From heparins to factor Xa inhibitors and beyond

Current and emerging drug strategies for the prevention of venous thromboembolism in acutely ill medical inpatients

INTRODUCTION

Venous thromboembolism (VTE) is a common complication in patients hospitalized for acute medical illnesses. Therefore, medical inpatients require a careful VTE and bleeding risk assessment to drive optimal strategies for VTE prevention. Low molecular weight heparin and fondaparinux have long been used for inhospital prophylaxis for patients at increased risk of VTE. The selection of patients who require post-discharge prophylaxis, and the role of direct oral anticoagulants remain debated. New molecules currently under development may contribute to improve the risk benefit of VTE prevention in this setting.

AREAS COVERED

This text summarizes the evidence on approved treatments and on other drugs for the prevention of VTE in acutely ill medical patients. The main focus is on their pharmacological proprieties, clinical efficacy and safety, and the current license approved by the FDA (Food and Drug Administration) and EMA (European Medicines Agency), giving the readers a way to compare available drugs to date. The trials presented consider both inhospital and extended prophylaxis.

EXPERT OPINION

Thanks to the potentially favorable safety profile, factor XI inhibitors may play a role in the prevention of VTE in this setting. The expert opinion section discusses pharmacological properties, prophylaxis trials, and potential clinical applications of this novel class of drugs.

Heparin and Derivatives for Advanced Cell Therapies

Heparin and its derivatives are saving thousands of human lives annually, by successfully preventing and treating thromboembolic events. Although the mode of action during anticoagulation is well studied, their influence on cell behavior is not fully understood as is the risk of bleeding and other side effects. New applications in regenerative medicine have evolved supporting production of cell-based therapeutics or as a substrate for creating functionalized matrices in biotechnology. The currently resurgent interest in heparins is related to the expected combined anti-inflammatory, anti-thrombotic and anti-viral action against COVID-19. Based on a concise summary of key biochemical and clinical data, this review summarizes the impact for manufacturing and application of cell therapeutics and highlights the need for discriminating the different heparins.

Anticoagulation via anti-Factor Xa inhibition

Hospital-acquired deep vein thrombosis (DVT) affects 10-25% of medical patients and up to 60% of surgical patients. While thromboprophylaxis is without a doubt under utilized in the hospital setting, there is also a need for more efficacious agents. Fondaparinux, the first of a new class of agents Factor Xa inhibitiors, has recently come into clinical use. It is a synthetic pentasaccharide and indirect Factor Xa inhibitor with a predictable antithrombotic action. Being a synthetic product, there are no concerns about supply, nor viral or prion protein contamination. Initial large international trials in orthopaedic patients demonstrated its superior efficacy to standard thromboprophylaxis. Further trials confirmed its superior efficacy in venous thromboembolism (VTE) prevention, both in medical and surgical patient groups, as well as treatment of pulmonary embolism and DVT. Its use has also recently been evaluated in acute coronary syndromes and angioplasty. Fondaparinux currently has licenses in the UK for thromboprophylaxis and treatment of VTE and a license for the management of acute coronary syndrome is likely to be forthcoming. It has a favourable side effect profile and if the price is acceptable, is likely to take over from low molecular weight heparins in these indications as the drug of choice on the grounds of efficacy and safety.

Popular naturally occurring antioxidants as potential anticoagulant drugs

Blood coagulation is a physiological process whose main task is prevention of blood loss from injured vessels. This process consists of a series of zymogens proteolytic activation leading to the generation of the main coagulation enzyme - thrombin. Besides its important role in blood coagulation process, thrombin is involved in many cardiovascular diseases, which are responsible for almost half of fatalities in economically developed countries. The evidence for the increased generation and in vivo activity of thrombin was observed in the plasma of individuals at high risk for clinically significant venous and arterial thromboembolic complications. Antioxidants activity of plants extracts has been well known for many years and was confirmed by many publications. However, during the last decade many research centers presented results suggesting anticoagulant potential of various plant extracts. Many researchers have also provided evidence that polyphenol compounds are able to inhibit the activity of many enzymes, including serine proteases. All research described in this review clearly indicate that polyphenols and polyphenol-rich extracts possess not only antioxidative but also anticoagulant properties and may be useful in creation of new therapeutic agents or dietary supplements. Based on described properties polyphenols would be very helpful with both prevention and treatment of thromboembolic complications associated with multiple failures of haemostasis, because the available therapeutic agents do not offer such double-effects (antioxidant and anticoagulant).

Acute Coronary Syndromes: Unstable Angina and Non-ST Elevation Myocardial Infarction

Non-ST elevation acute coronary syndromes (NSTE-ACS) encompass the clinical entities of unstable angina and non-ST elevation myocardial infarction. Several advances have occurred over the past decade, including the emergence of new antiplatelet and antithrombotic therapies and novel treatment strategies, leading to marked improvements in mortality. However, there has also been an increased incidence in NSTE-ACS as a result of the use of high-sensitivity troponins and the increase in cardiovascular risk factors. This article provides a focused update on contemporary management strategies pertaining to antiplatelet, antithrombotic, and anti-ischemic therapies and to revascularization strategies in patients with ACS.

Histones Differentially Modulate the Anticoagulant and Profibrinolytic Activities of Heparin, Heparin Derivatives, and Dabigatran

The antithrombin activity of unfractionated heparin (UFH) is offset by extracellular histones, which, along with DNA, represent a novel mediator of thrombosis and a structural component of thrombi. Here, we systematically evaluated the effect of histones, DNA, and histone-DNA complexes on the anticoagulant and profibrinolytic activities of UFH, its derivatives enoxaparin and fondaparinux, and the direct thrombin inhibitor dabigatran. Thrombin generation was assessed by calibrated automated thrombinography, inhibition of factor Xa and thrombin by synthetic substrates, tissue plasminogen activator-mediated clot lysis by turbidimetry, and thrombin-activatable fibrinolysis inhibitor (TAFI) activation by a functional assay. Histones alone delayed coagulation and slightly stimulated fibrinolysis. The anticoagulant activity of UFH and enoxaparin was markedly inhibited by histones, whereas that of fondaparinux was enhanced. Histones neutralized both the anti-Xa and anti-IIa activities of UFH and preferentially blocked the anti-IIa activity of enoxaparin. The anti-Xa activity of fondaparinux was not influenced by histones when analyzed by chromogenic substrates, but was potentiated in a plasma prothrombinase assay. Histones inhibited the profibrinolytic activity of UFH and enoxaparin and enhanced that of fondaparinux by acting on the modulation of TAFI activation by anticoagulants. Histone H1 was mainly responsible for these effects. Histone-DNA complexes, as well as intact neutrophil extracellular traps, impaired the activities of UFH, enoxaparin, and fondaparinux. Dabigatran was not noticeably affected by histones and/or DNA, whatever the assay performed. In conclusion, histones and DNA present in the forming clot may variably influence the antithrombotic activities of anticoagulants, suggesting a potential therapeutic advantage of dabigatran and fondaparinux over heparins.

Simple and rapid quality control of sulfated glycans by a fluorescence sensor assay--exemplarily developed for the sulfated polysaccharides from red algae Delesseria sanguinea

Sulfated polysaccharides (SP) from algae are of great interest due to their manifold biological activities. Obstacles to commercial (especially medical) application include considerable variability and complex chemical composition making the analysis and the quality control challenging. The aim of this study was to evaluate a simple microplate assay for screening the quality of SP. It is based on the fluorescence intensity (FI) increase of the sensor molecule Polymer-H by SP and was originally developed for direct quantification of SP. Exemplarily, 65 SP batches isolated from the red alga Delesseria sanguinea (D.s.-SP) and several other algae polysaccharides were investigated. Their FI increase in the Polymer-H assay was compared with other analytical parameters. By testing just one concentration of a D.s.-SP sample, quality deviations from the reference D.s.-SP and thus both batch-to-batch variability and stability can be detected. Further, structurally distinct SP showed to differ in their concentration-dependent FI profiles. By using corresponding reference compounds, the Polymer-H assay is therefore applicable as identification assay with high negative predictability. In conclusion, the Polymer-H assay showed to represent not only a simple method for quantification, but also for characterization identification and differentiation of SP of marine origin.

Delayed-type heparin allergy: diagnostic procedures and treatment alternatives-a case series including 15 patients

Delayed-type hypersensitivity reactions (DTHRs) after subcutaneous application of unfractionated heparins or low-molecular-weight heparins are not uncommon. Standard allergological testing usually includes intracutaneous skin tests and patch testing of different heparins, heparinoids, and thrombin inhibitors followed by subcutaneous and/or intravenous challenge with skin test-negative drugs. We present data from a single-center case series of 15 patients with DTHR after low-molecular-weight heparin administration. Intracutaneous testing that can be considered as gold standard identified the suspicious elicitor in 11 (73.4%) of 15 of the patients. Patch testing was positive in 5 (33.4%) of 15 of the patients and was only positive in patients who were also reacting in the intradermal testing. Intravenous challenge with heparin sodium was performed in 10 of 15 patients and was well tolerated in all cases, despite prior positive intracutaneous tests with the same substance. Intracutaneous documentation of DTHR was not an adequate predictor of intravenous challenge.

Use of Molecular Dynamics for the Refinement of an Electrostatic Model for the In Silico Design of a Polymer Antidote for the Anticoagulant Fondaparinux

Molecular dynamics (MD) simulations results are herein incorporated into an electrostatic model used to determine the structure of an effective polymer-based antidote to the anticoagulant fondaparinux. In silico data for the polymer or its cationic binding groups has not, up to now, been available, and experimental data on the structure of the polymer-fondaparinux complex is extremely limited. Consequently, the task of optimizing the polymer structure is a daunting challenge. MD simulations provided a means to gain microscopic information on the interactions of the binding groups and fondaparinux that would have otherwise been inaccessible. This was used to refine the electrostatic model and improve the quantitative model predictions of binding affinity. Once refined, the model provided guidelines to improve electrostatic forces between candidate polymers and fondaparinux in order to increase association rate constants.

Heparin-induced skin lesions

Heparins are widely used for prophylaxis and treatment of thromboembolic diseases. Besides bleeding complications, heparin-induced skin lesions are the most frequent unwanted adverse effects of subcutaneous heparin treatment. Evidence suggests that these lesions are more common than previously thought. Lesions are most frequently due to either allergic reactions or to possibly life-threatening heparin-induced thrombocytopenia. Early recognition and adequate treatment are highly important, because although both complications initially show a similar clinical picture, their treatment should be fundamentally different. Furthermore, risk factors associated with the patient, drug, and treatment regimen have been identified. We review the clinical range of heparin-induced skin lesions, emphasise evidence and controversies in epidemiology, diagnosis, and differential diagnosis, and discuss the management of patients with these skin lesions.

Antiplatelet and antithrombin strategies in acute coronary syndrome: state-of-the-art review

Antiplatelet and antithrombotic agents significantly alter the clinical course of patients with acute coronary syndrome (ACS) and hence form the bedrock of the management pathway of this closely related continuum of coronary pathologies. The contemporary therapeutic armamentarium for the treatment of ACS now reflects the many technical and pharmacological advances that took place over the last two decades. In the original 1996 American College of Cardiology/ American Heart Association (ACC/AHA) guidelines for the management of acute myocardial infarction, only one antiplatelet agent (Aspirin) and one anticoagulant (unfractionated heparin) were recommended as class I therapies. Since then many newer agents have been developed and approved for routine clinical use in ACS patients. Recent research has focussed on improving efficacy on one hand and reducing bleeding complications on the other. This review focuses on the mechanism, efficacy, safety profile and clinical trial evidence of P2 Y12 receptor antagonist antiplatelet agents, glycoprotein IIb/IIIa receptor inhibitors (GPI), protease-activated receptor-1 (PAR-1) inhibitors, thrombin inhibitor bivalirudin and Factor Xa inhibitors fondaparinaux and rivaroxaban.

New direct oral anticoagulants--current therapeutic options and treatment recommendations for bleeding complications

To date, clinical studies show that the incidence of spontaneous bleeding with new direct oral anticoagulants (DOAs) is comparable to that of established anticoagulants. However, unlike vitamin K antagonists, there are currently no clinically available antidotes or approved reversal agents for new DOAs. Restoring normal coagulation is important in many cases, such as emergency surgeries, serious bleedings, or anticoagulant overdosing. Attempts have been made to restore normal coagulation after treatment with new DOAs using compounds such as recombinant activated factor VII (rFVIIa), prothrombin complex concentrate (PCC), or FEIBA (factor eight inhibitor bypassing activity). Limited pre-clinical data and even less clinical evidence are available on the usefulness of these methods in restoring normal coagulation for the emergency management of critical bleeding episodes. Evaluating the utility of DOAs is further complicated by the fact that it is unknown how predictive established test systems are of the bleeding risks. Clinical practice requires further evaluation of the emergency management options for the new DOAs to define the agents and the doses that are most useful. Furthermore, patients receiving long-term treatment with a DOA are likely to undergo elective surgery at some point, and there is lack of evidence regarding perioperative treatment regimens under such conditions. This review summarises potential bleeding management options and available data on the new DOAs.

Anticoagulation with the oral direct thrombin inhibitor dabigatran does not enlarge hematoma volume in experimental intracerebral hemorrhage

BACKGROUND

The direct thrombin inhibitor dabigatran etexilate (DE) may constitute a future replacement of vitamin K antagonists for long-term anticoagulation. Whereas warfarin pretreatment is associated with greater hematoma expansion after intracerebral hemorrhage (ICH), it remains unclear what effect direct thrombin inhibitors would have. Using different experimental models of ICH, this study compared hematoma volume among DE-treated mice, warfarin-treated mice, and controls.

METHODS AND RESULTS

CD-1 mice were fed with DE or warfarin. Sham-treated mice served as controls. At the time point of ICH induction, DE mice revealed an increased activated partial thromboplastin time compared with controls (mean±SD 46.1 ± 5.0 versus 18.0 ± 1.5 seconds; P=0.022), whereas warfarin pretreatment resulted in a prothrombin time prolongation (51.4 ± 17.9 versus 10.4 ± 0.3 seconds; P<0.001). Twenty-four hours after collagenase-induced ICH formation, hematoma volume was 3.8 ± 2.9 μL in controls, 4.8 ± 2.7 μL in DE mice, and 14.5 ± 11.8 μL in warfarin mice (n=16; Welch ANOVA between-group differences P=0.007; posthoc analysis with the Dunnett method: DE versus controls, P=0.899; warfarin versus controls, P<0.001; DE versus warfarin, P=0.001). In addition, a model of laser-induced cerebral microhemorrhage was applied, and the distances that red blood cells and blood plasma were pushed into the brain were quantified. Warfarin mice showed enlarged red blood cell and blood plasma diameters compared to controls, but no difference was found between DE mice and controls.

CONCLUSIONS

In contrast with warfarin, pretreatment with DE did not increase hematoma volume in 2 different experimental models of ICH. In terms of safety, this observation may represent a potential advantage of anticoagulation with DE over warfarin.

Blood coagulation factor Xa as an emerging drug target

INTRODUCTION

Factor (F)Xa is well-known as an important player in the coagulation cascade responsible for thrombin generation. More recently, FXa emerged as an essential player in cell biology via activation of protease-activated receptors (PAR)-1 and -2. This pleiotropic role of FXa forms the basis for its potential contribution to the pathogenesis of several diseases.

AREAS COVERED

The role of FXa in pathophysiology is reviewed with special emphasis on its signal transduction properties. To this end, we first discuss the important role of FXa in the coagulation cascade, we continue with recent data on FXa induced signaling in pathophysiology with special emphasis on tissue remodeling and fibrosis and discuss the potential of FXa as an emerging drug target.

EXPERT OPINION

FXa is more than a passive intermediate in the coagulation cascade and FXa may in fact orchestrate fundamental processes during pathophysiology. Targeting FXa may be an exciting new therapeutic strategy in the treatment of (fibro)proliferative diseases for which current treatment options are limited.

Emerging anticoagulants for venous thromboembolism prevention

PURPOSE

To discuss the advantages and disadvantages of currently available anticoagulants, describe the characteristics of the ideal anticoagulant, and compare and contrast the mechanisms of action, pharmacokinetics, administration, efficacy, safety, and potential for drug interactions of currently available and emerging anticoagulants for prevention of venous thromboembolism (VTE).

SUMMARY

Despite the proven efficacy of currently available agents for VTE prevention, several shortcomings exist that may prevent their use under various circumstances. These include administration by injection, narrow therapeutic index, unpredictable pharmacokinetics and pharmacodynamics, need for laboratory monitoring, risk for bleeding, and drug interactions. The ideal anticoagulant would overcome many of these issues; in particular, it would be available as an oral formulation. Dabigatran, an oral direct thrombin (factor IIa) inhibitor, and apixaban and rivaroxaban, oral direct factor Xa inhibitors, represent new agents for anticoagulation that may address many of these issues. While not available as an oral agent, desirudin is an additional option and offers increased flexibility when a non-heparin-based injectable anticoagulant is desired. Current literature indicates that these agents generally do not require laboratory monitoring and are safe and effective for VTE prevention in clinical studies of patients undergoing major orthopedic surgery.

CONCLUSION

The development of new anticoagulants that may overcome limitations of existing agents represents an opportunity to further improve outcomes in patients at risk for VTE in orthopedic surgery.

Incidence and causes of heparin-induced skin lesions

BACKGROUND

Little is known about the incidence and causes of heparin-induced skin lesions. The 2 most commonly reported causes of heparin-induced skin lesions are immune-mediated heparin-induced thrombocytopenia and delayed-type hypersensitivity reactions.

METHODS

We prospectively examined consecutive patients who received subcutaneous heparin (most often enoxaparin or nadroparin) for the presence of heparin-induced skin lesions. If such lesions were identified, we performed a skin biopsy, platelet count measurements, and antiplatelet-factor 4 antibody and allergy testing.

RESULTS

We enrolled 320 patients. In total, 24 patients (7.5%, 95% confidence interval [CI] 4.7%-10.6%) had heparin-induced skin lesions. Delayed-type hypersensitivity reactions were identified as the cause in all 24 patients. One patient with histopathologic evidence of delayed-type hypersensitivity tested positive for antiplatelet-factor 4 antibodies. We identified the following risk factors for heparin-induced skin lesions: a body mass index greater than 25 (odds ratio [OR] 4.6, 95% CI 1.7-15.3), duration of heparin therapy longer than 9 days (OR 5.9, 95% CI 1.9-26.3) and female sex (OR 3.0, 95% CI 1.1-8.8).

INTERPRETATION

Heparin-induced skin lesions are relatively common, have identifiable risk factors and are commonly caused by a delayed-type hypersensitivity reaction (type IV allergic response).

Pharmacological profiles of animal- and nonanimal-derived sulfated polysaccharides--comparison of unfractionated heparin, the semisynthetic glucan sulfate PS3, and the sulfated polysaccharide fraction isolated from Delesseria sanguinea

Sulfated polysaccharides (SP) such as heparin are known to exhibit a wide range of biological activities, e.g., anticoagulant, anti-inflammatory, and antimetastastic effects. However, since the anticoagulant activity of heparin is dominating, its therapeutic use for other medical indications is limited due to an associated risk of bleeding. Further disadvantages of heparin are its animal origin, the shortage of resources, and its complex and variable composition. However, SP without these limitations may represent a substance class with good prospects for applications other than anticoagulation. In this study, the in vitro pharmacological profiles of two nonanimal-derived SP were investigated in comparison with unfractionated heparin. One is the natural SP fraction from the red algae Delesseria sanguinea (D.s.-SP). The other one is the chemically defined PS3, a semisynthetic β-1,3-glucan sulfate with proven in vivo anti-inflammatory and antimetastatic activities. All three polysaccharides were examined in vitro for their inhibitory effects on the coagulation and complement system, polymorphonuclear neutrophil elastase, hyaluronidase, matrix metalloproteinase-1, heparanase, and p-selectin-mediated cell adhesion. Compared with heparin, the nonanimal-derived polysaccharides have a four times weaker anticoagulant activity, but mostly exhibit stronger (1.4–224 times) effects on test systems investigating targets of inflammation or metastasis. According to their different structures, PS3 and D.s.-SP differ in their pharmacological profile with PS3 being the strongest inhibitor of heparanase and cell adhesion and D.s.-SP being the strongest inhibitor of hyaluronidase and complement activation. Considering both pharmacological profile and pharmaceutical quality parameters, PS3 represents a candidate for further development as an anti-inflammatory or antimetastatic drug whereas D.s.-SP might have perspectives for cosmetic applications.

Monitoring fondaparinux with the Sonoclot

Fondaparinux is a new anticoagulant that interacts with antithrombin III and activated coagulation factor X resulting in an inhibition of the coagulation system. It has been successful in doses of 2.5 mg for thromboprophylaxis as well as in higher therapeutic doses of 5-7.5 mg. No optimal method for monitoring the effects of fondaparinux has been proposed. The aim of the present study was to investigate whether a viscoelastic coagulation analyzer, the Sonoclot (Sienco, Denver, Colorado, USA), could be used for in-vitro monitoring of fondaparinux. Different concentrations of fondaparinux were added in vitro to whole blood taken from eight volunteers. The blood samples mixed with the various amounts of fondaparinux were analyzed using the Sonoclot. The whole-blood activated partial thromboplastin time with the Hemochron Jr (ITC, Edison, New Jersey, USA) was used as the reference coagulation analysis. All analyses were started expeditiously, within 30 s from sampling, and were performed at 37 degrees C. The values of the Sonoclot parameter clot rate, which measures the rate of fibrin formation, fibrin polymerization and platelet-fibrin interactions, were significantly correlated to increasing concentrations of fondaparinux (R = -0.90). The Sonoclot parameters of activated coagulation time, time to peak and clot retraction had weaker, but still significant, correlations to fondaparinux concentrations. At prophylactic doses (0.38 microg/ml blood) the clot rate decreased 15% compared with the initial unanticoagulated value, whereas at therapeutic doses (1.53 microg/ml blood) there was a 27% decrease. In conclusion, the Sonoclot parameter clot rate could be of clinical value to individualize the fondaparinux dosage, especially the higher, therapeutic, dosages.

P-selectin: a common therapeutic target for cardiovascular disorders, inflammation and tumour metastasis

P-selectin belongs to the family of selectin adhesion molecules, and is expressed by platelets and endothelial cells on stimulation. This pattern of expression may indicate an involvement of this molecule in inflammation and coagulation. Data from mice lacking P-selectin expression confirmed this assumption. In addition, a key role of P-selectin in the formation of tumour metastases has been established. Apparently unrelated, clinical experience has pointed towards a detrimental interaction of inflammation and cancer with thromboembolic diseases and vice versa. Therefore, targeting molecules such as P-selectin contributing to coagulation, inflammation and metastasis may offer novel therapeutic strategies to treat chronic inflammatory diseases and metastatic cancer. The authors aim to critically evaluate the contribution of P-selectin in these diseases, and discuss the value of therapeutic inhibition of P-selectin functions in coagulation, inflammation and metastasis.