Fluid-phase Thrombin Research Articles

Dabigatran Inhibits Both Clot-Bound and Fluid Phase Thrombin In Vitro: Effects Compared to Heparin and Hirudin.

Thrombin bound to fibrin either in an existing thrombus or on an artificial surface is thought to be protected from inhibition by heparin/ATIII. Thus, this surface can remain “active” despite heparin / LMWH therapy. As long as anticoagulant levels are maintained, fluid phase thrombin inhibition is sufficient to prevent progression of thrombosis. However, during times of trough levels or upon discontinued therapy, this active thrombin could initiate further prothrombotic events. Dabigatran is a reversible, univalent, direct inhibitor of thrombin. Considering its specificity and selectivity for the active site, it is to be expected that it can inhibit thrombin equally, both when bound to a clot and in fluid phase. Thus, this study investigated the ability of dabigatran to inhibit thrombin bound to fibrin, or thrombin in plasma as compared to heparin and hirudin in vitro. Clot-bound thrombin: Clots were generated in human platelet rich plasma by adding CaCl2 and incubating 2 hrs at 37°C (1). Clots were then washed to remove trapped FPA. Washed clots were then transferred to 0.5 ml platelet poor plasma (PPP) containing either dabigatran, heparin, hirudin or buffer and incubated at 37°C. Fluid-phase thrombin: Thrombin (20 pM) was added to PPP containing either dabigatran, heparin, hirudin or buffer and incubated for 1 hr at 37°C. Fibrinopeptide A (FPA) Measurement: Uncleaved fibrinogen was precipitated and fibrin formation was measured as FPA release using ELISA. The IC50 (concentration required to inhibit FPA release by 50%) in fluid phase and clot-bound conditions was calculated using nonlinear curve-fitting. Dabigatran, hirudin and heparin inhibited fluid and clot-bound thrombin with increasing concentrations. Dabigatran inhibited both clot-bound and fluid phase thrombin with similar affinity, IC50 values of 200 nM and 186 nM, respectively. The IC50 of heparin-induced inhibition of clot-bound thrombin was 98.7 nM and in fluid phase 8.4 nM. This is consistent with published results indicating that heparin is not as efficient in inhibiting thrombin bound to a clot as in the fluid phase. Hirudin is a potent, irreversible inhibitor of thrombin and in this study it inhibited both free and bound thrombin equally in low nM ranges, the IC50 of clot-bound thrombin was 0.59 nM and fluid phase thrombin 1.63 nM. The IC50 values were converted into a ratio of clot-bound vs fluid phase thrombin to allow a more direct comparison and are listed in the table.DabigatranHeparinHirudinRatio IC50 Clot-bound: Fluid Phase IIa1.0711.760.36Dabigatran is equally effective in inhibiting both clot-bound and fluid phase thrombin. These data are consistent with targeted inhibition of the active site of thrombin by a small molecule, irrespective of whether thrombin is bound via the exosite to fibrin or is present as free enzyme in plasma. In contrast, heparin is less effective in inhibiting of clot-bound than fluid phase thrombin, consistent with its mechanism of thrombin inhibition via ATIII. Hirudin more potently inhibits clot-bound than free thrombin at lower concentrations, at higher concentrations inhibition of fluid phase thrombin is favoured. Thus these data suggest that dabigatran could also be effective in clinical situations where surface-bound thrombin could play a role, such as during catheterization procedures or the treatment of established thrombosis.

Influence of warfarin therapy on activated factor VII clotting activity

Oral anticoagulant therapy has been widely employed to prevent and treat a variety of thrombotic disorders, although a new generation of antithrombotic drugs, which offer inhibition of clot-bound as well as fluid-phase thrombin, has been developed and tested in several clinical trials. Although most anticoagulant responses to hydroxycoumarin compounds are well established, there are controversial evidences on their influence on activated factor VII (FVIIa). After analyzing the prothrombin time (PT) (International Normalized Ratio reference range, 0.92-1.08), factor VII clotting activity (FVII:C) (reference range, 75-130 U/dl) and activated factor VII clotting activity (FVIIa:C) (reference range, 30-110 U/l) in 46 consecutive patients on stable warfarin therapy for atrial fibrillation, a consistent trend towards decreased values of both FVII:C and FVIIa:C was observed as PT values increased. At moderate-intensity anticoagulation, with international normalized ratios between 2 and 3, the mean activities of FVII:C and FVIIa:C dropped to 28 U/dl (range, 9-61 U/dl) and 5.8 U/l (range, 1-18 U/l), respectively. Results of our investigations indicate that warfarin therapy decreases FVIIa:C, highlighting the potential benefits of oral anticoagulants in thrombotic disorders and other clinical conditions characterized by increased levels of FVIIa. Owing to the good correlation with FVIIa:C, we also hypothesize that the PT and/or FVII:C might be employed for monitoring recombinant FVIIa therapy.

Argatroban

Argatroban is a direct, selective and reversible active site thrombin inhibitor derived from L-arginine. It is a representative of a new class of antithrombotic drugs which offer inhibition of clot-bound as well as fluid-phase thrombin. Argatroban is characterised by favourable pharmacokinetics (beta-elimination half-time approximately 40-50 min) undergoing hepatic metabolism and mainly biliary excretion. Renal impairment will not result in altered or delayed elimination. For many years, argatroban has been used in Japan and in the United States and is approved by the FDA for anticoagulation in patients with heparin-induced thrombocytopenia (HIT type II). The ease of monitoring with the activated partial thromboplastin time, lack of induction of antibodies and adequate safety in renal failure patients, make this drug a favourable mode therapy in comparison with other anticoagulants such as lepirudin or heparinoids. Since June 2005 argatroban has been approved in Germany for the treatment of patients with HIT type II. The main characteristics of the drug with special considerations for anaesthesiologists and intensive care physicians are presented in this review.

Percutaneous interventions in patients with immune-mediated heparin-induced thrombocytopenia.

The use of unfractionated heparin, the traditional antithrombotic agent during percutaneous coronary interventions (PCI), is associated with the risk of heparin-induced thrombocytopenia, a rare but often fatal clinical condition. This article focuses on several issues related to heparin-induced immune-mediated thrombocytopenia (HIT, type II) and alternative modes of periprocedural anticoagulation in patients with suspected or known HIT. The hypercoagulable state characterizing HIT, along with mechanical plaque disruption resulting from PCI place patients with HIT at particular risk of thrombosis during PCI. Given that a diagnosis of HIT precludes any further use of heparin, other treatment modalities are essential. Direct thrombin inhibitors are the drugs of choice in this challenging situation. These agents offer several advantages as anticoagulants for patients with HIT: (1) the ability to inhibit both thrombin that is bound to fibrin (clot-bound thrombin) and fluid-phase free thrombin; (2) rapid achievement of steady state; and (3) no cross-reactivity with HIT antibodies. Recent data on the use of bivalirudin, lepirudin, and argatroban in the setting of PCI in patients with HIT are encouraging. Optimal dosing regimens for argatroban, lepirudin, and bivalirudin should be further established in PCI patients.

A novel approach to thrombin inhibition

The inability of the heparin–antithrombin complex to inhibit fibrin-bound thrombin limits the utility of heparin for treatment of arterial thrombosis. In contrast with heparin, melagatran, a direct thrombin inhibitor (DTI), is equally effective at inhibiting fluid-phase thrombin and thrombin bound to fibrin. This reflects the ability of melagatran, a reversible, active site-directed DTI, to access the active site of thrombin even when it is fibrin-bound. Because bivalent DTIs, such as hirudin, compete with fibrin for access to the fibrin-binding site on thrombin, bivalent DTIs produce less inhibition of fibrin-bound thrombin than free thrombin when given in low doses. Consequently, higher doses of hirudin are needed for complete inhibition of fibrin-bound thrombin resulting in a steep dose–response curve. This phenomenon, combined with the fact that hirudin irreversibly inhibits thrombin, may account for hirudin's narrow therapeutic window. In a rabbit arterial thrombosis prevention and ear bleeding model, melagatran produced less bleeding than hirudin when the two agents were given at doses that produced nearly complete inhibition of thrombosis. The more favourable benefit-to-risk profile of melagatran in this model likely reflects better access to fibrin-bound thrombin and the reversible nature of the melagatran/thrombin complex. The theoretical advantages of melagatran may endow it with mechanistic advantages over heparin, and safety advantages over hirudin. Large-scale clinical trials are under way to evaluate the utility of an orally active prodrug form of melagatran for prevention and treatment of venous and arterial thrombosis.

Effect of Heparin on TAFI-Dependent Inhibition of Fibrinolysis: Relative Importance of TAFIa Generated by Clot-Bound and Fluid Phase Thrombin

Heparin has been proposed to enhance thrombolysis by inhibiting thrombin-dependent generation of activated TAFI (thrombin activatable fibrinolysis inhibitor), a carboxypeptidase that inhibits fibrinolysis. We evaluated the effect of heparin in an in vitro thrombolysis model consisting of a radiolabelled blood clot submerged in defibrinated plasma. Fibrinolysis was induced by adding t-PA (250 ng/ml) and calcium to the plasma bath. Control experiments indicated that thrombin generation induced by recalcification caused significant TAFI activation and inhibited clot lysis. Heparin (up to 1 U/ml), added to the plasma bath, failed to enhance clot lysis. Thrombin generation in the fluid phase was totally inhibited by heparin at concentrations > 0.5 U/ml. In contrast, thrombin generation on the clot surface was not inhibited by heparin (1 U/ml). TAFIa generation did occur in heparin-containing samples (1 U/ml) and amounted to about 10% of TAFIa formed in control samples. This low amount of TAFIa did exert antifibrinolytic activity as indicated by the observation that the addition of a specific TAFIa inhibitor (PTI) along with heparin enhanced clot lysis. Hirudin (10 micrograms/ml), at variance with heparin, inhibited clot-bound thrombin and enhanced clot lysis. These data show that heparin is unable to stimulate fibrinolysis through a TAFI-dependent mechanism, most likely because of its inefficiency in inhibiting thrombin generation on the clot surface. Moreover, they suggest that clot-bound thrombin plays a major role in TAFI-mediated inhibition of fibrinolysis through "localized" TAFIa generation.

Selective and Sustained Inhibition of Surface-bound Thrombin Activity by Intimatan/Heparin Cofactor II and Its Relevance to Assessing Systemic Anticoagulation In Vivo, Ex Vivo and In Vitro

We compare the relative activities of surface-bound and fluid-phase thrombin and their inhibition by heparin and Intimatan, a novel heparin cofactor II (HCII) agonist. In vitro, we compared the observed amidolytic activities of fluid-phase and surface-bound thrombin with the expected activities based upon 125I-specific activity. In vivo, we compared the inhibitory effects of heparin and Intimatan on thrombin activity bound to injured vessel walls. In vitro, the correlations between observed and expected activities of fluid-phase and surface-bound thrombin, were: r = 0.9974, p < 0.001; and r = 0.9678, p < 0.001; respectively. In vivo, injured vessel wall surface-bound thrombin activity persisted for > 24 h. This activity was not inhibited by heparin, but was inhibited by Intimatan, p < 0.001. We conclude that surface-bound thrombin is as active as fluid-phase thrombin and remains protected from inhibition by heparin, thereby contributing to vessel wall thrombogenicity following injury. In contrast, surface-bound thrombin is inhibited by Intimatan, thereby effectively decreasing vessel wall thrombogenicity following injury in vivo.

Desirudin: a review of its use in the management of thrombotic disorders.

Desirudin, a recombinant hirudin used in the prevention and management of thromboembolic disease, is a thrombin inhibitor which binds directly and with high affinity to clot-bound and fluid phase thrombin. As a prophylaxis in patients undergoing hip replacement surgery, desirudin was significantly more effective in reducing the incidence of deep vein thrombosis (DVT) than either unfractionated or low molecular weight heparin. However, results in patients with acute coronary syndromes are less conclusive. A significant reduction with desirudin compared with heparin in the incidence of death or non-fatal (re)infarction at 24 hours in patients with acute myocardial infarction (MI) was reported in the GUSTO (Global Use of Strategies to Open Occluded Coronary Arteries) IIb trial but not in the TIMI (Thrombolysis and Thrombin Inhibition in Myocardial Infarction) 9B trial. Despite the early reduction shown in GUSTO IIb, desirudin was not associated with an improved long term clinical benefit at 30 days compared with heparin. Similar results were seen in patients with unstable angina/non-Q-wave MI enrolled in the GUSTO IIb trial. In addition, desirudin and heparin showed similar efficacy in preventing restenosis 30 weeks after coronary angioplasty for unstable angina, despite desirudin being associated with a significant reduction in the rate of cardiac events within the first 96 hours. Desirudin is as well tolerated as heparin with a similar incidence of moderate and severe bleeding, intracranial haemorrhage or stroke reported when trialled in the prevention of DVT associated with hip replacement surgery or the treatment of acute coronary syndromes. However, in the GUSTO IIb trial a significantly higher incidence of transfusions was observed in patients with unstable angina/non-Q-wave MI. Desirudin is clearly more effective than heparin in the prevention of DVT in patients undergoing elective hip replacement, although cost factors may influence its ultimate place in therapy. In the treatment of acute coronary syndromes the role of desirudin is less certain; however, it may be useful for patients in whom heparin therapy is not a viable option.

Direct thrombin inhibitors for treatment of arterial thrombosis: potential differences between bivalirudin and hirudin

Given the central role of thrombin in arterial thrombogenesis, most treatment strategies for acute coronary syndromes are aimed at inhibiting its generation or blocking its activity. Although heparin has been widely used, it has limitations in the setting of arterial thrombosis. These limitations reflect the inability of heparin to inactivate thrombin bound to fibrin, a major stimulus for thrombus growth. In addition, the anticoagulant response to heparin varies from patient to patient, and heparin is neutralized by platelet Factor IV, large quantities of which are released from platelets activated at sites of plaque rupture. Consequently, heparin requires careful laboratory monitoring to ensure an adequate anticoagulant effect. Direct thrombin inhibitors, such as hirudin and bivalirudin, overcome the limitations of heparin. These agents inhibit fibrin-bound thrombin, as well as fluid-phase thrombin, and produce a predictable anticoagulant response. Bivalirudin has both safety and potential efficacy advantages over hirudin. Bivalirudin appears to have a wider therapeutic window than hirudin, possibly because bivalirudin only transiently inhibits the active site of thrombin. The better safety profile of bivalirudin permits administration of higher doses, which may give it an efficacy advantage. Hirudin prevents thrombin from activating protein C, thereby suppressing this natural anticoagulant pathway. In contrast, bivalirudin may promote protein C activation by transiently inhibiting thrombin until it can be bound by thrombomodulin. Differences between bivalirudin and hirudin, as well as other direct thrombin inhibitors, highlight the pitfalls of considering all direct thrombin inhibitors to have equivalent risk–benefit profiles.

Con: Heparin is not the best anticoagulant for cardiopulmonary bypass

T O SAY THAT heparin is the best anticoagulant for cardiopulmonary bypass (CPB) is to say that no further alternatives to heparin should be sought. However, this is clearly not the case. Heparm is not the 1deal anticoagulant, and the quest for better heparin alternatives should be continued. Extracorporeal circulation induces a total body inflammatory response, resulting in the activation of various inflammatory cascades, only one of which is the coagulation cascade. Thrombin is the major player in the coagulation cascade. Thrombin contributes not only to fibrin generation but also to platelet activation and to activation of the fibrinolytic system. Antithrombin (AT) is the major plasma inhibitor of thrombin. Neutralization of thrombm by AT proceeds slowly in the absence of heparin but is dramatically accelerated in the presence of heparln. I Thus, heparin serves to facilitate the reaction between thrombin and antithrombin. Once the thrombin/antithrombin (TAT) complex is generated, heparin dissociates from the TAT complex and is able to bind another molecule of AT, thus allowing one molecule of heparm to provide multiple rounds of thrombln inhibition. However, during CPB, inactivation of thrombin is incomplete. Despite seemingly adequate heparlnizatlon, there ~s evidence for continued thrombin and fibrin formation in the microcirculation during CPB, as evidenced by the continued generation of prothrombin fragment 1.2 (F1.2), TAT complex, and fibrinopeptide A (FPA). 2,3 This coagulation in the microcirculation places the microvasculature at risk for occlusion. Although heparin is very effective at catalyzing the inhibition of free, circulating, or fluid-phase thrombin by AT, it is a less effective catalyst if the thrombin is bound to fibrin or to a surface. 4 Heparin thus incompletely inhibits clot-bound, fibrin-bound, or solid-phase thrombin because thrombin is protected from inactivation by the heparm/AT complex. Consequently, the bound thrombin remains enzymatically active and is able to facihtate further thrombin generation. When a cardmc patient's blood comes in contact with the large artificial surface of the CPB circuit (approximately 6 m2), contact activation and the extrinsic coagulation cascade are also stimulated. 2 Any thrombin that is accreted onto the CPB circuit may escape inhibition by heparin/AT, thus allowing continued thrombm generation. The continued generation of F1.2, TAT, and FPA despite adequate heparinization for CPB suggests that significant amounts of thrombin escape inhibition and are therefore able to facdltate additional thrombin generation

Inhibition by Argatroban, a Specific Thrombin Inhibitor, of Platelet Activation by Fibrin Clot-associated Thrombin

Clot-associated thrombin retains amidolytic activity, and is resistant to inhibition by heparin, but not to low molecular weight thrombin inhibitors. We show that clot-associated thrombin induces platelet aggregation, is resistant to heparin:antithrombin III, less so to recombinant hirudin (rHV2Lys47) but not to argatroban, an active-site directed thrombin inhibitor. Fibrin clots prepared with human fibrinogen and thrombin were used to aggregate rabbit washed platelets assessed by single platelet counting, thromboxane B2 (TXB2) immunoassay and scanning electron microscopy. Fibrin clots decreased platelet counts, and released TXB2. Electron microscopy showed platelet aggregates on the clot surface. Argatroban concentration-dependently inhibited such aggregation with IC50s of 21 nM and 13 nM versus aggregation and TXB2 release respectively. The IC50s of Argatroban against fluid-phase thrombin producing similar aggregation were 12 nM (aggregation) and 33 nM (TXB2). rHV2Lys47 was less active against clot-induced aggregation (IC50 = 1.8 nM) than against fluid-phase thrombin (IC50 = 0.06 nM). Heparin had an IC50 of 0.02 mU/ml against aggregation induced by fluid-phase thrombin, but much greater concentrations are required to inhibit clot-induced aggregation (IC50 = 48 mU/ml). These data provide a basis for the superiority of direct-acting thrombin inhibitors over heparin in platelet rich thrombi.

Antithrombin activity of surface‐bound heparin studied under flow conditions

Polyacrylamide-grafted polyetherurethane sheets were modified by end-point and multipoint attachment of heparin. The surface-bound heparin was firmly attached. No release of heparin activity could be detected when the surface was rinsed at a wall shear rate of 2000 s-1. Uptake of antithrombin and thrombin inactivation were investigated under well-defined flow conditions by the use of a spinning device with an attached disk-shaped heparinized surface. It is demonstrated that the rate of thrombin inactivation at the antithrombin-heparin surface equals the maximal rate of transport of thrombin toward the surface when the surface coverage of antithrombin exceeds 10 pmol/cm2. This result indicates that a higher intrinsic catalytic efficiency of a surface does not necessarily result in a higher antithrombin activity. We varied the heparin content of the surfaces between 0 and 35 micrograms/cm2 by increasing the number of functional groups to which heparin could be covalently attached. The uptake of antithrombin increased with the heparin content of the surface, but the stoichiometry decreased from 2 to 0.5 pmol antithrombin/micrograms heparin. Apparently, antithrombin could not bind to heparins buried in the poly(acrylamide) layer. The rate of thrombin inactivation at surfaces with low heparin content (2 micrograms/cm2) fells below the transport limit of thrombin and became proportional with the heparin content of the surface. Although the contribution of surface-bound heparin to the neutralization of fluid-phase thrombin was found to be negligible compared with the effect of fluid-phase antithrombin at physiologic relevant concentrations, these heparinized surfaces markedly delayed the onset of thrombin generation in platelet-rich plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of clot-bound and free (fluid-phase thrombin) by a novel synthetic thrombin inhibitor (Ro 46-6240), recombinant hirudin and heparin in human plasma.

Clot-bound thrombin remains active and is less accessible to heparin-antithrombin III than fluid-phase thrombin. To determine whether clot-bound human thrombin is more susceptible to inactivation by direct thrombin inhibitors, the activity of a novel synthetic competitive thrombin inhibitor Ro 46-6240, recombinant hirudin and unfractionated heparin were compared with fluid-phase thrombin and clot-bound thrombin. Fibrinopeptide A generated in human plasma was used as an index of thrombin activity. Hirudin was the most potent inhibitor of fluid-phase and clot-bound thrombin. However, Ro 46-6240 inhibited clot-bound thrombin three times more potently than fluid-phase thrombin (IC50 19 vs 56 ng/ml) while hirudin was two times (IC50 8 vs 3 ng/ml) and heparin six times (IC50 1,205 vs 200 ng/ml) less active against clot-bound thrombin compared with fluid-phase thrombin. The relative selectivity for clot-bound thrombin is not a unique property of Ro 46-6240 since two other synthetic thrombin inhibitors tested inhibited clot-bound thrombin more effectively than fluid-phase thrombin and a third was equally active against both forms of thrombin. In contrast, the affinities of two chromogenic substrates were similar for both forms of thrombin. This study shows that direct thrombin inhibitors inhibit clot-bound thrombin more potently than heparin and suggests that an apparent selectivity for clot-bound thrombin can be achieved with some synthetic thrombin inhibitors. Further studies have to show whether the high potency of these direct thrombin inhibitor translates into antithrombotic efficacy in clinical situations with pre-existing clots.

Effects of heparin (H) and low-molecular-weight dermatan-sulfate (D) on clot-bound (CT) VS fluid-phase thrombin (FT): [formula omitted], G.Melandri, N. Candiotti,V. Cervi, A. Branzi, B. Magnani. Institute of Cardiology, University of Bologna, Italy

Effects of heparin (H) and low-molecular-weight dermatan-sulfate (D) on clot-bound (CT) VS fluid-phase thrombin (FT): [formula omitted], G.Melandri, N. Candiotti,V. Cervi, A. Branzi, B. Magnani. Institute of Cardiology, University of Bologna, Italy

Sustained Antithrombotic Activity of Hirudin After its Plasma Clearance: Comparison With Heparin

Thrombus extension in patients with venous thromboembolism is due to the accretion of fibrin onto existing thrombi. Extension is promoted by both circulating and thrombus-bound thrombin, which convert fibrinogen to fibrin. Heparin is an effective antithrombotic agent, but it requires continuous administration to achieve persistent inhibition of thrombus extension. Heparin is highly effective in inhibiting fluid phase thrombin, but is a relatively ineffective inhibitor of thrombus-bound thrombin. Hirudin, unlike heparin, inactivates both circulating and fibrin-bound thrombin and, therefore, has the potential to prevent thrombus extension even after a short course of treatment. The aim of this study was to evaluate the time course of the accretion of new fibrin onto preexisting rabbit jugular vein thrombi after a 3-hour infusion of saline, heparin, and hirudin. Heparin and recombinant (r)-hirudin (CGP 39399) were infused at doses that doubled the activated partial thromboplastin time (aPTT). At the end of the 3-hour infusions in rabbits treated with saline, heparin (0.75 mg/kg), or r-hirudin (1.25 mg/kg), accretion of 125I-fibrinogen was 59 +/- 5 micrograms, 34 +/- 4 micrograms, and 21 +/- 2 micrograms, respectively (heparin and r-hirudin v saline, P less than .01; r-hirudin v heparin, P less than .01). Three hours after the end of the infusions, the accreted 125I-fibrinogen in the saline-, heparin-, and hirudin-treated animals was 89 +/- 6 micrograms, 51 +/- 7 micrograms, and 23 +/- 3 micrograms, respectively; 9 hours after the end of the infusions, the accreted 125I-fibrinogen was 112 +/- 9 micrograms, 82 +/- 7 micrograms, and 25 +/- 3 micrograms, respectively. aPTT and thrombin clotting time (TCT) returned to the baseline value 90 minutes after the end of heparin or r-hirudin infusion. During in vitro experiments, human fibrin clots previously incubated in human plasma containing r-hirudin did not promote fibrinopeptide A (FPA) generation when washed and then incubated in human plasma in the absence of thrombin inhibitors. This persistent inhibition of FPA production was not observed after incubation in human plasma of human plasma clots preincubated with heparin. We conclude that heparin is effective in inhibiting thrombus extension while it is present in the circulation, but that this effect is rapidly lost after its plasma clearance. In contrast, the antithrombotic activity of r-hirudin is sustained beyond its plasma clearance, presumably because of its ability to inactivate thrombus-bound thrombin. Our findings indicate that r-hirudin might be an effective antithrombotic agent even when used for short periods.

Sustained antithrombotic activity of hirudin after its plasma clearance: comparison with heparin

Thrombus extension in patients with venous thromboembolism is due to the accretion of fibrin onto existing thrombi. Extension is promoted by both circulating and thrombus-bound thrombin, which convert fibrinogen to fibrin. Heparin is an effective antithrombotic agent, but it requires continuous administration to achieve persistent inhibition of thrombus extension. Heparin is highly effective in inhibiting fluid phase thrombin, but is a relatively ineffective inhibitor of thrombus- bound thrombin. Hirudin, unlike heparin, inactivates both circulating and fibrin-bound thrombin and, therefore, has the potential to prevent thrombus extension even after a short course of treatment. The aim of this study was to evaluate the time course of the accretion of new fibrin onto preexisting rabbit jugular vein thrombi after a 3-hour infusion of saline, heparin, and hirudin. Heparin and recombinant (r)- hirudin (CGP 39399) were infused at doses that doubled the activated partial thromboplastin time (aPTT). At the end of the 3-hour infusions in rabbits treated with saline, heparin (0.75 mg/kg), or r-hirudin (1.25 mg/kg), accretion of 125I-fibrinogen was 59 +/- 5 micrograms, 34 +/- 4 micrograms, and 21 +/- 2 micrograms, respectively (heparin and r- hirudin v saline, P less than .01; r-hirudin v heparin, P less than .01). Three hours after the end of the infusions, the accreted 125I- fibrinogen in the saline-, heparin-, and hirudin-treated animals was 89 +/- 6 micrograms, 51 +/- 7 micrograms, and 23 +/- 3 micrograms, respectively; 9 hours after the end of the infusions, the accreted 125I- fibrinogen was 112 +/- 9 micrograms, 82 +/- 7 micrograms, and 25 +/- 3 micrograms, respectively. aPTT and thrombin clotting time (TCT) returned to the baseline value 90 minutes after the end of heparin or r- hirudin infusion. During in vitro experiments, human fibrin clots previously incubated in human plasma containing r-hirudin did not promote fibrinopeptide A (FPA) generation when washed and then incubated in human plasma in the absence of thrombin inhibitors. This persistent inhibition of FPA production was not observed after incubation in human plasma of human plasma clots preincubated with heparin. We conclude that heparin is effective in inhibiting thrombus extension while it is present in the circulation, but that this effect is rapidly lost after its plasma clearance. In contrast, the antithrombotic activity of r-hirudin is sustained beyond its plasma clearance, presumably because of its ability to inactivate thrombus- bound thrombin. Our findings indicate that r-hirudin might be an effective antithrombotic agent even when used for short periods.

Clot-bound thrombin is protected from inhibition by heparin-antithrombin III but is susceptible to inactivation by antithrombin III-independent inhibitors.

Propagation of venous thrombi or rethrombosis after coronary thrombolytic therapy can occur despite heparin administration. To explore potential mechanisms, we set out to determine whether clot-bound thrombin is relatively protected from inhibition by heparin-antithrombin III but susceptible to inactivation by antithrombin III-independent inhibitors. Using plasma fibrinopeptide A (FPA) levels as an index of thrombin activity, we compared the ability of thrombin inhibitors to block FPA release mediated by fluid-phase thrombin with their activity against the clot-bound enzyme. Incubation of thrombin with citrated plasma results in concentration-dependent FPA generation, which reaches a plateau within minutes. In contrast, there is progressive FPA generation when fibrin clots are incubated with citrated plasma. Heparin, hirudin, hirudin dodecapeptide (hirugen), and D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone (PPACK) produce concentration-dependent inhibition of FPA release mediated by fluid-phase thrombin. However, heparin is much less effective at inhibiting thrombin bound to fibrin because a 20-fold higher concentration is necessary to block 70% of the activity of the clot-bound enzyme than is required for equivalent inhibition of fluid-phase thrombin (2.0 and 0.1 U/ml, respectively). In contrast, hirugen and PPACK are equally effective inhibitors of fluid- and solid-phase thrombin, while hirudin is only 50% as effective against the clot-bound enzyme. None of the inhibitors displace bound 125I-labeled thrombin from the clot. These studies indicate that (a) clot-bound thrombin is relatively protected from inhibition by heparin, possibly because the heparin binding site on thrombin is inaccessible when the enzyme is bound to fibrin, and (b) clot-bound thrombin is susceptible to inactivation by antithrombin III-independent inhibitors because the sites of their interaction are not masked by thrombin binding to fibrin. For these reasons, antithrombin III-independent inhibitors may be more effective than heparin in certain clinical settings.