BACKGROUND: New therapeutic approaches are urgently needed for recurrent GBM (rGBM). Recent studies suggest that GBM oncogenesis and recurrence are regulated by glioma initiating cells (GIC), a minority population of quiescent cells driven by embryonic signaling pathways and resistant to radiation and chemotherapy. Our hypothesis was that interrupting the sonic hedgehog signaling pathway (SHh), an integral mediator of GIC proliferation, would slow tumor progression and improve six month progression free survival (PFS6) in correlation with decreased GSC proliferation and self-renewal. METHODS: A two armed, randomized phase 0/II study of Vismodegib, an inhibitor of SMO was performed in 40 patients undergoing resection for rGBM. Arm I was randomized to receive Vismodegib for 7 days pre-operatively; Arm II was not treated with drug pre-operatively. All patients were treated with Vismodegib alone post-operatively until progression or death. Our primary objective was to estimate six month progression-free survival (PFS6), with secondary endpoints of median overall survival (mOS), response and toxicity. Secondary objectives included determination of: intra-tumoral PK and PD; inhibition of SHh signaling by RT-PCR and immunohistochemistry; and inhibition of GIC proliferation and self-renewal by neurospheres proliferation and limited dilution assays. RESULTS: PK/PD data and tumor specimens obtained from 39/40 patients demonstrated plasma and tissue levels for patients in Arm I (treated pre-operatively) were within therapeutic range. SHh signaling intermediates (Gli-1, Gli-2, PTCH-1b) as determined by RT-PCR and IHC were significantly lower in Arm I vs. Arm II (p < 0.0002). Finally, proliferating CD133 + neurospheres in serum free-media were derived from 17 of the 39 cultures assessed. Array CGH was consistent with GIC (rather than NSC) as the cells of origin, and the proportion of tumor-derived CD133+ neurospheres undergoing proliferation and self-renewal was decreased in Arms I vs. II (p <0.0053 & p < 0.003 respectively). Median PFS and OS was 1.8m and 8.3m respectively; toxicity was minimal. CONCLUSIONS: Vismodegib was well tolerated but poorly efficacious as a single agent in rGBM. However, phase PK and PD studies demonstrated that this agent achieved therapeutic intra-tumoral concentration in rGMB and had biological activity on proliferation and self-renewal of GBM derived CD133+ neurospheres. The phase O/II trial design facilitates a more thorough understanding of an agent's biological effects than the standard phase II trial, and has the potential to enable a more rational approach to this complex and deadly disease. Thus this data suggests that novel combinations pairing SHh pathway inhibition with other agents should be pursued despite lack of single agent efficacy. SECONDARY CATEGORY: Clinical Neuro-Oncology.