Handout 3: Anticoagulants Pharmacology

PHAR 740 January 25, 2024 Anticoagulants Pharmacology Required Reading: Basic & Clinical Pharmacology 15th edition, Katzung, Chapter 34 Key Concepts      Differentiate between the primary and secondary hemostasis based on its role in thrombosis Describe the events in coagulation cascade Compare and contrast the intrinsic and extrinsic pathways Explain the role and components of the common pathway List the anticoagulants pharmacological agents Warfarin, Heparin, Direct Thrombin Inhibitors, Factor Xa inhibitor Coagulation Cascade In the previous handout, we reviewed the primary Hemostasis which includes the activation and aggregation of platelets. The secondary hemostasis is the activation of coagulating cascade resulting in the formation of a stable fibrin clot at the site of vascular injury. In brief, once the endothelial is injured and blood is exposed to tissue factors, a cascade of events take place to form hemostatic plug. The primary response is vasoconstriction which serves to decrease blood flow, followed by platelets adherence to the ruptured endothelium and then platelet activation and aggregation. Concurrently, the thrombin becomes activated through extrinsic/intrinsic coagulating pathways to convert fibrinogen to fibrin, an insoluble protein. The end result of the clotting pathway is the formation of a stabilized fibrin clot. 1 Dr. Khansari, Handout 3 Copyrighted PHAR 740 January 25, 2024 Historically, coagulation pathways were divided into intrinsic and extrinsic pathways and a common pivotal point between the two. However, evidence suggests that coagulation starts with the extrinsic pathway (exposure to tissue factors) and is amplified through activation of intrinsic pathways (contact activation). All of the factors have roman numerals and the activated forms are signified by “a”. Most of the coagulation factors are made by the liver, with the exception of factor XIII derived from platelets and factor VIII derived from endothelial. Coagulation involves the stepwise activation of a series of inactive zymogen (proenzyme) which leads to the formation of fibrin matrix. This formation covalently cross-links and stabilizes the fibrin clot. A major difference between the intrinsic and extrinsic pathways is the requirement of tissue factors in extrinsic pathways. Tissue factor (factor III) is a cell surface glycoprotein which serves as the receptor for factor VII. Factor Xa is the common product of extrinsic and intrinsic pathways and is the active catalytic component which converts prothrombin to thrombin. Activated factor X (Xa) can be formed through either pathway to converts prothrombin to thrombin. This Xa-Va-Ca++PL (PL=phospholipids from platelets) complex is the pivotal step of the coagulation system. The essential cofactor for this reaction is factor Va. An addition of phospholipids and calcium accelerate the activation of factor X significantly by as near as 20,000 fold faster. 2 Dr. Khansari, Handout 3 Copyrighted PHAR 740 January 25, 2024 Anticoagulants  Heparin o Unfractionated o Low Molecular Weight  Direct Thrombin Inhibitors  Factor Xa inhibitor  Vitamin K antagonist Anticoagulants can also be categorized into two classes: oral anticoagulants which includes warfarin and the new drugs, rivaroxaban (Xa inhibitor) and dabigatran (block thrombin active site). The rest of the anticoagulants are intended for parental administration. 3 Dr. Khansari, Handout 3 Copyrighted PHAR 740 January 25, 2024 Learning Objectives  Describe the physiological process of endogenous fibrinolytics  Describe the mechanism by which each of the following regulates the hemostasis: Prostaglandin (PGI2), Antithrombin III, Protein C and Protein S, Tissue Factor Pathway Inhibitor (TFPI), Plasmin, thrombin.  List and describe the mechanism of action and major side effects of fibrinolytic agents Coagulation Cascade In the previous class, we reviewed the process by which injured endothelium cells initiate primary and secondary Hemostasis which results in the formation of a stable fibrin clot at the site of vascular injury. Endogenous Anticoagulant Mechanisms Under normal physiological condition, blood clot formation is prevented by several regulatory mechanisms. Furthermore, once a stable blood clot is formed, several anticoagulation mechanisms are activated by the adjacent intact endothelium to limit the size of the blood clot and the clot formation to the site of injury. These mechanisms include:  Release of Prostaglandin (PGI2)  Antithrombin III  Protein C and Protein S  Tissue Factor Pathway Inhibitor (TFPI)  Plasmin As we previously discussed PGI2 is a metabolite of arachidonic acid synthesized by the activity of COX enzymes and released from intact endothelium. It acts through G-Protein Couple receptors to increase the intracellular level of cAMP in the platelets to inhibit platelet aggregation, it is also a potent vasodilator. 4 Dr. Khansari, Handout 3 Copyrighted PHAR 740 January 25, 2024 Protein C, with its cofactor protein S are vitamin-K dependent proteins activated by thrombin, to cleave off factor Va and VIIIa into their inactive form. Thrombomodulin, a receptor on the surface of the endothelial cell binds to both Protein C and thrombin in such a way that thrombin cleaves protein C to activated protein C which then inactivate Va and VIIIa, essential coagulating factors in coagulation cascade. Antithrombin III is produced in the liver and serves as a suicide substrate for thrombin, Xa, and IXa. We reviewed its activities and effect of heparin on antithrombin III in detailed. Plasmin is formed from its own inactive precursor protein, plasminogen, by thrombin or by the activity of tissue plasminogen activator (t-PA), or urokinase plasminogen activator (u-PA). Tissue plasminogen activator (t-PA) is released from endothelial cells. t-PA improves tissue perfusion by dissolving the intra-arterial blood clots. The activity of t-PA is further regulated by release of plasminogen activator inhibitor-1 released form endothelial cells (PAI-1) which then inhibits the activation of t-PA. Tissue Factor Pathway Inhibitor (TFPI): Recall from the last session, that at the site of injury tissue factors (TF), (also known as coagulation factor III or tissue thromboplastin) are exposed and hence initiate the extrinsic pathway by forming a complex with VIIa. Through a feedback mechanism, a complex will be formed with Xa:TFPI:VIIa:TF to inactivate the VIIa:TF complex. Thrombolytic Agents    Alteplase (ACTIVASE) Reteplase (Retavase) Tenecteplase (Tnkase) 5 Dr. Khansari, Handout 3 Copyrighted PHAR 740 January 25, 2024 Fibrinolysis is the process of breaking down the fibrin clot characterized by the conversion of plasminogen to plasmin which, in turn, results in fibrin degradation. rt-PA (alteplase) is the recombinant tissue plasminogen activator which dissolves intra-arterial blood clots. Streptokinase is a bacterial enzyme synthesized by streptococci which also has the fibrinolytic characteristic. It lyses the blood clot by converting plasminogen to plasmin. The major side effect with streptokinase is potential activation of immune response. 6 Dr. Khansari, Handout 3 Copyrighted