Anti-Clotting Effect of Aspirin Wanes Over Time

NEW YORK (Reuters Health) - Aspirin is widely used to prevent platelets in the blood clumping together and forming clots, but physicians in Rome report that aspirin's anti-platelet action declines with prolonged treatment.

However, this is not seen with another anti-clotting drug called ticlopidine, also known as Ticlid.

Previous research has suggested a progressive decrease in aspirin's clinical efficacy after two years of treatment, Dr. Fabio M. Pulcinelli and his associates explain in the Journal of the American College of Cardiology. To investigate further, the research team at the University "La Sapienza" monitored blood samples from 150 patients for two years after they started anti-platelet treatment.

In the lab, the maximal percentage of platelets that could be made to clump together decreased from 88 percent at the start of the study to 38 percent after two months. Thereafter, clumping increased progressively to 46 percent at 6 months, 48 percent at 12 months and 62 percent at 24 months.

In contrast, in a group of 80 similar patients, the anti-platelet effects of ticlopidine did not change significantly during two years of treatment.

The authors point out that other researchers have observed a reduction in cardiovascular events when treatment with aspirin was combined with clopidogrel, another anti-platelet agent.

"Future studies should therefore address the question of whether the superiority of such a combination is dependent on a more complete and efficient inhibition of platelet aggregation or on a protective effect provided by clopidogrel in patients with a progressively reduced sensitivity to aspirin," Pulcinelli's team concludes.

SOURCE: Journal of the American College of Cardiology, March 17, 2004.

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