Common aspirin coating reported to reduce its effectiveness
Aspirin is a common over-the-counter medication used to reduce pain and inflammation; a low dose formulation (81 mg) is also taken to reduce the risk of heart attack and stroke. Many individuals favor the enteric coated form due to concern regarding stomach irritation from the uncoated form.
A new study has reported that the enteric coating may reduce the effectiveness of aspirin. Researchers affiliated with the University of Pennsylvania published their findings online on December 4.
The researchers noted that clinical studies have reported that low dose aspirin reduces the secondary incidence of myocardial infarction and stroke. They also noted that some researchers support the viewpoint that drug resistance to aspirin might result in treatment failure. For more than a decade, cardiologists and drug researchers have suggested that anywhere from 5-40% of the population is aspirin resistant. The study authors note that, despite this concern, no clear definition of aspirin resistance has been developed; furthermore, estimates of its incidence vary widely. Therefore, they designed a study to determine whether some individuals were truly aspirin resistant, possibly due to genetic factors.
The study group comprised 400 healthy volunteers who were screened for their response to a single oral dose of 325 mg immediate release or enteric coated aspirin. The activity of the aspirin was determined by measuring the activity of aspirin's molecular target, cyclooxygenase-1. If subjects were found to be aspirin resistant on one occasion, they underwent repeat testing and if still resistant were exposed to low dose enteric coated aspirin (81 mg) and clopidogrel (Plavix; 75 mg) for one week each. The investigators found that absorption varied to a significant degree (up to 40%) if a single dose of 325 mg enteric coated aspirin was given; however, variability was not found if the subjects were given an uncoated, immediate release aspirin (0% variability). All the subjects responded to aspirin upon repeated exposure, extension of the post dosing interval or addition of aspirin to their platelets ex vivo (aspirin added to a blood sample).
The authors concluded that aspirin resistance is extremely rare because the study failed to identify a single case of true drug resistance. However, they noted that pseudo-resistance, reflecting delayed and reduced drug absorption, occurred with enteric coated but not immediate release aspirin administration.
Take home message:
This study notes that an enteric coating may reduce the effectiveness of aspirin up to 40%; thus, the question becomes: is this an adequate dosage to reduce the risk of cardiovascular disease? Some individuals may find that an uncoated low dose aspirin tablet can be well tolerated. Another option would be to take one 325 mg enteric coated tablet daily; that would provide a more than adequate dose even if effectiveness was reduced by 40%.
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