Tissue factor controversies

Rationale for the Potential Use of Recombinant Activated Factor VII in Severe Post-Partum Hemorrhage

Severe post-partum hemorrhage (PPH) is a major cause of maternal mortality worldwide. Recombinant activated factor VII (rFVIIa) has recently been approved by the European Medicines Agency for the treatment of severe PPH if uterotonics fail to achieve hemostasis. Although large randomized controlled trials are lacking, accumulated evidence from smaller studies and international registries supports the efficacy of rFVIIa alongside extended standard treatment to control severe PPH. Because rFVIIa neither substitutes the activity of a missing coagulation factor nor bypasses a coagulation defect in this population, it is not immediately evident how it exerts its beneficial effect. Here, we discuss possible mechanistic explanations for the efficacy of rFVIIa and the published evidence in patients with severe PPH. Recombinant FVIIa may not primarily increase systemic thrombin generation, but may promote local thrombin generation through binding to activated platelets at the site of vascular wall injury. This explanation may also address safety concerns that have been raised over the administration of a procoagulant molecule in a background of increased thromboembolic risk due to both pregnancy-related hemostatic changes and the hemorrhagic state. However, the available safety data for this and other indications are reassuring and the rates of thromboembolic events do not appear to be increased in women with severe PPH treated with rFVIIa. We recommend that the administration of rFVIIa be considered before dilutional coagulopathy develops and used to support the current standard treatment in certain patients with severe PPH.

Coagulation Disorders, Testing, and Treatment in Exotic Animal Critical Care

Despite poor recognition in the literature, exotic companion animals are affected by many diseases that can result in disordered coagulation and fibrinolysis. This article outlines current knowledge of hemostasis, common diagnostic tests and reviews reported diseases associated with coagulopathy in small mammals, bird and reptiles. A range of conditions affect platelets and thrombocytes, endothelium and blood vessels, and plasma clotting factors. Improved recognition and monitoring of hemostatic disorders will enable targeted therapy and improved case outcomes.

Extracellular Vesicles of COVID-19 Patients Reflect Inflammation, Thrombogenicity, and Disease Severity

Severe COVID-19 infections present with cytokine storms, hypercoagulation, and acute respiratory distress syndrome, with extracellular vesicles (EVs) being involved in coagulation and inflammation. This study aimed to determine whether coagulation profiles and EVs reflect COVID-19 disease severity. Thirty-six patients with symptomatic COVID-19 infection with mild/moderate/severe disease (12 in each group) were analyzed. Sixteen healthy individuals served as controls. Coagulation profiles and EV characteristics were tested by nanoparticle tracking analysis (NTA), flow cytometry, and Western blot. While coagulation factors VII, V, VIII, and vWF were comparable, significant differences were found in patients' D-Dimer/fibrinogen/free protein S levels compared to controls. Severe patients' EVs displayed higher percentages of small EVs (<150 nm) with increased expression of exosome marker CD63. Severe patients' EVs displayed high levels of platelet markers (CD41) and coagulation factors (tissue factor activity, endothelial protein C receptor). EVs of patients with moderate/severe disease expressed significantly higher levels of immune cell markers (CD4/CD8/CD14) and contained higher levels of IL-6. We demonstrated that EVs, but not the coagulation profile, may serve as biomarkers for COVID-19 severity. EVs demonstrated elevated levels of immune- and vascular-related markers in patients with moderate/severe disease, and may play a role in disease pathogenesis.

Hemostasis based on a novel 'two-path unifying theory' and classification of hemostatic disorders

: Hemostasis is the most important protective mechanism for human survival following harmful vascular damage caused by internal disease or external injury. Physiological mechanism of hemostasis is partially understood. Hemostasis can be initiated by either intravascular injury or external bodily injury involving two different levels of damage [i.e., limited to the endothelium or combined with extravascular tissue (EVT)]. In intravascular injury, traumatic damage limited to local endothelium typically is of no consequence, but disease-induced endothelial damage associated with systemic endothelial injury seen in sepsis and other critical illnesses could cause generalized 'endotheliopathy'. It triggers no bleeding but promotes serious endothelial molecular response. If intravascular local trauma extends beyond the endothelium and into EVT, it causes intravascular 'bleeding' and initiate 'clotting' via normal hemostasis. In external bodily injury, local traumatic damage always extends to the endothelium and EVT, and triggers 'bleeding' and 'clotting'. Systemic endotheliopathy activates only unusually large von Willebrand factor multimers (ULVWF) path and mediates 'microthrombogenesis', producing 'microthrombi' strings. This partial activation of hemostasis with ULVWF path leads to vascular microthrombotic disease. But localized traumatic injury extending to the endothelium and EVT activates both ULVWF and tissue factor paths. Combined activation of ULVWF and tissue factor paths provides normal hemostasis in external bodily injury, but causes 'macrothrombus' formation in intravascular injury. This 'two-path unifying theory' concept succinctly elucidates simplified nature of hemostasis in intravascular and external bodily injuries. It also clarifies different pathogenesis of every hemorrhagic disease and thrombotic disorder related to internal vascular disease and external vascular injury.

Thrombogenesis and thrombotic disorders based on 'two-path unifying theory of hemostasis': philosophical, physiological, and phenotypical interpretation

: Hemostasis, endowed to human to protect lives, is a process of logical blood clotting system to prevent blood loss in vascular injury. However, the notion that deadly thrombosis occurs as a result of normal hemostasis in intravascular injury could encounter with conceptual skepticism because the term 'thrombosis' automatically conjures up as serious disease. According to 'two-path unifying theory', normal hemostasis is initiated only by vascular injury through activated unusually large von Willebrand factor (ULVWF) path and/or activated tissue factor (TF) path. When these two equally important paths are unified in normal hemostasis, clotting at external bodily injury site is initiated for wound healing, but in intravascular injury 'blood clots' is formed to produce a disease called 'thrombosis'. As microthrombi from ULVWF path and fibrin clots from TF path become unified, macrothrombus would be formed via thrombogenesis. However, if ULVWF path and TF path cannot be unified due to lone ULVWF path activation, partial hemostasis produces only microthrombi seen in endotheliopathy-associated vascular microthrombotic disease. In real life, in-vivo fibrin clot cannot be formed alone via normal hemostasis because bleeding vascular injury always activates both ULVWF and TF paths. Without vascular injury, microthrombi due to activated ULVWF path occur in ADAMTS13 deficiency in thrombotic thrombocytopenic purpura, and fibrin clots due to activated TF path occur in acute promyelocytic leukemia. These two conditions can be called pathologic hemostasis. Three thrombogenic pathways produce three thrombotic disorders, which include macrothrombosis, microthrombosis and true DIC through macrothrombogenesis, microthrombogenesis and fibrinogenesis in both physiologic and pathological hemostasis.

Methyl isocyanate inhalation induces tissue factor-dependent activation of coagulation in rats

Methyl isocyanate (MIC) is a highly toxic industrial chemical causing acute lethality after inhalation. The objective of this study was to determine whether alterations in hemostasis also occur in the immediate hours after exposure. Male rats were exposed to MIC (125-500 ppm) by nose-only vapor inhalation for 30 min. Arterial O₂ saturation was monitored prior to exposure, and hourly thereafter. Rats were euthanized at 1, 2, 4, and 8 hr and plasma analyzed for recalcification clotting time, tissue factor (TF) activity, and protein levels. Hypoxemia, as assessed by pulse oximetry, was an early feature of MIC inhalation. In contrast to sham or low (125 ppm) concentrations, 250 and 500 ppm MIC caused significant declines in blood oxygen saturation (% SpO₂) at 1 hr, which remained at deficit during the postexposure period. Commensurate with hypoxemia, plasma clotting time was significantly accelerated 1 hr after MIC inhalation (sham treatment: 955 ± 62.8 s; 125 ppm MIC: 790 ± 62 s; 250 ppm: 676 ± 28.0 s; 500 ppm: 581 ± 175 s). This procoagulant effect was transient, with no difference observed between sham and all MIC groups by 8 hr. Similarly, elevated TF activity and protein were detected in plasma 1 hr after MIC inhalation, each of which showed a progressive decline back to control levels at later timepoints. This study demonstrates that MIC inhalation resulted in hypoxemia and transient hypercoagulability of blood. Accelerated clotting occurred rapidly and was likely due to intravascular TF, which initiates the extrinsic coagulation pathway.

Microparticle-associated tissue factor activity in patients with metastatic pancreatic cancer and its effect on fibrin clot formation

Highly elevated microparticle (MP)-associated tissue factor (TF) activity was found in patients with pancreatic cancer, one of the most prothrombotic malignancies. It remains to be elucidated whether MP-TF activity reflects the prothrombotic state in these patients. MP-TF activity levels and the TF-dependent and -independent effect of MPs on fibrin clot formation were determined in patients with metastatic pancreatic cancer (n = 27), in healthy individuals (n = 10) and in plasma samples from lipopolysaccharide (LPS)-stimulated blood (LPS-plasma), which is rich in monocyte-derived TF-bearing MPs. The median MP-TF activity was 1.06 pg/mL (range, from 0.19 to 10.34 pg/mL) in patients with pancreatic cancer, 0.61 pg/mL (range, from 0.36 to 0.79 pg/mL) in LPS-plasma, and 0.18 pg/mL (range, from 0.04 to 0.39 pg/mL) in healthy individuals. MPs derived from LPS-plasma had the strongest impact on fibrin clot formation time (median, 157.6 seconds; range, from 149.5 to 170.4 seconds). Fibrin clot formation occurred significantly later in MPs derived from patients with pancreatic cancer (median, 273.4 seconds; range, from 146.6 to 354.4 seconds; P < 0.001) and in healthy individuals (median, 299.0 seconds; range, from 261.1 to 417.9 seconds; P < 0.001). Only in MPs derived from LPS-plasma the fibrin clot formation time dependent strongly on TF (median prolongation after TF blockade: 68% in LPS-plasma, 10% in patients with pancreatic cancer, and 4% in healthy individuals). In conclusion, highly elevated MP-TF activity was found in patients with metastatic pancreatic cancer, but TF-bearing MPs had a small effect on fibrin clot formation. TF-bearing MPs might not be the main mediators of the prothrombotic state associated with pancreatic cancer. However, the small but significant increase in coagulation potential by TF-bearing MPs might contribute to the multifactorial pathogenesis of venous thromboembolism in pancreatic cancer.

Selective tissue factor/factor VIIa Inhibitor, ER-410660, and its prodrug, E5539, have anti-venous and anti-arterial thrombotic effects with a low risk of bleeding

INTRODUCTION

Many anticoagulant drugs target factors common to both the intrinsic and extrinsic coagulation pathways, which may lead to bleeding complications. Since the tissue factor (TF)/factor VIIa complex is associated with thrombosis onset and specifically activates the extrinsic coagulation pathway, compounds that inhibit this complex may provide therapeutic and/or prophylactic benefits with a decreased risk of bleeding.

MATERIALS AND METHODS

The in vitro enzyme profile and anticoagulation selectivity of the TF/VIIa complex inhibitor, ER-410660, and its prodrug E5539 were assessed using enzyme inhibitory and plasma clotting assays. In vivo effects of ER-410660 and E5539 were determined using a TF-induced, thrombin generation rhesus monkey model; a stasis-induced, venous thrombosis rat model; a photochemically induced, arterial thrombosis rat model; and a rat tail-cut bleeding model.

RESULTS

ER-410660 selectively prolonged prothrombin time, but had a less potent anticoagulant effect on the intrinsic pathway. It also exhibited a dose-dependent inhibitory effect on thrombin generation caused by TF-injection in the rhesus monkey model. ER-410660 also reduced venous thrombus weights in the TF-administered, stasis-induced, venous thrombosis rat model and prolonged the occlusion time induced by arterial thrombus formation after vascular injury. The compound was capable of doubling the total bleeding time in the rat tail-cut model, albeit with a considerably higher dose compared to the effective dose in the venous and arterial thrombosis models. Moreover, E5539, an orally available ER-410660 prodrug, reduced the thrombin-anti-thrombin complex levels, induced by TF-injection, in a dose-dependent manner.

CONCLUSION

Selective TF/VIIa inhibitors have potential as novel anticoagulants with a lower propensity for enhancing bleeding.

Rapid activation of monocyte tissue factor by antithymocyte globulin is dependent on complement and protein disulfide isomerase

Lymphocyte depletion with antithymocyte globulin (ATG) can be complicated by systemic coagulation activation. We found that ATG activated tissue factor procoagulant activity (TF PCA) on monocytic cells more potently than other stimuli that decrypt TF, including cell disruption, TF pathway inhibitor inhibition, and calcium ionophore treatment. Induction of TF PCA by ATG was dependent on lipid raft integrity and complement activation. We showed that ATG-mediated TF activation required complement activation until assembly of the C5b-7 membrane insertion complex, but not lytic pore formation by the membrane attack complex C5b-9. Consistently, induction of TF PCA by ATG did not require maximal phosphatidylserine membrane exposure and was not correlated with the magnitude of complement-induced lytic cell injury. Blockade of free thiols, an inhibitory monoclonal antibody to protein disulfide isomerase (PDI), and the small-molecule PDI antagonist quercetin-3-rutinoside prevented ATG-mediated TF activation, and C5 complement activation resulted in oxidation of cell surface PDI. This rapid and potent mechanism of cellular TF activation represents a novel connection between the complement system and cell surface PDI-mediated thiol-disulfide exchange. Delineation of this clinically relevant mechanism of activation of the extrinsic coagulation pathway during immunosuppressive therapy with ATG may have broader implications for vascular thrombosis associated with inflammatory disorders.

Current consideration and management of disseminated intravascular coagulation

Disseminated intravascular coagulation (DIC) is a devastating clinical condition that is characterized by the loss of normal hemostatic control in response to sustained and systemic cell injury. The inciting injury may be from infection, trauma, or malignancy, but the consequent pathophysiology is multifactorial involving intertwined feedback loops between the coagulant, immune, and inflammatory pathways. Central to this is thrombin generation, but the ubiquitous nature of its in vivo functional consequences can make it difficult to dissect away the separate but overlapping components to the clinical problem. Therefore, early recognition and resolution of the precipitating events leading to DIC remains the central tenet to clinical care. This article refreshes our conceptual understanding of DIC pathogenesis and draws in recent advances in the cycle of cell death caused by extracellular nuclear proteins. It also aims to delineate recognition of response pathways that can be predominantly procoagulant or profibrinolytic to enable a more personalized and evidence-based approach to be delivered to the patient with DIC.