Antiplatelet Drugs

Antiplatelet agents inhibit platelet activation and aggregation, the dominant event in arterial (white) thrombus formation. They are the backbone of primary and secondary prevention in coronary artery disease, ischaemic stroke and peripheral arterial disease, and are among the most heavily tested topics in NEET PG pharmacology for the cardiovascular system.

Why platelets matter: the target pathway

Arterial thrombi are platelet-rich ("white thrombi"), forming on ruptured atherosclerotic plaques under high shear. Venous thrombi are fibrin- and RBC-rich ("red thrombi"), which is why arterial disease → antiplatelets, while venous thromboembolism → anticoagulants. This single distinction answers many one-liner questions.

Platelet activation proceeds through three amplifying loops, each a drug target:

1. Thromboxane A2 (TXA2) generated via cyclo-oxygenase-1 (COX-1) → blocked by aspirin.
2. ADP released from dense granules acting on the P2Y12 receptor → blocked by clopidogrel, prasugrel, ticagrelor, ticlopidine, cangrelor.
3. Final common pathway: activation of glycoprotein (GP) IIb/IIIa receptor, which binds fibrinogen and cross-links platelets → blocked by abciximab, eptifibatide, tirofiban.

High-yield: GP IIb/IIIa is the final common pathway of platelet aggregation, regardless of the initiating stimulus. Blocking it gives the most complete inhibition of aggregation.

A stepwise view of haemostasis at a plaque:

Plaque rupture → collagen/vWF exposure → platelet adhesion (GP Ib–vWF) → activation & shape change → TXA2 + ADP release (amplification) → GP IIb/IIIa activation → fibrinogen cross-linking → aggregation → thrombus.

Classification of antiplatelet drugs

Class	Mechanism	Examples	Reversibility
COX inhibitor	Irreversible acetylation of COX-1 → ↓TXA2	Aspirin	Irreversible
P2Y12 (ADP) blockers – thienopyridines	Irreversible P2Y12 block (prodrugs)	Clopidogrel, Prasugrel, Ticlopidine	Irreversible
P2Y12 blockers – non-thienopyridine	Reversible P2Y12 block	Ticagrelor, Cangrelor (IV)	Reversible
GP IIb/IIIa inhibitors	Block fibrinogen binding	Abciximab, Eptifibatide, Tirofiban	Varies
PDE inhibitors / others	↑cAMP, ↑cGMP, adenosine uptake block	Dipyridamole, Cilostazol	Reversible
PAR-1 (thrombin receptor) antagonist	Blocks protease-activated receptor-1	Vorapaxar	Reversible

Aspirin (acetylsalicylic acid)

Mechanism. Aspirin irreversibly acetylates a serine residue (Ser529) of COX-1 in platelets, abolishing thromboxane A2 synthesis. Because the platelet is anucleate, it cannot synthesise new enzyme, so inhibition lasts the entire 7–10 day platelet lifespan. Endothelial cells can regenerate COX, so prostacyclin (PGI2, antiaggregatory) recovers — the basis for using low doses.

Dose. Antiplatelet doses are 75–150 mg/day (often 75–100 mg). Higher doses inhibit endothelial PGI2 and add no antithrombotic benefit while increasing GI bleeding — the principle of "aspirin dilemma".

Loading dose in acute coronary syndrome (ACS): 150–325 mg chewed (chewing speeds absorption).

High-yield: Aspirin's antiplatelet effect persists ~7–10 days because platelets cannot resynthesise COX (no nucleus). Recovery of normal haemostasis requires generation of ~30–40% new platelets.

Uses: secondary prevention post-MI, post-stroke/TIA, stable angina, ACS, post-PCI (as part of DAPT), Kawasaki disease (anti-inflammatory + antiplatelet).

Adverse effects: GI erosions/bleeding, dyspepsia, bronchospasm in aspirin-sensitive asthma, Reye syndrome in children with viral illness (avoid in <16 yr with febrile illness — Kawasaki is the exception), tinnitus at high dose, prolonged bleeding time.

P2Y12 (ADP receptor) blockers

These block the P2Y12 receptor, preventing ADP-mediated amplification and GP IIb/IIIa activation.

Clopidogrel

• Prodrug: requires hepatic CYP2C19 activation (a two-step oxidation) to its active thiol metabolite, which irreversibly binds P2Y12.
• Onset slow unless a loading dose (300–600 mg) is given.
• Pharmacogenetics: CYP2C19 loss-of-function alleles (*2, *3) → poor metabolisers → reduced activation → "clopidogrel resistance" and higher stent thrombosis. PPIs, especially omeprazole, inhibit CYP2C19 and may blunt efficacy → prefer pantoprazole if a PPI is needed.

High-yield: Clopidogrel + omeprazole = pharmacokinetic interaction (CYP2C19 inhibition) reducing clopidogrel activation. Pantoprazole is the safer PPI choice.

Prasugrel

• Thienopyridine prodrug, but activation is more efficient and not dependent on CYP2C19 polymorphisms → faster, more consistent, more potent platelet inhibition.
• Contraindicated in prior stroke/TIA; avoid in age ≥75 and weight <60 kg (higher bleeding). Used in ACS managed with PCI.

Ticagrelor

• Non-thienopyridine (cyclopentyltriazolopyrimidine), directly acting (not a prodrug), and reversibly binds P2Y12 at an allosteric site.
• Twice-daily dosing (short half-life). Faster onset/offset than clopidogrel.
• Distinctive adverse effect: dyspnoea (adenosine-mediated) and ventricular pauses/bradyarrhythmia. Also raises serum uric acid and creatinine.
• Avoid aspirin doses >100 mg with ticagrelor (reduces efficacy).

Ticlopidine

• Older thienopyridine, now largely abandoned due to toxicity: neutropenia/agranulocytosis, thrombotic thrombocytopenic purpura (TTP), and aplastic anaemia.

High-yield: Ticlopidine → TTP and neutropenia/agranulocytosis is a classic exam fact. Clopidogrel can also rarely cause TTP but far less commonly.

Cangrelor

• IV, direct, reversible, ultra-short acting (half-life ~3–6 min); useful during PCI when a rapid on/off P2Y12 effect is needed.

Feature	Clopidogrel	Prasugrel	Ticagrelor	Cangrelor
Class	Thienopyridine	Thienopyridine	Cyclopentyltriazolopyrimidine	ATP analogue
Prodrug?	Yes (CYP2C19)	Yes (efficient)	No (direct)	No (direct, IV)
Binding	Irreversible	Irreversible	Reversible	Reversible
Onset	Slow	Fast	Fast	Immediate
Dosing	OD	OD	BD	IV infusion
Key issue	CYP2C19/PPI resistance	Bleeding; avoid prior stroke	Dyspnoea, pauses	Very short acting

Glycoprotein IIb/IIIa inhibitors

Given IV during high-risk PCI/ACS; block fibrinogen binding to activated GP IIb/IIIa, the final step of aggregation.

Drug	Nature	Half-life/notes
Abciximab	Monoclonal antibody (Fab fragment)	Long platelet-bound effect; irreversible-like; can cause thrombocytopenia
Eptifibatide	Cyclic peptide (KGD sequence)	Short, reversible; renally cleared
Tirofiban	Non-peptide (tyrosine derivative)	Short, reversible; renally cleared

High-yield: Abciximab is a monoclonal antibody Fab fragment and the prototype GP IIb/IIIa inhibitor; it can also block the vitronectin (αvβ3) receptor. Thrombocytopenia is a notable adverse effect of this class.

Mnemonic — the "-tide / -ban / abciximab" trio: AbEpTi (Abciximab, Eptifibatide, Tirofiban) = GP IIb/IIIa blockers.

Other antiplatelet agents

• Dipyridamole: inhibits phosphodiesterase (↑cAMP) and blocks adenosine reuptake → vasodilatation + weak antiplatelet effect. Combined with aspirin (Aggrenox) for secondary stroke prevention. Used in dipyridamole/thallium stress testing (coronary steal). Side effect: headache, flushing.
• Cilostazol: PDE-3 inhibitor; first-line drug therapy for intermittent claudication (peripheral arterial disease). Contraindicated in heart failure (like other PDE-3 inhibitors, can increase mortality).
• Vorapaxar: PAR-1 (thrombin receptor) antagonist; adjunct in secondary prevention. Contraindicated with prior stroke/TIA/intracranial bleed due to bleeding risk.

Dual antiplatelet therapy (DAPT)

DAPT = aspirin + a P2Y12 inhibitor. Rationale: blocking two independent activation pathways (TXA2 and ADP) gives synergistic inhibition, crucial after coronary stent placement to prevent stent thrombosis before endothelialisation.

Stepwise ACS antiplatelet approach:

1. Aspirin loading 150–325 mg chewed immediately.
2. Add a P2Y12 inhibitor (ticagrelor or prasugrel preferred over clopidogrel in ACS; clopidogrel if these are contraindicated/unavailable or if on oral anticoagulant).
3. GP IIb/IIIa inhibitor IV for selected high-thrombus-burden PCI.
4. Continue DAPT for ~12 months after ACS (duration individualised by bleeding vs ischaemic risk), then aspirin lifelong.

High-yield: After drug-eluting stent + ACS, standard DAPT duration is ~12 months; shorten if high bleeding risk, extend in high ischaemic/low bleeding risk.

Pre-operative and bleeding management

• Aspirin and thienopyridines are irreversible → effect outlasts the drug; only transfused/new platelets restore function.
• Stop clopidogrel ~5 days and prasugrel ~7 days before major/elective surgery; ticagrelor ~3–5 days (reversible but active metabolite). Aspirin is often continued for cardiac/vascular surgery but stopped ~7 days for high-bleeding-risk procedures (neurosurgery).
• Reversal of bleeding: platelet transfusion is the mainstay for irreversible agents (aspirin, clopidogrel, prasugrel). Platelet transfusion is less effective for ticagrelor because the reversible, circulating drug re-inhibits transfused platelets. Desmopressin (DDAVP) can help.

High-yield: For life-threatening bleeding on aspirin/clopidogrel → platelet transfusion. For ticagrelor, transfused platelets are inhibited by circulating drug, so it is less effective.

Aspirin resistance

A clinical/laboratory phenomenon where standard aspirin fails to adequately suppress TXA2 / platelet function. Proposed causes: poor compliance, enteric-coated absorption issues, COX-2-derived TXA2, drug interactions (ibuprofen taken before aspirin blocks COX-1 access to Ser529 — give aspirin first), and genetic polymorphisms. Associated with higher cardiovascular event rates. Tested as a concept rather than a defined entity.

Complications / adverse effects (class overview)

• Bleeding — the dose-limiting toxicity of all antiplatelets (GI, intracranial). Risk multiplies with combinations or with anticoagulants ("triple therapy").
• GI ulceration/haemorrhage — chiefly aspirin; co-prescribe PPI (pantoprazole with clopidogrel).
• Thrombocytopenia — GP IIb/IIIa inhibitors (esp. abciximab), ticlopidine.
• TTP / neutropenia — ticlopidine >> clopidogrel.
• Dyspnoea, bradyarrhythmias, ↑uric acid/creatinine — ticagrelor.
• Headache/flushing — dipyridamole, cilostazol.
• Reye syndrome — aspirin in children with viral illness.

Key differentials / "which drug" discriminators

• Irreversible vs reversible P2Y12: clopidogrel/prasugrel/ticlopidine (irreversible) vs ticagrelor/cangrelor (reversible).
• Prodrug vs direct: clopidogrel & prasugrel (prodrugs) vs ticagrelor (direct).
• Drug causing dyspnoea → ticagrelor. Drug causing TTP → ticlopidine. Monoclonal antibody antiplatelet → abciximab. Drug for claudication → cilostazol. Drug contraindicated in CHF → cilostazol.
• Antiplatelet vs anticoagulant: arterial/white thrombus & stents → antiplatelet; venous/AF/mechanical valves → anticoagulant.

Recently asked / exam angle

• Mechanism of aspirin = irreversible acetylation of COX-1 (Ser529); duration of action tied to anucleate platelet lifespan (7–10 days).
• Clopidogrel is a prodrug activated by CYP2C19; omeprazole reduces its efficacy.
• Ticagrelor — non-prodrug, reversible, twice daily, causes dyspnoea; keep aspirin ≤100 mg.
• Ticlopidine → TTP, neutropenia (avoided for this reason).
• Abciximab = Fab monoclonal antibody, GP IIb/IIIa final common pathway blocker.
• Cilostazol = drug of choice for intermittent claudication; contraindicated in heart failure.
• DAPT = aspirin + P2Y12 inhibitor, ~12 months after ACS/stent.
• Pre-op stoppage: clopidogrel 5 days, prasugrel 7 days; reversal = platelet transfusion (less useful for ticagrelor).
• Dipyridamole mechanism (PDE inhibition + adenosine uptake block) and use in stress testing (coronary steal).
• Prasugrel contraindicated in prior stroke/TIA.

Rapid revision

1. Arterial/white thrombus → antiplatelets; venous/red thrombus → anticoagulants.
2. Aspirin irreversibly acetylates COX-1 (Ser529); effect lasts platelet lifespan (~7–10 days).
3. Antiplatelet aspirin dose = 75–150 mg/day; ACS loading 150–325 mg chewed.
4. Clopidogrel = prodrug via CYP2C19; omeprazole ↓ efficacy → use pantoprazole.
5. Prasugrel = potent, CYP-independent; avoid in prior stroke, age ≥75, weight <60 kg.
6. Ticagrelor = direct, reversible, BD, causes dyspnoea & pauses; keep aspirin ≤100 mg.
7. Ticlopidine → TTP, neutropenia/agranulocytosis (largely abandoned).
8. GP IIb/IIIa blockers = abciximab (Fab mAb), eptifibatide, tirofiban — final common pathway, can cause thrombocytopenia.
9. DAPT = aspirin + P2Y12 inhibitor; ~12 months after ACS/stent to prevent stent thrombosis.
10. Bleeding reversal = platelet transfusion (poor for ticagrelor); ± desmopressin.
11. Cilostazol = intermittent claudication drug; contraindicated in heart failure.
12. Dipyridamole = PDE inhibitor + adenosine uptake blocker; used in stress imaging (coronary steal).