Chemical Synthesis and Structure-Activity Relationship Studies of the Coagulation Factor Xa Inhibitor Tick Anticoagulant Peptide from the Hematophagous Parasite Ornithodoros moubata.

Tick Anticoagulant Peptide (TAP), a 60-amino acid protein from the soft tick Ornithodoros moubata, inhibits activated coagulation factor X (fXa) with almost absolute specificity. Despite TAP and Bovine Pancreatic Trypsin Inhibitor (BPTI) (i.e., the prototype of the Kunitz-type protease inhibitors) sharing a similar 3D fold and disulphide bond topology, they have remarkably different amino acid sequence (only ~24% sequence identity), thermal stability, folding pathways, protease specificity, and even mechanism of protease inhibition. Here, fully active and correctly folded TAP was produced in reasonably high yields (~20%) by solid-phase peptide chemical synthesis and thoroughly characterised with respect to its chemical identity, disulphide pairing, folding kinetics, conformational dynamics, and fXa inhibition. The versatility of the chemical synthesis was exploited to perform structure-activity relationship studies on TAP by incorporating non-coded amino acids at positions 1 and 3 of the inhibitor. Using Hydrogen-Deuterium Exchange Mass Spectrometry, we found that TAP has a remarkably higher conformational flexibility compared to BPTI, and propose that these different dynamics could impact the different folding pathway and inhibition mechanisms of TAP and BPTI. Hence, the TAP/BPTI pair represents a nice example of divergent evolution, while the relative facility of TAP synthesis could represent a good starting point to design novel synthetic analogues with improved pharmacological profiles.

Activated protein C and free protein S in patients with mild to moderate bleeding disorders.

BACKGROUND: Underlying mechanisms for bleeding and impaired thrombin generation (TG) and plasma clot formation (PCF) in patients with mild to moderate bleeding disorders (MBDs) are still to be elucidated, especially in bleeding disorder of unknown cause (BDUC). The role of the natural anticoagulants activated protein C (APC) and free protein S (PS) has not yet been investigated in this patient population. AIMS: To analyze antigen levels of APC and PS in patients with MBDs and BDUC and investigate associations to clinical bleeding phenotype and severity as well as and hemostatic capacity. METHODS: Antigen levels of APC and free PS were measured in 262 patients from the Vienna Bleeding Biobank (VIBB), a single-center cohort study, by ELISA and compared to 61 healthy controls (HC). RESULTS: Antigen levels of APC were higher in MBD patients than in HC when adjusted for age, sex and BMI (median (IQR) 33.1 (20.6-52.6) and 28.6 (16.4-47.2) ng/mL). This was most pronounced in patients with BDUC (35.3 (21.7-54.3) ng/mL). No differences in PS antigen levels between patients and HC were seen overall, or according to specific diagnoses. Further, no association between APC or PS and bleeding severity or global tests of hemostasis or TG were identified, while paradoxically APC weakly correlated with shorter lag time and time to peak of PCF in BDUC. CONCLUSION: Our data demonstrate increased antigen levels of APC in BDUC, which might contribute to the bleeding tendency in some patients and could be a future therapeutic target in BDUC.

From haemadin to haemanorm: Synthesis and characterization of full-length haemadin from the leech Haemadipsa sylvestris and of a novel bivalent, highly potent thrombin inhibitor (haemanorm).

Hirudin from Hirudo medicinalis is a bivalent α-Thrombin (αT) inhibitor, targeting the enzyme active site and exosite-I, and is currently used in anticoagulant therapy along with its simplified analogue hirulog. Haemadin, a small protein (57 amino acids) isolated from the land-living leech Haemadipsa sylvestris, selectively inhibits αT with a potency identical to that of recombinant hirudin (KI = 0.2 pM), with which it shares a common disulfide topology and overall fold. At variance with hirudin, haemadin targets exosite-II and therefore (besides the free protease) it also blocks thrombomodulin-bound αT without inhibiting the active intermediate meizothrombin, thus offering potential advantages over hirudin. Here, we produced in reasonably high yields and pharmaceutical purity (>98%) wild-type haemadin and the oxidation resistant Met5 → nor-Leucine analogue, both inhibiting αT with a KI of 0.2 pM. Thereafter, we used site-directed mutagenesis, spectroscopic, ligand-displacement, and Hydrogen/Deuterium Exchange-Mass Spectrometry techniques to map the αT regions relevant for the interaction with full-length haemadin and with the synthetic N- and C-terminal peptides Haem(1-10) and Haem(45-57). Haem(1-10) competitively binds to/inhibits αT active site (KI = 1.9 µM) and its potency was enhanced by 10-fold after Phe3 → ß-Naphthylalanine exchange. Conversely to full-length haemadin, haem(45-57) displays intrinsic affinity for exosite-I (KD = 1.6 µM). Hence, we synthesized a peptide in which the sequences 1-9 and 45-57 were joined together through a 3-Glycine spacer to yield haemanorm, a highly potent (KI = 0.8 nM) inhibitor targeting αT active site and exosite-I. Haemanorm can be regarded as a novel class of hirulog-like αT inhibitors with potential pharmacological applications.

Investigating patients for bleeding disorders when most of the "usual" ones have been ruled out.

A State of the Art lecture titled "Investigating Patients for Bleeding Disorders When Most of the Usual Ones Have Been Ruled Out" was presented at the International Society on Thrombosis and Haemostasis Congress in 2023. Mild to moderate bleeding disorders (MBDs) in patients in whom no diagnosis of an established disorder, such as platelet function defect, von Willebrand disease, or a coagulation factor deficiency, can be identified are classified as bleeding disorders of unknown cause (BDUCs). Prospective data from the Vienna Bleeding Biobank and other studies have revealed a high proportion of BDUCs of up to 70% among patients with MBD who have a similar bleeding phenotype as other MBDs. As BDUC is a diagnosis of exclusion, the accuracy of the diagnostic workup is essential. For example, repeated testing for von Willebrand disease should be considered if von Willebrand factor values are <80 IU/dL. Current evidence does not support the clinical use of global assays such as thromboelastography, platelet function analyzer, or thrombin generation potential. Rare and novel bleeding disorders due to genetic variants in fibrinolytic factors or natural anticoagulants are rare and should only be analyzed in patients with specific phenotypes and a clear family history. In BDUC, blood group O was identified as a risk factor for increased bleeding severity and bleeding risk after hemostatic challenges. Future studies should improve the phenotypical characterization and ideally identify novel risk factors in BDUC, as a multifactorial pathogenesis is suspected. Finally, we summarize relevant new data on this topic presented during the 2023 International Society on Thrombosis and Haemostasis Congress.

Coagulation System Activation for Targeting of COVID-19: Insights into Anticoagulants, Vaccine-Loaded Nanoparticles, and Hypercoagulability in COVID-19 Vaccines.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), also known as COVID-19, is currently developing into a rapidly disseminating and an overwhelming worldwide pandemic. In severe COVID-19 cases, hypercoagulability and inflammation are two crucial complications responsible for poor prognosis and mortality. In addition, coagulation system activation and inflammation overlap and produce life-threatening complications, including coagulopathy and cytokine storm, which are associated with overproduction of cytokines and activation of the immune system; they might be a lead cause of organ damage. However, patients with severe COVID-19 who received anticoagulant therapy had lower mortality, especially with elevated D-dimer or fibrin degradation products (FDP). In this regard, the discovery of natural products with anticoagulant potential may help mitigate the numerous side effects of the available synthetic drugs. This review sheds light on blood coagulation and its impact on the complication associated with COVID-19. Furthermore, the sources of natural anticoagulants, the role of nanoparticle formulation in this outbreak, and the prevalence of thrombosis with thrombocytopenia syndrome (TTS) after COVID-19 vaccines are also reviewed. These combined data provide many research ideas related to the possibility of using these anticoagulant agents as a treatment to relieve acute symptoms of COVID-19 infection.

Classic Thrombophilias and Thrombotic Risk Among Middle-Aged and Older Adults: A Population-Based Cohort Study.

Background Five classic thrombophilias have been recognized: factor V Leiden (rs6025), the prothrombin G20210A variant (rs1799963), and protein C, protein S, and antithrombin deficiencies. This study aimed to determine the thrombotic risk of classic thrombophilias in a cohort of middle-aged and older adults. Methods and Results Factor V Leiden, prothrombin G20210A and protein-coding variants in the PROC (protein C), PROS1 (protein S), and SERPINC1 (antithrombin) anticoagulant genes were determined in 29 387 subjects (born 1923-1950, 60% women) who participated in the Malmö Diet and Cancer study (1991-1996). The Human Gene Mutation Database was used to define 68 disease-causing mutations. Patients were followed up from baseline until the first event of venous thromboembolism (VTE), death, or Dec 31, 2018. Carriership (n=908, 3.1%) for disease-causing mutations in the PROC, PROS1, and SERPINC1 genes was associated with incident VTE: Hazard ratio (HR) was 1.6 (95% CI, 1.3-1.9). Variants not in Human Gene Mutation Database were not linked to VTE (HR, 1.1; 95% CI, 0.8-1.5). Heterozygosity for rs6025 and rs1799963 was associated with incident VTE: HR, 1.8 (95% CI, 1.6-2.0) and HR, 1.6 (95% CI, 1.3-2.0), respectively. The HR for carrying 1 classical thrombophilia variant was 1.7 (95% CI, 1.6-1.9). HR was 3.9 (95% CI, 3.1-5.0) for carriers of ≥2 thrombophilia variants. Conclusions The 5 classic thrombophilias are associated with a dose-graded risk of VTE in middle-aged and older adults. Disease-causing variants in the PROC, PROS1, and SERPINC1 genes were more common than the rs1799963 variant but the conferred genetic risk was comparable with the rs6025 and rs1799963 variants.

Natural anticoagulants: A missing link in mild to moderate bleeding tendencies.

INTRODUCTION: There is a growing interest in natural anticoagulants as a cause of mild to moderate bleeding disorders (MBDs), particularly in patients with bleeding of unknown cause (BUC), which is defined as having a mild to moderate bleeding phenotype without a definite diagnosis despite exhaustive and repeated laboratory investigations. Recently, abnormalities in two natural anticoagulant pathways, thrombomodulin (TM), and tissue factor pathway inhibitor (TFPI), were identified in single patients or families as the underlying cause for a bleeding tendency. AIM: The objective of this review is to discuss the current understanding of the role of natural anticoagulants in MBDs using available clinical and translational data. METHODS: A Cochrane Library and PubMed (MEDLINE) search focusing on selected natural anticoagulants and their role in MBDs was conducted. RESULTS: Data on the influence of natural anticoagulants including protein C, protein S, antithrombin, TM, and TFPI or factors with anticoagulant properties like fibrinogen gamma prime (γ') on MBDs are scarce. Observations from sepsis treatment and from translational research highlight their importance as regulators of the haemostatic balance, especially via the activated protein C-related pathway, and suggest a role in some MBDs. CONCLUSION: Similar to the distinct genetic variants of natural anticoagulants linked to thrombosis, we hypothesize that novel variants may be associated with a bleeding tendency and could be identified using next generation sequencing.

Impact of erythrocytapheresis on natural anticoagulant levels in children with sickle cell disease: A pilot study.

Venous thromboembolism (VTE) is being increasingly recognized in children with sickle cell disease (SCD). In a retrospective cohort study, we identified bilateral central venous catheter (CVC) placement as an independent risk factor for VTE. At our institution, the only indication for bilateral CVC placement in children with SCD is erythrocytapheresis. To investigate the impact of erythrocytapheresis on coagulation, we measured levels of natural anticoagulants in 11 patients with SCD on chronic erythrocytapheresis, immediately before and after apheresis. We demonstrated a statistically significant reduction in most parameters. Additional studies are needed to further investigate the exact etiology and clinical impact of this acute decrease.

Predictors of stroke outcome: the role of hemorheology, natural anticoagulants, and serum albumin.

BACKGROUND: The early hours after an acute stroke are crucial; early accurate prediction of outcome in stroke patients can help health system providers and families to choose appropriate lines of management and plan for the future. The aim of this work is to assess the role of hemorheological parameters (such as blood viscosity, hematocrit, platelet aggregation, and leukocyte count), protein C, protein S, antithrombin III, and serum albumin as predictors of stroke outcome. METHODS: Thirty subjects, 20 patients with acute ischemic stroke within 24 h from the onset and 10 normal subjects, were included in this case control study. Clinical, functional, and radiological evaluation was done for the patients, and all patients and control were subjected to routine laboratory tests and assessment of blood viscosity, hematocrit level, platelet aggregation, protein C, protein S, and antithrombin III. RESULTS: Platelet aggregation was significantly higher and serum albumin was significantly lower in stroke patients compared to control (p value = 0.000 and 0.039) respectively. On comparing between patient with good and poor outcome, good outcome was associated with increased serum albumin level at admission (p value = 0.03) respectively. A significant negative correlation was found between total leukocyte count, hematocrit value, and stroke outcome (p value = 0.015 and 0.013) respectively. Only albumin was found to be a significant predictor for outcome by linear regression analysis. CONCLUSION: Serum albumin, hematocrit level, and total leukocyte count at the time of presentation of ischemic stroke are useful markers for stroke outcome.

ß-Antithrombin, subtype of antithrombin deficiency and the risk of venous thromboembolism in hereditary antithrombin deficiency: A family cohort study.

Hereditary antithrombin deficiency is associated with a high incidence of venous thromboembolism (VTE), but VTE risk differs between families. Beta-antithrombin is reported to be the most active isoform of antithrombin in vivo. Whether ß-antithrombin activity and subtypes in antithrombin deficiency have impact on VTE risk has not been investigated outside the proband setting. We performed a retrospective family cohort study to investigate whether subtypes of antithrombin deficiency or ß-antithrombin levels are associated with the risk of first or recurrent VTE. Eighty-one subjects from 21 families were included, of which 52 were antithrombin deficient. Βeta-antithrombin levels were decreased in most type I and type IIPE subjects, but normal levels were found in all subtypes of antithrombin deficiency. The annual incidence of VTE in antithrombin-deficient family members was 1.24%, 95%CI: 0.72-1.99%, in low ß-antithrombin 1.36% (95%CI: 0.76-2.25%) and in normal ß-antithrombin 0.79% (95%CI: 0.10-2.77). The annual incidence of recurrence in family members was 3.1% (95%CI: 0.9-7.1%). Duration of anticoagulation had an impact on recurrence risk: In family members annual recurrence with fixed duration was 10% (95%CI: 2.1-29.2%), with indefinite duration 1.5% (95%CI: 0.2-5.4%), p < 0.05. Beta-antithrombin levels were not associated with the risk for first or recurrent VTE in antithrombin deficient subjects. CONCLUSIONS: In this high-risk antithrombin-deficient population, both subjects with low and normal plasma ß-antithrombin activity had high risks of first and recurrent VTE. This puts the importance of ß-antithrombin into question. Long-term anticoagulation is warranted in antithrombin-deficient VTE patients.

Natural regulatory anticoagulants proteins among Sudanese patients with dengue virus infection.

THE COAGULATION SYSTEM: Abnormalities in natural physiologic anticoagulants are observed in dengue infection. Laboratory values such as protein C (PC), protein S (PS), and antithrombin (AT) indicate this problem on the coagulation system in dengue. Recently, an interrelationship between dengue and the levels of natural anticoagulants has been observed. OBJECTIVE: The study conducted to find out the effect of dengue on the natural anticoagulant proteins. METHODS: A case-control study was conducted in Port Sudan Teaching Hospital from February 2013 to June 2014 for 334 cases of dengue caused by dengue virus, 217 (65%) males and 117 (35%) were females along with 101 cases of control 64 (63.4%) males and 37 (36.6%) were females. Laboratory-positive dengue cases were confirmed by immunoglobulin (Ig) M and IgG immune chromatography rapid test and the WHO criteria were used for classifying the dengue severity. Platelet count (PLT), plasma prothrombin time (PT), partial thromboplastin time (PTT), fibrinogen, D-dimer (DD), aspartate transaminase, alanine transaminase, PC, PS, and AT were performed. RESULTS: Of 334, 289 patients had dengue fever (DF) and 45 patients had dengue hemorrhagic fever (DHF). Thrombocytopenia was present in 279 (83.5%). PLT was found to be significantly low in the case of dengue (P < 0.000). There was a highly significant difference between the prolongations of PT and PTT in DF (P < 0.000). Prolongations of PT and PTT were significantly higher (90% and 76.2%, respectively) in DF than DHF patients (10% and 23.8%, respectively). PC and PS were significantly higher in DHF 100% and 80% than DF 89% and 57%, respectively. CONCLUSION: The findings of this study suggest that lower levels of these proteins in patients with dengue are attributed to disseminated intravascular coagulation.

Classic thrombophilic gene variants.

Thrombophilia is defined as a condition predisposing to the development of venous thromboembolism (VTE) on the basis of a hypercoagulable state. Over the past decades, great advances in the pathogenesis of VTE have been made and nowadays it is well established that a thrombophilic state may be associated with acquired and/or inherited factors. The rare loss-of-function mutations of the genes encoding natural anticoagulant proteins (i. e. protein C, protein S and antithrombin) and the more common gain-of-function polymorphisms factor V Leiden and prothrombin G20210A are the main genetic determinants of thrombophilia. In addition, non-O blood group has been consistently demonstrated to be the most frequent inherited marker of an increased risk of VTE. The mechanism role of these inherited thrombophilia markers will be discussed in this narrative review.