Effect of a Selective Thrombin Inhibitor MCI-9038 on Fibrinolysis In Vitro and In Vivo

Factors influencing the efficacy of recombinant tissue plasminogen activator: Implications for ischemic stroke treatment

Intravenous thrombolysis with a recombinant tissue plasminogen activator (rt-PA) is the first-line treatment of acute ischemic stroke. However, successful recanalization is relatively low and the underlying processes are not completely understood. The goal was to provide insights into clinically important factors potentially limiting rt-PA efficacy such as clot size, rt-PA concentration, clot age and also rt-PA in combination with heparin anticoagulant. We established a static in vitro thrombolytic model based on red blood cell (RBC) dominant clots prepared using spontaneous clotting from the blood of healthy donors. Thrombolysis was determined by clot mass loss and by RBC release. The rt-PA became increasingly less efficient for clots larger than 50 μl at a clinically relevant concentration of 1.3 mg/l. A tenfold decrease or increase in concentration induced only a 2-fold decrease or increase in clot degradation. Clot age did not affect rt-PA-induced thrombolysis but 2-hours-old clots were degraded more readily due to higher activity of spontaneous thrombolysis, as compared to 5-hours-old clots. Finally, heparin (50 and 100 IU/ml) did not influence the rt-PA-induced thrombolysis. Our study provided in vitro evidence for a clot size threshold: clots larger than 50 μl are hard to degrade by rt-PA. Increasing rt-PA concentration provided limited thrombolytic efficacy improvement, whereas heparin addition had no effect. However, the higher susceptibility of younger clots to thrombolysis may prompt a shortened time from the onset of stroke to rt-PA treatment.

Iatrogenic air embolism: pathoanatomy, thromboinflammation, endotheliopathy, and therapies

Iatrogenic vascular air embolism is a relatively infrequent event but is associated with significant morbidity and mortality. These emboli can arise in many clinical settings such as neurosurgery, cardiac surgery, and liver transplantation, but more recently, endoscopy, hemodialysis, thoracentesis, tissue biopsy, angiography, and central and peripheral venous access and removal have overtaken surgery and trauma as significant causes of vascular air embolism. The true incidence may be greater since many of these air emboli are asymptomatic and frequently go undiagnosed or unreported. Due to the rarity of vascular air embolism and because of the many manifestations, diagnoses can be difficult and require immediate therapeutic intervention. An iatrogenic air embolism can result in both venous and arterial emboli whose anatomic locations dictate the clinical course. Most clinically significant iatrogenic air emboli are caused by arterial obstruction of small vessels because the pulmonary gas exchange filters the more frequent, smaller volume bubbles that gain access to the venous circulation. However, there is a subset of patients with venous air emboli caused by larger volumes of air who present with more protean manifestations. There have been significant gains in the understanding of the interactions of fluid dynamics, hemostasis, and inflammation caused by air emboli due to in vitro and in vivo studies on flow dynamics of bubbles in small vessels. Intensive research regarding the thromboinflammatory changes at the level of the endothelium has been described recently. The obstruction of vessels by air emboli causes immediate pathoanatomic and immunologic and thromboinflammatory responses at the level of the endothelium. In this review, we describe those immunologic and thromboinflammatory responses at the level of the endothelium as well as evaluate traditional and novel forms of therapy for this rare and often unrecognized clinical condition.

State-of-the-Art Review: The Preclinical and Clinical Pharmacology of Novastan (Argatroban): A Small-Molecule, Direct Thrombin Inhibitor

Because of the unsatisfactory options available for safe and effective antithrombotic therapy, recent, intense research and development efforts have been focused on direct thrombin inhibitors, also known as site-directed thrombin inhibitors. The intravenous agent Novastan (argatroban) is a small-molecule, reversible, direct thrombin inhibitor that is selective for the catalytic site of the thrombin molecule. Argatroban's molecular properties (small molecule; fast, selective, and reversible inhibition of the thrombin catalytic site; and similar in vitro potency for inhibiting both clot-bound and soluble thrombin) offer the potential for significant antithrombotic efficacy with minimal systemic anticoagulant ef fects. Its clinical pharmacologic properties offer the potential for minimal risk of bleeding, very rapid achievement of therapeutic antithrombotic efficacy, predictable dose-response, and rapid restoration of the hemostatic systems to normal upon termination of intravenous infusion. Argatroban is currently approved for clinical use in Japan for the treatment of peripheral arterial occlusive disease. It is in advanced clinical development in North America, South America, and Western Europe for several clinical indications, including (1) adjunctive therapy to thrombolytic agents in the treatment of acute myocardial infarction and (2) antithrombotic therapy for patients with heparin-induced thrombocytopenia and heparin-induced thrombocytopenia and thrombosis syndrome. Key Words: Molecular properties—Novastan (argatroban)—Pharmacology—Thrombin inhibitor.

Argatroban, a direct thrombin inhibitor for heparin-induced thrombocytopaenia: present and future perspectives

Heparin remains the most commonly used anticoagulant in the treatment of patients with acute vascular syndromes, including myocardial infarction, unstable angina and ischaemic stroke. However, heparin therapy is not always associated with a significant improvement of clinical outcomes, is linked with enhanced bleeding risk and can occasionally provoke the development of heparin-induced thrombocytopaenia, the most devastating complication of conventional therapy with unfractioned heparin. Understanding the key role of thrombin in clot formation and platelet activation has stimulated the development of a new class of drugs - direct thrombin inhibitors. The direct thrombin inhibitor argatroban has been known for decades. Similar to the unfractioned heparin, argatroban requires intravenous administration and activated partial prothrombin time-dependent dose adjustment; however, this pharmacological agent has a relatively short half-life that broadens its safety margins, as well as its low antigenic potential due to the small molecular weight of the compound. The efficacy of argatroban has been demonstrated among patients with acute coronary syndromes and stroke. However, this drug is currently approved by the FDA only for the treatment of patients with heparin-induced thrombocytopaenia. Indeed, in such patients, argatroban significantly improves clinical outcomes, and is associated with reduced mortality. Further clinical studies are needed to present more clinical evidence necessary to broad the indication spectrum of this agent.

Pharmacologic platelet anesthesia by glycoprotein IIb/IIIa complex antagonist and argatroban during in vitro extracorporeal circulation

OBJECTIVE

Contact between blood and the synthetic surfaces of a cardiopulmonary bypass circuit leads to platelet activation, and resultant platelet dysfunction contributes to postoperative bleeding. We compared the effects of various platelet inhibitors on preservation of platelet function during simulated cardiopulmonary bypass circulation.

METHODS

Fresh human blood was recirculated in an in vitro cardiopulmonary bypass model circuit. We measured various platelet activation markers including expressions of PAC-1 and P-selectin, annexin V binding, and microparticle formations by means of whole-blood flow cytometry.

RESULTS

Two types of glycoprotein IIb/IIIa complex antagonists, peptide-mimetic FK633 and abciximab and prostaglandin E(1), significantly prevented platelet loss and the increase in binding of PAC-1, an antibody specific for fibrinogen receptor on activated platelets, during extracorporeal circulation of heparinized blood. These antagonists significantly suppressed but did not abolish P-selectin expression, annexin V binding, and microparticle formation. Anti-von Willebrand factor monoclonal antibody and aurin tricarboxylic acid (an inhibitor of glycoprotein Ib) had no effect on platelet activation during simulated cardiopulmonary bypass circulation. These data suggest that inhibition of fibrinogen binding glycoprotein IIb/IIIa complex is partly effective in attenuating platelet activation in a heparinized cardiopulmonary bypass model circuit. The direct thrombin inhibitor argatroban prevented platelet loss and expression of P-selectin significantly more than did heparin. A combination of FK633 with argatroban as a substitute for heparin further prevented platelet loss and platelet secretion during simulated cardiopulmonary bypass circulation, although the inhibition of microparticle formation was less.

CONCLUSION

The inhibition of both platelet adhesion and thrombin may be effective to preserve platelet number and function during cardiopulmonary bypass circulation.

Studies on the different modes of action of the anticoagulant protease inhibitors DX-9065a and Argatroban. II. Effects on fibrinolysis

The accompanying paper (Nagashima, H. (2002) J. Biol. Chem. 277, 50439-50444) has demonstrated that argatroban can yield a stronger inhibitory effect on thrombin generation than DX-9065a during extrinsic pathway-stimulated human plasma coagulation, while these anticoagulant compounds have comparable abilities to prolong clot time. Since thrombin generation is known to be an important determinant for fibrinolytic resistance of clots formed during coagulation, the two compounds are compared by tissue plasminogen activator-induced clot lysis assays. The results demonstrated that, in the presence of thrombomodulin, argatroban dose dependently accelerated fibrinolysis of the clots, whereas DX-9065a did not. The activation of thrombin activatable fibrinolysis inhibitor (TAFI) determined in separate assays reflected the differential influence on thrombin generation by these compounds. Moreover, TAFI activation correlated closely with the fibrinolytic resistance observed during tissue plasminogen activator-induced clot lysis. This study demonstrates the differential effects of DX-9065a and argatroban on thrombin generation, which in turn results in a differential acceleration of fibrinolysis as well as TAFI activation in the clots formed under the influence of these compounds. The data implicate a possible difference in the antifibrinolytic properties of clots formed during treatment with these compounds.

No Effect of Clot Age or Thrombolysis on Argatroban’s Inhibition of Thrombin

The purpose of this study was to establish the effects of clot age and thrombolysis, with either streptokinase or tissue-type plasminogen activator (tPA), on argatroban’s ability to inhibit thrombin. The antithrombotic activity of argatroban has been quantified in fibrin clot permeation and fibrin clot perfusion systems as a function of clot age and composition. Analysis of the argatroban dose-response data with a competitive inhibition model has yielded IC50 values in the low micromolar range. Results obtained in a plasma clot permeation system have also shown that argatroban is a potent inhibitor of clot-bound thrombin, independent of either clot age or the presence of hemostatically active platelets. Treatment of aged plasma clots with either streptokinase or alteplase, at therapeutic levels, increased the available thrombin activity, yet argatroban still inhibited this clot-associated thrombin with IC50 values in the low micromolar range. Scanning electron microscopy/morphometric analyses demonstrated that permeation with argatroban had no significant effects on clot structure. We conclude that argatroban is an effective inhibitor of thrombin bound to aged fibrin clots, in purified systems and in plasma clots, as well as in clots that have been treated with the thrombolytic agents streptokinase and alteplase.

© 1998 by The American Society of Hematology.

No Effect of Clot Age or Thrombolysis on Argatroban’s Inhibition of Thrombin

The purpose of this study was to establish the effects of clot age and thrombolysis, with either streptokinase or tissue-type plasminogen activator (tPA), on argatroban’s ability to inhibit thrombin. The antithrombotic activity of argatroban has been quantified in fibrin clot permeation and fibrin clot perfusion systems as a function of clot age and composition. Analysis of the argatroban dose-response data with a competitive inhibition model has yielded IC50 values in the low micromolar range. Results obtained in a plasma clot permeation system have also shown that argatroban is a potent inhibitor of clot-bound thrombin, independent of either clot age or the presence of hemostatically active platelets. Treatment of aged plasma clots with either streptokinase or alteplase, at therapeutic levels, increased the available thrombin activity, yet argatroban still inhibited this clot-associated thrombin with IC50 values in the low micromolar range. Scanning electron microscopy/morphometric analyses demonstrated that permeation with argatroban had no significant effects on clot structure. We conclude that argatroban is an effective inhibitor of thrombin bound to aged fibrin clots, in purified systems and in plasma clots, as well as in clots that have been treated with the thrombolytic agents streptokinase and alteplase.

© 1998 by The American Society of Hematology.

Anticoagulant Effects of Argatroban on the Pre-DIC State in Patients with an Aortic Aneurysm: A Comparative Study of Heparin

We compared the efficacy of argatroban, a new synthetic thrombin-specific inhibitor, with that of heparin in pre-DIC state patients with abdominal aortic aneurysm (AAA). A pre-DIC state was diagnosed by a detection of soluble fibrin monomer complex (FM) and increased levels of thrombin-antithrombin III complex (TAT) of more than 20 ng/ml. Twelve patients showing a pre-DIC condition were treated with argatroban (40 mg/day, n = 6) or heparin (10,000 U/day, n = 6) for 5 days. Coagulation and fibrinolytic profiles were analyzed before and after drug administration. FM became negative in two (33%) patients after the argatroban treatment and in all (100%) of the heparin-treated patients. Plasma levels of TAT were significantly decreased after the heparin treatment, however, there was no significant alteration in this parameter after the argatroban treatment. In conclusion, the anticoagulant effects of heparin were superior to those of argatroban in controlling the pre-DIC state associated with AAA.

Inhibition of collagen-induced platelet aggregation by argatroban in patients with acute cerebral infarction

Platelet aggregation induced by collagen is enhanced in patients with cerebral thrombosis [1], and platelet aggregates and activation products such as beta-thromboglobulin appear in the circulation, particularly during the acute phase [2]. It is known that the presence of activated platelets markedly amplifies thrombin generation by the prothrombinase complex. Platelet aggregation, for which thrombin is the most potent stimulator, is effected by the activation of the thrombin receptor. Activated platelets likely provide the principal surfaces on which intrinsic coagulation factors and prothrombinase assemble in vivo. Moreover, the blood coagulation cascade is simultaneously enhanced during the acute phase of cerebral infarction, resulting in thrombin production and fibrin formation [3]. Cerebral arterial thrombosis is thought to be initiated by rupture of atherosclerotic plaque. Platelets adhere to the constituents of the plaque, and platelet aggregation and the formation of thrombin occur rapidly. The presence of activated platelets involving thrombin generation in cerebral infarction remains to be further clarified. The present study in patients with acute cerebral infarction was undertaken to determine whether argatroban, a direct thrombin inhibitor, is effective in inhibiting collagen-induced platelet aggregation. It is well known that argatroban inhibits not only thrombin cleavage of fibrinogen with a Ki of 19 nM, but also thrombin-mediated platelet activation with a Ki of 40 nM [4]. We evaluated whether the sensitivity of platelets to collagen is increased in the presence of a trace amount of thrombin associated on platelets, at a concentration that does not induce platelet aggregation in acute cerebral infarction. The study also measured the inhibitory effect, if any, of the addition of argatroban during platelet hyperactivation induced by collagen.

Report of successful use of argatroban as an alternative anticoagulant during coronary stent implantation in a patient with heparin-induced thrombocytopenia and thrombosis syndrome

Heparin-induced thrombocytopenia and thrombosis syndrome (HITTS) is a severe complication of heparin caused by an antibody response to the heparin-platelet factor 4 complex which results in severe thrombosis. Heparin rechallenge in HITTS patients carries a high risk of inducing thrombosis. Antithrombin agents represent treatment alternatives in HITTS patients who require anticoagulation. We report successful coronary stent implantation in a HITTS patient using the antithrombin agent argatroban.

Effect of argatroban on microthrombi formation and brain damage in the rat middle cerebral artery thrombosis model

Ischemic cerebral infarcts induce hypercoagulation and microthrombosis in the surrounding region, thus leading to vascular occlusion. We determined whether microthrombi contribute to the spreading of ischemic lesions following thrombotic middle cerebral artery (MCA) occlusion and also determined whether argatroban, a selective thrombin inhibitor, reduces the formation of the microthrombi and the area of the ischemic lesions. The rat left MCA was occluded by a platelet-rich thrombus formed following the photochemical reaction between rose bengal and green light. Microthrombi were histologically identified in the left hemisphere. The extent of ischemic lesions and microthrombi containing fibrin increased in a time-dependent manner after MCA occlusion. Argatroban inhibited the formation of microthrombi up to 3 hr after MCA occlusion; beyond 3 hr, it was ineffective. Argatroban also significantly (P < 0.01) reduced the size of ischemic cerebral lesions at 6 hr after MCA occlusion. It is concluded that the formation of microthrombi contributes to the progression of ischemic lesions in the early stage. It is likely that thrombin generated following thrombotic MCA occlusion contributes to the progression of ischemic lesions by promoting the formation of microthrombi. Argatroban can reduce the formation of microthrombi and ischemic lesions in the early stage.

Antithrombotic actions of argatroban in rat models of venous, 'mixed' and arterial thrombosis, and its effects on the tail transection bleeding time

1. The antithrombotic action of argatroban, a synthetic thrombin inhibitor, was studied in three models of thrombosis in the rat, and in the tail transection bleeding time test. Heparin was studied as a reference anticoagulant. 2. In the model of venous thrombosis induced by thromboplastin followed by stasis of the abdominal vena cava, argatroban had an ED50 of 125 micrograms kg-1, when administered as an i.v. bolus 5 min prior to the thromboplastin injection: the ED50 of heparin was 42 micrograms kg-1, where ED50 is the dose which reduces the weight of the thrombus by 50% compared with that of the control animals. When the two compounds were administered by continuous i.v. infusion, argatroban (ED50 = 1.5 micrograms kg-1 min-1) had the same potency as heparin (ED50 = 1.2 micrograms kg-1 min-1). 3. Argatroban was active in the arterio-venous shunt model with an ED50 of 0.6 mg kg-1 when the compound was given as a bolus. The ED50 of heparin was 0.04 mg kg-1 under the same conditions. The two compounds had ED50 values of 6 micrograms kg-1 min-1 (argatroban) and 3 micrograms kg-1 min-1 (heparin), when administered by continuous i.v. infusion. 4. When tested against occlusive arterial thrombus formation by electrical stimulation of the left carotid artery, both compounds given as either an i.v. bolus or a continuous infusion led to dose-dependent increases in the duration of post-lesion vessel patency. Heparin bolus was more active than argatroban on a weight basis, in that 2 mg kg-1 gave a similar increase in the time to occlusion as 8 mg kg-1 argatroban. As in the other models, when given as continuous infusions, argatroban (111% increase in time to occlusion at 20 tg kg-1, min-1) had similar activity to that of heparin (180% increase at 25 jg kg-1 min-1) on a weight basis. Hoever, the antithrombotic effects of argatroban were accompanied by only moderate changes in the coagulation parameters (thrombin time and activated partial thromboplastin time, APTT), whereas, even at a subthreshold dose of heparin (12.5 pg kg-1 min-1), both the thrombin time and the APTT were greater than 150 s.5. Infusions of both compounds caused dose-dependent increases in the tail transection bleeding time,with the dose of argatroban that doubles the bleeding time (11 I g kg-1 min-1) being five times greater than that of heparin (EDI, = 2.2 fig kg-1 min-1).6. These data show that, when administered as an intravenous infusion, argatroban is a potent antithrombotic agent in rat models of venous 'mixed' and arterial thrombosis, this effect can be obtained with a lower degree of systemic anticoagulation than with heparin in the arterial model, and argatroban has a lower haemorrhagic potential than that of heparin.

Inhibition of factor Xa-induced platelet aggregation by a selective thrombin inhibitor, argatroban

In the present study, a direct selective thrombin inhibitor, argatroban, and an indirect non-selective inhibitor, heparin, were examined for the inhibitory effect on factor Xa-induced platelet aggregation. Platelet aggregation was induced by thrombin or factor Xa+prothrombin in the presence of Ca++ using rabbit gel-filtered platelets. IC50 of argatroban on factor Xa-induced aggregation was 5-7 times higher than that on thrombin-induced aggregation, while IC50 of heparin in the presence of antithrombin III on factor Xa-induced aggregation was 90-200 times higher than that on thrombin-induced aggregation. This finding suggests that argatroban inhibits thrombin generating on the platelet surface more efficiently than heparin, and this may be one of the reasons for the higher efficacy of argatroban in arterial thrombosis as compared with heparin.

Comparison of hirudin and heparin as adjuncts to streptokinase thrombolysis in a canine model of coronary thrombosis

Recombinant desulfatohirudin (HI), a potent and specific thrombin inhibitor, was compared with heparin (HE) as an adjunct to streptokinase thrombolysis. In pentobarbital-anesthetized dogs, an occlusive thrombus (whole blood+thrombin) was introduced into the left anterior descending coronary artery (LAD) with superimposed endothelial damage and distal high-grade stenosis. Intravenous infusion of saline (vehicle), HI (0.3 mg/kg followed by 0.3 mg/kg per hour, 1 mg/kg followed by 1 mg/kg per hour, or 2 mg/kg followed by 2 mg/kg per hour), or HE (60 units/kg followed by 40 units/kg per hour or 100 units/kg followed by 60 units/kg per hour) was initiated 15 minutes before streptokinase (750,000 units for 60 minutes) administration. Vessel patency was monitored for 180 minutes after streptokinase administration with a volume flow probe on the proximal LAD. In dogs treated with no adjunctive agent (saline control), none of the vessels were recanalized with streptokinase. Both HI and HE promoted reperfusion, inhibited reocclusion, and reduced the residual thrombus mass in a dose-dependent fashion. However, at comparable levels of therapeutic anticoagulation (activated partial thromboplastin time [APTT] = 1.5-2.0 times baseline) HI exhibited a higher incidence of reperfusion (eight of eight dogs [100%] versus one of eight dogs [12%]), a shorter time to reperfusion (33 +/- 6 versus 59 minutes), a longer duration of initial reperfusion (106 +/- 21 versus 10 minutes), and a smaller residual thrombus mass than did HE. Likewise, the slope of the relation between the APTT prolongation and the total reperfusion time ("anticoagulation/antithrombosis profile") was almost five times higher for the combined HI data than for the HE data. Our results indicate that HI is more effective than HE in enhancing and sustaining coronary recanalization with streptokinase at a HI dose that modestly prolongs coagulation time and does not alter bleeding times.

Inhibition by fibrin coagulation of lung cancer cell destruction by human interleukin-2-activated killer cells

We examined the effect of fibrin coagulation on tumor cytotoxicity mediated by human lymphokine (IL-2)-activated killer (LAK) cells. LAK cells were induced from peripheral blood mononuclear cells (MNC) by culture with recombinant IL-2 for 4 or 5 days, and LAK cell-mediated cytotoxicity against tumor cells was assessed by 51Cr release assay in the presence or absence of plasma from normal subjects and lung cancer patients. Plasma did not affect the phase of induction of LAK activity by IL-2, but dose-dependently inhibited the effector phase of LAK cell-mediated cytotoxicity against Daudi cells. Similar inhibition of LAK cell-mediated cytotoxicity was observed on pretreatment of Daudi cells and human lung cancer cell lines with human fibrinogen plus thrombin. A parallel relationship was found between the amount of fibrinogen in plasma of lung cancer patients and inhibition of LAK cytotoxicity. This inhibition was reduced by addition of anticoagulants (heparin or argatroban). These findings suggest that fibrin coagulation on tumor cells protects them from LAK cell-mediated tumor cytotoxicity in malignant lesions and that a combination of an anticoagulant drug and IL-2/LAK therapy may be effective for treatment of lung cancer patients.

Heparin and the thrombin inhibitor argatroban enhance fibrinolysis by infused or bolus-injected saruplase (r-scu-PA) in rabbit femoral artery thrombosis

Enhancement by anticoagulation of thrombolysis with infused or bolus-injected saruplase (r-scu-PA) has been studied using heparin and the thrombin inhibitor argatroban. In a rabbit femoral artery thrombosis model infusion of saruplase (3 - 12 mg/kg, 60 min) caused a dose-dependent thrombolysis. Reperfusion rate after infusion of 3 mg/kg saruplase alone was 3/6, reperfusion time 42 +/- 3 min and reocclusion rate 2/3; final patency rate at 120 min was 17%. Combination of 3 mg/kg saruplase with heparin (150 U/kg + 100 U/kg.hr i.v.; 5.3-fold PTT-prolongation) resulted in a reperfusion rate of 6/6 after a reperfusion time of 39 +/- 7 min; reocclusion rate was 3/6 and final patency rate was 50%. Argatroban (1 mg/kg + 3 mg/kg.hr i.v.; 2.3-fold PTT prolongation) in combination with saruplase resulted in a reperfusion rate of 6/6 after 26 +/- 5 min; no reocclusion occured and final patency rate was 100% (p less than 0.05 vs saruplase alone). Bolus injection of 6 mg/kg saruplase achieved reperfusion in 5/6 arteries after 15 +/- 3 min, but reocclusion rate was 4/5; final patency rate was 17%. Combination of bolus-injected saruplase with heparin resulted in a reperfusion rate of 4/6 after 8 +/- 3 min and no reocclusion occured; patency rate was 67%. With combination of argatroban and bolus-injected saruplase 6/6 arteries were reperfused after 8 +/- 3 min; reocclusion was prevented and final patency rate was 100% (p less than 0.05 vs saruplase-bolus alone). Systemic fibrinogenolysis was more pronounced with bolus injection than infusion of saruplase. The results indicate that arterial thrombolysis with saruplase can be enhanced by heparin and the thrombin inhibitor argatroban. The bolus injection of saruplase resulted in persistent reperfusion when simultaneous anticoagulation was performed. Despite less PTT prolongation, enhancement of saruplase-induced thrombolysis was more effective with argatroban than with heparin in rabbit femoral artery thrombosis.

Prevention of arterial reocclusion after thrombolysis with recombinant lipoprotein-associated coagulation inhibitor

BACKGROUND

This study was designed to determine whether arterial reocclusion after thrombolysis can be prevented by lipoprotein-associated coagulation inhibitor (LACI), a physiological inhibitor of tissue factor-induced coagulation mediated by the extrinsic pathway.

METHODS AND RESULTS

Thrombosis was induced in femoral arteries of anesthetized dogs with the use of anodal current to elicit extensive vascular injury and formation of platelet-rich thrombi in one artery and with thrombogenic copper wire to elicit fibrin-rich thrombi without appreciable vascular injury in the contralateral artery. Recanalization of both vessels was induced with t-PA (1.7 mg/kg i.v. over 1 hour) and verified with Doppler flow probes. Reocclusion occurred within 2 hours in seven of seven arteries with electrical injury-induced thrombosis and in four of seven arteries with copper wire-induced thrombosis in the absence of LACI. In dogs given infusions of recombinant DNA-produced LACI (225 micrograms/kg over 15 minutes, followed by 4 micrograms/kg/min i.v.) after completion of the infusion of t-PA, no reocclusion occurred during the 2-hour interval of observation in any of the five arteries subjected to electrical injury (p less than 0.001), and cyclic partial occlusions were nearly abolished (0.4 +/- 0.4/hr in LACI-treated dogs compared with 13.7 +/- 5.5/hr in saline-treated dogs, p less than 0.0001). In contrast, reocclusion occurred in two of five arteries with indwelling copper wires, and cyclic partial occlusions were unaffected despite LACI. LACI prolonged the partial thromboplastin time modestly (1.7 +/- 0.2 x baseline) but did not affect platelet counts or aggregation assessed ex vivo.

CONCLUSIONS

Inhibition of the extrinsic pathway of coagulation with LACI prevents thrombotic arterial reocclusion after thrombolysis in vessels subjected to extensive vascular injury. Our results demonstrate that activation of the extrinsic pathway plays a critical role in thrombotic reocclusion and that LACI provides a highly targeted approach to facilitate sustained recanalization without directly inhibiting platelets.

Combined administration of aspirin and a specific thrombin inhibitor in man

BACKGROUND

Heparin is of limited value as an antithrombotic drug in the presence of platelet activation and residual thrombus. Greater anticoagulant activity can be achieved in vivo with more specific thrombin inhibitors. Heparin may also increase the risk of bleeding by an effect on platelets that is independent of its thrombin inhibitory activity.

METHODS AND RESULTS

The pharmacodynamic and pharmacokinetic effects of a novel thrombin inhibitor, argatroban, were examined alone and in combination with aspirin in normal male volunteers. Argatroban induced a dose-dependent prolongation of the thrombin time and the activated partial thromboplastin time (aPTT). aPTT had returned to its pretreatment value 1 hour after stopping the infusion of argatroban. Six male subjects received an infusion of 1 micrograms/kg/min argatroban after the administration of two doses of 162.5 mg aspirin or a matching placebo. At this dose, aspirin decreased serum thromboxane B2 by a mean of 99% and prolonged the bleeding time (230 +/- 52 versus 320 +/- 113 seconds, p less than 0.01). Argatroban given alone increased thrombin time by 454 +/- 18% and aPTT by 160 +/- 3%. Steady-state plasma concentrations were achieved at 1 hour and declined exponentially with an elimination half-life of 24 +/- 4 minutes. Neither the anticoagulant effects nor the plasma concentrations of argatroban were altered by aspirin. Furthermore, argatroban did not increase the bleeding time when given alone and did not further prolong the bleeding time when combined with aspirin.

CONCLUSION

The combination of aspirin and argatroban may prove to be an effective therapeutic strategy in the prevention of coronary thrombosis.

Relative efficacy of antithrombin compared with antiplatelet agents in accelerating coronary thrombolysis and preventing early reocclusion

BACKGROUND

Optimal coronary thrombolysis should be prompt and persistent. Although activation of platelets and increased thrombin activity have been associated with clinical thrombolysis, the role of each in delaying thrombolysis or inducing early coronary reocclusion has been difficult to define.

METHODS AND RESULTS

In conscious dogs with coronary thrombosis induced by electrical current, we assessed the impact on the rapidity of thrombolysis and the incidence of reocclusion of two types of adjunctive treatment given concomitantly with intravenous tissue-type plasminogen activator (t-PA): 1) inhibition of platelet function with a peptide mimetic antagonist of platelet glycoprotein IIb/IIIa receptors or with lysine acetylsalicylic acid (ASA) and 2) inhibition of thrombin activity with recombinant hirudin or with heparin. ASA but not the receptor antagonist shortened the time to thrombolysis with t-PA (20 +/- 13 [mean +/- SD] minutes with ASA, 36 +/- 15 minutes with receptor antagonist, and 43 +/- 16 minutes with the saline control). Reocclusion occurred promptly after completion of the infusion of t-PA in all seven dogs given saline. Reocclusion was delayed and prevented in some dogs within 90 minutes after the end of the infusion of t-PA by both antiplatelet agents but still occurred in 42% despite continued inhibition of platelet function (i.e., three of six dogs given ASA and two of six given receptor antagonist). In contrast, inhibition of thrombin activity with recombinant hirudin in a dose that prolonged the partial thromboplastin time modestly (1.5-2-fold) resulted in accelerated lysis (19 +/- 10 minutes) and prevention of reocclusion in each of six dogs. Heparin given in doses that elicited similar prolongation of the partial thromboplastin time did not accelerate lysis nor prevent reocclusion, which occurred in five of six dogs.

CONCLUSIONS

Inhibition of thrombin by recombinant hirudin facilitates thrombolysis and maintains patency of coronary arteries recanalized with t-PA particularly effectively. The benefit conferred may reflect direct anticoagulant effects plus diminished activation of platelets secondary to decreased thrombin activity.

Role of new anticoagulants as adjunctive therapy during thrombolysis

Procoagulant activity may persist during coronary thrombolysis and result in either delay in the time to recanalization or recurrent thrombosis. Although heparin and aspirin form the mainstay of current therapy, recurrent thrombosis occurs despite adjunctive heparin therapy during thrombolysis. Newer agents that inhibit thrombin by antithrombin III-independent mechanisms, or that inhibit earlier steps in the coagulation cascade, have been shown to be effective in the experimental preparation of coronary thrombolysis. Because heparin-antithrombin III is a relatively inefficient inhibitor of thrombin bound to fibrin, agents such as hirudin or small peptide inhibitors of the thrombin-active site appear to be more effective inhibitors of clot-associated thrombin activity. Inhibition of early steps in the coagulation cascade with the inhibitor of tissue factor-factor VIIa complex, or with activated protein C, also appears to be an effective anticoagulant strategy. In experimental preparations all of these agents have shown superiority in preventing recurrent thrombosis compared with heparin, and in some cases they appear to accelerate the rate of clot lysis.

In vivo thrombin inhibition enhances and sustains arterial recanalization with recombinant tissue-type plasminogen activator

The effects of heparin and the synthetic competitive thrombin inhibitor (2R,4R)-4-methyl-1-[N2-(3-methyl-1,2,3,4-tetrahydro-8-quinolinesulfon yl)-L-arginyl]-2-piperidinecarboxylic acid monohydrate (Argatroban) on thrombolysis with recombinant tissue-type plasminogen activator (rt-PA) was studied in groups of six or seven rabbits with arterial thrombosis. The model consisted of a whole-blood clot produced in a 1-cm isolated femoral arterial segment with superimposed endothelial damage and distal high-grade stenosis. rt-PA was injected as an intravenous bolus of 0.45 mg/kg body wt at 15-minute intervals until recanalization, or up to a maximum of four boluses. In seven rabbits given an intravenous injection of 17 mg/kg aspirin, rt-PA induced transient reflow in only one animal. In seven rabbits that received intravenous heparin (200 units/kg over 60 minutes), rt-PA administration produced reflow in five animals, which was persistent in two rabbits. Combined administration of aspirin and heparin in seven rabbits was associated with similar rt-PA-induced recanalization. rt-PA administration in six rabbits given intravenous Argatroban (100 micrograms/kg/min for 60 minutes) caused recanalization in five, with persistent patency in three. In six rabbits given aspirin and Argatroban, rt-PA caused recanalization in all, with persistent patency in five animals. Reflow occurred significantly more rapidly with Argatroban (14 +/- 7 minutes) than with heparin (35 +/- 11 minutes), reflow was obtained with fewer boluses of rt-PA in combination with Argatroban (median value of one bolus) than with heparin (median value, three boluses), and reocclusion after reflow was less frequent with Argatroban (0 of 11 versus 5 of 10 rabbits). Furthermore, the degree of thrombolysis determined by pathological analysis was significantly more extensive with Argatroban than with heparin, and patency persisted during a 3-hour observation period, despite elimination of Argatroban from the circulation. Thus, Argatroban, relative to heparin, enhances and sustains thrombolysis with rt-PA. It may offer promise as an adjunctive agent for thrombolytic therapy of arterial thrombosis.

Effect of a synthetic thrombin-inhibitor MD805 on the reaction between thrombin and plasma antithrombin-III

MD805, a synthetic thrombin-inhibitor, effectively retarded the time-dependent inactivation of thrombin which was generated endogeneously or added exogeneously in human plasma. The kinetical study of the time-dependent inactivation indicated that the type of inhibition was competitive and the obtained Ki of MD805 for thrombin was 3 x 10(-8)M. MD805 also inhibited the formation of thrombin-ATIII complex. These results indicated that the active site of thrombin was involved in the reaction between thrombin and ATIII, and that MD805 competed with ATIII for thrombin in exactly the same manner as it competed with fibrinogen or synthetic peptide substrates. As a result, MD805 would serve as a protective agent for ATIII from being consumed, in addition to its potent thrombin-inhibitory activity without the aid of ATIII. By contrast, heparin accelerated the time-dependent inactivation rate of thrombin and the formation of thrombin-ATIII complex, which indicates that heparin accelerates the consumption of ATIII.

Enhanced thrombolysis by a factor XIIIa inhibitor in a rabbit model of femoral artery thrombosis

Factor XIIIa (FXIIIa) catalyzes covalent crosslinking reactions of fibrin, affording the clot additional structural stability and resistance to plasmin-mediated degradation. Thus, inhibition of FXIIIa may render thrombi more susceptible to tissue-plasminogen activator (t-PA)-induced thrombolysis in vivo. We therefore examined thrombus weight and time to lysis in anesthetized rabbits undergoing arterial thrombosis produced by insertion of a copper coil into the lumen of the right femoral artery. The effects of t-PA alone (started 30 min after coil insertion) or in combination with a FXIIIa inhibitor (L722151) started 15 min before, 8 min after or 20 min after coil insertion were determined. Although t-PA alone (2 mg/kg over 2 hrs) lowered thrombus weight significantly, there was no evidence of flow restoration. Addition of L722151 to t-PA before, or 8 min after coil insertion, further lowered thrombus weights and produced thrombolysis in 50% of the animals. This beneficial effect was lost when L722151 administration was delayed until 20 min after thrombus formation, suggesting that the type(s) of crosslinking inhibited by L722151 was complete by this time. Infusion of L722151 alone had no significant effect on thrombus weight. These results demonstrate a time-dependent facilitation of t-PA-induced arterial thrombolysis by FXIIIa inhibition in a small animal model.

Induction of marked thrombin activity by pharmacologic concentrations of plasminogen activators in nonanticoagulated whole blood

Thrombin activity reflected by increased plasma concentrations in vivo of fibrinopeptide A (FPA) increases when streptokinase (SK) is administered to patients with acute myocardial infarction. Although procoagulant effects have been found in vitro, the use of anticoagulated plasma limits the extent to which the phenomena observed can be viewed to implicate procoagulant effects in vivo. Accordingly, we characterized the procoagulant effects of SK and tissue plasminogen activator (t-PA) in nonanticoagulated whole blood. Blood was collected from normal volunteers by venipuncture (No. 19 gauge steel needle) directly into polypropylene tubes containing either t-PA, SK, SK and heparin, t-PA and heparin, or saline. The concentration of FPA after 10 min of incubation with saline was 150 +/- 46 nM (n = 14)(SE). In contrast, in blood incubated with SK FPA was consistently and markedly increased after 10 min: 2318 +/- 416 nM (100 IU/ml SK) and 10,889 +/- 1156 nM (1000 IU/ml SK) (p less than 0.001 compared with control). Less marked elevations of FPA occurred after 10 min in blood incubated with t-PA (3171 +/- 604 nM with 2500 ng/ml t-PA, p less than 0.01 compared with 1000 IU/ml SK). Increases in FPA were less than 100 nM in blood incubated with activators and heparin. The extent to which plasminogen was activated, as measured by the release of the B beta 1-42 fibrinopeptide, was related to the magnitude of elevation of FPA. Procoagulant activity induced by extensive plasminogen activation may contribute to undesirable effects in vivo, such as a propensity to recurrent thrombosis or delayed fibrinolysis.

Differential effects of activation of prothrombin by streptokinase compared with urokinase and tissue-type plasminogen activator (t-PA)

We have previously reported paradoxical prothrombotic effects manifest by elevations of fibrinopeptide A (FPA) after administration of streptokinase to patients with acute myocardial infarction. To characterize mechanisms responsible and their dependence on streptokinase (SK) as opposed to other activators of the fibrinolytic system, the present study was performed to compare effects of streptokinase, tissue-type plasminogen activator (t-PA), and urokinase on plasma and on purified prothrombin in concentrations similar to those achieved pharmacologically. The effects of plasmin were assessed to determine the extent to which the elevations of FPA seen could be attributed to activation of plasminogen. Elevations of FPA were observed after incubation of each of the activators with citrated plasma at 37 degrees C for 60 minutes. However, they were most marked with streptokinase (64.5 +/- 4.6 pmol FPA/ml (mean +/- SE) with 100 IU SK/ml, and 77.6 +/- 5.0 pmol/ml with 500 IU SK/ml). Elevations of FPA induced by streptokinase were attenuated by 100 IU/ml heparin [15.2 +/- 1.9 pmol/ml after 100 IU of SK (p less than 0.001 compared with results with streptokinase without heparin)]. Human plasmin, 2.5 CTA/ml, caused changes similar to those induced by streptokinase. The minimal elevations of FPA induced by t-PA or urokinase (less than 10 pmol/ml without heparin) were not significantly attenuated by heparin. Incubation of barium citrate adsorbed plasma (vitamin K factor depleted) with streptokinase markedly attenuated elevation of FPA. Addition of prothrombin (1.5 microM) and streptokinase (100 IU/ml) to the barium citrate adsorbed plasma elicited elevations of FPA similar to those induced by streptokinase in citrated plasma. Amidolytic activity with the "thrombin" substrate H-D-phenylalanyl-L-pipecolyl-L-arginine-p-nitroanilide dihydrochloride (S-2238) was evident when streptokinase, plasminogen (0.24 microM), and prothrombin (1.5 microM) were incubated in buffer. Thus, concentrations of streptokinase that are low in terms of therapeutic blood levels activate prothrombin in plasma, likely due to activation of plasminogen. Neither tissue-type plasminogen activator nor urokinase in pharmacologically comparable concentrations increase thrombin activity appreciably perhaps because of less intense activation of plasminogen. Consideration of the prothrombotic effects observed may be relevant to selection of specific agents for therapeutic thrombolysis, to appropriate titration of dose, and to the need for the use of heparin conjointly with particular activators of the fibrinolytic system such as streptokinase.

Paradoxic elevation of fibrinopeptide A after streptokinase: evidence for continued thrombosis despite intense fibrinolysis

Elevated levels of fibrinopeptide A, a marker of thrombin activity associated with acute myocardial infarction, have been found to decrease after administration of streptokinase when reperfusion occurs. In contrast, in patients without reperfusion and those with reocclusion after streptokinase therapy, fibrinopeptide A remains elevated. In the present study early serial measurements of fibrinopeptide A were used to further characterize this paradoxic increase in thrombin activity after streptokinase and to characterize its response to heparin. In 19 patients with acute myocardial infarction fibrinopeptide A was elevated to 82.3 +/- 43.5 ng/ml (mean +/- SE) before therapy. Thirty minutes after the initiation of streptokinase, fibrinopeptide A increased to 300.1 +/- 117.4 ng/ml (p less than 0.01), consistent with extensive thrombin activity. Fibrinopeptide A remained elevated until 15 minutes after a heparin bolus injection when levels decreased to 15% of the poststreptokinase value (49.2 +/- 13.3 ng/ml) (p less than 0.001). These data document a prompt paradoxic increase in thrombin activity after administration of streptokinase that may be responsible for failure of therapy in some patients.