"In this work, we confirm reports by others that aspirin reduces the risk of breast cancer by about 20 percent in some women," said Dr. Marilie D. Gammon, professor of epidemiology at the University of North Carolina at Chapel Hill. "The risk reduction is most pronounced among daily aspirin users -- 27 percent."
Chief among new findings is that the reduction appears to be restricted to women with what are called ER-positive tumors, said Gammon, who works at the UNC School of Public Health. The discovery is biologically plausible because aspirin is thought to work through a chemical pathway in the body called Cox-2, and estrogen appears to interact with that pathway.
"ER-positive breast cancer is the predominant type of breast cancer among postmenopausal women, and postmenopausal women make up about 75 percent of all newly diagnosed breast cancer cases in the United States," she said. "In contrast, ER-positive breast cancer is not prevalent among postmenopausal women in Japan, for example, where breast cancer rates are much lower than they are in the U.S.
"If we can reduce the risk of ER-positive breast cancer through such efforts as taking an aspirin-like chemopreventive, we could potentially reduce the incidence breast cancer among American women," Gammon said. "This would be a big deal."
A report on the research appears as the lead article in the May 26 issue of the Journal of the American Medical Association.
Besides Gammon, authors include Drs. Mary Beth Terry, assistant professor of epidemiology, and Alfred I. Neugut, professor of epidemiology, both at Columbia.
Gammon is principal investigator of the Long Island Breast Cancer Study Project, one of the most comprehensive environmental epidemiologic studies ever done on that cancer. First results of the investigation, released in 2002, showed that exposure to air-polluting polycyclic aromatic hydrocarbons (PAHs) in the environment appeared to boost women's risk of breast cancer by a modest 50 percent in Suffolk and Nassau counties, N.Y. It uncovered no increased rate of the illness among area women who might have been exposed to organochlorine compounds.
The new study is based on data gathered during the Long Island project, said Gammon, deputy director of UNC's Environmental Health and Susceptibility Center and a Lineberger Comprehensive Cancer Center member.
"Starting with more than 3,000 women in the earlier research, we looked at blood samples taken from hundreds of new breast cancer patients and comparable women without breast cancer who served as controls," she said. "Our goal in that population-based, case-control study was to determine whether breast cancer incidence in women in the Long Island counties was associated with exposures to the environmental contaminants. We found only a small association."
In the new work, the scientists compared information the women provided about their use of aspirin and other nonsteriodal anti-inflammatory drugs with their breast cancer experiences. Statistical analyses revealed the apparent protective effect.
Strongest effects were seen among women who took aspirin recently and frequently -- seven or more tablets a week, Gammon said. Results for ibuprofen were generally weaker. Acetaminophen, a painkiller that does not inhibit prostaglandin synthesis and works differently in the body, offered no apparent cancer benefits.
Researchers could not tell from the women's recollections of aspirin use if dosage strength made a difference.
It is still too soon for doctors to recommend regular aspirin use to prevent the illness and more research needs to be done, including studies of more racially and ethnically diverse groups, the UNC scientist said. Others agreed.
"The American Cancer Society and other organizations do not recommend aspirin-like drugs for cancer prevention because the evidence remains incomplete, and these drugs have side effects that are uncommon but can be dangerous," said Dr. Michael Thun, vice president for epidemiology and surveillance at the society.
The National Cancer Institute and the National Institute of Environmental Health Sciences paid for the Long Island cancer research, which Congress mandated in 1993.