CHEMOPREVENTION OF COLON CANCER BY SELECTED ORGANOSELENIUM COMPOUNDS

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The overall objective is to develop an organoselenium compound with lowest toxicity and yet maximal chemopreventative efficacy against colon cancer, and to understand the mechanisms of cancer inhibition by this agent. Our studies indicate that 1,4-phenylenebis(methylene)selenocyanate (p-XSC) is superior to other organoselenium compounds against colon carcinogenesis. We hypothesize that the metabolism of p-XSC by glutathione leading to the formation of putative glutathione conjugate (p-XSe-SG) and/or metabolite of p-XSC, tetraselenocyclophane (TSC) are responsible for its chemopreventative activity. Initially, 40 and 80% MTD levels of these two agents and 80% MTD of p-XSC (parent compound) will be evaluated for their chemopreventative efficacy during initiation and post-initiation phases of AOM-induced colon carcinogenesis. Five week old male F344 rats will be fed high-fat diet or 40 and 80% MTD levels of organoselenium compounds and at 7 weeks of age, all animals except vehicle-treated groups will be administered s.c., AOM (15 mg/kg b.w., once weekly for 2 weeks) and one week later, animals on control diet will be transferred to organoselenium diets and those on organoselenium diets will be transferred to control diet and continued on this regimen until 50 weeks and sacrificed. Histopathological evaluation of colon tumors will be performed and organoselenium exhibiting highest chemopreventative index will be evaluated during promotion/progression phase. Groups of 5-week old male F344 rats will be fed a high-fat diet. At 7 weeks of age groups of rats in each dietary group will be treated s.c. with AOM as above. Fourteen weeks later, groups of animals on high-fat diet will be transferred to high-fat diet with and without organoselenium compound and continued on this regimen until termination of this study. Animals will be sacrificed at 30 and 50 (termination) weeks after second AOM injection. Colon tumor incidence and multiplicity among the dietary groups will be compared by appropriate statistical methods. The mechanisms of colon tumor inhibition by this compound will also be examined. These aspects will include measurement of colonic mucosal and tumor (a) glutathione peroxidase (b) cyclooxygenase (COX-1 and COX-2) isozymes (c) apoptosis, and (d) F/2- isoprostane as an indicator of lipid peroxidation and oxidative damage; and (e) DNA cytosine methylation and DNA methyl transferase during different time periods.