Drugs Used in Disorders of Coagulation – Study Notes (Transcript 09/04/2025)

Stages of Hemostasis

Vascular Phase (first response to vessel injury)
- Damaged endothelial cells release Endothelin → vasoconstriction
- Endothelial injury leads to platelet activation and release of mediators (see Platelet Phase)
Platelet Phase (Primary Hemostasis)
- Vessel injury exposes collagen; endothelial cells release von Willebrand factor (vWF) causing platelets to adhere to collagen in the vessel wall, undergo shape change, and become activated
- Activated platelets release mediators that amplify activation: ADP, thromboxane A2 (TxA2), serotonin
- Release of Ca^{2+} helps later coagulation and activates additional receptors
- Receptors on platelets activated:
- ADP Receptor (P2Y1/P2Y12)
- Thrombin Receptor (PAR-1/PAR-4)
- Fibrinogen Receptor (GP IIb/IIIa)
- Platelet Aggregation occurs via GP IIb/IIIa binding to Fibrinogen, forming a Platelet Plug
Coagulation Phase (Secondary Hemostasis)
- Intrinsic and Extrinsic pathways converge to activate Factor Xa
- Factor Xa converts Prothrombin into Thrombin
- Thrombin (major mediator) converts Fibrinogen → Fibrin
- Thrombin also converts Factor XIII into Factor XIIIa, which cross-links Fibrin, stabilizing the platelet plug into a firm fibrin clot
- Thrombin initiates a positive feedback loop, promoting a "thrombin burst" by activating additional coagulation factors (intrinsic pathway)
- Ca^{2+} is required for the whole coagulation pathway
- Endothelial regulation modulates coagulation (see Endothelial Regulation)
Fibrinolysis
- Endothelial cells release tissue plasminogen activator (tPA)
- tPA converts Plasminogen → Plasmin
- Plasmin digests Fibrin and dissolves the clot, restoring blood flow
Endothelial Regulation of Hemostasis
- Endothelial cells also release Prostacyclin (PGI_2)
- PGI_2 inhibits thrombus formation by binding to platelet receptors and increasing cAMP
- Increased cAMP leads to sequestration of intracellular Ca^{2+} and inhibits platelet activation and aggregation
True/False checks
- T/F: All the mechanisms of coagulation happen locally → True
- Diagrammatic understanding: Platelet adhesion and aggregation involves multiple mediators and receptors as described above

Synthesis of Thromboxane A2 and Platelet Activation Mediators

Thromboxane A2 (TxA2) synthesis
- Synthesized by COX-1 + Thromboxane A2 Synthase
- Question: What synthesizes TxA2? COX-1 + Thromboxane A2 Synthase
Activated platelets
- Activated platelets are platelets that have engaged collagen fibers or von Willebrand factor, changing shape, increasing surface area, and enabling platelet plug formation
- Activated platelets release: ADP, serotonin, and thromboxane A2
- Thrombin also causes further activation of platelets (positive feedback)
Receptors on platelets
- ADP Receptor (P2Y1/P2Y12)
- Thrombin Receptor (PAR-1/PAR-4)
- Fibrinogen Receptor (GP IIb/IIIa)
Roles of TxA2 and serotonin
- Released from activated platelets; cause vasoconstriction of blood vessels to aid hemostasis

Platelet Activation and the Amplification Cascade

What is paracrine activation?
- Platelets activating other platelets through ADP, serotonin, and thromboxane A2
What is autocrine activation?
- Platelets activating themselves through ADP, serotonin, and thromboxane A2
During platelet aggregation, mediators released to amplify activation
- ADP, Serotonin, TxA2
Role of Ca^{2+}
- Ca^{2+} is required for the coagulation pathway and for proper platelet activation

Platelet Adhesion, Aggregation, and Solubility of Fibrinogen/Fibrin

What is von Willebrand factor (vWF)?
- Endothelial cells release vWF; it promotes platelet adhesion at sites of vascular injury
Solubility of coagulation proteins
- Fibrinogen is soluble
- Fibrin is insoluble
During platelet aggregation, mediators that amplify activation are released
- ADP, Serotonin, TxA2
Receptors for platelet activation and their targets
- ADP Receptor (P2Y1/P2Y12) targeted by platelet activation pathways
- Other platelet receptors include GP IIb/IIIa (fibrinogen receptor) and PAR receptors for thrombin
What is paracrine activation? (definition as above)
What is autocrine activation? (definition as above)

Coagulation Pathways and Ca^{2+} Dependency

Coagulation cascade overview
- Intrinsic and Extrinsic pathways converge to activate Factor Xa, which converts Prothrombin to Thrombin
- Thrombin generates Fibrin from Fibrinogen and activates Factor XIII to XIIIa to cross-link fibrin
Ca^{2+} dependence
- Ca^{2+} is required for the coagulation cascade (throughout the pathway)
Receptors and targets in the coagulation pathway
- ADP Receptor (P2Y1/P2Y12) is a key receptor in platelet activation and is part of the pathway described as a target of the - grels in the diagram

Antithrombotic Regulation and Inhibitors

Thrombin roles in coagulation (4 roles)
1) Fibrin generation
2) Positive feedback loop leading to a thrombin burst
3) Activation of Factor XIII to XIIIa, stabilizing the clot
4) Activation of platelets by binding to PAR receptors on platelets
Inhibitors of plasmin and fibrinolysis regulators
- PAI-1 and PAI-2 are plasminogen activator inhibitors that inhibit t-PA, preventing excess plasmin generation
- α2-antiplasmin inhibits plasmin activity
Platelet activation inhibitors
- Prostacyclin (PGI_2) inhibits platelet activation and aggregation (via increasing cAMP)
Antithrombin and Protein C/S pathways
- Antithrombin inhibits factors including IIa (thrombin) and Xa
- Protein C and Protein S provide anticoagulant effects
Endothelial-derived regulators
- PGI_2 (as above) and Nitric Oxide (NO) produced by endothelial cells prevent platelet activation and aggregation

Therapeutic Targets and Agents Mentioned

Antiplatelet therapy (aspirin)
- Drug mentioned: Aspirin reduces platelet aggregation
- Note: Specific MOA (e.g., COX-1 inhibition) is not detailed in the provided content, but aspirin is identified as reducing platelet aggregation
Anticoagulant targets mentioned
- Warfarin, rivaroxaban, apixaban, dabigatran, and edoxaban target Factor Xa or thrombin
- Summary in content: These drugs target Factor Xa → formation of Thrombin
- Implication: Inhibit thrombin generation and downstream clot formation

Quick Reference: Key Equations and Concepts (LaTeX)

Prothrombin to Thrombin conversion
- ext{Prothrombin}
  ightarrow_{ ext{Factor~Xa}} ext{Thrombin}
Thrombin converts Fibrinogen to Fibrin
- ext{Fibrinogen}
  ightarrow_{ ext{Thrombin}} ext{Fibrin}
Cross-linking of fibrin by Factor XIII
- ext{Factor XIII}
  ightarrow ext{Factor XIIIa}
Plasminogen activation by tPA
- ext{Plasminogen}
  ightarrow_{ ext{tPA}} ext{Plasmin}
Plasmin-mediated fibrin breakdown
- Plasmin digests fibrin and dissolves the clot
Endothelial regulation via PGI_2 and cAMP
- ext{PGI}_2
  ightarrow ext{↑}cAMP
  ightarrow ext{Ca}^{2+} ext{ sequestration}
  ightarrow ext{inhibition of platelet activation}
Thrombin’s four roles in coagulation
- 1) Fibrin generation
- 2) Positive feedback (thrombin burst)
- 3) Activation of Factor XIII to XIIIa
- 4) Activation of platelets via PAR receptors

Connections to Foundational Concepts and Relevance

Integrates primary hemostasis (platelet plug formation) with secondary hemostasis (coagulation cascade and fibrin formation)
Highlights balance between pro-coagulant forces and endogenous anticoagulant/regulatory mechanisms (Antithrombin, Protein C/S, PGI_2, NO, PAI-1/2, α2-antiplasmin)
Demonstrates multiple drug targets: antiplatelet agents (e.g., aspirin) and anticoagulants (Factor Xa inhibitors, direct thrombin inhibitors, and vitamin K antagonists like warfarin) and their impact on thrombin generation and clot stabilization
Clinically relevant: understanding stages helps explain bleeding risk vs thrombosis risk and informs selection of therapeutic strategies in coagulation disorders