Treatment Of Thrombotic Disorders Research Articles

Treatment of thrombotic disorders in cancer patients.

Cancer patients are in a hypercoagulable state. The pathogenesis of thrombosis in malignancy is multifactorial with mechanisms including release of procoagulants by tumour cells, comorbid predisposing factors (bed rest, infection, surgery, etc.) and anti-cancer drugs. Cancer patients with established venous thromboembolism are more likely to develop recurrent venous thromboembolism during treatment with oral anticoagulants. This paper reviews the use of heparin for the treatment of thrombotic disorders in cancer patients. Treatment of acute venous thrombosis comprises initial heparin administration, which for a cancer patient should last for at least 5 days, followed by administration of oral anticoagulants. Low-molecular-weight heparins (LMWHs) have been shown to be as safe and effective as standard heparin for the treatment of acute deep vein thrombosis. Recent meta-analyses have revealed lower mortality rates with LMWH than with standard heparin, indicating that LMWH may exert an inhibitory effect on tumour growth that is not observed with standard heparin.

Limited Fibrin Specificity of Tissue-type Plasminogen Activator and Its Potential Link to Bleeding

Plasminogen activators initiate the fibrinolytic process by converting plasminogen to plasmin. Though plasminogen activators are effective in the treatment of thrombotic disorders, bleeding complications are associated with their use. The development of plasminogen activators with greater fibrin specificity was expected to reduce the incidence of bleeding complications; however, this has not occurred. In our rabbit model (a) bleeding from standardized ear incisions induced by tissue-type plasminogen activator (t-PA) is attenuated when fibrinogenolysis is reduced by the coadministration of alpha 2-antiplasmin and (b) when used in doses that produce equivalent thrombolysis, vampire bat plasminogen activator (b-PA), an agent that is more fibrin specific than t-PA, causes less bleeding than t-PA. In addition, we have found that the (DD)E complex formed as a result of degradation of crosslinked fibrin is a potent stimulator of t-PA-induced plasminogen activation but has no effect on b-PA. Fragment X, a high-molecular-weight clottable fibrinogen degradation product, accumulates after treatment with t-PA but not with t-PA given with alpha 2-antiplasmin or with b-PA. These findings suggest that there is a link between plasminogen activator-induced fibrinogenolysis and bleeding, and that the composition of fibrin within hemostatic plugs may influence susceptibility to lysis. Whether these results mean that fibrin-specific plasminogen activators like b-PA will have a better risk-to-benefit profile in humans requires rigorous testing in well-designed clinical trials. However, at the very least, our findings suggest that the development of plasminogen activators that are more fibrin specific than t-PA is a worth-while exercise.

Cardiovascular & Renal: Clopidogrel: A Novel Antiplatelet Drug for Prevention and Treatment of Thrombotic Disorders

Thrombotic events of the arterial circulation continue to be the major cause of cardiovascular death in spite of great efforts to prevent and treat thrombosis. Current antithrombotic strategies in clinical use are primarily based on aspirin (antiplatelet), heparin (anticoagulant) and coumadins (chronic anticoagulants). Recently, ReoPro®, an antibody blocking the platelet fibrinogen receptor, GpIIb/IIIa, has been introduced for limited acute use for prevention of ischaemic complications of coronary balloon angioplasty. Clopidogrel is a novel antiplatelet agent that has demonstrated antithrombotic efficacy beyond aspirin in preclinical studies and is now in clinical (Phase II) development for secondary prevention of broad ischaemic cardiovascular events including death. Clopidogrel is an ADP receptor antagonist with additional properties that produce effective and long-lasting antithrombotic actions. Clopidogrel seems to be free of the adverse side-effects that flawed ticlopidine, a closely related thieno-d...

Fibrinolytic Compromise by Synthetic and Recombinant Thrombin Inhibitors: Implications in the Treatment of Thrombotic Disorders

Site-directed synthetic and recombinant antithrombin agents are widely used to prevent reocclusion during thrombolytic therapy in acute myocardial infarction and other vascular occlusive disorders. However, minimal studies have been conducted to examine the interactions between these anticoagulant and thrombolytic agents. Because of the structural homology of serine proteases, some of these newly developed antithrombin agents are also capable of inhibiting fibrinolytic enzymes, which may lead to fibrinolytic compromise during thrombolytic therapy. In addition, inhibition of thrombomodutin-bound thrombin may also result in fibrinolytic deficit. Several thrombin inhibitors were studied in in vitro systems to assess whether their inactivating properties extend to fibrinolytic and profibinolytic enzymes, such as kallikrein, plasmin, urokinase, streptokinase, and tissue plasminogen activator. The thrombin inhibitors studied included hirudin, hirulog-I, argatroban, D-MePhe-Pro-Arg-H, and Ac-D-Phe-Pro-boroArg-OH. Their activities were compared with those of aprotinin, which is currently used clinically as an antifibrinolytic agent. Although argatroban, hirulog-I, and hirudin exhibited minimal inhibition of the nonthrombin enzymes studied, the tripeptide derivatives showed variable inhibitory activities, with the boronic acid derivative being the most potent and universal inhibitor. The in vivo antifibrinolytic activities of these thrombin inhibitors were also studied in a rabbit model of jugular vein clot lysis. In agreement with the in vitro studies, argatroban, hirulog-I, and hirudin exhibited minimal antifibrinolytic activities, while Ac-D-Phe-Pro-boroArg-OH and D-MePhe-Pro-Arg-H showed marked inhibition of the thrombolytic process. The results of these studies indicate that newly developed thrombin inhibitors with a broader serine protease spectrum may exhibit fibrinolytic compromise resulting in diminishment of the expected thrombolytic outcome. Key Words: Thrombolysis—Fibrinolysis—Thrombin inhibitors—Argatroban—Hirudin—Hirulog-I—Aprotinin.

Synthesis and characterisation of novel thrombin inhibitors based on 4-amidinophenylalanine.

Thrombin inhibitors have been thought to play a pivotal role in myocardial infarct and stroke incidences and their aftermath. Quite some time ago potent synthetic thrombin inhibitors became known based on peptide derivatives D-Phe-Pro-Arg and benzamidine. One of them, fairly well characterised was beta-naphthylsulphonylglycyl-D,L-4-amidino-phenylalanylpiperidi de (NAPAP). NAPAP was prone to being administered intravenously due to its short plasma half life. Drawbacks to this compound such as effects on histamine release and blood pressure may have obstructed its clinical use. Long half life and oral bioavailability would be desirable for prophylactic treatment of thrombotic disorders. We have used NAPAP as a template for new synthetic compounds to improve some characteristics of its profile. For screening purposes we have investigated fairly simple surrogate parameters, aspects that were considered to contribute to pharmacological effects. Potency was correlated to thrombin inhibition, side effects were addressed by specificity toward thrombin as well as reduction in basicity, and plasma half life was considered to be modulated by plasma stability of the compound. Oral bioavailability would be affected by instability during the passage through the gut wall. Chemical introduction of a carboxylic group and exchange of the naphthyl group for 4-methoxy-2,3,6-trimethylphenyl led to a compound that when compared to NAPAP, exhibited a 4-fold increase in thrombin inhibitory activity and a 3-fold increase in trypsin specificity. Plasma stability decreased to 22 h, however, sufficient enough not to play a major role in plasma half life. Gut homogenate stability of the compound has not changed. The potency increase did not translate into a reduction in IC50-values for the coagulation assay aPTT and TT, in contrast to the IC50-values for thrombin-induced platelet aggregation.

Inhibitory effect of clentiazem (TA-3090) on platelet aggregation -alone and in combination with aspirin or ticlopidine

Clentiazem (a novel calcium antagonist) and its basic metabolites (MB1-MB7) showed inhibitory effects on collagen-induced platelet aggregation in human platelets. All the basic metabolites (IC50:8–22μg/ml) had much stronger inhibitory effects than clentiazem itself (IC50:53μ/ml), but the acidic metabolites (MA1-MA4) had no inhibitory effects even at 300μg/ml. Other calcium antagonists (diltiazem, verapamil, nicardipine and nimodipine) also showed similar inhibitory effects although nicardipine and nimodipine were less active than the other drugs. The inhibitory effect of clentiazem was enhanced in the presence of aspirin or ticlopidine. Diltiazem and nicardipine also exhibited a similar potentiaton of the anti-platelet effect in combination with aspirin or ticlopidine. Clentiazem also inhibited collagen-induced thromboxaneB 2 production by the platelets, and this inhibition by clentiazem was additively enhanced by the presence of aspirin. When both clentiazem and aspirin were orally administered to rats, platelet aggregation was additively inhibited. These results indicate that a combination therapy with clentiazem plus aspirin or clentiazem plus ticlopidine may be useful for the prevention and/or treatment of thrombotic disorders.

Suppression by intradermal administration of heparin of eosinophil accumulation but not oedema formation in inflammatory reactions in guinea-pig skin.

1. Heparin is widely used in the treatment of thrombotic disorders and as an aid in surgery. Anti-inflammatory effects of heparin have also been described. In this study, we have investigated the effects of locally-injected heparin on the oedema formation and eosinophil accumulation induced by various inflammatory stimuli in guinea-pig skin. 2. Heparin dose-dependently suppressed the accumulation of 111In-labelled eosinophils induced by i.d. injection of zymosan-activated plasma (ZAP). The 111In-eosinophil accumulation induced by other inflammatory stimuli (compound 48/80, platelet activating factor, interleukin-8 and in a passive cutaneous anaphylaxis reaction) was also suppressed by locally-injected heparin. 3. Oedema formation in response to these same stimuli was not altered by the local injection of heparin. 4. Fucoidin, a negatively-charged sulphated algal polymer, had no effect on the 111In-eosinophil accumulation or oedema formation induced by ZAP. Nevertheless, fucoidin significantly suppressed the oedema formation induced by i.d. injection of cationic protein-containing extracts of Schistosoma mansoni larvae. Heparin also inhibited oedema induced by the extracts, suggesting that both fucoidin and heparin were effectively neutralizing the cationic protein of the extracts to inhibit their oedema-inducing activity. 5. Thus, heparin significantly inhibited the accumulation of 111In-eosinophils, but not oedema formation, induced by various inflammatory stimuli. This, taken together with the lack of effect of fucoidin, suggests that heparin interferes with the process of eosinophil trafficking by a mechanism that does not depend on neutralisation of the charge of the stimulatory molecules.

Antithrombins: Their Potential as Antithrombotic Agents

The inhibition of thrombin is the key to the prevention and treatment of thrombotic disorders. Although heparin is an extremely effective anticoagulant, it has certain limitations that are not shared by newer thrombin inhibitors. As a result, these novel inhibitors may have advantages over heparin in certain clinical settings.

Seminars in Thrombosis, Thrombolysis and Vascular Biology

In general terms, thrombotic disorders of the cardiovascular system are characterized by poorly regulated, nonphysiologic thrombus formation. Considered more specifically, however, pathologic thrombosis represents a critical imbalance, frequently at both the systemic and vascular levels, of coagulation, anticoagulation and fibrinolysis. Indeed, the balance is shifted toward coagulation, preventing normal physiologic blood flow. Anticoagulants have been a mainstay in the treatment of thrombotic disorders. However, emerging strategies have focused primarily on thrombus dissolution (fibrinolysis), which can be efficiently achieved with the administration of extrinsic plasminogen activators. Clearly, thrombolytic therapy is currently the most direct means of restoring blood flow, vital organ perfusion and hemostatic balance among patients with thrombotic disorders of the cardiovascular system.

Current perspectives in the treatment of thrombotic disorders.

New trends in antithrombotic therapy should reside in a better adaptation, both in potency and in target to the involved thrombogenic mechanism. Thrombogenesis as hemostasis is the result of cooperation between plasma coagulation factors and platelet functions, and these two systems are themselves in equilibrated antagonism with the vessel wall, mainly endothelial cells. These triangular relations between coagulation factors, platelet functions, and endothelial cell reactivity are quantitatively regulated by flow conditions. The relative importance of each of these protagonists in the genesis of vascular thrombosis varies along the vascular tree, mainly due to changes in flow characteristics, and explain the usual separation between venous and arterial thrombosis: venous thrombosis involves mainly coagulation factors and the vascular fibrinolytic response whereas arterial thrombosis involves the thromboresistant characteristics of the endothelial cell membrane and platelet functions. The real blood flow characteristics may be altered by local disease and influences the relative involvement of coagulation and fibrinolytic factors, platelet functions, and endothelial cells. Prevention of thrombosis must take into account all these phenomena and must be targeted to the predominant factor or factors. Depending on the local conditions, the therapeutical goal can be: (1) limitation of platelet functions or coagulation factors; (2) stimulation of thromboresistant properties of the endothelium (mainly its profibrinolytic characteristics); and (3) modification of the flow conditions. Several targets can be associated: the level of inhibition or stimulation of a function depends on the dysequilibrium, and efficient prevention does not always require complete inhibition of a function. Once a thrombus has developed, antithrombotic treatment will prevent its extension, and thrombolytic therapy will try to restore vascular patency. Once patency has been restored antithrombotic therapy is still needed to prevent recurrence of the thrombus. Even if the main targeting is on platelets, the choice in the molecule to be clinically used must be defined by the function of the platelet involved: thrombogenesis or vasospasm, and even by the metabolic pathway predominantly activated. In coagulation strategy, differences must be drawn between antithrombotic therapy directed against thrombin formation, complexes of coagulation, free enzymes or activation phases. In thrombolytic therapy all procedural uses of extrinsic thrombolytic agents (natural, modified, or artificial), increase of susceptibility to endogenous thrombolytic systems, and stimulation of endogenous thrombolytic activity do not bear the same efficiency. As a consequence, the responsibility for clinical use of new molecules with more specific activity can allow more efficient antithrombotic therapy directed at the condition of an indication targeted at the exact mechanisms involved.(ABSTRACT TRUNCATED AT 400 WORDS)

1 Arachidonate Metabolism in Blood Cells and the Vessel Wall

Arachidonic acid (AA) is metabolized by the cyclo-oxygenase and the lipoxygenase pathways to give a number of products, some of which have potent and sometimes opposing biological activities. Different cell types produce different metabolites, so that the chief AA metabolite produced by the platelet is the pro-aggregatory thromboxane A2 (TXA2), whereas that produced by the vascular endothelium is the anti-aggregatory prostacyclin. White blood cells, on the other hand, are the chief source of the leukotrienes, which are implicated in the inflammatory process. Generation of these products may be modified in certain pathological conditions, such as atherosclerosis and diabetes, where prostacyclin synthesis is reduced and TXA2 synthesis increased, resulting in a pro-thrombotic state. Synthesis of AA metabolites may be inhibited, either totally or selectively, using drugs which inhibit different enzymes in the metabolic pathway. These drugs may be beneficial in the treatment of thrombotic disorders and inflammation. AA metabolism may also be modified by dietary substitution with eicosapentaenoic acid, a fatty acid present in fish oils.

Inhibition of Human Platelet Functions by Verapamil

The effects of verapamil, a coronary vasodilator, on platelet functions was studied. Platelet aggregation induced by ADP, epinephrine or collagen was inhibited by verapamil in vitro. Calcium ionophore A23187-induced platelet aggregation was also inhibited by verapamil in a concentration dependent manner. In washed platelets, verapamil caused a dose-dependent inhibition of serotonin release induced either by thrombin or A23187 in the absence of extracellular calcium. Addition of 1 mM CaCl2 with A23187 or thrombin partially overcame this inhibition. Addition of 1 mM CaCl2 in the absence of verapamil had no effect on thrombin- or A23187-induced secretion. When verapamil was administered to the healthy volunteers at the dosage commonly used, inhibition of platelet aggregation was observed 2 hrs after the drug ingestion. It is of great interest that verapamil potentiated the anti-aggregating activity of prostacyclin in vitro. Our results may suggest a potential role for verapamil in the treatment of thrombotic disorders.

Antiplatelet Drugs in Arterial Thrombosis: A Review

The thrombotic process, basic mechanisms of action of antiplatelet drugs, and clinical trials with platelet inhibitors are reviewed. The role of prostaglandins in platelet reactions and the mechanisms of aspirin's, dipyridamole's and sulfinpyrazone's inhibition of platelet function are discussed. Clinical studies of the use of these three drugs, alone or in combination with each other or with anticoagulants, for the treatment of thrombotic disorders associated with valvular heart disease, prosthetic heart valves, cerebral vascular disease (e.g., transient ischemic attacks), coronary artery diseases (including myocardial infarction), peripheral vascular disease, renal disease and renal allograft rejection are reviewed and evaluated. Clinical evidence suggests that antiplatelet drugs are useful in preventing thromboembolism associated with coronary and cerebral vascular disorders. In peripheral vascular and renal diseases and in renal allografts, their benefits have not been proved. Further controlled clinical studies of the preventive use of these drugs for arterial thrombosis are needed.