Aspirin and Antiplatelet Medications
Aspirin for the Prevention and Treatment of Heart
Attacks and Strokes (Coronary and Cerebral Vascular Disease)
Medical Authors: Siamak Nabili, MD, MPH and Daniel
Kulick, MD, FACC, FSCAI
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
Additional Heart Attack Prevention Series Information (related articles)
What is aspirin?
Aspirin belongs to a class of medications called
nonsteroidal antiinflammatory drugs
(NSAIDs). Aspirin and other NSAIDs, for
example, ibuprofen (for
example, Motrin, Advil) and naproxen (for
example, Aleve), are widely
used to treat fever, pain, and inflammatory conditions such as arthritis,
tendonitis, and bursitis. Aspirin is known chemically as acetyl salicylic acid
and often abbreviated as ASA.
In addition to its effects on pain, fever, and
inflammation, aspirin also has
an important inhibitory effect on platelets in the blood. This antiplatelet
effect is used to prevent blood clot formation inside arteries, particularly in
individuals who have atherosclerosis (narrowing of the blood vessels) of their
arteries, or are otherwise prone to develop blood clots in their arteries.
What are antiplatelet agents?
Antiplatelet agents are medications that block
the formation of blood clots by preventing the clumping of platelets. There are
three types of antiplatelet agents:
- aspirin,
- thienopyridines, and
- glycoprotein IIb/IIIa inhibitors.
These agents differ in the way they work,
their potency (how strongly they prevent clumping), how rapidly they work, and
their cost.
What are platelets?
Platelets are particles (actually, remnants of cells) circulating in the
blood that are needed in order for blood clots to form. Platelets initiate the
formation of blood clots by sticking together (clumping or aggregating), a
process called platelet aggregation.
Clumps of platelets then are further bound together by a protein (fibrin) formed by clotting factors present in the blood.
The clumps of platelets and fibrin make up the blood clot.
Blood clots are important because they stop bleeding (for example, a cut or
laceration on the skin). When bleeding occurs from a cut, platelets become activated and form a network by attaching to the
blood vessel wall at the site
of bleeding, and by also attracting other clotting factors in the blood (such as
fibrin) to stop ongoing bleeding rapidly.
However, if a blood clot forms inside an artery, it blocks the
flow of blood to the tissue that the artery supplies, which can damage the
tissue. For example, a blood clot that forms in a coronary artery supplying
blood to the muscle of the heart causes a
heart attack, and a blood clot that
forms in an artery supplying blood to the brain causes a stroke.
How do antiplatelet agents work?
Aspirin
Aspirin prevents blood from clotting by blocking the production by platelets
of thromboxane A-2, the
chemical that causes platelets to clump. Aspirin accomplishes this by inhibiting
the enzyme cyclo-oxygenase-1 (COX-1) that
produces thromboxane A-2. While other NSAIDs also inhibit the COX-1 enzyme,
aspirin is the preferred NSAID for use as an antiplatelet agent because its
inhibition of the COX-1 enzyme lasts much longer than the other NSAIDs
(aspirin's antiplatelet effect lasts days while the other NSAIDs' antiplatelet
effects last only hours).
Thienopyridines
In addition to thromboxane A-2, platelets also produce adenosine diphosphate
(ADP). When ADP attaches to receptors on the surface of platelets, the platelets
clump. The thienopyridines, for example,
ticlopidine (Ticlid) and clopidogrel
(Plavix), block the ADP receptor. Blocking the ADP receptor prevents ADP from
attaching to the receptor and the platelets from clumping.
Glycoprotein IIb/IIIa inhibitors
The glycoprotein IIb/IIIa inhibitors, such as
abciximab
(Reopro) and eptifibatide (Integrilin), prevent clumping by inhibiting a
different receptor on the surface of platelets, the receptor for glycoprotein
IIb/IIIa. The glycoprotein IIb/IIIa inhibitors that are approved by the FDA must be given
intravenously (in the veins).
What is the relative potency of the antiplatelet agents?
- Since aspirin blocks only one of the several pathways by
which platelet aggregation can occur, aspirin is a weak antiplatelet agent
because platelet aggregation can be stimulated via another pathway.
- Since
glycoprotein IIb/IIIa inhibitors block the final common pathway for platelet
aggregation (platelet aggregation is blocked regardless of the nature of the initial stimuli),
glycoprotein IIb/IIIa inhibitors are the most potent antiplatelet agents. The
maximal antiplatelet effect of glycoprotein IIb/IIIa inhibitors is approximately
nine times that of aspirin.
- The maximal antiplatelet effect of thienopyridines is
in between that of aspirin and the glycoprotein IIb/IIIa inhibitors.
How quickly do antiplatelet agents work?
Aspirin
When aspirin is given in low doses (75 mg/day), the
complete inhibition of the COX-1 enzyme and hence maximal antiplatelet effect
may take several days. At a dose of 160-325 mg/day, the maximal antiplatelet
effect of aspirin occurs within 30 minutes. Thus, aspirin at low doses (75-150
mg/day) is used for the long term prevention of heart attacks and strokes,
whereas moderate doses (160-325 mg/day) of aspirin are given in situations where
an immediate anti-clotting effects are needed (such as in the treatment of acute
heart
attacks and unstable angina).
Thienopyridines
Like aspirin, the onset of action of clopidogrel (Plavix) is dose related.
Maximal antiplatelet effect occurs several days after initiation of clopidogrel
(75 mg/daily), but can occur within hours after larger doses of 300 or 600 mg.
Therefore, larger doses of clopidogrel are used initially when immediate
antiplatelet actions are needed (such as after placement of intracoronary
stents) while the lower doses are used as maintenance.
Glycoprotein IIb/IIIa inhibitors
The glycoprotein IIb/IIIa inhibitors have a rapid onset of
action. Their maximal antiplatelet effect occur within minutes after an
intravenous infusion, and are used mainly in patients with unstable angina or
acute heart attack (myocardial infarction).
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