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Why aspirin may not always prevent a heart attack: researchers uncover a new reason for "aspirin resistance".

When taking a drug is a matter of life and death, knowing that the drug will work is an absolute necessity. This is the case with aspirin, which is known to prevent heart attacks in patients at increased risk.

Aspirin is often prescribed to prevent platelets from clumping together and blocking blood flow through an artery and causing heart attack. It's particularly important in people with diabetes, especially when they are obese, because both factors can adversely affect how well aspirin works.

Studies suggest a higher dosage of aspirin taken twice daily may improve how well the drug works in patients with diabetes, but to date no clinical trials have been performed to determine whether this regimen prevents heart attack. Now it appears the formulation of aspirin may influence response.

How Aspirin's Efficacy Was Evaluated.

Researchers gave 40 obese patients with diabetes three different types of aspirin in 325 mg doses: plain, modified-release (liquid aspirin-filled capsules) and delayed-release enteric-coated (the kind designed to protect your stomach).

Each patient took one of the aspirin types for three days. After two weeks, they switched types. After another two weeks, they switched again until they had taken all three forms of aspirin.

How well the aspirin worked was determined by blood tests after each three-day dosing session. The tests evaluated levels of serum thromboxane, which is associated with risk of blood clotting. Aspirin has to lower those levels to a certain point to prevent a clot that can cause heart attack. If aspirin lowers levels somewhat, but not enough, it won't be protective.

Patients were considered nonresponsive (not protected) if their serum thromboxane level remained high enough to raise the risk of blood clotting within 72 hours after taking three doses of aspirin. In other words, the aspirin was not doing its job of reducing the risk of clotting.

What the Researchers Found. The modified-release type of aspirin did the best job by protecting 92 percent of patients. Normal aspirin also did well, with only 16 percent of patients failing to achieve adequate protection against clotting.

As explained in the Feb. 14, 2017, issue of Journal of the American College of Cardiology, the enteric-coated aspirin failed to completely inhibit thromboxane in 53 percent of patients, presumably because it was poorly absorbed. Moreover, there was tremendous individual variability in the rate of protection, or nonresponsiveness, with this type of aspirin, making its usefulness unpredictable. In fact, the researchers were unable to detect any aspirin in the blood in some patients taking enteric-coated aspirin.

Two patients failed all three kinds of aspirin, making them truly aspirin-resistant.

What This Means for You. Why aspirin sometimes fails has been well studied. Sometimes, it's because patients do not take the drug as prescribed. Others stop when it bothers their stomach or causes internal bleeding. But the underlying reason for aspirin resistance in compliant patients has long been questioned. This study implies that the type of aspirin a patient takes may, indeed, make a difference in how well it works. As the liquid form of aspirin capsules is not yet commercially available, plain aspirin would be preferred over the enteric-coated version.

Still Recommend Patients with Heart Disease Continue with Aspirin

"Heart attacks are commonly caused by blood clots in the heart's arteries. Aspirin, a relatively safe drug, has been proven to prevent blood clots and treat them after they occur, but aspirin may not work equally well among all patients. The findings by Bhatt et al suggest enteric-coated aspirin may be less effective at treating blood clots. This analysis underscores how much we still have to learn about aspirin and heart disease. The Duke Clinical Research Institute is coordinating a 20,000-patient clinical trial called ADAPTABLE (Aspirin Dosing: A Patient-centric Trial Assessing Benefits and Long-Term Effectiveness) and approximately 2,000 Duke patients with heart disease are being enrolled to assess the best aspirin dose (81mg or 325mg). Despite the findings of this small study, we recommend that patients with heart disease continue to take aspirin and ask their doctors about any new developments."
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Publication:Duke Medicine Health News
Date:Apr 1, 2017
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