BLOOD COAGULATION

1. BLOOD COAGULATION Assistant Professor Dr. Hayder B Sahib

2. The coagulation process that generates thrombin consists of two interrelated pathways, • 1- the extrinsic system • 2- intrinsic system • The extrinsic system is initiated by the activation of clotting factor VII by tissue factor (also known as thromboplastin). Tissue factor is a membrane protein that is normally separated from the blood by the endothelial cells that line the vasculature. • However, in response to vascular injury, tissue factor becomes exposed to blood. There it can bind and activate factor VII, initiating the extrinsic pathway. • The intrinsic system is triggered by the activation of clotting factor XII. This occurs when blood comes into contact with the collagen in the damaged wall of a blood vessel.

3. A. Formation of fibrin • Both the extrinsic and the intrinsic systems involve a cascade of enzyme reactions that sequentially transform various plasma factors • (proenzymes) to their active (enzymatic) forms. • Thrombin plays a key role in coagulation, because it is responsible for generation of fibrin, which forms the mesh-like matrix of the blood clot. • If thrombin is not formed or if its function is impeded (for example, by antithrombin III), coagulation is inhibited. • B. Inhibitors of coagulation • It is important that coagulation is restricted to the local site of vascular injury. Endogenously, there are several inhibitors of coagulation factors, including protein C, protein S, antithrombin III, and tissue factor pathway inhibitor.

4. The mechanism of action of several anticoagulant agents, including heparin and heparin-related products, involves activation of these endogenous inhibitors (primarily antithrombin III). • VI. ANTICOAGULANTS • The anticoagulant drugs inhibit either the action of the coagulation factors or interfere with the synthesis of the coagulation factors (for example, vitamin K antagonists such as warfarin).

5. A. Heparin and low molecular weight heparins • Heparin is an injectable, rapidly acting anticoagulant that is often used acutely to interfere with the formation of thrombi. • Unfractionated heparin is a mixture of straight-chain, anionic glycosaminoglycans with a wide range of molecular weights. It is strongly acidic because of the presence of sulfate and carboxylic acid groups. • The realization that low molecular weight forms of heparin (LMWHs) can also act as anticoagulants led to the isolation of enoxaparin, produced by enzymatic depolymerization of unfractionated heparin. • Other LMWHs include dalteparin and tinzaparinThe LMWHs are heterogeneous compounds about one-third the size of unfractionated heparin.

6. 1. Mechanism of action: • its anticoagulant effect is a consequence of binding to antithrombin III, with the subsequent rapid inactivation of coagulation factors. • Antithrombin III is an α globulin that inhibits serine proteases of thrombin (factor IIa) and factor Xa . • In the absence of heparin, antithrombin III interacts very slowly with thrombin and factor Xa. • When heparin molecules bind to antithrombin III, a conformational change occurs that catalyzes the inhibition of thrombin about 1000-fold • LMWHs complex with antithrombin III and inactivate factor Xa (including that located on platelet surfaces) but do not bind as avidly to thrombin. A unique pentasaccharide sequence contained in heparin and LMWHs permits their binding to antithrombin III.

7. 2. Therapeutic use: Heparin and the LMWHs • 1- limit the expansion of thrombi by preventing fibrin formation. These agents are used for the treatment of acute venous thromboembolism (DVT or PE). • 2- prophylaxis of postoperative venous thrombosis in patients undergoing surgery (for example, hip replacement) and those with acute MI. • 3- treating pregnant women, because they do not cross the placenta, due to their large size and negative charge. • LMWHs do not require the same intense monitoring as heparin, thereby saving laboratory costs and nursing time. These advantages make LMWHs useful for both inpatient and outpatient therapy.

8. 3. Pharmacokinetics: • Heparin must be administered subcutaneously or intravenously, because the drug does not readily cross membranes . • The LMWHs are administered subcutaneously. • Heparin is often initiated as an intravenous bolus to achieve immediate anticoagulation. This is followed by lower doses or continuous infusion of heparin, titrating the dose so that the activated partial thromboplastin time (aPTT) is 1.5- to 2.5-fold that of the normal control. [Note: The aPTT is the standard test used to monitor the extent of anticoagulation with heparin.] • Whereas the anticoagulant effect with heparin occurs within minutes of IV administration (or 1 to 2 hours after subcutaneous injection), the maximum anti–factor Xa activity of the LMWHs occurs about 4 hours after subcutaneous injection.

9. It is usually not necessary to monitor coagulation values with LMWHs because the plasma levels and pharmacokinetics of these drugs are more predictable. However, in renally impaired, pregnant, and obese patients, monitoring of factor Xa levels is recommended with LMWHs. • In the blood, heparin binds to many proteins that neutralize its activity, causing unpredictable pharmacokinetics. • Heparin binding to plasma proteins is variable in patients with thromboembolic diseases. Although generally restricted to the circulation, heparin is taken up by the monocyte/macrophage system, and it undergoes depolymerization and desulfation to inactive products.

10. The inactive metabolites, as well as some of the parent heparin and LMWHs, are excreted into the urine. • Renal insufficiency prolongs the half-life of LMWHs. Therefore, the dose of LMWHs should be reduced in patients with renal impairment. • The half-life of heparin is approximately 1.5 hours, whereas the half-life of the LMWHs is longer than that of heparin, ranging from 3 to 12 hours.

11. 4. Adverse effects: The chief complication of heparin and LMWH • 1- bleeding . Excessive bleeding may be managed by discontinuing the drug or by treating with protamine sulfate. • 2- chills • 3- fever • 4- urticaria • 5- anaphylactic shock. 6- Heparin-induced thrombocytopenia (HIT) is a serious condition, in which circulating blood contains an abnormally low number of platelets. This reaction • is immune-mediated and carries a risk of venous and arterial • embolism. • Heparin therapy should be discontinued when patients develop HIT or show severe thrombocytopenia. In cases of HIT, heparin can be replaced by another anticoagulant, such as argatroban.

12. B. Argatroban • Argatroban is a synthetic parenteral anticoagulant that is derived from l-arginine. It is a direct thrombin inhibitor. • Argatroban is used for • 1- the prophylaxis or treatment of venous thromboembolism • 2- in patients with HIT • 3- during PCI in patients who have or are at risk for developing HIT. • Argatroban is metabolized in the liver and has a half-life of about 39 to 51 minutes. Monitoring includes aPTT, hemoglobin, and hematocrit. • Because argatroban is metabolized in the liver, it may be used in patients with renal dysfunction, but it should be used cautiously in patients with hepatic impairment. As with other anticoagulants, the major side effect is bleeding.

13. C. Bivalirudin and desirudin • Bivalirudin and desirudin are parenteral anticoagulants that are analogs of hirudin, a thrombin inhibitor derived from medicinal leech saliva. • These drugs are selective direct thrombin inhibitors that reversibly inhibit the catalytic site of both free and clot-bound thrombin. • Bivalirudin is an alternative to heparin in patients undergoing PCI who have or are at risk for developing HIT and also in patients with unstable angina undergoing angioplasty.

14. In patients with normal renal function, the half-life of bivalirudin is 25 minutes. Dosage adjustments are required in patients with renal impairment. • Desirudin is indicated for the prevention of DVT in patients undergoing hip replacement surgery. • Like the others, bleeding is the major side effect of these agents. • fondaparinux. There is no available agent for the reversal of bleeding associated with fondaparinux. HIT is less likely with fondaparinux than with heparin but is still a possibility. • Fondaparinux should not be used in the setting of lumbar puncture or spinal cord surgery.

15. E. Dabigatran etexilate • 1. Mechanism of action: Dabigatran etexilate is the prodrug of the active moiety dabigatran, which is an oral direct thrombin inhibitor. Both clot-bound and free thrombin are inhibited by dabigatran. • 2. Therapeutic use: the prevention of stroke and systemic embolism in patients with nonvalvular atrial fibrillation. • Because of its efficacy, oral bioavailability, and predictable pharmacokinetic properties, dabigatran may be an alternative to enoxaparin for thromboprophylaxis in orthopedic surgery.

16. 3. Pharmacokinetics: Dabigatran etexilate is administered orally. • Due to the breakdown of the product and reduction of potency when exposed to moisture, capsules should be stored in the original container and swallowed whole. • It is hydrolyzed to the active drug, dabigatran, by various plasma esterases. The CYP450 system does not play a role in metabolism of dabigatran. • Instead, dabigatran is a substrate for P-glycoprotein (P-gp) and is eliminated renally.

17. 4. Adverse effects: • 1- bleeding • 2- should be used with caution in renal impairment or in patients over the age of 75 • 3- There is no approved antidote for reversing bleeding associated with dabigatran. • 4- Dabigatran does not require routine monitoring of the international normalized ratio (INR) and has fewer drug interactions as compared to warfarin. • 5- dyspepsia, abdominal pain, esophagitis, and GI bleeding. • 6- Abrupt discontinuation should be avoided, as patients may be at increased risk for thrombotic events. • This drug is contraindicated in patients with mechanical prosthetic heart valves and is not recommended in patients with bioprosthetic heart valves.

18. D. Fondaparinux • Fondaparinux is a pentasaccharide anticoagulant that is synthetically derived. • This agent selectively inhibits only factor Xa. By selectively binding to antithrombin III, fondaparinuxpotentiates (300- to 1000-fold) the innate neutralization of factor Xaby antithrombin III. • Fondaparinux is approved for use in the treatment of DVT and PE and for the prophylaxis of venous thromboembolism in the setting of orthopedic and abdominal surgery. • The drug is well absorbed from the subcutaneous route with a predictable pharmacokinetic profile and, therefore, requires less monitoring than heparin. • Fondaparinux is eliminated in the urine mainly as unchanged drug with an elimination half-life of 17 to 21 hours. It is contraindicated in patients with severe renal impairment. Bleeding is the major side effect

19. F. Rivaroxaban and apixaban • 1. Mechanism of action: Rivaroxaban and apixabanare oral inhibitors of factor Xa. Both agents bind to the active site of factor Xa, thereby preventing its ability to convert prothrombin to thrombin • 2. Therapeutic use: Rivaroxaban is approved for treatment and prevention of DVT and PE and for the prevention of stroke in nonvalvularatrial fibrillation. • Apixaban is used for stroke prevention in non-valvular atrial fibrillation. • 3. Pharmacokinetics: Both drugs are adequately absorbed after • oral administration and are highly protein bound. • Food may increase the absorption of rivaroxaban.

20. Rivaroxaban is metabolized mainly by the CYP 3A4/5 and CYP 2J2 isoenzymes to inactive metabolites. About one-third of the drug is excreted unchanged in the urine, and the inactive metabolites are excreted in the urine and feces. Apixaban is primarily metabolized by CYP 3A4, with CYP enzymes 1A2, 2C8, 2C9, 2C19, and 2J2 all sharing minor metabolic roles; approximately 27% is excreted renally. • Both rivaroxaban and apixaban are substrates for P-gp. Compared to warfarin, rivaroxaban and apixaban have fewer drug interactions. There are no laboratory monitoring requirements for either agent.

21. 4. Adverse effects: • Bleeding is the most serious adverse effect for the factor Xa inhibitors. • There is no antidote available to reverse bleeding caused by rivaroxaban or apixaban. • As both drugs are eliminated renally, declining kidney function can prolong the effect of the drugs and, therefore, increase the risk of bleeding. • Neither drug should be used in severe renal dysfunction (creatinine clearance less than 15 mL/min). Abrupt discontinuation of these agents should be avoided.

22. G. Warfarin • The coumarin anticoagulants owe their action to the ability to antagonize the cofactor functions of vitamin K. The only therapeutically relevant coumarin anticoagulant is warfarin . • Initially used as a rodenticide, warfarin is now widely used clinically as an oral anticoagulant. • The INR is the standard by which the anticoagulant activity of warfarin therapy is monitored. • The INR corrects for variations that occur with different thromboplastin reagents used to perform testing at various institutions.

23. The goal of warfarin therapy is an INR of 2 to 3 for most indications, with an INR of 2.5 to 3.5 targeted for some mechanical valves and other indications. • Warfarin has a narrow therapeutic index. • Therefore, it is important that the INR is maintained within the optimal range as much as possible, and frequent monitoring may be required.

24. 1. Mechanism of action: Factors II, VII, IX, and X require vitamin K as a cofactor for their synthesis by the liver. • Thesefactors undergo vitamin K–dependent posttranslational modification, whereby a number of their glutamic acid residues are carboxylatedto form γ-carboxyglutamic acid residues. • The γ-carboxyglutamyl residues bind calcium ions, which are essential for interaction between the coagulation factors and platelet membranes. In the carboxylation reactions, the vitamin K– dependent carboxylase fixes CO2 to form the new COOH group on glutamic acid. • The reduced vitamin K cofactor is converted to vitamin K epoxide during the reaction. Vitamin K is regenerated from the epoxide by vitamin K epoxide reductase, the enzyme that is inhibited by warfarin.

25. Warfarin treatment results in the production of clotting factors with diminished activity (10% to 40% of normal), due to the lack of sufficient γ-carboxyglutamyl side chains. • Unlike heparin, the anticoagulant effects of warfarin are not observed immediately after drug administration. • Instead, peak effects may be delayed for 72 to 96 hours, which is the time required to deplete the pool of circulating clotting factors. The anticoagulant effects of warfarin can be overcome by the administration of vitamin K.

26. reversal following administration of vitamin K takes approximately 24 hours (the time necessary for degradation of already synthesized clotting factors). • 2. Therapeutic use: Warfarin is used in the prevention and treatment of DVT and Pulmonery embolism PE, stroke prevention, stroke prevention in the setting of atrial fibrillation and/or prosthetic heart valves, protein C and S deficiency, and antiphospholipid syndrome. • It is also used for prevention of venous thromboembolism during orthopedic or gynecologic surgery.

27. 3. Pharmacokinetics: • 1- Warfarin is rapidly absorbed after oral administration • 2- (100% bioavailability with little individual patient variation). • 3- Warfarin is highly bound to plasma albumin, which prevents its diffusion into the cerebrospinal fluid, urine, and breast milk. • However, drugs that have a greater affinity for the albumin-binding site, such as sulfonamides, can displace the anticoagulant and lead to a transient, elevated activity. • Drugs that affect warfarin binding to plasma proteins can lead to variability in the therapeutic response to warfarin. • 4- Warfarin readily crosses the placental barrier.

28. 5- The mean half-life of warfarin is approximately 40 hours, but this value is highly variable among individuals. • 6- Warfarin is metabolized by the CYP450 system (including the 2C9, 2C19, 2C8, 2C18, 1A2, and 3A4 isoenzymes) to inactive components • 7- After conjugation to glucuronic acid, the inactive metabolites are excreted in urine and feces. Agents that affect the metabolism of warfarain may alter its therapeutic effects. • Warfarin has numerous drug interactions that may potentiate or attenuate its anticoagulant effect.

29. Adverse effects: • 1-hemorrhage, • 2-Skin lesions and necrosis are rare complications of warfarin therapy. • 3- Purple toe syndrome, • 4-a rare, painful, blue-tinged discoloration of the toe caused by cholesterol emboli from plaques, has also been observed with warfarin therapy. • 5- Warfarin is teratogenic and should never be used during pregnancy.

30. THROMBOLYTIC DRUGS • Acute thromboembolic disease in selected patients may be treated by the administration of agents that activate the conversion of plasminogen to plasmin, a serine protease that hydrolyzes fibrin and, thus, dissolves clots. • Streptokinase, one of the first such agents to be approved, causes a systemic fibrinolytic state that can lead to bleeding problems. • Alteplase acts more locally on the thrombotic fibrin toproduce fibrinolysis. • Urokinase is produced naturally in human kidneys and directly converts plasminogen into active plasmin.

31. 1. Mechanism of action: The thrombolytic agents share some common features. • 1- All act either directly or indirectly to convert plasminogen to plasmin, which, in turn, cleaves fibrin, thus lysing thrombi. • 2- Clot dissolution and reperfusion occur with a higher frequency when therapy is initiated early after clot formation because clots become more resistant to lysis as they age. • 3- Unfortunately, increased local thrombi may occur as the clot dissolves, leading to enhanced platelet aggregation and thrombosis. • Strategies to prevent this include administration of antiplatelet drugs, such as aspirin, or antithrombotics such as heparin.

32. 2. Therapeutic use: Originally used for the treatment of DVT and serious PE, thrombolytic drugs are now being used less frequently for these conditions. Their tendency to cause bleeding has also blunted their use in treating acute peripheral arterial thrombosis or MI. • For MI, intracoronary delivery of the drugs is the most reliable in terms of achieving recanalization. However, cardiac catheterization may not be possible in the 2- to 6-hour “therapeutic window,” beyond which significant myocardial salvage becomes less likely. • Thus, thrombolytic agents are usually administered intravenously. Thrombolytic agents are helpful in restoring catheter and shunt function, by lysing clots causing occlusions. They are also used to • dissolve clots that result in strokes.

33. 3. Adverse effects: The thrombolytic agents do not distinguish between the fibrin of an unwanted thrombus and the fibrin of a beneficial hemostatic plug. Thus, hemorrhage is a major side effect. • For example, a previously unsuspected lesion, such as a gastric ulcer, may hemorrhage following injection of a thrombolytic agent These drugs are contraindicated in pregnancy, and in patients with healing wounds, a history of cerebrovascular accident, brain tumor, head trauma, intracranial bleeding, and metastatic cancer. • B. Alteplase, reteplase, and tenecteplase • Alteplase(formerly known as tissue plasminogen activator or tPA) is a serine protease originally derived from cultured human melanoma cells. It is now obtained as a product of recombinant DNA technology. • Reteplaseis a genetically engineered, smaller derivative of recombinant tPA. Tenecteplaseis another recombinant tPA with a longer half-life and greater binding affinity for fibrin than alteplase. Alteplasehas a low affinity for free plasminogen in the plasma, but it rapidly activates plasminogen that is bound to fibrin in a thrombus or a hemostatic plug.

34. Thus, alteplase is said to be “fibrin selective” at low doses. Alteplase is approved for the treatment of MI, massive PE, and acute ischemic stroke. • Reteplase and tenecteplase are approved only for use in acute MI, although reteplase may be used off-label in DVT and massive PE. Alteplase has a very short half-life (5 to 30 minutes), and therefore, 10% of the total dose is injected intravenously as a bolus and the remaining drug is administered over 60 minutes. • Both reteplase and tenecteplase have longer half-lives and, therefore, may be administered as an intravenous bolus. • Alteplase may cause orolingual angioedema, and there may be an increased risk of this effect when combined with angiotensin-converting enzyme (ACE) inhibitors.

35. C. Streptokinase • Streptokinase is an extracellular protein purified from culture broths of group C β-hemolytic streptococci. It forms an active one-to-one complex with plasminogen. This enzymatically active complex converts uncomplexed plasminogen to the active enzyme plasmin. • In addition to the hydrolysis of fibrin plugs, the complex also catalyzes the degradation of fibrinogen, as well as clotting factors V and VII (Figure 22.22). With the advent of newer agents, streptokinase is rarely used and is no longer available in many markets. • D. Urokinase • Urokinase is produced naturally in the body by the kidneys. Therapeutic urokinase is isolated from cultures of human kidney cells and has low antigenicity.

36. Urokinase directly cleaves the arginine–valine bond of plasminogen to yield active plasmin. It is only approved for lysis of pulmonary emboli. • Off-label uses include treatment of acute MI, arterial thromboembolism, coronary artery thrombosis, and DVT. Its use has largely been supplanted by other agents with a more favorable benefit-to-risk ratio. • DRUGS USED TO TREAT BLEEDING • A. Aminocaproic acid and tranexamic acid • Fibrinolytic states can be controlled by the administration of aminocaproic acid or tranexamicacid. Both agents are synthetic, orally active, excreted in the urine, and inhibit plasminogen activation. Tranexamic acid is 10 times more potent than aminocaproic acid. A potential side effect is intravascular thrombosis.

37. B. Protamine sulfate • Protamine sulfate antagonizes the anticoagulant effects of heparin. This protein is derived from fish sperm or testes and is high in arginine content, which explains its basicity. • The positivelycharged protamine interacts with the negatively charged heparin, forming a stable complex without anticoagulant activity. Adverse effects of drug administration include hypersensitivity as well as dyspnea, flushing, bradycardia, and hypotension when rapidly injected. • C. Vitamin K • Vitamin K1 (phytonadione) administration can stop bleeding problems due to warfarin by increasing the supply of active vitamin K1, thereby inhibiting the effect of warfarin. Vitamin K1 may be administered via the oral, subcutaneous, or intravenous route. [Note: Intravenous vitamin K should be administered by slow IV infusion to minimize the risk of hypersensitivity or anaphylactoid reactions.] For the treatment of bleeding, the subcutaneous route of vitamin K1 is not preferred, as it is not as effective as oral or IV administration. The response to vitamin K1 is slow, requiring about 24 hours to reduce INR (time to synthesize new coagulation factors). Thus, if immediate hemostasis is required, fresh frozen plasma should be infused.