Effect of estrogens on DMBA induced breast tumors

Evidence is presented for the anti-carcinogenic activity of estriol when this hormone is administered to 50-55 day old SD rats prior to treatment with DMBA. In control and estradiol treated animals, 85-100% of animals developed breast tumors regardless of dose of steroid. Low doses of estriol (1 microgram/day) offered some protection against tumorigenesis but at levels exceeding 10 micrograms/day only 20-35% of animals developed tumors even when observed for 180 days following DMBA ingestion. Furthermore, the mean time for tumor production is greatly increased in the presence of E3. Short term (2 weeks) administration offered no protection against tumor formation, suggesting that the effect of estriol probably relates to the promotion stage of tumorigenesis.