Thrombolytics, Anticoagulant and Antiplatelet Drugs

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Thrombolytics, Anticoagulant and Antiplatelet
Drugs

• Phase III/Therapeutics

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AIMS of the lecture

• To introduce the major classes of drugs used in
  this area
• To highlight the evidence for their use
• To emphasise problems associated with their use
  especially WARFARIN

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Platelet Activation key role of Glycoprotein
IIb/IIIa

• Some 80,000 molecules of this integrin (?2b?3a)
  per platelet.
• All pathways to platelet activation converge on
  this receptor it is activated by conformational
  change.
• Activated high affinity GP IIb/IIIa binds to
  the RGD motif found in fibrinogen, vWF,
  fibronectin and vitronectin.

Loss of function mutations in the subunits
(?2b?3) cause the bleeding disorder of
Glanzmanns thrombasthenia.
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Aspirin

• Irreversibly inactivates platelet COX-1 -
  recovery requires production of new platelets
  (10d)
• Anti-platelet effect cumulative with low dose
  aspirin
• Even low dose aspirin affects bleeding time -
  increased peri-operative blood loss and doubling
  of the risk of a GI bleed
• Only alters clotting at high dose - PT can be
  prolonged in overdose
• COX-2 selective agents (Coxibs e.g. refecoxib
  and colecoxib) do not have antiplatelet effects
  I.e. patients will still need low-dose aspirin
  for cardiovascular protection.

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Newer Antiplatelets Thienopydrines

• 2 licensed agents (TICLOPIDINE CLOPIDOGREL)
• Function as ADP antagonists (P2Y receptors)
• Prodrugs - inactive against platelets in vitro
  in vivo effects dependent on active metabolites
• Ticlopidine largely replaced by Clopidogrel
  due to haematological SE neutropenia (1 cases)
  and rarely thrombotic thrombocytopenic purpura
  (TTP) (lt0.02 but up to 50 mortality rate).
  Clopidogrel is much safer but cases of TTP have
  been reported.

• Uses
• Prevention of stent occlusion

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2. In Acute Coronary Syndromes
CURE trial (NEJM 2001345492)
gt12,000 patients with unstable angina randomised
to aspirin /- clopidogrel and followed for 3-12
months. 1ary endpoint reached by 9.3 and 11.4
respectively , Plt0.001)
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3. In Secondary Prevention (Stroke MI) ?
CAPRIE trial (Lancet 19963481329-39)

• gt19,000 patients with recent stroke, MI or
  peripheal vacular disease randomised to aspirin
  (325mg) OR clopidogrel and followed for median
  1.9 yrs.
• 1ary endpoint (MI, stroke or vascular death)
  reached in 5.83 vs 5.32 i.e. RRR (relative risk
  reduction) of 8.7 (P0.043).
• No benefit in terms of side effects intracranial
  haemorrhage (0.33 vs 0.47), and
  gastrointestinal haemorrhage (0.52 vs 0.72)
• What are the cost implications ?
• NNT 115 (58-8647) - Note large confidence
  intervals
• Tablet cost of clopidogrel 1.26 (aspirin 1p)
• Drug cost to prevent 1 event 100k
• Cochrane review suggested ARR for MI as low as
  0.7 (cost to prevent MI 130k)

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GP IIb/IIIa Antagonists

• Fall into 3 chemically distinct groups
• Monoclonal ab (murine) abciximab slow
  dissociation from the receptor (antiplatelet
  effect last for 10s hrs after administration)
  and immunogenic. Also binds to other integrin
  receptors e.g. vitronectin blocking
  platelet-endothelium and platelet-SMC
  interactions.
• Peptide Antagonists (Eptafibatide) rapid
  receptor binding/dissociation and high systemic
  clearance. Only available for IV use.
• Non-peptide Antagonists (Tirofiban) developed
  with a view to oral administration, but have
  failed to show clinical efficacy by this route.

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What is the role of GP IIb/IIIa Antagonists in
Acute Coronary Syndromes?

• Acute Myocardial Infarction
• abciximab ½ dose tPA (reteplase) as effective
  as full does tPA alone in AMI (GUSTO V Trial
  data).
• - Both provided lt6 30-day mortality
• - Combination caused significantly less early
  reinfarction
• Unstable Angina/non-Q Wave AMI
• Useful adjunct for Percutaneous
  revascularisation
• - prevents early reocclusion following stenting
  or angioplasty.
• Patients must be stratified if used as 1ary Rx
• - only beneficial if high risk e.g. elevated
  Troponin.

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FIBRINOGENESIS
PROTHROMBINASE Complex
Factor Xa/Va
HIRUDIN
Fibrinogen
Thrombin
Soluble fibrin monomers
Factor XIII
Unstable fibrin polymer clot
Factor XIIIa
Stable fibrin polymer clot
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Anticoagulant action of HEPARIN
UFH
Pentasaccharide sequence of heparin (present in
UFH and LMWH) binds to AT causing conformational
change at its reactive centre accelerating
1000-fold its interaction with factor Xa. NB
Catalysis of AT-mediated inactivation of thrombin
(factor II) requires the formation of a ternary
heparinantithrombinthrombin complex with a
motif containing at least 18 saccharide units.
Hence LMWH has much less inhibitory activity
against factor IIa vs UFH - ratio anti-Xa to
anti-IIa typically 3.
LMWH
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Features of LMWH

• Generated by chemical (Enoxaprin) or enzymatic
  (heparinase, Tinzaperin) depolymerisation of
  UFHs.
• Average MW 5kD vs MW range 3-30kD for UFH
• Shorter chains cleared more slowly than longer
  chains
• - Reduced binding to plasma proteins,
  endothelial cells
• macrophages.
• - Not neutralised by PF4 released from
  activated platelets.

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LMWH vs. Unfractionated HEPARIN

• Cons
• Cannot be monitored by APTT
• - specific anti-Xa assay needed
• Not fully reversed by protamine
• Expensive (10-20-fold more than UFH)

• Pros
• Improved pharmacokinetics especially
  subcutaneous route
• Little effect in APTT so monitoring not usually
  required
• less likely to cause thrombo- cytopenia or
  osteoporosis long-term
• Possible exceptions include renal or hepatic
  failure and pregnancy

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Areas where efficacy of LMWH gt UFH

• Prophylaxis in Orthopaedic procedures on the
  lower limb
• - untreated up to 70 develop VTE.
• - LMWH efficacy exceeds low-intensity warfarin
  (INRlt2)
• Treatment of VTE (incl PE)
• Unstable angina

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Oral Anticoagulants - WARFARIN

• Dicoumarol first isolated from sweet clover
  silage
• - caused haemorrhagic disease in cattle.
• Subsequent synthesis of chemically related
  coumarin, WARFARIN
• - patent holder Wisconsin Alumni Research
  Foundation coumARIN.

The site of action of WARFARIN Vitamin-K
oxidation is coupled to ?-carboxylation of Glu
residues on clotting factor proteins, which is
necessary for full biological activity (as Ca
chelators). Warfarin blocks the vit K epoxide
reductase step in this cycle. The delayed onset
of Warfarins effect actually reflects the
half-lives of these modified clotting factors
(shortest, Factor VII 6h longest, Factor II
40-60h).
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Important points about WARFARINs Pharmacokinetics

• Rapidly and completely absorbed after oral
  administration
• Highly protein bound (gt99 to serum albumin)
• Crosses the placenta (teratogenic)
• Breast feeding OK (active W not detected in
  breast milk)
• Variable but usually slow systemic clearance
  t1/2 24-60hrs
• Clearance dependent on hepatic P450s (especially
  2C9)

Slow metabolism through some alleles explains
why 10 of patients have therapeutic INRs on low
doses of Warfarin lt1mg/d.
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Interactions with WARFARIN that matter
Reduced absorption cholestyramine or similar
resins. Reduced protein binding
hypoproteinaemic states e.g. nephrotic
syndrome Altered clearance P450 induction by
rifampicin, barbiturate or phenytoin P450
inhibition by amiodarone, metronidazole and
cimetidine. Altered vit K intake vitamin K
rich foods/supplements or antibiotic induced
reduction in gut-derived vitamin K. Altered
levels of clotting factors reduced in
hypermetabolic states e.g. hyperthyroidism
increased in pregnancy. Augmented bleeding
tendency in combination with antiplatelet
agents e.g. NSAIDs. Substitute non-NSAID
analgesics with care dextropropoxyphene and high
dose paracetamol (1.5-2g/d) can block W
metabolism.
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Reversal of WARFARIN-induced bleeding

• Is pharmacological reversal required?
• INR a poor guide unless very high (gt10)
• Bleeding into closed compartments most
  problematic especially intracerebral, into
  joints and retroperitoneal
• Dose of vitamin K1 (phytomenadione)?
• Slow onset hrs
• Full dose (10mg) may prevent rewarfarinisation
  e.g. prosthetic valves
• - Low-dose (1mg) /- FFP most flexible regime in
  this case

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FIBRINOLYSIS
fibrin
Kringle domains

Plasmin
FDPs
Plasmin
Alpha2-Antiplasmin
tPA
Plasminogen
Administering excess tPA can deplete anti-plasmin
levels leading to a systemic lysis state due to
the unrestrained protease action of plasmin.
Aminocaproic acid acts as a fibrinolysis
inhibitor by blocking these lys-rich sites
Endothelial cells
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tPA versus STREPTOKINASE (1)

• STREPOKINASE
• Product of ?-haemolytic strep hence anti-strep
  antibodies will neutralise it
• Forms a 11 complex with plasminogen this
  exposes its cleavage site promoting conversion to
  plasmin
• Has similar affinity for free or bound
  plasminogen - no clot selectivity
• tPA
• Binds to fibrin hence clot selectivity
• Activates plasminogen bound to fibrin - gt100-fold
  faster than circulating plasminogen enhancing
  clot selective fibrinolyis
• Levels of tPA during thrombolytic therapy are
  30-300x gt physiological levels hence some loss
  of clot selectivity

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tPA versus STREPTOKINASE (2)

• GUSTO trial supported tPA (accelerated alteplase
  over SK or alteplaseSK).
• But . . .
• Effect small (10 and 14 difference in 30-day
  mortality).
• Cost - recombinant tPAs 5-10 fold more expensive
  vs SK
• Increased risk of intracerebral bleed with tPAs (
  1 patients)
• Choice of rtPA over SK may be prompted by
• Age (lt75yr)
• Low risk of intracerebral bleed
• Size of infarct (especially large anterior MI)
• Early presentation (lt4hr)
• Previous SK (especially previous 6/12 or ever?)

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Who should NOT be thrombolysed?

• Risk of serious bleeding is low particularly in
  the absence of heparin (lt1 risk in major
  trials).
• It arises from
• Lysis of physiological clots
• A systemic lysis state (depleting fibrinogen,
  FV and FVIII)
• The following are generally contraindications
• Active bleeding or haemorrhagic disorder
• Aortic dissection
• Significant GI bleed in the previous 3 month
• Recent cardiovascular surgery or bowel resection
• Pericarditis
• Poorly controlled hypertension (DBP gt110 mmHg)
• Proliferative retinopathy
• CVA in past 3 months or SOL such as
  abscess/tumour
• Pregnancy

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Other newer agents

• New recombinant tPAs
• Most require infusion - often with complicated
  protocols e.g. accelerated loading of alteplase.
• Tenecteplase can be used as a single IV bolus
  c.f. anisoylated SK (APSAC).
• Synthetic Heparinoids
• Fondaparinux fully synthetic pentasaccharide
• Novel antithrombin agents (Direct Thrombin
  Inhibitors)
• peptide and non-peptide (orally active) analogs
  of hirudin . melagatran

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The Future Beyond LMWH?
Anti-Xa activity resides within the
pentasaccharide motif this is under development
as a novel antithrombotic (Org31540)

• Advantages of pentasaccaride
• Long t1/2 (15-20 h) compared even to LMWH
• High specificity - does not interact with AT
• No thrombocytopenia risk - does not bind PF4 or
  platelets

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XIMELAGATRAN

• Orally active pro-drug for Melagatran due to be
  licensed 2004/5?
• Does not require coagulation monitoring
• No significant drug interactions to date (but
  dose reduction in severe renal impairment)
• Equal efficacy with Warfarin in
• 1ary prevention VTE in orthopaedics
• 2ary prevention in establsihed VTE/PE
• embolic stroke prevention in AF
• THRIVE trial of 2480 patients with new VTE
  randomised to X (36mg bd), W and followed for
  6/12. No difference in recurrence rate (2),
  death or bleeding problems.

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Further web resources
These slides are available at www-clinpharm.medsch
l.cam.ac.uk

• Review of Ximelagatran
• Review on Pentasaccharides