Pharmacology · Coagulation

Coagulation Physiology and Pharmacological Targets

Hemostasis, the cascade, natural anticoagulants, fibrinolysis, and drug targets

Hemostasis arrests bleeding in two overlapping phases: primary hemostasis builds a platelet plug within seconds to minutes, and secondary hemostasis reinforces it with a cross-linked fibrin meshwork. The cellular events of each phase define the drug targets.
Primary hemostasis supplies three antiplatelet targets: thromboxane A2 synthesis through cyclooxygenase-1 (aspirin), the P2Y12 receptor (clopidogrel, prasugrel, ticagrelor, cangrelor), and glycoprotein IIb/IIIa, the final common pathway of aggregation.
Coagulation in vivo is initiated almost exclusively through the extrinsic pathway, with the intrinsic pathway amplifying thrombin generation rather than starting it. Thrombin is the central effector and is directly or indirectly the target of every major anticoagulant class.
The natural anticoagulant systems — antithrombin III, the protein C and protein S pathway, and tissue factor pathway inhibitor — are simultaneously the molecular route through which the heparins act and the origin of the inherited thrombophilias.
Arterial thrombosis is platelet-rich and forms under high shear at ruptured plaque, so antiplatelet therapy is primary; venous thrombosis is fibrin-rich and forms under stasis, so anticoagulation is primary. This single distinction drives drug selection across the thrombotic syndromes.

Primary Hemostasis — Platelet Pharmacological Targets

Aspirin Target

Cyclooxygenase-1

Converts arachidonic acid to thromboxane A2
Aspirin irreversibly acetylates cyclooxygenase-1
Thromboxane A2 blockade lasts full platelet lifetime (8–10 days)

P2Y12 Inhibitor Target

P2Y12 Receptor

Adenosine diphosphate receptor sustaining platelet activation
Clopidogrel, prasugrel: irreversible prodrugs
Ticagrelor, cangrelor: reversible, direct-acting

GPIIb/IIIa Inhibitor Target

Glycoprotein IIb/IIIa

Final common pathway of platelet aggregation
Binds fibrinogen and von Willebrand factor
Abciximab, eptifibatide, tirofiban: intravenous use

Coagulation Cascade — Drug Target Map

Cascade Node	Physiological Role	Drug Class	Agents
Vitamin K factors	Synthesis of factors II, VII, IX, X; proteins C, S	Vitamin K antagonist	Warfarin
Factor Xa	Convergence of extrinsic and intrinsic pathways; generates thrombin	Indirect (via antithrombin III) or direct	Heparins, fondaparinux; rivaroxaban, apixaban, edoxaban
Thrombin (factor IIa)	Cleaves fibrinogen; activates factors V, VIII, XIII; activates platelets	Indirect or direct thrombin inhibitor	Unfractionated heparin; dabigatran, bivalirudin, argatroban
Fibrinolysis	Plasmin dissolves fibrin clot	Thrombolytics / antifibrinolytics	Alteplase, tenecteplase; tranexamic acid, epsilon-aminocaproic acid

Natural Anticoagulant Systems

Serpin

Antithrombin III

Inhibits thrombin and factor Xa (slow alone)
Heparin accelerates inhibition ~1,000-fold
Deficiency: heparin resistance; antithrombin III concentrate needed

Vitamin K-Dependent Pathway

Protein C / Protein S

Thrombomodulin activates protein C when bound to thrombin
Activated protein C + protein S inactivate factors Va and VIIIa
Factor V Leiden: activated protein C resistance; 3–8× venous thromboembolism risk
Warfarin start without bridging in protein C deficiency: skin necrosis risk

Extrinsic Pathway Brake

Tissue Factor Pathway Inhibitor

Inhibits factor Xa first, then the tissue factor-factor VIIa complex
Limits duration of extrinsic pathway activation
Released from endothelium by heparin

Fibrinolysis — Thrombolytic and Antifibrinolytic Targets

Thrombolytic Target

Plasminogen and Plasmin

Plasmin cleaves fibrin into soluble degradation products including D-dimer
Tissue-type plasminogen activator released from endothelium by thrombin and shear stress
Alteplase, reteplase, tenecteplase: recombinant tissue-type plasminogen activator forms

Basis of Clot Selectivity

Fibrin Specificity

Tissue-type plasminogen activator has low activity until bound to fibrin
Ternary complex concentrates plasmin generation at the clot surface
Streptokinase activates circulating plasminogen nonspecifically: systemic lytic state

Antifibrinolytic Target

Lysine-Binding Sites

Tranexamic acid, epsilon-aminocaproic acid: lysine analogues
Competitively block plasminogen binding to fibrin
Endogenous brakes: plasminogen activator inhibitor-1 and alpha-2 antiplasmin

Arterial vs. Venous Thrombosis — Drug Selection Rule

Arterial thrombosis is platelet-rich (high shear, plaque rupture): antiplatelet therapy is primary. Venous thrombosis is fibrin-rich (low shear, stasis): anticoagulant therapy is primary. Direct oral anticoagulants are first-line for most venous thromboembolism. Warfarin remains required for mechanical heart valves.

Suggested References

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