Presentation on theme: "Mechanism of Action of Antithrombotic Drugs"— Presentation transcript:
1 Mechanism of Action of Antithrombotic Drugs Rabih R. Azar, MD, MSc, FACCDivision of CardiologyHotel Dieu de France Hospital
2 Mechanism of Action of Antithrombotic Drugs 1- The coagulation cascade2- Heparin3- Low molecular weight heparin4- Direct thrombin inhibitors5- Vitamin K antagonists6- Platelet antagonists7- Thrombolytics
3 Summary of the coagulation cascade Extrinsic pathway:VII, Tissue factorIntrinsic pathway:XII, XI, IX, VIIICommon Pathway:V, X, II (prothrombin)Coagulation occurs when thrombin is formed and transform fibrinogen into fibrin
4 The Central Role of Thrombin in the Coagulation Cascade Thrombin is formed from prothrombin after activation by factor XaHigh concentration of thrombin:activate fibrinogen into fibrinactivate factor XIII leading to fibrin cross-linkingresult in platelets activation and aggregation
5 Site of action of drugs Vascular Surface TXA2 ADP Fibrinogen Fibrin ClopidogrelTXA2ADPFibrinogenFibrinGP IIb/IIIaantagonistsThrombinProthrombinLMWHX XaLMWHSlide I.9Drugs used for the prevention of cardiac ischemic events in patients with UAP/NSTEMI act at various steps in the process of thrombus formation. Aspirin blocks formation of the platelet agonist TXA2 by irreversibly inhibiting the platelet cyclooxygenase pathway.20 The thienopyridine clopidogrel inhibits platelet activation by interfering with the mechanism mediated by ADP,20 another platelet agonist. Heparin primarily augments the activity of circulating antithrombin III, thereby enhancing its inhibitory effect on thrombin.21 Hirulog, a direct thrombin inhibitor, can block the activity of both free (circulating) and clot-bound thrombin.10 Low molecular weight heparin principally inhibits factor Xa, with a lesser effect on antithrombin III.22 Glycoprotein IIb/IIIa receptor antagonists act by preventing binding of fibrinogen to these receptors, the final common pathway for platelet aggregation.15,23Heparin–AT-IIIHirulogIntrinsicExtrinsicCollagenVascular SurfaceTXA2 = thromboxane A2; ADP = adenosine diphosphate; IIb/IIIa = glycoprotein IIb/IIIa; vWF = von Willebrand factor
6 Mechanism of Action of Antithrombotic Drugs 1- The coagulation cascade2- Heparin3- Low molecular weight heparin4- Direct thrombin inhibitors5- Vitamin K antagonists6- Platelet antagonists7- Thrombolytics
9 Mechanism of Action of Unfractionated Heparin Heparin = indirect thrombin inhibitorStimulate anti-thrombinAnti-thrombin binds to thrombin and other activated coagulation factors and forms inactivation complexesThe rate of formation of these inactivating complexes increases by a factor of several thousand in the presence of heparin
10 Mechanism of action of heparin on the thrombin anti-thrombin complex Thrombin and other clotting factors have an active center contained a serine amino acid. This center is inhibited by an arginine on the anti-thrombin (AT) molecule. Heparin binds to a lysine site on the AT molecule and produces a conformational change that converts AT from a slow to a very rapid thrombin inhibitor. Heparin then dissociates from the thrombin/AT complex and can be re-utilized.
12 Heparin Induced Thrombocytopenia Transient mild-moderate thrombocytopenia in 25%Severe thrombocytopenia in 5%Mild platelet reduction within the first 5 days may result from heparin-induced aggregation that is postulated to be benign and transientA smaller subset of pts may develop an antibody-mediated thrombocytopenia that is associated with paradoxical thrombosis. The antibody is directed against the heparin-platelet factor 4 complexThese antigen-antibody complexes bind to Fc receptors on adjacent platelets, causing aggregation and thromboembolism
13 Heparin Dosage and Reversal Therapeutic concentration: anti-Xa units/mL. This will prolong the aPTT by 2-2.5Bolus: units/kg; maintenance: units/kg/h (aPTT 2-2.5)Reversal of heparin:Protamin combines with heparin as an ion pair to form a stable complex devoid of anticoagulant activityFor every 100 units of heparin remaining in the patient, administer 1 mg of protamin sulfate IVThe rate of infusion should not exceed 50 mg in any 10 min period
14 Mechanism of Action of Antithrombotic Drugs 1- The coagulation cascade2- Heparin3- Low molecular weight heparin4- Direct thrombin inhibitors5- Vitamin K antagonists6- Platelet antagonists7- Thrombolytics
15 Mechanisms of inhibitory action of unfractionated heparin and low-molecular weight heparin on thrombin and factor XaInhibition of thrombin requires binding of heparin through a > 18 saccharide residueLMWH do not have that residue and are weak thrombin inhibitorsInhibition of Xa requires binding to AT only (without formation of a ternary complex).
16 Mechanism of Action of Low Molecular Weight Heparin (LMWH) Selective Inhibition of factor Xa via Anti-thrombin with a weak effect on thrombinLMWH anti-Xa/thrombin ratio >> 1In contrast, unfract heparin anti-Xa/thrombin ratio = 1Depending on the length of LMWH, the effect on thrombin may vary, and the anti-Xa/thrombin ratio varies
18 Why are LMWH considered superior to unfractionated heparin 1- Heparin does not inhibit clot bound thrombin or XaLMWH inhibits clot bound Xa2- Heparin binds extensively to plasma proteins including acute phase reactants and vascular and blood cellsLMWH binds much less to these proteins/cells3- LMWH are more resistant to neutralization by platelet factor 44- The longer half life and more predictable anticoagulant response of LMWH allow their administration at a fixed dose without need for laboratory monitoring
19 Dosing of LMWHWeight-based dosing of the LMW heparins results in predictable pharmcokinetics and plasma levels in patients with normal renal functionLevels are determined by anti-Xa units: peak therapeutic levels are unit/mL for twice daily dosing, determined 4 hours after administration, and 1.5 unit/mL for the once daily dosingNeutralization of LMWH by protamin is incomplete. Limited experience suggest that 1 mg of protamin sulfate may be used to partially neutralize 1 mg of enoxaparin. Definite neutralization requires FFP
20 Fondaparinux Is a synthetic pentasachharide Binds antithrombin with high specific activityInactivates XaLong half life of 15 hoursSuperior to enoxaparin in preventing DVT following orthopedic surgery
21 Mechanism of Action of Antithrombotic Drugs 1- The coagulation cascade2- Heparin3- Low molecular weight heparin4- Direct thrombin inhibitors5- Vitamin K antagonists6- Platelet antagonists7- Thrombolytics
22 Direct Thrombin Inhibitors: Mechanism of Action
23 Advantages of Direct Thrombin Inhibitors Do not necessitate anti-thrombin for their actionInactivate both free and fibrin-bound thrombinThey do not bind to plasma proteinsThey have a more predictable anticoagulant responseIndicated for the treatment of heparin induced thrombocytopenia
24 Direct thrombin inhibitors Hirudin (lepirudin): IV. Short half lifeBivalirudin: IV. Rapid onset and offset of action.Argatropan: IV. Short half life. Elevates INR because of test interference, rendering the transition to warfarin difficult.Melgatran: IV but has an oral form: Ximelagatran.Ximelagatran:is a prodrug.Predictable pharmacokinetics and bioavailability allowing for fixed dosing and predictable anticoagulant response. No need for routine coagulation monitoring.Rapid onset and offset of action allowing for immediate anticoagulation and thus no need for overlap with additional anticoagulant drugs.
25 Mechanism of Action of Antithrombotic Drugs 1- The coagulation cascade2- Heparin3- Low molecular weight heparin4- Direct thrombin inhibitors5- Vitamin K antagonists6- Platelet antagonists7- Thrombolytics
26 Mechanism of action of anti-vitamin K drugs - Inhibit Vit K epoxide reductase and quinone reductase-Block the formation of Vit KH2 which is a cofactor for the carboxylation of factors II, VII, IX, X to their active form
28 Clinical Use of Anti-vitamin K Vitamin K dependent proteins: VII, IX, X, prothrombin, prot C, SProt C and S have the shortest half life, followed by factor VIIThe initial effect of anti-vitamin K drugs is a hypercoagulability effect secondary to a deficiency in protein C and SA large loading dose of anti-vitamin K can decrease factor VII levels without affecting factors IX, X and II. This predisposes to bleeding without protecting against thrombosis because the activity of all 4 factors must be inhibited to achieve a clinically effective anticoagulation
29 Mechanism of Action of Antithrombotic Drugs 1- The coagulation cascade2- Heparin3- Low molecular weight heparin4- Direct thrombin inhibitors5- Vitamin K antagonists6- Platelet antagonists7- Thrombolytics
31 Platelet GP IIb/IIIa Receptor in Vascular Injury: Aggregation Fibrinogen (or von Willebrand factor)GP IIb/IIIaAggregationPlatelet activation causes changes in the shape of platelets and conformational changes in GP IIb/IIIa receptors, transforming the receptors from a ligand-unreceptive to a ligand-receptive state. Ligand-receptive GP IIb/IIIa receptors bind fibrinogen molecules, which form bridges between adjacent platelets and facilitate platelet aggregation. Inhibitors of GP IIb/IIIa receptors also bind to GP IIb/IIIa receptors, blocking the binding of fibrinogen and thus preventing platelet aggregation.`Coller. Heart Disease, UpdateColler B. Unstable angina. In: Braunwald E, ed. Inhibitors of Platelet Aggregation: GP IIb/IIIa Antagonists. Heart Disease, Update 4. Philadelphia, Pa: WB Saunders; 1995.
35 Effects of aspirin Ticlopidine Clopidogrel ADP ASPIRIN Collagen ThrombinEpinephrineThromboxane A2SerotoninPlatelet-activating factorASPIRINGP llb/lllaSlide I.10Multiple platelet agonists are involved in thrombus formation. Aspirin, a standard therapy for UAP and acute MI,24 interferes with the action of various agonists by inhibiting cyclooxygenase within platelets.20 By inhibiting platelet cyclooxygenase, aspirin prevents the actions of epinephrine, serotonin, and TXA2. Not all agonists are inhibited by aspirin, however.13,25 The thienopyridines ticlopidine and clopidogrel interfere with ADP receptors on the platelet surface to produce an antiplatelet effect,13,20,25,26 but, like aspirin, these agents interfere with only one of many intracellular pathways.FibrinogenADP= adenosine diphosphate; GP= glycoproteinThickness of line indicates strength of activatorAdapted from Ohman EM et al Eur Heart J 1995;16(suppl L):50-55.
36 Ticlopidine (ticlid) and clopidogrel (plavix) Ticlopidine Clopidogreltwice / day once / dayNeutropenia No neutropeniaCBC monitoring No need for CBC monitoringSkin reactions (rash) No skin reactionsDelayed onset of effect Rapid onset of effect
37 Platelet-fibrinogen interaction Platelet aggregationBlood vessel wallPlateletIIb/IIIaFibrinogenIb receptorSite of injuryPlatelet adhesionIIb/IIIa receptorblockersIbSlide I.13The combined effect of platelet adhesion and platelet recruitment by agonists such as thrombin exposes the integrin platelet glycoprotein IIb/IIIa receptor,13,15 which is present in as many as 50,000 copies on the surface of each platelet.32 Like other members of the integrin family, platelet glycoprotein IIb/IIIa receptors are specific for the amino-acid sequence arginine-glycine-aspartic acid, or RGD.32 Thus, adhesive proteins, such as fibrinogen, that contain an RGD sequence can bind to the glycoprotein IIb/IIIa receptor.23 Indeed, the primary function of this receptor is to provide a binding site for fibrinogen. Activated platelets form aggregates by cross-linking through fibrinogen's multiple glycoprotein IIb/IIIa receptor binding sites.23Once platelets begin cross-linking with fibrinogen, they rapidly join together into doublets, triplets, and multiple groups. These aggregates enlarge at the site of vessel damage.33 Following atherosclerotic plaque rupture, large platelet aggregates can potentially lead to a critical occlusion of the diseased vessel.16 Elucidation of the pathophysiology of thrombus formation has shown that the binding of fibrinogen to the platelet glycoprotein IIb/IIIa receptor is the final, obligatory pathway to platelet aggregation.15By preventing the binding or interaction of the receptor with fibrinogen, glycoprotein IIb/IIIa receptor antagonists help prevent platelet aggregation and subsequent thrombus formation.15
38 GP IIb/IIIa Inhibitors: Chemical Structures AbciximabTirofibanEptifibatideChimeric Monoclonal AntibodyMW 50,000 DNonpeptide Tyrosine Derivative MW 500 DCyclic Heptapeptide MW 800 DOOHHNSNHNHH2NH N–SO2–C4H9OCOOHHNRegardless of the stimulus that leads to platelet aggregation, the GP IIb/IIIa receptor—1 of a group of adhesion receptors known as integrins—plays a key role in the process. Although the 3 GP IIb/IIIa inhibitors currently in clinical use differ in their chemical structures, all 3 target this receptor and have a similar mechanism of action: inhibition of ligand binding to the receptor. Abciximab, the first GP IIb/IIIa blocker to be tested and used clinically, is the Fab fragment of a chimeric murine- and human-derived monoclonal antibody altered to reduce immunogenicity. With a molecular weight of approximately 50,000 daltons, abciximab is about 2 orders of magnitude larger than the other 2 agents. Eptifibatide, a peptide specific for the GP IIb/IIIa receptor, is an analog of the sequence at the extreme carboxyterminus of the -chain of fibrinogen, which mediates the binding of fibrinogen to the receptor. Tirofiban, a tyrosine derivative, was based on the Arg-Gly-Asp (RGD) recognition peptide for the GP IIb/IIIa receptor. The design of the molecule provided for GP IIb/IIIa specificity, high affinity for the target receptor, and elimination of peptide bonds.Topol et al. Lancet. 1999;353:Topol EJ, Byzova TV, Plow EF. Platelet GP IIb-IIIa blockers. Lancet. 1999;353:
39 Comparative Properties of GP IIb/IIIa Inhibitors Abciximab Tirofiban EptifibatideType of Chimeric monoclonal Nonpeptide Cyclicmolecule antibody tyrosine derivative heptapeptideAffinity for circulating platelets High Moderate UnknownKD (nmol/L)Molecules of drug per receptor 1.5 >100 Drug clearance t1/2 = h t1/2 = 2 h t1/2 = 2.5 hGP IIb/IIIa Nonspecific Specific Specific receptor specificity (RGD) (KGD)Among the properties that define GP IIb/IIIa inhibitor efficacy and safety are receptor specificity, antagonist affinity, and reversibility. These differ for the 3 agents currently approved for clinical use: the chimeric monoclonal antibody abciximab, the cyclic heptapeptide eptifibatide, and the nonpeptide tyrosine derivative tirofiban. While all 3 agents share a biologic target, they are different in other respects, including the physical binding site on the receptor (which regulates specificity), dissociation kinetics (governing affinity), stoichiometry for inhibition, clearance mechanisms, and plasma and biologic half-lives.Tirofiban and eptifibatide are both small molecules that fit into the binding pocket of the GP IIb/IIIa receptor, assuring high specificity. In contrast, abciximab is too large to enter the ligand-binding pocket; instead, it covers the receptor and interferes with a secondary fibrinogen-binding site. It thus binds less selectively than either of the other inhibitors. Abciximab also interferes with the function of several other ligand-binding sites, including the MAC-1 and vitronectin receptors; it is unknown whether this property of abciximab confers clinical benefit. Tirofiban and eptifibatide are competitive antagonists with a concentration-dependent antiplatelet effect. Both agents are cleared as active drug with lesser concentrations of inactive metabolites. There is no contribution to platelet blockade from metabolites of either drug. With abciximab, there is a slow loss of receptor blockade over time after termination of an infusion, with a concomitant prolongation of bleeding time. With tirofiban and eptifibatide, physiologic platelet aggregation and normal hemostasis generally return within 4 hours after the end of an infusion.Scarborough et al. Circulation. 1999;10: ; Tcheng. Am J Cardiol. 1999;83:7E-11E.Scarborough RM, Kleiman NS, Phillips DR. Platelet glycoprotein IIb/IIIa antagonists: what are the relevant issues concerning their pharmacology and clinical use? Circulation. 1999;100:Tcheng JE. Differences among the parenteral platelet glycoprotein IIb/IIIa inhibitors and implications for treatment. Am J Cardiol. 1999;83:7E-11E.
40 Mechanism of Action of Antithrombotic Drugs 1- The coagulation cascade2- Heparin3- Low molecular weight heparin4- Direct thrombin inhibitors5- Vitamin K antagonists6- Platelet antagonists7- Thrombolytics
41 The Fibrinolytic System Plasmin is the key protease enzyme of the coagulation system2 major activators of plasminogen: tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA)Both t-PA and plasminogen possess specific affinity for fibrin and thereby bind selectively to clots. In the absence of fibrin, t-PA activates plasminogen to plasmin relatively slowly
42 Thrombolytic DrugsBecause indiscriminate plasmin lysis of both fibrin and fibrinogen can produce a sysemic state of fibrin(ogen)olysis, which might cause a serious systemic bleeding tendency, attempts have been made to develop thrombolytic agents that generate plasmin preferentially at the fibrin surface in preformed thrombusStreptokinase and urokinase induce a systemic lytic stateIn contrast, t-PA activate plasminogen preferentially at the fibrin surface.The risk of hemorrhage is however not reduced, because of the inability of plasmin to discriminate between fibrin in pathological thrombi and fibrin in physiological hemostatic plugs
43 Streptokinase Isolated from hemolytic streptococci Streptokinase itself possesses no enzymatic activityStreptokinase forms a complex with plasminogen and it is the strepto-plasminogen complex that actually possesses enzymatic activityStreptokinase is anti-genic. Most individuals have preexisting antibodies from previous streptococcal infectionIt can cause allergic reactions: transient hypotension, serum sickness-type syndrome
44 tPA Produced by recombinant DNA technology Fibrin specific It activates plasminogen directlyThe efficiency of plasminogen activation by t-PA is enhanced in the presence of fibrinShort half life: minutes