Active site-blocked factor IXa prevents intravascular thrombus formation in the coronary vasculature without inhibiting extravascular coagulation in a canine thrombosis model

Factor IX(a) inhibitors: an updated patent review (2003-present)

INTRODUCTION

Anticoagulation with no bleeding complications is the current objective of drug discovery programs in the area of treating and/or preventing thromboembolism. Despite the promises of therapeutics targeting factors XI(a) and XII(a), none has been approved thus far. Clinically used thrombin- and/or factor Xa-based anticoagulants continue to be associated with a significant bleeding risk which limits their safe use in a broad range of thrombotic patients. Research findings in animals and humans indicate that it is possible to target factor IX(a) (FIX(a)) to achieve anticoagulation with a limited risk of bleeding.

AREAS COVERED

A review of patents literature has retrieved >35 patents on the development of molecules targeting FIX(a) since 2003. Small molecules, antibodies, and aptamers have been developed to target FIX(a) to potentially promote effective and safer anticoagulation. Most of these agents are in the pre-clinical development phase and few have been tested in clinical trials.

EXPERT OPINION

FIX(a) system is being considered to develop new anticoagulants with fewer bleeding complications. Our survey indicates that the number of FIX(a)-targeting agents is mediocre. The agents under development are diverse. Although additional development is essential, moving one or more of these agents to the clinic will facilitate achieving better clinical outcomes.

Review of the Target-Specific Oral Anticoagulants in Development for the Treatment and Prevention of Venous Thromboembolism

Anticoagulation therapy is often indicated for the treatment and prevention of venous thromboembolism (VTE). Despite advances in anticoagulant management with parenteral anticoagulants and vitamin K antagonists, limitations to their use still exists, leading to investigation of alternative anticoagulants such as factor Xa inhibitors and direct thrombin inhibitors. To date, 3 target-specific oral anticoagulants (TSOACs) are Food and Drug Administration approved; several other agents are currently in development to optimize VTE management and minimize bleeding risks. The objective of this systematic review article is to provide clinicians an overview of the clinical evidence on the investigational TSOACs for the treatment and prevention of VTE. Of the agents in development, edoxaban holds the most promise due to robust data supporting its clinical benefit with a similar bleeding risk to currently approved agents. Clinicians should understand the TSOACs under investigation, since differences in pharmacokinetics and pharmacodynamics may influence clinical decision making and agent selection for management of VTE. Currently, no direct comparisons between TSOACs have been conducted. Agents under investigation have yet to overcome the major limitations of the currently existing TSOACs. Further studies are necessary to clarify which TSOAC agent is best for management of VTE in clinical practice.

The Future of Anticoagulation

Anticoagulant agents, such as unfractionated heparin and warfarin, have been in use for roughly 50 years. Over the past decade, injectable agents such as low-molecular-weight heparins, pentasaccharide, and direct thrombin inhibitors have been major advances in preventing and treating thrombosis. Despite these somewhat recent additions, there is still enormous potential to improve on the pharmacokinetic and pharmacodynamic properties of these agents, as well as improve patient outcomes. There are currently a large number of anticoagulant agents (injectable and oral) that could be available for use in the next several years. Many of these new agents have unique mechanisms that may provide practitioners with anticoagulant alternatives. This review gives a detailed analysis of the anticoagulant agents that may add to our armamentarium in the management of thrombosis.

Translation and Clinical Development of Antithrombotic Aptamers

Thrombosis is a necessary physiological process to protect the body from uncontrolled bleeding. Pathological thrombus formation can lead to devastating clinical events including heart attack, stroke, deep vein thrombosis, pulmonary embolism, and disseminated intravascular coagulation. Numerous drugs have been developed to inhibit thrombosis. These have been targeted to coagulation factors along with proteins and receptors that activate platelets. While these drugs are effective at preventing blood clotting, their major side effect is inadvertent hemorrhage that can result in significant morbidity and mortality. There exists a need for anticoagulants that are not only effective at preventing thrombosis but can also be readily reversed. Aptamers offer a potential solution, representing a new class of drug agents that can be isolated to any protein and where antidote oligonucleotides can be designed based on the sequence of the aptamer. We present a summary of the anticoagulant and antithrombotic aptamers that have been identified and their stage of development and comment on the future of aptamer-based drug development to treat thrombosis.

Effect of the REG1 anticoagulation system versus bivalirudin on outcomes after percutaneous coronary intervention (REGULATE-PCI): a randomised clinical trial

BACKGROUND

REG1 is a novel anticoagulation system consisting of pegnivacogin, an RNA aptamer inhibitor of coagulation factor IXa, and anivamersen, a complementary sequence reversal oligonucleotide. We tested the hypothesis that near complete inhibition of factor IXa with pegnivacogin during percutaneous coronary intervention, followed by partial reversal with anivamersen, would reduce ischaemic events compared with bivalirudin, without increasing bleeding.

METHODS

We did a randomised, open-label, active-controlled, multicentre, superiority trial to compare REG1 with bivalirudin at 225 hospitals in North America and Europe. We planned to randomly allocate 13,200 patients undergoing percutaneous coronary intervention in a 1:1 ratio to either REG1 (pegnivacogin 1 mg/kg bolus [>99% factor IXa inhibition] followed by 80% reversal with anivamersen after percutaneous coronary intervention) or bivalirudin. Exclusion criteria included ST segment elevation myocardial infarction within 48 h. The primary efficacy endpoint was the composite of all-cause death, myocardial infarction, stroke, and unplanned target lesion revascularisation by day 3 after randomisation. The principal safety endpoint was major bleeding. Analysis was by intention to treat. This trial is registered at ClinicalTrials.gov, identifier NCT01848106. The trial was terminated early after enrolment of 3232 patients due to severe allergic reactions.

FINDINGS

1616 patients were allocated REG1 and 1616 were assigned bivalirudin, of whom 1605 and 1601 patients, respectively, received the assigned treatment. Severe allergic reactions were reported in ten (1%) of 1605 patients receiving REG1 versus one (<1%) of 1601 patients treated with bivalirudin. The composite primary endpoint did not differ between groups, with 108 (7%) of 1616 patients assigned REG1 and 103 (6%) of 1616 allocated bivalirudin reporting a primary endpoint event (odds ratio [OR] 1·05, 95% CI 0·80-1·39; p=0·72). Major bleeding was similar between treatment groups (seven [<1%] of 1605 receiving REG1 vs two [<1%] of 1601 treated with bivalirudin; OR 3·49, 95% CI 0·73-16·82; p=0·10), but major or minor bleeding was increased with REG1 (104 [6%] vs 65 [4%]; 1·64, 1·19-2·25; p=0·002).

INTERPRETATION

The reversible factor IXa inhibitor REG1, as currently formulated, is associated with severe allergic reactions. Although statistical power was limited because of early termination, there was no evidence that REG1 reduced ischaemic events or bleeding compared with bivalirudin.

FUNDING

Regado Biosciences Inc.

Factor IXa as a target for anticoagulation in thrombotic disorders and conditions

From acute coronary syndrome (ACS) to the prevention of cardioembolic events in patients with atrial fibrillation and thrombosis of mechanical heart valves, there is a quest to develop a new generation of anticoagulants. Perhaps the 'holy grail' of antithrombotic therapy is not only a drug that will prevent coagulation without promoting bleeding but also an anticoagulant that is easily reversible should the clinical need arise. Further, an optimally designed anticoagulant would have broad applications to include arterial, venous, hybrid conditions (atrial flutter and fibrillation) and nonbiological materials. Factor (F)IXa plays a pivotal part in tissue factor (TF)-mediated thrombin generation, and therefore represents a potentially promising target for drug development. FIXa activity has been targeted by multiple modalities, including oral inhibitors, RNA aptamers, monoclonal antibodies and synthetic active-site-blocking competitive inhibitors. Herein, we summarize the biochemistry of FIXa as it applies to thrombotic disorders and conditions, as well as the evolution of targeted therapies.

Anticoagulant drugs: an update

Thromboembolic disorders continue to be a major cause of morbidity and mortality, resulting in an increased need for anticoagulant therapy. In recent years, new anticoagulant drugs have been developed at a rapid pace, prompted by the recognition of many undesirable properties of currently used agents, and by a greater knowledge of the active enzymatic sites of clotting factors. Furthermore, the structure of a thrombus is better understood, so that newer drugs can inhibit thrombin or Factor Xa not only on the surface of a thrombus, as in the case of heparin, but also the fibrin-bound thrombin or Factor Xa within the thrombus. These agents are usually small molecules synthesized on the basis of their inhibition of specific active sites in the respective coagulation factors. They possess many improved characteristics, such as greater efficacy and safety, oral administration, reliable pharmacokinetics, less need for laboratory monitoring and minimal interactions with other drugs and diet. Prominent among these are lepuridin (Refludan, Pfizer), fondaparinux (Arixtra, Sanofi-Synthelabo) and ximelagatran (Exanta, Astra Zeneca). However, these new drugs are still far from fulfilling the desired objectives. Most of them possess some but not all of the needed properties. Furthermore, many do not have specific antidotes for immediate reversal of their pharmacologic actions, and all are much more costly than conventional agents. Development of newer agents with properties closer to that of the ideal drug remains a challenge.

Low molecular weight heparin inhibits plasma thrombin generation via direct targeting of factor IXa: contribution of the serpin-independent mechanism

BACKGROUND

Although heparin possesses multiple mechanisms of action, enhanced factor Xa inhibition by antithrombin is accepted as the predominant therapeutic mechanism. The contribution of FIXa inhibition to heparin activity in human plasma remains incompletely defined.

OBJECTIVES

To determine the relevance of FIXa as a therapeutic target for heparins, particularly serpin-independent inhibition of intrinsic tenase (FIXa-FVIIIa) activity.

PATIENTS/METHODS

Thrombin generation was detected by fluorogenic substrate cleavage. The inhibitory potencies (EC(50) s) of low molecular weight heparin (LMWH), super-sulfated LMWH (ssLMWH), fondaparinux and unfractionated heparin (UFH) were determined by plotting concentration vs. relative velocity index (ratio ± heparin). Inhibition was compared under FIX-dependent and FIX-independent conditions (0.2 or 4 pm tissue factor [TF], respectively) in normal plasma, and in mock-depleted or antithrombin/FIX-depleted plasma supplemented with recombinant FIX.

RESULTS

UFH and fondaparinux demonstrated similar potency under FIX-dependent and FIX-independent conditions, whereas LMWH (2.9-fold) and ssLMWH (5.1-fold) demonstrated increased potency with limiting TF. UFH (62-fold) and fondaparinux (42-fold) demonstrated markedly increased EC(50) values in antithrombin-depleted plasma, whereas LMWH (9.4-fold) and ssLMWH (two-fold) were less affected, with an EC(50) within the therapeutic range for LMWH. The molecular target for LMWH/ssLMWH was confirmed by supplementing FIX/antithrombin-depleted plasma with 90 nm recombinant FIX possessing mutations in the heparin-binding exosite. Mutated FIX demonstrated resistance to inhibition of thrombin generation by LMWH and ssLMWH that paralleled the effect of these mutations on intrinsic tenase inhibition.

CONCLUSIONS

Therapeutic LMWH concentrations inhibit plasma thrombin generation via antithrombin-independent interaction with the FIXa heparin-binding exosite.

The REG1 anticoagulation system: a novel actively controlled factor IX inhibitor using RNA aptamer technology for treatment of acute coronary syndrome

Antithrombotic agents including anticoagulants and antiplatelets are the cornerstone of treatment of acute coronary syndromes. Currently available anticoagulants have several important limitations including unpredictable pharmacodynamics, immunogenicity, and difficulty in reversibility. A potent anticoagulant that has predictable efficacy, is easily reversible should the clinical need arise, and reduces ischemic events without an increase in bleeding risk would overcome many of the current limitations. Inhibition of factor IX in the coagulation cascade has shown promise as a target for development of a novel anticoagulant with a favorable bleeding risk. Aptamers are small oligonucleotides that can be developed to inhibit specific protein targets with high affinity and used as active drugs. Because aptamers are made of oligonucleotide sequences, they provide the code for their own complement (reversal agent) that can be developed and used to inhibit their function. The REG1 anticoagulation system is a novel, aptamer-based, factor IXa inhibitor that is being developed for use in patients undergoing percutaneous coronary intervention and the treatment of acute coronary syndrome.

Tirofiban versus abciximab: tirofiban is administered at suboptimal dosages when evaluated in an arterial thrombosis model in non-human primates

To prevent thrombosis in high-risk acute coronary syndrome patients undergoing percutaneous coronary intervention for re-vascularisation, concomitant administration of a glycoprotein IIb/IIIa inhibitor, such as abciximab, tirofiban or eptifibatide, is recommended. Abciximab and eptifibatide are mostly preferred over tirofiban, which is less effective in preventing ischaemic events. We compared the efficacy and bleeding potential of escalating doses of tirofiban and abciximab in non-human primates. The efficacy of tirofiban and abciximab in inhibiting cyclic flow reductions (CFRs) was tested in a high shear arterial thrombosis model. Bleeding was evaluated with the template bleeding time and an incision bleeding model. Abciximab completely inhibited arterial thrombosis after injection of its therapeutic bolus dose. With tirofiban, a dose three times higher than the recommended therapeutic dose caused weak inhibition characterised by a return of CFRs after re-injury. At nine times the recommended therapeutic dose, complete inhibition was observed, and the efficacy of tirofiban was comparable to abciximab at its therapeutic bolus dose. Blood loss was less than with abciximab at its effective dose. In this model, tirofiban compared favourably with abciximab, although only at a dose of 3-9 times the therapeutic dose, and caused less bleeding than abciximab.

Factor IXa as a target for pharmacologic inhibition in acute coronary syndrome

Anticoagulant therapy, combined with platelet-directed inhibitors, represents a standard-of-care in the management of patients with acute coronary syndrome, particularly those who require percutaneous coronary interventions. While a vast clinical experience, coupled with large clinical trials have collectively provided guidance, an optimal anticoagulant drug and applied strategy, defined as one that reduces thrombotic and hemorrhagic events consistently, with minimal off-target effects and active control of systemic anticoagulation according to patient and clinical-setting specific need, remains at large. An advancing knowledge of coagulation, hemostasis, and thrombosis suggests that factor IXa, a protease that governs thrombin generation in common thrombotic disorders may represent a prime target for pharmacologic inhibition.

Antithrombotic drug candidate ALX-0081 shows superior preclinical efficacy and safety compared with currently marketed antiplatelet drugs

Neutralizing the interaction of the platelet receptor gpIb with VWF is an attractive strategy to treat and prevent thrombotic complications. ALX-0081 is a bivalent Nanobody which specifically targets the gpIb-binding site of VWF and interacts avidly with VWF. Nanobodies are therapeutic proteins derived from naturally occurring heavy-chain-only Abs and combine a small molecular size with a high inherent stability. ALX-0081 exerts potent activity in vitro and in vivo. Perfusion experiments with blood from patients with acute coronary syndrome on standard antithrombotics demonstrated complete inhibition of platelet adhesion after addition of ALX-0081, while in the absence of ALX-0081 residual adhesion was observed. In a baboon efficacy and safety model measuring acute thrombosis and surgical bleeding, ALX-0081 showed a superior therapeutic window compared with marketed antithrombotics. Pharmacokinetic and biodistribution experiments demonstrated target-mediated clearance of ALX-0081, which leads to a self-regulating disposition behavior. In conclusion, these preclinical data demonstrate that ALX-0081 combines a high efficacy with an improved safety profile compared with currently marketed antithrombotics. ALX-0081 has entered clinical development.

First clinical application of an actively reversible direct factor IXa inhibitor as an anticoagulation strategy in patients undergoing percutaneous coronary intervention

BACKGROUND

The ideal anticoagulant should prevent ischemic complications without increasing the risk of bleeding. Controlled anticoagulation is possible with the REG1 system, an RNA aptamer pair comprising the direct factor IXa inhibitor RB006 and its active control agent RB007.

METHODS AND RESULTS

This phase 2a study included a roll-in group (n=2) treated with REG1 plus glycoprotein IIb/IIIa inhibitors followed by 2 groups randomized 5:1 to REG1 or unfractionated heparin. In group 1 (n=12), RB006 was partially reversed with RB007 after percutaneous coronary intervention and fully reversed 4 hours later. In group 2 (n=12), RB006 was fully reversed with RB007 immediately after percutaneous coronary intervention. Femoral sheaths were removed after complete reversal. Patients were pretreated with aspirin and clopidogrel. End points included major bleeding within 48 hours; composite of death, myocardial infarction, or urgent target vessel revascularization within 14 days; and pharmacodynamic measures. All cases were successful, with final Thrombolysis in Myocardial Infarction grade 3 flow and no angiographic thrombotic complications. There were 2 ischemic end points in the REG1 group and 1 in the unfractionated heparin group, with 1 major bleed in the unfractionated heparin group. Median activated clotting time values rose from 151 to 236 seconds after RB006. Administration of the partial RB007 dose reversed anticoagulation to an intermediate activated clotting time value of 186 seconds. Complete reversal with RB007 returned the median activated clotting time value to 144 seconds. Both reversal strategies enabled scheduled femoral sheath removal.

CONCLUSIONS

This study demonstrates the clinical translation of a novel platform of anticoagulation targeting factor IXa and its active reversal to percutaneous coronary intervention and provides the basis for further investigation.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT00715455.

Coagulation factor IXa as a target for treatment and prophylaxis of venous thromboembolism

Venous thromboembolism remains a frequent cause of vascular death. Despite advances in anticoagulant drug development, unmet needs remain, including limited treatment options for patients with severe renal impairment and the inability to fully reverse the effects of anticoagulants approved or in late-stage development. Because coagulation factor IXa plays a pivotal role in tissue factor-mediated thrombin generation, it represents an attractive target for anticoagulant development. This article discusses the rationale for factor IXa as an anticoagulant target and the potential role in venous thromboembolism prevention or management of the 2 factor IXa inhibitors that have undergone testing in phase 1 or 2 trials: TTP889, an oral, small-molecule compound, and RB006, an aptamer-based compound, the intravenous and subcutaneous formulations of which are the anticoagulant components of the REG1 and REG2 anticoagulation systems, respectively.

Fucosylated chondroitin sulfate inhibits plasma thrombin generation via targeting of the factor IXa heparin-binding exosite

Depolymerized holothurian glycosaminoglycan (DHG) is a fucosylated chondroitin sulfate with antithrombin-independent antithrombotic properties. Heparin cofactor II (HCII)-dependent and -independent mechanisms for DHG inhibition of plasma thrombin generation were evaluated. When thrombin generation was initiated with 0.2 pM tissue factor (TF), the half maximal effective concentration (EC(50)) for DHG inhibition was identical in mock- or HCII-depleted plasma, suggesting a serpin-independent mechanism. In the presence of excess TF, the EC(50) for DHG was increased 13- to 27-fold, suggesting inhibition was dependent on intrinsic tenase (factor IXa-factor VIIIa) components. In factor VIII-deficient plasma supplemented with 700 pM factor VIII or VIIIa, and factor IX-deficient plasma supplemented with plasma-derived factor IX or 100 pM factor IXa, the EC(50) for DHG was similar. Thus, cofactor and zymogen activation did not contribute to DHG inhibition of thrombin generation. Factor IX-deficient plasma supplemented with mutant factor IX(a) proteins demonstrated resistance to DHG inhibition of thrombin generation [factor IX(a) R233A > R170A > WT] that inversely correlated with protease-heparin affinity. These results replicate the effect of these mutations with purified intrinsic tenase components, and establish the factor IXa heparin-binding exosite as the relevant molecular target for inhibition by DHG. Glycosaminoglycan-mediated intrinsic tenase inhibition is a novel antithrombotic mechanism with physiologic and therapeutic applications.

Cardiovascular disease in patients with hemophilia

Mortality due to ischemic heart disease in hemophilia patients is lower as compared to the general male population. Differences in the prevalence of cardiovascular risk factors cannot explain this finding. The hypocoagulable state of hemophilia patients might have a protective effect on thrombus formation, which precipitates infarction. It remains unclear whether the deficiency of coagulation factor VIII or IX exerts a protective effect on the development of atherosclerosis. Despite the relative protection against cardiovascular events, the incidence of ischemic cardiovascular disease in hemophilia patients is increasing, because life expectancy of these patients now approaches that of the general population. This review focuses on what is currently known about cardiovascular risk factors, atherosclerosis, arterial thrombosis and ischemic cardiovascular disease in hemophilia patients.

A randomized, repeat-dose, pharmacodynamic and safety study of an antidote-controlled factor IXa inhibitor

BACKGROUND

Active and safe reversibility of anticoagulation is an unmet need in clinical care. Factor IXa, required for rapid thrombin generation on platelet surfaces, is a novel target for modulating coagulation. REG1 comprises RB006 (drug) and RB007 (antidote). RB006, a ribonucleic acid aptamer, exerts its anticoagulant effect by selectively binding FIXa. RB007, the complementary oligonucleotide antidote, binds to RB006 by Watson-Crick base pairing, neutralizing its anti-FIXa activity.

OBJECTIVE

To test the multiple repeat-dose safety, intraindividual pharmacodynamic reproducibility and graded active reversibility of REG1.

METHODS

We randomized 39 healthy volunteers to receive either three consecutive weight-adjusted, drug-antidote treatment cycles, or double placebo. Each treatment cycle included an intravenous bolus of 0.75 mg kg(-1) RB006, followed 60 min later by a descending dose of RB007, ranging from a 2 : 1 to 0.125 : 1 antidote/drug ratio (1.5 mg kg(-1) to 0.094 mg kg(-1) RB007). Serial clinical assessments and coagulation measurements were performed through 14 days postrandomization.

RESULTS

Repeat doses of RB006 achieved highly reproducible activated partial thromboplastin time (APTT) levels with low intrasubject variability (coefficient of variation 5.5%, intraclass correlation coefficient 5.8 at 15 min postdose), while repeat doses of RB007 reversed the APTT levels dose-dependently and reproducibly. There was no major bleeding and there were no other serious adverse events.

CONCLUSIONS

This is the first human study demonstrating multiple repeat-dose safety, intraindividual pharmacodynamic reproducibility and graded active reversibility of an RNA aptamer-oligonucleotide antidote pair. The results lay the foundation for studying the translation of this novel anticoagulation platform to a wide variety of clinical applications.

Partial factor IXa inhibition with TTP889 for prevention of venous thromboembolism: an exploratory study

BACKGROUND

Inhibitors of factor (F) IXa show potent antithrombotic activity with a low risk of bleeding in preclinical models. We investigated the anticoagulant potential of oral TTP889, a small molecule that inhibits up to 90% of FIXa activity at therapeutic doses, using a clinical model of extended prophylaxis in hip fracture surgery (HFS).

METHODS

In this multicenter, randomized, double-blind study, 261 patients received oral TTP889 (300 mg once daily) or placebo starting 6-10 days after HFS, and standard thromboprophylaxis for 5-9 days. Treatment was continued for 3 weeks and all patients then underwent mandatory bilateral venography. The primary efficacy outcome was venous thromboembolism (VTE; venographic or symptomatic deep vein thrombosis or pulmonary embolism) during treatment, and it was evaluated centrally by an independent adjudication panel. The main safety outcome was bleeding (major, clinically relevant non-major, and minor events).

RESULTS

Two hundred and twelve patients with an evaluable venogram were included in the efficacy analysis. The primary efficacy outcome occurred in 32.1% (35/109) of patients who had been allocated TTP889, and 28.2% (29/103) of patients on placebo (P = 0.58). There were no major bleeding events, and only two clinically relevant non-major bleeding events with TTP889.

CONCLUSION

Partial FIXa inhibition with TTP889 300 mg daily was not effective for extended prevention of VTE after standard prophylaxis for up to 9 days. Coupled with the low incidence of bleeding episodes, this suggests a lack of antithrombotic potential. Further investigation of TTP889 in different clinical settings is needed. (Clinical trial registration information URL: http://www.clinicaltrials.gov. Unique identifier: NCT00119457).

Computationally derived points of fragility of a human cascade are consistent with current therapeutic strategies

The role that mechanistic mathematical modeling and systems biology will play in molecular medicine and clinical development remains uncertain. In this study, mathematical modeling and sensitivity analysis were used to explore the working hypothesis that mechanistic models of human cascades, despite model uncertainty, can be computationally screened for points of fragility, and that these sensitive mechanisms could serve as therapeutic targets. We tested our working hypothesis by screening a model of the well-studied coagulation cascade, developed and validated from literature. The predicted sensitive mechanisms were then compared with the treatment literature. The model, composed of 92 proteins and 148 protein-protein interactions, was validated using 21 published datasets generated from two different quiescent in vitro coagulation models. Simulated platelet activation and thrombin generation profiles in the presence and absence of natural anticoagulants were consistent with measured values, with a mean correlation of 0.87 across all trials. Overall state sensitivity coefficients, which measure the robustness or fragility of a given mechanism, were calculated using a Monte Carlo strategy. In the absence of anticoagulants, fluid and surface phase factor X/activated factor X (fX/FXa) activity and thrombin-mediated platelet activation were found to be fragile, while fIX/FIXa and fVIII/FVIIIa activation and activity were robust. Both anti-fX/FXa and direct thrombin inhibitors are important classes of anticoagulants; for example, anti-fX/FXa inhibitors have FDA approval for the prevention of venous thromboembolism following surgical intervention and as an initial treatment for deep venous thrombosis and pulmonary embolism. Both in vitro and in vivo experimental evidence is reviewed supporting the prediction that fIX/FIXa activity is robust. When taken together, these results support our working hypothesis that computationally derived points of fragility of human relevant cascades could be used as a rational basis for target selection despite model uncertainty.

Factor IXa inhibitors as novel anticoagulants

Currently available anticoagulants are limited by modest therapeutic benefits, narrow clinical applications, increased bleeding risk, and drug-induced thrombophilia. Because factor IX plays a pivotal role in tissue factor (TF)-mediated thrombin generation, it may represent a promising target for drug development. Several methods of attenuating factor IX activity, including monoclonal antibodies, synthetic active site-blocked competitive inhibitors, oral inhibitors, and RNA aptamers, have undergone investigation. This review summarizes present knowledge of factor IX inhibitors with emphasis on biology, pharmacology, preclinical data, and early-phase clinical experience in humans.

The status of new anticoagulants

Currently available anticoagulants include heparin, low-molecular weight heparin, fondaparinux and warfarin. Despite advances with low-molecular weight heparin and fondaparinux, the currently available agents have limitations that have provided the impetus for the development of new drugs for prevention and treatment of both venous and arterial thromboembolism. Novel anticoagulants targeting specific steps in coagulation are in various stages of development. This paper reviews the pharmacology of these new agents and describes the results of clinical trials with new anticoagulants in more advanced stages of clinical testing.

Depolymerized holothurian glycosaminoglycan and heparin inhibit the intrinsic tenase complex by a common antithrombin-independent mechanism

Depolymerized holothurian glycosaminoglycan (DHG) is a fucosylated chrondroitin sulfate that possesses antithrombin-independent antithrombotic properties and inhibits factor X activation by the intrinsic tenase complex (factor IXa-factor VIIIa). The mechanism and molecular target for intrinsic tenase inhibition were determined and compared with inhibition by low-molecular-weight heparin (LMWH). DHG inhibited factor X activation in a noncompetitive manner (reduced V(max(app))), with 50-fold higher apparent affinity than LMWH. DHG did not affect factor VIIIa half-life or chromogenic substrate cleavage by factor IXa-phospholipid but reduced the affinity of factor IXa for factor VIIIa. DHG competed factor IXa binding to immobilized LMWH with an EC(50) 35-fold lower than soluble LWMH. Analysis of intrinsic tenase inhibition, employing factor IXa with mutations in the heparin-binding exosite, demonstrated that relative affinity (K(i)) for DHG was as follows: wild type > K241A > H92A > R170A > > R233A, with partial rather than complete inhibition of the mutants. This rank order for DHG potency correlated with the effect of these mutations on factor IXa-LMWH affinity and the potency of LMWH for intrinsic tenase. DHG also accelerated decay of the intact intrinsic tenase complex. Thus, DHG binds to an exosite on factor IXa that overlaps with the binding sites for LMWH and factor VIIIa, disrupting critical factor IXa-factor VIIIa interactions.

New anticoagulants

The limitations of heparin and warfarin have prompted the development of new anticoagulant drugs for prevention and treatment of venous and arterial thromboembolism. Novel parenteral agents include synthetic analogs of the pentasaccharide sequence of heparin that mediates its interaction with antithrombin. Fondaparinux, the first synthetic pentasaccharide, is licensed for prevention of venous thromboembolism (VTE) after major orthopedic surgery and for initial treatment of patients with VTE. Idraparinux, a long-acting pentasaccharide that is administered subcutaneously once-weekly, is being compared with warfarin for treatment of VTE and for prevention of cardioembolic events in patients with atrial fibrillation. New oral anticoagulants include direct inhibitors of thrombin, factor Xa and factor IXa. Designed to provide more streamlined anticoagulation than warfarin, these agents can be given without routine coagulation monitoring. Ximelagatran, the first oral direct thrombin inhibitor, is as effective and safe as warfarin for prevention of cardioembolic events in patients with atrial fibrillation. However, ximelagatran produces a three-fold elevation in alanine transaminase levels in 7.9% of patients treated for more than a month, the long-term significance of which is uncertain. Whether other direct thrombin inhibitors or inhibitors of factors Xa or IXa also have this problem is under investigation. After a brief review of coagulation pathways, this paper focuses on new anticoagulants in advanced stages of clinical testing.

Structure-based design and synthesis of pyrazinones containing novel P1 ‘side pocket’ moieties as inhibitors of TF/VIIa

We describe the structure-based design, synthesis, and enzymatic activity of a series of substituted pyrazinones as inhibitors of the TF/VIIa complex. These inhibitors contain substituents meta to the P(1) amidine designed to explore additional interactions with the VIIa residues in the so-called 'S(1) side pocket'. A crystal structure of the designed inhibitors demonstrates the ability of the P(1) side pocket moiety to engage Lys192 and main chain of Gly216 via hydrogen bond interactions, thus, providing additional possibility for chemical modification to improve selectivity and/or physical properties of inhibitors.

New anticoagulants: beyond heparin, low-molecular-weight heparin and warfarin

The limitations of traditional anticoagulants, heparin and warfarin, have prompted the development of new anticoagulant drugs for prevention and treatment of both venous and arterial thromboembolism. After a brief review of thrombogenesis and its regulation, this paper focuses on new anticoagulant agents in more advanced stages of clinical testing.

Antithrombotic agents in the treatment of severe sepsis

Sepsis, a systemic inflammatory syndrome, is a response to infection and when associated with multiple organ dysfunction is termed severe sepsis. It remains a leading cause of mortality in the critically ill. The response to the invading microorganisms may be considered as a balance between a pro-inflammatory and an anti-inflammatory reaction. While an inadequate pro-inflammatory reaction and a strong anti-inflammatory response could lead to overwhelming infection and the death of the patient, a strong and uncontrolled pro-inflammatory response, manifested by the release of pro-inflammatory mediators may lead to microvascular thrombosis and multiple organ failure. Endotoxin triggers sepsis via the release of various mediators such as tumour necrosis factor-alpha and interleukin-1 (IL-1). These cytokines activate the complement and coagulation systems, release adhesion molecules, prostaglandins, leukotrienes, reactive oxygen species and nitric oxide. Other mediators involved in the sepsis syndrome include IL-1, -6 and -8; arachidonic acid metabolites; platelet activating factor; histamine; bradykinin; angiotensin; complement components and vasoactive intestinal peptide. These pro-inflammatory responses are counteracted by IL-10. Most of the trials targeting the different mediators of the pro-inflammatory response have failed due to a lack of correct definition of sepsis. Understanding the exact pathophysiology of the disease will enable more advanced treatment options. Targeting the coagulation system with various anticoagulant agents including, activated protein C, and tissue factor pathway inhibitor (TFPI) is a rational approach. Many clinical trials have been conducted to evaluate these agents in severe sepsis. While trials on antithrombin and TFPI were not so successful, the double-blind, placebo-controlled, Phase III trial of recombinant human activated Protein C Worldwide Evaluation in Severe Sepsis (PROWESS) was successful, creating a significant decrease in mortality when compared to the placebo group. A better understanding of the pathophysiologic mechanism of severe sepsis will provide better treatment options, and combination antithrombotic treatment may provide a multipronged approach for the treatment of severe sepsis.

Emerging anticoagulant and thrombolytic drugs

Since its discovery, heparin has been used intensely as an anticoagulant for several medical and surgical indications. However, efforts are in progress to replace heparin because of its serious complications, such as intraoperative and postoperative bleeding, osteoporosis, alopecia, heparin resistance, heparin rebound, heparin-induced thrombocytopenia (HIT) and thrombosis syndrome (HITTS), and other disadvantages. Significant developments in the field of new anticoagulants have resulted in the evaluation and introduction of low molecular weight heparins (LMWHs) and heparinoids, hirudin, ancrod, synthetic peptides and peptidomimetics. However, despite significant progress in the development of these new anticoagulants, a better or an ideal anticoagulant for cardiovascular patients is not yet available and heparin still continues to amaze both basic scientists and the clinicians. To minimise the adverse effects of heparin, newer approaches to optimise its use in combination with the new anticoagulants may provide better clinical outcome. In our experience, the off-label use of argatroban at a dose of 300 microg/kg iv. bolus followed by 10 microg/kg/minute infusion in combination with aggrastat (a glycoprotein [GP] IIb/IIIa inhibitor) at a dose of 10 microg/kg iv. bolus followed by an infusion of 0.15 microg/kg/minute in patients with HIT undergoing percutaneous coronary interventions resulted in elevation of celite activated clotting time (ACT) to 300 seconds followed by a gradual decline and the ACT remained above 200 seconds even after 200 min of drug administration. A bewildering array of newer anticoagulants now exist, such as LMWHs and heparinoids, indirect or direct thrombin inhibitors, oral thrombin inhibitors, such as melagatran (AstraZeneca) and HC-977 (Mitsubishi Pharmaceuticals), Factor IXa inhibitors, indirect or direct Factor Xa inhibitors, Factor VIIa/tissue factor (TF) pathway inhibitor, newer antiplatelet agents, such as GPIIb/IIIa inhibitors, fibrin specific thrombolytic agent, such as tenecteplase and modulation of the endogenous fibrinolytic activity by thrombin activatable fibrinolytic inhibitor (TAFI), Factor XIIIa inhibitors and PAI-1 inhibitors. The quest for newer anticoagulant, antiplatelet and fibrinolytic agents will continue until ideal agents are found.

The Factor IXa Heparin-Binding Exosite Is a Cofactor Interactive Site: Mechanism for Antithrombin-Independent Inhibition of Intrinsic Tenase by Heparin

Therapeutic heparin concentrations selectively inhibit the intrinsic tenase complex in an antithrombin-independent manner. To define the molecular target and mechanism for this inhibition, recombinant human factor IXa with alanine substituted for solvent-exposed basic residues (H92, R170, R233, K241) in the protease domain was characterized with regard to enzymatic activity, heparin affinity, and inhibition by low molecular weight heparin (LMWH). These mutations only had modest effects on chromogenic substrate hydrolysis and the kinetics of factor X activation by factor IXa. Likewise, factor IXa H92A and K241A showed factor IXa-factor VIIIa affinity similar to factor IXa wild type (WT). In contrast, factor IXa R170A demonstrated a 4-fold increase in apparent factor IXa-factor VIIIa affinity and dramatically increased coagulant activity relative to factor IXa WT. Factor IXa R233A demonstrated a 2.5-fold decrease in cofactor affinity and reduced ability to stabilize cofactor half-life relative to wild type, suggesting that interaction with the factor VIIIa A2 domain was disrupted. Markedly (R233A) or moderately (H92A, R170A, K241A) reduced binding to immobilized LMWH was observed for the mutant proteases. Solution competition demonstrated that the EC(50) for LMWH was increased less than 2-fold for factor IXa H92A and K241A but over 3.5-fold for factor IXa R170A, indicating that relative heparin affinity was WT > H92A/K241A > R170A >> R233A. Kinetic analysis of intrinsic tenase inhibition demonstrated that relative affinity for LMWH was WT > K241A > H92A > R170A >> R233A, correlating with heparin affinity. Thus, LMWH inhibits intrinsic tenase by interacting with the heparin-binding exosite in the factor IXa protease domain, which disrupts interaction with the factor VIIIa A2 domain.

5-Amidinobenzo[b]thiophenes as dual inhibitors of factors IXa and Xa

Syntheses and SAR studies of 5-amidinobenzo[b]thiophene analogs provided compounds with low submicromolar factor IXa activity and equal or slightly better selectivity relative to factor Xa.

The impact of vitamin K-dependent factor depletion by warfarin on platelet-rich thrombosis after deep arterial injury

Although the central role of thrombin in arterial thrombosis is well established, the efficacy of vitamin K-dependent factor depletion by warfarin at preventing this process has not been established. To assess the efficacy of warfarin in the prevention of arterial thrombosis, two intensities of anticoagulation were compared in a well-characterized porcine model of carotid angioplasty. For 10 days prior to angioplasty, pigs received either high-dose warfarin (n = 9), low-dose warfarin plus aspirin (n = 9), or control tablets (n = 10). Injured arteries were assessed for (111)In-platelet ( x 10(6) cm(-2)) and (125)I-fibrin(ogen) (molecules x 10(12) cm(-2)) deposition and the incidence of macroscopic thrombus. Platelet (30 +/- 7 vs. 332 +/- 137; P = 0.001) and fibrinogen (156 +/- 17 vs. 365 +/- 90; P < 0.05) deposition were significantly reduced in animals receiving high-intensity warfarin whereas low-intensity warfarin/ASA (520 +/- 240 and 1193 +/- 638) was similar to control (P =NS). At the time of angioplasty, the PT-INR and vitamin K-dependent factors varied over a broad range. The greatest reduction of platelet and fibrinogen deposition occurred as the PT-INR increased from 1.0 to 2.2. Increasing the PT-INR beyond 3.0 resulted in little, if any, incremental reduction of either platelet or fibrinogen deposition. Macroscopic thrombus was abolished at PT-INR > 2.2. Despite a broad range of vitamin K factor activities, no single factor was predictive of either platelet or fibrinogen deposition. Warfarin at PT-INR > 2.2 effectively eliminates thrombosis following deep arterial injury. Arterial thrombosis correlates poorly with any single vitamin K-dependent factor but rather appears to be a function of the entire extrinsic coagulation pathway as measured by the PT-INR.

Prolonged bleeding time induced by anticoagulant glycosaminoglycans in dogs is associated with the inhibition of thrombin-induced platelet aggregation

INTRODUCTION

The clinical use of unfractionated heparin (UFH) is complicated by hemorrhage. This has led to a search for safer alternatives, one of which, the recently identified depolymerized holothurian glycosaminoglycan (DHG), causes less bleeding and exhibits a better antithrombotic-hemorrhagic ratio in rats and dogs than UFH and low-molecular-weight heparin (LMWH). In contrast to UFH and LMWH, which exert their anticoagulant effects by inhibiting thrombin in the presence of antithrombin III (AT), DHG exerts its anticoagulant effect by inhibiting the intrinsic factor Xase complex and thrombin in the presence of heparin cofactor II (HCII).

MATERIALS AND METHODS

The hemorrhagic effect of DHG was compared with those of UFH and LMWH in healthy dogs, and the mechanism responsible for prolonging bleeding time was examined both in dogs and with human platelets.

RESULTS

DHG prolonged template-bleeding time in dogs less than UFH and LMWH do. Although the maximum noneffective concentrations of each glycosaminoglycan (GAG) that prolong the bleeding time are almost the same as the concentrations that inhibit thrombin-induced platelet aggregation, they are not related to those that inhibit ADP-induced platelet aggregation. Results of experiments on gel-filtered platelets from humans indicate that the inhibition of thrombin-induced platelet aggregation caused by UFH and LMWH in the presence of AT is more prominent than that caused by DHG with HCII.

CONCLUSIONS

These results suggest that the prolongation of bleeding time caused by GAGs are associated with the inhibition of thrombin-induced platelet aggregation, and DHG may cause less bleeding than UFH and LMWH because of its different thrombin inhibition mechanism in platelet-rich plasma (PRP).

Antidotes to haemorrhage: recombinant factor VIIa

Recombinant Factor VIIa (rFVIIa) concentrates were originally developed to treat the refractory bleeding complications associated with allo-antibody inhibitors in hemophilias A and B. As experience was gained in the hemophilias, the physiology of rFVIIa and its successes in controlling bleeds stimulated rFVIIa use in other challenging medical conditions complicated by bleeding. Thus, rFVIIa has assumed the role of a 'universal pancoagulant' without sufficient evidence-based data from well-designed, adequately powered clinical trials. This chapter discusses the anecdotal experience with rFVIIa based upon the few controlled trials that do exist, and emphasizes that these empirical dosing strategies have not yielded the best approach to achieve effective control of bleeding. Evidence-based data are necessary to establish the cost-benefit and risk-benefit profiles of rFVIIa, and to establish it as a standard treatment for bleeding.

Heparin Inhibits the Intrinsic Tenase Complex by Interacting with an Exosite on Factor IXa

The specific molecular target for direct heparin inhibition of factor X activation by intrinsic tenase (factor IXa-factor VIIIa) was investigated. Comparison of size-fractionated oligosaccharides demonstrated that an octasaccharide was sufficient to inhibit intrinsic tenase. Substitution of soluble dihexanoic phosphatidylserine (C6PS) for phospholipid (PL) vesicles demonstrated that inhibition by low-molecular weight heparin (LMWH) was independent of factor IXa-factor VIIIa membrane assembly. LMWH also inhibited factor X activation by the factor IXa-PL complex via a distinct mechanism that required longer oligosaccharides and was independent of substrate concentrations. The apparent affinity of LMWH for the factor IXa-PL complex was higher in the absence of factor VIIIa, suggesting that the cofactor adversely affected the interaction of heparin with factor IXa-phospholipid. LMWH did not interact directly with the active site, as it failed to inhibit chromogenic substrate cleavage by the factor IXa-PL complex. LMWH induced a modest decrease in factor IXa-factor VIIIa affinity [K(D(app))] on PL vesicles that did not account for the inhibition. In contrast, LMWH caused a substantial reduction in factor IXa-factor VIIIa affinity in the presence of C6PS that fully accounted for the inhibition. Factor IXa bound LMWH with significantly higher affinity than factor X by competition solution affinity analysis, and the K(D(app)) for the factor IXa-LMWH complex agreed with the K(I) for inhibition of the factor IXa-PL complex by LMWH. Thus, LMWH binds to an exosite on factor IXa that antagonizes cofactor activity without disrupting factor IXa-factor VIIIa assembly on the PL surface. This exosite may contribute to the clinical efficacy of heparin and represents a novel target for antithrombotic therapy.

Emerging anticoagulant drugs

The limitations of traditional anticoagulants, heparin and warfarin, have prompted the development of new anticoagulant drugs for prevention and treatment of both venous and arterial thromboembolism. After a brief review of thrombogenesis and its regulation, this study focuses on new anticoagulant agents in more advanced stages of clinical testing.

Current anticoagulant therapy--unmet clinical needs

Heparin and the vitamin K antagonist warfarin have been in clinical use for more than 50 years. However, both are associated with several well-documented drawbacks that limit their use. Warfarin can be administered orally, making it the agent of choice for long-term management of thromboembolic conditions, but frequent coagulation monitoring is necessary because of its unpredictable anticoagulant effect--the result, in part, of food and drug interactions-and its narrow therapeutic window. Heparin and low-molecular-weight heparin (LMWH) can be administered parenterally only. Coagulation monitoring is also required with heparin although not with LMWH, due to reduced levels of plasma protein binding. In the last 10 years, in the quest to develop new agents that are at least as effective as those currently available, with improved safety and greater ease of use, anticoagulants that target almost every step in the coagulation pathway have been developed. These include inhibitors of the factor VIIa (FVIIa)/tissue factor complex, FIXa inhibitors, direct and antithrombin-dependent FXa inhibitors, agents that enhance the protein C anticoagulant pathway, and direct thrombin inhibitors (DTIs) that inhibit the activity of thrombin. Of the new agents, three DTIs-hirudin, bivalirudin, and argatroban-and the synthetic pentasaccharide (Arixtra) are approved for clinical use. Three other new agents-activated protein C (APC), tissue factor pathway inhibitor (TFPI), and the oral DTI ximelagatran (Exanta, AstraZeneca)-have been evaluated in Phase III studies. The mechanism of action and properties of these new anticoagulants and their potential to replace those in current use will be reviewed here.

Inhibition of arterial thrombosis by polyamines in a canine coronary artery injury model

Polyamines are polycations present in all living organisms and have been shown to play an important role in various physiological functions. Previous studies have shown that various amines including polyamines inhibit platelet activation. Among the amines tested tetra-amine, spermine is the potent inhibitor of platelet aggregation. In spite of vast literature on the anti-aggregatory effect of amines, there are no definitive studies testing their efficacy in an in vivo thrombosis model. In the present study, we investigated if polyamines could inhibit in-vivo thrombosis. A partially occlusive thrombus was generated by application of electric current in canine coronary artery. In control animals, the artery was completely in 76+/-14 min after the current was discontinued. When 40 mg/kg (1.44 mM) spermine was given immediately after stopping the current blood flow remained patent for >240 min. At equimolar concentration, triamine, spermidine and diamine putrescine are also equally effective in preventing thrombus development. The anti thrombic effect of polyamines was not associated with increased bleeding tendency, as judged by the amount of blood adsorbed by a gauge pad placed in a surgical incision extending to the muscle tissue and by a standard template bleeding. These results indicate that apart from inhibiting in-vitro platelet aggregation polyamines can also inhibit in-vivo thrombus formation.

New anticoagulants: current status and future potential

Arterial and venous thrombosis are a major cause of morbidity and mortality. Anticoagulants are a cornerstone of treatment in patients with these disorders. The two most frequently used anticoagulants, heparin and warfarin, have pharmacological and/or biophysical limitations that make them difficult to use in day-to-day clinical practice. Development of new anticoagulants, which were designed to overcome these limitations, has been facilitated by an increased understanding of the coagulation cascade, the advent of molecular modeling and structure-based drug design, and the realization that the treatment of thrombosis and its complications consumes billions of dollars in annual healthcare expenditures. New anticoagulants target various steps in the coagulation pathway. Coagulation is triggered by the factor VIIa/tissue factor complex and propagated by factors Xa and IXa, together with their activated cofactors, factor Va and VIIIa, respectively. Thrombin, the final effector in coagulation, then converts soluble fibrinogen into insoluble fibrin, the major matrix protein of the clot. New anticoagulation drugs that target each of these clotting enzymes have been developed. This review will focus on those drugs in more advanced stages of clinical evaluation. These include inhibitors of initiation of coagulation (tissue factor pathway inhibitor, nematode anticoagulant peptide and active-site blocked factor VIIa), inhibitors of propagation of coagulation (active-site blocked factor IXa, antibodies against factor IX/IXa, fondaparinux sodium, direct factor Xa inhibitors, protein C derivatives and soluble thrombomodulin), and thrombin inhibitors (hirudin, bivalirudin, argatroban and ximelagatran).

Effect of polyamines on in vitro platelet aggregation and in vivo thrombus formation

INTRODUCTION

Polyamines are polycations present in all living organisms and have been shown to play an important role in various physiological functions. Previous studies have shown that various amines including polyamines inhibited platelet activation, but there were no definitive studies testing their efficacy in an in vivo thrombosis model. We carried out detailed in vitro platelet aggregation studies using various concentrations of polyamines as well as agonists.

METHODS

Platelet aggregation was measured by a turbidimetric method. Electric current induced in vivo thrombosis model is used for assessing antithrombotic effect. Incidence of bleeding was evaluated by template bleeding and incisional bleeding.

RESULTS

Polyamines inhibited agonist-induced platelet aggregation in a dose-dependent manner. The inhibitory effect of polyamines is inversely proportional to the concentration of the agonist used. Among the polyamines, spermine is the potent inhibitor of platelet aggregation. A partially occlusive thrombus was generated by application of electric current in canine coronary artery. In control animals, the artery was completely occluded in 70 +/- 11 min after the current was discontinued. Blood flow remained patent for >240 min when 2 mg/kg spermine was given immediately after stopping the current. The antithrombotic effect of spermine was not associated with increased bleeding tendency.

CONCLUSION

These results indicate that apart from inhibiting in vitro platelet aggregation polyamines can also inhibit in vivo thrombus formation. To our knowledge, this is the first study demonstrating this phenomenon.

Overview of anticoagulant drugs for the future

More efficacious, safer, and easier to use anticoagulants are under development. Multiple agents have been shown to be effective in ex vivo or animal thrombosis models and several have progressed to clinical studies. Investigators have not yet determined if pharmaceuticals that inhibit coagulation factor activity earlier in the cascade (for example, inhibitors of tissue factor/factor VIIa, factor IXa, or Xa) are superior to those that block the cascade at a later point. Orally bioavailable drugs for the long-term treatment of thrombotic disorders, particularly those that do not require monitoring, are needed and are under development. Local delivery of anticoagulants or genes modulating anticoagulant control at sites of increased thrombogenicity, such as in diseased arteries, is a promising treatment modality that may decrease systemic bleeding problems. Much about the initiating pathophysiologic events leading to venous thrombotic disease needs to be elucidated before such local therapy can be tested in the venous vasculature. While awaiting better anticoagulants to become routinely available, we need to improve patient management with existing drugs by instituting anticoagulation clinics, promoting patient self-monitoring, and improving efforts to educate patients and health care providers about the use of anticoagulant drugs.

Antithrombotic effects of controlled inhibition of factor VIII with a partially inhibitory human monoclonal antibody in a murine vena cava thrombosis model

The human monoclonal antibody mAb-LE2E9 partially inactivates human factor VIII (FVIII), leaving approximately 10% residual activity. The antithrombotic efficacy of the antibody was evaluated in mouse models of inferior vena cava thrombosis. Thrombi were induced in wild-type mice given either the antibody or saline. No thrombi occurred in any of 8 mice treated with mAb-LE2E9, whereas 6 of 8 control mice developed thrombi (P =.007). Treatment with mAb-LE2E9 did not result in a severe bleeding phenotype: a tail-cutting experiment that resulted in death of C57BL/6 FVIII-deficient (FVIII(-/-)) mice did not cause hemorrhagic death in mice treated with mAb-LE2E9. To evaluate the antithrombotic effect of mAb-LE2E9 in presence of human FVIII, thrombus formation was induced in FVIII(-/-) mice reconstituted intravenously with recombinant human FVIII (rhFVIII) or rhFVIII preincubated with mAb-LE2E9. Only 1 of 9 mice produced a thrombus in the rhFVIII/antibody complex-treated group, compared with 7 of 9 in the control group (P =.015). FVIII(-/-) mice were also reconstituted with rhFVIII and then injected with either mAb-LE2E9 or saline. One of 14 mice in the group treated with the antibody developed a thrombus, compared with 10 of 14 in the control group (P =.001). The thrombi occurring in antibody-treated animals were smaller than in controls (P <.01). All animals survived, and there were no bleeding complications. Thus, the mAb-LE2E9 antibody inhibits thrombosis without causing an overt bleeding tendency.

A human antibody that inhibits factor IX/IXa function potently inhibits arterial thrombosis without increasing bleeding

10C12, a human antibody F(ab')2, which specifically binds to the gamma-carboxyglutamic acid domain of factor IX/factor IXa (F.IX/IXa), interferes with all known coagulation processes in which F.IX/IXa is involved. In a rabbit model of carotid artery injury, intravenous administration of 10C12 or heparin decreased thrombosis dose dependently. The dose that resulted in a 90% reduction of thrombus mass (ED90) was a 30-microg/kg bolus of 10C12 or a 100-U/kg bolus plus 1.0 U x kg(-1) x min(-1) infusion of heparin. Heparin, at and below the ED90, significantly prolonged coagulation times and cuticle bleeding times. In contrast, 10C12 had no effect on coagulation or bleeding times at doses up to 4 times the ED90. To further evaluate the effect of 10C12 on bleeding, it was compared with heparin in a novel model of blood loss. At the ED90 of heparin, blood loss induced by a standardized injury to the vasculature of the rabbit tibia increased to more than 2 times that of saline controls. In contrast, the dose of 10C12 required to produce a similar increase in blood loss was more than 30 times the ED90. The antithrombotic potency and relative safety of this fully human antibody suggests that it may have therapeutic value for treatment of thrombotic disorders.

Inhibition of factor IX(a) is protective in a rat model of thromboembolic stroke

BACKGROUND AND PURPOSE

Although used clinically to prevent stroke, there are few examples of anticoagulant investigations in the treatment of acute thromboembolic stroke in animal models. The treatment of thromboembolic stroke in experimental models has been investigated almost exclusively around the use of tissue plasminogen activator (tPA). In this study, using a rat thromboembolic stroke model, we investigated the use of an inhibitory anti-factor IX(a) monoclonal antibody (SB 249417) for the treatment of thromboembolic stroke and compared its efficacy to that of tPA.

METHODS

Stroke was initiated by delivering 6 clots into the internal carotid artery. After 2, 4, or 6 hours, rats received either intravenous vehicle, 10.0 mg/kg tPA, or 1.0, 2.0, or 3.0 mg/kg SB 249417. At 24 hours after stroke, infarct volumes and neurological deficits were assessed.

RESULTS

Treatment with tPA 2, 4, or 6 hours after stroke reduced infarct volumes by 35% (P=NS), 45%, and 39%, respectively. tPA treatment did not improve neurological deficits at any time point. Treatment with SB 249417 (3.0 mg/kg) 2, 4, or 6 hours after stroke reduced infarct volumes by 44%, 50%, and 13% (P=NS), respectively. Neurological deficits were reduced by 49%, 42%, and 13% (P=NS), respectively. Neither mortality nor hemorrhage was affected by either treatment.

CONCLUSIONS

The data indicate that the inhibition of factor IX(a) within 4 hours of thromboembolic stroke produced a more favorable outcome than tPA. When treatment was initiated 6 hours after stroke, the benefits of factor IX(a) inhibition were lost, whereas tPA continued to suppress lesion development, albeit without a corresponding improvement in functional deficits. This study suggests that cerebral ischemia and the resultant perfusion deficit are exacerbated by the activation of blood coagulation and that anticoagulants like SB 249417 may find utility in the treatment of ischemic stroke.

New anticoagulant drugs

Arterial and venous thrombosis are major causes of morbidity and mortality. Arterial thrombosis is the most common cause of myocardial infarction, stroke, and limb gangrene. Venous thrombosis leads to potentially fatal pulmonary embolism and to post-phlebitic syndrome. Because arterial thrombi consist of platelet aggregates held together by small amounts of fibrin, strategies to inhibit arterial thrombogenesis focus mainly on blocking platelet function, but often include anticoagulants to prevent fibrin deposition. Anticoagulants are used for the prevention and treatment of venous thrombosis because venous thrombi consist mainly of fibrin and red blood cells. This paper (1) reviews arterial and venous thrombogenesis, (2) outlines new anticoagulant strategies, and (3) provides clinical perspectives on these new strategies.

Hypersulfated low molecular weight heparin with reduced affinity for antithrombin acts as an anticoagulant by inhibiting intrinsic tenase and prothrombinase

In buffer systems, heparin and low molecular weight heparin (LMWH) directly inhibit the intrinsic factor X-activating complex (intrinsic tenase) but have no effect on the prothrombin-activating complex (prothrombinase). Although chemical modification of LMWH, to lower its affinity for antithrombin (LA-LMWH) has no effect on its ability to inhibit intrinsic tenase, N-desulfation of LMWH reduces its activity 12-fold. To further explore the role of sulfation, hypersulfated LA-LMWH was synthesized (sLA-LMWH). sLA-LMWH is not only a 32-fold more potent inhibitor of intrinsic tenase than LA-LMWH; it also acquires prothrombinase inhibitory activity. A direct correlation between the extent of sulfation of LA-LMWH and its inhibitory activity against intrinsic tenase and prothrombinase is observed. In plasma-based assays of tenase and prothrombinase, sLA-LMWH produces similar prolongation of clotting times in plasma depleted of antithrombin and/or heparin cofactor II as it does in control plasma. In contrast, heparin has no effect in antithrombin-depleted plasma. When the effect of sLA-LMWH on various components of tenase and prothrombinase was examined, its inhibitory activity was found to be cofactor-dependent (factors Va and VIIIa) and phospholipid-independent. These studies reveal that sLA-LMWH acts as a potent antithrombin-independent inhibitor of coagulation by attenuating intrinsic tenase and prothrombinase.

Comparing the antithrombotic efficacy of a humanized anti-factor IX(a) monoclonal antibody (SB 249417) to the low molecular weight heparin enoxaparin in a rat model of arterial thrombosis

A humanized inhibitory anti-factor IX(a) antibody (SB 249417) has been compared to enoxaparin (Lovenox) in a rat model of arterial thrombosis. Pretreatment of rats with either SB 249417 (3.0 mg/kg, i. v.) or enoxaparin (30.0 mg/kg, i.v. or s.c.) resulted in comparable and significant reductions in thrombus formation. However, the efficacious dose of enoxaparin resulted in >30-fold increase in the aPTT over baseline, while the efficacious dose of SB 249417 prolonged the aPTT by only approximately 3-fold. Additionally, pretreatment with SB 249417 resulted in sustained blood flow and arterial patency throughout the experiment in >80% of rats treated. In contrast, <30% of rats pretreated with enoxaparin remained patent throughout the experiment. The data in this report indicate that the selective inhibition of factor IX(a) with the monoclonal antibody SB 249417 produces a superior antithrombotic profile to that of the low molecular weight heparin enoxaparin.