Gurusamy Lecture-1 Physiology of Coagulation Sep 4 2024 PDF

Drugs Used in Disorders of Coagulation (PHRC-5832) Dr. Narasimman Gurusamy Assistant Professor College of Pharmacy 954-262-1322 [email protected] Learning Objectives ✓ 1. Mechanisms of coagulation and hemostasis....

Drugs Used in Disorders of Coagulation (PHRC-5832) Dr. Narasimman Gurusamy Assistant Professor College of Pharmacy 954-262-1322 [email protected] Learning Objectives ✓ 1. Mechanisms of coagulation and hemostasis. ✓ 2. Major classes of antiplatelet (antithrombotic) and anticoagulant medications and their mechanisms of action. ✓ 4. Toxicities and drug interactions associated with the various anti-thrombotic and anticoagulant agents.  Hemostasis Hemo=Blood Stasis=Stop or Still Hemostasis is a complex process involving a number of clotting factors that are activated in a sequential steps, or cascade, to prevent or stop bleeding. Drugs are used to modify, interrupt, or enhance this process.  Normal Hemostasis When a small blood vessel is injured, hemorrhage is prevented. It occurs in Four phases Vascular phase (vasospasm reduces blood flow and facilitates platelet aggregation and coagulation). Platelet phase (platelet aggregation and formation of a platelet plug to arrest bleeding). Coagulation phase (formation of a fibrin clot; exposure of the blood to tissue factors to arrest bleeding until the vessel is repaired). Fibrinolytic phase (fibrinolysis; breakdown of the clot, after the vessel is repaired).  Vascular phase Cutting or damaging blood vessels leads to vascular spasm (vasoconstriction) of the smooth muscle in the vessel wall. This vasoconstriction slow or even stop blood flow. Endothelin, a potent vasoconstrictor, released from endothelial cells causes the smooth muscle in blood vessel walls to contract Thromboxane A2, Serotonin released from activated platelets also has vasoconstrictive properties. This response will last up to 20-30 minutes and is localized to the damaged area. Overview of platelets adhesion and activation at the site of injury 1. Vasoconstriction: Blood vessel narrows to reduce blood flow. 2. Collagen Exposure: Injury exposes vessel wall collagen. 3. Adhesion: Platelets bind to exposed collagen. 4. Shape Change: Platelets change to spiky shape. 5. Granule Release: Platelets release activating substances (ADP (adenosine diphosphate), serotonin, and thromboxane A2) 6. Aggregation: Platelets stick together at injury site. 7. Thromboxane A2: Released, enhances activation. 8. Coagulation Cascade: Initiates fibrin clot formation. 9. Fibrin Clot: Mesh traps cells, reinforces plug. 10. Healing: Clot retracts, aiding vessel repair. Platelet phase Platelets adhere to the damaged surface and form a temporary plug Thrombus formation at the site of the damaged vascular wall (EC, endothelial cell) Platelets become “sticky” and adhere to: Collagen, the basement membrane and microfibrils in the subendothelium of blood vessels (platelet adhesion) at the injured site. Platelet membrane receptors include the ✓ glycoprotein (GP) Ia / GPVI receptor, binding to collagen (C); ✓ GP Ib receptor, binding von Willebrand factor (vWF); and ✓ GP IIb/IIIa, which binds fibrinogen and other macromolecules. Collagen also activates platelets (platelet aggregation). Collagen activates receptors on the surface of platelets triggering the release of platelet granules containing adenosine diphosphate (ADP) and serotonin (5HT). Thromboxane A2 (TXA2) is also formed and released into the plasma. ADP, 5HT and TXA2 promote platelet aggregation that is necessary for the rapid formation of a hemostatic plug. TXA2 also increases release of aggregating factors from granules. Antiplatelet prostacyclin (PGI2) is released from the endothelium. Platelet adhesion and aggregation. Glycoprotein GPVI and GPIb are platelet receptors that bind to collagen and von Willebrand factor (vWF), causing platelets to adhere to the subendothelium of a damaged blood vessel. Protease-activated receptor-1 (PAR-1 and PAR-4), thrombin receptors on human platelets that respond to thrombin (IIa); P2Y1 and P2Y12 are receptors for ADP; when stimulated by agonists, these receptors activate the fibrinogen-binding protein GPIIb/IIIa and COX-1 to promote platelet aggregation and secretion. Thromboxane A2 (TxA2) is the major product of COX-1 involved in platelet activation. Prostacyclin (PGI2), synthesized by endothelial cells, inhibits platelet activation.  Coagulation phase The overall process involves the formation of the insoluble protein fibrin from the plasma protein fibrinogen through the action of the enzyme thrombin. Fibrin forms a network of fibers which traps blood cells and platelets forming a thrombus or clot. This process depends on the presence in the blood of 11 different clotting factors (proteins) and calcium (Factor IV). Ultimately, these factors will generate the production of prothrombin activator (Factor X). The pathways leading to the formation of the thrombus; the Extrinsic Pathway, Intrinsic Pathway and Common pathway  General mechanism of Coagulation of Blood or Fibrin Formation Most coagulation factors are Protease from the preceding step cleaves zymogen creating next enzyme in cascade. Protease cleaves one glycoproteins that normally exist as or more peptide bonds in the precursor molecule. inactive pre-enzymes known as Cofactor provides an anchor (receptor) for the zymogens. reaction to take place- binds to platelets. Also Each step requires the Localizes reaction ✓ protease from the preceding step, Calcium needed by proteases for binding of the ✓ a zymogen, zymogen to the phospholipid surface- allows it to ✓ a nonenzymatic protein cofactor become activated (eg factor V, and tissue factor), Organizing surface Platelets provide phospholipids. ✓ calcium and They also release stored clotting factors. - reaction is ✓ an organizing surface (e.g. localized by this anchor platelets). Activated zymogen catalyzes the conversion of the next zymogen in the series to its active enzymatic form until thrombin converts fibrinogen to fibrin. Extrinsic pathway The extrinsic pathway The extrinsic pathway is initiated outside the bloodstream Tissue factor (TF) is an integral membrane protein, normally separated from the blood by the vascular endothelium, which plays a key role in the initiation of blood coagulation. With a perforating vascular injury, TF becomes exposed to blood and binds plasma factor VIIa. The resulting complex initiates a series of enzymatic reactions leading to activation of factor X, and clot formation through common pathway. The intrinsic pathway Within blood vessels, thrombin generated from previous activation of the extrinsic pathway also activates the intrinsic pathway. The intrinsic pathway can be activated by damage to the vascular endothelium leads to exposure of clotting factors to negatively charged subendothelial surfaces, mediated by the molecule Kallikrein. Reactions in the intrinsic pathway include: Surface contact activates factor XII Factor XIIa activates factor XI Factor XIa activates factor IX Factor IXa combines with factor VIIIa, platelet membrane phospholipid and Ca2+ ions to activate factor X Factor Xa activates the common pathway generating more thrombin Vast amounts of thrombin can be generated from a single initial stimulus through the creation of a positive feedback loop. The common pathway Factor Xa combines with factor V, platelet membrane phospholipids and Ca2+ ions to convert prothrombin into thrombin. Thrombin then converts fibrinogen into fibrin strands which form an important structural component of a thrombus. Fibrin monomers rapidly polymerize to fibrin strands, which forms the mesh-like matrix of the blood clot. Clot is further stabilized by the crosslinking action of factor XIII which is activated by thrombin. Polymerizati on of fibrin and cross linking Pathway Activation Components Outcome Tissue factor: released Tissue factor Extrinsic due to vascular damage Factor VII Activation of factor X Surface contact: Factor XII exposure of coagulation Factor XI factors to negatively Factor IX charged vascular Factor VIII Intrinsic subendothelial surfaces Platelet Activation of factor X Thrombin: released at membrane the end of the common phospholipids pathway (positive Ca2+ ions feedback loop) Generation of Factor X thrombin: Factor V Converts fibrinogen Factor Xa: activation of Prothrombin to fibrin factor X by either Platelet Activates intrinsic Common the extrinsic or intrinsic membrane pathway pathway phospholipids Activates factor XIII Ca2+ ions Platelet activation Fibrinogen Regulation of clot formation Actions of thrombin Thrombin plays a significant role in many aspects of coagulation and haemostasis: Fibrin generation: thrombin converts fibrinogen into fibrin Intrinsic pathway activation: thrombin generates a positive feedback loop by initiating the intrinsic pathway leading to the formation of large amounts of additional thrombin (a ‘thrombin burst’). Factor XIII activation: thrombin converts factor XIII into factor XIIIa (fibrin stabilising factor) which then cross-links fibrin to generate a ‘fibrin mesh’ which encapsulates activated platelets creating a thrombus and stopping bleeding Platelet activation: thrombin receptors on platelets cause activation and aggregation, further enhancing the haemostatic effects of the coagulation cascade Regulation of clot formation  Fibrinolytic phase Anticlotting mechanisms are activated to allow clot disintegration and repair of the damaged vessel. Fibrinolysis involves the conversion of inactive plasminogen to plasmin. Plasmin, a powerful proteolytic enzyme, which dissolves /breakdown the clot Plasmin is formed through the same chemical pathway that produces thrombin. Fibrinolysis Plasmin digests fibrin to limit the extent of thrombosis. Fibrinolysis. Endothelial cells secrete t-PA at sites of injury. t-PA binds to fibrin and converts plasminogen to plasmin, which digests fibrin. PAI-1 and PAI-2 inactivate t-PA; α2-AP inactivates plasmin. Natural (endogenous) mechanisms In normal blood vessels there is a delicate balance between procoagulant and anticoagulant processes. Platelets remain quiescent or active depending on the balance between activating and inhibiting chemicals. Endothelial Prostacyclin Platelet ADP: a powerful inducer (PGI2): inhibits thrombus of platelet aggregation formation Platelets Thromboxane A2 Antithrombin (AT) (TXA2): induces thrombogenesis ✓ Inactivates factors like IIa, and vasoconstriction IXa, Xa, XIa & XIIa Platelets Serotonin (5-HT): Endogenous anticoagulants: stimulating further aggregation protein C and S and vasoconstriction ✓ Inactivates factors like Va & VIIIa  Platelet Signals Activators Inhibitors Prostacyclin (PGI2) ADP Nitric Oxide Collagen cAMP Thrombin- needs it!! Decreased levels of thrombin and Thromboxanes (TXA2) thromboxanes Serotonin PGI2 binds to platelet receptors and Epinephrine increase cAMP. This leads to Antigen-Antibody Complex sequestering of Ca2+ which inhibits Viruses and Bacteria platelet aggregation. Nitric Oxide (NO) is also produced by EC which prevents platelet activation and aggregation. Hemostasis DEPENDENT UPON: 1. Vessel Wall Integrity 2. Adequate Numbers of Platelets 3. Proper Functioning Platelets 4. Adequate Levels of Clotting Factors 5. Proper Function of Fibrinolytic Pathway Anticoagulant components of hemostasis 1. Smooth, intact endothelium - an undamaged endothelial lining prevents the initiation of hemostasis. 2. Thrombin adsorption to fibrin - 90% of thrombin formed during hemostasis is adsorbed to fibrin preventing the diffusion of thrombin to surrounding areas. 3. Heparin - released from mast cells (tissue basophils) inactivates thrombin. 4. Activated thrombin - stimulates endothelial cells to release a prostaglandin -prostacyclin. Prostacyclin prevents adherence of platelets to surrounding, uninjured endothelial cells, inhibits the aggregation of platelets and produces vasodilation. 5. Fibrinolytic system - is turned on as a direct outcome of the clotting process. Hageman factor (XII) activates plasminogen forming the fibrin digesting enzyme plasmin.  Regulation of Hemostasis Localized hemostasis in response to bleeding or trauma is achieved through fibrin inhibition and fibrinolysis. Coagulation proteins are inactivated by: ✓ 2-macroglobin (broad-spectrum protease inhibitor) ✓ 2-antiplasmin (plasmin proteolysis, inhibition of plasminogen binding to fibrin, and cross-linking fibrin) ✓ Antithrombin III (intrinsic and common pathway); ✓ Protein C (degrades factors Va and VIIIa) ✓ Protein S inhibits factor IXa  Pathological Thrombus Formation Thrombus, a clot that adheres to a vessel wall Embolus, a clot that floats within the blood Both thrombi and emboli are dangerous because they may occlude blood vessels and deprive tissues of oxygen and nutrients. If formed in lungs, known as pulmonary embolism Anticoagulants are drugs that retard coagulation and thereby prevent the occurrence of a thrombus.  Treatment Strategies The nature of a clot depends on the site at which it is formed: White clot is found in rapidly flowing (arterial) blood and consists mainly of aggregated platelets. Red clot is found in slow moving (venous) or stagnant blood mainly consisting of fibrin strands with trapped blood cells. In general, a white clot is treated with antiplatelet drugs and anticoagulants and a red clot is treated with anticoagulants and fibrinolytic drugs.  Deep vein thrombosis (DVT) often occur in veins due to factors like stasis, endothelial damage, and hypercoagulability. Excessive of hemostasis results in disseminated intravascular coagulation (DIC).  Classification of anticoagulants There are a number of different Anti-coagulants = Drugs used to reduce categories of drugs which the coagulability of blood modify the coagulation process: Anti-thrombotics (Anti-platelets) = I. Anticoagulants Drugs which interfere with platelet II. Antiplatelet agents functions (Thrombocytes – Platelets) III. Thrombolytics Thrombolytics (Fibrinolytics) = Drugs used to lyse thrombin clot (reduces blood clots) 29 Anticoagulant Drugs Parenteral:  Oral: Indirect thrombin inhibitors Warfarin Unfractionated Heparin (UFH) Dabigatran etexilate Low molecular weight heparins Rivaroxaban Enoxaparin Dalteparin Apixaban Tinzaparin Edoxaban Fondaparinux Betrixaban Direct thrombin inhibitors Hirudin  Reversal agents for oral Bivalirudin anticoagulants: Desirudin Idarucizumab Argatroban Andexanet alfa Classification of antiplatelets (based on mechanism of action) Protease-activated receptor-1 (PAR-1) inhibitors Aspirin Ticlopidine Ticagrelor Abciximab Eptifibatide Dipyridamole Vorapaxar Clopidogrel Cangrelor Tirofiban Cilostazol Prasugrel Thrombolytics: Streptokinase Anysoylated plasminogen activator Antifibrinolytic agents: complex Aminocaproic acid Tissue plasminogen activator Tranexamic acid Tenecteplase Aprotinin Reteplase and Urokinase