Charting aspirin's value as colorectal drug.

Despite accumulating evidence that aspirin wards off colon cancer, negative data from a new study suggest that the whole story on aspirin and colorectal cancer has yet to unfold.

In recent years, several research teams have demonstrated that regular aspirin use appears to help guard against colorectal cancer. In 1991, epidemiologists at the American Cancer Society (ACS) in Atlanta reported that people who take aspirin may reduce by half their risk of dying from colon cancer (SN: 12/7/91, p.374). Earlier that same year, a Boston University team showed that regular use of aspirin or aspirin-containing drugs seems to help prevent the development of colon cancer (SN: 3/16/91, p.166).

In addition, researchers postulated a mechanism for aspirin's anticancer action. Laboratory evidence indicates that aspirin and aspirin-like compounds interfere with the synthesis of prostaglandins, hormone-like substances that can fuel the growth of tumors.

Proponents of the drug note that aspirin has already proved its mettle against a range of ills from headaches to heart disease (SN: 7/27/91, p.55). Yet the new study, published in the Aug. 4 JOURNAL OF THE NATIONAL CANCER INSTITUTE, hints that for colorectal cancer, aspirin is far from a proven preventive.

Epidemiologist Peter H. Gann, now at the Northwestern University Medical School in Chicago, and his colleagues at the Harvard Medical School in Boston analyzed data from 22,071 male physicians participating in the Physicians' Health Study, a long-term effort designed to test aspirin's value in reducing the risk of cardiovascular disease. Half those volunteers took an adult aspirin tablet and half took a placebo pill every other day. The researchers kept track of the recruits and noted how many cases of colorectal cancer developed during a five-year period.

They discovered 118 cases of invasive colorectal cancer and 253 cases of colorectal polyps, small wart-like growths that can lead to cancer.

The team found no statistically significant reduction in invasive colorectal cancer among physicians taking aspirin. The researchers did find a small decrease in colorectal polyps among the aspirin takers; however, chance could account for this finding, Gann points out.

"The mounting enthusiasm for the potential of aspirin is tempered by these results," Gann says. "I think if there were a very large and fairly immediate impact of aspirin, we would have seen it."

This study represents the first randomized, placebo-controlled investigation of whether aspirin can prevent colorectal cancer, notes epidemiologist E. Robert Greenberg of the Dartmouth Medical School in Hanover, N.H. Previous studies have often been case-control or observational in nature. Such epidemiological studies, in which participants are not randomly assigned to an active treatment group, can be subject to confounding factors that skew the results, he says. Greenberg and his Dartmouth colleague John A. Baron wrote an editorial that appears in the same issue of the journal.

The new study also has a downside. First of all, the researchers tracked the physicians for just five years, a period that may have been too short to show aspirin's value, says Clark W. Heath Jr., one author of the earlier ACS study on aspirin and colorectal cancer. In addition, the physicians received a low dose of aspirin -- 325 milligrams every other day. It may be that aspirin's cancer-fighting abilities show up only at higher doses, says Heath, an epidemiologist with ACS.

All involved believe that additional studies will eventually reveal aspirin's true role in colorectal cancer. Until then, Heath warns against popping aspirin to gain an edge against gastrointestinal cancers. Most people view aspirin as harmless, he says. Yet this drug can cause severe bleeding, damage to the lining of the stomach, and other ill effects, he points out.