Abstract C11: Combined inhibition of MEK and PI3 kinase signaling results in improved survival in a preclinical orthotopic model for human glioblastoma multiforme.

Glioblastoma multiforme (GBM; astrocytoma grade IV) is the most frequent and aggressive brain tumor for which no effective therapy is currently available. It has been shown that PI3K, RTK/RAS, and Rb signaling are commonly altered in human GBM. We utilized a genetically engineered mouse (GEM) model for GBM, designated “TRP,” that expresses GFAP-T121 (T; for Rb suppression), the KrasG12D mutation (R), and is heterozygous for a PTEN null allele (P), to develop an orthotopic mouse model for the preclinical evaluation of potential therapeutics for GBM treatment. Although the de novo TRP brain tumor GEM model recapitulates features of human GBM including tissue invasion, pseudopalisading necrosis, and dense vascularization, the latency to tumorigenesis (4-6 months) makes its use as a preclinical model for drug screening challenging. Therefore we isolated primary GBM cells from TRP GEMs and injected cells intracranially (IC) into syngeneic mouse brains. Recipient mice developed grade IV astrocytomas and recapitulated TRP GEM tumor histopathology. The orthotopic tumors presented linear foci of necrosis with peudopalisading by neoplastic cells that are hallmarks of human GBM, and were highly proliferative, invasive, and vascular. In addition, immunohistochemistry analysis of TRP orthotopic tumors identified markers characteristic of human GBM, and tumor progression was readily examined by serial MRI. We used primary tumor cells derived from the TRP model in cell proliferation assays and found that PI3K and MAPK pathway inhibitors used as single agents inhibited cell growth alone, but did not result in significant apoptosis. However, when cells were treated with a combination of agents such as BKM120 (a pan-PI3K inhibitor currently in clinical trials for solid tumors) and PD0325901 (a MEK inhibitor), potency was enhanced and there was a substantial increase in cell death. Analysis of downstream targets revealed a synergistic effect on target downregulation in the PI3K pathway. The drug combination was also evaluated in vivo in the orthotopic model. We found that it delayed tumor growth by increasing GBM cell apoptosis, resulting in significantly increased survival. Our results show that combined targeting of the PI3K and MAPK pathways can have a synergistic effect in glioblastoma, and that our primary cells were a valuable tool to predict the in vivo outcome. We also validated this new orthotopic model of GBM and showed that it is tractable for the assessment of potential therapeutic regimens for human GBM. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C11. Citation Format: Rajaa El Meskini, Anthony Iacovelli, Alan Kulaga, Michelle Gumprecht, Philip L. Martin, Maureen L. Baran, Deborah B. Householder, Terry Van Dyke, Zoe Weaver Ohler. Combined inhibition of MEK and PI3 kinase signaling results in improved survival in a preclinical orthotopic model for human glioblastoma multiforme. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C11.