Direct thrombin inhibitors for anticoagulation

A Non-competitive Serpin-Like Thrombin Inhibitor Isolated from Moringa oleifera Exhibit a High Affinity for Thrombin

The majority of the clotting factors involved in blood coagulation pathways are serine proteases and thrombin is one of the key serine proteases involved in blood clotting. Many synthetic and chemical drugs targeting these proteases as therapeutics are known. However, they are associated with serious side effects such as bleeding, haemorrhage, edema etc. Serine protease inhibitors from plants have been suggested as one of the potential anticoagulant molecules against thrombosis. In the present work, a direct thrombin inhibitor from Moringa oleifera was isolated, purified and characterized. The homogeneity of the inhibitor is confirmed on native- PAGE. The purified inhibitor (5 µg) showed 63% thrombin inhibition at pH 7.2 at 37 °C. The IC₅₀ value of the isolated inhibitor was determined as 4.23 µg. The inhibitor on SDS-PAGE appeared as a single protein-stained band corresponding to 50 kDa thereby indicating its molecular weight as 50 kDa. Purified thrombin inhibitor (5 µg) showed 12% inhibition of trypsin, and 17% inhibition of chymotrypsin. This suggests more specificity of purified inhibitor towards thrombin. The isolated inhibitor showed a non-competitive mode of inhibition against thrombin as determined by the Dixon plot. The inhibition constant (Ki) was calculated as 4.35 × 10^-7 M. The present work reports for the first time a direct thrombin inhibitor from M. oleifera which may be further explored as an antithrombotic drug.

Monitoring Direct Thrombin Inhibitors With Calibrated Diluted Thrombin Time vs Activated Partial Thromboplastin Time in Pediatric Patients

OBJECTIVES

Activated partial thromboplastin time (aPTT) is the primary test used to monitor intravenous (IV) direct thrombin inhibitors (DTIs) but has many limitations. The plasma diluted thrombin time (dTT) has shown better correlation with DTI levels than aPTT. This study compared dose-response curves for dTT and aPTT in pediatric patients receiving argatroban and bivalirudin.

METHODS

A retrospective review of pediatric patients treated with argatroban (n = 45) or bivalirudin (n = 14) monitored with dTT and aPTT.

RESULTS

The dTT assay was calibrated to report DTI concentrations in µg/mL for argatroban and bivalirudin with good analytic sensitivity and specificity. The dTT was fivefold more likely to show a stable dose-response slope than the aPTT (P < .0002; odds ratio, 4.9). For patients in whom both dTT and aPTT showed a significant correlation between dose and assay results, dTT had a higher average correlation factor compared with aPTT (P = .007). Argatroban dose-response slopes showed more inter- and intrapatient variation than bivalirudin (dose-response slope coefficient of variation, 132% vs 52%).

CONCLUSIONS

The dTT assay was more likely to show a stable dose response and have a stronger correlation with DTI dose than aPTT. Argatroban shows more variation in dose response than bivalirudin.

Proteolysis in Reproduction: Lessons From Gene-Modified Organism Studies

The physiological roles of proteolysis are not limited to degrading unnecessary proteins. Proteolysis plays pivotal roles in various biological processes through cleaving peptide bonds to activate and inactivate proteins including enzymes, transcription factors, and receptors. As a wide range of cellular processes is regulated by proteolysis, abnormalities or dysregulation of such proteolytic processes therefore often cause diseases. Recent genetic studies have clarified the inclusion of proteases and protease inhibitors in various reproductive processes such as development of gonads, generation and activation of gametes, and physical interaction between gametes in various species including yeast, animals, and plants. Such studies not only clarify proteolysis-related factors but the biological processes regulated by proteolysis for successful reproduction. Here the physiological roles of proteases and proteolysis in reproduction will be reviewed based on findings using gene-modified organisms.

From Animal Poisons and Venoms to Medicines: Achievements, Challenges and Perspectives in Drug Discovery

Animal poisons and venoms are comprised of different classes of molecules displaying wide-ranging pharmacological activities. This review aims to provide an in-depth view of toxin-based compounds from terrestrial and marine organisms used as diagnostic tools, experimental molecules to validate postulated therapeutic targets, drug libraries, prototypes for the design of drugs, cosmeceuticals, and therapeutic agents. However, making these molecules applicable requires extensive preclinical trials, with some applications also demanding clinical trials, in order to validate their molecular target, mechanism of action, effective dose, potential adverse effects, as well as other fundamental parameters. Here we go through the pitfalls for a toxin-based potential therapeutic drug to become eligible for clinical trials and marketing. The manuscript also presents an overview of the current picture for several molecules from different animal venoms and poisons (such as those from amphibians, cone snails, hymenopterans, scorpions, sea anemones, snakes, spiders, tetraodontiformes, bats, and shrews) that have been used in clinical trials. Advances and perspectives on the therapeutic potential of molecules from other underexploited animals, such as caterpillars and ticks, are also reported. The challenges faced during the lengthy and costly preclinical and clinical studies and how to overcome these hindrances are also discussed for that drug candidates going to the bedside. It covers most of the drugs developed using toxins, the molecules that have failed and those that are currently in clinical trials. The article presents a detailed overview of toxins that have been used as therapeutic agents, including their discovery, formulation, dosage, indications, main adverse effects, and pregnancy and breastfeeding prescription warnings. Toxins in diagnosis, as well as cosmeceuticals and atypical therapies (bee venom and leech therapies) are also reported. The level of cumulative and detailed information provided in this review may help pharmacists, physicians, biotechnologists, pharmacologists, and scientists interested in toxinology, drug discovery, and development of toxin-based products.

Discovery of potent thrombin inhibitors from a protease-focused DNA-encoded chemical library

To rapidly identify small-molecule lead compounds to target healthcare-associated proteases, we constructed a unique 9.8-million-membered protease-focused DNA-encoded chemical library. Affinity selection of this library with a healthcare-relevant protease (i.e., thrombin, a key protein necessary for blood coagulation) revealed potent inhibitors in the first screening attempt. Our results emphasize the utility of a structurally focused DNA-encoded chemical library approach to rapidly uncover hits for healthcare targets (e.g., proteases) where no drug exists (e.g., male contraception) and for emerging diseases (e.g., coronavirus disease 2019).

DNA-encoded chemical libraries are collections of compounds individually coupled to unique DNA tags serving as amplifiable identification barcodes. By bridging split-and-pool combinatorial synthesis with the ligation of unique encoding DNA oligomers, million- to billion-member libraries can be synthesized for use in hundreds of healthcare target screens. Although structural diversity and desirable molecular property ranges generally guide DNA-encoded chemical library design, recent reports have highlighted the utility of focused DNA-encoded chemical libraries that are structurally biased for a class of protein targets. Herein, a protease-focused DNA-encoded chemical library was designed that utilizes chemotypes known to engage conserved catalytic protease residues. The three-cycle library features functional moieties such as guanidine, which interacts strongly with aspartate of the protease catalytic triad, as well as mild electrophiles such as sulfonamide, urea, and carbamate. We developed a DNA-compatible method for guanidinylation of amines and reduction of nitriles. Employing these optimized reactions, we constructed a 9.8-million-membered DNA-encoded chemical library. Affinity selection of the library with thrombin, a common protease, revealed a number of enriched features which ultimately led to the discovery of a 1 nM inhibitor of thrombin. Thus, structurally focused DNA-encoded chemical libraries have tremendous potential to find clinically useful high-affinity hits for the rapid discovery of drugs for targets (e.g., proteases) with essential functions in infectious diseases (e.g., severe acute respiratory syndrome coronavirus 2) and relevant healthcare conditions (e.g., male contraception).

Evaluation of antithrombotic activity of thrombin DNA aptamers by a murine thrombosis model

Aptamers are nucleic acid based molecular recognition elements with a high potential for the theranostics. Some of the aptamers are under development for therapeutic applications as promising antithrombotic agents; and G-quadruplex DNA aptamers, which directly inhibit the thrombin activity, are among them. RA-36, the 31-meric DNA aptamer, consists of two thrombin binding pharmacophores joined with the thymine linker. It has been shown earlier that RA-36 directly inhibits thrombin in the reaction of fibrinogen hydrolysis, and also it inhibits plasma and blood coagulation. Studies of both inhibitory and anticoagulation effects had indicated rather high species specificity of the aptamer. Further R&D of RA-36 requires exploring its efficiency in vivo. Therefore the development of a robust and adequate animal model for effective physiological studies of aptamers is in high current demand. This work is devoted to in vivo study of the antithrombotic effect of RA-36 aptamer. A murine model of thrombosis has been applied to reveal a lag and even prevention of thrombus formation when RA-36 was intravenous bolus injected in high doses of 1.4–7.1 µmol/kg (14–70 mg/kg). A comparative study of RA-36 aptamer and bivalirudin reveals that both direct thrombin inhibitors have similar antithrombotic effects for the murine model of thrombosis; though in vitro bivalirudin has anticoagulation activity several times higher compared to RA-36. The results indicate that both RA-36 aptamer and bivalirudin are direct thrombin inhibitors of different potency, but possible interactions of the thrombin-inhibitor complex with other components of blood coagulation cascade level the physiological effects for both inhibitors.

Direct thrombin inhibitors: patents 2002-2012 (Review)

Acute vascular diseases and other thromboses of the blood system constitute major health risks in developing countries. Thrombin plays a central role in blood coagulation, which is a crucial process involved in thrombosis. Direct thrombin inhibitors (DTIs) such as argatroban, dabigatran, dabigatran etexilate, lepirudin, desirudin and bivalirudin, which bind to thrombin and block its enzymatic activity, are widely and effectively used in the treatment of thromboembolic diseases. DTIs appear to overcome the disadvantages of indirect thrombin inhibitors such as unfractionated heparins (UFH). Although these DTIs show specific advantages over indirect inhibitors, they still present limitations, such as a narrow therapeutic window, and bleeding and anaphylaxis as side-effects. Novel anticoagulant drugs need thus to be developed to overcome these limitations. In the search for additional candidate agents with improved efficacy, safety and high bioavailability in oral administration, a high number of compounds has been identified, such as those derived from the tripeptide template D-Phe-Pro-Arg, aptamers and peptides isolated from blood-sucking animals. These candidates may prove the new agents of choice for the treatment of cardiovascular diseases.

Management of anticoagulation in the critically ill patient

This article presents a review of the most commonly used anticoagulants in the intensive care unit setting. The difference between agents as well as the advantages and disadvantages are reviewed. For each agent, the mechanism of action, dosing, monitoring, adverse effects, and reversal strategies are discussed.

Dabigatran etexilate: an oral direct thrombin inhibitor for the management of thromboembolic disorders

BACKGROUND

Until recently, warfarin was the only oral anticoagulant available in the United States. Its narrow therapeutic index, interpatient variability in dose response, and drug and food interactions make it difficult to use. Dabigatran etexilate (DE) is a new oral direct thrombin inhibitor that was approved in the US and in Canada for the prevention of thromboembolic events in patients with atrial fibrillation (AF), as well as in Europe and Canada for the prevention of venous thromboembolism (VTE).

OBJECTIVE

To discuss the role of DE for the prevention and treatment of VTE, as well as for the prevention of stroke in patients with AF.

METHODS

Peer-reviewed clinical trials, review articles, and treatment guidelines were identified from MEDLINE and the Current Contents database (both 1966-February 15, 2012) using the search terms dabigatran, VTE, Afib, pharmacokinetics, pharmacodynamics, pharmacoeconomics, and cost-effectiveness. Citations from available articles were also reviewed for additional references.

RESULTS

For VTE prophylaxis, DE 150 or 220 mg orally daily has demonstrated either superiority or noninferiority to subcutaneous enoxaparin once daily in most studies. However, one study failed to demonstrate noninferiority to subcutaneous enoxaparin dosed BID in the composite end point of VTE, and all-cause mortality. For VTE treatment, DE 150 mg BID orally was shown to be noninferior to warfarin in preventing recurrent events. For AF, DE 150 mg BID orally is superior to warfarin in the prevention of thromboembolism, whereas 110 mg BID is noninferior to warfarin. Pharmacoeconomic analyses performed in the United Kingdom and Ireland found that DE can be cost-saving compared with enoxaparin in the prevention of VTE. Adverse effects of DE reported in clinical studies include dyspepsia (12%-13%) and bleeding (minor bleeding: 6%-22%).

CONCLUSIONS

DE exhibited a safety profile and efficacy comparable to enoxaparin for VTE prophylaxis; comparable safety profile and efficacy to warfarin for VTE treatment; and superiority (150 mg BID orally) in the prevention of stroke and systemic embolism compared with warfarin in patients with AF. The relative ease of oral administration, no need for routine monitoring, and lack of significant drug interactions, may favor use of DE over other anticoagulants. However, there is no antidote for DE currently available.

Translatability scoring in drug development: eight case studies

Translational medicine describes the transfer of basic in vitro and in vivo data into human applications. In the light of low rates of market approvals for new medical entities, better strategies to predict the risk of drug development should be used to increase output and reduce costs. Recently, a scoring system to assess the translatability of early drug projects has been proposed. Here eight drugs from different therapeutic areas have been subjected to a retrospective test-run in this system fictively located at the phase II-III transition. The scores gained here underline the importance of biomarker quality which is pivotal to decrease the risk of the project in all cases. This is particularly evident for gefitinib. The EGFR mutation status is a breakthrough biomarker to predict therapeutic success which made this compound clinically acceptable, and this is plausibly reflected by a considerable increase of the translatability score. For psychiatric and Alzheimer's drugs, and for a CETP-inhibitor, the lack of suitable biomarkers and animal models is reflected by a low translatability score, well correlating with the excessive translational risk in these areas. These case studies document the apparent utility of the scoring system, at least under retrospective conditions, as the scores correlate with the outcomes at the level of market approval. Prospective validation is still missing, but these case studies are encouraging.

Direct thrombin inhibitors

Heparins and vitamin K antagonists have been the primary agents used for anticoagulation in certain cardiovascular and thromboembolic diseases for over 50 years. However, they can be difficult to administer and are fraught with limitations. In response to the need for new anticoagulants, direct thrombin inhibitors (DTIs) have been developed and investigated for their utility in prophylaxis and treatment of venous thromboembolism (VTE), heparin-induced thrombocytopenia (HIT), acute coronary syndromes (ACS), secondary prevention of coronary events after ACS, and nonvalvular atrial fibrillation. Currently, four parenteral direct inhibitors of thrombin activity are FDA-approved in North America: lepirudin, desirudin, bivalirudin and argatroban. Of the new oral DTIs, dabigatran etexilate is the most studied and promising of these agents. This review discusses the clinical indications and efficacy of these direct thrombin inhibitors as well as future directions in anticoagulant therapy.

Laboratory monitoring of new anticoagulants

Maintaining a balance between bleeding and clotting has always been a challenge in treating coagulation disorders. A perturbation in that balance can be associated with substantial morbidity and mortality. As a result, anticoagulant monitoring is extremely important, and inappropriate testing may lead to complications. There are now a variety of new anticoagulant drugs in clinical use including several direct thrombin inhibitors (DTIs), such as argatroban, bivalirudin, and hirudin, as well as a Factor Xa inhibitor, fondaparinux. There are pitfalls associated with some of the currently used laboratory monitoring tests, and newer alternative laboratory monitoring tests have been investigated (Walenga and Hoppensteadt, Semin Thromb Hemost 2004;30:683-695). In addition, laboratory testing can assist with transitioning patients from DTI to warfarin therapy.

Use of stent grafts and coils in vessel rupture and perforation

Vessel rupture and perforation are important complications of percutaneous treatment of coronary and peripheral arterial disease. These complications can result in abrupt vessel closure, distal organ injury, bleeding into the surrounding tissue, and death. Prompt management of such complications is therefore critically important. This paper reviews the management of vessel rupture and perforation, including the use of different types of covered stents (balloon-expandable and self-expanding), as well as the various types of embolization coils. Particular focus will be placed on percutaneous coronary artery and peripheral arterial interventions.

Recombinant activated factor VII effectively reverses the anticoagulant effects of heparin, enoxaparin, fondaparinux, argatroban, and bivalirudin ex vivo as measured using thromboelastography

Bleeding is the major adverse reaction to anticoagulants, leading to significant morbidity and even mortality. Protamine is a specific antidote for heparin yet is only partially effective for enoxaparin, and the activated factor X inhibitor fondaparinux and the direct thrombin inhibitors argatroban and bivalirudin lack specific antidotes. We evaluated the ability of recombinant activated factor VII (rFVIIa), a general hemostatic agent, to reverse the anticoagulant effects of heparin, enoxaparin, fondaparinux, argatroban, and bivalirudin, as measured by thromboelastography. Whole-blood samples containing each test anticoagulant, with or without rFVIIa 1.5-4.5 microg/ml, were prepared ex vivo (n >or= 48, each anticoagulant) and analyzed by thromboelastography. The thromboelastography parameters of clot initiation, propagation, rigidity and elasticity were compared for the ex-vivo samples for each anticoagulant. The reversal ability of rFVIIa was also assessed using the standard clinical assay used to monitor each anticoagulant. Thromboelastography was performed on blood from eight stably anticoagulated patients, with and without exogenous rFVIIa. For each anticoagulant, rFVIIa significantly improved and, in some cases, completely normalized all thromboelastography parameters (P < 0.001). rFVIIa significantly (P < 0.01) decreased the activated partial thromboplastin time for argatroban-containing, bivalirudin-containing, or heparin-containing blood yet did not affect the anti-activated factor X levels for enoxaparin-containing or fondaparinux-containing blood. By thromboelastography, rFVIIa exerted generally similar reversal effects on the anticoagulated patient samples as on the ex-vivo samples. In conclusion, rFVIIa effectively reverses the anticoagulant effects of heparin, enoxaparin, fondaparinux, argatroban, and bivalirudin, and should be considered for patients with excessive bleeding associated with these anticoagulants.

Effects on coagulation of a synthetic heparan mimetic given intraperitoneally or orally

OTR4120, which belongs to a family of heparan sulfate-mimetic polymers, promotes tissue repair when injected locally in doses of a few micrograms. As OTR4120 is a sulfated polysaccharide, we investigated its possible role on the coagulation cascade. We used both in vitro and in vivo assays. Increases in clotting times (thrombin time, prothrombin time, and activated partial thromboplastin time) occurred with OTR4120 in doses at least 10 times lower than heparin. OTR4120 dose-dependently inhibited the biological activity of thrombin and bound thrombin with an affinity of 14 +/- 2 nM. SDS-PAGE showed that OTR4120 induced the formation of covalently linked complexes between antithrombin III or heparin cofactor II and thrombin. OTR4120 induced anticoagulant effects, and antithrombin activity was greatest 90 min after intraperitoneal injection. No bleeding or significant platelet count changes occurred with doses smaller than 55 mg/kg. Interestingly, orally administered OTR4120 crossed the gastrointestinal barrier and, in a dose of 70 mg/kg, induced significant ex vivo antithrombotic activity in the bloodstream.

Differential effects of direct thrombin inhibitors and antithrombin-dependent anticoagulants on the dynamics of clot formation

New anticoagulants, including the direct thrombin inhibitors (DTIs) and fondaparinux, are increasingly replacing unfractionated heparin and enoxaparin. We examined the effects of argatroban (n = 60), bivalirudin (n = 44), heparin (n = 14), enoxaparin (n = 22), and fondaparinux (n = 24) on clot formation utilizing thromboelastography. Blood samples containing anticoagulants at clinically relevant concentrations were prepared ex vivo and analyzed using kaolin or tissue factor activation. Thromboelastography parameters of clot initiation (R), clot propagation (K and angle), clot rigidity (maximum amplitude) and clot elasticity (G) were compared between anticoagulants. Thromboelastography was also performed on blood from eight patients receiving anticoagulants. Each anticoagulant exerted significant concentration-dependent effects on R, K and angle. Only heparin, enoxaparin, and fondaparinux significantly affected maximum amplitude and G. Significant differences existed for all parameters between heparin and each anticoagulant and between fondaparinux and each DTI (P < 0.001), and for angle, maximum amplitude, and G between enoxaparin and each DTI (P < 0.008). Thromboelastography responses in ex-vivo samples and patient samples were comparable. In conclusion, whereas argatroban, bivalirudin, heparin, enoxaparin and fondaparinux each delay clot formation, the DTIs do not alter clot rigidity or elasticity. The reduced bleeding reported with DTIs versus heparin may relate to the fact that clots form with normal rigidity and elasticity.

Recombinant factor VIIa for refractory bleeding after cardiac surgery secondary to anticoagulation with the direct thrombin inhibitor lepirudin

A 56-year-old man with heparin-induced thrombocytopenia with thrombosis syndrome (HITTS) received anticoagulation with recombinant hirudin (lepirudin) for emergency coronary artery bypass graft (CABG) surgery and aortic valve replacement. The patient experienced life-threatening refractory bleeding that was successfully treated with recombinant factor VIIa. He had a history of infective endocarditis that resulted in severe aortic insufficiency, three-vessel coronary artery disease, and acute renal failure requiring hemodialysis. The patient was transferred from another hospital for the emergency surgery, but before his transfer, he developed HITTS secondary to therapeutic heparin for a deep vein thrombosis of the lower extremity. The presence of HITTS, the urgent nature of the case, and the availability of the direct thrombin inhibitor led the surgical team to select lepirudin for anticoagulation to facilitate cardiopulmonary bypass. After separation from cardiopulmonary bypass, the patient was in a coagulopathic state due to the inability to reverse the lepirudin and the slowed elimination of the drug secondary to inadequate renal function. As a result, the patient experienced excessive generalized oozing that was unresponsive to traditional therapies and blood product transfusions. Recombinant factor VIIa 35 microg/kg was given as rescue therapy. The bleeding slowed, which allowed placement of chest tubes and closing of the sternum. The patient was transferred to the intensive care unit in stable condition with no evidence of thrombosis in the freshly placed bypass grafts or on the bioprosthetic valve. Recombinant factor VIIa appears to be a suitable option as salvage therapy in patients with refractory bleeding secondary to anticoagulation with a direct thrombin inhibitor during cardiac surgery.

Antithrombotic action of annexin V proved as efficient as direct inhibition of tissue factor or thrombin

The role of phospholipid platelet membrane and tissue factor in thrombin generation and thrombus formation is accepted. In the present study we have explored antithrombotic action of strategies aimed to block exposure of negatively charged phospholipids and we compared effects with those obtained through tissue factor or a direct thrombin inhibition. Type III collagen was exposed to flowing blood (5 min, 300 s(-1)). Effects of inhibition of platelet deposition by annexin A5 (ANXA5), hirudin (HIR) or by an antibody against tissue factor (TF) were evaluated. Prothrombin fragment F1 + 2 (F1 + 2) was monitored. Pre-incubation of whole blood with HIR or ANXA5 resulted in a statistically significant reduction of platelet deposition (12.2 +/- 0.6% in control experiments vs. 8.3 +/- 0.4% and 8.5 +/- 0.5%, respectively, P < 0.05). A similar decrease was found when blood was incubated with an antibody against TF. Furthermore, ANXA5 and HIR inhibited the recruitment of platelets into forming aggregates. The height of platelet aggregates generated was decreased in the presence of HIR or ANXA5, but only incubation with both inhibitors reached levels of statistical significance. The presence of ANXA5 or HIR decreased levels of F1 + 2 suggesting a reduced activation of the coagulation system. In our experimental studies, the inhibitory potential of ANXA5 on platelet-thrombus formation was as effective as that of a direct thrombin inhibitor, as HIR, or an antibody against TF. Negatively charged phospholipids exposed on activated platelets potentiate the formation of platelet aggregates on a collagen surface and further suggest that inhibition of platelet procoagulant activity might be a specific target for antithrombotic drugs.

Novel chemo-enzymatic oligomers of cinnamic acids as direct and indirect inhibitors of coagulation proteinases

Thrombin and factor Xa, two important procoagulant enzymes, have been prime targets for regulation of clotting through the direct and indirect mechanism of inhibition. Our efforts on exploiting the indirect mechanism led us to study a carboxylic acid-based scaffold, which displayed major acceleration in the inhibition of these enzymes [J. Med. Chem.2005, 48, 1269, 5360]. This work advances the study to chemo-enzymatically prepared oligomers of 4-hydroxycinnamic acids, DHPs, which display interesting anticoagulant properties. Oligomers, ranging in size from tetramers to pentadecamers, were prepared through peroxidase-catalyzed oxidative coupling of caffeic, ferulic, and sinapic acids, and sulfated using triethylamine-sulfur trioxide complex. Chromatographic, spectroscopic, and elemental studies suggest that the DHPs are heterogeneous, polydisperse preparations composed of inter-monomer linkages similar to those found in natural lignins. Measurement of activated thromboplastin and prothrombin time indicates that both the sulfated and unsulfated derivatives of the DHPs display anticoagulant activity, which is dramatically higher than that of the reference polyacrylic acids. More interestingly, this activity approaches that of low-molecular-weight heparin with the sulfated derivative showing approximately 2- to 3-fold greater potency than the unsulfated parent. Studies on the inhibition of factor Xa and thrombin indicate that the oligomers exert their anticoagulant effect through both direct and indirect inhibition mechanisms. This dual inhibition property of 4-hydroxycinnamic acid-based DHP oligomers is the first example in inhibitors of coagulation. This work puts forward a novel, non-heparin structure, which may be exploited for the design of potent, dual action inhibitors of coagulation through combinatorial virtual screening on a library of DHP oligomers.

Acute thrombotic disorders

The acquired hypercoagulable states are responsible for a broad range of thrombotic and thromboembolic disorders. Symptoms and signs of acute ischemia or organ dysfunction will lead many of these patients to seek care in EDs. Proper diagnosis and therapy must be based on an understanding of epidemiology and pathophysiology. Immediate anticoagulation with heparin may not always be the treatment of choice; careful analysis of clinical and laboratory parameters is necessary to arrive at the safest and most effective course of action. Newer anticoagulants, including low-molecular-weight heparins and nonheparin compounds, are changing the therapeutic approach to many of these disorders.

Lepirudin Anticoagulation for Heparin-Induced Thrombocytopenia After Cardiac Surgery in a Pediatric Patient

A 21-month-old boy received anticoagulation with lepirudin for heparin-induced thrombocytopenia (HIT) after undergoing cardiac surgery and receiving extracorporeal membrane oxygenation (ECMO). This report illustrates the significance of HIT in pediatric patients after cardiac surgery and the successful administration of lepirudin in this setting. To our knowledge, this is the first published report of lepirudin administered to treat HIT in a child after cardiac surgery and ECMO. Although guidelines exist that suggest the potential administration of lepirudin as treatment for children with HIT, further studies are needed to determine the safest yet most effective dosage for this population.

New Anticoagulant Agents: Direct Thrombin Inhibitors

Decades of research have been devoted to developing effective, safe, and convenient anticoagulant agents. In recent years, much emphasis has been placed on the development of direct thrombin inhibitors (DTIs) that offer benefits over agents like heparin and warfarin including the inhibition of both circulating and clot-bound thrombin; a more predictable anticoagulant response, because they do not bind to plasma proteins and are not neutralized by platelet factor 4; lack of required cofactors, such as antithrombin or heparin cofactor II; inhibiting thrombin-induced platelet aggregation; and absence of induction of immune-mediated thrombocytopenia. Various injectable DTIs are currently available and used for many indications. In addition, research is now focusing on oral DTIs that seem promising and offer various advantages, such as oral administration, predictable pharmacokinetics and pharmacodynamics, a broad therapeutic window, no routine monitoring, no significant drug interactions, and fixed-dose administration.

Ximelagatran: An orally active direct thrombin inhibitor

PURPOSE

The chemistry, pharmacology, pharmacokinetics, clinical efficacy, dosage and administration, contraindications, and adverse effects of ximelagatran are reviewed.

SUMMARY

Ximelagatran is the first orally active direct thrombin inhibitor to be tested in Phase III clinical trials. After oral administration, ximelagatran is rapidly converted to its active metabolite, melagatran. Melagatran (after oral ximelagatran administration) predictably inhibits thrombin function without need for routine anticoagulation monitoring. Melagatran effectively inhibits both free and clot-bound thrombin-a potential pharmacodynamic advantage over heparin products. Melagatran has a half-life of 2.4-4.6 hours, necessitating twice-daily administration. Melagatran is primarily eliminated by the kidneys and has not been studied clinically in patients with severe renal failure. Ximelagatran has undergone 10 Phase III trials (6 for prophylaxis of venous thromboembolism [VTE] due to orthopedic surgery, 1 for initial treatment of VTE, 1 for long-term prevention of VTE recurrence, and 2 for stroke prophylaxis due to atrial fibrillation). Results were generally positive. AstraZeneca applied in December 2003 for marketing approval of ximelagatran for prevention of VTE after total knee replacement surgery, long-term prevention of VTE recurrence after standard therapy, and stroke prevention due to atrial fibrillation. FDA denied approval of ximelagatran for all indications, mainly because of increased rates of coronary artery disease events in ximelagatran recipients in some studies and the possibility of hepatic failure when the medication is used for long-term therapy.

CONCLUSION

Ximelagatran has shown promise as a possible alternative to warfarin and other anticoagulants but will require further study to ensure its safety.

New blood thinner offers first potential alternative in 50 years: ximelagatran

Traditional anticoagulants employed in the treatment of thrombosis include the injectable heparins and oral warfarin. Though effective, these traditional agents are fraught with limitations in their ease of use in the clinical setting. Warfarin, for example, has many pharmacokinetic properties and food-and-drug interactions that result in unpredictable patient response and the need for expensive and time-consuming monitoring of coagulation status. Ximelagatran is a novel, promising, orally active, direct thrombin inhibitor currently in development that, for the first time in 50 years, offers a potential alternative to the mainstay oral agent "warfarin." Advantages of ximelagatran over warfarin include predictable pharmacokinetics and pharmacodynamics, a broad therapeutic window, no routine anticoagulant monitoring, no clinically significant drug interactions, and fixed-dose administration. Ximelagatran has been evaluated for thromboprophylaxis following orthopedic surgery, acute treatment and secondary prevention of venous thrombosis, stroke prevention in atrial fibrillation, and acute coronary syndromes. Results of clinical trials suggest that ximelagatran is equally or more efficacious than warfarin and/or low-molecular-weight heparin therapy without increasing rates of minor or major bleeding. Although postmarketing surveillance will provide the final test of this drug, the future looks promising for addition of a new anticoagulant with the potential to provide excellent efficacy, predictable response, and reduced adverse effects. Pending regulatory approval, ximelagatran may help overcome barriers to appropriate anticoagulant therapy, thereby decreasing morbidity and mortality associated with thrombotic diseases.

The Direct Thrombin Inhibitors: Their Role and Use for Rational Anticoagulation

Major clinical advantages are achieved when direct thrombin inhibitors are used in venous thromboembolism. These agents provide more reliable anticoagulant response patterns because they are not significantly bound to plasma proteins and few, if any, drug-drug interactions are seen. The studies to date confirm that not all direct thrombin inhibitors are the same. The new reversible, short-acting catalytic site-specific drugs provide an excellent safety profile and high degree of efficacy for the prophylaxis and treatment of venous thromboembolism and pulmonary embolic states. The availability of the oral prodrug ximelagatran allows reproducible, effective, and safe direct thrombin inhibition without the requirement for coagulation laboratory monitoring; it appears destined to be the oral anticoagulant of the future.

Projecting future drug expenditures—2005

PURPOSE

Drug expenditure trends in 2003 and 2004 and projected drug expenditures for 2005 are discussed.

SUMMARY

Various factors are likely to affect drug costs, including drug prices, drugs in development, and generic drugs. In 2003 there was a continued moderation of the increase in drug expenditures. Drug expenditures increased by 11.4% from 2002 to 2003. Through the first nine months of 2004, expenditures increased by only 8.7% compared with 2003. This moderation can be attributed to many factors, particularly patent expirations, prescription-to-nonprescription conversions and a continued slowdown in new drug approvals. Higher cost sharing for consumers and continued weaknesses in several sectors of the U.S. economy affecting employment levels and insurance coverage also contributed to this smaller increase in drug utilization. It is expected that 2005 drug expenditure growth will out-pace the growth in overall health care expenditures and growth in the economy.

CONCLUSION

In 2005, there should be a 10-12% increase in drug expenditures in outpatient settings, a 12-15% increase in clinics, and a 6-9% increase in hospitals.

Evolving concepts in the treatment of venous thromboembolism: the role of factor Xa inhibitors

Anticoagulation is an essential component of the care of patients with venous thromboembolism (VTE). Traditional anticoagulants for the treatment of VTE include unfractionated heparin (UFH), low-molecular-weight heparin (LMWH), and the oral vitamin K antagonist, warfarin. A variety of anticoagulant agents with improved pharmacologic and clinical profiles are emerging and offer benefits over the traditional therapies. One of the most recent advances has been the development of new agents, such as oral direct thrombin inhibitors and factor Xa inhibitors, that have a more selective and targeted effect on the coagulation cascade. Recent clinical trials have evaluated fondaparinux, the first commercially available factor Xa inhibitor, in the treatment of patients with deep vein thrombosis and pulmonary embolism and indicate efficacy and safety as compared with traditional options such as UFH and LMWH. Fondaparinux is a welcomed addition to the available antithrombotic options.

Ximelagatran: Pharmacology, Pharmacokinetics, and Pharmacodynamics

Thromboembolism is the largest cause of morbidity and mortality in the western world, yet oral anticoagulation is currently available only with vitamin K antagonists--most often, warfarin. Warfarin has been used for treatment of thrombotic disease for about 50 years. However, despite its widespread use, it is associated with several limitations, such as varied patient response, a narrow therapeutic window, numerous drug and food interactions, and need for frequent therapeutic monitoring. In addition, its full anticoagulant effect usually takes at least 4-5 days after the start of therapy or any dosage change, and it has a slow offset of therapy. A new oral anticoagulant, ximelagatran, has considerable advantages compared with warfarin. The agent requires no therapeutic monitoring, has a wide therapeutic window, and is not known to interact with food or drugs. The advantages ximelagatran brings to clinical practice should be a welcome addition to the options for management of thrombotic disease.

Transitioning from Traditional to Novel Anticoagulants: The Impact of Oral Direct Thrombin Inhibitors on Anticoagulation Management

Limitations of traditional anticoagulants have led to the need for structured services to provide anticoagulation monitoring. Anticoagulation management services have historically filled this role. Novel antithrombotic agents currently in development have the potential to improve on the limitations of vitamin K antagonists and injectable forms of heparin and to change the field of anticoagulation management. Of the emerging agents, oral direct thrombin inhibitors present the most promise and have a more practical clinical profile. The introduction of the oral direct thrombin inhibitors may present both an opportunity and a threat to anticoagulation management services and anticoagulation service providers. Traditional anticoagulation monitoring services will have to retool for the future if they are to remain relevant in the new era of novel antithrombotic.agents.

Ximelagatran: an oral direct thrombin inhibitor

OBJECTIVE

To present the chemistry, pharmacology, and pharmacokinetics of ximelagatran, an oral direct thrombin inhibitor (DTI), and to review available comparative clinical trial data evaluating its efficacy and safety relative to other antithrombotic agents in the prevention and treatment of thromboembolism.

DATA SOURCES

A search of the PubMed and Cochrane databases (1995-August 2004), supplemented by a manual search of article bibliographies, conference abstracts, and data on file from the manufacturer, was conducted. Key search terms were ximelagatran, melagatran, H376/95, and direct thrombin inhibitors.

STUDY SELECTION AND DATA EXTRACTION

Pertinent information from available clinical trials, including study design, patient demographics, dosing regimens, anticoagulant comparators, methods for evaluating effectiveness, treatment outcomes, adverse events, and pharmacokinetic and pharmacodynamic evaluations, was extracted.

DATA SYNTHESIS

Ximelagatran is an orally administered DTI under development for use in the treatment of venous thromboembolism (VTE), long-term prevention of a second VTE event, stroke secondary to atrial fibrillation, prevention of VTE after orthopedic procedures, and recurrent ischemic events after acute myocardial infarction.

CONCLUSIONS

Ximelagatran, in twice-daily doses of 24 or 36 mg, is an alternative to low-molecular-weight heparins or warfarin in thromboprophylaxis following orthopedic knee replacement, atrial fibrillation, or initial treatment of VTE. Improved outcomes versus placebo were seen in the long-term prevention of VTE in patients who completed an initial 6 months of treatment. Liver-related effects need further clarification.

Impact of Oral Direct Thrombin Inhibitors on Anticoagulation Clinics

OBJECTIVES

To characterize the services and activities performed by anticoagulation clinics (ACCs) across the United States, examine the anticipated impact that oral direct thrombin inhibitors (DTIs) will have on the clinical services of ACCs, and elicit possible management strategies to realign services provided by ACCs and opportunities for restructuring clinical services.

METHODS

A survey was developed in consultation with content experts in the field, pretested by several ACC providers, and subsequently refined. Surveys were mailed to 400 randomly selected ACC-based providers.

RESULTS

The final usable response rate was 34.5% (115 of 333 surveys). Respondents anticipated that the number of patient visits/month would decrease from a median of 336 (interquartile range [IQR] 151-775) to 150 (IQR 71-350, p<0.001) after the introduction of oral DTIs. In addition, time spent on many direct and indirect patient care activities currently performed by ACCs was expected to decline. Respondents indicated that ACCs may find new roles by providing guidance on individual suitability for therapy, managing the transition to oral DTIs, providing education for patients and health care providers on thrombotic disease state management, monitoring patients for recurrent thrombosis and risk of bleeding complications, monitoring patient compliance, and providing counseling and safety surveillance for patients receiving treatment with oral DTIs.

CONCLUSION

The advent of oral DTIs is likely to have a significant impact on the structure and delivery of antithrombotic services. Clinics that are proactive and redesign their patient care services to consider emerging anticoagulant agents will be more likely to remain relevant and viable.