Effects on Thrombin Generation of Single Injections of Hirulog™ in Patients with Calf Vein Thrombosis

Bivalirudin: a review

Bivalirudin (Angiomax) is a thrombin-inhibiting oligopeptide that was developed via rational drug design as a hirudin analogue ('hirulog'). Similar to hirudin, it is a bivalent thrombin inhibitor, as its 20-amino acid structure combines a carboxy-terminal region that recognises thrombin's fibrin(ogen)-binding site, and an amino-terminal tetrapeptide that inhibits the active site of thrombin, connected by a tetraglycine spacer. It has certain pharmacological advantages over hirudin, including enzymic metabolism (less dependence on renal clearance) and low immunogenicity (reduced potential for anaphylaxis). Bivalirudin is approved for use in percutaneous transluminal coronary angioplasty (PTCA), and is undergoing active investigation for anticoagulation during cardiac surgery, both 'off-pump' and with cardiopulmonary bypass ('on-pump'). Anecdotal 'off-label' experience for the treatment of heparin-induced thrombocytopenia shows promise.

Synthetic and recombinant antithrombin drugs

As the final enzyme in the activation of the coagulation system, the serine protease, thrombin, is believed to be an important target for the development of new anticoagulant/antithrombotic drugs. Direct thrombin inhibitors are either derived from natural sources, such as hirudin or are chemically synthesised, such as argatroban. The coupling of hirudin or parts of it with other entities leads to novel agents with different pharmacokinetic and pharmacodynamic characteristics, such as polyethylene glycol (PEG)-hirudin or the hirulogs. Due to the reversible or irreversible inactivation of the enzyme, thrombin inhibitors exert strong anticoagulant effects that can be measured in global clotting assays. Furthermore, these compounds inhibit thrombin-induced platelet reactions and influence other cellular, receptor-mediated actions of thrombin, e.g., on vascular cells. Directly acting thrombin inhibitors prevent blood clotting and are also capable of inhibiting clot-associated thrombin; however, they do not effectively block the further generation of the enzyme. Comprehensive experimental studies suggest that thrombin inhibitors may be effective drugs in a wide range of intravascular thrombus formation, also including the inhibition of vascular restenosis. Recent clinical trials revealed the effectiveness of direct thrombin inhibitors in various thrombotic and cardiovascular indications, but also a tendency to an increased risk of bleeding complications. At present, thrombin inhibitors are the most promising class of drugs for the initial therapy of patients with heparin-induced thrombocytopaenia (HIT) or the heparin-induced thrombocytopaenia and thrombosis syndrome (HITTS). They are also useful for the management of venous thrombosis and for acute ischaemic syndromes as well as for invasive procedures. However, with regard to the long-term outcome, a superiority of thrombin inhibitors over heparin has not yet been demonstrated. Several important issues, such as monitoring, pharmacological antagonism and drug interactions will also play an important role in the development of these new drugs. Further clinical trials are required to confirm the effectiveness of direct thrombin inhibitors in the prophylaxis and treatment of various thromboembolic and cardiovascular disorders.

Prevention of Venous Thromboembolism Following Orthopaedic Surgery

Patients undergoing total hip or total knee replacement are at high risk of venous thromboembolism (VTE), and are therefore considered to be populations well suited for the evaluation and dose optimisation of new anticoagulants. Deep vein thrombosis may lead to life-threatening pulmonary embolism, disabling morbidity in the form of the post-thrombotic syndrome, and risk of recurrent thrombotic events. There is increasing evidence that anticoagulant treatment for the prevention of VTE should be extended from 1 to at least 4 weeks after surgery. Anticoagulation with vitamin K antagonists (such as warfarin), low molecular weight heparin or unfractionated heparin effectively lowers the risk of VTE, but these anticoagulants have limitations such as the need for coagulation monitoring and subsequent dose adjustment (vitamin K antagonists), difficulty of continuing prophylaxis out of hospital because of the requirement for parenteral administration, and risk of heparin-induced thrombocytopenia. The development of new anticoagulants has been pursued with the aim of finding more effective, safer and/or more convenient therapies. Thrombin is a central regulator in the coagulation and inflammation process and several direct thrombin inhibitors (DTIs) with distinct pharmacological profiles, as well as pharmacological differences from the conventional anticoagulants, are currently in clinical use for certain indications or are under development. Clinical experience with parenterally administered DTIs has accumulated since the mid 1990s, although only desirudin (a recombinant hirudin) is currently approved for use in patients undergoing orthopaedic surgery. Two oral DTIs, ximelagatran and dabigatran etexilate, are in clinical development. Dabigatran etexilate has recently been evaluated in phase II clinical trials in patients undergoing total hip replacement. Several large phase III trials have now demonstrated the efficacy and safety of ximelagatran in the prevention of VTE following total hip or knee replacement. Ximelagatran can be used with an oral fixed dose without the need for coagulation monitoring or dose adjustment. Hence, it offers significant potential to facilitate the management of anticoagulation in or out of hospital.

Pharmacological characterization of a novel factor Xa inhibitor, FXV673

FXV673 is a novel, potent, and selective factor Xa (FXa) inhibitor. FXV673 inhibited human, dog, and rabbit FXa with a K(i) of 0.52, 1.41, and 0.27 nM, respectively. FXV673 also displayed excellent specificity toward FXa relative to other serine proteases. It showed selectivity of more than 1000-fold over thrombin, activated protein C (aPC), plasmin, and tissue-plasminogen activator (t-PA). FXV673 prolonged plasma activated partial thromboplastin time (APTT) and prothrombin time (PT) in a dose-dependent fashion. In the APTT assays, the concentrations (microM) required for doubling coagulation time were 0.41 (human), 0.65 (monkey), 1.12 (dog), 0.25 (rabbit), and 0.80 (rat). The concentrations (microM) required in the PT assays were 1.1 (human), 1.32 (monkey), 2.31 (dog), 0.92 (rabbit), and 1.69 (rat). A coupled-enzyme assay was performed to measure thrombin activity following prothrombinase conversion of prothrombin to thrombin. FXV673 showed IC(50)s of 1.38 and 2.55 nM, respectively, when artificial phosphatidylserine/phosphatidylcholine (PS/PC) liposomes or fresh platelets were used as the phospholipid source for prothrombinase complex formation. It was demonstrated that FXV673 could inhibit further thrombin generation in the prothrombinase complex using PS/PC liposomes. FXV673 dose-dependently prolonged the time to vessel occlusion and inhibited thrombus formation in well-characterized canine models of thrombosis. Interspecies extrapolation (approximately 2.5-fold higher sensitivity for FXa inhibition in human than in dog) suggested that 100 ng/ml of FXV673 would be an effective plasma concentration for clinical studies. Currently FXV673 is undergoing clinical studies to be developed as an antithrombotic agent.

In vitro characterization of a novel factor Xa inhibitor, RPR 130737

RPR 130737 inhibited factor Xa (FXa) with a Ki of 2.4 nM and also displayed excellent specificity toward FXa relative to other serine proteases. It showed selectivity of more than 1000-fold over thrombin, activated protein C, plasmin, tissue-plasminogen activator and trypsin. RPR 130737 prolonged plasma activated partial thromboplastin time and prothrombin time in a dose-dependent fashion. In the activated partial thromboplastin time assay, the concentrations required for doubling coagulation time were 0.32 microM (human), 0.61 microM (monkey), 0.44 microM (dog), 0.15 microM (rabbit), and 0.82 microM (rat). The concentrations required to double prothrombin time were 0.86 microM (human), and 1.26 microM (monkey), 1.15 microM (dog), 0.39 microM (rabbit) and 7.31 microM (rat). Kinetic studies revealed that RPR 130737 was a fast binding, reversible and competitive inhibitor for FXa when Spectrozyme FXa, a chromogenic substrate, was used. A coupled-enzyme assay measuring thrombin activity following prothrombinase conversion of prothrombin to thrombin indicated that RPR 130737 was a potent inhibitor for prothrombinase-bound FXa. In this assay, RPR 130737 showed IC50s of 17 nM and 35.9 nM, respectively when artificial phosphatidylserine/phosphatidylcholine (PS/PC) liposomes or gel-filtered platelets were used as the phospholipid source. An FX-deficient plasma clotting-time correction assay further demonstrated that RPR 130737 was a specific inhibitor of FXa. RPR 130737 showed no effect on platelet aggregation in vitro. These results indicate that RPR 130737 has the potential to be developed as an antithrombotic agent based on its potent and selective inhibitory effect against FXa.

Bivalirudin: a new generation antithrombotic drug

Bivalirudin (Angiomax, The Medicines Company) is a synthetic 20 amino acid peptide rationally designed on the basis of structural studies of hirudin, a naturally occurring anticoagulant. Bivalirudin represents a new class of anticoagulant drugs that directly inhibits thrombin, a key component in blood clot formation and extension. With its high binding affinity and specificity for thrombin, bivalirudin acts directly on thrombin, rather than via other clotting factors. The compound has a variety of potential uses as an alternative to heparin in the management of cardiovascular disease and related medical procedures i.e., unstable angina (UA), myocardial infarction (MI) and percutaneous transluminal coronary angioplasty (PTCA).

Treatment of deep venous thrombosis and pulmonary embolism. Current recommendations

The therapy of deep venous thrombosis consists of several elements and depends on the localization, the age and the extent of the thrombus. This article discusses various types of initial therapy and long-term treatment of venous thromboembolism and also reviews future perspectives of pharmacological treatment. The initial treatment regimens comprise thrombolysis, thrombectomy, inferior vena cava filters and the anticoagulation with either unfractionated heparin or low molecular weight heparins. Various thrombin-inhibitors have been tested for initial treatment of thrombosis, however, further investigations of their efficacy, safety and cost-effectiveness will have to provide firm evidence on their superiority when compared to unfractionated or low molecular weight heparins.

Inhibition by hirulog-1 of generation of plasminogen activator inhibitor-1 from vascular smooth-muscle cells induced by thrombin

Hirulog-1 effectively prevents thrombosis in coronary artery disease and is associated with a low incidence of bleeding complications. Our study characterized the effect of Hirulog-1 on thrombin-induced production of plasminogen activator inhibitor-1 (PAI-1) in cultured baboon aortic smooth-muscle cells (BASMCs). Thrombin increased the steady-state levels of PAI-1 messenger RNA (mRNA) and the release of PAI-1 antigen from BASMCs. Treatments with 10-20 mg/L of Hirulog-1 inhibited >80% of thrombin-induced PAI-1 generation from BASMCs. Hirulog-1 alone did not significantly alter PAI-1 production in the absence of thrombin. Significant reduction of thrombin-induced PAI-1 release was observed in cultures treated with Hirulog-1 for 1 h. The maximal effect of Hirulog-1 on thrombin-induced PAI-1 release was achieved in cultures treated with thrombin plus Hirulog-1 for 3 to 6 h, associated with the normalization of PAI-1 mRNA levels induced by thrombin treatment. Strong inhibition by Hirulog-1 on thrombin-induced PAI-1 release remained in cultures with 8 h of the treatment, but the effect was attenuated 16 h after a single addition of the inhibitor. Our study demonstrates that Hirulog-1 effectively inhibited thrombin-induced PAI-1 production in cultured vascular SMCs at mRNA and protein levels. Vascular SMCs may be exposed to high concentrations of thrombin when endothelium is injured. The information generated from this study suggests that Hirulog-1 potentially prevents intravascular thrombogenesis through inhibiting thrombin-induced PAI-1 production in vascular SMCs, especially when hypercoagulation and endothelial injury occurs.

Sustained inhibition of whole-blood clot procoagulant activity by inhibition of thrombus-associated factor Xa

Progression of arterial thrombosis partly depends on thrombus-associated thrombin and activated factor X (Xa) activity. However, whether Xa or thrombin is the most appropriate target for inhibition of recurrent thrombosis is unknown. This study was designed to determine whether inhibition of Xa results in more sustained attenuation of thrombus-associated precoagulant activity than does inhibition of thrombin. Clots prepared ex vivo from human whole blood and pathological arterial thrombi from patients were preincubated in citrated plasma containing no inhibitor, 0.5 to 1 U/mL heparin, 0.5 to 1 mumol/L hirudin, 5 to 10 mumol/L tick anticoagulant peptide (TAP), 0.15 to 3 mumol/L tissue factor pathway inhibitor (TFPI), or a combination of 1 mumol/L hirudin and 10 mumol/L TAP for 2 hours. After preincubation the clots were removed from first-stage plasma, extensively washed in phosphate-buffered saline, and added to nonanticoagulated whole blood. Clots preincubated in plasma without inhibitors induced marked activation of the coagulation system in whole blood, as characterized by greater increases in the concentration of fibrinopeptide A (FPA) over 7 minutes than in blood without added clots (1522 +/- 568 compared with 117 +/- 170 ng/mL, P < .01). Preincubation of clots with heparin or hirudin did not attenuate the increases in FPA in wholeblood. In contrast, compared with incubation without an inhibitor, preincubation of clots with TAP or TFPI markedly attenuated the increases in FPA when clots were added to whole blood (551 +/- 316 and 508 +/- 208 ng/mL, respectively, P < .01). Similar results were obtained with arterial thrombi from patients. Inhibition of Xa but not of thrombin results in sustained attenuation of thrombus-associated procoagulant activity. Uninhibited thrombus-associated Xa activity may account for the increases in thrombin activity that are commonly observed in clinical trials after discontinuation of thrombin inhibitors.