Treating healthy women at high risk of developing breast cancer with the drug exemestane can cut their risk of developing the disease by two-thirds, a new study shows.
"The results are extremely exciting because they have the potential to impact thousands of women," said Dr . Michael Wosnick, vice-president of research at the Canadian Cancer Society.
He said using the drug exemestane, which suppresses estrogen production, offers some women an "important new option" for reducing their risk.
The research was presented Saturday at the American Society of Clinical Oncology conference in Chicago and published online simultaneously in the New England Journal of Medicine
Dr. Paul Goss of the Massachusetts General Hospital Cancer Center in Boston, lead author of the study said the reductions in cancers were "massive."
However, impressive relative risk reductions always need to be kept in perspective.
The research involved 4,560 post-menopausal women from Canada, the U.S. and Europe. Only 43 developed invasive breast cancer during the three-year study period - including 11 women taking exemestane and 32 taking a placebo (sugar pill).
Stated another way, among study participants taking a placebo, 0.55 per cent developed cancer annually; among those taking exemestane, the annual rate of breast cancer diagnosis was 0.19 per cent.
That translates into a 65 per cent relative risk reduction.
All the women in the study were considered to be at high-risk of developing breast cancer because they had at least one of three risk factors: A mastectomy to treat a non-invasive breast cancer like ductal carcinoma in-situ, a Gail five-year risk score of 1.66 or higher (a tool that considers family history, age at first menstruation, number of pregnancies, etc.), or they were over age 60.
Breast cancer chemoprevention - the use of drugs to prevent cancer or cancer recurrence - is an area that has generated tremendous research interest.
The new study shows three years of exemestane (brand name Aromasin) can reduce breast cancer incidence up to 65 per cent, while previous research has shown that five years of tamoxifen (Nolvadex) can shrink incidence by about 50 per cent in high-risk women, and raloxifene (Evista) can trim breast cancer cases by 38 per cent over the same period.
Yet, despite these promising results the number of healthy women taking drugs to prevent cancer is "vanishingly rare," Dr. Nancy Davidson of the University of Pittsburgh Cancer Institute, wrote in an editorial, also published in the New England Journal of Medicine.
"We have run out of excuses. What are we waiting for?" she wrote.
The problem with these drugs is they have a lot of side effects, including an increased risk of fractures, heart attacks and strokes and a higher risk of developing other cancers.
Because they suppress estrogen, the drugs can also worsen menopausal symptoms like hot flashes, fatigue and insomnia.
The new research, funded in part by drug maker Pfizer Inc., as well as by the Canadian Cancer Society, showed women taking exemestane has fewer serious side effects. However, the study lasted only three years and side effects tend to be more common in the real world than in clinical studies.
Exemestane costs about $180 a month, while tamoxifen costs about $50 monthly.
In 2011, an estimated 23,400 Canadian women will be diagnosed with breast cancer and 5,100 will die of it, according to the Canadian Cancer Society.