Anticoagulants and HIT

1
Anticoagulants and HIT

• Doug Tollefsen
• August 25, 2006

2
Heparin

• Administered to 12 million patients/year in the
  U.S.
• 120,000 patients/year will develop HIT (60,000
  with thrombosis)
• Repeating polymer of uronic acid and glucosamine
• Highly heterogeneous in terms of sulfation and
  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
5
Allosteric Effect of Heparin
Jin et al., Proc Natl Acad Sci USA 1997 94 14683
Sufficient for inhibition of factor Xa
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Template Effect of Heparin
Required for inhibition of thrombin
Thrombin
Heparin (gt18 monosaccharide units)
Antithrombin
Petitou and van Boeckel, Angew Chem Int Ed 2004
43 3118
7
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
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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 of patients 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
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
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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)
If platelets lt20,000, consider
post-transfusion purpura sepsis another
drug reaction
Warkentin, Br J Haematol 2003 121 535
16
Typical Time Course of HIT
Thrombosis may occur 1-2 days before
thrombocytopenia
Warkentin et al., N Engl J Med 1995 332 1330
17
Temporal Profiles of HIT
All patients received heparin during surgery (day
0) Detectable antibodies were present on or after
day 5
(up to 3 wk after heparin exposure)
2/3 of pts Recent induction of antibodies
1/3 of pts Previous induction of antibodies
(100 d)
Rare Heparin absent at onset of HIT
Warkentin, Br J Haematol 2003 121 535
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Management of HIT

• High index of suspicion
• Unexplained thrombocytopenia (usually 50
  decrease)
• Onset 5-14 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 (highest risk for
  thrombosis occurs after stopping heparin)
• Continue until thrombocytopenia resolved, then
  begin warfarin (30-60 days) overlap DTI
  warfarin 5 days
• LMW heparin contraindicated fondaparinux may be
  safe
• Anecdotal use of plasma exchange or IVIG in
  critically ill patients
• ACCP recommends treatment of isolated HIT
  without thrombosis
• Obtain patients blood for confirmatory tests
• ELISA for antibodies that bind to heparin/PF4
  (strength of reaction may be important)
• Platelet serotonin release assay (SRA)

Complete ACCP recommendations are published in
Chest 2004 126 311S-337S
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http//hematology.im.wustl.edu
20
Warfarin-induced Venous Limb Gangrenein HIT
Patients
Arterial pulses detectable Occurred in 8 of 158
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 based on hirudin 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) Minimal prolongation of the PT (target INR
2-3 during overlap with warfarin) Argatroban Synth
etic 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)
Greinacher and Warkentin recommend (Thromb. Res.
2006 118 165) (1) omitting bolus, decreasing
initial infusion rate, and adjusting lepirudin
dose to aPTT 1.5-2.0 (2) administering IV
vitamin K to patients on warfarin.
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Efficacy of Direct Thrombin Inhibitorsin HIT
with thrombosis
NS
NS
Plt0.001
Plt0.05
Reviewed in Greinacher and Warkentin, Thromb.
Res. 2006 118 165.
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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
25
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)
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
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)
29
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)
30
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 that decrease the
  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