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Drugs and blood clotting

Teaching slide set shared by Clare Guilding, Newcastle University UK

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Drugs and blood clotting

  1. 1. Dr Clare Guilding e.mail: clare.guilding@ncl.ac.uk Drugs used in haematology; anticoagulants, antiplatelet agents and thrombolytic agents
  2. 2. Learning Outcomes This session should assist you in acquiring the knowledge & understanding to: ― Describe the basic pharmacology of anticoagulant, antiplatelet and fibrinolytic drugs Lesson outline 1. Disorders of inappropriate blood clotting (thrombosis) 2. Drugs developed to prevent and/or reverse thrombus formation: - Anticoagulants - Antiplatelet agents - Thrombolytic agents Drugs used in haematology
  3. 3. Drugs and blood clotting Before you learn the pharmacology of anticoagulant drugs you should ensure you understand the following physiological processes: • Normal haemostatic processes 1. Vasoconstriction – reduces blood flow therefore reduces blood loss 2. Platelet plug formation – involving platelet adhesion, platelet release reaction and platelet aggregation 3. Stable clot formation (coagulation cascade) – end result is activation of thrombin which : a) converts soluble fibrinogen to insoluble fibrin b) induces more platelet recruitment and activation • Normal fibrinolytic mechanisms
  4. 4. Thrombosis • Thrombosis – pathogenic state in which the normal haemostatic processes are activated inappropriately Atrial Fibrillation Deep Vein Thrombosis Pulmonary Embolus Myocardial infarction
  5. 5. Atrial Fibrillation Deep Vein Thrombosis Pulmonary Embolus Myocardial infarction Anticoagulants Antiplatelet agents Fibrinolytic agents Thrombosis • Thrombosis – pathogenic state in which the normal haemostatic processes are activated inappropriately
  6. 6. Formation of thrombus • Thrombus - a blood clot formed in vivo - has a distinct structure – white head, red tail There are 2 main types of thrombus: 1. Arterial thrombus 2. Venous thrombus Adapted from© Can Stock Photo / rob3000 Tissue factor Collagen Endothelium Subendothelial matrix Smooth muscle
  7. 7. Formation of thrombus 1. Arterial thrombus: — Thrombus has large head, formed from platelets — Primary trigger of arterial thrombosis is rupture of an atherosclerotic plaque (seen in yellow) Thrombus in the lumen of a coronary artery — Mainly treated with antiplatelet drugs Adapted from © Can Stock Photo / megija Foam cells Rupture of plaque Thrombus Atheroscleroti c plaque Necrotic core Endothelium Vessel wall
  8. 8. Formation of thrombus 2. Venous thrombus: — Thrombus consists of a fibrin web enmeshed with red blood cells and platelets — Tail can break off giving rise to embolisms A thromboembolus filling a pulmonary artery — Mainly treated with anticoagulant drugs Abnormal blood flow Increased coagulability erythrocytes Altered vessel wall Adapted from © Can Stock Photo / alila
  9. 9. Drugs and blood clotting • Drugs have been developed to prevent and/or reverse thrombus formation. These drugs fall into 3 classes: 1. Anticoagulants e.g Heparin and oral anticoagulants - modify blood clotting mechanisms 2. Antiplatelet agents e.g Aspirin include clopidrogel next year - inhibit COX-1 activity to inhibit platelet aggregation 3. Fibrinolytic agents e.g Alteplase - break down fibrin
  10. 10. Anticoagulants • There are 4 main classes of anticoagulant 1. Heparin and low molecular weight heparins 2. Warfarin 3. Selective factor Xa inhibitors 4. Direct thrombin (factor IIa) inhibitors • Anticoagulants target various factors in the coagulation cascade, thereby preventing formation of a stable fibrin meshwork.
  11. 11. Heparin Pharmacodynamics • Family of sulphated mucopolysaccharides, found in the secretory granules of mast cells • Commercial preparations vary in MW from 3000 to 30,000Da • Inhibits coagulation by activating antithrombin III (AT III)  AT III is a naturally occurring inhibitor of thrombin and clotting factors IX, Xa, XI and XII  In the presence of heparin, AT III becomes ~1000x more active and inhibition of clotting factors is instantaneous
  12. 12. XIIXIIa XIXIa X Xa IXIXa II (Prothrombin) IIa (Thrombin) Fibrinogen Fibrin Stabilised fibrin XIIIa XIII ContactTissue damage Extrinsic pathway Intrinsic pathway Blood clotting cascade VII VIIa
  13. 13. XIIXIIa XIXIa X Xa IXIXa II (Prothrombin) IIa (Thrombin) Fibrinogen Fibrin Stabilised fibrin XIIIa XIII Intrinsic pathway AT III Heparin AT III Heparin AT III Heparin AT III Heparin AT III Heparin Blood clotting cascade Tissue damage Extrinsic pathway VII VIIa Contact
  14. 14. Pharmacodynamics • Low molecular weight heparins (LMWH; fragments or synthetic heparin) have more consistent activity e.g. enoxaparin* • LMWHs inactivate factor Xa (and thrombin) (also via activation of antithrombin III) • Heparin and LMWH have immediate onset of action Low molecular weight heparins (LMWH)
  15. 15. XIXIa X Xa IXIXa II (Prothrombin) IIa (Thrombin) Fibrinogen Fibrin Stabilised fibrin XIIIa XIII Contact (e.g. with glass) Intrinsic pathway Blood clotting cascade ATIII LMWH Tissue damage Extrinsic pathway VII VIIa XIIXIIa
  16. 16. Heparin and LMWH: Pharmacokinetics • Inactive given orally (not absorbed from GI tract) • Administered IV or SC (SC for LMWHs) • Heparin has a short half life (t ½ <1h low doses, 2h large doses) ― Heparin must be given frequently or as a continuous infusion • LMWH have longer duration of action (t ½ ~4-5h) ― Allows once daily dosing • Eliminated mainly by renal excretion ― Care needed in patients with renal disease • Side effects include bleeding and hypersensitivity • Overdose treated by IV protamine (strongly basic protein)
  17. 17. Heparin: Clinical use • Treatment of established venous thromboembolism • Prevention of venous thromboembolism – LMWHs used to prevent post-operative venous thrombosis • Cardiac disease – reduces risk of venous thromboembolism in patients with angina and following acute MI Because of the need for frequent dosing, if long-term anticoagulation is required, heparin is often used only to commence anticoagulation therapy until an oral anticoagulant takes effect
  18. 18. Oral anticoagulants: Vitamin K cycle Inactive clotting factors II, VII, IX, X Active clotting factors IIa, VIIa, IXa, Xa • Vitamin K levels in humans are maintained by the action of the enzyme Vitamin K reductase which ‘recycles’ Vitamin K • Vitamin K is required to activate the clotting factors II, VII, IX, X Inactive clotting factors can’t bind stably to the blood vessel endothelium and cannot activate clotting
  19. 19. Oral anticoagulants: Mechanism of action  Warfarin inhibits Vitamin K reductase thus prevents the activation of the clotting factors II, VII, IX, X Inactive clotting factors II, VII, IX, XInactive clotting factors can’t bind stably to the blood vessel endothelium and cannot activate clotting
  20. 20. XIIXIIa XIXIa X Xa IXIXa II (Prothrombin) IIa (Thrombin) Fibrinogen Fibrin Stabilised fibrin XIIIa XIII Tissue damage Extrinsic pathway Intrinsic pathway VII VIIa Blood clotting and warfarin Contact
  21. 21. XIIXIIa XIXIa X Xa IXIXa II (Prothrombin) IIa (Thrombin) Fibrinogen Fibrin Stabilised fibrin XIIIa XIII ContactTissue damage Extrinsic pathway Intrinsic pathway VII VIIa W W W W Blood clotting and warfarin
  22. 22. • Inhibits the activation of Vitamin K1 dependent clotting factors II, VII, IX and X • The shift in haemostatic balance in favour of anticoagulation doesn’t take place until all active vitamin K dependant proteins, made before the drug was administered have been cleared from the circulation. • The process occurs at different rates for different clotting factors e.g. half life VII ~6h, half life IX and X 9 ~ 8-24h, half life prothrombin (II) ~72h Hence there is a 1-2 day lag period before warfarin is pharmacologically effective • A small population of patients is genetically resistant to warfarin, due to reduced binding to Vitamin K reductases Warfarin: Pharmacodynamics and pharmacokinetics
  23. 23. • Absorption - rapidly and almost totally absorbed from the GI tract - levels peak in blood ~0.5-4h after administration • Distribution – low volume of distribution as ~ 99% plasma protein bound (mainly to albumin) • Metabolism – action is terminated by metabolism in the liver by CYP450 enzymes (e.g. CYP2C9, 2C19, 3A4) • Excretion – metabolites are conjugated to glucuronide and excreted in urine and faeces • Half life – variable ~ 15-80 hours • Dose is highly variable - (2-112 mg/week) Warfarin: Pharmacokinetics
  24. 24. Warfarin: Clinical use • To prevent the progression or reoccurrence of: — Venous thrombosis — Pulmonary embolus • To prevent: — Arterial thromboemboli in patients with atrial fibrillation or cardiac disease (including mechanical heart valves) • At least 6 weeks anticoagulation is recommended for calf vein thrombosis and at least 3 months for DVT or PE
  25. 25. • Acute anticoagulation usually starts with heparin and an oral anticoagulant e.g. warfarin • Heparin is rapidly effective – Effect of heparin is monitored by activated partial thromboplastin time (APTT) – LMMH – usually no monitoring needed, has less side effects and fewer bleeding complications • Warfarin takes several days to achieve full anticoagulation – Effect of anticoagulant monitored using the prothrombin time (converted to INR: International normalised ratio) • Heparin therefore covers the lag period and can then be withdrawn Clinical practice
  26. 26. Your patient is prescribed warfarin. Would the addition of the following drugs to the dosing regimen increase or decrease anticoagulation within the body? 1. Drugs which have a high affinity for the plasma binding protein albumin e.g. sulfonamides 2. Drugs which reduce absorption e.g. sucralfate 3. Drugs which decrease platelet aggregation e.g. aspirin 4. Drugs which inhibit CYP 2C9 e.g. St John’s Wort Warfarin prescribing questions
  27. 27. Your patient is prescribed warfarin. Would the addition of the following drugs to the dosing regimen increase or decrease anticoagulation within the body? 1. Drugs which have a high affinity for the plasma binding protein albumin e.g. sulfonamides Increase. Warfarin is highly plasma protein bound. Sulfonamides comptete with warfarin for binging to albumin, so more warfarin is free/unbound (=more pharmacologically available) 2. Drugs which reduce absorption e.g. sucralfate 3. Drugs which decrease platelet aggregation e.g. aspirin 4. Drugs which inhibit CYP 2C9 e.g. St John’s Wort Warfarin prescribing questions
  28. 28. Your patient is prescribed warfarin. Would the addition of the following drugs to the dosing regimen increase or decrease anticoagulation within the body? 1. Drugs which have a high affinity for the plasma binding protein albumin e.g. sulfonamides 2. Drugs which reduce absorption e.g. sucralfate Decrease. Less warfarin would be absorbed from the GI tract 3. Drugs which decrease platelet aggregation e.g. aspirin 4. Drugs which inhibit CYP 2C9 e.g. St John’s Wort Warfarin prescribing questions
  29. 29. Your patient is prescribed warfarin. Would the addition of the following drugs to the dosing regimen increase or decrease anticoagulation within the body? 1. Drugs which have a high affinity for the plasma binding protein albumin e.g. sulfonamides 2. Drugs which reduce absorption e.g. sucralfate 3. Drugs which decrease platelet aggregation e.g. aspirin Increase. Both drugs ultimately lead to a reduction in blood clot formation, so addition of aspirin would potentiate warfarin’s actions 4. Drugs which inhibit CYP 2C9 e.g. St John’s Wort Warfarin prescribing questions
  30. 30. Your patient is prescribed warfarin. Would the addition of the following drugs to the dosing regimen increase or decrease anticoagulation within the body? 1. Drugs which have a high affinity for the plasma binding protein albumin e.g. sulfonamides 2. Drugs which reduce absorption e.g. sucralfate 3. Drugs which decrease platelet aggregation e.g. aspirin 4. Drugs which inhibit CYP 2C9 e.g. St John’s Wort Increase. CYP2C9 metabolises (inactivates) warfarin. Less CYP2C9 = less inactivation = more pharmacologically active warfarin remains Warfarin prescribing questions
  31. 31. 3. Selective factor Xa inhibitors 4. Direct thrombin (factor IIa) inhibitors e.g. dabigatran Newer oral anticoagulants Dabigatran • Competitive reversible inhibitor of thrombin • Used for: — Prevention of stroke and embolism in patients with atrial fibrillation — Prophylaxis of venous thromboembolism after hip or knee replacement surgery • Has a rapid onset of action • Does NOT require routine oral anticoagulant monitoring • No way to reverse anticoagulation in the event of a significant bleed
  32. 32. XIIXIIa XIXIa X Xa IXIXa II (Prothrombin) IIa (Thrombin) Fibrinogen Fibrin Stabilised fibrin XIIIa XIII Tissue damage Extrinsic pathway Intrinsic pathway VII VIIa Direct thrombin (factor IIa) inhibitors Contact
  33. 33. XIIXIIa XIXIa X Xa IXIXa II (Prothrombin) IIa (Thrombin) Fibrinogen Fibrin Stabilised fibrin XIIIa XIII Tissue damage Extrinsic pathway Intrinsic pathway VII VIIa Contact Dabigatran Direct thrombin (factor IIa) inhibitors
  34. 34. Drugs and blood clotting Lesson outline 1. Disorders of inappropriate blood clotting (thrombosis) 2. Drugs developed to prevent and/or reverse thrombus formation: - Anticoagulants - Antiplatelet agents - Thrombolytic agents
  35. 35. Antiplatelet drugs • Platelets provide the initial haemostatic plug at sites of vascular injury Inhibition of platelet function is a useful prophylactic and therapeutic strategy against MI and stroke caused by thrombosis 1 2 3 From Lippincott's Illustrated Review Pharmacology
  36. 36. Recruits platelets into plug Antiplatelet drugs e.g. aspirin • Platelet-derived thromboxane A2 (TXA2) promotes aggregation
  37. 37. Antiplatelet drugs e.g. aspirin • Aspirin irreversibly inhibits COX-1, therefore inhibits the synthesis of TXA2 • Because platelets do not contain DNA or RNA they cannot cannot synthesise new COX-1 • The inhibition is irreversible and effective for the life of the circulating platelet (7-10 days) Clinical use: Used prophylactically to prevent arterial thrombosis leading to: – transient ischemic attack – stroke – myocardial infarction xx x
  38. 38. • Thromboses are dynamic - balance between breakdown (fibrinolysis) and formation • Thrombolytic drugs potentiate the effects of the fibrinolytic system • They activate conversion of plasminogen to plasmin which breaks down fibrin, thus dissolves clots Fibrinolytic (thrombolytic) drugs e.g. streptokinase, alteplase Fibrinolysis
  39. 39. • Administered iv; immediate effect • Short half-lives (<10-90 mins) • Main hazard is bleeding • Main uses nowadays: — restoring catheter and shunt function, by lysing clots causing occlusions — To dissolve clots that result in strokes Fibrinolytic (thrombolytic) drugs e.g. streptokinase, alteplase
  40. 40. Key points Think back through the presentation and write some of the key points you have learned in your own words • … • … • … • …
  41. 41. Key points • Heparin works by activating antithrombin III, a naturally occurring inhibitor of clotting factors • Warfarin works by inhibiting vitamin K reductase • Heparin is immediately effective, warfarin can take 1-2 days to be effective • Heparin must be administered parenterally, warfarin orally • Thrombolytic agents stimulate fibrinolysis by activating conversion of plasminogen to plasmin (which breaks down fibrin) • Aspirin inhibits COX mediated production of TXA2 , inhibiting TXA2 mediated platelet aggregation
  42. 42. Recommended reading Rang, Dale, Ritter and Flower. Pharmacology. Relevant sections within the chapter ‘Haemostasis and thrombosis’. Golan et al. Principles of Pharmacology. Relevant sections within the chapter ‘Pharmacology of Haemostasis and Thrombosis’. Additional images come from Lippincott's Illustrated Review Pharmacology

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