The results "strongly support" the idea that the main blood thinner used in the study, a drug called warfarin, blocks the growth of cancer cells, a team of researchers led by Dr. Sam Schulman, of the Karolinska Hospital, in Stockholm, Sweden, reports in the June 29th issue of The New England Journal of Medicine.
The findings are bound to be controversial, according to the report, however, since the mechanism behind the drug's apparent protective effects is still uncertain. Only large studies that track the effects of blood thinners over time will resolve the issue, the researchers note.
The study included more than 800 people who experienced a blood clot in a lung-- a condition known as pulmonary embolism--or a leg, a disorder known as deep-vein thrombosis. To reduce the risk of having another clot, all the patients took a blood-thinning drug--in most cases warfarin. But some study participants were randomly assigned to take a blood thinner for just 6 weeks, while others took the medicine for 6 months.
A little more than 8 years later, 13% of people in the study were diagnosed with cancer for the first time, but the rate was greatest in people who had taken a blood thinner for just 6 weeks, the researchers report. About 10% of people who took the longer course of the drug developed cancer, compared with nearly 16% of people treated for 6 weeks. Most of the difference in the cancer rate was due to an increased rate of cancers of the kidney, bladder, prostate, ovary and uterus in the 6-week group, according to the report. Several other factors were also associated with an increased risk of cancer, including older age and having a blood clot recur.
Despite the difference in cancer rates, however, the rate of cancer-related deaths was similar in both groups.
Blood-thinning drugs hold promise as a way to prevent cancer, although there is not enough evidence to justify prescribing blood-thinning drugs to cancer patients or to healthy people who have an increased risk of cancer, according to Drs. Christoph C. Zielinski and Michael Hejna, of University Hospital, in Vienna, Austria.
But they note in an editorial that accompanies the study that "we must take the current results seriously enough to encourage the study of warfarin in controlled clinical trials, perhaps primarily with the aim of assessing prevention rather than treatment of cancer."
"It may very well be that we are holding in our hands a drug that could be added to the armamentarium of chemoprotective agents," they conclude.
SOURCE: The New England Journal of Medicine 2000;342:1953-1958, 1991-1993.