Consequences of thrombus

Anticoagulant Therapy

Coagulation CascadeCoagulation Cascade

Anticoagulant therapy is aimed to: Anticoagulant therapy is aimed to: Prevent clot in patients at risk Prevent clot extension/ embolism Deep venous thrombosis (DVT) & pulmonary embolism (PE)

Activated factor X (FXa) + FVa + Ca++ + phospholipids

Prothrombin (FII) thrombin (FIIa)

Fibrinogen fibrin blood clot

Present on platelets’ surfaces. Act by

accelerating thrombus formation.

This complex and Ca2+ comprise the

prothrombinase complex

Thrombin stimulates platelet aggregation

Phospholipids on platelets stimulate clot formation

Clotting factors are serine protease enzymes

The Key Clotting Factor is:

THROMBIN

Because it acts on many aspects of the coagulation

cascade

Scheme of anticoagulant drugs

THROMBIN INHIBITORS

Thrombin inhibitors can either inactivate thrombin directly or block thrombin formation

Thrombin can be inhibited irreversibly by glycosaminoglycans like heparin through an antithrombin III-dependent mechanism

The enzyme can be inhibited reversibly by hirudin and hirudin derivatives in an antithrombin III-independent manner (direct acting)

In addition to inhibiting thrombin, glycosaminoglycans also block thrombin generation

Antithrombin-III Dependent Antithrombin-III Dependent Thrombin InhibitorsThrombin Inhibitors

Standard Unfractionated Heparin (UFH) Heparin is a mixture of glycosaminoglycan molecules,

which are heterogenous in molecular size The mean molecular weight of heparin is 15,000 D Antithrombin III (ATIII) binding is necessary for its

anticoagulant activity Antithrombin III (ATIII) is a slow endogenous a slow endogenous

progressive inhibitorprogressive inhibitor of thrombin and other clotting enzymes (FXa)

Mode of Action of Heparin

It binds to ATIII through a unique pentasaccharide conformational change in ATIII ↑ activity of ATIII

It binds to ATIII through a unique pentasaccharide conformational change in ATIII ↑ activity of ATIII

N.B. ATIII alone can inhibit thrombin but in a very slow reaction

N.B. ATIII alone can inhibit thrombin but in a very slow reaction

Heparin acts as a template to create (thrombin-ATIII complex)Heparin acts as a template to

create (thrombin-ATIII complex)

N.B. only 1 ATIII bind to 1 thrombin (1:1)

N.B. only 1 ATIII bind to 1 thrombin (1:1)

Then heparin dissociates and is reused againThen heparin dissociates and is reused again

Heparin inactivates thrombin by binding both ATIII and thrombin

To inactivate thrombin

1. Heparin binds to ATIII by the unique penta-saccharidepenta-saccharide

2. Also binds to thrombin through the heparin-binding domainheparin-binding domain

3. Conversely, to inactivate factor Xa, heparin only needs to bind with ATIII through its pentasaccharide sequence

Anti-IIAnti-IIaa activity activity = Anti-X = Anti-Xaa activity activity

Every heparin molecule contains :

Pentasaccharide + heparin binding domain

Targets for Heparin-ATIII Complex

Heparin inhibits several coagulation enzymes including thrombin (factor IIa) and factors IXa, Xa, XIa & XIIa

The enzyme most sensitive to inhibition is factor IIa

The next most sensitive enzyme is factor Xa

By inhibiting these two enzymes heparin inhibits both thrombin activity & thrombin formation

Limitations to Heparin’s useLimitations to Heparin’s use

LIMITATION CAUSE CONSEQUENCE

Pharmacokinetic

- Binding to plasma proteins

- Binding to endothelium and macrophages

- Poor bioavailability at low doses, Marked variability in dose response, Dose-dependent clearance

Biophysical

- Heparin is unable to inactivate thrombin bound to fibrin or fibrin degradation products and factor Xa bound within the prothrombinase complex

- Limited efficacy in preventing arterial thrombosis & reocclusion after successful thrombolysis

Antihemostatic - Heparin binds to platelets and inhibits their function

Heparin-induced bleeding

Need to be given in hospital and monitoring

the patient

Low molecular weight heparin have a mean molecular weight of 5000 D.

Prepared by controlled chemical or enzymatic depolymerization of standard unfractioned heparin are about ⅓ the size of starting material

20% of LMWH molecules contain the pentasaccharide domain

Enoxaparin is the most used LMWHEnoxaparin is the most used LMWH

Low Molecular Weight Heparins (LMWHs)

They contain pentasaccharide inactivation of Factor Xa In contrast, only 25% to 50% of LMWH molecules that have

the pentasaccharide sequence are long enough to interact with both ATIII & thrombin

Mechanism of Action of Low Molecular Weight Heparin (LMWH)

Anti-IIAnti-IIa a < Anti-X < Anti-Xaa activity activity

25% of LMWH can interact with both ATIII and thrombin

The rest only inactivate factor X

Pharmacokinetic Profile of Pharmacokinetic Profile of LMWHLMWH

LMWH exhibit less binding to plasma proteins & cell surfaces (better than heparin)

The reduced binding to plasma proteins results in Better bioavailability (90% vs. 20% of heparin) more predictable anticoagulant response Laboratory monitoring of LMWH activity is not required LMWH has low resistance in comparison to heparin T1/2 = 4 hours (more than heparin) Given at fixed doses once to twice daily by S.C. route, and

is given for both inpatients as well as for outpatients.

Comparison of UFH & LMWH

CharacterCharacter UFHUFH LMWHLMWH

Average Mol wtAverage Mol wt 15,000 5,000

Anti-XAnti-Xaa/anti-II/anti-IIaa activity activity 1/1 2-4/1

aPTT monitoring requiredaPTT monitoring required Yes No

Inactivation of platelet-bound XInactivation of platelet-bound Xaa No Yes

Protein bindingProtein binding Powerful) 4+) Weak (+)

Endothelial cell bindingEndothelial cell binding Powerful) 4+) No

Dose-dependent clearanceDose-dependent clearance Yes No

Elimination half-lifeElimination half-life 30-150 min 2-5 times longer

Biophysical Limitations of Heparin and LMWH

Both heparin and LMWH can’t degrade fibrin-bound thrombin (only free thrombin is degraded) nor Factor Xa

within the prothrombinase complex.

Other Injectable Antithrombotic AgentsOther Injectable Antithrombotic Agents

FondaparinuxFondaparinux, a pentasaccharide, is an AT-III-dependent selective for factor Xselective for factor Xaa

Prevents venous thrombosis associated with orthopedic surgery

Administered > 6 hours postoperatively and the dose is adjusted for patients with renal impairment.

To be revised.……

Therapeutic Uses

o Heparin should be given either IV or S.C. injection.o onset of action: few minutes (IV) 1-2 hours (S.C.)o LMWHs are given by S.C. routeo I.M. injection hematoma formation (thus is avoided)

Treatment of deep vein thrombosis Treatment of pulmonary embolism Prevention of postoperative venous thrombosis in patients

with acute MI phase or one undergoing elective surgery(not emergency surgery)

Reduction of coronary artery thrombosis after thrombolytic treatment

Heparin is the anticoagulant of choice in pregnant women

Adverse Effects

Bleeding: they both lead to bleeding but the bleeding is less in LMWH To treat bleeding: inject antidote protamine sulphateprotamine sulphate (1mg IV for each

100 units of UFH) (reversal effect)

Thrombosis: heparin ↓ ATIII ↑ risk of thrombosis

Cont’d

Thrombocytopenia:: Heparin-induced thrombocytopenia (HIT) is a lifethreatening immune reactionOccurs in 3% of patientsUsually occurs a week from starting heparin therapy HIT ↑ platelet activation platelet aggregation thrombosis.HIT endothelial damageHIT may occur in the early stages of treatmen (within 5 days) but it’s non-immune reaction (not life threatening)LMWHs, though of lower risk, are contraindicated with HIT.

How does HIT occur? Heparin injection immune reaction with

body produce antibody against heparin& also bind to platelet receptor activation of platelet thrombosis

Cont’d

OsteoporosisOsteoporosis occurs with large doses of UFH >20,000 U/day for 6 months or longer (chronic use)

HyperkalemiaHyperkalemia rarely occurs with UFH It is attributed to inhibition of aldosterone secretion It occurs with both low- & high-dose UFH therapy Onset is quick within a week after therapy initiation It is reversible by therapy discontinuation Diabetic & renal failure patients are at higher risk Hypersensitivity: (Antigenicity due to animal source) rarely occurring reactions include urticaria, rash, rhinitis,

angioedema & reversible alopecia(hair loss)

Clinically Approved Direct Clinically Approved Direct Thrombin InhibitorsThrombin Inhibitors

Lepirudin, recombinant hirudin*-like peptide. Direct acting thrombin inhibitor Used in HIT patients (IV injection) Has renal clearance It acts on free thrombin and thrombin bound to fibrin It has potential use in unstable angina patients and after

thrombolysis

* hirudin: is a leech derived anticoagulant

It binds to active site and substrate site of thrombin

DIRECT FACTOR Xa INHIBITORS

They are available by recombinant technology

They are available by recombinant technology

Oral Anticoagulants Vitamin K Antagonists (The Coumarins)Vitamin K Antagonists (The Coumarins)

Vitamin K is co-factor for the hepatic synthesis of clotting factors II, VII, IX & X

Vit. K Vit. K epoxide (active form)By Vit.k reductase

warfarin

Warfarin inhibits Vit. K reductase no active form of Vit. K no synthesis of clotting factorsWarfarin inhibits Vit. K reductase no active

form of Vit. K no synthesis of clotting factors

Vitamin K AntagonistsWarfarin

Onset:Onset: starts after 12-16 hours lasts for 4-5 days Clinical anticoagulant activity needs several days to

develop (due to the already circulating clotting factors)

o So the action of warfarin will appear after the elimination of prior clotting factors.

o Elimination time (factor II needs: 60 hours factor X: 40 hours)

Overlap heparin & warfarin therapyOverlap heparin & warfarin therapy taken together until the effect of warfarin appears (after 5 days) then stop taking heparin.

Vitamin K AntagonistsWarfarin

Warfarin has 100% oral bioavailability, high plasma protein binding & long plasma t1/2 of 36 hours

A high loading dose followed by an adjusted maintenance dose

Warfarin is contraindicated with pregnancy as it crosses the placental barrier and is teratogenic in the first trimester & and induce intracranial hemorrhage in the baby during delivery

Warfarin is metabolized by hepatic Cytochrome P450 enzymes with half-life of 40 hrs

Warfarin Drug Pharmacokinetic & Pharmacodynamic Interactions

Potentiating warfarin Inhibitors of hepatic P450

enzymes (cimetidine, cotrimoxazole, imipramine, amiodarone)

Platelet aggregation inhibitors (NSAIDs e.g. aspirin)

3rd generation cephalosporins* Drugs displacing warfarin from

binding sites (NSAIDs) Drugs reducing the availability of

vitamin K Hepatic disease(↓ clotting factors)&

hyperthyroidism (↑ basal metabolic rate)

Inhibiting Warfarin Vitamin K in some

parenteral feed Inducers of hepatic P450

enzymes (rifampicin, barbiturates, … etc)

Reduction of GIT absorption (cholestyramine)

Diuretics Hypothyroidism

*Cephalosporins potentiate warfarin’s effect by killing vit.k producing normal flora

Warfarin Side-EffectsWarfarin Side-Effects

Drug-drug interactions Bleeding disorder (thus should be monitored)

Treatment for bleeding• Minor bleeding: stop therapy + oral Vitamin K• Severe Bleeding: stop therapy + I.V. Vitamin K

fresh frozen plasma, recombinant factor VIIa or prothrombin complex may be used