Radiotherapy combined concurrently with chemotherapy has become standard treatment for esophageal cancer (EC), but the efficacy is limited by treatment-induced toxicity. To improve therapy of EC, novel approaches are being investigated including targeting of specific molecules or molecular pathways whose structure or functions are abnormal in cancer cells. To investigate this approach in preclinical settings, we have explored the ability of flavopiridol, a wide spectrum cyclin-dependent kinase (cdks) inhibitor, to enhance in vitro sensitivity of SEG-1, an EC cell line, to radiation and docetaxel and in vivo response to these agents of xenografts generated in nude mice by these cells. Flavopiridol (100 - 300 nM, 1d) by itself exhibited cytotxoic activity by reducing the survival of SEG-1 cells by 20 - 50%. It also greatly enhanced in vitro radiosensitivity of these cells in a dose dependent manner; the enhancement factor (EF) ranged from 1.20 to 1.86. Flow cytometry analysis of SEG-1 cells showed that flavopiridol increased the percent of G2/M phase cells, the most radiosensitive phase of the cell cycle. At the molecular level, flavopiridol down regulated the levels of phosphorylated forms of cdk-1 and cdk-2, Rb protein and cyclin D1 protein. Flavopiridol also abolished the phosphorylation of the RNA polymerase II at its carboxyl terminus domain that was induced by radiation implying that it also inhibited the kinase activity of cdk-9. In vivo experiments using SEG-1 xenografts showed that flavopiridol given by i.p. at a dose of 15 mg/kg, enhanced tumor radioresponse by a factor of 2.11. Docetaxel, a potent chemotherapeutic agent was also effective in increasing radioresponse of SEG-1 xenografts (EF was 1.17). Histology of tumors treated with flavopiridol and radiation showed a reduction in mitotic activity of tumor cells, loss of tumor cells, and increased percentage of necrosis. Thus, flavopiridol has the potential to improve the efficacy of radiotherapy for esophageal cancer. Investigations on the efficacy of triple agent combination therapy with flavopiridol, docetaxel and radiation in the treatment of EC cells in vitro and tumor xenografts in vivo are underway. (Supported in part by Aventis Pharmaceuticals and MDACC MRP esophageal grant)

[Proc Amer Assoc Cancer Res, Volume 46, 2005]