Consequences of thrombus consequencesangina Myocardial infaction stroke Deep venous thrombosis

Anticoagulant Therapy Treatment of thrombus Prevent Thrombosis Anticoagulants Antiplatelets Removing the occlusion Thrombolysis Coronary angioplasty Vasodilation Nitrates CCB

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

This complex and Ca 2+ comprise the prothrombinase complex Thrombin stimulates platelet aggregation Phospholipids on platelets stimulate clot formation Clotting factors are serine protease enzymes

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

Scheme of anticoagulant drugs Anticoagulant drugs oral warfarin injected Direct acting: Lepirudin Antithrombin III dependent: heparin Low molecular weight heparin

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

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 a slow endogenous progressive inhibitor Antithrombin III (ATIII) is a slow endogenous progressive inhibitor of thrombin and other clotting enzymes (FXa)

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

Heparin inactivates thrombin by binding both ATIII and thrombin To inactivate thrombin penta-saccharide 1. Heparin binds to ATIII by the unique penta-saccharide heparin-binding domain 2. Also binds to thrombin through the heparin-binding domain Conversely, to inactivate factor Xa, heparin only needs to bind with ATIII through its pentasaccharide sequence Anti-II a activity = Anti-X a activity Every heparin molecule contains : Pentasaccharide + heparin binding domain

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

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

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

 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 Mechanism of Action of Low Molecular Weight Heparin (LMWH) Anti-II a < Anti-X a activity 25% of LMWH can interact with both ATIII and thrombin The rest only inactivate factor X

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  T 1/2 = 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.

Comparison of UFH & LMWH CharacterUFHLMWH Average Mol wt 15,0005,000 Anti-X a /anti-II a activity 1/12-4/1 aPTT monitoring required YesNo Inactivation of platelet-bound X a NoYes Protein binding Powerful) 4+)Weak (+) Endothelial cell binding Powerful) 4+)No Dose-dependent clearance YesNo Elimination half-life min2-5 times longer

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.

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

Therapeutic Uses o Heparin should be given either IV or S.C. injection. o onset of action: few minutes (IV) 1-2 hours (S.C.) o LMWHs are given by S.C. route o 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

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

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.

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

Cont’d  Osteoporosis  Osteoporosis occurs with large doses of UFH >20,000 U/day for 6 months or longer (chronic use)  Hyperkalemia  Hyperkalemia rarely occurs with UFH  It is attributed to inhibition of aldosterone secretion  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)

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 * hirudin: is a leech derived anticoagulant It binds to active site and substrate site of thrombin

DIRECT FACTOR Xa INHIBITORS FXa inhibitors Tick anticoagulant peptide (TAP) antistatin They are available by recombinant technology Differs from heparin Independent of AT III Inhibit FXa within prothrombinase complex

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

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

Vitamin K Antagonists Warfarin  Warfarin has 100% oral bioavailability, high plasma protein binding & long plasma t 1/2 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

Warfarin Drug Pharmacokinetic & Pharmacodynamic Interactions Potentiating warfarin  Inhibitors of hepatic P450 enzymes (cimetidine, cotrimoxazole, imipramine, amiodarone)  Platelet aggregation inhibitors (NSAIDs e.g. aspirin)  3 rd 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 *Cephalosporins potentiate warfarin’s effect by killing vit.k producing normal flora

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 VII a or prothrombin complex may be used