Regular use of aspirin may prevent the progression of breast cancer, according to results of a study done by researchers in Kansas City. The study found that aspirin significantly reduced the growth of tumors in mice. It also slowed the growth of breast cancer cell lines in vitro. Gargi Maity (Veterans Affairs Medical Center, Kansas City, MO) presented the results on 21 April 2013 at the annual meeting of the American Society for Biochemistry and Molecular Biology, held in conjunction with the Experimental Biology 2013 conference (Boston, MA).

Aspirin, or acetylsalicylic acid, is used to treat or prevent a number of different conditions, including heart attack and stroke. Its role in the prevention and treatment of cancer has intrigued researchers since it was found that people who regularly used aspirin were less likely to develop colorectal cancer, squamous cell esophageal cancer and prostate cancer. There is also anecdotal evidence to suggest that breast cancer is less likely to return in women who use aspirin. But the physiological mechanisms underlying these effects are not well understood.

The Kansas City study found that aspirin may interfere with cancer's ability to take on an aggressive state. In a mouse model of cancer, treatment with aspirin inhibited the formation of stem cells, which are believed to drive tumor growth and spread. Destruction of stem cells is critical in preventing cancer regrowth, but many cancer chemotherapies fail to target stem cells, allowing relapse.

The researchers also evaluated the effects of aspirin on two different breast cancer cell lines. One of the lines tested is a relatively uncommon but difficult to treat form of the disease called triple-negative breast cancer. These cancer cells lack receptors for the hormones estrogen, progesterone and Her2. “We are interested in triple negative breast cancer because the prognosis is very poor,” said Sushanta Banerjee (University of Kansas Medical Center), senior author of the study. The study also looked at a line of hormone receptor–positive breast cancer cells.

In lab tests, aspirin blocked the proliferation of both cell lines. Aspirin also improved the effectiveness of tamoxifen, a drug commonly used to treat hormone receptor–positive breast cancers.

Banerjee commented that aspirin's efficacy in preventing cancer growth may be due to its ability to attack multiple metabolic pathways. “Cancer is not a single-gene disease,” he says.

Because aspirin has side effects, researchers must assess whether the positive effects of regular use of the drug outweigh the risks.