Research unravels aspirin's role in preventing certain bowel cancers
Several large studies have suggested that aspirin can help protect against the disease in high-risk individuals. But new results suggest the drug only prevents cancers that have a certain molecular defect - over-production of an enzyme called COX-2.
Aspirin is known to affect several of the COX enzymes - which are involved in regulating inflammation - but this is the first major study to link COX enzymes to aspirin's ability to prevent bowel cancer.
The result could allow doctors to target aspirin-based bowel cancer prevention to people who need it.
Scientists from the Massachusetts General Hospital (MGH), Dana-Farber Cancer Institute and Brigham and Women's Hospital analysed data from two ongoing prospective research studies - the Nurses Health Study (NHS) and the Health Professionals Follow-Up Study (HPFS), both of which gather health data on participants every two years.
They focussed on nearly 83,000 NHS participants and around 47,000 HPFS participants, and analysed tumour specimens from over 600 participants who had been diagnosed with colorectal cancer for expression of the COX-2 enzyme.
Dr Andrew Chan, lead author and member of the MGH gastrointestinal unit, explained: "We knew that aspirin can block COX-2 function and that COX-2 is present in the vast majority of colorectal tumours but not in normal colon tissue.
"Therefore we hypothesised that, if blocking the COX-2 pathway was the mechanism underlying aspirin-associated risk reduction, it should preferentially reduce the incidence of those tumours that rely on COX-2."
The study showed that those participants who took at least two standard aspirin tablets a week had about three quarters the risk of colorectal cancer than those who did not take aspirin.
In addition, the reduction in risk only applied to tumours that expressed the COX-2 enzyme, and the incidence of tumours that did not express the enzyme was virtually the same, regardless of whether or not the participant took aspirin.
Dr Charles Fuchs, senior author and Dana-Farber researcher, said: "These results will allow us to test another hypothesis: that in patients who have had colorectal cancer or polyps in the past, expression of COX-2 in the earlier lesion might indicate those for whom aspirin could reduce the risk of recurrence.
"We hope that our future research will further clarify who would benefit most from regular aspirin therapy and that understanding the mechanism of this effect will lead to new preventive and treatment strategies."
However, the researchers emphasised that they do not generally recommend aspirin therapy to reduce the risk of colorectal cancer.
A Cancer Research UK spokesperson pointed out, "It's important to remember that long term use of aspirin can have risks as well as benefits and people should not take regular large doses over a long period of time without proper medical advice.
"Poor diet can contribute to bowel cancer risk. Eating a diet rich in vegetables and fruit, taking regular exercise and maintaining a healthy weight can help to reduce your risk."