NEW YORK (Reuters Health), May 11 - At a dose of 300 mg daily for at least five years, treatment with aspirin markedly reduces the incidence of colorectal cancer a decade later, according to long-term follow-up of randomized trials and observational studies.
Previously published findings from large randomized trials and some observational studies revealed no effect of aspirin on colorectal cancer. But Drs. Enrico Flossmann and Peter M. Rothwell, writing in the May 12th issue of the Lancet, contend that aspirin doses were too low and follow-up inadequate.
The two British neurologists, from Radcliffe Infirmary in Oxford, theorized that aspirin would not significantly reduce the risk until after a latency period of 10 years.
They evaluated two controlled trials with follow-up data for a median of 23 years. In the British Doctors Aspirin Trial (n = 5139) and the U.K. Transient Ischaemic Attack (UK-TIA) Aspirin Trial (n = 937), the treatment period lasted for at least five years, with randomization in a 2:1 ratio for aspirin versus control treatment. Doses were 300 mg or higher.
In the intention-to-treat analysis, neither study alone nor combined revealed a reduction in incidence of colorectal cancer during the first 10 years of follow-up. However, significant differences during the period from 10 to 14 years were observed in both trials, separately and together (adjusted hazard ratio for combined data = 0.37, p = 0.002).
The risk reduction was no longer statistically significant during continued assessment for more than 15 years, the investigators state.
Dr. Flossmann and Dr. Rothwell also reviewed observational studies, including 19 case-control studies (n > 20,800) and 11 cohort studies (n > 1 million). Findings were similar to the results from the two clinical studies, with 50% to 70% reductions in odds of colorectal cancer after 10 years.
In subgroup analysis, the preventive effect was still significant when analyzing outcomes by age, sex, race, or country. It remained significant in a subgroup of subjects with a family history of colorectal cancer. The data also suggest that the risk reductions were strengthened when aspirin was continued for up to at least 20 years.
In an accompanying editorial, Dr. Andrew T. Chan, at the Massachusetts General Hospital in Boston, states that these data provide "proof of principal" that aspirin, "at biologically relevant doses, can reduce the incidence of colorectal cancer."
Both the investigators and the editorialist state that further research is required before aspirin can be recommended as primary prophylaxis for colorectal cancer.
Last Updated: 2007-05-10 19:01:13 -0400 (Reuters Health)
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