The inhibitors of the tissue factor:factor VII pathway

It is widely accepted that blood coagulation in vivo is initiated during normal hemostasis, as well as during intravascular thrombus formation, when the cell-surface protein, tissue factor (TF), is exposed to the blood as a consequence of vascular injury. In addition to its essential role in hemostasis, tissue factor may be also implicated in several pathophysiological processes, such as intracellular signaling, cell proliferation, and inflammation. For these reasons, the tissue factor:factor VIIa complex has been the subject of intense research focus. Many experimental studies have demonstrated that inhibition of tissue factor:factor VIIa procoagulant activity are powerful inhibitors of in vivo thrombosis and that this approach usually results in less pronounced bleeding tendency, as compared to other "more classical" antithrombotic interventions. Alternative approaches may be represented by transfecting the arterial wall with natural inhibitors of tissue factor:factor VIIa complex, such as tissue factor pathway inhibitor (TFPI), which may result in complete inhibition of local thrombosis without incurring in potentially harmful systemic effects. Additional studies are warranted to determine the efficacy and safety of such approaches in patients.     

Endothelium-derived relaxing factor modulates platelet aggregation in an in vivo model of recurrent platelet activation.

It has been shown that endothelium-derived relaxing factor (EDRF) may inhibit platelet aggregation in vitro through activation of platelet-soluble guanylate cyclase. To assess whether EDRF may also affect platelet function in vivo, intravascular platelet aggregation was initiated by placing an external constrictor around endothelially injured rabbit carotid arteries. Carotid blood flow velocity was measured continuously by a Doppler flow probe placed proximal to the constrictor. After placement of the constrictor, cyclic flow reductions (CFRs), due to recurrent platelet aggregation, developed at the site of the stenosis. After CFRs were observed for 30 minutes, a solution of authentic nitric oxide (NO, n = 10) was infused into the carotid artery via a small catheter placed proximally to the stenosis. Before infusion of NO, CFR frequency averaged 18.3 +/- 2.9 cycles per hour, and CFR severity (lowest carotid blood flow as percentage of baseline values) was 6 +/- 1%. NO completely inhibited CFRs in all animals, as shown by the normal and constant pattern of carotid blood flow (CFR frequency, 0 cycles per hour, p < 0.001; carotid blood flow, 92 +/- 5%, p = NS versus baseline). These effects were transient; CFRs were restored spontaneously within 10 minutes after cessation of NO infusion. After CFRs returned, S-nitroso-cysteine (S-NO-cys), a proposed form of EDRF, was infused into the carotid artery. S-NO-cys also abolished CFRs in all animals but at a significantly lower dose than NO (0.3 +/- 0.1 versus 12 +/- 4 nmol/min). The role of endogenously released EDRF in modulating in vivo platelet function was then tested in additional experiments. In 10 animals, endogenous release of EDRF was stimulated by infusing acetylcholine into the aortic root during CFRs. Infusion of acetylcholine was also associated with a complete inhibition of CFRs, similar to that observed during exogenous infusion of NO or S-NO-cys. These antithrombotic effects of acetylcholine were completely lost when EDRF synthesis was prevented by administration of the L-arginine analogue NG-monomethyl L-arginine (L-NMMA). Furthermore, in six additional rabbits the basal release of EDRF was blocked by L-NMMA after CFRs had been previously abolished with aspirin or the combination of aspirin and ketanserin, a serotonin S2 receptor antagonist. L-NMMA caused restoration of CFRs in all animals, indicating that even the basal release of EDRF is important in modulating platelet reactivity in vivo. Taken together, the data of the present study demonstrate that endogenous EDRF might importantly contribute to the modulation of platelet function in vivo.     

Expression of exogenous tissue factor pathway inhibitor in vivo suppresses thrombus formation in injured rabbit carotid arteries

OBJECTIVES: The aim of the present study was to test the hypothesis that retrovirus-mediated in vivo tissue factor pathway inhibitor (TFPI) gene transfer to the arterial wall would efficiently inhibit thrombosis without causing significant changes in systemic hemostatic variables. BACKGROUND: Acute coronary syndromes (unstable angina and acute myocardial infarction) are usually caused by atherosclerotic plaque rupture, with consequent activation of the coagulation cascade and circulating platelets. Tissue factor (TF) exposure represents an early event in this pathophysiologic sequence, leading to activation of the extrinsic coagulation pathway and thrombin formation. Tissue factor pathway inhibitor is a naturally occurring inhibitor of the extrinsic pathway. METHODS: In the present study, the gene coding for rabbit TFPI was inserted in a retroviral vector under control of a tetracycline-inducible promoter. Replication-defective, infectious, recombinant retroviruses were used to transfect rabbit carotid arteries with either TFPI or a reporter gene--green fluorescent protein (GFP). RESULTS: Retroviral-mediated arterial gene transfer of TFPI resulted in potent inhibition of intravascular thrombus formation in stenotic and injured rabbit carotid arteries, whereas transfection of the contralateral carotid artery with GFP had no effect on thrombosis. No significant changes in systemic hemostatic variables (prothrombin time and partial thromboplastin time) were observed when thrombosis was inhibited. CONCLUSIONS: These data suggest that retroviral-mediated transfection of the arterial wall with TFPI might represent an attractive approach for the treatment of thrombotic disorders.     

Local platelet activation causes vasoconstriction of large epicardial canine coronary arteries in vivo. Thromboxane A2 and serotonin are possible mediators

The goal of the present study was to demonstrate that intracoronary platelet deposition may trigger intense vasoconstriction of large epicardial coronary arteries in vivo and that this is largely mediated by thromboxane A2 and serotonin released by activated platelets. Cyclic flow variations (progressive declines in blood flow followed by sudden restorations of flow) due to recurrent intracoronary platelet activation and thrombus formation were induced by damaging the endothelium and placing a cylindrical constrictor on the left anterior descending coronary artery (LAD) in open-chest, anesthetized dogs. Coronary diameters were measured in vivo by means of ultrasonic crystals sutured on the LAD immediately distal to the constrictor (LAD1) and 1 cm below (LAD2) and on the circumflex coronary artery (Cx). Coronary artery diastolic diameters were measured continuously before and during cyclic flow variations and after they were abolished by administration of LY53857, a serotonin-receptor antagonist (group 1, n = 7), or SQ29548, a thromboxane-receptor antagonist (group 2, n = 7). During cyclic flow variations, at the nadir of coronary flow, LAD1 (a site of maximal platelet accumulation) cross-sectional area decreased by 52 +/- 10% and 38 +/- 6% in group 1 and 2 animals, respectively (p less than 0.001 compared with values recorded during a brief LAD occlusion obtained by a suture snare), whereas LAD2 (a site of minimal or no platelet accumulation) cross-sectional area did not differ from that recorded during the brief LAD occlusion. SQ29548 abolished cyclic flow variations in seven of seven dogs and LY53857 in six of seven, but they affected the increased coronary vasoconstriction differently: LAD1 cross-sectional area increased by 32 +/- 6% of the control value in SQ29548-treated animals, whereas it returned to baseline dimension values in the LY53857-treated group as these interventions also abolished the cyclic flow variations. We conclude that a marked coronary vasoconstriction may be triggered by local platelet deposition and that thromboxane A2 and serotonin are mediators of this vasoconstriction.     

Inflammation and Cardiovascular Disease: From Pathogenesis to Therapeutic Target

Atherosclerosis represents the most common pathological substrate of coronary heart disease (CHD), and the characterization of the disease as a chronic low-grade inflammatory condition is now largely accepted. A number of mediators of inflammation have been widely studied, both as surrogate biomarkers and as causal agents, in the pathophysiological network of atherogenesis and plaque vulnerability. The epidemiological observation that biomarkers of inflammation are associated with clinical cardiovascular risk supports the theory that targeted anti-inflammatory treatment appears to be a promising strategy in reducing residual cardiovascular risk on the background of traditional medical therapy. A large number of randomized controlled trials have shown that drugs commonly used in cardiovascular disease (CVD), such as statins, may be effective in the primary and secondary prevention of cardiovascular events through an anti-inflammatory effect. Moreover, several anti-inflammatory drugs are being tested for their potential to reduce residual cardiovascular risk on the background of validated medical therapy for atherosclerotic disease. In this paper, we review relevant evidence with regard to the relationship between inflammation and CVD, from pathogenesis to therapeutic strategies.     

Evidence that combined thromboxane A2 and serotonin receptor blockade might prevent coronary artery thrombosis and the conversion from chronic to acute coronary heart disease syndromes

Evidence that combined thromboxane A2 and serotonin blockade may prevent coronary artery thrombosis and the conversion from chronic to acute coronary heart disease syndromes is considered. Available data from clinical and experimental animal studies are consistent with the hypothesis that interference with thromboxane and serotonin's contributions to platelet aggregation and dynamic coronary artery constriction might prevent the conversion from chronic to acute disorders, including the development of unstable angina and myocardial infarction in some patients at risk. Substantial protection preventing the conversion from stable to unstable angina and myocardial infarction may very well require both thromboxane and serotonin receptor antagonists or the combination of a thromboxane synthesis inhibitor and receptor antagonist with a serotonin receptor antagonist. Future clinical studies should test this hypothesis directly.