New anticoagulants: current status and future potential

Direct thrombin inhibitors versus vitamin K antagonists for preventing cerebral or systemic embolism in people with non-valvular atrial fibrillation

BACKGROUND

Chronic anticoagulation with vitamin K antagonists (VKAs) prevents ischaemic stroke and systemic embolism in people with non-valvular atrial fibrillation (AF) but dose adjustment, coagulation monitoring and bleeding limits its use. Direct thrombin inhibitors (DTIs) are under investigation as potential alternatives.

OBJECTIVES

To assess (1) the comparative efficacy of long-term anticoagulation using DTIs versus VKAs on vascular deaths and ischaemic events in people with non-valvular AF, and (2) the comparative safety of chronic anticoagulation using DTIs versus VKAs on (a) fatal and non-fatal major bleeding events including haemorrhagic strokes, (b) adverse events other than bleeding and ischaemic events that lead to treatment discontinuation and (c) all-cause mortality in people with non-valvular AF.

SEARCH METHODS

We searched the Cochrane Stroke Group Trials Register (July 2013), the Cochrane Central Register of Controlled Trials (CENTRAL), (The Cochrane Library, May 2013), MEDLINE (1950 to July 2013), EMBASE (1980 to October 2013), LILACS (1982 to October 2013) and trials registers (September 2013). We also searched the websites of clinical trials and pharmaceutical companies and handsearched the reference lists of articles and conference proceedings.

SELECTION CRITERIA

Randomised controlled trials (RCTs) comparing DTIs versus VKAs for prevention of stroke and systemic embolism in people with non-valvular AF.

DATA COLLECTION AND ANALYSIS

All three review authors independently performed data extraction and assessment of risk of bias. Primary analyses compared all DTIs combined versus warfarin. We performed post hoc analyses excluding ximelagatran because this drug was withdrawn from the market owing to safety concerns.

MAIN RESULTS

We included eight studies involving a total of 27,557 participants with non-valvular AF and one or more risk factors for stroke; 26,601 of them were assigned to standard doses groups and included in the primary analysis. The DTIs: dabigatran 110 mg twice daily and 150 mg twice daily (three studies, 12,355 participants), AZD0837 300 mg once per day (two studies, 233 participants) and ximelagatran 36 mg twice per day (three studies, 3726 participants) were compared with the VKA warfarin (10,287 participants). Overall risk of bias and statistical heterogeneity of the studies included were low.The odds of vascular death and ischaemic events were not significantly different between all DTIs and warfarin (odds ratio (OR) 0.94, 95% confidence interval (CI) 0.85 to 1.05). Sensitivity analysis by dose of dabigatran on reduction in ischaemic events and vascular mortality indicated that dabigatran 150 mg twice daily was superior to warfarin although the effect estimate was of borderline statistical significance (OR 0.86, 95% CI 0.75 to 0.99). Sensitivity analyses by other factors did not alter the results. Fatal and non-fatal major bleeding events, including haemorrhagic strokes, were less frequent with the DTIs (OR 0.87, 95% CI 0.78 to 0.97). Adverse events that led to discontinuation of treatment were significantly more frequent with the DTIs (OR 2.18, 95% CI 1.82 to 2.61). All-cause mortality was similar between DTIs and warfarin (OR 0.91, 95% CI 0.83 to 1.01).

AUTHORS' CONCLUSIONS

DTIs were as efficacious as VKAs for the composite outcome of vascular death and ischaemic events and only the dose of dabigatran 150 mg twice daily was found to be superior to warfarin. DTIs were associated with fewer major haemorrhagic events, including haemorrhagic strokes. Adverse events that led to discontinuation of treatment occurred more frequently with the DTIs. We detected no difference in death from all causes.

Pharmacology of argatroban

Argatroban is a synthetic, small-molecule direct thrombin inhibitor that is approved in the USA, the EU and Japan for prophylaxis or treatment of thrombosis in patients with heparin-induced thrombocytopenia (HIT), and for anticoagulation of HIT patients undergoing PCI. Argatroban binds reversibly to, and inhibits both soluble and clot-bound thrombin. Argatroban does not generate antibodies, is not susceptible to degradation by proteases and is cleared hepatically. It has a predictable anticoagulant effect and there is a good correlation between dose, plasma concentration and pharmacodynamic effect. Initial clinical studies suggest that further investigations to establish the use of argatroban in ischemic stroke, acute coronary syndrome, hemodialysis, blood oxygenation, off-pump cardiac surgery and other clinical indications are warranted.

Thrombin receptor (PAR-1) antagonists as novel antithrombotic agents

In addition to its central role in haemostasis and wound healing, thrombin activates platelets and smooth muscle cells by proteolytic activation of cell surface protease-activated receptor-1 (PAR-1), which is also known as the thrombin receptor. Thrombin is the most potent activator of human platelets and, as such, a thrombin receptor antagonist is likely to exert potent antithrombotic effect in platelet-rich arterial thrombosis. As thrombin receptor antagonism does not inhibit the ability of thrombin to generate fibrin, such an agent is likely to have less bleeding liability than conventional anticoagulants. The proof-of-concept of the antithrombotic effect of PAR-1 antagonists has been established in several non-human primate models. The current success of PAR-1 research is underscored by the advancement of two candidates into clinical trails for acute coronary syndrome by Schering-Plough and Eisai Company.

Error in body weight estimation leads to inadequate parenteral anticoagulation

Parenteral anticoagulation is a cornerstone in the management of venous and arterial thrombosis. Unfractionated heparin has a wide dose/response relationship, requiring frequent and troublesome laboratorial follow-up. Because of all these factors, low-molecular-weight heparin use has been increasing. Inadequate dosage has been pointed out as a potential problem because the use of subjectively estimated weight instead of real measured weight is common practice in the emergency department (ED). To evaluate the impact of inadequate weight estimation on enoxaparin dosage, we investigated the adequacy of anticoagulation of patients in a tertiary ED where subjective weight estimation is common practice. We obtained the estimated, informed, and measured weight of 28 patients in need of parenteral anticoagulation. Basal and steady-state (after the second subcutaneous shot of enoxaparin) anti-Xa activity was obtained as a measure of adequate anticoagulation. The patients were divided into 2 groups according the anticoagulation adequacy. From the 28 patients enrolled, 75% (group 1, n = 21) received at least 0.9 mg/kg per dose BID and 25% (group 2, n = 7) received less than 0.9 mg/kg per dose BID of enoxaparin. Only 4 (14.3%) of all patients had anti-Xa activity less than the inferior limit of the therapeutic range (<0.5 UI/mL), all of them from group 2. In conclusion, when weight estimation was used to determine the enoxaparin dosage, 25% of the patients were inadequately anticoagulated (anti-Xa activity <0.5 UI/mL) during the initial crucial phase of treatment.

Coagulation Monitoring

Monitoring hemostasis is now possible by different modalities, of which the point of care devices seem most helpful to the clinician in the operating room. Most of these monitors are being used in the cardiac population, and their significance in other fields remains to be assessed.

Update on the clinical applications of argatroban

The small molecule, arginomimetic drug argatroban is the first synthetic direct antithrombin to be approved for clinical use. Argatroban reversibly binds to and inhibits both soluble and clot-bound thrombin. In contrast to other direct thrombin inhibitors, argatroban upregulates nitric oxide, enhancing its antithrombotic effect, does not generate antibodies, is not susceptible to degradation by proteases and is hepatically cleared. It has a predictable anticoagulant effect. Argatroban has proven efficacy and safety for prophylaxis and treatment of patients with thrombosis associated with heparin-induced thrombocytopenia (HIT), and for percutaneous coronary intervention in HIT and non-HIT patients. Pilot studies suggest that further investigations to establish the use of argatroban in ischemic stroke, acute coronary syndrome, hemodialysis, blood oxygenation, off-pump cardiac surgery and other clinical indications are warranted.

Bivalirudin as an Anticoagulation Agent: Safety and Efficacy in Peripheral Interventions

PURPOSE

Traditionally, unfractionated heparin is used to prevent thrombotic complications in peripheral interventions. The purpose of this study is to evaluate the use of bivalirudin as the anticoagulant agent for peripheral interventions.

MATERIALS AND METHODS

A retrospective analysis of 108 patients who underwent 110 peripheral interventions between January 2002 and January 2004 and received bivalirudin as the sole anticoagulation agent was conducted at Baptist Cardiac and Vascular Institute. Interventions were performed in the following areas: iliac, femoropopliteal, and distal (n = 55), carotid (n = 31), vertebral (n = 1), renal (n = 14), aorta (n = 7), and subclavian (n = 2). The following procedural and clinical endpoints were examined: death, requirement of urgent surgery or surgery during the same admission, urgent percutaneous revascularization in the same treated vessel, thrombotic or embolic events, bleeding events, and groin complications.

RESULTS

A total of 266 lesions were dilated in 185 arteries. There were no procedural mortalities, procedural success was 99.1%, and the complication rate was 3.6%. There was one embolic stroke (0.9%), one thrombosis (0.9%), and two groin hematomas (1.8%). No patient required urgent surgery or reintervention in the same treated vessel. No complications were noted at 7 days after the procedure. There were two interventions by postprocedure day 30: toe amputation and groin debridement.

CONCLUSION

Bivalirudin is a safe alternative to unfractionated heparin as the anticoagulation agent in peripheral interventions. This study shows that the complication profile is comparable to other bivalirudin studies. Bivalirudin is effective, easy to use, and is associated with few bleeding complications.

Tissue factor produced by the endocrine cells of the islets of Langerhans is associated with a negative outcome of clinical islet transplantation

There are strong indications that only a small fraction of grafts successfully engraft in clinical islet transplantation. One explanation may be the instant blood-mediated inflammatory reaction (IBMIR) elicited by tissue factor, which is produced by the endocrine cells. In the present study, we show that islets intended for islet transplantation produce tissue factor in both the transmembrane and the alternatively spliced form and that the membrane-bound form is released as microparticles often associated with both insulin and glucagon granules. A low-molecular mass factor VIIa (FVIIa) inhibitor that indirectly blocks both forms of tissue factor was shown in vitro to be a promising drug to eliminate the IBMIR. Thrombin-antithrombin complex (TAT) and FVIIa-antithrombin complex (FVIIa-AT) were measured in nine patients who together received 20 infusions of isolated human islets. Both the TAT and FVIIa-AT complexes increased rapidly within 15-60 min after infusion. When the initial TAT and FVIIa-AT levels were plotted against the increase in C-peptide concentration after 7 days, patients with an initially strong IBMIR showed no significant increase in insulin synthesis after 7 days. In conclusion, tissue factor present in both the islets and the culture medium and elicits IBMIR, which affects the function of the transplanted islets.

Small-molecule direct antithrombins: argatroban

Argatroban represents the first antithrombin agent that was approved for clinical use. It belongs to the peptidomimetic (arginomimetic) group of drugs with multiple pharmacological properties. Unlike the other antithrombin drugs, such as hirudins and hirulogs, argatroban is a reversible antithrombin agent and therefore exhibits a considerably different pharmacological profile. Although argatroban is considered to be a member of the antithrombin family, its mechanisms of action include several other processes that have not been explored fully to date. These include the inhibition of non-thrombin serine proteases, a direct effect on endothelial cells and the vasculature (generation of nitric oxide), and downregulation of various inflammatory and thrombotic cytokines. Due to its lower molecular weight, argatroban is capable of passing through endovascular and cellular barriers and may, therefore, be more effective than heparins and hirudins in the antithrombotic management of microvascular disorders. Argatroban is an effective anticoagulant agent that produces a stronger anticoagulant effect than heparins and hirudins at equivalent anticoagulant levels, as measured by the activated clotting time (ACT) and activated partial thromboplastin time (APTT). At comparable ACT (300 seconds) and APTT (75-90 seconds), argatroban produces stronger inhibition of thrombin generation, as measured by in-vitro assays. Argatroban does not generate any neutralizing or non-neutralizing antibodies, and has predictable antithrombotic effects in different patients. In addition to the inhibition of thrombogenesis, argatroban also facilitates blood flow, inhibition of platelet activation and endothelial cell stimulation, mechanisms that are not necessarily related to thrombin inhibition. Despite these pharmacological advantages, additional clinical investigations are needed to validate the use of argatroban in clinical indications other than those for which it is currently approved, namely, heparin-induced thrombocytopenia and support of percutaneous coronary angioplasty.

Oral direct thrombin inhibition: an effective and novel approach for venous thromboembolism

Venous thromboembolism is a serious illness that affects patient morbidity and mortality and presents a significant management challenge to healthcare providers world-wide. Despite major achievements in the significant reduction of thromboembolic complications, the most common therapies currently used for prevention and treatment of venous thromboembolism--heparins and vitamin K antagonists such as warfarin--have several limitations. In particular, unfractionated heparin and warfarin show significant inter-patient variability in pharmacokinetics and pharmacodynamics, which makes regular coagulation monitoring necessary. Furthermore, only warfarin is suitable for long-term use, as it is administered orally. A new class of anticoagulants has been developed that directly target thrombin, a key enzyme in the blood coagulation cascade. Unlike warfarin and heparin, these direct thrombin inhibitors are able to inhibit fibrin-bound thrombin and so produce more effective inhibition of coagulation. Importantly, some members of this class of drugs have been developed for oral administration. Ximelagatran, which is converted to its active form melagatran, has predictable pharmacokinetics and pharmacodynamics. Therefore, ximelagatran can be administered in fixed doses with no need for coagulation monitoring. Its efficacy and safety profile have been demonstrated in preclinical and clinical studies. As the first oral agent in the new class, direct thrombin inhibitors, ximelagatran has significant potential for improving the prevention and treatment of venous thromboembolism.

Hydrogen/Deuterium Exchange Mass Spectrometry for Investigating Protein-Ligand Interactions

Amide hydrogen/deuterium exchange mass spectrometry is rapidly becoming a powerful method for high-resolution analyses of protein dynamics, structure, and function. Hydrogen/deuterium exchange approaches can provide information that greatly augments and refines information derived from high-resolution structural studies, and can provide detailed information on native protein structure when structural information is unavailable. Application of this method for rapid analyses of protein-ligand complexes could prove useful for studies of important disease-related protein complexes. The following review covers fundamentals of hydrogen/deuterium exchange and its applications to the study of protein-ligand complexes. In addition, hydrogen/deuterium exchange mass spectrometry studies on a protein-inhibitor complex are presented.