Understanding Anticoagulants: The Good And The Bad

Overview of Hemostasis

Understanding Anticoagulants

Blood Vessel Injury Neural Mechanism

The Good . . . and the Bad

Contact/ Tissue Factor

Platelet Activation

Blood Vessel Constriction

Platelet Aggregation

Coagulation Cascade

Primary hemostatic plug

Alan P. Agins, Ph.D. President, PRN Associates Continuing Medical Education, Tucson, AZ

Reduced Blood flow

Fibrin formation

Stable Hemostatic Plug

The Platelet Aggregation Step

Fibrinogen

Fibrinogen Thrombin

Thromboxane A2

ADP

ADP

VWF

Endothelium

AA

Ca++

Ca++ Ca++

Glycoproteins Ilb, IIIa

Thromboxane A2

Ca++ Glycoproteins Ilb, IIIa

Exposed Collagen VWF

Ti F a ssu ct e or

Thrombin

Exposed Collagen

cyclooxygenase

TxA2

Collagen

Glycoproteins Ilb, IIIa

Collagen VWF

Exposed Collagen

ADP

TxA2

VWF

Exposed Collagen

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The exciting, but really complicated biochemistry involving “factors” steps Exposed Phospholipid Surface site for coagulation cascade activity Intrinsic Pathway ADP

TxA2

Collagen

Ti Fa ssu ct e or

Contact Activation Pathway

VWF

Extrinsic Pathway

Exposed Collagen

Intrinsic

Extrinsic

Final Common Pathway

Hemostasis

Checks and Balances • Five mechanisms keep platelet activation and the coagulation cascade in check. • Abnormalities can lead to an increased tendency toward thrombosis: – Protein C – Antithrombin – Plasmin – Tissue factor pathway inhibitor – Prostacyclin

Causes of Thrombosis

Hemostatic plug Platelets

Endothelial cell Fibrin RBC

• Composition of the blood (hypercoagulability) • Quality of the vessel wall (endothelial cell injury) Virchow's • Nature of the blood flow triad (hemostasis)

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Arterial Thrombus Usually occur in association with pre-existing vascular disease, the most common of which is atherosclerosis ~ plaque rupture Produce clinical manifestations by inducing tissue ischemia, either by obstructing flow or by embolizing into the distal microcirculation

Venous thrombus Usually in the lower limbs - often asymptomatic Can produce acute symptoms if they cause inflammation of the vessel wall, obstruct flow, or embolize into the pulmonary circulation

May lead to MI, occlusive stroke or other ischemic events

Risks for Thrombosis CAD / plaque rupture Recent Surgery Immobilization causing stasis of blood. Travel Obesity Malignancy, especially adenocarcinoma Previous history of DVT or pulmonary embolus (PE) Pregnancy (up to 2 months postpartum) Fracture Heart failure (causes stasis) Oral contraceptive / Estrogen use

Indications For Anticoagulant Therapy Venous thromboembolic disease • Deep venous thrombosis (DVT) • Pulmonary embolism (PE) • Primary prophylaxis of DVT or PE

Indications For Anticoagulant Therapy Arterial thromboembolic disease • Prosthetic heart valves • Mitral valve disease, especially with atrial fibrillation • Congestive cardiomyopathies, especially with AF • Atrial fibrillation • Mural cardiac thrombi • Transient ischemic attacks • Stroke in evolution

Heparin Mucopolysaccharide: MW from 6,000 to 40,000 Da. Average MW of commercial preps: 12,000 - 15,000. Key structural unit of heparin is a unique pentasaccharide sequence

• Disseminated intravascular coagulation • Maintenance of patency of vascular grafts, shunts, bypasses

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Heparin Mechanism Heparin’s Limitations

Binds to plasma proteins – variable bioavailability

Heparin’s Limitations

Heparin - Adverse Effects • Hemorrhagic events – Antidote = Protamine Sulfate

• Non-hemorrhagic side-effects

The Heparin : Antithrombin complex can only bind to and inhibit soluble thrombin – NOT firbrin-bound thrombin

Heparin Monitoring • Activated partial thromboplastin time (aPTT) • Termed "partial" due to the absence of tissue factor from the reaction mixture. • Indicator of the efficacy of both the "intrinsic" (contact activation pathway) and the common coagulation pathways. • Adequate therapeutic effect = aPTT ratio of 2.02.5 times of the baseline aPTT • Monitor aPTT every 4 hours until therapeutic range has been achieved. • Thereafter, monitor aPTT and platelet count daily.

– Elevation of serum aminotransferase levels • reported in as many as 80% of patients receiving heparin • not associated with liver dysfunction • disappears after the drug is discontinued. – Hyperkalemia • 5 to 10% of patients receiving heparin • Due to aldosterone suppression. • Can appear within a few days

• Rare side-effects- alopecia and osteoporosis

Heparin Monitoring • Anti-Xa levels may be more reliable than aPTTs for monitoring heparin in newborns and some children: • At birth, aPTT is prolonged, reflecting the immaturity of the coagulation system. • Children requiring heparin therapy frequently have underlying disorders that influence the baseline aPTT and therefore the response to heparin. • Elevated factor VIII may cause subtherapeutic aPTT despite adequate anti-Xa level. • In these children, aPTT may not correlate well with anti-Xa levels and thus checking both parameters may be helpful. In such conditions, anti-Xa should be used.

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Low Molecular Weight Heparin • LMWH: 4000-5000 (vs Heparin: 15,000+) LMWHs inactivate Xa but have less effect on thrombin (some molecules not long enough) – ratio of anti-Xa to anti-thrombin activity of 3:1 – Do not prolong PTT unless dose high

• Advantages over heparin: – Easier to administer: sc, BID dosing – Dosage and anticoagulant effect easier to predict; dose based on body weight – Lab monitoring not necessary in all patients – Less chance of inducing immune-mediated thrombocytopenia – Smaller risk of osteoporosis in long-term use

Names of LMWHs • Enoxaparin (Lovenox) • Dalteparin (Fragmin) • Tinzaparin (Innohep) – Differ chemically and pharmacokenetically but unsure if these differences are clinically significant

• Other products not yet approved here: – Fraxiparin, reviparin, nadroparin, bemiparin, certoparin

Why do we need newer Anticoagulants? • UFH and LMWHs are inconvenient for the outpatient setting (IV or sq only)

LMWH Rx monitoring • Uncomplicated patients do not require monitoring • Who may need to be? – Newborns, children, pregnant women – Conditions: obesity, renal insufficiency, malignancy, myeloproliferative disorders – Pts with hemorrhagic complications or with initial therapy to confirm appropriate levels

• Anti-Xa for monitoring

Categories of New Drugs –Factor Xa inhibitors: • fondaparinux, idraparinux

–Direct Thrombin Inhibitors: • UFH and LMWHs can cause HIT: – Risk 0.2% with LMWH vs. 2.6 % with UFH – Pts with HIT still need to be anticoagulated

• hirudin, lepirudin, desirudin, bivalirudin, argatroban, ximelagatran

–Heparinoids: • Danaparoid (discontinued)

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Factor Xa inhibitors

Fondaparinux (Arixtra) Intrinsic pathway

• Fondaparinux (Arixtra) – Synthetic polysaccharide: – The drug is the unique pentasaccharide sequence that UFH and LMWH use to bind to AT

Extrinsic pathway

Antithrombin ATIII

ATIII

ATIII

Xa

Xa

Fondaparinux

II Too short to inactivate thrombin (much like LMWH); need >18 saccharide units to inactivate thrombin – Reacts with strong affinity to AT (reversible) → Induces conformational change in AT →

IIa

Fibrinogen

Platelets

Fibrin clot

IIA

Increased ability to inactivate Xa

Fondaparinux – Does not interact with plasma proteins, platelets, or platelet factor IV = useful in HIT (although not yet formally approved) – FDA approved in 2001 • Prevention of post op VTE (DVT and PE) in orthopedic surgery – Hip fracture, hip replacement, knee replacement – Fondaparinux vs. enoxaparin in one study decreased VTE in knee replacement from 12.5 to 27.8%

• 2004/5 approval: – VTE treatment if administered with warfarin – Anticoagulation in abdominal surgery

• Potential uses being studied: MI, PCI, UA

Factor Xa inhibitors - Summary

Fondaparinux • Drug monitoring: • APTT and PT are insensitive • PT/INR may or may not be proportional to the clinical safety or efficacy—more studies needed • Anti-factor Xa assay –must be calibrated with fondaparinux – Long half-life (17 hours) = qd dosing (LMWH = BID)

Idraparinux – Longer acting analogue (q week dosing) currently being developed

Comparison Parenteral Anticoagulants

• Alternative agent for LMWH for prophylaxis or initial treatment of venous thromboembolism • Treatment of HIT • No true antidote although Factor VIIa may be of benefit • Ongoing trials with Idraparinux

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Direct Thrombin Inhibitors

Direct Thrombin Inhibitors

Heparin binding site

• Two types

IIa

– Bivalent DTIs (hirudin and analogs) – Univalent DTIs bind only to the active site

• Direct thrombin inhibitors block both circulating thrombin and clot-bound thrombin. • There is no therapeutic drug monitoring widely available for DTIs • The ecarin clotting time, although not in general clinical use, would be the most appropriate monitoring test.

Bivalent DTIs

IIa

IIa

Catalytic site

Substrate Recognition Site

Hirudins

Argatroban Melagatran

Univalent DTIs Ximelagatran

• All bind in active site and exosite I • IV, IM, SC

• Reversible: Bivalirudin • Irreversible: Lepirudin, Desirudin • Minor differences in structure

IIa

IIa

– First oral direct thrombin inhibitor – Prodrug of melagatran – Discontinued (2006) due to Liver Toxicity

Argatroban

• Approved for acute coronary syndrome ("unstable angina") • Less suitable for long-term treatment

Advantages of direct thrombin inhibitors No nonspecific binding to plasma proteins

Predictable anticoagulant response

Not neutralized by platelet factor 4 (PF4)

Retains activity in presence of platelet-rich thrombi

Ability to inactivate free and bound thrombin

Completely inhibits fluid-phase and fibrin-bound thrombin

Inhibits thrombin-mediated platelet activation

No activation of clotting cascade or release of binding proteins

No formation of heparin-PF4 complexes

No heparin-induced thrombocytopenia

– IV – Second agent (the first is lepirudin) to be indicated for heparin-induced thrombocytopenia (HIT). – Hepatically eliminated and can be used in patients with end-stage renal disease.

Oral Anticoagulants

Warfarin

Courtesy of R Mehran, MD.

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WARFARIN

Warfarin: Mechanism of Action

Serendipitous history “WARF” = Wisconsin Alumni Research Foundation (the holder of original patent) Taken by six – seven million patients in United States

Vitamin K VII Synthesis of NonIX Functional X Coagulation Factors II

THERAPEUTIC USES: Prophylaxis and treatment of venous thrombosis Treatment of atrial fibrillation with embolism Prophylaxis & treatment of pulmonary embolism Adjunctive therapy for coronary occlusion

Warfarin

Prophylaxis in patients with prosthetic valves

Warfarin • Racemic mixture of two active optical isomers R & S forms • Both isomers active, however S-warfarin has 5X the potency of the R-isomer and modulates the in vivo activity of warfarin. • Each isomer is cleared by different cytochrome P450 pathways. • CYP2C9 is the principle enzyme that metabolizes S-warfarin • CYP1A2 & CYP3A4 metabolize the R-isomer.

Warfarin - Adverse Effects “narrow therapeutic index” ƒ Hemorrhage Risk of severe bleeding small (1-2%/yr) but definite Any benefit needs to outweigh this risk when warfarin is considered as a therapeutic measure.

ƒ ƒ ƒ ƒ ƒ

Skin necrosis Purple toe syndrome / Cholesterol embolism Teratogenecity Osteoporosis Agranulocytosis, leukopenia, diarrhea, nausea, anorexia

Warfarin Monitoring Prothrombin Time • The time it takes plasma to clot after addition of tissue factor • Measures the extrinsic pathway and final common pathway (factors II, V, VII, X and fibrinogen) • An estimated 800 million PT/INR assays are performed annually worldwide

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Warfarin Monitoring Terms: PT ratio (PTR) = Patient’s PT Control PT ISI - International Sensitivity Index (standard for thromboplastin reagent)

INR = PTRISI

normal range for the INR is 0.8-1.2

Timing of INR Monitoring Coagulation factors t1/2 vary from 6 – 72 hours T1/2 of warfarin ranges from 1- 2.5 days 1st INR – 2 to 3 days – then daily until therapeutic for at least 2 consecutive days Followed by - INR 2-3 times weekly for 1 – 2 weeks Gradually reduce frequency to q4weeks (if stable) Remember Changes made in Warfarin dose are not completely reflected in the INR until day 3 or 4

Genetic Variability Cytochrome P450

Desired Therapeutic Range Indication

INR

Prophylaxis of venous thromboembolism Treatment of venous thromboembolism Atrial fibrillation

2.0-3.0

Mitral valve stenosis

2.0-3.0

Heart valve replacement Bioprosthetic valve Mechanical valve Myocardial infarction

2.0-3.0 2.0-3.0

2.0-3.0 2.5-3.5 2.0-3.0 2.5-3.5 (high risk patients)

Genetic Variability Vitamin K epoxide reductase • Polymorphisms in the vitamin K epoxide reductase complex 1 (VKORC1) gene explain 30% of the dose variation between patients • The Good: Polymorphisms lead to a more rapid achievement of a therapeutic INR • The Not-so-Good: Shorter time to reach an INR > 4, which is associated with bleeding • African-Americans are relatively less sensitive to warfarin • Asian Americans are more sensitive

Warfarin Interaction 9 Alter metabolic clearance

Genetic polymorphism of CYP2C9 may play a role in the interpatient variability of response to warfarin and predisposition to drug interactions.

9 Reduce absorption from the intestine

Polymorphism of CYP2C9 exists in approx 10% of Caucasians (very rare in African American or Asian populations)

9 Increase metabolic clearance of vitamin Kdependent coagulation factor

CYP2C9 polymorphisms do not influence time to reach effective INR (as opposed to VKORC1) but do shorten the time to INR >4

9 Interfere with other pathways of hemostasis

9 Inhibit synthesis of vitamin K-dependent coagulation factors

9 Unknown mechanisms

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Drugs / Lifestyle / Dietary Interaction

Drugs / Lifestyle / Dietary Interaction

Agents that may reduce the effectiveness of oral anticoagulants – Increase risk of Thrombosis

Increase hemorrhage risk by inhibiting metabolic clearance of warfarin (increase INR) CYP 1A, 2C, 3A

Increase Metabolic Clearance – – – – – –

Carbamazepine Rifampin Phenytoin St John’s wort Cigarette smoking Cruciferous vegetables

Reduce Absorption – Bile Acid Resins Direct Antagonism – Vitamin K (dietary) Independent Risk Factors – Oral contraceptives – Estrogen / SERMs

Drugs / Lifestyle / Dietary Interaction

Omeprazole Amiodarone Ketoconazole Itraconazole Fluconazole Metronidazole Cimetidine

Erythromycin Clarithromycin Fluoroquinolones Protease Inhibitors Grapefruit juice Many others

Risks Related to Anticoagulants

Increase hemorrhage risk by other mechanisms (pharmodynamic, pharmacokinetic, multiple pathways, etc)

• Antiplatelet Drugs • Botanical supplement affecting platelets • Ginger, ginkgo, garlic, feverfew, St John’s wort • Glucosamine / chondroitin supplements (increases INR)

• • • • •

Alcohol Some cephalosporin and sulfa antibiotics COX-2 inhibitors Valproate Many others

Education is the key to keeping patients safe – Proper dosing – Compliance – Potential for interactions with other drugs, foods, lifestyle, etc – Need for routine bloodwork – Plan for handling bleeding

• Warfarin has a narrow therapeutic index • Between 1993 & 2006 ~ 9,766 reports of bleeding complications related to warfarin therapy. – 86% of these were considered serious / 10% were fatal.

• In April 2006, Warfarin ranked ninth in the list of drugs with the most reported adverse events (4,861 cases). • In October 2006, a black box warning of the risk of bleeding was added to the package inserts for proprietary and generic warfarin products in the U.S.

Antiplatelet Drugs

• Minor • Major

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ADP receptor

As pir in

TxA2 (Thromboxane) Increases calcium flux • Adhesion • Aggregation • Vasoconstriction

Why Low Dose ASA?

Clopidogrel Ticlopidine

Dipyradimole Ca++

cyclooxygenase Aspirin AA

TxA2

ReoPro Aggrestat Integralin

AA

X

cyclooxygenase

PGI2 (Prostacyclin) Increases cAMP • Prevents Adhesion • Prevents Aggregation • Vasodilation

TxA2

Ca++ Collagen VWF

Endothelium

Why Low Dose ASA?

No nucleus

X

cyclooxygenase No new protein synthesis AA TxA2 (ie., cyclooxygenase) for life of platelet

Endothelium

TxA2 (Thromboxane) Increases calcium flux • Adhesion • Aggregation • Vasoconstriction PGI2 (Prostacyclin) Increases cAMP • Prevents Adhesion • Prevents Aggregation • Vasodilation

AA

cyclooxygenase

X

PGI2

Aspirin – Antiagregation occurs within 1hr – Side Effects: Allergy, GI discomfort, GI bleed – OTC, easy to crush, inexpensive, small tablet, enteric coated available to decrease stomach upset – Remember: Aspirin’s effect on cyclooxygenase is irreversible. It will take at least 7 - 8 days after stopping therapy to completely restore platelet function

Nucleated Cells cyclooxygenase AA PGI2 Produce mRNA Synthesize new cyclooxygenase Continue to make beneficial PGI2

X

ADP receptor

Clopidogrel

Clopidogrel Ticlopidine

Ca++

AA

cyclooxygenase

Ca++ Collagen VWF

TxA2

– Takes 2 to 3days; maximum inhibition between 4 and 6 days – Oral loading dose of 300mg results in faster platelet inhibition (2-3hrs) – Alternative in ASA allergy / aspirin resistance – Fewer gastrointestinal hemorrhages than aspirin, but more diarrhea and rash – Once daily, prescription required, expensive – Drug requires conversion to active metabolite (CYP 3A4) – Like ASA, effect is irreversible. It will take at least 7 - 8 days after stopping therapy to completely restore platelet function

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Antiplatelet Therapy: Common Oral Agents

Ticlopidine – MOA not entirely known (similar to clopidogrel) – Takes 2 to 3days; maximum inhibition between 4 and 6 days – many adverse events (diarrhea 20%, other GI sx, rash, neutropenia is rare 1% but severe – occurs in first 2-3mos, also thrombocytopenic purpura) – Due to serious and common side effects, and the fact that it’s not much better than alternatives, it’s now rarely used

ADP receptor

Dipyradimole Ca++

AA

Class

Acetylsalicylic acid (ASA)

Clopidogrel

Ticlopidine

Salicylate

Thienopyridine

Thienopyridine

Formulation

Active Drug

Pro-Drug

Active Drug

Maintenance Dose

75-325 mg daily

75 mg daily

250 mg twice daily

2-3%

1- 4% alone 3-5% w/ ASA

1% alone 2-6% w/ ASA

Major Bleeding Risk (%)

Dipyridamole + Aspirin

cyclooxygenase

Ca++ Collagen VWF

TxA2

– Increases cAMP = decreased calcium entry = decreases platelet activation – 38% risk reduction for combo, better than either agent alone – Headache is a common SE; similar bleeding to ASA – Twice daily, capsule may be opened up & tablet crushed but granules must not be crushed – issues with tubes, ASA allergy

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