Antiplatelet drugs

Antiplatelet drugs prevent the aggregation of platelets and play a major role in conditions of arterial thrombosis such as myocardial infarction.

Many of these drugs can be taken on regular basis for long term prevention of platelet aggregation and thereby preventing attacks of myocardial infarction.

Most commonly used antiplatelet agent on long-term basis is low-dose aspirin in the dose of 75 mg or 81 mg or 100 mg per day. It inhibits formation of Thromboxane A2, and thus inhibits aggregation of platelets. (Anti-aggregatory action on platelets / antiplatelet action).

Aspirin is useful not only on long-term basis to prevent the chances of myocardial infarction; but is also of value in Acute myocardial infarction/Acute coronary syndrome. In Acute myocardial infarction it is used in the dose of 150 to 325 mg.

The preferred form of aspirin is soluble/dispersible aspirin, which will reach the circulation as rapidly as possible.

[Be careful on  your concept: Aspirin is not used as analgesic ; it is used as antiplatelet agent ] (!!!!)

Mechanism of action:

Aspirin inhibits Thromboxane A2 formation.  Thus In Acute myocardial infarction, Aspirin inhibits –  ongoing platelet aggregation if any in process, and thus halts the thrombus extension, and the progress of the event; and helps in restricting the infarct size. This is valuable in decreasing the mortality.

SAQ/LAQ/Viva

Mention the examples of antiplatelet drugs based on their mechanism of action.

1.TXA2 inhibitor

• Low dose aspirin: Secondary/primary prevention MI, Acute MI to halt progression, reduce infarct size. Low dose aspirin is the safest, most efficacious, and least expensive antiplatlet drug.

2. Drugs inhibiting ADP binding (ADP receptor antagonists – P2Y12 Antagonists)

• -Clopidogrel/Prasugrel/Ticagrelor:
• Clopidogrel is useful in patients with aspirin allergy-(ess incidence of neutropenia as compared to Ticlopidine). Sometimes combined with Aspirin
• -Ticlopidine: is a prodrug. Useful in patients with aspirin allergy. Can produce adverse effects like Neutropenia, Thrombotic thrombocytopenic purpura, Bleeding, skin rashes, diarrhea, and liver dysfunction

3. Drugs inhibiting GP IIb/IIIa receptor: (Useful in acute coronary syndrome: Rapid acting)

• Abciximab (monoclonal antibody to GP IIb/IIIa receptor)
• Eptifibatide, Tirofiban (Antagonists of GPIIb/IIIa receptor)

4. Protease Activated Receptor Antagonist: (PAR-1 Antagonists):

• Vorapaxar

5. PGI2 analogue (Prostacyclin analogue):

• Epoprostanol: Used to prevent platelet aggregation during hemolysis, and in  treatment of pulmonary hypertension

6. Phosphodiesterase Inhibitors:

• Dipyridamole