Abstract Gliomas are difficult to eradicate due to several factors: a) the lack of efficient delivery of anti-tumor agents to the tumors without affecting the normal brain tissue; b) the inherent or acquired resistance of tumor cells to anti-tumor agents; and c) the highly invasive nature of tumor cells that escape the primary tumor mass and subsequently cause recurrence. We previously established that a secreted form of tumor necrosis factor-related apoptosis inducing ligand (S-TRAIL) specifically eradicates glioma cells when delivered by various stem cell types in culture and in orthotopic glioma models. However, recent evidence has shown that a number of established and primary glioma lines have varying degrees of resistance to TRAIL, due to the heterogeneity of genetic alterations and possibly due to the over-activation of phosphatidylinositol 3 kinase (PI3K) pathway. In this study, we report the effect of a novel PI3K inhibitor, PI-103, alone or in combination with neural stem cell (NSC)-delivered S-TRAIL in mouse models of malignant and invasive gliomas. Using established and primary CD133+ glioma initiating cells engineered to express fluorescent and bioluminescent markers, we show that PI-103 alone inhibits proliferation and invasion, and causes G0-G1 arrest in cell cycle of most of the glioma cells tested. Furthermore, PI-103 augments the response of glioma cells to stem cell delivered S-TRAIL in co-cultures of NSCs and glioma cells suggesting that it can synergize with S-TRAIL-induced apoptosis. Using orthotopic malignant glioma models and bioluminescence imaging, PI-103 treatment in combination with NSC-derived S-TRAIL was shown to result in significant reduction in tumor volumes compared to either treatment alone in vivo. Our results reveal that combination of the novel PI3K inhibitor, PI-103, and stem cell delivered TRAIL might represent a novel therapy for malignant gliomas. 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 3495.