Title: Anticoagulant Therapy
1Anticoagulant Therapy
Deep venous thrombosis
Pulmonary embolism
2Coagulation Cascade
- Antiagoagulant therapy is aimed at
- preventing clot in patients at risk
- Prevent clot extension/ embolisation
- Deep venous thrombosis (DVT) pulmonary embolism
(PE) - Prothrombinase complex comprises the mixture of
FVa/FXa in addition to calcium phospholipid - The presence of phospholipid accelerates thrombin
formation by 780-fold
3(No Transcript)
4THROMBIN 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 - In addition to inhibiting thrombin, the
glycosaminoglycans also block thrombin generation
5Antithrombin-III Dependent Thrombin Inhibitors
- Standard Unfractionated Heparin (UFH)
- Heparin is a mixture of glycosaminoglycan
molecules, which are heterogenous in molecular
size - Antithrombin III (ATIII) binding is a necessary
requirement for its anticoagulant activity - The mean molecular weight of heparin is 15,000 D
6Mode of Action of Heparin
- Antithrombin III (ATIII) is a slow progressive
inhibitor of thrombin and other clotting enzymes.
- Heparin binds to ATIII through a unique
pentasaccharide (light blue areas) ?
conformational change in the reactive center of
ATIII ? accelerating the rate of ATIII-mediated
inactivation of the clotting enzymes - Heparin also promotes the formation of the
thrombin-ATIII complex by serving as a template
that binds both thrombin and ATIII - ATIII forms a 11 irreversible complex with the
coagulation enzymes - Once this occurs, the heparin dissociates and can
be reused
Heparin
7Heparin inactivates thrombin by binding both
ATIII and thrombin
- To inactivate thrombin, heparin serves as a
template and binds both anti-thrombin III (ATIII)
and thrombin - Binding to ATIII is mediated by the unique
penta-saccharide sequence on heparin - Binding to thrombin occurs through the
heparin-binding domain on the enzyme - Conversely, to inactivate factor Xa, heparin
needs only to bind to ATIII through its
pentasaccharide sequence
Anti-IIa Anti-Xa activity
8Targets for Heparin-ATIII Complex
- Heparin/ATIII inactivates several coagulation
enzymes including thrombin (factor IIa) and
factors Xa, IXa, XIa - 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
9Limitations to the Use of Heparin
10Low Molecular Weight Heparins (LMWHs)
- Low molecular weight heparins (mean molecular
weight 5000 D), prepared by controlled chemical
or enzymatic depolymerization of standard
unfractionated heparin are about one third the
size of starting material - Whereas about one third of the molecules of
unfractionated heparin have the unique
antithrombin III (ATIII)-binding pentasaccharide,
only about 20 of low molecular weight heparin
chains contain the pentasaccharide - Enoxaparin, dalteparin tinzaparin are available
LMWHs products
11Mechanism of Action of Low Molecular Weight
Heparin (LMWH)
- All LMWH molecules, which contain the unique
pentasaccharide, can catalyze the inactivation of
factor Xa by antithrombin III (ATIII) - In contrast, only 25 to 50 of LMWH molecules
that have the pentasaccharide sequence also
contain at least 13 additional saccharide units
to bind to both ATIII FIIa - As a result, the antithrombin (anti-factor IIa)
- activity of LMWH is less than its anti-factor Xa
activity - Standard heparin has equivalent
- anti-factor IIa and anti-factor Xa activity
- because all of the heparin chains that
- contain the pentasaccharide are long
- enough to interact with both ATIII thrombin
12Pharmacokinetic Profile of LMWH
- LMWH has a more favorable pharmacokinetic profile
than standard heparin because LMWH exhibits less
binding to plasma proteins cell surfaces - The reduced binding to plasma proteins results in
- Better bioavailability (90 vs. 20 for heparin)
- more predictable anticoagulant response
- Laboratory monitoring of LMWH activity is not
required - Heparin resistance is rare for LMWH
- The reduced binding of LMWH to cell surfaces
explains why it has a longer half-life than
heparin (4 hr vs. 2 hr for heparin), - Given at fixed doses once to twice daily by S.C.
route
13Biophysical Limitations of Heparin and LMWH
- Both heparin and low molecular weight heparin
preparations have biophysical limitations because
they are unable to inactivate thrombin bound to
fibrin, or to subendothelial matrix and to
inhibit factor Xa within the prothrombinase
complex - Thrombin binds to fibrin where it remains
catalytically active - Thrombin bound to fibrin is protected
- from inactivation by heparin/antithrombin III
14Other Injectable Antithrombotic Agents
- Fondaparinux, a pentasaccharide, is an
AT-III-dependent selective factor Xa inhibitor - It is indicated for the prevention of venous
thrombosis associated with orthopedic surgery - Administered gt6 hours postoperatively and dose
adjusted for renal impairment - Tests for Monitoring Antithrombotic Therapy
- Prothrombin time (PT)/International Normalization
Ratio (INR), usual target is 2-3 times normal - Activated partial thromboplastin time (aPTT)-
(serum UFH) - Anti-Xa activity for LMWHs-treatment in cases of
unexpected bleeding pregnant women
15Therapeutic Uses
- Heparin should be given either by IV or S.C.
injection with onset of action of few minutes and
1-2 hr respectively - LMWHs is given by S.C. route
- I.M. injection produces hematoma formation
- Treatment of deep-vein thrombosis pulmonary
embolism - Prevention of postoperative venous thrombosis in
patients in acute MI phase or one undergoing
elective surgery - Reduction of coronary artery thrombosis after
thrombolytic treatment - Anticoagulant of choice in pregnant women
16Adverse Effects
- Bleeding Bleeding time monitoring is essential.
Treatment involves injection of antidote
protamine sulphate (1mg Iv for each 100 units of
UFH) (reversal of effect) - Thrombosis AT-III inactivation may lead to
potent activation of many clotting factors
hence increasing thrombosis risk - Thrombocytopenia UFH-induced thrombocytopenia
(HIT) is a life-threatening immune reaction that
occurs in up to 3 of patients on heparin therapy
for 5-14 days - It induces platelet activation endothelial
damage with enhanced thrombi formation
paradoxical thrombosis - A non-immunologic reversible HIT may occur in
early phase of therapy due to direct effect of
UFH on platelets - LMWHs, though of lower risk, are contraindicated
with HIT
17Adverse Effects
- Osteoporosis occurs with large doses of UFH
gt20,000 U/day for 6 months or longer - Hyperkalemia rarely occurs with UFH
- It is attributed to inhibition of aldostetone
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
18Contraindications
- Hypersensitivity to heparin
- Active bleeding or hemophilia
- Significant throbocytopenia, purpura
- Severe hypertension
- Intracranial hemrrhage
- Ulcerative GIT lesions
- Active TB
- Recent surgery in CNS, eye
- Advanced hepatic or renal disease
19Direct Thrombin InhibitorsHirudin Hirugen
Hirlug
- A, Hirudin
- A leech-derived protein, a potent specific
inhibitor of thrombin - It binds to both the substrate recognition site
and the catalytic center. The hirudin-thrombin
complex slowly dissociates - B, Hirugen
- A synthetic peptide analogue of the carboxy
terminal of hirudin - It binds to the substrate recognition site of
thrombin
20Direct Thrombin Inhibitors (DTI)
- C, Hirulog is a synthetic bivalent inhibitor of
thrombin comprised of a catalytic site inhibitor
linked to hirugen. Thus, hirulog interacts with
both the substrate recognition site and the
catalytic center of thrombin. - D, Catalytic site inhibitors interact with the
active center of thrombin
21Inhibition of Bound Thrombin
- Neither heparin/ATIII nor LMWH/ATIII are an
effective inhibitor of fibrin-bound thrombin
because the heparin-binding site on thrombin is
masked when the enzyme is bound to fibrin - In contrast, the ATIII-independent thrombin
inhibitors are able to inactivate fibrin-bound
thrombin as well as free thrombin
22In vivo studies with direct thrombin inhibitors
- In experimental animals, hirudin, hirulog, and
inhibitors of the catalytic site of thrombin are
more effective than heparin in preventing
extension of venous thrombosis, preventing
platelet-dependent arterial thrombosis, and
accelerating thrombolysis - Preliminary studies in humans also suggest that
the direct thrombin inhibitors are more effective
than heparin in venous thrombosis, in unstable
angina, and in the setting of thrombolytic therapy
23Clinically Approved Direct Thrombin Inhibitors
- Lepirudin, recombinant hirudin-like peptide, has
been approved for IV anticoagulant use in HIT
patients, has renal clearance - It has potential use in unstable angina patients
(Circulation 2001 103 1479) - Bivaluridin, a bivalent DTI, used by IV route for
patients undergoing percutaneous coronary
intervention - Argatroban, a small monovalent (thrombin active
site only) molecule, with DTI activity, used
similarly in HIT patients, has hepatic clearance - aPTT is used to monitor activity for these agents
24DIRECT FACTOR Xa INHIBITORS
- There are two direct factor Xa inhibitors, the
tick anticoagulant peptide (TAP), originally
isolated from the soft tick Ornithodoros moubata
and antistasin, derived from the Mexican leech - Both inhibitors are now available by recombinant
technology - Studies in animals indicate that both TAP and
antistasin are effective antithrombotic agents in
experimental models of arterial thrombosis
25DIRECT FACTOR Xa INHIBITORS
- Differ from heparin and low molecular weight
heparins in two ways 1) they inactivate factor
Xa independent of antithrombin III (ATIII) and
2) in addition to inactivating free factor Xa,
there is evidence that these agents also are able
to inactivate factor Xa within the prothrombinase
complex
26Oral Anticoagulants Vitamin K Antagonists (The
Coumarins)
- Vitamin K is crucial co-factor for the hepatic
synthesis of clotting factors II, VII, IX X - Vitamin K catalyses the ?-carboxylation of
glutamic acid residues in the mentioned factors
via a vitamin K-dependent carboxylase - The ?-carboxyglutamyl residues bind Ca2 to
enable interaction with phosphlipids
27Vitamin K AntagonistsWarfarin
- The reduced vit K is converted into vitamin K
epoxide which is reduced back by vitamin K
reductase the target enzyme which warfarin
inhibits - This results in the production of inactive
clotting factors lacking ?-carboxyglutamyl
residues
28Vitamin K AntagonistsWarfarin
- Onset Effect of a single dose starts only after
12-16 hrs (unlike heparin) lasts for 4-5 days
although its quick GIT absorption - Clinical anticoagulant activity needs several
days to develop (four half-lives of clotting
factors needed to elapse before steady state) - This may be related to the elimination half-lives
of the concerned clotting factors (6-72 hrs)
(Factor II 40-72 hrs, Xlt48hrs) - Overlap heparin warfarin therapy to overcome
delayed warfarin activity warfarin-inhibition
of the anticoagulant protein C S
29Vitamin K AntagonistsWarfarin
- Warfarin has 100 oral bioavailability, powerful
plasma protein binding long plasma t1/2 of 36
hrs - A loading high dose followed by maintenance dose
is adjusted - 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
30Warfarin Drug Pharmacokinetic Pharmacodynamic
Interactions
- Potentiating warfarin
- Inhibitors of hepatic P450 enzymes (cimetidine,
co-trimoxazole, imipramine) - Platelet aggregation inhibitors (NSAIDs, aspirin)
- 3rd G cephalosporins
- Drugs displacing warfarin from binding sites
(NSAIDs) - Drugs reducing the availability of vitamin K
- Hepatic disease hyperthyroidism
- Inhibiting Warfarin
- Vitamin K in some parenteral feed
- Inducers of hepatic P450 enzymes (rifampicin,
barbiturates, etc) - Reduction of GIT absorption (colestyramine)
- Diuretics
- Hypothyroidism
31Warfarin Side-Effects
- Drug-drug interactions
- Bleeding disorder monitor anticoagulant effect
by measuring PT or INR, reversal of action - 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
32Comparison of UFH LNWH
Character UFH LMWH
Average Mol wt 15,000 5,000
Anti-Xa/anti-IIa activity 1/1 2-4/1
aPTT monitoring required Yes No
Inactivation of platelet-bound Xa No Yes
Protein binding Powerful) 4) Weak ()
Endothelial cell binding Powerful) 4) No
Dose-dependent clearence Yes No
Elimination half-life 30-150 min 2-5 times longer