line treatment of newly diagnosed glioblastoma, the first part of the presented project aims at analyzing combined mTOR/radiotherapy in the syngeneic, orthotopic VM/Dk/SMA-560 mouse glioma model. The combined treatment of CCI-779, a small-molecule inhibitor of the mTOR kinase complex 1 (mTORC1) approved for advanced renal cell carcinoma and mantle cell lymphoma, at 20 mg/KG from day 3 until day 17 and focal irradiation at 6 Gy on day 5 after tumor inocculation demonstrated remarkable antiangiogenic and antitumoral activity as well as prolonged survival of tumor bearing animals of 9 days, significant compared with irradiation- or CCI-779-treatment alone.
Loss of phosphatase and tensin homologue on chromosome ten (PTEN), which is a common event in glioblastoma, results in activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/mTOR signaling pathway, leading to neovascularisation, cell cycle progression and escape from apoptosis. So far, conflicting data on the sensitivity of PTEN wild-type (wt) versus mutant cells exist. Similarly, the relevance of the feedback activation of Akt by mTOR inhibition is debated. Here, analysis of PTEN on mRNA, promoter methylation as well as protein levels clearly demonstrates for cell lines as well as primary glioma cells that proliferation of PTEN wt cells is also sensitive to mTOR inhibition albeight at higher concentrations. We further demonstrate by differential effects on the mTOR complexes (mTORC) 1 and 2 by shRNA as compared to sole mTORC 1 inhibition by CCI-779 that feedback activation of Akt, which is more prominent in PTEN mutant than in wild-type cells, may in fact have additional therapeutic antiinvasive and angiogenic effects via inhibition of a G-protein-interacting protein and vascular endothelial growth factor receptor (VEGFR)-2, respectively. CCI-779 exerted marked anti-angiogenic effects both by reducing levels of VEGFR and by inhibiting radiation-enhanced proliferation of brain endothelial cells. Moreover, CCI-779 applied after radiosensibilization inhibited glioma invasiveness in a supra-additive way and reverted the proinvasive effect of sublethal irradiation alone.
The results support the clinical evaluation of combined targeted mTOR inhibition with CCI-779 and radiotherapy in patients with newly diagnosed glioblastomas that is going to be conducted in the European Organization for Research and Treatment of Cancer.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1308A.