1. ANTITHROMBOTIC THERAPY
R. Whorton

2. Mechanisms of Thrombus Formation and Hemostasis
Vessel Disruption
Thrombosis
Endothelial Injury/Dysfunction
Platelet Adherence, Aggregation, Activation
Thrombin Generation and Fibrin Formation
Hemostatic Plug
Arrested Bleeding
Thrombus
Occluding (in situ, embolic)
Infarction, ischemia, embolism
Plasmin Generation and Fibrinolysis
Dissolution of Hemostatic Plug or Occluding Thrombus

3. Platelet Adherence, Aggregation and Activation
Adherence: Interaction between GPIb-V-IX complex and subendothelial
collagen mediated in part by vWF. Leads to platelet activation and
activation of GPIIb/IIIa.
Aggregation: Binding of circulating, bifunctional figrinogen dimers -
platelet - platelet interactions. Leads to platelet activation.
Activation: Activated as above and in response to soluble factors
(thrombin, ADP, epinephrine, PAF, thromboxane A2, etc).
Activation leads to shape change, release of dense granule contents
(calcium, ADP, serotonin), production of thromboxane A2, and
activation of GPIIb/IIIa. Thromboxane A2 leads to further activation.
ADP, Calcium
Serotonin
ADP
P2T
Fibrinogen dimers
vWF
P2T AA
Cyclooxygenase
GPIIb/IIIa
GPIb-V-IX
Collagen
vWF
TxA2 Synthase
GPIIb/IIIa*
GPIb-V-IX
Collagen
vWF
TxA2
ADHERENCE
AGGREGATION

4. Thrombin Generation and Fibrin Formation
Endothelium
Contact
Phase
Tissue
Factor
Intrinsic
Coagulation
Extrinsic
Prothrombin
Thrombin
Fibrinogen
Fibrin
Acidic
Phospholipids
Coagulation:
Thrombin Generation and Fibrin Formation
Platelet Activation

5. Coagulation Cascade Intrinsic System (Contact Pathway) XII XIIa
HMW-K
Contact Ka
HMW-K
Pre-K
XII
XIIa
Extrinsic System
HMW-K
(Tissue Factor Pathway) XI
XIa
XIIa, XIa Ka
Ca2+
VIIa
VII IX
IXa
VIII, PL
Ca2+
Ca2+, III X Xa X
PL, V
Ca2+
(Prothrombin) II
IIa (Thrombin)
(Fibrinogen) I
Fibrin (soluble)
IIa
Factors in boxes are vitamin K -
dependent clotting factors
XIIIa
XIII
Fibrin (insoluble)
Coagulation Cascade

6. Regulation of Thrombin Activity:
1. Inactivation by binding to ATIII
2. Binding to receptors on endothelial cells
3. Endothelial facilitated ATIII-thrombin interaction (surface)
4. Activation of protein C by thrombin in combination with thrombomodulin. Protein C inactivates factors V and VIII and thus limits thrombin formation

7. Other Factors Regulating Thrombus Formation and Size
Shear forces caused by blood flow activate platelets, activate aggregation
and affect adherence
High flow rates reduce coagulation factor concentration - Low flow rates
(venous side) may allow coagulation factor concentrations to reach
levels which activate coagulation leading to thrombus formation
Arterial thrombi are comprised mainly of platelets - result of activated
platelet aggregation
Venous thrombi contain platelets, red cells and fibrin - result, in part,
from activation of coagulation cascade

8. - CLOT LYSIS Tissue Plasminogen Activator Plasmin Plasminogen Plasmin
2 antiplasmin
Fibrin
Split
Products
Fibrinogen
Split
Products
THROMBIN
Fibrinogen
Fibrin
Fibrinolysis is limited primarily to the clot by the
protease inhibitor, 2 antiplasmin, which inactivates
circulating plasmin.

9. Rationale for Antithrombotic Therapy Therapeutic Approaches
Thrombogenic Mechanisms
Therapeutic Approaches
Vascular Injury
Platelet Adherence,
Aggregation
and Activation
Thrombin Generation
and Fibrin Activation
Plasmin Generation
and Fibrinolysis
Reduce Risk Factors
(Hypertension, atherosclerosis, diabetes, etc)
Alter Platelet Function
Inhibitors of Adhesion
Inhibitors of Aggregation
Inhibitors of Activation
Affect Coagulation Factors
Disrupt Coagulation Factor synthesis
Direct Thrombin Inhibitors
Xa inhibitors
AT-III Activators
Fibrinogen Depletion
Stimulate Fibrinolysis
Fibrinolytic Agents

10. + - + - - - - + + - ANTIPLATELET DRUGS Inhibitors of Activation:
TxA2
Thrombin +
Inhibitors of Activation:
Aspirin
Dipyridamole
Inhibitors of Aggregation
Clopidogrel
Abciximab (Reopro)
Epifibatide
Tirobifan
General Use:
Prevention of arterial
thrombogenesis
[Ca2+]i - PL AA
Aspirin
PLA2
Cyclooxygenase
TxA2 Synthase
PGI2
Adenylate
Cyclase +
TxA2
cAMP
AMP -
Phospho-
diesterase -
Clopidogrel
Contraction
Release
Dipyridamole - -
P2T
GMP
cGMP + +
Guanylate
Cyclase
GPIIb/IIIa - NO
Abciximab,
Epifibatide,
Tirofiban

11. Antiplatelet Drugs
Inhibitors of Activation: These drugs interfere with platelet function and are used primarily for arterial thrombotic diseases to prevent the formation of platelet thrombi and platelet-derived vasoactive substances (Thromboxane A2 and serotonin).
Aspirin - Irreversible inhibition of cyclooxygenase which lasts the lifetime of the platelet.
Dipyridamole - Inhibits adenosine uptake and cyclic nucleotide phosphodiesterase and elevates cAMP levels. Usually used in combination with other drugs, especially warfarin, for prevention of thrombus/embolus formation in patients with prosthetic heart valves.

12. Vascular Disorders for Which Aspirin Has Been Shown to be Effective
Minimum Effective Daily Dose, mg
Men at high cardiovascular riska
Hypertension
Stable angina
Unstable anginab
Acute MI
TIA and ischemic strokeb
Severe carotid artery stenosisb
Acute ischemic strokeb 75
160 50
aAspirin use not recommended for primary prevention in people at low risk.
bHigher doses have been tested and not found to confer any greater reduction in risk.
Chest 126: 234S-264S, 2004.

13. Antiplatelet Drugs cont’d
Inhibitors of Aggregation: These drugs prevent thrombus/embolus formation by inhibition of aggregation. Used in situations where it is important to prevent arterial thrombogenesis.
Clopidogrel (Plavix) and Prasugrel (Effient): P2T purinergic receptor antagonists.
Uses: Arterial thrombotic complications associated with unstable angina, transient cerebral ischemia, non-cardioembolic stroke, peripheral vascular disease, valvular disease and coronary bypass surgery. Primary prevention in acute coronary syndromes. In some situations may be more effective than aspirin. Especially useful in patients who do not tolerate aspirin. Also used in combination with aspirin in angioplasty or to prevent reocclusion after fibrinolytic treatment of myocardial infarction. Both are prodrugs which require activation by the liver (2 steps for Clopidogrel, 1 for prasugrel making this drug more predictable with fewer potential interactions).
Side effects: Diarrhea, skin rash, occasional severe but reversible neutropenia, thrombotic thrombocytopenic pupurea.

14. Antiplatelet Drugs cont’d
Abciximab (ReoPro): Chimeric antibody with binding regions of a mouse monoclonal antibody directed against GPIIb/IIIa and the constant region of human IgG (to reduce antigenicity). Must be given iv.
Eptifibatide (Intergilin) and Tirofiban (aggarstat): Epifibatide is a cyclic heptapeptide with a KGD (lys-gly-asp) sequence similar to that in barbourin. Tirofiban is a non-peptide drug which binds to RGD recognition domains in integrin receptors. Both bind rapidly to and block the GPIIb/IIIa receptor. Both drugs must be given iv. As these drugs are relatively specific as inhibitors of aggregation, adherence is not affected and bleeding times are minimally elongated. Eptifibatide is rapidly cleared from circulation.
Uses: These drugs are primarily used in conjunction with aspirin and/or heparin in preventing reocclusion following angioplasty. Also used to prevent reocclusion after fibrinolytic therapy in patients with acute MI.

15. Anticoagulant Drugs ATIII IIa Xa Heparin LMWH
Heparin, oral anticoagulants (direct thrombin inhibitors, factor Xa inhibitors, and warfarin)
Heparin
Heparin is a mucopolysaccharide available as either unfractionated (3-40kDa) heparin (UFH) , fractionated low molecular weight heparin (<6kDa) – LMWH and synthetic pentasaccharides (fundaparinux). All inhibit coagulation by inactivating clotting factors. For UFH inactivation of thrombin and activated factor X are most important. In contrast LMWH and fundaparinux do not inactivate thrombin but inhibit coagulation by inactivating factor Xa.
LMWH
ATIII
IIa
Heparin Xa
Heparin binds both ATIII and
thrombin (IIa) accelerating the
inactivation of thrombin
LMWH can not bind both ATIII
and thrombin and is not as effective
In accelerating interaction
LMWH does increase the rate
of inactivation of Xa
Heparin and LMWH dissociate leaving an irreversible complex between ATIII and the clotting factor. As such, ATIII is a suicide substrate for these clotting factors.

16. Comparison Unfractionated Heparin – LMWH and Fondaparinux
Route of administration
Unfractionated Heparin – IV or subcutaneously. Bioavailability by subcutaneous route is only 30%.
LMWH (enoxaparin) and Fondaparinux) – Subcutaneously. Bioavailability is at least 90% - longer half life than heparin.
Pharmacokinetics
Unfractionated Heparin – rapid saturable binding to proteins and vascular surface reduces effectiveness and may lead to sudden unexpected increase in antithrombotic effect – must monitor aPTT.
LMWH and Fondaparinux – predictable clearance by renal and hepatic mechanisms (minor)
Anticoagulant effect
Unfractionated Heparin – must be carefully titrated and followed by laboratory evaluation of coagulation – complex pharmacokinetics make it difficult to predict plasma levels
LMWH and Fondaparinux – Effects and plasma levels are predictable (dose related) and usually do not require monitoring.
Uses
Used for venous thromboembolism (DVT, PE), unstable angina, cardiac vascular surgery, angioplasty. Because LMWH and Fondaparinux can be administered subcutaneously with predictable anticoagulation, often preferred and can be used in outpatient setting. These drugs are also used in combination with aspirin or other antiplatelet drugs with appropriate monitoring of clotting activity.
Risks
Bleeding (bigger problem with UFH). Up to 5% of patients receiving UFH or LMWH develop an immunologic reaction leading to thrombocytpenia (heparin induced thrombocytopenia – HIT) – not seen with fondaparinux making it useful in patients with HIT.
Antidote – Protoamine sulfate – not as effective with LMWH

17. Anticoagulant Drugs Cont’d – Direct Thrombin Inhibitors:
Hirudin, Bivalirudin: Peptides mimics of anticoagulant isolated from medicinal leach. Produced by recombinant techniques. Hirudin binds to the fibrinogen recognition site of thrombin blocking both protease activity and access to substrate. Bivalirudin is a synthetic derivative of hirudin. Two forms of recombinant hirudin – lepirudin and desirudin.
Uses: Hirudin and bivalirudin are used in patients with or at risk for heparin induced thrombocytopenia. Bivalirudin is also used as a adjunct to thrombolytic therapy in acute MI, for treatment of unstable angina and as an antithrombotic agent in conjunction with GP IIb/IIIa inhibitor in high risk angioplasty.
Pharmocokinetics: Given either iv (half-life of 1-2 hrs) or sc where peak plasma concentrations occur in 2-3 hrs.

18. Anticoagulant Drugs – Direct Thrombin Inhibitors:
Argatroban: Argatroban is a competitive inhibitor of thrombin. It is administered iv and has a half-life of 50 minutes. Used in patients with heparin induced thrombocytopenia and in high risk angioplasty (especially in patients with HIT).
Dabigatran: Dabigatran (Pradaxa) is a newly approved, orally available, potent, irreversible inhibitor of thrombin. Approved for use in stroke prevention in patients with non valvular atrial fibrillation and for prevention of venous thromboembolism after hip or knee replacement. More effective and more predictable than warafrin.
Anticoagulant Drugs - Factor Xa inhibitors
Rivaroxaban and Apixaban: Rivaroxaban (Xarelto) and Apixaban (Eliquis) are inhibitors of factor Xa. They are given orally and like dabigatran are essentially irreversible inhibors. They are approved for prevention of stroke and systemic embolism in patients with nonvalvular artrial fibrillation and for prevention of venous thromboembolism after knee or hip relacement.

19. ORAL ANTICOAGULANTS
Warfarin (Coumadin): Derivative of 4-hydroxycoumarin whose major pharmacologic effect is to inhibit the formation of  - carboxyglutamate residues (about 10/protein) in vitamin K - dependent clotting factors. Required for proper association of Ca2+ and interaction between the clotting factor and PL.
COO-
CH2
Prothrombin
Glu
-OOC
 Gla O2
CO2
Chloro-K
Warfarin
NAD+
NADH
Viatmin K (KH2)
Vitamin K epoxide OH O
Pharmacokinetics: Warfarin is well absorbed with an elimination half-life or hrs. Therapeutic effect delayed from 8 to 12 hours after administration since fully carboxylated clotting factors normally present must be degraded. Steady state anticoagulant activity reached slowly (2-3 weeks) due to long circulating half-lives of 6, 24, 40, and 60 hrs for factors VII, IX, X, and II.
Adverse effects: Hemorrhage
Interactions: Hazardous to administer along with any other antithrombotic agents.
Antidote: Vitamin K1 - must discontinue warfarin. Prothrombin times return to normal within 24 hrs. - -

20. Warfarin
Uses: DVT (venous thromboembolic disease), pulmonary embolism, valvular disease, atrial stasis, atrial fibrillation, prophylaxis in patients with prostheric heart valves and in patients with a history of stroke. In DVT, warfarin therapy is initiated within the first 24 hours in patients receiving heparin. Warfarin is continued for 3-6 months or longer depending on presence of risk factors.
Factors Which Increase Risk for Bleeding in Patients Taking Warfarin
1. Vitamin K deficiency – Diseases which affect vitamin K uptake, certain antibiotics which inhibit vitamin K synthesis by the intestinal flora.
2. Liver Disease – Altered synthesis of clotting factors
3. Drug Interactions (serious bleeding complications are possible)
Antiplatelet drugs including aspirin, NSAIDS, others: clopidogrel, ticlopidine
4. Drugs which inhibit warfarin metabolism – cimetidine, salicylates, imipramine, antifungal azoles.
5. Drugs that inhibit reduction of vitamin K – cephalosporines
Individual responses to warfarin vary greatly - thus it is important to monitor coagulation.

21. Comparison of Oral Anticoagulants:
Drug
Dabigatran
Rivaroxaban
Warfarin
Mechanism of action
Direct thrombin inhibitor
Factor Xa inhibitor
Inhibits formation of vitamin K-dependent clotting factors
Advantages/
disadvantages
Predictable level of anticoagulation; frequent monitoring not required
Effects are reversible with idarucizumab (Praxbind®)
Not reversible
Frequent monitoring of PT/INR and adjustment in dosing required
8 days to reach steady-state
Drug and dietary interactions
Extensive plasma protein binding
Effects are reversible
Adverse effects
Hypersensitivity to dibigatran
Dyspepsia and gastritis
Bleeding complications
Hypersensitivity to rivaroxaban
Syncope

22. Mechanism of Action of Fibrinolytic Drugs
Circulating Plasminogen
Hypofibrinogenemia
Streptokinase +
Plasminogen
Urokinase
Fibrinogen
Fibrinogen
Split Products
Plasmin
Pre-existing
antistreccocal
antibodies
Free
Plasmin
Inactive complexes
2-antiplasmin
Fibrin
FSP
TPA
Clot

23. Primary use of fibrinolytic agents
Acute myocardial infarction
Acute ischemic stroke
Acute pulmonary embolism
Acute deep vein thrombosis
SK, tPA
tPA
SK, UK, tPA SK
Given intra-arterially.
During continuous infusion of streptokinase (DVT, PE) plasminogen levels fall, reducing the effectiveness (partially due to requirement of complex formation).

24. Fibrinolytic Therapy for Acute MI
Drug
Loading Dose
Maintenance Dose
Duration of Infusion
Concurrent Heparin
Streptokinse No
1.5 million IU
1 hr
tPA (Alteplase)
15 mg
50 mg over 30 min and 35 mg over next hr
90 min
Yes
tPA (Reteplase)
10 U over 2 min, wait 30 min and repeat 10 U over 2 min
34 min
tPA (Tenecteplase)
30-50 mg by single bolus body weight
5-10 sec
Alteplase, reteplase and tenecteplase are recombinant forms of tPA.
Risk of bleeding, especially intracranial bleeding, seen with streptokinase and urokinase after prolonged treatment.
Patients given mg aspirin as soon as diagnosis is suspected.