Monotherapies have proven largely ineffective for the treatment of glioblastomas, suggesting that increased patient benefit may be achieved by combining therapies. Two protumorigenic pathways known to be active in glioblastoma include RAS/RAF/mitogen-activated protein kinase and phosphatidylinositol 3-kinase/AKT/target of rapamycin (TOR). We investigated the efficacy of a combination of novel low molecular weight inhibitors LBT613 and RAD001 (everolimus), which were designed to target RAF and TOR, respectively. LBT613 decreased phosphorylation of extracellular signal-regulated kinase 1 and 2, downstream effectors of RAF, in a human glioma cell line. RAD001 resulted in decreased phosphorylation of the TOR effector S6. To determine if targeting RAF and TOR activities could result in decreased protumorigenic glioma cellular behaviors, we evaluated the abilities of LBT613 and RAD001 to affect the proliferation, migration, and invasion of human glioma cells. Treatment with either LBT613 or RAD001 alone significantly decreased the proliferation of multiple human glioma cell lines. Furthermore, LBT613 and RAD001 in combination synergized to decrease glioma cell proliferation in association with G 1 cell cycle arrest. Glioma invasion is a critical contributor to tumor malignancy. The combination of LBT613 and RAD001 inhibited the invasion of human glioma cells through Matrigel to a greater degree than treatment with either drug alone. These data suggest that the combination of LBT613 and RAD001 reduces glioma cell proliferation and invasion and support examination of the combination of RAF and TOR inhibitors for the treatment of human glioblastoma patients. [Mol Cancer Ther 2007;6(9):2449 -57]

Malignant gliomas are uncommon, but extremely lethal, cancers. Current standard-of-care includes surgery, radiation and chemotherapy, but recent research has generated a shift towards targeting the aberrant signal transduction components that underlie the pathogenesis of malignant gliomas. Protein kinases are a family of enzymes that are key elements in signal transduction-regulated cellular homeostasis subdivided based on their catalytic activity into tyrosine kinases and serine/threonine kinases. Protein kinases can be deregulated by several mechanisms, including genomic rearrangement, mutations of oncogenes or loss of tumour suppressor genes and overexpression or mutation of growth factor receptors to contribute to cancer initiation and maintenance. In malignant gliomas, several protein kinases are commonly over activated and may represent new therapeutic targets. Two main classes of agents targeting protein kinases are monoclonal antibodies and small-molecule inhibitors. In clinical trials, these molecularly targeted therapies have demonstrated limited efficacy as single agents in unselected malignant glioma patient populations. Several mechanisms of the failure of targeted agent monotherapies have been elucidated as new therapeutic strategies have emerged to overcome the resistance. Multi-targeted kinase inhibitors and combinations of single-targeted kinase inhibitors with one another or with traditional cytotoxics may increase treatment efficacy. Identification of biomarkers of response or resistance will be of paramount importance to enrich patients for specific targeted agents based on their genetic/molecular signature. In this review, the authors discuss the role of protein kinases in malignant glioma and how to target aberrant protein kinases with novel therapeutics.