Expansion
Aspirin blocks thromboxane, clopidogrel blocks ADP, abciximab blocks GPIIb/IIIa
Mnemonic
Three mechanisms, three targets on the platelet:
- Aspirin - irreversibly inhibits COX-1, blocking thromboxane A2. Lasts the platelet lifespan of 7 to 10 days, which is why it is stopped a week before surgery
- Clopidogrel, ticagrelor, prasugrel - block the P2Y12 ADP receptor. Clopidogrel is a prodrug needing CYP2C19, so it is less effective in poor metabolisers and with omeprazole
- Abciximab, tirofiban - block glycoprotein IIb/IIIa, the final common pathway of aggregation
“Aspirin blocks thromboxane, clopidogrel blocks ADP, the -ximabs block the final receptor.”
Aspirin is irreversible; ticagrelor is reversible, which matters when urgent surgery is needed.
Dipyridamole works differently again, inhibiting phosphodiesterase and adenosine uptake, and is used with aspirin after stroke.
Expansion
| Drug | Target | Notes |
|---|---|---|
| Aspirin | COX-1, irreversibly | Effect lasts the platelet lifespan of 7 to 10 days |
| Clopidogrel | P2Y12 ADP receptor, irreversibly | Prodrug requiring CYP2C19; interaction with omeprazole |
| Ticagrelor, prasugrel | P2Y12, more potent | Not prodrugs; ticagrelor causes dyspnoea; prasugrel avoided after stroke |
| Dipyridamole | Phosphodiesterase | Used with aspirin after stroke |
| Abciximab, tirofiban | GPIIb/IIIa, the final common pathway | Intravenous, used in percutaneous coronary intervention |
Clinical use
- Acute coronary syndrome: dual antiplatelet therapy with aspirin plus ticagrelor or clopidogrel, typically for 12 months
- Post stent: duration determined by stent type and bleeding risk
- Secondary prevention: lifelong aspirin, or clopidogrel monotherapy after stroke or in peripheral arterial disease
- Atrial fibrillation requires anticoagulation, not antiplatelets, which are far less effective at preventing cardioembolic stroke
The key contrast to hold onto is antiplatelets for arterial disease, where thrombi are platelet-rich and form under high shear, and anticoagulants for venous and cardioembolic disease, where thrombi are fibrin-rich.