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The extracellular signal-regulated kinase 1/2 (ERK 1/2) mitogen activated protein kinase (MAPK) signaling pathway is known to be activated by and thought to be a cell survival mechanism after exposure to ionizing radiation. AZD6244 is a highly selective inhibitor of MEK 1/2 kinases (MEK 1 IC50 14 nM) with minimal effects on a variety of other kinases at doses as high as 10 μM. AZD6244 has been shown to inhibittumor cell proliferation and inhibition of tumor growth in a range ofxenograft tumors when delivered as a single agent and greater than additive effect when combined with cytotoxic chemotherapies. However, there are limited data on the use of AZD6244in combination with ionizing radiation, therefore an evaluation and understanding of the interaction between MEK inhibition and radiotherapy is clearly warranted. In thepresent study, we report the effects of combined treatment withradiation and AZD6244 in three human cancer cell lines MiaPaCa-2 (pancreatic), DU-145 (prostate), and A549 (lung). AZD6244 had significant radiation modifying effects in clonogenic cell survival experiments. Cells weretreated with vehicle alone or with AZD6244 at 250 nM for 16hrs. Clonogenic cell survival assay showed that AZD6244enhanced tumor cell radiosensitivity with the survival factorat 2Gy (SF2) being reduced from 81% ± 0.5, 79% ± 7, and 62% ±0.9 in vehicle treated to 40% ±5, 60% ±3, and 45% ± 1.7 in AZD6244-treatedA549, DU145, and MiaPaCa-2 cells respectively corresponding to a dose enhancement factor at a surviving fraction of 0.1 of 2.0, 1.16, and 1.38. We examined potentialmechanisms that may contribute to the enhanced radiation responseinduced by AZD6244. Radiation activated ERK1/2 inthese cell lines, and this activation was suppressed byAZD6244 correlating with its radiosensitizing effects. Cell cycle assay was performed after exposure to radiation alone or AZD6244 in combination with radiation. A significant increase in the proportion of cells in G1 was observed in both conditions with no difference between the treatments. There is no evidence of inhibited DNA repair capacity as measured by gamma H2AX foci count at various timepoints after exposure to radiation alone or AZD6244 in combination with radiation. Overall,our results suggest that AZD6244 interactswith radiation and enhances the radioresponse in a variety of human cancerhistologies. A detailed investigation of the mechanism and determinants of magnitude of effect are ongoing. Insight gained from the analyses of these molecular mechanismswill be useful in understanding the importance of the MEK1/2 pathway in radiation response and the potential clinical role of agents such as AZD6244 in combination with ionizing radiation.

99th AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA