BACKGROUND: Glioblastoma represents a compelling disease for signal transduction targeted therapy. Despite the evidence defining the PI3K network as a high quality target, regulatory feedback loops as well as cross talk with other networks make targeting the PI3K network a challenging task. Complete control of GBM is dependent on the elucidation of the escape mechanisms and the rational design of combination therapies. A major objective in our laboratory is to develop robust predictive molecular and genetic markers/signature that will lead to selection precision therapy. These predictive markers will allow us to determine rational combinatorial approaches to target aberrations in the PI3K network and at the same time prevent deleterious effects of feedback loops and cross talk. METHODS: We have utilized a large panel of glioma stem cell (GSC) lines to perform drug sensitivity/resistance screens. Reponses profiles of GSCs using multiple PI3K inhibitors in cytotoxicity assay were generated. Molecular targets/pathways contributing to alternate signaling pathway activation by inhibitors were identified using gene expression profiling and protein expression/activity profiling. Top gene targets were subjected to loss of function studies using RNAi. Synergistic effect of knocking down the target along with PI3K inhibitors on cell proliferation, and tumorigenicity were assessed. RESULTS: We have utilized several system approaches to identify targets in compensatory or parallel pathways after PI3K inhibition. Our results suggest that different molecular profiles in GSC dictate the differences in viability observed after drug treatment, despite a common blockade of PI3K pathway. Resistant and sensitive cell lines were profiled using the Affymetrix and RPPA platforms to identify genes that are differentially expressed in sensitive and resistant GSCs responsible for intrinsic resistance. Using gene expression profiling, we also identified adaptive response gene targets in pre- and post-treatment resistant GSC. This approach has uncovered alternate pathways and shutting down these pathways would synergize the effect on cell growth when combined with PI3K pathway inhibitors. We have selected the top 10 candidate genes and two targets are being studied for rational combination with PI3K inhibitors CONCLUSIONS: Failure of PI3K pathway inhibitors is due to escape through overlapping or parallel signaling pathways, and that complete control of GBM depend on the elucidation of these escape mechanisms and we show that therapies to target essential pathways overcome or circumvent treatment resistance and can provide the next generation of improved therapeutic options. SECONDARY CATEGORY: Tumor Biology.