An overview of antithrombotic therapy

Many of the common problems in clinical practice today relate to thrombosis. The underlying final pathophysiological process in myocardial infarction and stroke is thrombus formation (thrombogenesis). Common cardiovascular disorders such as atrial fibrillation and heart failure are also associated with thrombogenesis. Thrombosis is also a clinical problem in various cancers and after surgery, especially orthopaedic.

Pathophysiology

Over 150 years ago Virchow recognised three prerequisites for thrombogenesis: abnormal blood flow, vessel wall abnormalities, and blood constituent abnormalities. This concept has been extended by modern knowledge of the endothelial function, flow characteristics, and blood constituents including haemorheological factors, clotting factors, and platelet physiology. As thrombus consists of platelets and fibrin (and often bystanding erythrocytes) optimum antithrombotic prophylactic therapy can and should be directed towards both.

Antiplatelet drugs

Aspirin and agents acting on the cyclo-oxygenase pathway

Aspirin irreversibly inhibits cyclo-oxygenase by acetylation of amino acids that are next to the active site. In platelets, this is the rate limiting step in synthesis of thromboxane A₂, and inhibition occurs in the megakaryocyte so that all budding platelets are dysfunctional. Because platelets are unable to regenerate fresh cyclo-oxygenase in response, the effect of aspirin remains as long as the lifespan of the platelet (generally about 10 days). A severe weakness of aspirin is that its specificity for cyclo-oxygenase means it has little effect on other pathways of platelet activation. Thus aspirin fails to prevent aggregation induced by thrombin and only partially inhibits that induced by ADP and high dose collagen. Antithrombotic doses used in clinical trials have varied widely from less than 50 mg to over 1200 mg/day, with no evidence of any difference in clinical efficacy. Absorption is over 80% with extensive presystemic metabolism to salicylic acid. Only the parent acetylsalicylic acid has any significant effect on platelet function.

Contraindications to aspirin

Absolute	Relative
• Active gastrointestinal ulceration	•  History of ulceration or dyspepsia
• Hypersensitivity	•  Children over 12 years old
• Thrombocytopenia	•  Bleeding disorders
	•  Warfarin treatment

Adverse effects of aspirin include haemorrhage, hypersensitivity and skin rashes, alopecia, and purpura.

Sulfinpyrazone also inhibits cyclo-oxygenase (thus producing an aspirin-like state), but is reversible, and also inhibits serotonin uptake by platelets. Iloprost is a prostacyclin analogue that exerts its effects by promoting vasodilatation and inhibiting platelet aggregation induced by ADP, thereby opposing the effects of thromboxane A₂.

Dipyridamole

Dipyridamole inhibits phosphodiesterase, thus preventing the inactivation of cyclic AMP, intraplatelet levels of which are increased, resulting in reduced activation of cytoplasmic second messengers. However, it may also exert its effect in other ways, such as stimulating prostacyclin release and inhibiting thromboxane A₂ formation. The influence of this drug on these pathways causes reduced platelet aggregability and adhesion invitro with increased platelet survival in vivo. Its effect is relatively short lasting, and repeated dosing or slow release preparations are needed to achieve 24 hour inhibition of platelet function.

Clopidogrel and ticlopidine

These thienopyridine derivatives inhibit platelet aggregation induced by agonists such as platelet activating factor and collagen, and also dramatically reduce the binding of ADP to a platelet surface purinoreceptor. The mechanism of this inhibitory action seems to be independent of cyclo-oxygenase. There is also impairment of the platelet response to thrombin, collagen, fibrinogen, and von Willebrand factor. The peak action on platelet function occurs after several days of oral dosing. Adverse effects include evidence of bone marrow suppression, in particular leucopenia, especially with ticlopidine.

Other receptor blockers

Signal transduction generally occurs when specific receptors on the surface are occupied by ligands such as ADP, leading to structural modification of the glycoprotein IIb/IIIa receptor on the surface of the platelet. This is the commonest receptor on the platelet surface and represents the final common pathway for platelet aggregation, resulting in crosslinking of platelets.

Factors that influence the efficacy of warfarin

Patient factors

• Enhanced anticoagulant effect—Weight loss, increased age (>80 years), acute illness, impaired liver function, heart failure, renal failure, excess alcohol ingestion

• Reduced anticoagulant effect—Weight gain, diarrhoea and vomiting, relative youth ( <40 years), Asian or African-Caribbean background

Drug interactions with warfarin

• Reduced protein binding—Aspirin, phenylbutazone, sulfinpyrazone, chlorpromazine

• Inhibition of metabolism of warfarin—Cimetidine, erythromycin, sodium valproate

• Enhanced metabolism of warfarin—Barbiturates, phenytoin, carbamazepine

• Reduced synthesis of factors II, VII, IX, X—Phenytoin, salicylates

• Reduced absorption of vitamin K—Broad spectrum antibiotics, laxatives

• Enhanced risk of peptic ulceration—Aspirin, NSAIDs, corticosteroids

• Thrombolytics—Streptokinase, tissue plasminogen activator

• Antiplatelet drugs—Aspirin, NSAIDs

After intravenous administration of glycoprotein IIb/IIIa receptor inhibitors such as abciximab, platelet aggregation is 90% inhibited within two hours, but function recovers over the course of two days. The major adverse effect is haemorrhage, and concurrent use of oral anticoagulants is contraindicated. Eptifibatide is a cyclic heptapeptide that mimics the part of the structure of fibrinogen that interacts with glycoprotein IIb/IIIa. Thus it is a fraction of the size of abciximab and is targeted at the same structure on the platelet surface.

Clinical trials with oral glycoprotein IIb/IIIa receptor inhibitors have been disappointing, with no beneficial effects seen and even some evidence of harm.

Anticoagulant drugs

Warfarin

This 4-hydroxycoumarin compound, the most widely used anticoagulant in Britain and the Western world, inhibits the synthesis of factors dependent on vitamin K (prothrombin; factors VII, IX, and X; protein C; protein S). Factor VII levels fall rapidly (in <24 hours) but factor II has a longer half life and only falls to 50% of normal after three days. Warfarin is approximately 97% bound to albumin, and free warfarin enters liver parenchymal cells and is degraded in microsomes to an inactive water soluble metabolite that is conjugated and excreted in the bile. Partial reabsorption is followed by renal excretion of conjugated metabolites.

There is a considerable variability in warfarin's effect on patients, its effectiveness being influenced by age, racial background, diet, and co-medications such as antibiotics. Thus it demands frequent laboratory monitoring, the prothrombin time being compared with a standard to produce the international normalised ratio. The degree of anticoagulation required varies with clinical circumstance, but the target international normalised ratio usually ranges from 2 to 4. Phenindione is an alternative oral vitamin K antagonist, but concerns regarding the potential for hepatotoxicity, nephrotoxicity, and blood dyscrasias have reduced its role largely to individuals with documented hypersensitivity to warfarin.

Adverse effects of warfarin include haemorrhage, hypersensitivity and skin rashes, alopecia, and purpura.

Melagatran

This oral thrombin inhibitor undergoing phase III trials seems to be well tolerated, with few clinically significant bleeding problems, in patients with venous thromboembolism. Although considerable pharmacokinetic and animal data exist, solid evidence of its effectiveness compared with warfarin and heparin in patients at high or low risk is still awaited.

Heparin

Heparin is a glycosaminoglycan whose major anticoagulant effect is accounted for by a pentasaccharide with a high affinity for antithrombin III. This binding results in a conformational change in antithrombin III so that inactivation of coagulation enzymes thrombin (IIa), factor IXa, and factor Xa is markedly enhanced. Its short half life means it must be given continuously, and its extensive first pass metabolism means it must be given parenterally, preferably by continuous intravenous infusion, and it is therefore inappropriate for home use. The effect on the intrinsic clotting cascade must be monitored carefully by measuring the activated partial thromboplastin time (APTT), generally aiming for a value 1.5 to 2.5 times that of control.

Unfractionated heparin consists of a heterogeneous mixture of polysaccharides with an average molecular weight of 15 000 Da. Low molecular weight heparins (4000-6000 Da) are weaker inhibitors of thrombin but inhibit factor Xa to a similar extent. Different commercial preparations of low molecular weight heparin vary in the ratio of anti-Xa to antithrombin activity, although the clinical relevance of this is uncertain. Better absorption after subcutaneous administration and reduced protein binding result in greatly improved bioavailability. The effective half life after subcutaneous injection is four hours, allowing an injection once daily in most circumstances. These more predictable pharmacokinetics allow the dose to be calculated on the basis of the patient's weight and reduce the requirement for frequent monitoring. In those rare cases where monitoring is deemed necessary, measurement of plasma levels of anti-Xa activity is needed. Tests of APTT are unhelpful.

Comparison of low molecular weight and unfractionated heparins

	Unfractionated heparin	Low molecular weight heparin
Action	Anti-XIIa, XIa, IXa, VIIa, antithrombin	Mostly anti-Xa
Route of administration	Subcutaneous Intravenous	Subcutaneous
Absorption from subcutaneous route	Slow	Improved
Protein binding	Proteins in plasma and on endothelium	Reduced
Bioavailability	Subcutaneous—10-30% at low doses, 90% at higher doses Intravenous—100% by definition	>90%
Effective half life	Subcutaneous—1.5 hours Intravenous—30 min	4 hours
Between and within individual variation	Extensive	Minimal
Monitoring	APTT	Not required (anti-Xa activity)
Elimination	Liver and kidney	Kidney

Adverse effects of heparin include haemorrhage, osteoporosis, alopecia, thrombocytopenia, and hypersensitivity. At present, the risk of haemorrhage seems to be similar with low molecular weight and unfractionated heparin. However, the risk of heparin induced thrombocytopenia seems to be less with the low molecular weight form.

Hirudin and direct thrombin inhibitors

Hirudin, a 65 amino acid residue anticoagulant peptide with a relative molecular mass of 7000 Da purified from the leech Hirudo medicinalis, binds thrombin with high specificity and sensitivity. With a true half life of about an hour and a half life effect on the APTT of two to three hours, it may be seen as an alternative to heparin in indications such as unstable angina and in coronary angioplasty.

Many derivatives are available, with hirulog and argatroban among the best developed. However, trials of the former have been discouraging: no clear benefit over heparin was shown. Conversely, argatroban may have a role in the anticoagulation of patients unable to tolerate heparin as a result of heparin induced thrombocytopenia. Furthermore, in a clinical trial of patients with heparin induced thrombocytopenia, use of argatroban was associated with a reduction in levels of plasma platelet activation markers.

Thrombolytic agents

These agents lyse pre-existing thrombus, either by potentiating the body's own fibrinolytic pathways (such as streptokinase) or by mimicking natural thrombolytic molecules (such as tissue plasminogen activator). The common agents in clinical use are derived from bacterial products (streptokinase) or manufactured using recombinant DNA technology (recombinant tissue plasminogen activator). Newer drugs aim to be less antigenic and more thrombus specific in an attempt to increase efficacy and specificity of various agents; on present evidence, however, the differences between thrombolytic agents are only marginal. Because of the lack of site specificity for these drugs, the major adverse effect is that of haemorrhage (gastrointestinal, intracranial, etc). The other important adverse effect is that of hypersensitivity reaction, especially with streptokinase. This usually manifests as flushing, breathlessness, rash, urticaria, and hypotension. Severe anaphylaxis is rare. Hypersensitivity reactions are avoided by using tissue plasminogen activator or recombinant tissue plasminogen activator, which are not antigenic.

Fibrinolytic drugs

Examples	Source	Mechanism of action
Streptokinase	Group C β haemolytic streptococci	Complexes with and activates plasminogen
Urokinase	Trypsin-like chemical produced by kidney	Direct acting plasminogen activator
Reteplase (recombinant tissue plasminogen activator)	Recombinant DNA technology	Acivates plasminogen, non-immunogenic

Contraindications to thrombolysis

Absolute	Relative
• Recent or current haemorrhage, trauma, or surgery	•  Previous peptic ulceration
• Active peptic ulceration	•  Warfarin
• Coagulation defects	•  Liver disease
• Oesophageal varices	•  Previous use of anistreplase or streptokinase within four years (use alternative agent)
• Coma	•  Hypersensitivity (anistreplase, streptokinase)
• Recent or disabling cerebrovascular accident	•  Heavy vaginal bleeding
• Hypertension
• Aortic dissection

Streptokinase

Derived from streptococci, this product is an effective thrombolytic agent for the treatment of acute myocardial infarction and pulmonary thromboembolism. Acting by converting plasminogen to plasmin, the main fibrinolytic enzyme, it potentiates fibrinolysis. However, it is not site specific, lysing thrombus anywhere in the body. Being bacteria derived, it is antigenic, and repeated administration results in neutralising antibodies and allergic reactions. For example, a single administration of 1.5 MU for acute myocardial infarction results in neutralising antibodies that have been shown to persist for up to four years and are sufficient to neutralise a repeat administration of a similar dose of the agent in half of cases.

Tissue plasminogen activator

In clinical use this is produced by recombinant DNA technology and mimics an endogenous molecule that activates the fibrinolytic system. Thus, recombinant tissue plasminogen activator does not elicit an allergic response and is considered more clot specific. Nevertheless, it has a short half life and needs continuous infusion to achieve its greatest efficacy. Accelerated administration of tissue plasminogen activator gives a slight mortality advantage over streptokinase at the cost of a marginal increase in stroke rate.  

Further reading

• Antiplatelet Trialists' Collaboration. Collaborative overview of randomised trials of antiplatelet therapy, I: Prevention of death, myocardial infarction, and stroke by prolonged antiplatelet therapy in various categories of patients.

• Blann AD, Lip GYH. Virchow's triad revisited: the importance of soluble coagulation factors, the endothelium, and platelets. Thromb Res 2001;101:321-7

• CAPRIE Steering Committee. A randomised, blinded, trial of clopidogrel versus aspirin in patients at risk of ischaemic events (CAPRIE). Lancet 1996;348:1329-39

• Catella-Lawson F. Direct thrombin inhibitors in cardiovascular disease. Coron Artery Dis 1997;8:105-11

• Eriksson H, Eriksson UG, Frison L. Pharmacokinetic and pharmacodynamics of melagatran, a novel synthetic LMW thrombin inhibitor, in patients with a DVT. Thromb Haemost 1999;81:358-63

• International Stroke Trial Collaborative Group. The international stroke trial (IST): a randomised trial of aspirin, subcutaneous heparin, or both, or neither among 19 435 patients with acute ischaemic stroke. Lancet 1997;349:1569-81

• Lewis BE, Wallis DE, Berkowitz SD, Matthai WH, Fareed J, Walenga JM, et al. Argatroban anticoagulant therapy in patients with heparin-induced thrombocytopenia. Circulation 2001;103:1838-43

• Nurden AT. New thoughts on strategies for modulating platelet function through the inhibition of surface receptors. Haemostasis 1996;20:78-88

• Stirling Y. Warfarin-induced changes in procoagulant and anticoagulant proteins. Blood Coagul Fibrinolysis 1995;6:361-73

Figure.

Routes to inhibiting platelet function

Figure.

Key components of Virchow's triad (VWF=von Willebrand factor)

Figure.

Platelet metabolism influenced by aspirin

Figure.

Vitamin K metabolism and the effect of warfarin

Figure.

Simplified coagulation cascade

Figure.

The three low molecular weight heparins that have been evaluated in clinical trials of acute coronary syndromes are shown with their respective anti-Xa and antithrombin activity (PF4=platelet factor 4)

Figure.

Simplified fibrinolysis (PAI-1=plasminogen activator inhibitor, tPA=tissue plasminogen activator, uPA=urokinase plasminogen activator)