New Anticoagulants:Beyond Warfarin and Heparin

Rachel LaCount

Resident in Pathology

August 4, 2005

Overview

• Older anticoagulants: Warfarin & Heparin, LMWH

• Why we need new drugs

• Newer drugs– Direct Thrombin Inhibitors– Xa inhibitors– Heparinoids– The future

Brief Review: Warfarin MOA

• Blocks vitamin K- dependent glutamate carboxylation of precursor factors II, VII, IX, X

• Vit K = cofactor

• Warfarin blocks the reduction of Vit K

• Oral administration

Review: Heparin MOA• Indirect thrombin inhibitor: IV only• Called UFH (unfractionated heparin)• Complexes with AT (heparin co-factor I)• AT by itself inactivates SLOWLY!

– Thrombin – Factor Xa– XIIa, XIa, IXa (lesser extent)

• AT + Heparin: conformational change in AT = 1000-4000 fold acceleration in inactivation

• At high concentrations: Also binds to platelets and heparin co-factor II—which inhibits thrombin

• Heparin made of polysaccharide chains of varying lengths • Unique pentasaccharide sequence binds to AT

– Sequence is randomly distributed along heparin chains

• Inactivation of Xa—Heparin doesn’t have to bind to it• Inactivation of Thrombin--Requires heparin to bind to both

AT and itself– Heparin must be >18 monosaccharides long to do this– Virtually all heparin molecules are > 18

Limitations of Warfarin and Heparin:

• Both have narrow therapeutic windows• Highly variable dose responses: requires

laboratory monitoring (PT, APTT)– Heparin can bind to other plasma proteins making

bioavailability variable– Warfarin has numerous food, drug interactions

• Limited ability to stop a clot from propagating:– Heparin does not inactivate thrombin bound to fibrin

or Xa bound to platelets very well

LMWHs• Molecular wt:

Heparin: 15,000 vs LMWH: 4000-5000

• 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: sq, 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

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

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– People with hemorrhagic complications or

with initial therapy to confirm appropriate levels

LMWH Rx monitoring

• Levels measured by chromogenic-based anti-factor Xa assays

• Clot based APTT only sensitive to very high levels of LMWHs

• Calibration is done with the same brand of LMWH that the patient is using—and not with Heparin!

Anti Xa Assay

• Pt plasma + known amount of excess Factor Xa and antithrombin

• UFH/LMWH binds antithrombin & inhibits Factor Xa

• Residual Factor Xa is measured– Factor Xa cleaves a chromogenic substrate similar to

its natural substrate, releasing color detected by a spectrophotometer

• Residual Xa is inversely proportional to the amount of LMWH (or UFH)

Why new drugs?

• UFH and LMWHs are inconvenient for the outpatient setting (IV or sq only)

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

Why new drugs?• Warfarin is underused in pts who need it

most:– Only 47% of patients with afib are taking

warfarin– This is often due to hemorrhagic

contraindications– Convenience issues due to the need for

frequent monitoring– Difficulty in maintaining optimal

anticoagulation

The ideal anticoagulant

• Effective• Minimal complications/side effects• Convenient administration (ie: oral for outpatients)• Rapid absorption • Fast on and offset action• Predictable pharmacokinetics • No interactions with food or drugs• No HIT• No coagulation monitoring

Categories of new drugs

– Direct Thrombin Inhibitors:• hirudin, lepirudin, desirudin, bivalirudin,

argatroban, ximelagatran

– Xa inhibitors:• fondaparinux, idraparinux

– Heparinoids:• Danaparoid (discontinued)

Objectives for each drug

• Mechanism of Action

• Current uses

• Limitations

• Monitoring in the lab

Direct Thrombin Inhibitors

Direct Thrombin Inhibitors• 3D structure of thrombin:

100s of thrombin inhibitors in last 15 yrs

• Most are peptidomimetic compounds– Mimic the fibrinogen

sequence which interacts with the thrombin active site

– Peptidomimetic: A compound that mimics binding and biological activity of the natural peptide

3 important areas:

1. Active site: fibrinogen binding

2. Exosite I: major docking site--interaction with fibrinogen and other receptors; fibrinogen recognition site

3. Exosite II: interacts with heparin

Thrombin

Hirudin• Medicinal leeches:

– Used since ancient times to relieve body of “bad humors”• Egyptians, Greeks

– Reached peak popularity in mid-19th century

Hirudo medicinalis

• 1884: John Haycraft in Birmingham demonstrated that medicinal leeches, Hirudo medicinalis, secrete a substance that prevents blood from clotting

• 1904: Substance named hirudin

• 1957: Markwardt isolated the active anticoagulant substance, determined it to be a polypeptide 65 AAs long which inhibited thrombin

• Estimated to require 50,000 leeches annually for diagnostics and treatment

• 1986: DNA isolated and cloned

• Today recombinant hirudin is made in yeast cells

– Lepirudin, desirudin, bivalirudin

R-Hirudins

• All bind in active site and exosite I• Irreversible: Lepirudin, Desirudin• Reversible: Bivalirudin• Minor differences in structure between them

– ie: Lepirudin has one extra oxygen molecule than desirudin and one AA difference

• Lepirudin (Refludan)– Approved for use in HIT

• No binding to platelet factor IV• 89% of patients with rapid increase in plt count

– Monitored with APTT daily• Measure 4 hr after dose• Target: Pt’s APTT to be 1.5-2.5 x the lab’s median APTT• Caution in pts with renal insufficiency• Dose dependent relationship

– Antihirudin antibodies develop in 40-70% • Drug is made of non-human proteins• Can cause irritation to skin• Not neutralizing; may enhance drug potency by delaying

clearance • These patients need to be monitored with APTT

• Desirudin (Iprivask)– Studied in DVT prophylaxis for total hip

• Lower rate of DVT than LMWH and no increase in bleeding complications

– Also used in HIT– Also monitored with APTT

• Monitor especially in pts with renal insufficiency• APTT twice upper limit of normal = stop and

restart at reduced dose• Dose dependent relationship

– Antihirudin antibodies can also develop

• Bivalirudin (Angiomax)

– Previously called hirulog– Binds reversibly to thrombin

• Thrombin slowly cleaves the drug from its active site• Short half-life 20-30 min

– Less immunogenic (is only 20 AAs long)– FDA approved in 2002 for use in angioplasty for

patients with unstable angina

• Bivalirudin, cont…– REPLACE-2 (2004)

• Randomized Evaluation in PCI Linking Angiomax to Reduced Clinical Events

• Pts randomized to receive either bivalirudin or heparin + GP IIb/IIIa inhibitor during PCI

• Significant reduction in in-hospital bleeding• Endpoints about equal: MI, urgent repeat

revascularization• Trend towards decreased mortality

– Undergoing study for use in cardiac surgery for use both “on pump” and “off pump”

– Monitored with APTT or ACT

Direct Thrombin Inhibitors, cont…

• Argatroban (Novastan) – Binds at active site reversibly – Does not bind at exosites– First introduced in Japan in 1990 for treatment of

peripheral vascular disorders

• Argatroban, cont…– FDA approved in 2000: Anticoagulant for pts

with HIT needing prophylaxis or treatment for thrombosis

– 2002 approved for pts with or at risk for HIT undergoing PCI

– Small, without immunogenicity – Monitoring with APTT or ACT recommended

• Desired APTT: 1.5 to 3 times the initial baseline value (not to exceed 100 seconds)

• Ximelagatran (Exanta) – ORAL !!!!! BID dosing– Only binds to active site; reversible– Prodrug for melagatran– Numerous Phase III trials (10):

• 6 for prophylaxis of venous thromboembolism [VTE] due to orthopedic surgery

• 1 for initial treatment of VTE• 1 for long-term prevention of VTE recurrence

• 2 for stroke prophylaxis due to atrial fibrillation – The good news: Some studies have shown

benefit over warfarin or LMWH

Ximelagatran, cont…

The bad news:• Studies have shown increase in bilirubin, ALT • FDA: Estimates rate of liver injury 1 in 200 • 10% could progress to liver failure, need liver

transplantation, or death– 1 in 2000 patients tx’d long-term could have overt

liver failure

• 3 of 6948 patients did die under circumstances that FDA felt could reasonably be related to ximelagatran – Consistent with the 1-in-2000 rate

DTIs…a bit more on lab testing• Interference with APTT:

– Other drugs: warfarin, abx– Pts with LA– High factor VIII levels leading to falsely low APTT

• Why not do a Factor IIa assay to measure drug activity?– For example, chromogenic anti-factor IIa assay– Limited reproducibility, linearity, and sensitivity

• Why not use PT/INR?– Little to no sensitivity to some Rx and less

than optimal to others– INR can be markedly different depending on

reagent used– One recent study of lepirudin, argatroban, &

bivalirudin observed a dose dependent effect on the INR

• PT/INR most affected by argatroban at therapeutic concentrations

• Lepirudin had the least overall effect on PT/INR

DTIs…

DTIs & Lab tests, cont…

• TCT (thrombin clotting time)– Seems logical, but is overly sensitive to these drugs

and doesn’t provide useful clinical info

• ACT (activated clotting time)– Has been used successfully in the OR

• ECT (ecarin clotting time)– Not standardized for clinical use, but shows promise– Venom from snake Echis carinatus:

• Converts prothrombin to a meizothrombin that is sensitive to thrombin inhibitors

Factor Xa inhibitors

Factor Xa inhibitors• Fondaparinux (Arixtra)

– Synthetic polysaccharide: – The drug is the unique pentasaccharide sequence that

UFH and LMWH use to bind to AT

– Reacts with strong affinity to AT (reversible) → Induces conformational change in AT → Increased ability to inactivate Xa

Fondaparinux: Too short to inactivate thrombin (much like LMWH); need >18 saccharide units to inactivate thrombin

• Fondaparinux, cont…– 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

• Fondaparinux, cont…– 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

Danaparoid (Orgaran)

• A LWM heparinoid

• Derived from porcine gut mucosa – (can’t use if have a pork allergy)

• A mixture of heparan sulfate, dermatan sulfate, chondroitin sulfate

• Does not have heparin or heparin fragments (heparan differs from heparin by sulfur groups on the sugar molecules)

Danaparoid: MOA• Binds to AT and heparin cofactor II• More selective at inhibiting Xa than LMWH, not as

selective as fondaparinux:

• Ability to prevent extension of thrombi • Minimal effect on platelet function and aggregability

Drug Anti-Xa/anti-IIa ratio

UFH 1:1

LMWH 2-4:1

Danaparoid 22:1

Fondaparinux ∞

Danaparoid

• FDA approved in 1996– Proven effective in DVT prophylaxis in pts

undergoing hip surgery

• Used off label in patients with HIT

• Discontinued manufacturing in USA 4/2002 due to problems obtaining raw material– Available in Germany

Future possible drugs

• Ticks Isolated from the saliva or

blood of various species:– Thrombin inhibitors– Factor Xa inhibitors, – Tissue factor pathway

inhibitor (TFPI)

• Razaxaban– Oral drug– Inhibitor of factor Xa without requiring AT– In phase II trials

• DX-9065a – Xa inhibitor; also in phase II trials; IV

• Potential targets being developed– TF/fVIIa

• Recombinant tissue factor inhibitor (FTPI)• Other specific TF/fVIIa or fVIIa inhibitors being developed

– Recombinant APC• In phase III trials—inactivates Va and VIIIa

Future possible drugs

Future possible drugs

• Aptamers: – From root word aptus (“fit”)– Single stranded nucleic acids that fold into

specific 3D structures which bind and inhibit a protein target

– Anti factor VIIa, IXa, and thrombin aptamers have been developed

– Should be nonimmunogenic—small and similar to endogenous molecules

– Possible use in HIT

References• Angelli, G. Current issues in anticoagulation. Pathophysiology and

Haemostasis and Thrombosis. 2005;34(suppl 1):2-9.• Bauer, K. Clinical uses of fondaparinux. Uptodate.• Davidson, B. Preparing for the new anticoagulants. J of Thrombosis and

Thrombolysis. 2003;16 (1/2): 49-54.• DiNardo, J. Fondaparinux. Newsletter. Society of Cardiovascular

Anesthesiologists. 2003 Dec. (http://www.scahq.org/sca3/newsletters/2003dec/drug4.shtml)

• Gosselin, RC, et al. Effect of direct thrombin inhibitors, bivalirudin, lepirudin, and argatroban, on prothrombin time and INR values. Am J Clin Pathol. 2004;121:593-99.

• Hirsh, J. et al. Heparin and low-molecular-weight heparin. Chest. 2004;126:188S-203S.

• Kikelj, D. Peptiomimetic thrombin inhibitors. Pathophysiology of Haemostasis and Thrombosis. 2003/2004;33:487-491.

• Lai, R. et al. A thrombin inhibitor from the ixodid tick, Amblyomma hebraeum. Gene. 2004 Nov 24; 342(2):243-9.

• Lawrence, LK. New anticoagulants. Uptodate.

• Markwardt, F. Past, present and future of hirudin. Haemostasis. 1991;21 Suppl 1:11-26.

• Martel, N. et al. Risk of heparin induced thrombocytopenia with unfractionated and low molecular weight heparin thromboprophylaxis: a meta-analysis. Blood. 2005 Jun 28 (Epub ahead of print).

• Nimjee, S. et al. The potential of aptamers as anticoagulants. TCM. 2005; 15(1):41-45.

• Starke, K. The beginnings of hirudin. Trends Pharmacol Sci. 1989 Mar; 10(3):99.

• Walenga, J. et al. Monitoring the new antithrombotic drugs. Seminars in Thrombosis and Hemostasis. 2004; 30(6): 683-695.

• Weitz, J. et al. Treatment of venous thromboembolism: New anticoagulants for treatment of venous thromboembolism. Circulation. 2004;110:I-19 – I-26.

• White, CM. Thrombin-directed inhibitors: Pharmacology and clinical use. American Heart Journal. 2005 Jan;149(15):S54-60.

• Valentine, K. et al. Clinical use of heparin and low molecular weight heparin. Uptodate

References, cont…