Cardio L19 Antocoagulants 1 Flashcards Preview

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Flashcards in Cardio L19 Antocoagulants 1 Deck (50):
1

Haemostasis

prevention of bleeding due to vessel damage

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Haemostasis a result of 3 componenets:

1. Formation of platelet plug – to block hole
2. Formation of fibrin clot from proteins present in plasma-reinforces plug (fibrin mesh)
3. Contraction of blood vessels – to limit blood flow

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Drugs used in clotting and bleeding disorders: to reduce clotting

Anticoagulants
Antiplatelet drugs
Thrombolytics

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To facilitate clotting

Replacement factors 9VIII, IX)
Plasminogen inhibitors

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Thrombosis:

an unwanted haemostatic plug in blood vessel or heart

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Venous → thrombosis

predominantly fibrin/associated with blood flow stasis (anticoagulants).

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Arterial thrombosis

predominantly platelets/ associated with atherosclerosis/causes ischemia tissue infarction (antiplatelet drugs).

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Blood coagulation:

1. Conversion of fluid blood to solid gel or clot.
2. Conversion of soluble fibrinogen to fibrin
3. Insoluble meshwork of fibrin traps blood cells stabilizing clots

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Clotting cascade endpoint →

thrombin converts fibrinogen to fibrin monomers and then into fibrin polymers

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2 pathways of blood coagulation

1. In vivo (extrinsic) pathway) → Tissue damage
2. The invitro (intrinsic pathway) → Contact (e.g. with exposed collagen)

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Pathways:
Intrinsic pathway →

Contact (e.g. with collagen/damaged surface)
1. T Factor XII converted to XIIa via contact (e.g. via exposed collagen) (12 → 12a)
2. Factor XIIa converts factor XI to Xia (11 to 11a)
3. Factors Xia converts factor Ix to IXa
4. Factor VIIA, phosphlopids and Calcium ions aid to convert factor X to XA
5. Factor X via Va, phospholipis and Calcium ions convert prothrombin (factor II) to Factor IIA (Thrombin)
6. Thrombin converts fibrinogen to fibrin and thrombin also converts XII via calcium ions to initiate the stabilisation of fibrin.

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Extrinsic pathway:

1. Tissue Damamge/Trauma causes the release of TF VIIa, phospholipids and Calcium to covert:
2. Converts X to Xa and follows steps above.
3. Prothrombin to thrombin
4. Fibrinogen to fibrin

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Clotting Cascade: Important points:

1. Inactive precursors activated in series by proteolysis
2. Two distinct pathways:
3. Both pathways converge and activate factor X
4. Calcium ions and –vely charged phospholipids (PL) required for three steps:
5. PL provided by activated platelets
6. Some factors promote coagulation by binding to PL, calcium and serine protease (i.e. Factor II activation of X required calcium/PL and protease factor Va

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End point fibrinogen to Fibrin

caused by the conversion of Tissue factor IIa (Thrombin) from its inactive precursor (TF II (Prothrombin)

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1. Platelet activated

a. Changes shape and expresses negatively charged phospholipid → allows calcium to bind
i. TF can bind to activated platelets (with calcium) e.g. TF10
ii. TF I → prothrombin
b. Expression to TF5a
c. Leads to downstream activation of clotting cascade above and thrombin formation therefore fibrin clot.

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TF binding requires →

1. Negative phospholipids
2. Calcium
3. Gamma-carboxyglutamic acid residues on surface of TF’s → critical for binding to activated platelets

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Carboxylation of factors II, VII, IX and X in liver →
How

• Tissue factors when made are uncarboxylated.
• In the liver TF 2,7,9,10 are convert to the gamma form
• How:
o Involves cyclic reaction using vit K via vitamin K reductase.
o Allows TF to bind to activated platelets properly

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warfarin Mechanism of action

Inhibit carboxylation of Factors II, VII, IX and X (2,7,9,10)
Blocks vit K reductase action

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warfarin Administration

Oral, rapidly absorbed by GI tract

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warfarin Pharmacokinetics
1. Binding power
2. Binding strength
3. Metabolisation
4. Measurement of action

• Rate of onset – depends on factor half-life: I.e. shortest half-life is factor VII at 6 hrs.
→ Start 12-16 hr Duration 4-5 days
• Strongly binds to plasma proteins → makes drug interaction complex
• Metabolized in liver (t0.5 ~40 hrs)
• Measurement of action – measure prothrombin time (PT)

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Prothrombin time (PT): clotting time

1. Clotting time of plasma from patient following addition of calcium/ hromboplastin (standardised reference amount)

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Prothrombin time (PT): expressed as

ratio (International normalized ratio) of clotting time compared with healthy subjects.

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Prothrombin time (PT): INR values


a. 2-2.5 for prophylaxis of deep vein thrombosis
b. 2.5 treatment of DVT/ pulmonary embolism
c. 3.5 for recurrent DVT/ PE

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Prothrombin time (PT): 4. INR Determined:

a. Daily initially
b. Then at longer intervals
c. Then upto every 12 weeks

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Warfarin actions Potentiated by



Drugs which displace it from plasma proteins (Aspirin)
Drugs which interfere with liver function (sulphonamides)
Drugs which interfere with platelet functions (NSAID)
Liver disease (decreases factor production and warfarin clearance)
Decreased Vit K availability

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Warfarin actions Decreased by

Drugs which induce metabolized enzymes (Barbituates)
Promoted clotting factor synthesis (Vitamin K)
Reduced warfarin absorption (colestipol)

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Warfarin actions Unwanted side effects

Haemorrhage – bowel or brain

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Warfarin actions Side effects Major bleeding action

1. Stop warfarin administration
2. Give Vit K
3. Give replacement tissue factors

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Warfarin actions Side effects INR 8.0

4. Stop warfarin until <5.0
5. Give Vit K

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Warfarin actions Side effect INR 6.0-8.0

Stop warfarin until < 5.0
Teratogenic
Contraindicated ui pregnancy

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Injectable anticoagulants: examples

Heparin
LMW Heparin

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Heparin: Mechanism of Action


→Binds to antithrombin III and changes the structure which allows it to bind to IIa
→Binds to IIa directly
→ Antithrombin III can bind to Xa

Forms a tight complex making it a potnet complex in the removal of TF IIa and antithrombin III, which removes IIa from the blood stream.
Heparin then dissociates and repeats the process else where.

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Heparin: Potent action

More potent at Factor IIa than factor Xa

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Heparin: Use

In vivo or (in virtro to clear blocked IV catheter)

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Heparin: Limitations

Activity modified by platelet factor 4 release from platelets inhibits heparin activity
If factor Xa already bound to fibrin cannot interact with ATIII/heparin complex

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Heparin: Administration

Due to charge and large size not absorbed in gut.
IV or SC (LMW heparin)

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Heparin: Pharmacokinetics

• Complex due to plasma protein binding
• Initial rapid removal due to binding to endothelial/ macrophage cells (therefore big bolus initially and then an infusion (for renal excretion)
• Slower subsequent removal by renal excretion

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Heparin: Onset of action

Immediate if IV

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Heparin: Unwanted effects

• Haemorrhage (treat with heparin antagonist protamine sulphate – strong basic protein binds heparin)
• Thrombosis – rare (associated with antibodies against heparin causing endothelial tissue damage
• Occasional osteoporosis, Hypersensitivity and hypoaldosteronism

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Heparin: Examples

Heparin
Calciparine
Minihep
Monoparin

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LMW heparin: MOA of LMW Heparin

Cannot get antithrombin III to interact with IIa and therefore doesn’t inhibit IIa but it can inhibit Xa

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LMW heparin: Pharmacokinetics

LMW does no bind to plasma proteins so kinetics simpler

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LMW heparin: Problem

Only acts on Factor X

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LMW heparin: Good

Not neutralised by platelet factor 4/ Can administer subcutaneously
Much less complex pharmacokinetics

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LMW heparin: Examples

Certoparin
Dalteparin
Enoxaparin

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Antithrombin III – independent: Hirudin

From leech, inhibits thrombin by binding to active site

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Antithrombin III – independent: Hirugen

Synthetic peptide derived from hirudin

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Antithrombin III – independent: Bivalirudin (angiomax)

Bivalirudin (angiomax)

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Antithrombin III – independent: Actions/benefits of bivalrudin

• Potent arterial and venous antithrombotic with fewer bleeding problems (like heparin)
• Clinical trials showed no greater efficacy over pre-existing therapies therefore limited clinical uptake
• Still may prove useful in patients with hepain antibody production
Usage in percutaneous coronary intervention (coronary angioplasty) on the increase due to rapid on/off effects.

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Clinical use of anticoagulants:

1. Heparin → acutely (shortterm therapy)
2. Warfarin → prolonged therapy

1. Prevention of DVT (peri-operatively)
2. In treatment of DVT/ prevention of PE
3. Prevention of thrombosis on prosthetic heart valves.

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