ANTITHROMBOTIC THERAPY
R. Whorton
Mechanisms of Thrombus Formation and Hemostasis
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
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
Fibrinogen dimers
vWF
P2T
P2T
AA
Cyclooxygenase
GPIIb/IIIa
GPIb-V-IX
TxA2 Synthase
GPIIb/IIIa*
vWF
GPIb-V-IX
TxA2
Collagen
vWF
ADHERENCE
Collagen
AGGREGATION
Coagulation:
Thrombin Generation and Fibrin Formation
Prothrombin
Platelet Activation
Thrombin
Fibrinogen
Acidic
Phospholipids
Extrinsic
Coagulation
Tissue
Factor
Intrinsic
Coagulation
Fibrin
Contact
Phase
Endothelium
Intrinsic System
(Contact Pathway)
Contact
HMW-K
Ka
HMW-K
Pre-K
XIIa
XII
Extrinsic System
(Tissue Factor Pathway)
HMW-K
XI
XIIa, XIa
Ka
XIa
Ca2+
IX
VIIa
IXa
X
VIII, PL
Ca2+
VII
Ca2+, III
X
Xa
PL, V
Ca2+
(Prothrombin) II
IIa (Thrombin)
(Fibrinogen) I
Fibrin (soluble)
XIIIa
Factors in boxes are vitamin K dependent clotting factors
IIa
Fibrin (insoluble)
Coagulation Cascade
XIII
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
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
CLOT LYSIS
Tissue Plasminogen
Activator
Plasminogen
Fibrinogen
Split
Products
Plasmin
Plasmin
THROMBIN
Fibrinogen
2 antiplasmin
Fibrin
Fibrinolysis is limited primarily to the clot by the
protease inhibitor, 2 antiplasmin, which inactivates
circulating plasmin.
Fibrin
Split
Products
Rationale for Antithrombotic Therapy
Thrombogenic Mechanisms
Therapeutic Approaches
Vascular Injury
Reduce Risk Factors
(Hypertension, atherosclerosis, diabetes, etc)
Platelet Adherence,
Aggregation
and Activation
Alter Platelet Function
Inhibitors of Adhesion
Inhibitors of Aggregation
Inhibitors of Activation
Thrombin Generation
and Fibrin Activation
Plasmin Generation
and Fibrinolysis
Affect Coagulation Factors
Disrupt Coagulation Factor synthesis
Direct Thrombin Inhibitors
Xa inhibitors
AT-III Activators
Fibrinogen Depletion
Stimulate Fibrinolysis
Fibrinolytic Agents
ANTIPLATELET DRUGS
TxA2
Thrombin
+
[Ca2+]i
PL
AA
PLA2
Aspirin
Cyclooxygenase
Inhibitors of Aggregation
Clopidogrel
Abciximab (Reopro)
Epifibatide
Tirobifan
TxA2 Synthase
PGI2
+
Adenylate
Cyclase
TxA2
cAMP
AMP
-
Phosphodiesterase
Dipyridamole
GMP
Release
+
NO
Clopidogrel
Contraction
cGMP
Guanylate
Cyclase
-
Inhibitors of Activation:
Aspirin
Dipyridamole
P2T
-
+
GPIIb/IIIa
-
Abciximab,
Epifibatide,
Tirofiban
General Use:
Prevention of arterial
thrombogenesis
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.
Vascular Disorders for Which Aspirin Has Been Shown to be Effective
Disorder
Men at high cardiovascular riska
Hypertension
Stable angina
Unstable anginab
Acute MI
TIA and ischemic strokeb
Severe carotid artery stenosisb
Acute ischemic strokeb
aAspirin
Minimum Effective
Daily Dose, mg
75
75
75
75
160
50
75
160
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.
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.
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.
Anticoagulant Drugs
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.
IIa
ATIII
Heparin
Heparin binds both ATIII and
thrombin (IIa) accelerating the
inactivation of thrombin
ATIII
Heparin
IIa
ATIII
IIa
ATIII
LMWH
LMWH
LMWH can not bind both ATIII
and thrombin and is not as effective
In accelerating interaction
ATIII
Xa
LMWH does increase the rate
of inactivation of Xa
Xa
LMWH
Heparin and LMWH dissociate leaving an irreversible complex between AT III and the
clotting factor. As such, ATIII is a suicide substrate for these clotting factors.
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
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.
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.
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.
COOGlu
-OOC
COO-
CH2
CH2
CH2
CH2
Prothrombin
 Gla
Prothrombin
CO2
O2
Chloro-K
OH
O
Pharmacokinetics: Warfarin is well absorbed
with an elimination half-life or 35-45 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.
O
OH
O
Vitamin K epoxide
Viatmin K (KH2)
Interactions: Hazardous to administer along with
any other antithrombotic agents.
Warfarin
NAD+
Adverse effects: Hemorrhage
NADH
Antidote: Vitamin K1 - must discontinue warfarin.
Prothrombin times return to normal within 24 hrs.
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.
Comparison of Oral Anticoagulants:
Drug
Dabigatran
Rivaroxaban
Warfarin
Mechanism of action
Direct thrombin
inhibitor
Factor Xa inhibitor
Advantages/
disadvantages
Predictable level of
anticoagulation;
frequent monitoring
not required
Predictable level of
anticoagulation;
frequent monitoring
not required
Inhibits formation of
vitamin K-dependent
clotting factors
Frequent monitoring of
PT/INR and
adjustment in dosing
required
Effects are reversible
with idarucizumab
(Praxbind®)
Not reversible
8 days to reach
steady-state
Drug and dietary
interactions
Extensive plasma
protein binding
Adverse effects
Hypersensitivity to
dibigatran
Hypersensitivity to
rivaroxaban
Dyspepsia and
gastritis
Syncope
Bleeding
complications
Bleeding
complications
Effects are reversible
Bleeding
complications
Mechanism of Action of Fibrinolytic Drugs
Circulating Plasminogen
Streptokinase
+
Plasminogen
Urokinase
Hypofibrinogenemia
Fibrinogen
Fibrinogen
Split Products
Plasmin
Pre-existing
antistreccocal
antibodies
Free
Plasmin
Fibrin
FSP
TPA
Clot
Inactive complexes
2-antiplasmin
Fibrinolytic Agents
Primary use of fibrinolytic agents
Acute myocardial infarction
SK, tPA
Acute ischemic stroke
tPA
Acute pulmonary embolism
SK, UK, tPA
Acute deep vein thrombosis
SK
Given intra-arterially.
During continuous infusion of streptokinase (DVT, PE) plasminogen
levels fall, reducing the effectiveness (partially due to requirement of
complex formation).
Fibrinolytic Therapy for Acute MI
Drug
Loading
Dose
Maintenance
Dose
Duration of
Infusion
Concurrent
Heparin
Streptokinse
No
1.5 million IU
1 hr
No
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
Yes
tPA
(Tenecteplase)
30-50 mg by single bolus
body weight
5-10 sec
Yes
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 160-325 mg aspirin as soon as diagnosis is suspected.