Pharmacology · Antithrombotic Agents

Anticoagulants, Thrombolytics & Antiplatelet Drugs

Comprehensive USMLE-focused notes on the clotting cascade, mechanism-based drug classes, key monitoring parameters, reversal strategies, and high-yield clinical pearls.
foundation

🩸 Clotting Cascade Overview

  • Coagulation transforms soluble fibrinogen into insoluble fibrin mesh
  • Circulating zymogens undergo limited proteolysis → active serine proteases
  • Cascade organized into:
    • Intrinsic pathway (factors XII, XI, IX, VIII)
    • Extrinsic pathway (factor VII + tissue factor)
    • Common pathway (factors X, V, II, I)
  • Thrombin (factor IIa) is the master effector:
    • Converts fibrinogen → fibrin
    • Activates platelets via PAR receptors
    • Amplifies its own production (positive feedback on factors V, VIII, XI)
  • Endogenous anticoagulants:
    • Antithrombin III (ATIII) — inhibits thrombin, IXa, Xa, XIa, XIIa
    • Protein C + Protein S — proteolyze factors Va and VIIIa
    • Tissue factor pathway inhibitor (TFPI) — inhibits Xa and VIIa/TF complex
Vascular injury → TF exposure → Extrinsic pathway (VIIa) → Xa → Thrombin (IIa) → Fibrin clot
  • Warfarin affects factors II, VII, IX, X (all vitamin K–dependent)
  • Heparin accelerates ATIII-mediated inhibition of IIa and Xa
  • Protein C & S are also vitamin K–dependent — warfarin can transiently lower protein C before factors IX/X, creating a prothrombotic state
anticoagulant

💉 Heparin & Low‑Molecular‑Weight Heparin

  • Chemistry: sulfated polysaccharide (15–20 kDa)
  • Route: IV or SC (not oral)
  • Half‑life: ~2 hours
  • Metabolism: hepatic + reticuloendothelial
  • Placental transfer: no (safe in pregnancy)
  • Mechanism:
    • Binds antithrombin III → conformational change
    • Accelerates ATIII-mediated inactivation of IIa, Xa, IXa, XIa, XIIa
  • Monitoring: aPTT (partial thromboplastin time)
  • Reversal: protamine sulfate (chemical antagonist, fast onset)
  • Uses:
    • Acute thromboses (DVT, PE)
    • Unstable angina / NSTEMI
    • Disseminated intravascular coagulation (DIC)
    • Open‑heart surgery / cardiopulmonary bypass
  • Toxicity:
    • Bleeding (major)
    • Heparin‑induced thrombocytopenia (HIT) — immune-mediated, paradoxical thrombosis
    • Osteoporosis (long‑term use)
    • Hypersensitivity
  • HIT: suspect when platelet count drops >50% or <100,000/μL on days 5–10 — stop heparin immediately; use direct thrombin inhibitors (argatroban) instead
  • Enoxaparin (LMWH): SC administration, longer half‑life, less thrombocytopenia, enhanced anti‑Xa activity, no aPTT monitoring required
anticoagulant

💊 Warfarin (Coumarin)

  • Chemistry: small molecule, lipid‑soluble vitamin K derivative
  • Route: oral (PO)
  • Bioavailability: 98% plasma protein bound
  • Half‑life: >30 hours
  • Metabolism: hepatic via CYP450 (2C9, 1A2, 3A4)
  • Placental transfer: yes (teratogenic — bone dysmorphogenesis)
  • Mechanism:
    • Inhibits vitamin K epoxide reductase
    • Prevents γ‑carboxylation of factors II, VII, IX, X
    • No effect on pre‑existing circulating factors
  • Monitoring: PT / INR (prothrombin time)
  • Reversal:
    • Vitamin K (slow, hours to days)
    • Fresh frozen plasma (FFP) — rapid restoration
  • Uses:
    • Long‑term anticoagulation: DVT, PE, post‑MI, atrial fibrillation, mechanical heart valves
  • Toxicity:
    • Bleeding (major — intracranial, GI)
    • Skin necrosis (early therapy, protein C deficiency state)
    • Teratogenicity (fetal bone dysplasia, CNS defects)
    • Drug interactions (see below)

Drug Interactions

MechanismEffect on INRExamples
↓ Oral absorption↓ INRCholestyramine
Protein binding displacement↑ INR (↑ free fraction)ASA, sulfonamides, phenytoin
CYP450 induction↓ INR (accelerated clearance)Barbiturates, carbamazepine, rifampin
CYP450 inhibition↑ INR (reduced clearance)Cimetidine, macrolides, azole antifungals
  • Protein C has the shortest half‑life (~8 h) among vitamin K–dependent factors — warfarin initiation can cause transient hypercoagulability and skin necrosis
  • Bridge with heparin when starting warfarin in high‑risk patients
direct inhibitors

🎯 Direct Inhibitors of Activated Clotting Factors

Direct Thrombin Inhibitors

  • Argatroban
    • IV; used in HIT (no cross‑reactivity with heparin antibodies)
    • Does not require ATIII
  • Dabigatran
    • Oral, no PT/INR monitoring
    • Atrial fibrillation (alternative to warfarin)
    • Reversal: idarucizumab (monoclonal antibody)
  • Bivalirudin
    • IV; used with aspirin in unstable angina / PCI
    • Short half‑life, reversible

Direct Factor Xa Inhibitors ("-xabans")

  • Rivaroxaban (and apixaban, edoxaban)
    • Oral, no PT/INR monitoring required
    • Indications: DVT prophylaxis (knee/hip surgery), stroke prevention in non‑valvular AF
    • Reversal: andexanet alfa (recombinant modified factor Xa decoy)
  • Direct inhibitors target the common pathway (IIa or Xa) and do not require ATIII
  • They have predictable pharmacokinetics and fewer drug–food interactions than warfarin
thrombolytic

🧬 Thrombolytics (Fibrinolytics)

  • Convert plasminogen → plasmin (serine protease)
  • Plasmin degrades fibrin → dissolves clots
  • Used IV in emergency settings:
    • Acute MI (ST‑elevation)
    • Massive PE with hemodynamic instability
    • Ischemic stroke (within 3–4.5 hours)

Streptokinase

  • Bacterial protein from β‑hemolytic streptococci
  • Acts on free and fibrin‑bound plasminogen (not clot‑specific)
  • Depletes circulating plasminogen and factors V + VIII
  • Antigenic — pre‑existing antibodies may reduce efficacy
  • May cause hypersensitivity and hypotension

Alteplase (tPA)

  • Recombinant tissue plasminogen activator
  • Clot‑specific — preferentially activates fibrin‑bound plasminogen
  • Low allergy risk (human protein)
  • First‑line for ischemic stroke (time‑dependent)
  • Effectiveness is time‑critical: >60% mortality reduction if given within 3 hours of MI onset
  • Major complication: bleeding (intracerebral hemorrhage most serious)
  • Antidotes for excessive bleeding: aminocaproic acid, tranexamic acid (antifibrinolytics)
  • Adjuvant therapies in MI: ASA, beta‑blockers, nitrates (↓ mortality); adenosine (↓ infarct size)
  • tPA is preferred over streptokinase for stroke due to clot specificity and lower immunogenicity
antiplatelet

🧫 Antiplatelet Agents

  • Platelet activation cascade:
    • Adhesion to injured endothelium (von Willebrand factor, collagen)
    • Activation → release of ADP, TXA₂, 5‑HT, thrombin
    • Conformational change of GP IIb/IIIa receptors
    • Cross‑linking via fibrinogen → aggregation
  • Endogenous inhibitors: PGI₂ (prostacyclin), ↑ cAMP

Aspirin (ASA)

  • Irreversibly inhibits COX‑1 in platelets → ↓ TXA₂
  • Low‑dose: primary/secondary prevention of MI, stroke; atrial arrhythmias; TIAs
  • Adverse: GI bleeding, hypersensitivity (aspirin‑exacerbated respiratory disease)

ADP Receptor Blockers

  • Clopidogrel, prasugrel, ticagrelor
  • Inhibit P2Y₁₂ ADP receptor → ↓ platelet activation
  • Used in ACS, post‑MI, TIAs (alternative to ASA)
  • Dual antiplatelet therapy (ASA + ADP blocker) in NSTEMI / post‑stenting
  • Adverse: bleeding, leukopenia, TTP (rare)

GP IIb/IIIa Inhibitors

  • Abciximab, eptifibatide, tirofiban
  • Bind to glycoprotein IIb/IIIa receptors → block fibrinogen cross‑linking → prevent aggregation
  • Used in acute coronary syndromes and percutaneous coronary intervention (PCI)
  • Administered IV; bleeding is the main adverse effect
  • Dual antiplatelet therapy (ASA + P2Y₁₂ inhibitor) is standard after drug‑eluting stent placement — duration depends on bleeding risk
reference

⚖️ Comparison Tables

Heparin vs. Warfarin

FeatureHeparinWarfarin
Chemical natureLarge polysaccharideSmall lipophilic molecule
RouteIV / SCOral
Half‑life~2 h>30 h
Placental transferNoYes (teratogenic)
MechanismATIII‑mediated inhibition of IIa, Xa↓ synthesis of factors II, VII, IX, X
MonitoringaPTTPT / INR
ReversalProtamine sulfateVitamin K / FFP

Antiplatelet Drug Targets

DrugTargetReversibilityKey Use
AspirinCOX‑1 (TXA₂)IrreversiblePrimary/secondary prevention
ClopidogrelP2Y₁₂ (ADP receptor)IrreversibleACS, post‑stent
TicagrelorP2Y₁₂ReversibleACS
AbciximabGP IIb/IIIaReversiblePCI, ACS
high‑yield

💎 Clinical Pearls & Exam Traps

  • Warfarin effect lags 2–3 days because existing factors must be cleared
  • Heparin acts immediately — use for acute thrombosis, bridge to warfarin
  • Protein C half‑life ~8 h → warfarin can cause skin necrosis in first days
  • LMWH (enoxaparin) is preferred in pregnancy over warfarin (no placental transfer)
  • HIT is a clinical diagnosis: platelet drop + thrombosis — stop all heparin, use argatroban
  • Direct oral anticoagulants (DOACs) do not require routine monitoring but reversal agents exist: idarucizumab (dabigatran), andexanet alfa (Xa inhibitors)
  • tPA for stroke: must rule out hemorrhage before administration — time window is critical
  • Antiplatelet agents increase bleeding risk — hold before surgery based on drug half‑life
  • Coagulation factor half‑lives (high‑yield):
    • Factor VII: ~6 h (shortest) → warfarin affects PT/INR first
    • Protein C: ~8 h → transient hypercoagulability
    • Factor II: ~60 h → warfarin effect on thrombin generation lags
    • Factor IX: ~24 h, Factor X: ~40 h
  • Thrombolytic time sensitivity: MI < 3 h → greatest benefit; stroke 3–4.5 h window
  • Dual antiplatelet therapy (DAPT): ASA + P2Y₁₂ inhibitor reduces stent thrombosis but increases bleeding — duration individualized