Fibrin(ogen) as a Therapeutic Target: Opportunities and Challenges

Fibrinogen is one of the key molecular players in haemostasis. Thrombin-mediated release of fibrinopeptides from fibrinogen converts this soluble protein into a network of fibrin fibres that form a building block for blood clots. Thrombin-activated factor XIII further crosslinks the fibrin fibres and incorporates antifibrinolytic proteins into the network, thus stabilising the clot. The conversion of fibrinogen to fibrin also exposes binding sites for fibrinolytic proteins to limit clot formation and avoid unwanted extension of the fibrin fibres. Altered clot structure and/or incorporation of antifibrinolytic proteins into fibrin networks disturbs the delicate equilibrium between clot formation and lysis, resulting in either unstable clots (predisposing to bleeding events) or persistent clots that are resistant to lysis (increasing risk of thrombosis). In this review, we discuss the factors responsible for alterations in fibrin(ogen) that can modulate clot stability, in turn predisposing to abnormal haemostasis. We also explore the mechanistic pathways that may allow the use of fibrinogen as a potential therapeutic target to treat vascular thrombosis or bleeding disorders. Better understanding of fibrinogen function will help to devise future effective and safe therapies to modulate thrombosis and bleeding risk, while maintaining the fine balance between clot formation and lysis.

Plasma fibrin clot properties and cardiovascular mortality in patients with type 2 diabetes: a long‐term follow‐up study

Background Patients with type 2 diabetes mellitus (T2DM) are at high risk of cardiovascular mortality, but the mechanisms behind this remain unclear. Prothrombotic fibrin clot properties have been shown in T2DM and cardiovascular disease. We hypothesized that formation of denser clots, which are resistant to fibrinolysis, has a negative impact on cardiovascular mortality in T2DM. Methods We studied 133 T2DM patients aged 43–83 years. Plasma fibrin clot turbidity, permeation, compaction, and efficiency of clot lysis using 3 assays including the determination of maximum concentration (D-Dmax) and rate of increase in D-dimer concentration (D-Drate) released during tissue plasminogen activator-induced degradation, were evaluated at the time of enrollment, along with thrombin generation and fibrinolytic proteins. During a median follow-up period of 72 months, cardiovascular mortality was recorded. Results Cardiovascular deaths (n = 16, 12%) occurred more frequently in patients with increased D-Dmax (> 4.26 mg/l, hazard ratio [HR] 5.43, 95% confidence interval [CI] 1.99–14.79), or decreased D-Drate (< 0.07 mg/l/min, HR 2.97, 95% CI 1.07–8.23), or increased peak thrombin (> 283.5 nM, HR 5.65, 95% CI 2.07–15.51). These predictors had an even more potent impact on cardiovascular mortality in patients with prior cardiovascular disease (64.7%) and with corresponding risks as follows: HR 6.18, 95% CI 2.02–18.96; HR 8.98, 95% CI 2.99–26.96; and HR 5.35, 95% CI 1.62–17.72, respectively. Other investigated fibrin variables and fibrinolytic proteins did not associate with cardiovascular mortality. In multivariable analysis, cardiovascular mortality was predicted by D-Dmax > 4.26 mg/l, age > 65 years, prior cardiovascular disease, and C-reactive protein > 3 mg/l. Conclusions This study is the first to show that formation of denser fibrin clots resistant to fibrinolysis could be a risk factor for long-term cardiovascular mortality in T2DM.

Extensive Characterization of the Hemostatic Derangement Occurring in COVID-19 Patients Admitted to the Bergamo Hospital

INTRODUCTION The occurrence of a hypercoagulable state in hospitalized COVID-19 patients is supported by studies conducted with routine coagulation tests, including plasma D-dimer and fibrinogen, and platelet count. AIM In this study we performed an extensive characterization of the hemostatic alterations by both global and specific assays in a cohort of 78 patients hospitalized for COVID-19. The aims were to: 1) clarify mechanisms underlying the coagulopathy, and 2) identify predictive factors of disease severity and thrombotic events (i.e. deep vein thrombosis [DVT], pulmonary embolism [PE] or arterial thromboembolism [ATE]). METHODS COVID-19 patients admitted to the Hospital Papa Giovanni XXIII in Bergamo, Italy, from March 23 to May 30, 2020, were enrolled prospectively, providing informed consent. As a global assay, thromboelastometry (ROTEM) was performed in whole blood by EXTEM, INTEM, and FIBTEM tests. Specific assays included plasma levels of intrinsic and extrinsic pathway coagulation factors, von Willebrand factor (vWF) antigen and activity, anticoagulant proteins (i.e. protein C [PC], free-protein S [PS], and antithrombin [AT]), fibrinolytic proteins (i.e. tissue plasminogen activator [t-PA], and inhibitor [PAI-1]), and hypercoagulation biomarkers (i.e. prothrombin fragment 1+2 [F1+2], and D-dimer). In addition, biomarkers of immunoinflammation (i.e. neutrophil extracellular traps [NETs], CRP and procalcitonin) were measured. Occurrence of thrombotic events and death were monitored during follow up. RESULTS 78 patients (56M/22F), median age 62.7 years (25-87), were analyzed. According to disease severity, 45 were ICU, and 33 non-ICU patients. Sixty-three of them were on thromboprophylaxis. Global hemostasis analysis by ROTEM showed a prothrombotic profile in patients compared to controls, with a significantly shorter clot formation time (CFT), and increased maximum clot firmness (MCF), which were significantly greater in the ICU vs non-ICU patients. The occurrence of an 'in vivo' hypercoagulable state was confirmed by increased plasma levels of F1+2 and D-dimer, with the highest values of D-dimer in the ICU subjects. Hypercoagulability, rather than factors' consumption, was also shown by the findings of significantly higher plasma procoagulant factors V, VIII, IX and fibrinogen in ICU compared to non-ICU patients (p&lt;0.001). Endothelium activation was shown by extremely elevated vWF antigen and activity levels in all patients (highest values in ICU subjects). Moreover, the concentrations of fibrinolytic proteins, t-PA, and its inhibitor PAI-1, were elevated (p&lt;0.01) in patients compared to normal controls, without difference between ICU and non-ICU subjects. Finally, the inflammatory parameters' analysis in the ICU group demonstrated significantly increased plasma levels of NETs, CRP, and procalcitonin, compared to non-ICU patients. Of note, NETs levels significantly (p&lt;0.02) correlated with vWF, D-dimer and t-PA, while CRP and procalcitonin inversely correlated with anticoagulant PC. After a median time of 8.8 days, 19 (24%) patients experienced thrombosis (3 DVT, 8 PE, 8 ATE). Thirteen (17%) patients from total population died after a median time of 33 days of hospitalization. Baseline D-dimer and t-PA levels were significantly higher in patients developing VTE, while baseline FVIII, vWF and D-dimer levels were greater in subjects who died during follow-up. By Cox analysis, high D-dimer and younger age were significantly associated with mortality. CONCLUSIONS Our study provides for the first time an extensive overview of the hypercoagulable state induced by SARSCoV-2 infection, demonstrating alterations in all of the different hemostatic compartments analyzed. The viral infection-induced hemostatic abnormalities are exacerbated by the severity of the disease and strongly correlate with the proinflammatory status, demonstrating the link between coagulation and inflammation. This link is further supported by the clear correlation found between NETosis and markers of endothelial and blood clotting activation. Finally, these data add evidence to the role of D-dimer as a significant predictor of intra-hospital mortality. Disclosures No relevant conflicts of interest to declare.

Mathematical Modelling of Intravenous Thrombolysis in Acute Ischaemic stroke: Effects of Dose Regimens on Levels of Fibrinolytic Proteins and Clot Lysis Time

Thrombolytic therapy is one of the medical procedures in the treatment of acute ischaemic stroke (AIS), whereby the tissue plasminogen activator (tPA) is intravenously administered to dissolve the obstructive blood clot. The treatment of AIS by thrombolysis can sometimes be ineffective and it can cause serious complications, such as intracranial haemorrhage (ICH). In this study, we propose an efficient mathematical modelling approach that can be used to evaluate the therapeutic efficacy and safety of thrombolysis in various clinically relevant scenarios. Our model combines the pharmacokinetics and pharmacodynamics of tPA with local clot lysis dynamics. By varying the drug dose, bolus-infusion delay time, and bolus-infusion ratio, with the FDA approved dosing protocol serving as a reference, we have used the model to simulate 13 dose regimens. Simulation results are compared for temporal concentrations of fibrinolytic proteins in plasma and the time that is taken to achieve recanalisation. Our results show that high infusion rates can cause the rapid degradation of plasma fibrinogen, indicative of increased risk for ICH, but they do not necessarily lead to fast recanalisation. In addition, a bolus-infusion delay results in an immediate drop in plasma tPA concentration, which prolongs the time to achieve recanalisation. Therefore, an optimal administration regimen should be sought by keeping the tPA level sufficiently high throughout the treatment and maximising the lysis rate while also limiting the degradation of fibrinogen in systemic plasma. This can be achieved through model-based optimisation in the future.

Reduced plasma levels of small HDL particles transporting fibrinolytic proteins in pulmonary arterial hypertension

BackgroundAberrant lipoprotein metabolism has been implicated in experimental pulmonary hypertension, but the relevance to patients with pulmonary arterial hypertension (PAH) is inconclusive.ObjectiveTo investigate the relationship between circulating lipoprotein subclasses and survival in patients with PAH.MethodsUsing nuclear magnetic resonance spectroscopy, 105 discrete lipoproteins were measured in plasma samples from two cohorts of patients with idiopathic or heritable PAH. Data from 1124 plasma proteins were used to identify proteins linked to lipoprotein subclasses. The physical presence of proteins was confirmed in plasma lipoprotein subfractions separated by ultracentrifugation.ResultsPlasma levels of three lipoproteins from the small high-density lipoprotein (HDL) subclass, termed HDL-4, were inversely related to survival in both the discovery (n=127) and validation (n=77) cohorts, independent of exercise capacity, comorbidities, treatment, N-terminal probrain natriuretic peptide, C reactive protein and the principal lipoprotein classes. The small HDL subclass rich in apolipoprotein A-2 content (HDL-4-Apo A-2) exhibited the most significant association with survival. None of the other lipoprotein classes, including principal lipoprotein classes HDL and low-density lipoprotein cholesterol, were prognostic. Three out of nine proteins identified to associate with HDL-4-Apo A-2 are involved in the regulation of fibrinolysis, namely, the plasmin regulator, alpha-2-antiplasmin, and two major components of the kallikrein–kinin pathway (coagulation factor XI and prekallikrein), and their physical presence in the HDL-4 subfraction was confirmed.ConclusionReduced plasma levels of small HDL particles transporting fibrinolytic proteins are associated with poor outcomes in patients with idiopathic and heritable PAH.

Haemostatic alterations induced by treatment with asparaginases and clinical consequences

SummaryThe benefit of asparaginase for treating acute lymphoid leukaemia (ALL) has been well established. Native asparaginase derives from Escherichia coli (colaspase) or Erwinia chrysanthemi (crisantaspase); in a third preparation, colaspase is pegylated. Depletion of asparagine leads to decreased synthesis of procoagulant, anticoagulant, and fibrinolytic proteins, with resultant hypercoagulability and greater risk of venous thromboembolism (VTE). Colaspase and crisantaspase are not dose-equivalent, with crisantaspase displaying haemostatic toxicity only at dosages much higher and administered more frequently than those of colaspase. Cerebral venous thrombosis and pulmonary embolism are two life-endangering manifestations that occur during treatment with asparaginase particularly in children and in adults with ALL, respectively. Approximately one-third of VTEs are located in the upper extremities and are central venous line-related. Other risk factors are longer duration of asparaginase treatment and concomitant use of prednisone, anthracyclines, and oral contraceptives. The risk associated with inherited thrombophilia is uncertain but is clearly enhanced by other risk factors or by the use of prednisone. VTE prevention with fresh frozen plasma is not recommended; the efficacy of antithrombin (AT) concentrates has occasionally been reported, but these reports should be confirmed by proper studies, and AT should not be routinely employed. Therapeutic or prophylactic heparin doses are only partially effective, and direct thrombin or factor Xa inhibitors could play significant roles in the near future.

Association among Fibrinolytic Proteins, Metabolic Syndrome Components, Insulin Secretion, and Resistance in Schoolchildren

We investigated the role of urokinase plasminogen activator (uPA) and its soluble receptors (suPAR) and plasminogen activator inhibitor-1 (PAI-1) in metabolic syndrome (MetS) components, insulin secretion, and resistance in schoolchildren. We enrolled 387 children, aged 10.3 ± 1.5 years, in Taipei. Anthropometry, fibrinolytic proteins, MetS components, insulin secretion, and resistance were measured. Subjects were divided into normal, overweight, and obese groups. Finally, the relationship between fibrinolytic proteins and metabolic syndrome in boys and girls was analyzed. In boys, PAI-1 was positively associated with body mass index (BMI) percentile, hypertriglyceride, insulin secretion, and resistance. In girls, PAI-1 was positively associated with obesity, hypertriglyceridemia, and insulin secretion. In girls, uPA was positively associated with insulin secretion. suPAR was positively associated with high-sensitivity C-reactive protein in both boys and girls, and with BMI percentile and body fat in girls. The obese boys had higher suPAR and PAI-1 levels than the normal group. The obese girls had higher uPA, suPAR, and PAI-1 than the normal group. Boys and girls with MetS had higher PAI-1. Fibrinolytic proteins, especially PAI-1, are associated with MetS components and insulin secretion in children. Fibrinolytic proteins changes were more likely to occur in girls than in boys.