5MeCDDO Blocks Metabolic Activation but not Progression of Breast, Intestine, and Tongue Cancers. Is Antioxidant Response Element a Prevention Target?

The preventive efficacy of the triterpenoid 5MeCDDO was tested in two models of mammary cancer, the Min model of intestinal cancer, and a chemically induced model of head and neck cancer. In one model of mammary cancer, female Sprague-Dawley rats were administered MNU at 50 days of age, and 5MeCDDO (27 ppm) was administered in the diet beginning 5 days later for the duration of the study; 5MeCDDO was ineffective. In contrast, in a model examining initiation of mammary cancers by the procarcinogen dimethyl-benzanthracene, 5, 6-benzoflavone (500 ppm, an Ah receptor agonist) or 5MeCDDO (27 or 2.7 ppm) decreased tumor multiplicity by 90%, 80%, and 50%, respectively. This anti-initiating effect which is presumably mediated by altered metabolic activation parallels our observation that 5MeCDDO induced proteins of various antioxidant response element (ARE)-related phase II drug-metabolizing enzymes [e.g., GST Pi, AKR 7A3 (aflatoxicol), epoxide hydrolase, and quinone reductase] in the liver. 5MeCDDO tested in the 4-nitroquinoline-l-oxide (4-NQO) head and neck cancer model failed to decrease tumor incidence or invasiveness. In the Min mouse model of intestinal cancer, a high dose of 5MeCDDO (80 ppm) was weakly effective in reducing adenoma multiplicity [∼30% (P < 0.05)]; however, a lower dose was totally ineffective. These findings question whether measuring increased levels of certain ARE-related genes (e.g., quinone reductase, GST Pi), indicating decreased carcinogen activation are sufficient to imply general chemopreventive efficacy of a given agent or mixture. Cancer Prev Res; 9(7); 616-23. ©2016 AACR.