Aspirin exerts its therapeutic effects by inhibiting cyclooxygenase enzymes; this property makes aspirin useful as a pain reliever and a preventive for strokes, heart attacks, and blood clots.
Cyclooxygenase is responsible for the formation of prostaglandin-like molecules, which take part in a wide variety of metabolic and immunologic activities, including inflammation, cellular growth, and coagulation.
Colon Cancer and COX-2 (Cyclooxygenase-2)
- The majority of colon cancers begin as adenomatous polyps along the inner wall of the large intestine. Ostensibly, the cells in these polyps eventually undergo transformation to become malignant tumors; screening prior to transformation forestalls the development of full-blown cancer.
- Most colon cancers—and supposedly most colon polyps—produce cyclooxygenase 2 (COX-2), which helps to promote the growth and spread of tumor cells.
- By inhibiting COX-2, aspirin seems to prevent the formation of adenomas in the colon and apparently slows the proliferation of established colon cancers.
- According to the authors of a recently completed observational study at Massachusetts General Hospital and Harvard Medical School, aspirin probably reduces the risk of death for individuals who already have colon cancer. (Chan A, et al. Aspirin use after colorectal cancer diagnosis associated with improved survival. JAMA. 2009;302[6]:649-659)
- Dr. Chan and his colleagues suggest that aspirin may have therapeutic potential for selected patients with newly-diagnosed colorectal cancers and even for those whose cancers have already metastasized.
- However, in keeping with consensus opinion surrounding this issue, these investigators concede that routine use of aspirin or similar agents cannot be recommended in light of concerns over aspirin’s known side effects.
The USPSTF and Current Recommendations Regarding Aspirin for Preventing Colon Cancer
The U.S. Preventive Services Task Force (USPSTF) is an independent panel of experts in primary medical care and prevention that systematically reviews available scientific data and develops recommendations for clinical preventive services.
USPSTF recommendations are graded according to the strength of available information (the “quality of evidence”); while USPSTF recommendations are typically adapted into mainstream medical practice, physicians and other health care practitioners are not necessarily bound by them.
USPSTF recommendations may conflict with those of other scientific organizations, such as the National Institutes of Health, the American College of Physicians, or medical specialty groups.
In developing its recommendation, the USPSTF considered the following:
Evidence for Purported Benefits of Aspirin and NSAID Use
- There is fair to good evidence that aspirin and non-steroidal anti-inflammatory drugs (NSAIDs), taken in higher doses for longer periods, reduce the incidence of adenomatous colon polyps.
- There is good evidence that low-dose aspirin does not lead to a reduction in the incidence of colorectal cancer.
- There is fair evidence that aspirin, used in doses higher than those recommended for prevention of cardiovascular disease, and NSAIDs may be associated with a reduction in the incidence of colorectal cancer.
- There is fair evidence that aspirin used over longer periods may be associated with a reduction in the incidence of colorectal cancer.
- There is poor-quality evidence that aspirin and NSAID use leads to a reduction in colorectal cancer-associated mortality rates (the strength of this evidence may now change, in light of the 2009 JAMA study and another trial currently ongoing in Singapore).
Evidence for Harm from Aspirin and NSAID Use
- There is good evidence that aspirin increases the incidence of gastrointestinal bleeding in a dose-related manner, and fair evidence that aspirin increases the incidence of hemorrhagic stroke.
- There is good evidence that NSAIDs (including aspirin) increase the incidence of gastrointestinal bleeding and renal impairment (kidney failure), especially in older persons.
- There is good evidence that COX-2 inhibitors increase the incidence of renal impairment. COX-2 inhibitors appear to be associated with an increased risk of cardiovascular events (heart attack and stroke).
- Overall, there is good evidence of at least moderate harms associated with aspirin and NSAIDs.
Given the strength of current evidence, the USPSTF recommends against the routine use of aspirin and non-steroidal anti-inflammatories (NSAIDs) to prevent colorectal cancer in persons at average risk of colorectal cancer.
Clearly, recommendations from expert bodies can change as new research changes the strength of available evidence. While the body of evidence now weighs against the routine use of aspirin for preventing colon cancer, aspirin (and other NSAIDs) might find a home in the treatment of already-established tumors.
And, to add complication to an already convoluted issue, there are dozens of herbal sources of COX-2 inhibitors…one of them might even offer all of the benefits of aspirin without carrying the same burden of harm.
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