Antithrombotic Compounds Research Articles

Toward a Novel Class of Antithrombotic Compounds with Dual Function. Discovery of 1,4-Benzoxazin-3(4H)-one Derivatives Possessing Thrombin Inhibitory and Fibrinogen Receptor Antagonistic Activities

A novel class of potential antithrombotic compounds with moderate thrombin inhibitory and fibrinogen receptor antagonistic activity is described. Combination of anticoagulant and antiaggregatory activity in the same molecular entity is presented as a new promising approach in the search for novel antithrombotic agents.

Protein Palmitoylation Participates in PAR1-Mediated Platelet Activation.

Platelet accumulation at sites of vascular injury is the primary event in arterial thrombosis. We have performed a screen of 16,320 small molecules to identify compounds that inhibit platelet activation mediated by the PAR1 specific agonist, SFLLRN. JF081204{5C} (9-methylene-4-pentyl-2,3,4,9-tetrahydro-1H-cyclopenta(b)quinoline) was identified to be a potent antiplatelet and antithrombotic compound. We have previously shown that JF0812405{5C} inhibited thrombosis in a murine model by up to 80% with an IC50 of 2 mg/kg. Assays of structure activity relationships revealed that length of the alkyl chain of the compound is a critical determinant of its activity. Initial target-directed studies suggested that this compound inhibited protein palmitoylation. We therefore sought to clarify the effects of this compound on platelet protein palmitoylation and determine the role of palmitoylation in signaling induced by PAR1. We first analyzed the effects of JF081204{5C} on palmitoylation in resting platelets. Resting platelets were incubated with a cell-permeable peptide (MyrGCK(NBD)) that renders cells fluorescent upon palmitoylation. By confocal microscopy, the peptide was observed within platelets primarily localized around the periphery. Incubation of resting platelets with JF081204{5C} inhibited palmitoylation-induced fluorescence of the MGC peptide. The inhibitory effect of JF081204{5C} on palmitoylation was confirmed by 3H-palmitate labeling experiments. Resting platelets were incubated with 3H-palmitate in the presence or absence of JF081204{5C} and proteins were extracted by acetone precipitation. 3H-palmitate incorporation was decreased in platelets incubated with JF081204{5C} as evidenced by scintillation counting. We next analyzed the effects of JF081204{5C} on PAR1-mediated activation. JF081204{5C} inhibited SFLLRN-induced P-selectin expression, 14C-serotonin release, aggregation, and αIIbβ3 activation with an IC50 of 5 μM. In contrast, JF081204{5C} failed to inhibit platelet activation induced by ADP, PMA, A23187, or collagen. However, JF081204{5C} inhibited epinephrine-induced platelet aggregation with an IC50 of 5 μM, indicating that JF081204{5C} acts on an intracellular target. Stimulation with SFLLRN resulted in increased 3H-palmitate-incorporation into platelet proteins as evidenced by scintillation counting and autoradiography. In contrast, this increase was not observed following stimulation with U46619 or epinephrine. Autoradiography of the labeled proteins separated by SDS-PAGE demonstrated that JF081204{5C} inhibited SFLLRN-induced 3H-palmitate incorporation into select bands by up to 50%, while not affecting 3H-palmitate incorporation into other bands. We next sought to assess the role of palmitoylation in signaling through PAR1 using alternative inhibitors. 2-Bromopalmitate, a palmitic acid analog, abolished 3H-palmitate-incorporation into platelet protein and inhibited SFLLRN-induced platelet activation with an IC50 of 30 μM. Acyl-protein thioesterase 1 (APT1) is an enzyme that specifically depalmitoylates proteins. Infusion of APT1 into permeabilized platelets inhibited SFLLRN-induced P-selectin expression in a dose-dependent manner with an IC50 of 350 nM. The observations that SFLLRN is a more potent inducer of protein palmitoylation than other platelet agonists and that inhibitors of protein palmitoylation block SFLLRN-mediated platelet activation demonstrate that protein palmitoylation plays a prominent role in signaling downstream of PAR1.

Fondaparinux: a Factor Xa inhibitor for antithrombotic therapy

Fondaparinux (Arixtra®) is the first of a new class of antithrombotic compounds – Factor Xa inhibitors. This synthetic pentasaccharide acts by inhibiting Factor Xa selectively. Its efficacy and safety have been demonstrated in animal models of venous and arterial thromboses and bleeding risk. In humans, its pharmacokinetic profile after subcutaneous injection shows a rapid onset of antithrombotic activity and an elimination half-life that reliably allows once-daily dosing. As the inter-subject variability is limited, no laboratory monitoring of coagulation parameters is needed. The efficacy and safety of fondaparinux have been examined in several Phase II and III clinical trials. So far, the largest programme has involved ∼ 9000 patients undergoing major orthopaedic surgery of the lower limbs. In the setting of short-term prophylaxis, the efficacy of fondaparinux for preventing venous thromboembolism was significantly superior to that of the low-molecular-weight heparin, enoxaparin (common reduction in risk: 50.6%; p < 0.001). The benefit of extended thromboprophylaxis with fondaparinux in hip fracture surgery patients was also demonstrated. Fondaparinux also showed benefit in the prevention of venous thromboembolism in other surgical and medical settings and in the treatment of patients with venous thromboembolism. Overall, fondaparinux therapy was as well-tolerated as currently available treatments. In conclusion, selective inhibition of Factor Xa is an effective antithrombotic strategy. Fondaparinux may substantially improve the prevention and treatment of thrombosis. Fondaparinux 2.5 mg once-daily s.c. has been approved for the prevention of venous thromboembolism after major orthopaedic surgery. Fondaparinux use in extended prophylaxis (4 weeks) after hip fracture surgery has also been recently approved.

New Pentasaccharides for Prophylaxis of Deep Vein Thrombosis: Pharmacology

Fondaparinux is the first of a new class of antithrombotic compounds, the synthetic pentasaccharides. By binding rapidly and strongly to antithrombin, its sole physiologic target in plasma, fondaparinux catalyzes specifically the inhibition of factor Xa, which results in effective and linear dose-dependent inhibition of thrombin generation. Fondaparinux does not bind to platelets. Its antithrombotic effect has been demonstrated in several animal models of arterial and venous thrombosis. At equivalent antithrombotic concentrations, fondaparinux induced less bleeding than unfractionated heparin in experimental bleeding models. Furthermore, it did not cross-react with sera from patients with heparin-induced thrombocytopenia. Administered subcutaneously, the absorption of fondaparinux is complete, rapid, and independent of dose. It has a linear pharmacokinetic profile, and its half-life of approximately 17 h allows for once-daily dosing. Fondaparinux is almost completely excreted by the kidneys. Owing to the limited intrasubject and intersubject variability, routine monitoring and dose adjustments should not be required for most patients. Fondaparinux has been approved for use in thromboprophylaxis after major orthopedic surgery, where it has demonstrated its efficacy compared to a low-molecular-weight heparin. Its clinical development in other indications is ongoing.

New Pentasaccharides for the Prophylaxis of Venous Thromboembolism: Clinical Studies

Fondaparinux, a selective inhibitor of factor Xa, is the first of a new class of antithrombotic compounds, the synthetic pentasaccharides. Its benefit-to-risk ratio in preventing venous thromboembolism after major orthopedic surgery was investigated in four randomized, double-blind international phase III trials in patients undergoing surgery for hip fracture, elective hip replacement, and major knee surgery. Compared to enoxaparin, fondaparinux administered at a subcutaneous dose of 2.5 mg qd, starting postoperatively, reduced the overall incidence of venous thromboembolism up to day 11 by 55.2% (p < 0.001). The incidence of clinically relevant bleeding was low and did not differ between the two groups. Overall, fondaparinux achieved optimal efficacy and safety when treatment was initiated > or =6 h after the surgical procedure. In a further randomized double-blind trial, 4 weeks of prophylaxis with fondaparinux after hip fracture surgery reduced the risk of venous thromboembolism by 96% as compared to 1 week of prophylaxis, and was well tolerated. Fondaparinux has been recently approved for use in thromboprophylaxis after major orthopedic surgery. The clinical development of fondaparinux in other thromboprophylactic indications is ongoing.

The antiaggregating activity of clopidogrel is not affected by N-acetyl L-cysteine.

N-acetyl L-cysteine (NAC) is widely used to treat obstructive bronchopulmonary diseases. It has thiol reactive properties, accounting for its mucolytic activity. Clopidogrel is a potent antithrombotic compound, metabolised by the liver which generates an active metabolite containing a thiol reactive group, responsible for an irreversible interaction with the platelet P2Y(12) ADP receptor. The aim of this study was to determine if NAC interferes with the antiaggregating activity of clopidogrel. For this purpose, NAC (100 micro M) was incubated with platelets from rats treated or not with clopidogrel (5 mg/kg, PO, -2 h). Clopidogrel treatment strongly inhibited aggregation but this effect was not modified by NAC. In another experiment, a low concentration of the active metabolite of clopidogrel (0.3 micro g/ml) was incubated with platelets from men or rats, in the absence or presence of NAC (100 micro M). When stimulated by ADP (2.5 micro M), platelet aggregation was inhibited by the active metabolite when incubated alone. In the presence of NAC, the inhibition by the active metabolite was not modified, therefore clearly indicating that NAC cannot reduce the thiol reactive part of the active metabolite of clopidogrel and does not interfere with its anti-aggregating activity. Moreover, in rats treated for 5 days with NAC (150 mg/kg), the activity of clopidogrel (5 or 10 mg/kg) against ADP-induced platelet aggregation was neither inhibited nor increased. This demonstrates that the generation of the active metabolite of clopidogrel is not affected by NAC. In conclusion, we have found that NAC does not restore the "normal" properties of P2Y(12) on platelets from clopidogrel-treated animals, it does not interfere with the antiaggregating activity of the active metabolite of clopidogrel, and does not interfere with the generation of the active metabolite.

D-003, a potential antithrombotic compound isolated from sugar cane wax with effects on arachidonic acid metabolites

D-003 is a natural mixture of higher primary saturated aliphatic acids purified from sugar cane wax, whose main component is octacosanoic acid followed by triacontanoic, dotriacontanoic, and tetratriacontanoic acids. D-003 inhibits platelet aggregation and arterial thrombosis experimentally induced in a dose-dependent fashion. This study was undertaken to investigate the effects of D-003 (25 and 200 mg/kg) in experimental models of venous thrombosis and on plasma levels of two metabolites from arachidonic acid (AA) : thromboxane A 2 (TxA 2) and prostacyclin (PgI 2). D-003 orally administered as single doses of 200 mg/kg, but not at 25 mg/kg, significantly increased plasma levels of 6 keto PgF1α levels, a stable metabolite of PgI 2 in PPP obtained from collagen-stimulated blood (4 μg/ml) compared with control group. Nevertheless, levels of 6 keto PgF1α levels determined after 10 days of oral treatment with both doses of D-003 were significantly larger than those of the controls. Likewise, single and repeated oral doses of D-003 (25 and 200 g/kg) significantly reduced the TxB 2 and MDA plasma levels obtained from whole blood stimulated by collagen. Hence, TxB 2/6 keto PgF1α ratio significantly decreased in animals treated with D-003. Single and repeated oral doses of D-003 (25 and 200 mg/kg) significantly reduced the weight of venous thrombus experimentally induced in rats. D-003 at single doses (400 mg/kg but not 200 mg/kg) significantly protected from death induced by endovenous infusion of collagen plus epinephrine in mice. The present results support that these effects of D-003 on AA metabolites could explain, at least partially, its antiplatelet and antithrombotic effects.

Inhibition of Platelet Adhesion and Aggregation by a Defined Region (Gly-486–Lys-502) of High Molecular Weight Kininogen

Proteolytic cleavage of single chain high molecular weight kininogen (HK) by kallikrein releases the short-lived vasodilator bradykinin and leaves behind two-chain high molecular weight kininogen (HKa). HKa and particularly its His-Gly-Lys-rich domain 5 have been previously reported to exert anti-adhesive properties by binding to the extracellular matrix protein vitronectin (VN). In this study the ability of HKa and domain 5 to interfere with platelet adhesion and aggregation was investigated. In a purified system HKa and particularly domain 5 but not HK inhibited the binding of VN to the alpha(IIb)beta(3) integrin, whereas the binding of fibrinogen to this integrin was not affected. The region Gly-486-Lys-502 from the carboxyl terminus of the domain 5 was identified as responsible for inhibition of the VN-alpha(IIb)beta(3)-integrin interaction, as this portion was also found to mediate kininogen binding to VN. Through these interactions, HKa, the isolated domain 5, and the peptide Gly-486-Lys-502 abrogated the alpha(IIb)beta(3)-integrin-dependent adhesion of human platelets to VN but not to fibrinogen. The codistribution of VN and HKa at sites of ex vivo platelet aggregation was demonstrated by transmission immune electron microscopy, indicating that the described interaction is likely to take place in vivo. Moreover, domain 5 and the peptide Gly-486-Lys-502 dose-dependently blocked platelet aggregation, resembling the inhibitory effect of monoclonal antibody 13H1 against multimeric VN. Finally, treatment of mice with isolated domain 5 resulted in a significantly prolonged tail bleeding time. Taken together, our data emphasize the inhibitory role of HK domain 5 on platelet adhesion and aggregation; new anti-thrombotic compounds may become available on the basis of peptide Gly-486-Lys-502 of HK domain 5.

Assessment of ex vivo Pharmacodynamic Markers during Inhibition of Thrombosis by CI-1031 (ZK-807834), a Novel Direct Factor Xa Inhibitor

CI-1031 (ZK-807834) is a novel, synthetic factor Xa (FXa) inhibitor with a Ki of 0.11 nM against human FXa. In human plasma in vitro, CI-1031 doubled PT and aPTT at 0.23 and 0.49 µM, respectively. The in vivo antithrombotic effect of CI-1031 was evaluated in a veno-venous shunt model of thrombosis in anesthetized rabbits. After thrombus formation was verified in the first shunts, rabbits received either vehicle or CI-1031 intravenously (bolus injection of 60, 240, or 480 µg/kg followed by an infusion of 2, 8, or 16 µg/kg/min for 140 min, respectively). The second shunts were inserted after 20 min of infusion of CI-1031 or vehicle. CI-1031 dose-dependently prolonged time to occlusion (TTO) in the second shunts (35 ± 21, 62 ± 24, and 120 ± 0 min for the three dose groups, respectively, vs. 10 ± 1 min for vehicle). Thrombus mass (TM) was reduced in a dose-dependent manner by CI-1031 (42 ± 7, 27 ± 6, and 18 ± 4 mg vs. 50 ± 4 mg for vehicle). Maximal TM reduction was 70% with an IC₅₀ of 0.6 µg/ml. Among all the coagulation parameters tested, PT had the best correlation with plasma CI- 1031 concentration (r = 0.97). Ex vivo plasma anti-FXa activity was also well correlated with plasma concentration of CI-1031 and with PT (r = 0.96 and 0.98, respectively). These results indicate that CI-1031, which is currently undergoing clinical evaluation, is an effective antithrombotic compound with a favorable efficacy-to-bleeding ratio. In addition, CI-1031 concentration in plasma can be monitored using PT or anti-Xa assays, thereby providing reliable methods to ensure safe and accurate dose titration of CI-1031.

Recent advances in the status and targets of antithrombotic agents

Cardiovascular diseases associated with intravascular thrombosis are the most common cause of death in both developed and developing countries, Clinical and experimental studies indicate that abnormalities in the normal blood flow, activation of platelets, coagulation cascade or fibrinolysis contribute to the pathogenesis of intravascular thrombosis/thromboembolism. Although the treatment strategy for thrombosis has improved with newer diagnostic and surgical tools, effective antithrombotic therapy with minimal side effects still poses a challenge to scientists around the globe. There has been an intense effort in the last decade to understand mechanisms involved in thrombosis and also to develop novel antithrombotic agents for the treatment and prevention of these disorders. Advanced understanding of the molecular mechanisms of thrombosis has helped in the development of newer and specific experimental models for evaluating the antithrombotic agents with high specificity. These developments are also expected to minimize time and the cost incurred on the newer potential antithrombotic drugs. This review describes various important targets for effective antithrombotic action. Methods for evaluating potential antithrombotic compounds against each target and status of the newer compounds developed in the respective classes are also covered.

Activation of Caspase-3/Caspase-3-Like Activity in Rat Cardiomyocytes by an RGD Peptide, but not the GPIIb/IIIa Antagonist Lotrafiban

Platelet glycoprotein GPIIb/IIIa antagonists are antithrombotic compounds that interfere with fibrinogen binding to this integrin receptor and prevent subsequent platelet aggregation [1]. Several intravenous GPIIb/IIIa antagonists (including abciximab, eptifibatide, and tirofiban) are currently employed during acute coronary syndrome and/or percutaneous coronary intervention in order to reduce the incidence of associated thrombotic events. These acute treatments are well tolerated and have been shown to decrease the rates of subsequent myocardial infarction (MI) and death (for review, see Refs. [2–4]). In contrast, clinical trials examining the potential benefit of chronic treatment with orally active GPIIb/IIIa antagonists for the prevention of thrombotic events (i.e., stroke, MI) have all failed due to lack of efficacy, often with an increase in the incidence of mortality and MI [5–8].

Coagulation factor Xa inhibition: biological background and rationale.

Ischemic heart disease and cerebrovascular disease are the leading causes of death in the world. Surprisingly, these diseases are treated by relatively antiquated drugs. However, due to our improved understanding of the underlying pathology of these diseases, and a number of technological advances in tools for drug discovery and chemical optimization, an exciting new wave of antithrombotic compounds is beginning to emerge in clinical trials. These agents, referred to as direct coagulation factor Xa inhibitors, appear to provide an enhanced risk-benefit margin compared to conventional therapy. Preclinical and early clinical data gathered over the past few years suggests that direct fXa inhibitors will provide the necessary advancements in efficacy, safety, and ease of use required to displace conventional therapy. Whether or not these agents will succeed will be determined as this class of agents advances through clinical trials in the near future. This review describes some of the key studies that sparked an interest in fXa as a therapeutic target, highlighting the findings that provided important rationale for continuing the development of potent and selective direct fXa inhibitors.

Antithrombotic effect of LB-30057 (CI-1028), a new synthetic thrombin inhibitor, in a rabbit model of thrombosis: comparison with inogatran.

LB-30057 (CI-1028) is a novel, orally bioavailable, direct thrombin inhibitor with a Ki of 0.38 nM against human thrombin. The effects of LB-30057 on thrombus formation and hemostasis were evaluated in a veno-venous shunt model of thrombosis in rabbits, and compared with inogatran, another direct inhibitor of thrombin. Each compound was studied at 5 or 6 different doses with 5 or 6 rabbits in each group. After administration as a bolus i.v. injection followed by continuous infusion, both LB-30057 and inogatran dose-dependently inhibited thrombus formation, which was measured as an increase in time to occlusion (TTO) and a decrease in thrombus weight. Both compounds also improved vena caval blood flow and reduced the overall incidence of thrombotic occlusion. LB-30057 significantly prolonged TTO from 23 +/- 4 min (before dose) to 110 +/- 10 min at the highest dose (0.7 mg/kg + 47 microg/kg/min) (p < 0.001), and reduced thrombus weight from 57 +/- 2 mg to 15 +/- 5 mg (p < 0.001). Occlusive thrombus formed in only one of six rabbits that received the highest dose of LB-30057 (vs. 13/13 in the control group, p < 0.01). At the dose that produced the maximum antithrombotic effect (0.7 mg/kg + 47 microg/kg/min), LB-30057 increased aPTT and bleeding time approximately 2-and 2.5-fold above baseline, respectively. On a gravimetric basis, LB-30057 and inogatran displayed comparable in vivo antithrombotic efficacy. When compared to equally effective anti thrombotic doses of inogatran, LB-30057 caused less prolongation in aPTT, had no effect on PT, and tended to have less of effect on bleeding time. These results indicate that LB-30057 is an effective antithrombotic compound and it appears to have a better benefit/risk profile than inogatran in this experimental model.

Perivascular graft heparin delivery using biodegradable polymer wraps

Heparin remains the gold-standard inhibitor of the processes involved in the vascular response to injury. Though this compound has profound and wide-reaching effects on vascular cells in culture and animal models, its clinical utility has been questionable at best. It is clear that the mode of heparin delivery is critical to its potential and it may well be that routine forms of administration are insufficient to observe benefit given the heparin's short half-life and complex pharmacokinetics. When ingested orally, heparin is degraded to inactive oligomer fragments while systemic administration is complicated by the need for continuous infusion and the potential for uncontrolled hemorrhage. Thus alternative heparin delivery systems have been proposed to maximize regional effects while limiting systemic toxicity. Yet, as heparin is such a potent antithrombotic compound and since existing local delivery systems lack the ability to precisely regulate release kinetics, even site-specific therapy is prone to bleeding. We now describe the design and development of a novel biodegradable system for the perivascular delivery of heparin to the blood vessel wall with well-defined release kinetics. This system consists of heparin-encapsulated poly( dl lactide- co-glycolide) (pLGA) microspheres sequestered in an alginate gel. Controlled release of heparin from this heterogeneous system could be obtained over a period of 25 days in vitro. The experimental variables affecting heparin release from these matrices were investigated. Gel permeation chromatography (GPC) and scanning electron microscopy (SEM) were used to monitor the degradation process and found to correlate well with the release kinetics. Heparin-releasing gels inhibited growth of bovine vascular smooth muscle cells in tissue culture in a dose-dependent manner. Moreover, gel release controlled vascular injury in denuding and interposition vascular graft animal models of disease even when uncontrolled bleeding was evident with standard matrix-type release. This system may therefore provide an effective means of examining the effects of various compounds in the control of smooth muscle cell proliferation in accelerated arteriopathies and also shed light on the biologic nature of these processes.

Cloning, characterization, and functional studies of human and mouse glycoprotein VI: a platelet-specific collagen receptor from the immunoglobulin superfamily

Injuries to the vessel wall and subsequent exposure of collagen from the subendothelial matrix result in thrombus formation. In physiological conditions, the platelet plug limits blood loss. However, in pathologic conditions, such as rupture of atherosclerotic plaques, platelet–collagen interactions are associated with cardiovascular and cerebral vascular diseases. Platelet glycoprotein VI (GPVI) plays a crucial role in collagen-induced activation and aggregation of platelets, and people who are deficient in GPVI suffer from bleeding disorders. Based on the fact that GPVI is coupled to the Fc receptor (FcR)-γ chain and thus should share homology with the FcR chains, the genes encoding human and mouse GPVI were identified. They belong to the immunoglobulin (Ig) superfamily and share 64% homology at the protein level. Functional evidence demonstrating the identity of the recombinant protein with GPVI was shown by binding to its natural ligand collagen; binding to convulxin (Cvx), a GPVI-specific ligand from snake venom; binding of anti-GPVI IgG isolated from a patient; and association to the FcR-γ chain. The study also demonstrated that the soluble protein blocks Cvx and collagen-induced platelet aggregation and that GPVI expression is restricted to megakaryocytes and platelets. Finally, human GPVI was mapped to chromosome 19, long arm, region 1, band 3 (19q13), in the same region as multiple members of the Ig superfamily. This work offers the opportunity to explore the involvement of GPVI in thrombotic disease, to develop alternative antithrombotic compounds, and to characterize the mechanism involved in GPVI genetic deficiencies.

Cloning, characterization, and functional studies of human and mouse glycoprotein VI: a platelet-specific collagen receptor from the immunoglobulin superfamily

AbstractInjuries to the vessel wall and subsequent exposure of collagen from the subendothelial matrix result in thrombus formation. In physiological conditions, the platelet plug limits blood loss. However, in pathologic conditions, such as rupture of atherosclerotic plaques, platelet–collagen interactions are associated with cardiovascular and cerebral vascular diseases. Platelet glycoprotein VI (GPVI) plays a crucial role in collagen-induced activation and aggregation of platelets, and people who are deficient in GPVI suffer from bleeding disorders. Based on the fact that GPVI is coupled to the Fc receptor (FcR)-γ chain and thus should share homology with the FcR chains, the genes encoding human and mouse GPVI were identified. They belong to the immunoglobulin (Ig) superfamily and share 64% homology at the protein level. Functional evidence demonstrating the identity of the recombinant protein with GPVI was shown by binding to its natural ligand collagen; binding to convulxin (Cvx), a GPVI-specific ligand from snake venom; binding of anti-GPVI IgG isolated from a patient; and association to the FcR-γ chain. The study also demonstrated that the soluble protein blocks Cvx and collagen-induced platelet aggregation and that GPVI expression is restricted to megakaryocytes and platelets. Finally, human GPVI was mapped to chromosome 19, long arm, region 1, band 3 (19q13), in the same region as multiple members of the Ig superfamily. This work offers the opportunity to explore the involvement of GPVI in thrombotic disease, to develop alternative antithrombotic compounds, and to characterize the mechanism involved in GPVI genetic deficiencies.

Structure–Activity Relationships for Selected Sulfur-Rich Antithrombotic Compounds

We assessed the antithrombotic activity of some simple organosulfur compounds which have some of the functionality found in the disulfide ajoene, a pharmacologically active compound isolated from garlic. The results establish that antithrombotic activity is associated with disulfides directly attached to a phenyl ring and is further enhanced by an α-sulfonyl group. CH3SO2CH2SSPh proved to be a potent inhibitor of platelet aggregation with an IC50 of 5 μM.

Contribution of in vivo models of thrombosis to the discovery and development of novel antithrombotic agents

Cardiovascular and cerebrovascular diseases continue to be the leading cause of death throughout the world. Over the past two decades, great advances have been made in the pharmacological treatment and prevention of thrombotic disorders (e.g., tissue plasminogen activators, platelet GPIIb/IIIa antagonists, ADP receptor antagonists such as clopidogrel, low-molecular weight heparins, and direct thrombin inhibitors). New research is leading to the next generation of antithrombotic compounds such as direct coagulation FVIIa inhibitors, tissue factor pathway inhibitors, gene therapy, and orally active direct thrombin inhibitors and coagulation Factor Xa (FXa) inhibitors. Animal models of thrombosis have played a crucial role in discovering and validiting novel drug targets, selecting new agents for clinical evaluation, and providing dosing and safety information for clinical trials. In addition, these models have provided valuable information regarding the mechanisms of these new agents and the interactions between antithrombotic agents that work by different mechanisms. This review briefly presents the pivitol preclinical studies that led to the development of drugs that have proven to be effective clinicallly. The role that animal models of thrombosis are playing in the discovery and development of novel antithrombotic agents is also described, with specific emphasis on FXa inhibitors. The major issues regarding the use of animal models of thrombosis, such as the use of positive controls, appropriate pharmacodynamic markers of activity, safety evaluation, species-specificity, and pharmacokinetics, are highlighted. Finally, the use of genetic models in thrombosis/hemostasis research and pharmacology is presented using gene-therapy for hemophilia as an example of how animal models have aided in the development of these therapies that are now being evaluated clinically. In summary, animal models have contributed greatly to the discovery of currently available antithrombotic agents and will play a primary role in the discovery and characterization of the novel antithrombotic agents that will provide safe and effective pharmacological treatment for life-threatening thrombotic diseases.

Successful Attenuation of Venous Thrombus Growth in Rabbits after the Administration of a Novel Oral Thrombin Inhibitor

Current antithrombotic compounds have several limitations in clinical practice. The present study was designed to investigate a novel orally available direct thrombin inhibitor, BSF 208791. Intravenous administration of BSF 208791 showed superior antithrombotic properties as compared with Polyethylenglycol-Hirudin (PEG-Hirudin) and low molecular weight heparin (LMWH) in a model of venous thrombosis in rabbits. The thrombus growth was 22%, 30%, 37% and 50% after BSF 208791, PEG-Hirudin. LMWH, and saline administration, respectively. Moreover, bleeding time was less affected after administration of BSF 208791 as compared with PEG-Hirudin. The oral administration of BSF 208791 resulted in adequate bioavailability and significantly reduced venous thrombus growth to 36% as compared with 60% in the saline treated rabbits. The antithrombotic effect of BSF 208791 appears to be superior to PEG-Hiridin and LMWH without affecting the bleeding time. BSF 208791 is an orally available agent that might be a promising candidate for future antithrombotic therapy.

Effect of Factor Xa Inhibitors on the Platelet-derived Microparticles Procoagulant Activity In Vitro&gt; and In Vivo in Rats

The aim of this study was to investigate the effect of factor Xa inhibitors on the prothrombinase activity of platelet-derived microparticles in vitro and in vivo. The factor Xa inhibitors studied were DX9065A (a direct factor Xa inhibitor) and Sanorg34006 (an antithrombin (AT)-mediated factor Xa inhibitor). Microparticles formed from the platelet surface following activation were isolated by size exclusion gel chromatography. After purification, their presence was detected by their procoagulant activity and by flow cytometry. Our results show that factor Xa and/or factor Va were present at the surface of the platelet-derived microparticles. Prothrombinase formed on the microparticles was inhibited by factor Xa inhibitors at IC50 values of 0.45+/-0.05 and 0.045+/-0.005 microM for DX9065A and AT-Sanorg34006 respectively. In an experiment aimed at determining the kinetics of microparticles formation we demonstrated that thrombin traces were sufficient to induce the formation of a significant quantity of microparticles. Both factor Xa inhibitors delayed the formation of microparticles by delaying thrombin generation. The thrombogenic effect of the microparticles were studied in vivo in a modified arterio-venous shunt model in the rat. In this model, the increase in the thrombus weigh due to microparticles or phospholipids did not differ significantly (33% and 23% respectively). In these conditions, prothrombinase activity seemed to play a lesser role in the thrombogenic effect than phospholipids. Nevertheless, factor Xa inhibitors were efficient and inhibited thrombus formation in a dose-dependent manner. These results demonstrate that platelet-derived microparticles display a potent prothrombotic effect in vivo and show that factor Xa inhibitors are potent antithrombotic compounds when thrombosis was induced by microparticles.