Aspirin slashes colon-cancer death rates.

People who regularly take aspirin cut in half their risk of dying from colon cancer, according to the largest, most definitive epidemiologic study to investigate this link. The finding suggests a relatively inexpensive strategy for battling a scourage that currently claims the lives of about 50,000 Americans annually.

The new work also adds weight to the findings of a much smaller study reported earlier this year. Although that retrospective study of about 6,000 men and women did not address cancer-survival rates, it did indicate that regular aspirin use may reduce the incidence of colon cancer (SN: 3/16/91, p.166).

Spurred by those findings, epidemiologist Michael J. Thun and his colleagues at the American Cancer Society in Atlanta correlated aspirin use and colon-cancer deaths among the 662,424 men and women they had been tracking since 1982 as part of the society's Cancer Prevention Study. Upon entering the study, participants had answered a questionnaire covering a range of behavioral, dietary and lifestyle factors, including frequency of aspirin use during the past year.

The risk of dying from colon cancer decreased with increasing aspirin use, Thun's team reports in the Dec. 5 NEW ENGLAND JOURNAL OF MEDICINE. Colon-cancer death rates among the most frequent users -- those taking aspirin 16 times or more per month -- were only 60 percent as high as that seen in the study's aspirin asbtainers. And when the researcher controlled for risk factors such as diet, obesity and family history of colon cancer, the risk dropped to 50 percent of the colon-cancer death rate of the aspirin abstainers.

"It's an interesting finding," says epidemiologist John A. Baron of Dartmouth Medical School in Hanover, N.H. He notes, however, that the new study does not prove that aspirin itself helps prevent colon cancer. For example, he points out that side effects of frequent aspirin use, such as intestinal bleeding, may have caused members of this subgroup to seek medical attention more frequently than other volunteers, thereby increasing the likelihood that any developing colon cancer would receive early diagnosis. And early diagnosis increases the chance that a person with colon cancer will survive.

A large, randomized trial in which some people take aspirin and others take placebo pills would provide firmer evidence of aspirin's protection against colon cancer. But such a gold-standard study might be difficult to conduct, Baron says, because people who know about aspirin's widely publicized heart benefits (SN: 7/27/91, p.55) might balk at the prospect of receiving placebo pills instead.

For now, Baron and Thun remain cautious about advocating regular aspirin use for cancer prevention, since it can cause potentially dangerous side effects. At the same time, they note that aspirin may provide secondary anticancer benefits for people who already take it to manage arthritis pain or to reduce the risk of heart disease.

Even if epidemiologists can demonstrate that aspirin fights colon cancer, there remains the question of how it exerts this effect. Like other nonsteroidal anti-inflammatory drugs (NSAIDs), aspirin inhibits the synthesis of compounds called prostaglandins, which spur body cells -- including colon ceels -- to proliferate. Thun suggests that aspirin might prevent rampant cell division -- a key attribute of cancers -- by interfering with prostaglandin production.

A number of laboratory studies have shown that aspirin and other types of NSAIDs inhibit the growth of chemically induced colon tumors in rats and mice. Others studies have shown that an NSAID called sulindac can shrink large-bowel polyps in people. Such polyps, though benign, can develop into cancers, Thun notes. Taken together, the individual pieces of evidence provide scientists with compelling reasons to further explore the link between aspirin and colon cancer, he contends.