Enteric-coated aspirin is less potent than plain aspirin

Enteric-coated aspirin is less potent than plain aspirin, which may make it less effective in preventing heart disease, according to a preliminary small study presented today at the American Heart Association’s 5th Annual Conference on Arteriosclerosis, Thrombosis and Vascular Biology.

 “At the low doses of 75 milligrams (mg) a significant percentage of people are not receiving sufficient aspirin,” said lead author Dermot Cox, B.Sc., Ph.D., lecturer in the department of clinical pharmacology at the Royal College of Surgeons in Dublin.  “This is a problem particularly for overweight patients.  A 200-pound patient has a 40 percent probability of not being adequately treated on low-dose, enteric-coated aspirin.”

Several large clinical studies have shown the benefits of moderate and high-risk patients taking aspirin daily to prevent heart attacks.   “However, it is also known that a significant number of patients—up to 30 percent—are resistant to the effects of aspirin,” Cox said.  This study may help explain one reason for resistance.

“Since aspirin is known to cause stomach ulcers, low doses of 75 mg are used to try and reduce these effects.  Some manufacturers also coat aspirin with an acid-resistant coating.  This means that the aspirin is absorbed in the colon rather than in the stomach.”  

Cox and colleagues looked at the effectiveness of “coated” aspirin because the Irish Medicines Board (the Irish equivalent of the U. S. Food and Drug Administration) was concerned that manufacturers of new enteric-coated aspirins needed to prove that their products would be as effective as aspirin brands already on the market.

Researchers studied 75 healthy volunteers who were on no other medications.   They studied five different aspirin preparations: a 75 mg uncoated aspirin, a 75 mg dose of three enteric-coated brands (Nu-Seals, Caprin and ProTek), and twice-daily Asasantin (an anti-platelet drug that includes 25mg aspirin and 100 mg dipyridamole). 

There were three separate studies:   Protek and Caprin were tested on at least 23 volunteers each in separate studies.  The comparison drug in each case was 75 mg Nu-Seals.  The third comparison was between Asasantin and 75 mg of soluble aspirin.  The three comparisons were done in a “cross-over” design, so that volunteers took one type of aspirin for 14 days, then spent the next 14 days free of aspirin before “crossing over” to take the other aspirin preparation for 14 days.  Blood samples were taken at the start and end of each study.   

After each aspirin regimen, researchers measured the participants’ levels of serum thromboxane, which is the product of cyclooxygenase, the enzyme inhibited by aspirin.   Lower levels of thromboxane mean blood platelets are less likely to stick together, thus a more effective aspirin preparation.  The lowest level of thromboxane, at 0.28 nanograms per milliliter(ng/ml) of blood, was found in participants taking 75 mg plain aspirin.  People taking Asasantin had an average level of 1.85 ng/ml; Caprin, 2.24 ng/ml; and those taking Nu-Seals had a thromboxane level of 2.75 ng/ml.  People taking Protek had the highest thromboxane level, at 5.5 ng/ml.

“Thus, plain aspirin and Asasantin had the highest levels of thromboxane inhibition and Protek had the lowest levels of inhibition,” Cox said.

Doctors should be aware of the potential problems associated with low-dose, enteric-coated aspirin preparations, he said.   In heavier patients a doctor should consider assessing platelet function, using a plain aspirin or a higher dose enteric-coated preparation.

People who have been told by their doctor to take aspirin should not change their dose without talking to their doctor.

Co-authors are Andrew Maree, Michelle Dooley, Michael F. Byrne, Ronan Conroy and Desmond J. Fitzgerald. http://www.americanheart.org

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