Public Release: 

Estrogen may protect against breast cancer

Study designed to mimic the protective effects of pregnancy

American Association for Cancer Research

Boston, MA (October 15, 2002) - Short-term use of pregnancy-level estrogen, designed to mimic pregnancy, is highly effective in protecting women from breast cancer, according to a study presented at the American Association for Cancer Research's first annual Frontiers in Cancer Prevention Research meeting.

A full-term pregnancy at an early age is the only natural physiological condition known to significantly decrease the lifetime risk of breast cancer in women of all ethnicities worldwide. Results of this study showed that when administered one microgram of estradiol per day for three weeks, rats that had not previously borne offspring had no mammary cancers nine months after being injected with chemical carcinogens.

"We found that daily sustained treatment with pregnancy levels of estrogen for three weeks is a simple, safe, short-term, inexpensive procedure for hormonal prevention of mammary cancer," according to Rajkumar Lakshmanaswamy, research assistant in the cancer research laboratory and lead investigator of the study which was conducted in Nandi Laboratory at the Cancer Research Laboratory, University of California at Berkeley. "This procedure is as effective as full-term pregnancy, removal of the ovaries or long-term tamoxifen treatment, without any loss of ovarian function including the potential for future successful pregnancies and lactation."

Full-term pregnancy in humans, as well as rats, results in a long-term decrease in blood levels of growth hormone and prolactin, resulting in a reduced promotional environment for development of breast cancer. The study was conducted in rats that had never borne babies, to test the duration and level of estradiol needed to induce protection against carcinogen-induced mammary cancer.

"The study concluded that this treatment can be used as a paradigm for developing strategies for human breast cancer prevention," said Dr. Lakshmanaswamy.

The gestation period of rats is 21 days. Rats were injected with the chemical carcinogen, N-methyl-N-nitrosourea (MNU), at 7 weeks of age. Two weeks later, the rats were treated with daily total release of 10 nanogram, 100 nanogram, or one microgram of estradiol for three weeks. All animals also received a 30 mg progesterone silastic capsule for three weeks. Blood levels of estradiol immediately after treatments were 5, 30 and 118 pg/ml, respectively, indicating that that only treatment with one microgram of estradiol resulted in pregnancy levels in circulation. Control rats had a 90 percent mammary cancer incidence, while rats treated with one microgram of estradiol per day had no mammary cancers. Treatments with 10 nanogram or 100 nanogram of estradiol per day did not significantly decrease the mammary cancer incidence compared to the controls; cancer incidence was 82 percent and 67 percent respectively. These results clearly show that only one microgram of estradiol is needed per day for three weeks to confer protection against mammary cancer and indicates that this study can be used as a paradigm for developing prevention strategies for human breast cancer.


More than 203,500 new cases of breast cancer will be diagnosed and more than 39,600 women will die in 2002, according to the American Cancer Society. Breast cancer is the second leading cause of cancer-related death in women.

Founded in 1907, the American Association for Cancer Research (AACR) is a professional society of more than 19,000 laboratory and clinical scientists engaged in cancer research in the United States in more than 60 other countries. AACR's mission is to accelerate the prevention and cure of cancer through research, education, communication and advocacy. Its principal activities include the publication of five major peer-reviewed scientific journals (Cancer Research; Clinical Cancer Research; Molecular Cancer Therapeutics; Molecular Cancer Research; and Cancer Epidemiology, Biomarkers & Prevention.

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