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Laboratories routinely use the (MNU) induced model of mammary cancers in rats to screen for potential preventive/therapeutic agents. In order to develop a rapid screening method, we determined the effects of short-term treatment with various agents on cell proliferation and apoptosis in small palpable MNU induced mammary cancers. Female Sprague-Dawley rats were administered MNU (75 mg/kg BW, IV) at 50 days of age. When a rat developed a small palpable mammary cancer (within 2-4 months), it was randomized to vehicle or agent treatment for 7 days. The agents were administered either in the diet or by daily gavage. Rats received BRDU by IP injection 2 hours prior to sacrifice. Both proliferation indices (PIs) [by BRDU labeling] and apoptotic indices (AIs) [byTUNEL assay] were determined (10 tumors/group). A wide variety of agents were tested that 1) target the hormonal axis (tamoxifen, vorozole, ovariectomy); 2) target specific proteins (Iressa, Tipfarnitib, celecoxib, sulindac); 3) induce Phase I/II drug metabolizing enzymes (indole-3-carbinol, 5,6 benzoflavone, diindoylmethane); 4) are RXR agonists (Targretin, UAB-30); and 5) are a less specific group of agents including meclizine, vitamin C and sodium phenylbutyrate. In general, there was a strong correlation between inhibition of cell proliferation and preventive/therapeutic activity (R squared 0.91, P<0.001). In contrast, the AI index was increased (3-5X) only by agents with strikingly high preventive/therapeutic activity. Agents that strongly inhibited PI and increased AI were also therapeutic in this model. Interestingly, vorozole (an aromatase inhibitor) and ovariectomy greatly increased both apoptotic and necrotic cell death in contrast to most agents which caused minimal necrotic cell death. Thus, measurements of PI and AI in small palpable mammary cancers following short-term agent treatment allows one to predict the preventive/therapeutic efficacy of that agent. This method uses less than 10% of the amount of agent required for a standard prevention assay. This approach was recently used in our laboratories to screen a variety of RXR agonists for their potential preventive efficacy. The assay could both identify effective agents and be used to screen for known toxicities of the rexinoids. These results will also be contrasted with similar studies in normal mammary glands that are technically more difficult due to the low numbers of epithelial cells.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA