Pharmacology III (PHL 418)

1. Pharmacology III (PHL 418) Drugs to Reduce Blood Clotting (تخثر الدم) Dr. Mohammad Nazam Ansari

2. Drugs used to reduce clotting DVT: Deep venous thrombosis

3. Thrombus Formation • Arterial formation: Begins with platelet adhesion to arterial vessel wall  Adenosine diphosphate (ADP) released from platelets  more platelet aggregation Blood flow inhibited  fibrin, platelets & RBC’s surround clot  build up of size structure  occludes blood vessels  tissue ischemia • The result of Arterial Thrombus is localized tissue injury from lack of perfusion

4. Thrombus Formation • Venous Formation: Usually from slow blood flow Can occur rapidly  Stagnation of the blood flow initiate the coagulation cascade production of fibrin enmeshes RBC’s & platelets to form the thrombus. Venous thrombus has a long tail that can break off to produce an embolus. These travel to faraway sites then lodge in lung (capillary level)  inadequate O2 & CO2 exchange occur (i.e. pulmonary embolism & cerebral embolism) • Oral & parenteral anticoagulants (Heparin/Warfarin) primarily act by preventing venous thrombosis • Antiplatelet drugs primarily act by preventing arterial thrombosis

5. Hemostasis • Involves formation of blood clots to stop bleeding from damaged vessels, and activation of natural anticoagulation and fibrinolytic systems to limit clot formation to sites of injury • Bleeding disorders are due to defects in clot formation or overactive fibrinolytic systems • Hypercoagulability disorders are due to defects in anticoagulant system or underactive fibrinolytic systems

6. Normal hemostasis • Initial step is formation of platelet plug to stop bleeding from damaged vessel • Then, platelet plug is reinforced by fibrin clot • Then, fibrin clot is stabilized by activated factor XIII (Fibrin stabilizing factor), which cross-links fibrin strands • Fibrin clot may occur via either intrinsic or extrinsic pathway (or both), • Coagulation factors in intrinsic or extrinsic pathway assemble on surface of activated platelets, which are usually at site of vascular injury • Many coagulation reactions also require calcium as a cofactor

7. The coagulation factors (proteins) are manufactured by the liver. • The liver must be able to use Vitamin K to produce Factors II, VII, IX, and X. • Dietary vitamin K is widely available from plant and animal sources.

8. Clotting Factor • Factor I: fibrinogen • Factor II: prothrombin • Factor III: tissue thromboplastin(tissue factor and phospholipid) • Factor IV: ionized calcium • Factor V: occasionally called labile factor or proaccelerin • Factor VI: unassigned • Factor VII: occasionally called stable factor or proconvertin • Factor VIII: antihemophilic factor or von Willebrand factor • Factor IX: plasma thromboplastin component, Christmas factor • Factor X: occasionally called Stuart-Prower factor • Factor XI: occasionally called plasma thromboplastin antecedent • Factor XII: Hageman factor • Factor XIII: fibrin-stabilizing factor

9. Coagulation cascade Extrinsic Pathway • Initiating factor is outside the blood vessels: tissue factor • Clotting: faster in Seconds Intrinsic Pathway • All clotting factors are within the blood vessels • Clotting: slower

10. Intrinsic pathway • Involves factors VIII, IX, XI, XII, prekallikrein, high molecular weight kininogen • Merges with extrinsic pathway into common pathway • Activated when factor XII binds to negatively charged “foreign” surface exposed to blood • Then sequentially activates factors XI, IX, X, then factor II (prothrombin to thrombin), which converts fibrinogen to fibrin • Once extrinsic pathway is inhibited by TFPI-Xa complex, factor VIIIa / IXa complex becomes dominant generator of factor Xa, thrombin and fibrin • Factor XIIa also converts prekallikrein to kallikrein, which activates more factor XIIa; both require high molecular weight kininogen as cofactors

11. Schematic showing the intrinsic and extrinsic pathways of the coagulation cascade leading to fibrin formation. Factor III - tissue thromboplastin (tissue factor) Factor IV – ionized Ca++ (Factor I)

12. Extrinsic pathway • Involves tissue factor (TF), originally considered “extrinsic” to blood since it is present on cell surfaces not normally in contact with (i.e. extrinsic to) the circulatory system • The primary mechanism of the coagulation pathway in vivo is tissue factor binding to activated factor VII (factor VIIa) • TF-Factor VIIa complex activates factors X and IX • Activated factor IX activates more factor X, • Activated factor X converts prothrombin to thrombin, with activated factor V, anionic phospholipids and calcium as cofactors • After initial activation, pathway is inhibited by the binding of tissue factor pathway inhibitor (TFPI) to factor Xa, which inhibits TF-VIIa complex, and further coagulation is dependent on the intrinsic pathway

13. Common pathway • Involves fibrinogen (factor I), factors II (prothrombin), V, X • Thrombin converts soluble fibrinogen to insoluble fibrin; remaining fibrin monomers polymerize to form fibrin; thrombin also binds to antithrombin, which inhibits thrombin to prevent excessive clotting • Factor XIII cross links fibrin to increase stability of fibrin clot

15. Drugs used to reduce clotting DVT: Deep venous thrombosis

16. Intrinsic Pathway Extrinsic Pathway Tissue Injury Blood Vessel Injury Tissue Factor XIIa XII Thromboplastin XIa XI IXa IX VIIa VII Xa X X Prothrombin Thrombin Factors affected By Heparin Fribrin monomer Fibrinogen Vit. K dependent Factors Affected by Oral Anticoagulants Fibrin polymer XIII

17. Definition of Anticoagulation • Therapeutic interference ("blood-thinning") with the clotting mechanism of the blood to prevent or treat thrombosis and embolism.

18. Indications of Anticoagulant Therapy • Treatment and Prevention of Deep Venous Thrombosis • Pulmonary Emboli • Prevention of stroke in patients with atrial fibrillation, artificial heart valves, cardiac thrombus. • Ischemic heart disease • During procedures such as cardiac catheterisation and apheresis.

19. 1. Standard Heparin • Heterogenous mixture of polysaccharide chains • MW 3,000 to 30,000 • Administration • Parenteral: only IV or deep s.c. • Do not inject IM (danger of hematoma formation)- • Metabolism • Partially in the liver by heparinase to uroheparin, which has only slight antithrombin activity. • Adverse effect • haemorrhage - antidote - protaminesulphate

20. Mechanism of action: • Primarily: interaction with antithrombin III: alters the molecular configuration of antithrombin III, making it 1,000 to 4,000 times more potent as an inhibitor of thrombin formation: limits conversion of fibrinogen to fibrin: prolongs aPTT (activated partial thromboplastin time) • Also inhibits the effects of factor Xa on the coagulation cascade & limits platelet aggregation.

21. Heparin mechanism of action Heparin Antithrombin III Thrombin

22. Complications of Heparin • Haemorrhage(bleeding from ruptured blood vessels) • Heparin-induced thrombocytopenia (HIT) [an abnormally low amount of platelets] • Most significant adverse effect of heparin after haemorrhage • Begins 3 - 15 days after start of heparin • Treatment • Stop heparin immediately: Platelet count returns 4 days after stopping heparin • Use other drugs for anticoagulation • Osteoporosis (long-term only) [thinning of bone tissue and loss of bone density ]

23. Low Molecular Weight Heparin • Advantages • Standard (Unfractionated) heparin 3k to 30k • LMWH contains polysaccharide chains MW 5k • Same incidence of bleeding • Lower incidence of thrombocytopenia • Lower incidence of bone loss • Safe for use during pregnancy {does not cross the placenta} • Higher Bioavailability: 90% Vs 30% • Longer Plasma Half life: 4-6 h Vs 0.5-1 h • Renal (Slower) Vs Hepatic clearance

24. LMWH Administration • Subcutaneous injection • Once every 12 or 24 hours • Outpatient • Patients that need monitoring • Pregnant • Pediatric • Renal • Prolonged therapy • Those at risk for bleeding • Mechanism of action • Primarily by inhibiting factor Xa, which is higher in the coagulation cascade than antithrombin: LMWH is more efficient than UFH (Un-fractionated). • {the molecular configuration of antithrombin III is not altered by LMWH}

25. 2. Oral anticoagulantsVitamin K-Dependent Clotting Factors Vitamin K VII Synthesis of Functional Coagulation Factors IX X II

26. Warfarin Mechanism of Action Vitamin K Antagonism of Vitamin K VII Synthesis of Non Functional Coagulation Factors IX X II Warfarin

28. Warfarin Enhances Antithrombin Activity PT: Prothrombin time aPTT: Activated partial thromboplastin time

29. Warfarin: Major Adverse Effect—Haemorrhage • Factors that may influence bleeding risk: • Intensity of anticoagulation • Concomitant clinical disorders • Concomitant use of other medications • Quality of management Warfarin-induced Skin Necrosis

30. Warfarin Dosing & Monitoring • Start low • Initiate 5 mg daily • Educate patient • Stabilize • Titrate to appropriate INR (International normalized ratio) • Monitor INR frequently (daily then weekly) • Adjust as necessary Relative Contraindications • Pregnancy • Situations where the risk of hemorrhage is greater than the potential clinical benefits of therapy • Uncontrolled alcohol/drug abuse • Unsupervised dementia/psychosis

31. Signs of Warfarin Over dosage • Any unusual bleeding: • Blood in stools or urine • Excessive menstrual bleeding • Bruising (Minor hematoma) • Excessive nose bleeds/bleeding gums • Persistent oozing from superficial injuries • Bleeding from tumor, ulcer, or other lesion

32. Why do we need new anticoagulation drugs? • Heparin-induced thrombocytopenia • Heparin prophylaxis is imperfect • Heparin-associated osteoporosis • Warfarin takes several days for its effect • Warfarin interacts with many other drugs • Warfarin is dangerous if not monitored

33. New Anticoagulation Drugs • Direct Thrombin Inhibitors • Synthetic pentasaccharide • Enhanced Protein C pathway • Tissue Factor Pathway Inhibitor (TFPI) • Thrombolytic Drugs • Antiplatelet Drugs

34. Direct Thrombin Inhibitors • Advantages • Bound thrombin readily inhibited • More predictable patient response • Not neutralized by PF4 (Platelet factor 4) • Disadvantages • Higher cost of the drugs Ximelagatran • Promising oral direct thrombin inhibitor • Converted to the active form melagatran in vivo • No dosing problems • No monitoring needed. • Recent atrial fibrillation study showed it to possibly be superior to warfarin.

35. Ximelagatran Enhances Antithrombin Activity

36. Dabigatranetexilate • Oral dosing • Absorbed from GI tract • Transforms to active dabigatran • Future – replace warfarin • Wider therapeutic range • Acceptable bleeding risk • Little or no lab monitoring Argatroban • Small molecule • A synthetic derivative of L-arginine with antithrombotic activity • univalent and direct inhibitor of fibrin-bound thrombin • Used to treat HIT

37. Hirudin and Derivatives • Hirudin is a naturally occurring peptide in the salivary glands of medicinal leeches (such as Hirudomedicinalis) • has a blood anticoagulant property • Thrombin is produced from prothrombin. • A key event in the final stages of blood coagulation is the conversion of fibrinogen into fibrin by the thrombin. • Fibrin is then cross linked by factor XIII to form a blood clot. • The principal inhibitor of thrombin in normal blood circulation is antithrombin III. • Similar to antithrombin III, the anticoagulatant activity of hirudin is based on its ability to inhibit the procoagulant activity of thrombin.

38. Hirudin is the most potent natural inhibitor of thrombin. • Therefore, hirudin has therapeutic value in blood coagulation disorders, in the treatment of skin hematomas, either as an injectable or a topical application cream. • It is difficult to extract large amounts of hirudin from natural sources, so a method for producing and purifying this protein using recombinant biotechnology has been developed. This has led to the development and marketing of a number of hirudin-based anticoagulant pharmaceutical products, such as lepirudin (Refludan), and desirudin (Revasc/Iprivask). • Several other direct thrombin inhibitors are derived chemically from hirudin.

39. Hirudin and Derivatives 1. Lepirudin (Refludan) • It is almost identical to hirudin extracted from Leech saliva of Hirudomedicinalis • Cleared by kidneys • Inhibits clot bound thrombin • Clot associated Xa will trigger generation of more thrombin once treatment stops

40. Hirudin and Derivatives 2. Bivalirudin (Angiomax) • Semisynthetic • Cleared by liver • Half-life • Shorter than lepirudin • Safer drug • No risk of HIT

41. Hirudin and Derivatives • Indications: • HIT (heparin-induced thrombocytopenia ) • Cardiopulmonary bypass • Hip replacement surgery • Unstable angina

42. New Anticoagulation Drugs • Direct Thrombin Inhibitors • Synthetic pentasaccharide • Enhanced Protein C pathway • Tissue Factor Pathway Inhibitor (TFPI) • Thrombolytic Drugs • Antiplatelet Drugs

43. Synthetic Pentasaccharide E.g. Fonaparinux (Arixtra) • Synthetic, single molecular entity • Targets Factor Xa • Does not cause thrombocytopenia • Shown promise in DVT prevention during orthopedic procedures. • Also being examined in ischaemic heart disease • Approved for surgery prophylaxis • General • Total hip replacement • Total knee replacement

44. New Anticoagulation Drugs • Direct Thrombin Inhibitors • Synthetic pentasaccharide • Enhanced Protein C pathway • Tissue Factor Pathway Inhibitor (TFPI) • Thrombolytic Drugs • Antiplatelet Drugs

45. The protein C/protein S anticoagulant pathway Thrombin-thrombomodulin (TM) complex activates protein C. Activated protein C with its cofactor, free protein S, degrades factors Va and VIIIa. In addition, when thrombin binds thrombomodulin, thrombin loses its procoagulant functions.

47. New Anticoagulation Drugs • Direct Thrombin Inhibitors • Synthetic pentasaccharide • Enhanced Protein C pathway • Tissue Factor Pathway Inhibitor (TFPI) • Thrombolytic Drugs • Antiplatelet Drugs

48. Inhibition of VIIa/TF • TFPI – tissue factor pathway inhibitor