Antiplatelet Drugs

Platelets are central components of primary haemostasis. After vascular injury, they adhere to the subendothelial matrix, become activated, release mediators and bind to one another to form the platelet plug. The same process can produce an arterial thrombus after rupture of an atherosclerotic plaque; several stages therefore represent important pharmacological targets.

Learning objective: distinguish adhesion, activation and aggregation, and understand where the major antiplatelet drug classes intervene in platelet signalling.

Key features of platelets

Platelets are anucleate cell fragments approximately 2–4 µm in diameter and originate from bone-marrow megakaryocytes.
The usual reference range is approximately 150–400 × 10⁹/L. A value below 150 × 10⁹/L is termed thrombocytopenia, although bleeding risk does not depend on platelet count alone.
Their average lifespan is about 7–10 days, and a healthy adult produces approximately 10¹¹ platelets per day.
Because platelets lack a nucleus, they cannot initiate new gene transcription. However, they contain megakaryocyte-derived mRNA and ribosomes and therefore retain limited protein-synthesis capacity.

Stages of primary haemostasis

1. Adhesion

Vascular injury exposes subendothelial collagen. Under high shear, the interaction between collagen-bound von Willebrand factor (vWF) and the platelet GPIb–IX–V complex supports initial tethering. More stable collagen adhesion is mediated mainly by the GPVI receptor and the α₂β₁ integrin, formerly termed GPIa/IIa.

High shear and platelet activity dominate arterial thrombosis. Venous stasis primarily promotes activation of the coagulation system; consequently, the mechanism of venous thrombosis is not identical to platelet adhesion after vascular injury.

2. Activation and secretion

Adhesion and agonist signalling induce shape change, granule secretion, formation of a procoagulant surface and an increase in cytosolic Ca²⁺.

Source	Important components	Role
Dense (δ) granules	ADP, ATP, Ca²⁺, serotonin	Recruitment of additional platelets, signalling and local vasoconstriction.
α-granules	Fibrinogen, vWF, factor V, P-selectin and numerous growth factors	Adhesion, aggregation, coagulation, inflammatory cell interactions and tissue repair.
Mediator synthesised from membrane phospholipids	Thromboxane A₂ (TXA₂)	Produced from arachidonic acid by COX-1 and thromboxane synthase; it is not a pre-stored granule constituent.

3. Aggregation

The platelet α_IIbβ₃ integrin (GPIIb/IIIa) is already present as a heterodimer on resting platelets, but in a low-affinity conformation. Inside-out signalling induces a conformational change that enables high-affinity binding of fibrinogen and vWF. Fibrinogen bridges then link activated platelets to one another.

Key point: GPIIb/IIIa is not formed by the dimerisation of two separate monomers during activation. It is a pre-existing α_IIbβ₃ heterodimer whose conformation, membrane distribution and ligand-binding capacity change upon activation.

Major platelet signalling pathways

Agonist or inhibitory mediator	Receptor	Signalling	Main effect
ADP	P2Y₁	G_q	Ca²⁺ signal and shape change; initiation of aggregation.
ADP	P2Y₁₂	G_i	Inhibition of adenylyl cyclase, reduction of cAMP and stabilisation of aggregation.
TXA₂	TP	Mainly G_q and G_12/13	Activation, secretion, aggregation and vasoconstriction.
Thrombin	PAR-1, PAR-4	Multiple G-protein pathways	Potent platelet activation.
Prostacyclin (PGI₂)	IP	G_s	Increased cAMP, platelet inhibition and vasodilation.
NO	Soluble guanylyl cyclase	Increased cGMP	Platelet inhibition and vasodilation.

In general, an increase in cytosolic Ca²⁺ promotes platelet activation, whereas increases in cAMP and cGMP inhibit it. The process comprises several interconnected kinases, phosphatases, ion-transport mechanisms and cytoskeletal changes and cannot be reduced to a single “cAMP-dependent calcium pump”.

Pharmacological targets

1. Irreversible COX-1 inhibition: acetylsalicylic acid

Platelets use COX-1 and thromboxane synthase to produce TXA₂ from arachidonic acid. TXA₂ is highly unstable and rapidly hydrolyses to the more stable TXB₂; TXB₂ metabolites are therefore often measured when assessing thromboxane production.

Acetylsalicylic acid (ASA, aspirin) irreversibly acetylates a serine residue in COX-1. Low-dose ASA, usually 75–100 mg/day, therefore produces sustained suppression of TXA₂ synthesis in exposed platelets. Function recovers gradually as new platelets enter the circulation.

Important clarification: enteric coating is not required for the antiplatelet mechanism and may delay or increase the variability of absorption. ASA can exert a substantial presystemic effect on platelets in the portal circulation and is rapidly hydrolysed to salicylate, but this does not mean that low-dose ASA has no systemic effects or adverse effects. Bleeding and gastrointestinal injury remain clinically relevant risks.

TXA₂-synthase and TP-receptor inhibition are pharmacologically feasible, but agents such as ridogrel and ifetroban have not become foundations of routine antiplatelet therapy.

2. P2Y₁₂-receptor inhibitors

P2Y₁₂ inhibition interrupts ADP-mediated G_i signalling, preserving adenylyl-cyclase activity, increasing cAMP and reducing stabilisation of α_IIbβ₃ activation.

Drug	Binding and activation	Important feature
Clopidogrel	Irreversible; prodrug	CYP2C19 contributes importantly to activation. Omeprazole and esomeprazole may reduce exposure to the active metabolite; concomitant use should therefore be avoided or carefully assessed.
Prasugrel	Irreversible; prodrug	Rapid and more predictable action, but bleeding risk and contraindications require particular attention.
Ticagrelor	Reversible, direct-acting; not a prodrug	Rapid onset and faster functional recovery than with thienopyridines.
Cangrelor	Reversible, direct, intravenous	Very rapid onset and short duration; used in selected PCI settings.
Ticlopidine	Irreversible; prodrug	An older agent largely replaced because of unfavourable haematological safety.

In acute coronary syndrome, dual antiplatelet therapy (DAPT) usually combines ASA with a P2Y₁₂ inhibitor. Drug selection and treatment duration depend on ischaemic and bleeding risk, the intervention performed and current guidelines.

3. Phosphodiesterase inhibitors

Platelets express PDE2, PDE3 and PDE5, among other isoenzymes. PDE inhibition reduces cyclic-nucleotide degradation and consequently inhibits platelet activation.

Dipyridamole: inhibits phosphodiesterases and cellular uptake of adenosine. Increased extracellular adenosine supports cAMP elevation and vasodilation through A_2A receptors. An extended-release formulation combined with ASA is used in selected cerebrovascular indications.
Cilostazol: acts mainly as a PDE3 inhibitor and has antiplatelet and vasodilator effects. Approved indications and use vary by region.

4. GPIIb/IIIa inhibitors

Abciximab, eptifibatide and tirofiban inhibit the common final pathway of aggregation by preventing ligand binding to activated α_IIbβ₃. They are potent intravenous antiplatelet agents with substantial bleeding risk and are now used mainly in selected PCI situations or thrombotic complications rather than routinely.

5. Pharmacology of the prostacyclin pathway

PGI₂ increases cAMP through G_s-coupled IP receptors, inhibiting platelet activation and causing vasodilation. Iloprost is a stable prostacyclin analogue used mainly for pulmonary arterial hypertension and, in some countries, severe Raynaud phenomenon or critical limb ischaemia. It is not a standard antiplatelet drug for routine arterial antithrombotic therapy.

6. PAR-1 antagonist

Vorapaxar is a selective PAR-1 antagonist that inhibits thrombin-mediated platelet activation. Although receptor binding is reversible, its very long effective half-life results in prolonged action. Use is limited by major bleeding risk; it is contraindicated in patients with previous stroke, TIA or intracranial haemorrhage and is not part of routine practice in the European Union.

Summary

Drug class	Target	Consequence
ASA	Irreversible COX-1 acetylation	Reduced TXA₂ synthesis.
Clopidogrel, prasugrel, ticagrelor, cangrelor	P2Y₁₂	Inhibition of ADP-mediated stabilisation of aggregation.
Dipyridamole, cilostazol	PDEs and partly adenosine transport	Reduced cAMP/cGMP degradation and stronger inhibitory signalling.
Abciximab, eptifibatide, tirofiban	α_IIbβ₃ (GPIIb/IIIa)	Inhibition of fibrinogen bridging, the final step of aggregation.
Vorapaxar	PAR-1	Partial inhibition of thrombin-mediated platelet activation.

Educational note: clinical antiplatelet therapy always depends on the indication, the individual patient and bleeding risk. This material explains pharmacological mechanisms and is not a guide to individual treatment or medication.

Professional sources

You need to be logged in to view this exercise.