Prevention of Venous Thromboembolism Following Orthopaedic Surgery

Fibrin polymerization simulation using a reactive dissipative particle dynamics method

The study on the polymerization of fibrinogen molecules into fibrin monomers and eventually a stable, mechanically robust fibrin clot is a persistent and enduring topic in the field of thrombosis and hemostasis. Despite many research advances in fibrin polymerization, the change in the structure of fibrin clots and its influence on the formation of a fibrous protein network are still poorly understood. In this paper, we develop a new computational method to simulate fibrin clot polymerization using dissipative particle dynamics simulations. With an effective combination of reactive molecular dynamics formularies and many body dissipative particle dynamics principles, we constructed the reactive dissipative particle dynamics (RDPD) model to predict the complex network formation of fibrin clots and branching of the fibrin network. The 340 kDa fibrinogen molecule is converted into a spring-bead coarse-grain system with 11 beads using a topology representing network algorithm, and using RDPD, we simulated polymerization and formation of the fibrin clot. The final polymerized structure of the fibrin clot qualitatively agrees with experimental results from the literature, and to the best of our knowledge this is the first molecular-based study that simulates polymerization and structure of fibrin clots.

Thrombin inhibition and cyclophosphamide synergistically block tumor progression and metastasis

Cancer is often associated with an increased risk of thrombotic events which are exacerbated by treatment with chemotherapeutics such as cyclosphosphamide (CP). Evidence suggests that thrombin can stimulate tumor progression via formation of fibrin and activation of protease-activated receptors (PARs) and platelets. We examined the effect of co-treatment with CP and dabigatran etexilate, a direct inhibitor of thrombin, using the murine orthotopic 4T1 tumor model. Mice receiving co-treatment with both low dose CP and dabigatran etexilate had significantly smaller mammary tumors and fewer lung metastases than mice treated with CP or dabigratran etexilate alone. Co-treatment with dabigatran etexilate and low dose CP also significantly decreased the number of arginase(+)Gr-1(+)CD11b(+) myeloid derived suppressor cells as well as levels of TGF-β in spleens from tumor bearing mice. 4T1 tumors express procoagulant tissue factor (TF) and spontaneously release TF(+) microparticles which are potent procoagulant factors that promote thrombin generation. Treatment with dabigatran etexilate alone prevented tumor-induced increases in circulating TF(+) microparticles and also decreased the numbers of tumor-induced activated platelets by 40%. These results show that co-treatment with dabigatran etexilate and CP synergistically inhibits growth and metastasis of mammary tumors, suggesting that oral administration of the thrombin inhibitor dabigatran etexilate may be beneficial in not only preventing thrombotic events in cancer patients but also in treating malignant tumors themselves.

The use of novel oral anticoagulants for thromboprophylaxis after elective major orthopedic surgery

Venous thromboembolism is a common cause of morbidity and mortality among patients undergoing elective orthopedic surgery. Due to the high incidence of venous thromboembolism in this setting, perioperative anticoagulation is the recommended approach for thromboprophylaxis. Low molecular weight heparin (LMWH), fondaparinux and warfarin are the agents commonly used for thromboprophylaxis. The well-recognized limitations of warfarin and the inconvenience and discomfort associated with the subcutaneous administration of low molecular weight heparin and fondaparinux inspired intense investigation to develop novel oral anticoagulants (NOACs) with more predictable pharmacokinetics, fewer drug interactions and no need for regular laboratory monitoring. Three NOACs have been demonstrated to be effective for thromboprophylaxis after total hip arthroplasty (THA) and total knee arthroplasty (TKA) in large randomized controlled trials. Here we review the pharmacology of rivaroxaban, dabigatran, and apixaban, summarize the major clinical trials of these agents in thromboprophylaxis after THA and TKA, and discuss the clinical factors to be considered by providers when selecting a NOAC for their patients.

Dabigatran etexilate: a review of its use for the prevention of venous thromboembolism after total hip or knee replacement surgery

Dabigatran etexilate (Pradaxa®, Pradax™, Prazaxa®) is indicated for the primary prevention of venous thromboembolic (VTE) events in adults who have undergone elective total hip replacement (THR) or total knee replacement (TKR) surgery. This article reviews the clinical efficacy and tolerability of oral dabigatran etexilate in patients undergoing major orthopaedic surgery, as well as summarizing its pharmacological properties and results of a cost-utility analysis. The discussion of clinical trial data focuses on comparative trials with the EU approved dosage regimen of once-daily subcutaneous enoxaparin sodium 40 mg. Dabigatran etexilate is an oral prodrug of the potent, rapidly acting, reversible, competitive inhibitor of thrombin, dabigatran. Dabigatran has predictable and consistent anticoagulant effects and does not require routine coagulation monitoring or dose titration. In the large, randomized, double-blind, phase III, noninferiority trials, RE-MODEL, RE-NOVATE and RE-NOVATE II, oral dabigatran etexilate, at dosages of 150 and 220 mg once daily, initiated postoperatively was shown to be noninferior to subcutaneous enoxaparin sodium 40 mg once daily (initiated prior to surgery) with regard to the incidence of the composite of total VTE events and all-cause mortality in patients undergoing TKR or THR surgery. In general, oral dabigatran etexilate was well tolerated in clinical trials of patients undergoing major orthopaedic surgery. There were no significant between-group differences in bleeding complications, including major bleeding, clinically relevant non-major bleeding or minor bleeding between the dabigatran etexilate or enoxaparin sodium groups. In addition, a cost-utility analysis from the perspective of the UK National Health Service indicated that dabigatran etexilate 220 mg once daily is dominant (i.e. more effective and less costly) to enoxaparin sodium 40 mg once daily in patients undergoing THR or TKR. Additional head-to-head comparisons would be beneficial to more definitively position dabigatran etexilate relative to other available oral treatment options for VTE prophylaxis, such as rivaroxaban and apixaban. In the meantime, dabigatran etexilate offers a convenient, effective and generally well tolerated treatment option for the prophylaxis of VTE in patients undergoing TKR and THR surgery.

Incidence and management of ischemic stroke and intracerebral hemorrhage in patients on dabigatran etexilate treatment

Dabigatran etexilate is an oral, reversible direct thrombin inhibitor and has been recently approved for the prevention of stroke in patients with non-valvular atrial fibrillation. This review describes the incidence and management of stroke and related complications in patients on dabigatran etexilate. Dabigatran is a rapidly acting, and highly selective and reversible inhibitor of thrombin. It also has a potent inhibitory effect on thrombin-induced platelet aggregation, making it effective in preventing both venous and arterial thrombosis. The activated partial thromboplastin time, ecarin clotting time and thrombin time are sensitive tests to evaluate the anticoagulant effects of dabigatran. The rate of ischemic stroke is significantly lower in patients on 150 mg of dabigatran etexilate as compared to 110-mg dose or warfarin (9.2, 13.4, 12 per 1,000 patients, respectively). As there is no standard coagulation test for dabigatran; treatment of acute stroke in such patients is debatable. Careful clinical consideration is required before administering thrombolytic therapy in this patient population. The rate of hemorrhagic stroke was 1.2 and 1.0 per 1,000 patients treated on 110 and 150 mg of dabigatran, respectively. As there is no specific antidote, the only treatment option is discontinuation of the drug and supportive management. Other treatment options, though not clinically proven, include specific reversal agents, which can be individualized according to the severity of the hemorrhage. Dabigatran should be discontinued before invasive procedures depending on the degree of renal impairment and risk of bleeding.

Looking at the proteases from a simple perspective

Proteases have received enormous interest from the research and medical communities because of their significant roles in several human diseases. Some examples include the involvement of thrombin in thrombosis, HIV-1 protease in Acquired Immune Deficiency Syndrome, cruzain in Trypanosoma cruzi infection, and membrane-type 1 matrix metalloproteinase in tumor invasion and metastasis. Many efforts has been undertaken to design effective inhibitors featuring potent inhibitory activity, specificity, and metabolic stability to those proteases involved in such pathologies. Protease inhibitors usually target the active site, but some of them act by other inhibitory mechanisms. The understanding of the structure-function relationships of proteases and inhibitors has an impact on new inhibitor drugs designing. In this paper, the structures of four proteases (thrombin, HIV-protease, cruzain, and a matrix metalloproteinase) are briefly reviewed, and used as examples of the importance of proteases for the development of new treatment strategies, leading to a longer and healthier life.

Review article: thromboprophylaxis after total hip replacement

The risk of secondary haematologic complications such as deep vein thrombosis or pulmonary embolism increases significantly when a transient period of hypercoagulability is induced after total hip replacement (THR). A number of drug-based anticoagulant approaches are available to modulate this risk, but the optimal length of therapy for such approaches remains unclear. The literature was reviewed for evidence-based support of the routine use of an extended course (>14 days) of thromboprophylaxis after THR. Electronic databases and real-time online literature searches were performed, using the PubMed (Medline), EMBASE, CINAHL and GoogleScholar, and the Cochrane and British Medical Journal Clinical Evidence libraries. Author-defined key word searches were performed. Only articles in the English language, for which full text could be retrieved, were reviewed. There is robust evidence to support an extended course (>14 days) of thromboprophylaxis after THR. Such recommendations have been translated into the guidelines of key professional bodies, including those of the American College of Chest Physicians. Meta-review suggests a clear benefit of such regimens and supports wider adoption, even when weighed against a small increase in adverse events.

Oral Anticoagulants in Development

Current anticoagulant provision is dominated by parenteral heparin and oral warfarin, which act by inhibiting several steps of the coagulation pathway indirectly. Recent research efforts have focused on the identification of small molecule inhibitors of the coagulation enzymes as novel therapies for thrombotic disorders. There has been particular success in developing nonpeptidic, orally available, small molecules to directly inhibit the key proteases, factor IIa and factor Xa. Of the new oral anticoagulants in development, the two agents in the most advanced stage are dabigatran etexilate (BIBR 1048) and rivaroxaban (BAY 59-7939), which inhibit factor IIa and factor Xa, respectively. Other agents in the early stages of development include several Xa inhibitors (LY-517717, YM150, DU-176b and apixaban [BMS-562247]), a factor IXa inhibitor (TTP889), and an orally active glycosaminoglycan enhancer (odiparcil [SB-424323]), which indirectly enhances thrombin inhibition via heparin cofactor II. Results have been reported from important, phase II dose-finding studies, and a number of registration-track phase III studies have been initiated, reflecting the drive towards potentially more effective, but primarily safer and more convenient therapies for the prevention and treatment of venous and arterial thrombosis. Indeed, two unmet needs for anticoagulation that can be easily identified are safety and ease of use. Safety relates primarily to the incidence of major bleeding and this remains the key concern of orthopaedic surgeons, over and above any efficacy advantage, and convenience of use, which centres on oral administration replacing the need for injections. The clinical development of these new anticoagulants is following the well tested strategy of dose-ranging and registration studies in major orthopaedic surgery, prior to development in arterial indications. There are a number of subtle issues, including the timing of the first perioperative dose, duration of prophylactic treatment and definition/assessment of study endpoints that can influence study outcome and require careful consideration when evaluating study results with new agents and in the comparison with established agents, and which are considered in this review. It is anticipated that over the next 3 years, at least one of these agents will be successfully licensed for the prevention of venous thromboembolism after major orthopaedic surgery, which will act as a springboard for the gradual replacement of current anticoagulants.

Assessment of the Risk and the Prophylaxis of Venous Thromboembolism in Surgical Patients

This review article first deals with risk factors for developing postoperative venous thromboembolism. The various pharmacological prophylactic methods available today are summarized with focus on dominating method low molecular weight heparin and the two new ones, the Xa inhibitor fondaparinux and the IIa inhibitor ximelagatran, both approved in orthopaedic surgery. The optimal timing to start prophylaxis is between 2 hours before and 10 hours after surgery. Evidence is accumulating that some in risk groups prolonged prophylaxis is indicated, ie. elective hip surgery, probably hip fracture surgery and probably also patients operated on for abdominal/pelvic cancer.

Peptidomimetic Thrombin Inhibitors

The central position of thrombin in the coagulation cascade has made it a popular target for discovery of novel antithrombotic agents. Starting with hirudin, a natural peptide isolated from the medicinal leech, its shorter synthetic analogue hirulog, and argatroban,the first therapeutically used synthetic small-molecule thrombin active site inhibitor, hundreds of direct thrombin inhibitors have been discovered over the last 20 years. Most of them are peptidomimetic compounds,based on the amino acid sequence of fibrinogen which binds into the thrombin active site. Since elucidation of the crystal structure of human thrombin in 1989, the structure-based design of low-molecular-weight peptidomimetic thrombin inhibitors has been greatly aided by the use of x-ray crystallographic analysis of thrombin-inhibitor complexes. The ultimate goal of most research programmes and drug optimization strategies is to develop an orally bioavailable, small-molecule,direct thrombin inhibitor that would be suitable for once or twice daily dosing. An overview of the most advanced peptidomimetic direct thrombin inhibitors bivalirudin, argatroban, ximelagatran and dabigatran is presented.

Ximelagatran in Orthopaedic Surgery

Ximelagatran represents the first new oral anticoagulant since the introduction of warfarin almost 60 years ago, and has been evaluated for the treatment and prevention of a range of venous and arterial thromboembolic disorders. The MElagatran THRomboprophylaxis in Orthopaedic surgery (METHRO) and EXpanded PRophylaxis Evaluation Surgery Study (EXPRESS) studies have investigated the efficacy and safety of subcutaneous (s.c.) melagatran followed by oral ximelagatran in preventing venous thromboembolism (VTE) in patients undergoing total hip replacement or total knee replacement. In METHRO II, immediate pre-operative-initiated s.c. melagatran followed by post-operative ximelagatran dose-dependently reduced VTE, with the highest dose (melagatran 3 mg/ximelagatran 24 mg twice daily) associated with a significantly reduced incidence of VTE compared with the low-molecular-weight heparin (LMWH) dalteparin (15.1 vs. 28.2%; p < 0.0001). In METHRO III, the efficacy of s.c. melagatran 3 mg/ximelagatran 24 mg twice daily initiated post-operatively (4-12 h after surgery) was comparable to that of the LMWH enoxaparin initiated 12 h before surgery (total VTE incidence, 31.0 and 27.3%, respectively). Rates of severe bleeding were also comparable between treatments (melagatran/ximelagatran = 1.4%; enoxaparin = 1.7%). Treatment with melagatran/ximelagatran was significantly more effective when initiated earlier (4-8 h) rather than later (8-12 h) after surgery (total VTE incidence, 27.5 vs. 35.4%; p = 0.0034). Based on the results of METHRO II and III, the EXPRESS study evaluated the efficacy and bleeding profile of s.c. melagatran 2 mg immediately before surgery, followed by s.c. melagatran 3 mg on the evening of the day of surgery and then ximelagatran 24 mg twice daily. This regimen was significantly more effective than enoxaparin (total VTE incidence, 20.3 vs. 26.6%; p < 0.0004). Excessive bleeding (as judged by the investigator) was more frequent with melagatran/ximelagatran, but rates of fatal bleeding, critical-site bleeding and bleeding requiring re-operation did not differ between the groups. Taken together, the METHRO and EXPRESS studies demonstrate that melagatran/ximelagatran has comparable or superior efficacy to LMWHs in the prevention of VTE in orthopaedic surgery patients, and that the timing and dose of melagatran is important in optimizing the balance of efficacy and bleeding risk.

New approaches to anticoagulation in atrial fibrillation

Oral direct thrombin inhibitors (DTIs) are a potential alternative to vitamin K antagonists, such as warfarin, for anticoagulant therapy. The oral DTI at the most advanced stage of clinical development is ximelagatran, which is rapidly absorbed and bioconverted to the active form melagatran. Oral ximelagatran has been evaluated in randomized, controlled trials for several indications, including stroke prevention in atrial fibrillation (AF). Recently, two pivotal phase III trials demonstrated that fixed-dose oral ximelagatran, 36 mg twice daily without coagulation monitoring, prevents stroke and systemic embolic events in patients with nonvalvular AF as effectively as well-controlled, adjusted-dose warfarin. Oral ximelagatran was generally well tolerated and caused less total (major plus minor) bleeding than warfarin. In a minority of ximelagatran-treated patients, elevated serum alanine aminotransferase levels were reported, but were typically not associated with specific symptoms, and returned toward the pretreatment baseline whether treatment was continued or discontinued. In AF, oral ximelagatran promises a better benefit to risk ratio than warfarin.