Does aspirin prolong survival after diagnosis of colon cancer?

Colon cancer, unfortunately, is a common and potentially lethal disease. The risk of colon cancer is 50% higher in overweight individuals, relative to people who are not overweight. Obesity also increases the risk of dying from colon cancer.

The good news is that aspirin appears to increase lifespan in people who have had non-metastatic colon cancer, according to a study published in this week’s JAMA (Journal of the American Medical Association).

Dr. Andrew Chan and colleagues conducted an observational study of 1279 men and women who were diagnosed with colon cancer, designed to examine the impact of aspirin use after the diagnosis on both colon cancer-specific, and overall, survival.

They found that after 12 years of follow up, aspirin users had a 29% lower colon cancer-specific mortality and a 21% lower overall mortality than nonusers. In people who did not use aspirin before diagnosis, but started using it after diagnosis, the effect was greatest, with a 47% decrease in the risk of colon cancer-specific death.

The link between aspirin and colon cancer is an enzyme called COX-2 (or cycloogenase 2), which promotes inflammation and multiplication of cells. COX-2 is overexpressed by 80-85% of colon cancers, and in these colon cancers, is likely promoting growth of the tumor.

Aspirin works by inhibiting the COX enzymes, of which there are actually 2 types:

COX-1 is a ‘housekeeping’ enzyme that is expressed in most tissues in the body, regulating normal cellular functions, including maintaining protection of the lining of the stomach, as well as blood clotting.
COX-2 is the enzyme that mediates the inflammatory response (as above). This mechanism is also what makes us feel better when we take aspirin for headaches or inflammatory pain.

In light of aspirin’s ability to decrease COX-2 activity, it is therefore not surprising that Chan’s study found benefit for aspirin use only in those patients whose colon cancers produce COX-2 (remembering that most colon cancers do express COX-2).

The benefits of aspirin, of course, have to be balanced against the potential risks. The side effects of aspirin stem from the unwanted effect that aspirin has on COX-1. The most significant of these effects is the risk of bleeding, especially from stomach ulcers.

It is important to point out that this study was an observational study. In other words, it was not the gold standard ‘randomized controlled trial’ that we like to hang our hat on – there is potential for error in the conclusions. While there is certainly a suggestion that aspirin is beneficial to patients who have had colon cancer (and this has been suggested in other studies as well), we are not at a point where it can be recommended routinely. Further studies are required, and in fact, one such randomized trial is already underway.