Antiplatelet Drug Mechanisms
How antiplatelet drugs work at the molecular level, from aspirin's COX-1 inhibition to P2Y12 receptor antagonists and GP IIb/IIIa inhibitors.
## Overview
Platelets are central to arterial thrombosis, which underlies myocardial infarction and ischemic stroke. Antiplatelet drugs interrupt platelet activation and aggregation at distinct molecular steps. Dual antiplatelet therapy (DAPT) combining two agents from different classes is standard after acute coronary syndromes and coronary stenting.
## Aspirin: COX-1 Inhibition
Aspirin irreversibly acetylates cyclooxygenase-1 (COX-1) in platelets, permanently blocking thromboxane A2 (TXA2) synthesis. TXA2 is a potent platelet activator and vasoconstrictor. Because platelets lack nuclei and cannot synthesize new COX-1, aspirin's effect lasts the platelet's entire 7-10 day lifespan. Low-dose aspirin (75-100 mg daily) selectively inhibits platelet COX-1 while preserving endothelial prostacyclin (COX-2-derived) production.
## P2Y12 Receptor Antagonists
ADP released from activated platelets binds P2Y12 receptors, amplifying aggregation. Blocking this pathway provides additive antiplatelet effect to aspirin.
- **Clopidogrel**: Prodrug requiring CYP2C19 activation. Irreversible binding. Onset 2-6 hours. Pharmacogenomic variability (poor metabolizers have reduced efficacy).
- **Prasugrel**: Prodrug with more efficient activation. Faster, stronger, more consistent than clopidogrel. Higher bleeding risk. Contraindicated in prior stroke/TIA.
- **Ticagrelor**: Direct-acting reversible antagonist. No prodrug conversion needed. Requires twice-daily dosing. Dyspnea occurs in approximately 14% of patients.
- **Cangrelor**: IV direct-acting agent with 3-6 minute half-life. Used during PCI when oral agents cannot be administered.
## GP IIb/IIIa Inhibitors
Glycoprotein IIb/IIIa is the final common pathway of platelet aggregation, cross-linking platelets via fibrinogen. Abciximab (monoclonal antibody), eptifibatide (peptide), and tirofiban (non-peptide) block this receptor. Used IV during high-risk PCI. Their role has diminished with potent oral P2Y12 antagonists and improved stent technology.
## Other Targets
Dipyridamole inhibits phosphodiesterase and adenosine reuptake in platelets, raising cAMP. Used in combination with aspirin for secondary stroke prevention. Vorapaxar is a protease-activated receptor-1 (PAR-1) antagonist that blocks thrombin-mediated platelet activation. It reduces ischemic events but increases bleeding significantly.
## Dual Antiplatelet Therapy
DAPT (aspirin plus a P2Y12 inhibitor) is standard for 6-12 months post-ACS and after drug-eluting stent implantation. Duration is balanced between stent thrombosis risk and bleeding risk. Extended DAPT beyond 12 months is considered in high ischemic risk patients.
## Key Takeaways
- Aspirin irreversibly blocks COX-1 for the platelet's entire lifespan
- P2Y12 antagonists differ in activation, reversibility, and pharmacogenomic sensitivity
- GP IIb/IIIa inhibitors block the final aggregation pathway but increase bleeding
- DAPT duration is individualized based on ischemic versus bleeding risk