Antiplatelet Therapy

Before we take a look at Antiplatelet Therapy, let's get an understanding of what Platelets are.

Your blood contains key components including...

• red cells
• white cells
• platelets and...
  
• plasma

Platelets are fragments of bone marrow cells called Megakaryocytes. Platelets break off from the Megakaryocytes and enter the blood stream after being stimulated to do so by a hormone called Thrombopoietin

A platelet will circulate in your blood stream for about 7 to 10 days..the extent of its lifespan...after which, it is removed from the blood by the liver or spleen.

Healthy human blood contains about 150,000-450,000 platelets per milliliter of blood

Blood Clotting:

Platelets provide a mixture of hormones and proteins that are necessary for blood to coagulate.

When a blood vessel is damaged...which could be the result of external trauma...or internal injury caused by a ruptured plaque in an artery wall or from blood vessel injury sustained as part of the normal angioplasty and stent implantation procedure...platelets rush to that damaged area and begin to form a clump.

Antiplatelet Therapy interferes with the platelet 'clumping mechanism'.

When Platelets are stimulated to clot... they stick to each other and also release chemicals which...

• cause the blood vessels to narrow to reduce the amount of blood loss
• attract even more platelets to form a larger clump
• stimulate the synthesis of a protein called Fibrin.....causing the platelets to form a clot

Collagen is also released during blood vessel injury. Platelets recognize this collagen release and are stimulated to begin the coagulation process.

If a blood clot forms inside an artery it can cause a partial or complete blockage impeding the flow of blood to the tissue that the artery supplies. This process can damage the tissue because it is being starved of required oxygen and nutrients contained in the blood.

If this situation is not rectified quickly, the affected tissue can die. This is what happens during the process of a Heart Attack or Myocardial Infarction (MI)...antiplatelet therapy can be used to great effect in this setting.

What are Antiplatelet Therapy Agents?

Medications or pharmaceutical drugs that interfere with the process that forms blood clots and inhibit the clumping together of Plateletes are called Antiplatelet Therapy Agents.

There are three categories of Antiplatelet drugs available:

• NSAIDs - particularly Aspirin
• Thienopyridines
• Glycoprotein IIb/IIIa Inhibitors

Aspirin

Aspirin is a member of the family of Nonsteroidal Anti Inflammatory drugs known as NSAID's which also have an antiplatelet effect.

Aspirin interferes with the clotting process by inhibiting the action of an enzyme that is involved in the production of a chemical that causes platelets to clump together.

As far as Coronary Heart Health is concerned, Aspirin is commonly used as an antiplatelet agent in otherwise healthy people with significant risk factors for developing the disease. It is thought that daily low to moderate doses are safe...although there are risks of bleeding and GI tract irritation associated with aspirin use in some individuals.

Thienopyridines

Platelets store a chemical called Adenosine Diphosphate also known as ADP.

ADP is stimulated to release when the platelet becomes activated. The ADP chemical positions itself on the platelet cell surface, locking onto ADP cell receptors. This causes the platelets to clump.

A couple of examples of Thienopyridines are...

• Ticlopidine - Ticlid
• Clopidogrel - Plavix

These drugs block the ADP receptor sites on the platelet cell surface called the P2Y12 receptor...thereby inhibiting the platelet clumping process. There is an association of increased risk of developing a condition called Thrombotic Thrombocytopenic Purpura or TTP with the use of Ticlopidine. This is a disorder of the blood-coagulation process which results in the formation of extensive array of microscopic blood clots in the small blood vessels throughout the body.

because of this, Clopidogrel tends to be favored because the risk of developing this condition while on Clopidogrel therapy is very low.

Both these drugs are indicated for use in patients that...

• carry a high risk of having a Coronary Ischemic Event due to advanced Atherosclerosis
• are having a Heart Attack - Myocardial Infarction (MI). Aspirin may also be used as an associated Antiplatlet medication in this setting
• have just undergone Coronary Angioplasty and / or Coronary Stent implantation. After these procedures, Clopidegrel may be prescribed as an ongoing medication

Glycoprotein IIb IIIa Inhibitors

A Glycoprotein is a molecule that consists of a carbohydrate or a sugar that is joined to a protein. Glycoproteins are 'tree - like' structures that reside on the surface of the cells in your body...such as platelets in your blood.

Glycoproteins provide receptor sites for other cells, hormones, viruses, bacteria and drugs to attach themselves to the cell and communicate directly with it...affecting its operation...either positively or negatively.

Glycoproteins are very important for the correct function of your body systems. For example, in the immune system, almost every key molecule involved in the immune system response is a glycoprotein.

Glycoprotein IIb IIIa inhibitors inhibit the stimulation response mechanism...the clumping of platelets...by competing with the IIb IIIa Glycoprotein receptors on the surface of a platelet

These platelet cell surface receptors become activated when the platelet is stimulated by...

• Adenosine Diphosphate or ADP
  
• Epinephrine
  
• Collagen
  
• Thrombin

Here are Three GP IIb/IIIa inhibitors commonly available:

• Abciximab - ReoPro
  
• Eptifibatide - Integrilin
• Tirofiban - Aggrastat

Abciximab - ReoPro:

• for use in Percutaneous Coronary Intervention - PCI...also known as angioplasty...with or without stent implantation.
• associated with a decreased incidence of ischemic complications (partial or complete blockage of the treated artery) as a direct result of the procedure
  
• associated with decreased need for repeat PCI procedures up to four weeks immediately after the initial procedure

Possible side effects may include...

• general increased risk of bleeding
• gastrointestinal hemorrhage

Eptifibatide - Integrilin:

• derived and synthesized from a protein in the venom of the Pygmy rattlesnake - Sistrurus miliarus barbouri
• used to reduce the risk of acute cardiac ischemic events - Heart Attack - in patients with unstable (uncontrollable) angina
•  used in patients who are undergoing PCI procedures and in those who are being treated conservatively...with no surgical intervention.
• usually used in association with Aspirin or Clopidegrel and Heparin

Possible side effects may include...

• Bleeding, possibly severe
• Hypotension

Tirofiban - Aggrastat:

• for use in the management of patients with unstable angina and myocardial infarction.
• for patients undergoing PCI procedures
• helps decrease the rate of ischemia and myocardial infarction due to platelet aggregation.
• used in combination with heparin and aspirin

Glycoprotein IIb IIIa Inhibitors are the most potent antiplatelet therapy agents available. Because of potential bleeding and hypotension complications...and because of the serious nature of the condition of many of the patients that this antiplatlet drug category is used on, you will find GP IIb IIIa Inhibitors used in a clinical or hospital setting.

Aspirin on the other hand is generally quite safe to use in low to moderate doses on a daily basis...its antiplatlet effect much milder and slower acting than that of GP IIb IIIa Inhibitors.

Thienopyridines are somewhere between GP IIb IIIa Inhibitors and Aspirin in terms of potency. Patients can safely use Clopidegrel in the home setting as long as the prescription guidelines are strictly followed.

Antiplatelet Therapy is an important treatment option available which can help to reduce ischemic injury risk associated with platelet aggregation ..or stickiness.

Mild Antiplatelet Therapy such as Aspirin is generally safe in the otherwise healthy individual. More potent forms of Antiplatelet medication require the expert guidance of a health care professional.

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