Three reinforcing activation loops. (1) Thromboxane A2 from platelet COX-1: aspirin irreversibly acetylates COX-1 → no TXA2 for the platelet's 7-10 d life span. (2) ADP from dense granules → P2Y12 receptor (Gi-coupled) → CLOPIDOGREL (irreversible, prodrug requiring CYP2C19 activation), PRASUGREL (irreversible, more reliable activation), TICAGRELOR (reversible, direct); cangrelor IV. (3) Thrombin → PAR1 (vorapaxar antagonist). Final common path: αIIbβ3 (GPIIb-IIIa) conformational change → fibrinogen binding → cross-linking + aggregation. Abciximab + eptifibatide + tirofiban block αIIbβ3 (PCI use). Combined antiplatelet: dual = aspirin + P2Y12i (standard post-PCI; CV mortality benefit). Bleeding risk scales with intensity + number of agents.
Organ Systems
immune-hematologic
cardiovascular
Pathway Steps
platelet-activation → txa2-release — via COX-1 (platelet) → TXA2; ASPIRIN-INHIBITED (irreversibly). Activated platelets synthesize thromboxane A2 (TXA2) from arachidonic acid via COX-1, which recruits and activates more platelets in a positive-feedback loop. This COX-1/TXA2 step is irreversibly blocked by aspirin — the mechanistic basis of low-dose aspirin’s antiplatelet effect.
platelet-activation → adp-release — via dense granule exocytosis → P2Y12 on neighboring platelets; CLOPIDOGREL / TICAGRELOR target. Activated platelets also secrete ADP from dense granules, which acts on the P2Y12 receptor to amplify and sustain activation. P2Y12 is the target of clopidogrel, prasugrel, and ticagrelor — the other pillar of dual antiplatelet therapy alongside aspirin.
platelet-activation → thrombin-par1 — via thrombin generation + PAR1 cleavage; vorapaxar antagonist. Thrombin, generated by the coagulation cascade, is the most potent platelet activator, acting through protease-activated receptors (PAR1/PAR4). This links secondary hemostasis (coagulation) to platelet activation — and PAR1 antagonists (vorapaxar) are a newer antiplatelet class.
platelet-activation → gpiib-iiia-binding — via αIIbβ3 conformational change → fibrinogen cross-linking; ABCIXIMAB blocks. All activation pathways converge on the integrin GPIIb/IIIa, which changes conformation to bind fibrinogen, cross-linking platelets into an aggregate — the final common step. GPIIb/IIIa inhibitors (abciximab, eptifibatide) block it directly, and its deficiency causes Glanzmann thrombasthenia.
Known Modulators
aspirin (inhibitor) — COX-1 (platelet TXA2) — irreversible for life of platelet