ANTICOAGULANTS
Jill Gregory
Medicinal Chemistry
Dr. Buynak
4.23.09
Overview
• Review Blood basics
– composition
– how blood clots
• Note Instances where blood clotting is bad
• Discuss Anticoagulants
– Heparin
– Warfarin
– New Drugs
• F Xa inhibitors
• thrombin inhibitors
• Show example Clinical Trial
Blood Composition
• connective tissue with cells suspended in plasma
Plasma (55%)
Cellular Elements (45%)
water
erythrocytes (red blood cells)
Ions / electrolytes (K+ Ca++)
leukocytes (white blood cells)
plasma proteins (Fibrinogen)
platelets
transported substances
How blood clots
• Damage to endothelium (vessels)
• Platelet plug
• Coagulation Factors from
– plasma
– platelets
– and damaged cells
interact and activate a cascade that leads
to…
• Activated Fibrin: fibers woven into a patch
http://www.advanced-supplements.com/neprinol.html
Coagulation Cascade
Intrinsic pathway
Extrinsic pathway
TF
XIIa
XII
VIIa
XIa
IXa
XI
IX
VIIIa
X
Xa
Va
Common pathway
II (prothrombin)
Each step requires:
• Zymogen (precursor protein)
• Activated Protease from step before
• Cofactors like Ca++
IIa (thrombin)
Fibrinogen
Fibrin
XIII
XIIIa
Stabilized
Fibrin
When would we not want clots to
form??
• Clotting is good at sites of injury because it minimizes blood
loss and prevents infectious agents from getting into our
systems.
• Clotting can be bad when occurs inappropriately in normal
vessels [called thrombus] …clot can break free [called
embolus] and lodge in heart, causing myocardial infarction, or
in brain, causing stroke.
• Thrombosis is caused by abnormalities in blood composition,
vessel wall quality, and/or nature of the blood flow
– Different types of thrombosis:
• Venous [i.e. VTE = venous thromboembolism;
DVT = Deep-vein thrombosis]
• Arterial
When would we not want clots to
form??
• Certain people are at a
higher risk for
thromboembolism:
Virchow triad for the
pathophysiology of thrombus
formation.
http://emedicine.medscape.com/article/959501overview
– Cardiovascular disease
– Atrial fibrillation
– Prosthetic device (i.e.
replacement heart valve)
– Post-surgery (i.e. hip/knee
replacements)
Thromboembolism in the U.S.
• “Annually, more individuals may die from DVT
complications than from the combination of AIDS,
breast cancer, and motor vehicle accidents
combined”
– 900,000 to 2,000,000 VTE cases per year in U.S.
– Estimates of death rates per year vary from 50,000 to 300,000
– 700,000 Strokes per year
• 15% of strokes are in people with Atrial Fibrillation
– 1.2 mil Heart Attacks per year
• Help prevent thromboembolism with the antithrombotic drugs:
Anticoagulants, Antiplatelets, Thrombolytics
Anticoagulants
Inhibit clotting factor activation reactions in the blood
•
CURRENT DRUGS
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•
TARGETED FACTOR
Unfractionated Heparin______________
Antithrombin (indirectly Xa and IIa)
Low Molecular Weight Heparin________
Antithrombin (indirectly Xa and IIa)
Lepirudin (DTI)____________________Thrombin (IIa)
Bivalirudin (DTI) ___________________
Thrombin (IIa)
Argatroban(DTI)____________________
Thrombin (IIa)
Danaparoid_______________________
Antithrombin
Drotrecogin Alfa____________________
Va, VIIIa
Vitamin K antagonists (Warfarin)_______
Prothrombin (II), VII, IX, X
NEW/ in DEVELOPMENT DRUGS
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Fondaparinux_____________________Xa
Idraparinux_______________________Xa
SSR 126517______________________
Xa
Rivaroxaban______________________
Xa
Apixaban_________________________
Xa
LY517717________________________
Xa
YM150__________________________Xa
DU-176b_________________________
Xa
Betrixaban________________________
Xa
Ximelagatran*_____________________
Thrombin (IIa)
Dabigatran etexilate________________Thrombin (IIa)
*taken off the market
Italics are Oral Drugs
Aside: other Anti-thrombotic drug types
• Anti-platelet agents include:
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Aspirin (acetylsalicylic acid)
clopidogrel
dipyridamole
ticlopidine
glycoprotein IIb/IIIa inhibitors
• Thrombolytic (/fibrinolytic) drugs include:
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tissue plasminogen activator - t-PA - alteplase (Activase)
reteplase (Retavase)
tenecteplase (TNKase)
anistreplase (Eminase)
streptokinase (Kabikinase, Streptase)
urokinase (Abbokinase)
Heparin
And other current Parenteral
Anticoagulants
Natural anticoagulation mechanisms
•
Prostacyclin:
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•
made by the endothelial cells and prevents platelets
from collecting and secreting sticky substances
Antithrombin, a plasma protein
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•
stimulated by Heparan sulfate proteoglycans from
endothelial cells, inhibits coagulation factors of
intrinsic and common pathways
Activated plasma zymogen Protein C with
cofactor protein S
–
•
degrades certain cofactors and therefore greatly
inhibits activation of prothrombin and factor X
Tissue Factor Pathway Inhibitor (TFPI)
–
a lipoprotein that can inhibit tissue factor bound
factor Va or VIIa
http://en.wikipedia.org/wiki/Coagulation
Basics of Heparin
• Derived from mucosal tissues of slaughtered meat animals.
Also exists in your body and is secreted form mast cells at the site of
tissue injury
• Increases Antithrombin activity. (Indirect inhibition method of IIa & Xa)
• Usually intravenous adminstration
• Glycosaminoglycan chains attached to a protein core = proteoglycans
(750 kiloDaltons to 1000 kiloDaltons)
• Low molecular weight heparin: different composition; more
predictable; subcutaneous injection twice daily; use preferred over
unfractionated heparin
Action of Heparin and similar drugs
Dark shaded region is a
high-affinity pentasaccharide
sequence
2: Unfractionated Heparin
3: Low-molecular weight Heparin
4: Fondaparinux (discussed later)
Image from Sterling T. Bennett’s chapter on Monitoring Anticoagulant Therapy
Crystal structure of the antithrombin-pentasaccharide complex
Essentials of Glycobiology
Second Edition
Chapter 35, Figure 2
More about Heparin
• Fast action intravenously or by injection (not absorbed
through the stomach or intestinal wall)
• peak after injection 2 - 4 hr
• Complex metabolism (long chains broken up)
• half life 1 - 5 hr; cleared by the reticuloendothelial system
and some excreted in urine.
• A few Drug-drug interactions
• Toxicities: Bleeding & Heparin-Induced Thrombocytopenia
Other Parenteral Anticoagulants
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•
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Lepirudin (DTI) derived from hirudin from leech salivary
glands
Bivalirudin (DTI) approved for use during heparin-induced
thrombocytopenia (HIT) & percutaneous coronary
interventions
Argatroban (DTI) can be used in patients with risk of (HIT)
Danaparoid no longer available in the U.S.
Drotrecogin Alfa used in patients with sepsis; recombinant
form of activated protein C that inhibits f Va and f VIIIa
Dicoumarol
Phenprocoumon
Warfarin Sodium
Vitamin K antagonists:
Parent Molecules:
4-Hydroxycoumarin & indan-1,3-dione
Oral Anticoagulants
Acenocoumarol
Anisindione
Vitamin K antagonists
Mechanism of Action
• Some clotting factors need a carboxyl group added to their
carboxyl-terminal glutamates after synthesis in the liver in
order to be able to bind Ca++ during coagulation activation
[these are called Vitamin K dependent clotting factors]
• This γ-carboxylation reaction requires reduced Vitamin K,
which leaves the reaction as a Vitamin K epoxide.
• Vitamin K epoxide is then converted back to its reduced form
via the enzyme vitamin K epoxide reductase and NADH
• The Vitamin K antagonists inhibit the action of the reductase
enzyme
http://www.vet.uga.edu/VPP/clerk/Harrell/index.php
http://tollefsen.wustl.edu/projects/coagulation/coagulation.html
Vitamin K antagonists
Effect on Coagulation
• Vitamin K dependent clotting factors: Factors II, VII, IX, and X
Intrinsic pathway
Extrinsic pathway
TF
XIIa
XII
VIIa
XIa
IXa
XI
IX
VIIIa
X
Xa
Va
Common pathway
II (prothrombin)
IIa (thrombin)
Fibrinogen
Fibrin
XIII
XIIIa
Stabilized
Fibrin
Vitamin K antagonists
History of Warfarin
• 1930s: cows hemorrhaging after eating spoiled
sweet clover silage
• 1939: bishydroxycoumarin (dicoumarol) identified
• 1948: potent form as rodenticide
– Called Warfarin (Wisconsin Alumni Research
Foundation)
Anticoagulant in humans? No, too toxic!?
• 1951: Army inductee’s failed attempt at suicide
with high dose of warfarin rodenticide
• Clinical use for over 60 years
Vitamin K antagonists
Warfarin
• Adminstered orally, intravenously, or rectally
• Bioavailabily nearly complete; absorption dampered by
food
• Peak concentration 2 - 8 hr
• Binds to albumin 99% of time
• Can cross placental barrier
• Racemic mixture: S form by CYP2C9; R by CYP1A2,
minor pathway CYP2C19, and minor pathway CYP3A4
• half-life: 25 - 60 hr; Excreted in urine and stool
• Food-drug & drug-drug interactions: extensive!!
• Toxicities: bleeding, fetal bone abnormalities
Vitamin K antagonists
Problems with Warfarin
•
Food and drug interactions
•
Genetic variation in metabolism
•
narrow therapeutic window
dosage adjustments &
freq. monitor with INR
overlap with parenteral drugs
•
slow onset of action
Newer Anticoagulants
Targeting specific factors
Factor Xa Inhibitors
• Parenteral or Oral
• Direct or Indirect mech. of inhibition
Intrinsic pathway
Extrinsic pathway
TF
XIIa
XII
VIIa
XIa
IXa
XI
IX
VIIIa
X
Xa
Va
Common pathway
II (prothrombin)
IIa (thrombin)
Fibrinogen
Fibrin
XIII
XIIIa
Stabilized
Fibrin
Factor Xa Inhibitors
Fondaparinux
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Parenteral anticoagulant
first selective factor Xa inhibitor approved by the FDA (2001)
55% better than enoxaparin (LMWH) at reducing risk of VTE
synthetic pentasaccharide: “represents the oligosaccharide
consensus sequence of heparin”
• Indirect inhibition: binds to antithrombin and
increases antithrombin’s affinity for
factor Xa by 300-fold
http://www.fda.gov/cder/foi/nda/2001/21-345_Arixtra_chemr.pdf
Factor Xa Inhibitors
Indraparinux & SSR 126517
• Hypermethylated form of fondaparinux = tighter
binding to antithrombin
• Status: phase III clinical trials
• 80 hr half-life = subcutaneous injection once
weekly
• SSR 126517 is a biotinylated form of
indraparinux, prepared to have an antidote
(avidin)
Factor Xa Inhibitors
Oral factor Xa inhibitors
• Developed off of success of fondaparinux
• Can affect free factor Xa OR Xa bound to
prothrombinase complex!
• Many different drugs of this type being
developed
– Rivaroxaban
– Apixaban
– LY517717, YM150, DU-176b, and betrixaban
Factor Xa Inhibitors
Rivaroxaban & Apixaban
Rivaroxaban
Apixaban
Company sponsor
Bayer, Johnson & Johnson
Bristol-Myers Squibb; Pfizer
# clinical trials
21
15
Molecular weight
436
460
80%
50%
Bioavailability
Peak , half-life (hr) 3, 5-9
3, 9-14
metabolism
CYP 3A4, CYP2J2, CYP-ind.mech.
CYP 3A4, CYP-ind.mech.
excretion
66% kidney, rest in feces
25% kidney, rest in feces
Factor Xa Inhibitors
Two synthesis schemes of
rivaroxaban
Direct Thrombin Inhibitors
Intrinsic pathway
Extrinsic pathway
TF
XIIa
XII
VIIa
XIa
IXa
XI
IX
VIIIa
X
Xa
Va
Common pathway
II (prothrombin)
IIa (thrombin)
Fibrinogen
Fibrin
XIII
XIIIa
Stabilized
Fibrin
Direct Thrombin Inhibitors
Ximelagatran
• First target-specific oral anticoagulant in trials
• Ximelagatran is the oral prodrug of Melagatran
• Hepatatoxicity
– Did not receive FDA approval in 2004
– On the market in Europe but pulled in 2006
• ‘proof of principle’
– “efficacious” as warfarin
– Wider therapeutic index
– Little dosage adjustment/ no monitoring
Direct Thrombin Inhibitors
Dabigatran etexilate
• Prodrug converted to Dabigatran (471 mw) by esterases in
blood after absorbed in stomach in microenvironment from
tartaric acid capsule
• Peak 2hr; Half life 14 – 17 hr
• Not metabolized by cytochrome system = no anticipated drug
interactions
• Farthest along in trials
• History:
– early 1990s: German scientists at Boehringer-Ingleheim began
research
– 1996 Synthesized novel drug
– 1999 Phase I trials
– 2001-3 Phase II trials
– 2004-present Phase III trials
– 2008 announced to be entering European market soon as Pradaxa®
– RE-LY trial results in 2009
• Promising; improvement in liver safety
Direct Thrombin Inhibitors
Dabigatran etexilate clinical trial example
“RELY: Randomized Evaluation of Long term anticoagulant therapy
(RE-LY) comparing the efficacy and safety of two blinded doses of
dabigatran etexilate with open label warfarin for the prevention of
stroke and systemic embolism in patients with non-valvular atrial
fibrillation
Study Design:
Prospective, multi-centre, randomized, open label, controlled
parallel group, non-inferiority trial.
Study Objective:
Demonstrate that the efficacy and safety of two blinded doses of
dabigatran etexilate in patients with non-valvular atrial fibrillation are
non-inferior to warfarin treatment for the prevention of stroke and
systemic embolism.
Scope:
A total of 15, 000 patients will be randomized from approximately
1,000 clinical sites within 44 countries. Patients will be randomized
over a 2-year period with a minimum 1 year follow-up, maximum of 3
years and mean of 1.5 years of follow-up.”
https://www.rely-trial.com/RelyWeb/resources/jsp/emergency/dabigatran_bg.jsp
Summary Slide
Intrinsic pathway
Extrinsic pathway
TF
XIIa
XII
VIIa
XIa
IXa
XI
IX
VIIIa
X
Xa
Va
Common pathway
II (prothrombin)
Vitamin K antagonists
Heparin derivatives
Factor Xa inhibitors
Thrombin inhibitors
IIa (thrombin)
Fibrinogen
Fibrin
XIII
XIIIa
Stabilized
Fibrin
Future of Anticoagulants
• Clinical trials of novel anticoagulants will
continue
• New drugs to be on the market soon but cost will
determine how wide spread the use will be
• Parallel development of f Xa inhibitors and direct
thrombin inhibitors
• Drugs with other targets (f VIIa - TF, f Va - VIIIa,
f IXa) will go to trials
• Utilize crystal structures/docking algorithms
References
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http://tollefsen.wustl.edu/projects/coagulation/coagulation.html (Great on blood coagulation)
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http://www.accessdata.fda.gov/scripts/cder/drugsatfda/ (approved drugs)
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http://clinicaltrials.gov/ct2/search (searchable database for clinical trials)
http://en.wikipedia.org/wiki/Coagulation
http://my.clevelandclinic.org/drugs/antiplatelet_drugs/hic_antiplatelet_drugs.aspx
http://www.vet.uga.edu/VPP/clerk/Harrell/index.php
http://en.wikipedia.org/wiki/Thrombolytic_drug
http://www.americanheart.org/presenter.jhtml?identifier=4451
http://www.natfonline.org/frequency_te.php
http://emedicine.medscape.com/article/959501-overview
https://www.rely-trial.com/RelyWeb/resources/jsp/emergency/dabigatran_bg.jsp
Brunton, Lazo, Parker. Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 11th
ed. Chapter 54: Blood Coagulation and Anticoagulant, Thrombolytic, and Antiplatelet Drugs” by
Philip W. Majerus and Douglas M. Tollefsen.1467-1488.
Bauer, Kenneth A. “New Anticoagulants.” Current Opinion in Hematology 2008, 15:509-515.
Turpie, Alexander. “New oral anticoagulants in atrial fibrillation.” European Society of Cardiology,
European Heart Journal 2007, 29:155-165.
Esko, Jeffrey D and Robert J. Linhart. “Proteins that bind Sulfated Glycosaminoglycans.”
accessed online from Essentials of Glycobiology 2nd edition. CSH Press 2009.
<http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=glyco2&part=ch35>
“Rivaroxaban.” Prous Science. Drugs of the Future 2006, 31 (6): 484-493.
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