GBM6 and GBM12 cells (250–1500 cells/well) were plated in sextuplicate and allowed to attach for 12h. Cells were treated with OSU-03012 (OSU, 0.5–2.0 μM); sildenafil (SIL, 0.5–2.0 μM); or the drugs combined for 24h. Media was removed, plates washed, and cells cultured for the next 10–14 days in media lacking drugs. After ~10 days media was removed, plates washed, and cells fixed and stained. The number of colonies per plate (> 50 cells per colony) was counted, plating efficiencies determined, and the relative changes in colony formation entered into the Calcusyn for Windows program to determine synergy (n = 3). A CI (combination index) of less than 0.70 indicates a strong level of synergy.