Aspirin seems to reduce the risk of colorectal cancer by blocking an enzyme that causes inflammation, researchers have found.

The COX-2 enzyme plays a role in causing arthritis pain and swelling, and ASA blocks the function of the enzyme.

Dr. Andrew Chan of Massachusetts General Hospital in Boston and his team knew COX-2 is found in the majority of colorectal tumours but not normal tissues.

Chan and his colleagues hypothesized that if Aspirin helps colorectal cancer by blocking COX-2 then the drug should work on tumours relying on the enzyme.

As suspected, Aspirin did lower the risk of developing colorectal cancer in people with tumours showing above normal amounts of the COX-2 enzyme, the team reports in Thursday's issue of the New England Journal of Medicine.

The researchers found the link by analyzing medical records and pathology reports from people diagnosed with colorectal cancer in two ongoing studies, the Nurses Health Study and the Health Professionals Follow-Up Study.

As in previous studies, among the more than 120,000 participants, those who took at least two standard tablets of Aspirin a week showed about three-quarters the risk of colorectal cancer as people who did not take the drug.

The age-standardized incidence rate for COX-2 cancers was 37 per 100,000 person-years among regular Aspirin users, compared with 56 per 100,000 person-years among those who did not use Aspirin regularly.

Preventing recurrence?

The reduced risk was only found in tumours positive for COX-2, while the the rate for cancers in COX-2 negative tumours was about the same between the two groups.

The study's authors are not recommending that people in general take Aspirin to lower their chance of developing colorectal cancer. For most people, screening is the best prevention, since polyps can be treated before they become cancerous.

People who have had colorectal cancer and are trying to prevent a recurrence should talk to their doctor about the advisability of taking Aspirin, said the study's senior author, Dr. Charles Fuchs of the Dana-Farber Cancer Institute in Boston.

Testing whether Aspirin reduces recurrence among people who have had colorectal cancer or polyps that are positive for COX-2 is the next step, Fuchs said.

Colorectal cancer is the third most common cancer in Canada, with about 20,000 new cases diagnosed last year, and an estimated 8,500 deaths annually across the country.