1. Heparin Benedict R. Lucchesi, M.D., Ph.D. Department of Pharmacology
University of Michigan Medical School

2. Heparin
Chemistry
Low molecular weight fractions of heparin have a high affinity for ACTIVATED FACTOR X (Xa), but have less of an effect on thrombin.

3. Antithrombin-Binding Structure of Heparin
L-Iduronic
Acid H N S O 3 -
CH2OSO3 I V
D-Glucosamine
Unit
D-Glucuronic OH
COO-
OSO-3 *
H2CO R’
NH R’’ *
Antithrombin binding region
R’= H or -SO3-
R’’= COCH3 or -SO3-
Groups essential for high
affinity binding of antithrombin

4. Interaction of Heparin-ATIII-Clotting Factors
IIa
Heparin needs to interact with both ATIII and Thrombin (IIa)
To enhance its effect on Factor Xa, heparin needs only to interact with ATIII
LMWH can only increase the action of ATIII on Factor Xa and not on thrombin (IIa).
Heparin
ATIII Xa
LMWH
ATIII Xa

5. Low Molecular Weight Heparinb i n 5
13 or more saccharide units
Heparin
Lysine
Sites A T I F a c t o r X 5
Low Molecular Weight Heparin
Lysine
Sites
<13
Inhibition of thrombin and Factor Xa by the Heparin/AT III complex through a unique pentasaccharide unit. Binding to thrombin requires a minimum of 13 saccharide units. Low molecular weight heparin acts to inhibit Factor Xa and requires that the latter only bind to AT III.

6. Heparin-Thrombin-Fibrin Complex
Ternary heparin/thrombin/fibrin complex.
Thrombin binds to fibrin via exosite 1.
Heparin bridges thrombin to fibrin by binding simultaneously to both fibrin and exosite 2 on thrombin.

7. Anticoagulant Therapy
Heparin
Actions of Heparin
Inactive by itself as an anticoagulant
Requires the presence of a plasma cofactor- ANTITHROMBIN III (AT III)
Heparin potentiates the action of AT III
Heparin-AT III-complex neutralizes the actions of: Factors II, IX, X, XI, XII and XIII
Binds to lysine sites on AT III, leads to conformational change at the arginine reactive center

8. Fibrinogen Prothrombin Thrombin Fibrin
INTRINSIC PATHWAY EXTRINSIC PATHWAY
IXa VIIa / Tissue Factor
VIIIa
TFPI
Platelet
Aggregation
aPC
= sites of
Heparin/ATIII
Inhibition
FACTOR Xa
Fibrinogen
Prothrombin Thrombin Va
aPC
XIIIa
PC / Thrombomodulin
Fibrin
Protein S aPC Inactivation Va & VIIIa

9. AT III Heparin Complex XIIa XIa IXa VIII VIIIa Xa V Va IIa
Fibrinogen Fibrin H e p a r i n / A T I I I C o m p l e x i n a c t i v a t e s t h e c o a g u l a t i o n e n z y m e s F a c t o r X I I a , F a c t o r X I a , F A C T O R X a a n d T H R O M B I N ( F a c t o r I I a ) . T h r o m b i n - i n d u c e d a c t i v a t i o n o f F a c t o r s V a a n d V I I I a i s i n h i b i t e d b y
the heparin/ATIII complex.

10. Actions of Heparin
Low concentrations of heparin increase the activity of AT III considerably, especially against Factor Xa and THROMBIN - these are the most sensitive components of the coagulation cascade
Rationale for the clinical use of “mini-dose” heparin
Inhibition of THROMBIN requires that both the AT III complex and the ENZYME bind to heparin
Inhibition of FACTOR Xa requires that heparin only bind to AT III

11. Actions of Heparin (continued)
Binds strongly to AT III - leads to conformational change of AT III
Active site of AT III is exposed
The active AT III inhibits the proteases involved in coagulation - Factors II, IX, X, XI, XII and XIII
Heparin is NOT consumed, but is released from the AT III complex and is available to react to AT III.

12. Heparin Pharmacokinetics
Heparin binds to saturable sites on the endothelial cells
It is internalized and depolymerized
It displaces platelet factor 4 from the endothelial cells - a protein that neutralizes heparin

13. Heparin-Anticoagulant action is modified by:
Fibrin -
Clot bound fibrin binds thrombin and protects it from inactivation by heparin-AT III.
Platelets -
Bind factor Xa and protect it from heparin-AT III complex inhibition and by secreting platelet factor 4
Not the case with HIRUDIN (AT III independent).
Subendothelial thrombin is protected from heparin-AT III as well.

14. Heparin-Contraindications
Patients who are hypersensitive
Presence of active bleeding or hemophilia
Thrombocytopenia
Purpura
Severe hypertension
Intracranial hemorrhage
Bacterial endocarditis
Active tuberculosis
Ulcerative lesions of GI tract

15. Heparin-Contraindications
Threatened abortion
Visceral carcinoma
During or after surgery on the brain, spinal cord or eye
Patients undergoing lumbar puncture or regional anesthesia block
History of heparin-induced thrombocytopenia

16. Heparin-Adverse Effects
Side Effects Dose Related Frequency
Major Bleeding Yes %
Thrombocytopenia Yes %
with thrombosis Yes %
Osteoporosis Yes Rare
Anaphylaxis No Rare
Skin necrosis ? Rare
Local urticaria ? Rare
Hypoaldosteronism ? Rare

17. Heparin-Induced Thrombocytopeniavs
Heparin-Induced Thrombocytopenia and
Thrombosis

18. Heparin-Adverse Effects (continued)
Heparin-Induced Thrombocytopenia - TWO FORMS:
Mild reduction in platelet count, 2-15 days after initiation of full-dose heparin therapy
Platelet count usually remains above 100,000/µl. Bleeding risk is minimal

19. Heparin-Adverse Effects (continued)
Heparin-Induced Thrombocytopenia-and Thrombosis
Severe reduction in platelet count, 7-14 days after initiation of therapy with full-dose or low-dose heparin
May be associated with thrombotic complications, including arterial thrombosis with platelet-fibrin clots that may cause MI or stroke
Presence of antiplatelet IgG in patients with severe form ?
May be less common with heparin from pork.

20. Heparin - Laboratory Monitoring
aPTT / TCT
Therapy is routinely monitored by means of the aPTT (at UofM it is the TCT)
A clotting time of 1.5 to 2.0 times the normal mean aPTT value ( seconds) is therapeutic
Initially the aPTT should be measured and the infusion rate adjusted every 4 hours.
Once a steady state is achieved, daily monitoring is sufficient.

21. Resistance to Heparin
Some patients may not show a prolongation of the aPTT unless very high doses of heparin are used
Presence of an increased concentration of FACTOR VIII will give rise to a very short control aPTT - they may not be truly resistant to heparin

22. Heparin-Resistance to Heparin (continued)
Accelerated clearance of heparin may exist - as in the case of massive pulmonary embolism
Inherited AT III deficiency have % of the normal plasma concentration of AT III. They respond normally to heparin
Acquired AT III deficiency as with hepatic cirrhosis, nephrotic syndrome or disseminated intravascular coagulation; large doses of heparin may not prolong the aPTT

23. Heparin - Managing Over-Anticoagulation
Anticoagulant effect of heparin disappears within hours after discontinuation of the drug.
Mild bleeding due to heparin can be controlled without administration of an antagonist.
Antagonists are used if bleeding is life-threatening.

24. Heparin - Managing of Over-Anticoagulation
Management depends on:
Degree of over-anticoagulation
Presence or absence of bleeding
A specific, immediate heparin antagonist
Protamine Sulfate
Use mg intravenously
Side-effects largely allergic in nature

25. Heparin: Managing Overanticoagulation (cont’d)
Protamine binds to the acidic (negatively charged) heparin molecule - neutralizes heparin.
Protamine also interacts with platelets, fibrinogen, and other plasma proteins.
Use smallest dose, give by slow IV infusion - do not exceed 50 mg over 10 min. - Causes, flushing, bradycardia, dyspnea, hypotension, anaphylaxis.
Use 1 mg of protamine for every 100 units of heparin remaining in the patient.
Protamine sulfate is a low molecular weight, basic (positively charged) protein.

26. Heparin - Reversal - CABG
Heparinase I (Neutralase™) In Clinical Development
°Specific heparin degrading enzyme
°Produced by gram negative soil bacterium, Flavobacterium heparinum
°Cleaves heparin through a lyase reaction at alpha glycosidic bond linkages between N-sulfated-O-glucosamine and L-iduronic acid residues.
°Each heparinase I molecule cleaves about 70 heparin bond linkages per second at 30° C.
°Eleven heparinase I sensitive cleavage sites per heparin molecule including on located directly within the ATIII-binding pentasaccharide.
°Reverses aPTT - ACT within seconds - Dose ~ 5-30 µg/kg
°t1/2 ~ 6-7 min

27. Heparin - Clinical Uses
Effective for the prevention and treatment of:
venous thrombosis and pulmonary embolism
mural thrombosis after acute MI
managing unstable angina
prevention of coronary artery rethrombosis
used to prevent blood clotting in extracorporeal circulation - e.g. surgery, hemodialysis
treat selected cases of disseminated intravas-cular coagulation (DIC)
treat fetal growth retardation in pregnant women

28. Heparin - Recommendations for Clinical Use
Pregnancy - heparin is the anticoagulant of choice
does not cross the placenta
no untoward effects in the fetus or newborn
given in therapeutic doses - 15,000 U sc q 12 hrs to women with prosthetic heart valves or venous thromboembolism
doses in excess of 20,000 U per 24 hrs for more than 5 months is questionable -due to risk of OSTEOPOROSIS

29. Low Molecular-Weight Heparins
Enoxaparin (Lovenox™)
Dalteparin (Fragmin™)
contain a lower proportion of the critical pentasaccharide sequence than the parent compound.
they increase the action of ATIII on factor Xa, but not its action on thrombin.

30. Low Molecular-Weight Heparins
The LMWHs are not inactivated by platelet factor 4, therefore activity extends to factor Xa bound to platelet membranes.
In clinical doses, no affect on platelet reactivity, PT or aPTT.
Currently approved for prevention of deep vein thrombosis:
After hip or knee surgery or abdominal surgery.
Unstable angina (NQWMI).

31. Low Molecular Weight Heparins
Do not require routine monitoring of INR, PT, or aPTT.
One fixed dose administered subcutaneously.
30 mg every 12 hours.
Must not be administered IM and is not intended for IV administration.
Use with caution in patients with a history of heparin-induced thrombocytopenia.
Reversed by protamine, 1 mg for each mg of LMWH.