Consequences of thrombus

1. Consequences of thrombus

2. Anticoagulant Therapy

3. Coagulation Cascade • Anticoagulant therapy is aimed to: • Prevent clot in patients at risk • Prevent clot extension/ embolism • Deep venous thrombosis (DVT) & pulmonary embolism (PE) Present on platelets’ surfaces. Act by accelerating thrombus formation. Coagulation cascade Activated factor X (FXa) + FVa + Ca++ + phospholipids Prothrombin (FII) thrombin (FIIa) Fibrinogen  fibrin  blood clot

4. Cont’d This complex and Ca2+ comprise the prothrombinase complex Thrombin stimulates platelet aggregation Coagulation  ↑ platelet aggregation Phospholipids on platelets stimulate clot formation Platelets aggregation ↔ coagulation Clotting factors are serine protease enzymes

5. The Key Clotting Factor is: THROMBIN Because it acts on many aspects of the coagulation cascade

6. Scheme of anticoagulant drugs

7. THROMBIN INHIBITORS • Thrombin inhibitors can either inactivate thrombin directly or block thrombin formation • Thrombin can be inhibited irreversibly by glycosaminoglycans like heparin through an antithrombin III-dependent mechanism • The enzyme can be inhibited reversibly by hirudin and hirudin derivatives in an antithrombin III-independent manner (direct acting) • In addition to inhibiting thrombin, glycosaminoglycans also block thrombin generation

8. Antithrombin-III Dependent Thrombin Inhibitors Standard Unfractionated Heparin (UFH) • Heparin is a mixture of glycosaminoglycan molecules, which are heterogenous in molecular size • The mean molecular weight of heparin is 15,000 D • Antithrombin III (ATIII) binding is necessary for its anticoagulant activity • Antithrombin III (ATIII) is a slow endogenous progressive inhibitor of thrombin and other clotting enzymes (FXa)

9. Mode of Action of Heparin It has 2 mechanisms It binds to ATIII through a unique pentasaccharide conformational change in ATIII  ↑ activity of ATIII Heparin acts as a template to create (thrombin-ATIII complex) N.B. only 1 ATIII bind to 1 thrombin (1:1) N.B. ATIII alone can inhibit thrombin but in a very slow reaction Then heparin dissociates and is reused again

10. To inactivate thrombin Heparin binds to ATIII by the unique penta-saccharide Also binds to thrombin through the heparin-binding domain Conversely, to inactivate factor Xa, heparin only needs to bind with ATIII through its pentasaccharidesequence Heparin inactivates thrombin by binding both ATIII and thrombin Every heparin molecule contains : Pentasaccharide + heparin binding domain Anti-IIa activity = Anti-Xa activity

11. Targets for Heparin-ATIII Complex • Heparin inhibits several coagulation enzymes including thrombin (factor IIa) and factors IXa, Xa, XIa & XIIa • The enzyme most sensitive to inhibition is factor IIa • The next most sensitive enzyme is factor Xa • By inhibiting these two enzymes heparin inhibits both thrombin activity & thrombin formation

12. Limitations to Heparin’s use Need to be given in hospital and monitoring the patient

13. Low Molecular Weight Heparins (LMWHs) • Low molecular weight heparin have a mean molecular weight of 5000 D. • Prepared by controlled chemical or enzymatic depolymerization of standard unfractionedheparin are about ⅓ the size of starting material • 20% of LMWH molecules contain the pentasaccharide domain • Enoxaparin is the most used LMWH

14. Mechanism of Action of Low Molecular Weight Heparin (LMWH) • They contain pentasaccharide  inactivation of Factor Xa • In contrast, only 25% to 50% of LMWH molecules that have the pentasaccharide sequence are long enough to interact with both ATIII & thrombin 25% of LMWH can interact with both ATIII and thrombin The rest only inactivate factor X Anti-IIa< Anti-Xaactivity

15. Pharmacokinetic Profile of LMWH • LMWH exhibit less binding to plasma proteins & cell surfaces (better than heparin) • The reduced binding to plasma proteins results in • Better bioavailability (90% vs. 20% of heparin) • more predictable anticoagulant response • Laboratory monitoring of LMWH activity is not required • LMWH has low resistance in comparison to heparin • T1/2 = 4 hours (more than heparin) • Given at fixed doses once to twice daily by S.C. route, and is given for both inpatients as well as for outpatients.

16. Comparison of UFH & LMWH

17. Biophysical Limitations of Heparin and LMWH • Both heparin and LMWH can’t degrade fibrin-bound thrombin (only free thrombin is degraded) nor Factor Xa within the prothrombinase complex.

18. Other Injectable Antithrombotic Agents • Fondaparinux, a pentasaccharide,is an AT-III-dependentselective for factor Xa • Prevents venous thrombosis associated with orthopedic surgery • Administered > 6 hours postoperatively and the dose is adjusted for patients with renal impairment. To be revised …….

19. Therapeutic Uses • Heparin should be given either IV or S.C. injection. • onset of action: few minutes (IV) 1-2 hours (S.C.) • LMWHs are given by S.C. route • I.M. injection hematoma formation (thus is avoided) • Treatment of deep vein thrombosis • Treatment of pulmonary embolism • Prevention of postoperative venous thrombosis in patients with acute MI phase or one undergoing elective surgery(not emergency surgery) • Reduction of coronary artery thrombosis after thrombolytic treatment • Heparin is the anticoagulant of choice in pregnant women

20. Adverse Effects • Bleeding:they both lead to bleeding but the bleeding is less in LMWH • To treat bleeding: inject antidote protaminesulphate(1mg IV for each 100 units of UFH) (reversal effect) • Thrombosis: heparin  ↓ ATIII  ↑ risk of thrombosis

21. Cont’d • Thrombocytopenia: Heparin-induced thrombocytopenia (HIT) is a lifethreatening immune reaction • Occurs in 3% of patients • Usually occurs a week from starting heparin therapy • HIT  ↑ platelet activation  platelet aggregation  thrombosis. • HIT  endothelial damage • HIT may occur in the early stages of treatmen (within 5 days) but it’s non-immune reaction (not life threatening) • LMWHs, though of lower risk, are contraindicated with HIT. For patients with HIT we use lepirudin

22. How does HIT occur? • Heparin injection immune reaction with body produce antibody against heparin& also bind to platelet receptor activation of platelet thrombosis

23. Cont’d • Osteoporosis occurs with large doses of UFH >20,000 U/day for 6 months or longer (chronic use) • Hyperkalemia rarely occurs with UFH • It is attributed to inhibition of aldosteronesecretion • It occurs with both low- & high-dose UFH therapy • Onset is quick within a week after therapy initiation • It is reversible by therapy discontinuation • Diabetic & renal failure patients are at higher risk • Hypersensitivity:(Antigenicity due to animal source) • rarely occurring reactions include urticaria, rash, rhinitis, angioedema & reversible alopecia(hair loss)

25. Clinically Approved Direct Thrombin Inhibitors • Lepirudin, recombinant hirudin*-like peptide. • Direct acting thrombin inhibitor • Used in HIT patients (IV injection) • Has renal clearance • It acts on free thrombin and thrombin bound to fibrin • It has potential use in unstable angina patients and after thrombolysis Mechanism of action It binds to active site and substrate site of thrombin * hirudin: is a leech derived anticoagulant

26. DIRECT FACTOR Xa INHIBITORS They are available by recombinant technology

27. Oral Anticoagulants Vitamin K Antagonists (The Coumarins) • Vitamin K is co-factor for the hepatic synthesis of clotting factors II, VII, IX & X warfarin By Vit.kreductase Vit. K Vit. K epoxide (active form) Warfarin inhibits Vit. K reductase no active form of Vit. K  no synthesis of clotting factors

28. Vitamin K AntagonistsWarfarin • Onset:starts after 12-16 hours • lasts for 4-5 days • Clinical anticoagulant activity needs several days to develop (due to the already circulating clotting factors) • So the action of warfarin will appear after the elimination of prior clotting factors. • Elimination time (factor II needs: 60 hours factor X: 40 hours) • Overlap heparin & warfarin therapytaken together until the effect of warfarin appears (after 5 days) then stop taking heparin.

29. Vitamin K AntagonistsWarfarin • Warfarin has 100% oral bioavailability, high plasma protein binding & long plasma t1/2 of 36 hours • A high loading dose followed by an adjusted maintenance dose • Warfarin is contraindicated with pregnancy as it crosses the placental barrier and is teratogenic in the first trimester & and induce intracranial hemorrhage in the baby during delivery • Warfarin is metabolized by hepatic Cytochrome P450 enzymes with half-life of 40 hrs

30. Potentiating warfarin Inhibitors of hepatic P450 enzymes (cimetidine, cotrimoxazole, imipramine, amiodarone) Platelet aggregation inhibitors (NSAIDs e.g. aspirin) 3rd generation cephalosporins* Drugs displacing warfarin from binding sites (NSAIDs) Drugs reducing the availability of vitamin K Hepatic disease(↓ clotting factors)& hyperthyroidism (↑ basal metabolic rate) Inhibiting Warfarin Vitamin K in some parenteral feed Inducers of hepatic P450 enzymes (rifampicin, barbiturates, … etc) Reduction of GIT absorption (cholestyramine) Diuretics Hypothyroidism Warfarin Drug Pharmacokinetic & Pharmacodynamic Interactions *Cephalosporins potentiate warfarin’s effect by killing vit.k producing normal flora

31. Warfarin Side-Effects • Drug-drug interactions • Bleeding disorder (thus should be monitored) Treatment for bleeding • Minor bleeding: stop therapy + oral Vitamin K • Severe Bleeding: stop therapy + I.V. Vitamin K fresh frozen plasma, recombinant factor VIIa or prothrombin complex may be used