Anticoagulants

1
Anticoagulants

• Doug Tollefsen
• July 30, 2004

2
Heparin

• Repeating polymer of uronic acid and glucosamine
• Highly heterogeneous in terms of
• sulfation
• polymer length
• Isolated from mammalian tissues rich in mast
  cells
• Stabilizes enzymes in mast cell granules
• Similar molecules (heparan sulfate) on surface of
  endothelial cells
• Binds to the plasma protein antithrombin
• Antithrombin-heparin complex rapidly inhibits
  thrombin and/or factor Xa
• Only 1/3 of heparin polymers bind and activate
  antithrombin

3
Biosynthesis of Heparin
Adapted from Lindahl et al., Ann NY Acad Sci
1989 36 556
4
Antithrombin-binding Heparin Pentasaccharide
Petitou and van Boeckel, Angew Chem Int Ed 2004
43 3118
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Allosteric Effect of Heparin
Jin et al., Proc Natl Acad Sci USA 1997 94 14683
Sufficient for inhibition of factor Xa
6
Template Effect of Heparin
Required for inhibition of thrombin
Petitou and van Boeckel, Angew Chem Int Ed 2004
43 3118
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Inhibition by Deformation of the Protease
Huntington et al., Nature 2000 407 923
Active protease (thrombin or Xa)
Proteolytic attack
Cleaved antithrombin
Antithrombin
Inactive protease covalently bound to antithrombin
8
Heparin Preparations Used Clinically
Thrombin inhibition (gt18 monosaccharide units)
Factor Xa inhibition (gt5 monosaccharide units)
Unfractionated Heparin
LMW Heparin
Penta
3000
6000
9000
12000
15000
18000
21000
Molecular Weight
9
Low-molecular-weight Heparins
10
Synthetic Heparin Pentasaccharides
11
Pharmacologic Properties of Heparin
12
Complications of Heparin Therapy

• Bleeding
• Major bleeding (5-10 day course) 2 with UFH
  1 with LMWH
• Increased risk with aPTT gt2.5 times normal
• Usually controlled by discontinuation of heparin
• Osteoporosis
• Decreased bone density in 30 after 1 month of
  full-dose UFH therapy
• Vertebral fractures in 2
• Lower incidence with LMWH
• Heparin resistance (gt40,000 U/day of UFH with
  subtherapeutic aPTT)
• Elevated factor VIII or other plasma proteins
• Antithrombin deficiency
• Increased clearance
• Monitor with anti-Xa assay (0.3-0.7 U/ml target)
• Thrombocytopenia
• Immune (onset after 5-10 days HIT antibodies
  present thrombosis likely)
• Non-immune (immediate onset HIT antibodies
  absent thrombosis unlikely)

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Pathogenesis of HIT
Formation of IgG to neoepitopes on PF4 bound to
heparin Activation of platelets by binding of
immune complexes to Fc receptor Induction of
tissue factor on endothelial cells Generation of
thrombin
Warkentin, Annu Rev Med 1999 50 129
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Iceberg Model of HIT
Only a subset of PF4/heparin antibodies activate
platelets. Activation assays are more specific
than antigen assays for clinical HIT. Thrombosis
does not occur in the absence of a significant
fall in platelet count. Incidence of HIT varies
with the clinical setting.
Warkentin, Br J Haematol 2003 121 535
15
Severity of Thrombocytopenia in HIT
Patients who tested positive for HIT antibodies
by SRA
(Thrombosis, skin lesion, or acute systemic
reaction to heparin)
(All other patients had a 50 or greater fall in
platelet count)
Warkentin, Br J Haematol 2003 121 535
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Typical Time Course of HIT
Warkentin et al., N Engl J Med 1995 332 1330
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Temporal Profiles of HIT
All patients received heparin during surgery (day
0) Detectable antibodies were present on or after
day 5
2/3 of pts Recent induction of antibodies
1/3 of pts Previous induction of antibodies
(100 d)
Rare Heparin absent at onset
Warkentin, Br J Haematol 2003 121 535
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Management of HIT

• High index of suspicion
• Unexplained thrombocytopenia (usually 50
  decrease)
• Onset 5-10 days after beginning heparin (any
  type, route or dose)
• or immediate onset if 100 after
  prior exposure to heparin
• Thrombosis, skin lesions at heparin injection
  site, or acute systemic reaction to IV heparin
• Immediately stop all heparin (including IV
  flushes)
• Immediately begin treatment with a direct
  thrombin inhibitor (not warfarin)
• Lepirudin or argatroban (thrombosis may occur
  after stopping heparin)
• Continue until thrombocytopenia resolved, then
  begin warfarin if indicated
• LMW heparin contraindicated fondaparinux may be
  safe
• Treatment of isolated HIT (without thrombosis)
  is recommended
• Obtain patients blood for confirmatory tests
• ELISA for antibodies that bind to heparin/PF4
• Platelet serotonin release assay (SRA)
• Use of heparin in patients with a history of HIT

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Warfarin-induced Venous Limb Gangrene
Arterial pulses detectable Occurred in 8 of 158
of patients with HIT Associated with warfarin
treatment while the patients were still
thrombocytopenic May be caused by depletion of
protein C in the setting of ongoing platelet
activation and thrombin generation
Microvascular thrombosis (venules)
Warkentin et al., Ann Intern Med 1997 127 804
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Parenteral Direct Thrombin Inhibitors
Lepirudin (Refludan) Recombinant 65 amino acid
polypeptide Given IV to achieve aPTT 1.5-2.5
times the patients baseline Half-life 80 min
(increased in renal failure) May develop
antibodies that prolong the half-life and
increase the aPTT (check aPTT daily) Prolongs PT
(target INR 4 during overlap with
warfarin) Argatroban Synthetic arginine
analog Given IV to achieve aPTT 1.5-3.0 times the
patients baseline Half-life 40-50 min (unchanged
in renal failure increased in liver
failure) Prolongs PT (target INR 4 during
overlap with warfarin)
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Warfarin

• Identified (1924) as a toxic substance in spoiled
  sweet clover that caused bleeding in cattle
• Pharmacokinetics
• Plasma concentration peaks 2-8 h after an oral
  dose
• 99 bound to plasma proteins (albumin)
• Half-life in plasma 25-60 h
• Inhibits biosynthesis of vitamin K-dependent
  zymogens (delayed onset of action)
• Prothrombin
• Factor VII
• Factor IX
• Factor X
• Protein C
• Protein S

procoagulant
anticoagulant
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Structures of Warfarin and Vitamin K
Warfarin
Vitamin K1 (phytonadione) R 20-carbon phytyl
chain
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Vitamin K Cycle
Zymogen O2 CO2
g-Carboxylated Zymogen
vitamin K epoxide
reduced vitamin K
vitamin K epoxide reductase (mutations cause
warfarin resistance)
vitamin K reductase
Warfarin inhibits
Warfarin inhibits
vitamin K
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Clearance of Vitamin K-dependent Proteins
100
90
80
70
60
Activity ()
50
Prothrombin
Factor X
40
Factor IX
Protein C
Factor VII
30
0
20
40
60
80
Time after administration of warfarin (hours)
25
Clearance of Vitamin K-dependent Proteins
100
INR
90
80
70
Antithrombotic effect
60
Activity ()
50
Prothrombin
Factor X
40
Factor IX
Protein C
Factor VII
30
0
20
40
60
80
Time after administration of warfarin (hours)
26
Clearance of Vitamin K-dependent Proteins
100
INR
90
80
70
Antithrombotic effect
60
Activity ()
50
Prothrombin
Factor X
40
Prothrombotic effect
Factor IX
Protein C
Factor VII
30
0
20
40
60
80
Time after administration of warfarin (hours)
27
International Normalized Ratio (INR)
C International Sensitivity Index
40
ISI 1.2 (sensitive PT reagent)
35
30
ISI 2.5 (insensitive PT reagent)
25
PT
20
15
10
Therapeutic range
5
0
1
2
3
4
5
6
7
INR
28
Conditions that Alter the Responseto Warfarin

• Compliance
• Drugs
• Affect hepatic metabolism of warfarin
• Affect binding to plasma proteins
• Diet
• Availability of vitamin K
• Other conditions
• Nephrotic syndrome (low plasma albumin)
• Pregnancy (high levels of coagulation factors)
• Liver disease (low levels of coagulation factors)

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Warfarin Resistance and Sensitivity

• Warfarin resistance (gt20 mg QD with
  subtherapeutic INR)
• Non-compliance
• Lab error
• Excessive vitamin K (PO or parenteral)
• Mutations in vitamin K epoxide reductase (rare)
• Rost et al., Nature 2004 427 537
• Warfarin sensitivity (lt2 mg QD with
  supratherapeutic INR)
• Cytochrome P450 polymorphisms (affect rate of
  metabolism)
• 10-20 Caucasians
• lt5 African-Americans or Asians

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Cytochrome P450 Polymorphisms
Effect on daily maintenance warfarin dose
Higashi et al., JAMA 2002 287 1690
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Complications of Warfarin Therapy

• Bleeding
• Incidence varies (4 major bleeding during
  3-month course of treatment for VTE 3-4 annual
  risk thereafter)
• Risk increases with INR gt 4
• Treated with vitamin K (delayed response) or
  fresh-frozen plasma (immediate response)
• Birth defects and abortion
• Skeletal and CNS abnormalities (hypoplastic nose,
  flat face, altered calcification)
• Contraindicated during pregnancy (heparin may be
  used)
• Skin necrosis
• Microvascular thrombosis
• May occur in patients with heterozygous protein C
  or S deficiency if a high initial dose is used or
  heparin overlap is inadequate

Embryopathy
Skin necrosis
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Factors to Consider Before Prescribing
Anticoagulants

• Benefit vs risk
• How well does the anticoagulant prevent
  undesirable events? (varies)
• Risk of bleeding and other complications
• Duration and intensity of therapy
• Depends on the estimated risk of recurrence of
  VTE
• Temporary risk factors usually do not require
  prolonged anticoagulation
• Choice of drug for long-term anticoagulation

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Risk-Benefit Analysis
Retrospective analysis of 1608 patients with
mechanical heart valves treated with oral
anticoagulants
95 confidence interval
Thrombo- embolism
Major bleeding
Cannegieter et al., N Engl J Med 1995 333 11
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Risk of Recurrent Venous Thromboembolism
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Treatment of First Episode of Idiopathic VTE
Three months vs extended anticoagulation
Definition of idiopathic excluded trauma immo
bilization cancer known deficiency
of antithrombin protein C protein S
Kearon et al., N Engl J Med 1999 340 901
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Treatment of First Episode of Idiopathic VTE
Three months vs extended anticoagulation
Kearon et al., N Engl J Med 1999 340 901
37
Dose-intensity of Long-term Warfarin
Prevention of recurrent idiopathic VTE
Median INR 1.0
Median INR 1.8
INR 1.5-1.9
Median INR 1.7
Median INR 2.4
INR 1.5-2.0
INR 2.0-3.0
PREVENT Ridker et al., N Engl J Med 2003 348
1425
ELATE Kearon et al., N Engl J Med 2003 349 631
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Dose-intensity of Long-term Warfarin
Prevention of recurrent idiopathic VTE
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Treatment of VTE Associated with aTransient Risk
Factor
One vs three months of warfarin
Transient risk factors LE fracture or
cast Hospitalization with confinement to bed x 3
days Surgery with general anesthesia gt30
min Exclusions Known molecular defects or
anti-phospholipid antibodies (not routinely
tested)
Kearon et al., J Thromb Haemost 2004 2 743
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Treatment of VTE in Cancer Patients
Patients with active cancer (excl. basal/squam.
skin) in the previous 6 mo presenting with
symptomatic DVT and/or PE
N 338
Major bleeding N 12 Oral anticoagulant N 19
Dalteparin P 0.27
N 338
Lee et al., N Engl J Med 2003 349 146
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Ximelagatran

• First oral direct thrombin inhibitor
• Pharmacology
• Well absorbed from the GI tract
• Converted to melagatran in vivo
• Peak absorption in 15-30 minutes
• Peak levels in 2-3 hours
• Not protein bound
• Half-life 3-4 hours
• Eliminated via kidneys

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Prevention of Recurrent VTE with Ximelagatran
N 611
3 fatal PE
N 612
no fatal PE
Schulman et al., N Engl J Med 2003 349 1713
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Prevention of Recurrent VTE with Ximelagatran
Elevated gt3 x normal 1.0 Placebo (6 pts) 6.0
Ximelagatran (37 pts) all but 4
normalized drug continued in 24 pts
Major bleeding 1.1 Ximelagatran 1.3
Placebo No deaths
Impressive results for an oral anticoagulant
Does not require monitoring of
anticoagulation Real world effects may be better
than warfarin Unclear significance of LFT
abnormalities