Antiplatelet Drugs

• Platelet Activation Cascade: Platelets respond to vascular injury through a coordinated sequence of activation events. Initial adhesion occurs via von Willebrand factor (vWF) binding to glycoprotein Ib (GPIb) on the platelet surface, anchoring platelets to exposed subendothelial collagen. This binding event triggers "shape change," converting platelets from discoid to spherical morphology. Subsequent collagen engagement via glycoprotein VI (GPVI) and integrin α2β1 (via direct collagen binding) initiates intracellular signaling cascades, including phosphoinositide 3-kinase (PI3K) and protein kinase C (PKC) activation. These pathways trigger the release of dense granule contents (adenosine diphosphate [ADP], serotonin, calcium) and alpha granule contents (fibrinogen, von Willebrand factor, P-selectin), creating a self-amplifying activation loop through paracrine signaling to adjacent platelets.

• ADP and P2Y12 Signaling: ADP released from dense granules binds two distinct platelet surface receptors—P2Y1 (coupled to phospholipase C via Gq proteins) and P2Y12 (coupled to adenylyl cyclase inhibition via Gi proteins). P2Y1 activation produces inositol 1,4,5-trisphosphate (IP3) and diacylglycerol, causing calcium mobilization and protein kinase C activation. P2Y12 signaling decreases cAMP levels, removing inhibition on platelet activation. Critically, P2Y12 signaling is essential for sustained platelet aggregation, making it a primary target for antiplatelet therapy. Clopidogrel, prasugrel, and ticagrelor all target this pathway by blocking P2Y12, preventing the amplification of platelet activation and substantially reducing thrombotic burden.

• Thromboxane A2 (TXA2) Synthesis and COX-1 Inhibition: Platelet activation triggers phospholipase A2 (PLA2)-mediated release of arachidonic acid from platelet membrane phospholipids. Arachidonic acid is rapidly metabolized by cyclooxygenase-1 (COX-1) to prostaglandin H2, which is subsequently converted to thromboxane A2 (TXA2) by thromboxane synthase. TXA2 is a potent platelet agonist that binds the thromboxane-prostanoid receptor (TP receptor), amplifying platelet activation and promoting vasoconstriction. Aspirin irreversibly acetylates a serine residue (Ser529) on COX-1, permanently inhibiting TXA2 synthesis in platelets. Since platelets lack nuclei, they cannot resynthesis new COX-1, making aspirin's antiplatelet effect irreversible for the lifespan of the platelet (7-10 days). This mechanism contrasts sharply with nucleated endothelial cells, which can regenerate COX-2 and continue producing prostacyclin (an anti-aggregatory agent), creating a favorable therapeutic window.

• Glycoprotein IIb/IIIa Receptor and Final Common Pathway: The final common pathway of platelet aggregation converges on fibrinogen-mediated cross-linking of adjacent platelets. Fibrinogen is a soluble plasma protein with two sets of RGD (arginine-glycine-aspartate) sequences that bind integrin αIIbβ3 (glycoprotein IIb/IIIa) on the platelet surface. Upon platelet activation, integrin αIIbβ3 undergoes conformational change from a low-affinity to high-affinity state, permitting fibrinogen binding and bridging adjacent platelets. This integrin-fibrinogen interaction is the ultimate step common to all platelet aggregation pathways, irrespective of the initial activating stimulus (ADP, thrombin, TXA2, collagen). Glycoprotein IIb/IIIa inhibitors block this final common pathway by competitively antagonizing fibrinogen binding, producing rapid and potent antiplatelet effects. These agents can inhibit aggregation induced by all agonists, making them broadly applicable in acute coronary syndromes.

• Phosphodiesterase-3 Inhibition and cAMP Elevation: Cilostazol increases intracellular cyclic adenosine monophosphate (cAMP) by inhibiting phosphodiesterase-3 (PDE-3). Elevated cAMP activates protein kinase A (PKA), which phosphorylates and inactivates myosin light-chain kinase and other proteins essential for platelet contraction and aggregation. cAMP also suppresses intracellular calcium mobilization, further inhibiting activation cascades. The net result is platelet inhibition and modest vasodilation, distinguishing cilostazol from pure antiplatelet agents.

• Adenosine Reuptake Inhibition (Dipyridamole): Dipyridamole inhibits the nucleoside transporter that reuptakes adenosine from the extracellular space into cells. This mechanism increases local adenosine concentrations, which activate A2A adenosine receptors on platelets and adenosine receptors on vascular endothelium. Adenosine receptor activation increases cAMP through Gs-coupled signaling, producing antiplatelet and vasodilatory effects. Dipyridamole also inhibits phosphodiesterase, further elevating cAMP. These mechanisms contribute modest antiplatelet activity, which is why dipyridamole is typically combined with aspirin (as in the stroke prevention agent "aggrenox") rather than used as monotherapy.

• Acute Coronary Syndrome (ACS): Rupture of a lipid-rich atherosclerotic plaque exposes tissue factor and collagen to circulating blood, triggering both coagulation cascade activation and platelet aggregation. The resulting intracoronary thrombus may partially or completely occlude the coronary artery, causing acute myocardial infarction. ACS includes unstable angina, non-ST-elevation MI (NSTEMI), and ST-elevation MI (STEMI). All forms of ACS mandate immediate dual antiplatelet therapy (DAPT) with aspirin and a P2Y12 inhibitor. The pathophysiology involves dynamic interplay between platelet-rich thrombi (particularly important in STEMI) and fibrin-rich thrombi.

• Ischemic Stroke and Transient Ischemic Attack (TIA): Atherosclerotic plaques in cerebral arteries (particularly at branch points in the carotid arteries and major intracranial vessels) can rupture or erode, triggering local platelet aggregation and thrombus formation. Additionally, atrial fibrillation promotes cardioembolism through blood stasis and endothelial injury. Antiplatelet therapy reduces recurrent stroke risk in patients with non-cardioembolic ischemic stroke or TIA. Aspirin is the cornerstone of acute stroke management (when not contraindicated by hemorrhagic stroke) and long-term secondary prevention.

• Coronary Stent Placement: Placement of a bare-metal stent (BMS) or drug-eluting stent (DES) creates an intracoronary foreign body that triggers platelet aggregation and thrombus formation on the stent struts. This stent thrombosis can occur acutely (within hours to days) or late (months to years after placement). Dual antiplatelet therapy with aspirin and a P2Y12 inhibitor is mandated following stent placement to prevent stent thrombosis, a catastrophic complication with high mortality. The duration of DAPT depends on stent type (shorter for BMS, longer for DES) and bleeding risk.

• Peripheral Arterial Disease (PAD): Atherosclerosis affecting the aorta, iliac, femoral, and tibial arteries predisposes to thrombotic complications and limb-threatening ischemia. Antiplatelet therapy reduces cardiovascular and cerebrovascular events in symptomatic PAD patients. Cilostazol is specifically indicated for claudication, as it improves walking distance through both antiplatelet and vasodilatory mechanisms.

• Atrial Fibrillation with Low Bleeding Risk: While anticoagulation is the primary strategy for stroke prevention in atrial fibrillation, antiplatelet monotherapy may be considered in very low-risk patients (CHA2DS2-VASc score 0-1) or those with contraindications to anticoagulation. However, this indication has been progressively de-emphasized as anticoagulants (both warfarin and DOACs) are far more effective than antiplatelet agents for AF-related stroke prevention.

• Diabetes Mellitus: Diabetic patients have enhanced platelet reactivity, elevated TXA2 production, and increased P2Y12 signaling, resulting in a prothrombotic phenotype. Additionally, diabetic patients have higher prevalence of atherosclerotic disease. These factors justify more aggressive antiplatelet therapy in diabetic patients with established atherosclerotic disease. Some guidelines suggest aspirin for primary prevention in diabetics, though recent evidence has questioned this practice.

• Smoking: Smoking acutely increases platelet reactivity through multiple mechanisms, including increased TXA2 production and enhanced P2Y12 signaling. Chronic smoking accelerates atherosclerosis. Smokers presenting with ACS or undergoing coronary intervention require antiplatelet therapy, and smoking cessation is critical for long-term outcomes.

• Acute Myocardial Infarction: Acute plaque rupture and intracoronary thrombus formation produce acute coronary insufficiency. Patients experience acute-onset substernal chest pain or pressure, often described as "crushing" or "elephant sitting on my chest." Pain typically radiates to the left arm, jaw, or back and may be accompanied by dyspnea, diaphoresis, nausea, and a sense of impending doom. In STEMI, acute transmural myocardial necrosis produces characteristic electrocardiographic changes (ST-segment elevation, T-wave inversions) and elevation of cardiac biomarkers (troponin, myoglobin). NSTEMI presents with less dramatic ECG changes (ST depression, T-wave changes, or normal ECG) and less extensive myocardial necrosis. Women, elderly patients, and diabetics may present atypically with dyspnea, fatigue, or vague chest discomfort without classic chest pain. The pathophysiology underlying symptom presentation relates to acute regional myocardial ischemia and the inflammatory cascade triggered by myocardial infarction.

• Ischemic Stroke: Acute arterial occlusion in the cerebral circulation produces sudden focal neurological deficits. Depending on the vascular territory affected, patients may present with contralateral hemiparesis, facial droop, aphasia (dominant hemisphere), neglect (non-dominant hemisphere), diplopia, ataxia, or dysarthria. The NIH Stroke Scale quantifies symptom severity. Transient ischemic attacks present with identical symptoms that resolve completely within 24 hours (typically within minutes to hours), without acute infarction on diffusion-weighted imaging (DWI). The symptom onset is abrupt, distinguishing ischemic stroke from hemorrhagic stroke (which may be associated with headache) or migraine aura (which evolves gradually over 20-30 minutes).

• Unstable Angina: Patients experience acute-onset or accelerating anginal chest pain at rest or with minimal exertion, representing dynamic coronary obstruction (from plaque rupture or transient thrombosis) with preserved perfusion. The pain quality and location are identical to stable angina but occur with greater frequency, severity, or unprovoked (at rest). By definition, unstable angina shows no troponin elevation, distinguishing it from NSTEMI on the basis of cardiac biomarkers.

• Acute Coronary Stent Thrombosis: Patients present with acute-onset chest pain and electrocardiographic changes identical to acute MI, representing thrombotic occlusion of the stented vessel. Acute stent thrombosis (within 30 days of stent placement) is a catastrophic complication typically related to inadequate antiplatelet therapy, premature DAPT discontinuation, or stent malpositioning. Late stent thrombosis (>30 days) is rarer but may occur in association with DES, particularly if DAPT is prematurely discontinued.

• Claudication: Patients with peripheral arterial disease experience reproducible leg pain with walking that resolves with rest. The pain reflects muscular ischemia during exercise when blood flow cannot meet metabolic demand. The symptom is classic for intermittent claudication and drives the use of cilostazol to improve walking distance and symptom tolerance.

• Physical Examination Findings: In ACS, patients appear anxious or uncomfortable, often with diaphoresis. Cardiac examination may reveal tachycardia, new cardiac murmurs (from papillary muscle rupture or ventricular septal defect in MI), signs of heart failure (bibasilar crackles, S3 gallop), or hypotension (in cardiogenic shock). In acute stroke, neurological examination reveals focal deficits corresponding to the vascular territory affected. Patients with claudication demonstrate normal examination at rest but may show signs of chronic ischemia (cool extremities, weak pulses, hair loss, skin atrophy).

• Important Clinical Variants: Silent ischemia (particularly in diabetics and elderly patients) represents a major clinical challenge, as patients may present without chest pain but with biomarker elevation and ECG changes. Spontaneous coronary artery dissection (SCAD) is an increasingly recognized cause of ACS in young women and peripartum patients, presenting identically to atherosclerotic ACS. Takotsubo cardiomyopathy ("stress cardiomyopathy") mimics STEMI with chest pain, ST elevation, and troponin elevation, though coronary angiography reveals unobstructed coronaries.

• Electrocardiography (ECG): The 12-lead ECG is the initial diagnostic test in suspected ACS. STEMI demonstrates ST-segment elevation ≥1 mm in ≥2 contiguous leads (or ≥2 mm in precordial leads), reflecting acute transmural myocardial injury. T-wave inversions and reciprocal ST depression may accompany STEMI. NSTEMI shows ST-segment depression or T-wave inversions without ST elevation. Unstable angina has ECG changes or normal baseline ECG. The ECG has high specificity (~95%) for STEMI but moderate sensitivity (~80%) for NSTEMI. In acute stroke, non-contrast head CT excludes hemorrhage (which is an absolute contraindication to thrombolysis) and guides acute management decisions.

• Cardiac Biomarkers (Troponin, Myoglobin, CK-MB): Cardiac troponins (troponin I and troponin T) are the gold standard biomarkers for myocardial necrosis. High-sensitivity troponin assays achieve negative predictive value >99% for acute MI within 3 hours of symptom onset, enabling rapid rule-out of MI and safe early discharge. Troponins begin rising 2-4 hours after symptom onset, peak at 24-48 hours, and remain elevated for 7-14 days. Serial troponin measurements (delta troponin) improve sensitivity for detecting rising and falling patterns characteristic of acute MI. Myoglobin rises earlier (within 1-2 hours) than troponin but is less specific. CK-MB, an older marker, is rarely used in modern practice. Normal troponin in the setting of ongoing symptoms warrants repeat measurement at 3 hours to exclude evolving MI. Troponin elevation ≥99th percent

Aspirin

• GI mucosal injury and bleeding: COX-1 blockade removes prostaglandin E2/prostacyclin-mediated mucosal cytoprotection (bicarbonate, mucus, mucosal blood flow), producing dyspepsia, gastric erosions, and ulcer hemorrhage. A proton pump inhibitor is added for high GI bleeding risk.
• Salicylate toxicity: direct respiratory-center stimulation plus uncoupling of oxidative phosphorylation gives the classic mixed respiratory alkalosis and anion-gap metabolic acidosis with tinnitus, hyperpnea, and fever. Management is alkalinization of urine with sodium bicarbonate (ion trapping) and hemodialysis for severe or refractory toxicity.
• Reye syndrome: encephalopathy plus microvesicular hepatic steatosis in children given aspirin during viral illness (influenza, varicella) — hence aspirin is avoided in children except in Kawasaki disease.
• Aspirin-exacerbated respiratory disease: COX inhibition shunts arachidonate toward leukotrienes, precipitating bronchospasm and nasal congestion in patients with asthma and nasal polyps (Samter triad).
• Hyperuricemia: low doses inhibit tubular urate secretion and may precipitate gout.

P2Y12 inhibitors

• Clopidogrel: a prodrug requiring CYP2C19; loss-of-function alleles blunt platelet inhibition (FDA boxed warning), and strong CYP2C19-inhibiting PPIs such as omeprazole are avoided in favor of pantoprazole. TTP is rare.
• Ticlopidine: neutropenia/agranulocytosis and TTP require serial CBC monitoring; essentially obsolete for this reason.
• Prasugrel: greatest bleeding risk; contraindicated with prior stroke/TIA; generally not recommended at age ≥75 years (unless high ischemic risk, e.g., diabetes or prior MI); for weight <60 kg the FDA label advises reducing the maintenance dose to 5 mg daily rather than avoiding the drug.
• Ticagrelor: reversible binding; adenosine reuptake inhibition causes dyspnea and ventricular pauses. Boxed warning — maintenance aspirin above roughly 100 mg/day reduces its efficacy.

Other agents

• GP IIb/IIIa inhibitors: bleeding and acute profound thrombocytopenia (immune-mediated, especially abciximab) within hours — check a platelet count.
• Cilostazol: contraindicated in heart failure of any severity (PDE-3 inhibitor class mortality signal); headache, palpitations, diarrhea.
• Dipyridamole: vasodilation with coronary steal, exploited in pharmacologic stress testing but hazardous in active ischemia.

Reversal

• No specific antidote: effects of aspirin, clopidogrel, and prasugrel are irreversible and abate only as new platelets appear. Per the 2021 ACC/AHA/SCAI revascularization guideline, before elective CABG hold clopidogrel ≥5 days, prasugrel ≥7 days, and ticagrelor ≥3 days (ticagrelor's reversible binding permits the shorter interval); in urgent CABG, at least 24 hours of discontinuation is recommended. Platelet transfusion (± desmopressin) is reserved for life-threatening bleeding; the PATCH trial showed harm from routine platelet transfusion in spontaneous intracerebral hemorrhage.

• Aspirin prolongs bleeding time with normal PT/aPTT: platelet function, not the coagulation cascade, is impaired — the classic lab triad on a stem describing mucocutaneous bleeding after NSAID use.
• Irreversible vs reversible matters: aspirin, clopidogrel, and prasugrel covalently modify their targets, so effect persists for the 7–10 day platelet lifespan; ticagrelor and cilostazol are reversible and wash out faster. Ibuprofen taken before aspirin can competitively block COX-1 access and blunt cardioprotection.
• Clopidogrel is a CYP2C19 prodrug: a patient with recurrent stent thrombosis despite adherence is testing loss-of-function CYP2C19 or a concomitant omeprazole/esomeprazole interaction. Best next step is switching to pantoprazole or to prasugrel/ticagrelor, which do not depend on CYP2C19.
• Dyspnea on a new P2Y12 inhibitor = ticagrelor (adenosine reuptake inhibition), not heart failure or pulmonary embolism. Also remember its aspirin dose cap.
• Prior stroke or TIA is a contraindication to prasugrel — the single association examiners most reliably test for that drug.
• Samter triad (asthma, nasal polyps, aspirin sensitivity) is managed with leukotriene-receptor antagonists and, when aspirin is required for coronary disease, aspirin desensitization.
• Aspirin plus a P2Y12 inhibitor is standard after ACS and stenting under ACC/AHA revascularization guidance, with duration shortened when bleeding risk is high; for minor stroke or high-risk TIA, AHA/ASA supports short-course DAPT followed by single-agent therapy — long-term DAPT for stroke only increases hemorrhage.
• Distractors to avoid: abciximab-induced thrombocytopenia is not HIT and is not treated with a direct thrombin inhibitor; antiplatelet agents do not replace anticoagulation for atrial-fibrillation stroke prevention; and Glanzmann thrombasthenia (GP IIb/IIIa defect: absent aggregation to ADP/collagen/epinephrine but normal ristocetin-induced agglutination) versus Bernard–Soulier (GP Ib defect: impaired ristocetin agglutination not corrected by normal plasma, giant platelets) mirrors the pharmacology but is a distinct genetic disease, not a drug effect.