11506 Background: ATO is effective in the treatment of acute promyelocytic leukemia, but its role in the management of solid tumors has yet to be defined. Post-operative therapy consisting of TMZ with and without RT is considered the standard of care for patients with newly-diagnosed glioblastoma multiforme (GBM). ATO has been shown to potentiate the effects of RT in preclinical models. Therefore, we investigated the cytotoxicity of ATO and TMZ both together and in combination with RT in the U251T human glioma cell line. METHODS U251T cells in log phase growth were treated with ATO (0.05-5 μM) and/or TMZ (0.05-5 μM) for 24 hours or sequentially according to the following schedules: ATO for 18 hours followed by TMZ for 6 hours or the reverse sequence. For RT experiments, cells were treated with 0-10 Gy of ionizing RT using a Cesium-137 source. Following drug and/or RT treatment, cells were replated onto 1.5 cm tissue culture dishes in triplicate and incubated at 37oC for 8 to 10 days to form colonies. Colonies of more than 40 cells each were counted and the IC50 values and combination indices (CI) were derived using Calcusyn software (Biosoft, Cambridge, UK). Each combination was repeated at least 3 times. RESULTS The mean single agent IC50 for ATO, TMZ, and RT were 1.10 ± 0.28 μM, 1.88 ± 0.91 μM, and 2.62 ± 0.23 Gy, respectively. Simultaneous treatment with ATO and TMZ was slightly synergistic, while sequential treatment was either additive (TMZ→ATO) or synergistic (ATO→TMZ). Combinations of RT with ATO, TMZ or both were all synergistic, with the triple agent combination showing the greatest synergy. CONCLUSIONS RT followed by ATO and TMZ given sequentially demonstrated striking synergy in U251T human glioma cells. A clinical trial of this combination therapy in patients with newly-diagnosed GBM is currently under development. [Table: see text] No significant financial relationships to disclose.