Comparative Pharmacodynamics and Pharmacokinetics of Oral Direct Thrombin and Factor Xa Inhibitors in Development

New anticoagulants and regional anesthesia

PURPOSE OF REVIEW

The use of pharmacological thromboprophylaxis in the perioperative period may conflict with regional anesthetic techniques in which maintaining hemostatic integrity is essential. Recently, new anticoagulants have been developed with more efficacy and a better safety profile. This article reviews the basis for the actual recommendations and the current status and management of these new drugs.

RECENT FINDINGS

Recent studies have outlined that the risk of epidural hematoma after neuraxial anesthesia may be higher than estimated. Therefore, it is imperative to follow the published recommendations. The use of new anticoagulant drugs may take into account the pharmacological profile of each one to safely perform regional anesthesia, mainly the time to reach peak plasma level and half-life.

SUMMARY

When new anticoagulant drugs are used for thromboprophylaxis in orthopedic surgery, the performance of neuraxial anesthetic techniques should be based on their pharmacology. If a peripheral blockade is chosen, these recommendations should be followed when a block is performed in a noncompressible area.

Phenyltriazolinones as potent factor Xa inhibitors

We have discovered that phenyltriazolinone is a novel and potent P1 moiety for coagulation factor Xa. X-ray structures of the inhibitors with a phenyltriazolinone in the P1 position revealed that the side chain of Asp189 has reoriented resulting in a novel S1 binding pocket which is larger in size to accommodate the phenyltriazolinone P1 substrate.

New pharmacotherapy for stroke prevention in atrial fibrillation: update 2010

Atrial fibrillation (AF) is the most commonly encountered arrhythmia in clinical practice and is associated with substantial morbidity and mortality. Its prevalence increases with age, affecting about 1% of patients aged <60 years and almost 10% of patients >80 years. AF is associated with a fivefold increasing risk of embolism or stroke with absolute risk ranging from less than 1% to 20% per year, depending on patient age and the presence of clinical risk factors including congestive heart failure, systemic hypertension, diabetes mellitus and prior history of cardioembolic events. Vitamin K antagonists (VKAs) and acetyl salicylic acid are currently the only licensed antithrombotic therapies for stroke prevention in patients with AF. Anticoagulants are very effective for stroke prevention in patients with AF, overall a 64% relative risk reduction. Nonetheless, approximately 50% of patients with AF who have an indication for VKA receive anticoagulant therapy, of which only 50% maintain adequate therapeutic ranges. Furthermore, 50% will discontinue VKAs within 3 to 5 years regardless of appropriate international normalized ratio control. Underutilization of VKAs is related, in part, to their numerous limitations and difficulty in maintaining adequate therapeutic control, prompting the development of new antithrombotic strategies that are equally effective and safer, and easier to manage than VKAs. This review focuses on new antithrombotic therapies for stroke prevention in patients with AF.

New direct thrombin inhibitors

Direct thrombin inhibitors (DTIs) are a class of anticoagulants that bind selectively to thrombin and block its interaction with its substrates. Dabigatran etexilate and AZD0837, the new generation of DTIs, are now under intense development, and are potentially of great interest for internists. Dabigatran etexilate is a potent, non-peptidic small molecule that specifically and reversibly inhibits both free and clot-bound thrombin by binding to the active site of thrombin molecule. It has been already licensed in the European Union and in Canada for the prevention of VTE in patients undergoing hip- and knee-replacement surgery. Ongoing trials are evaluating its efficacy and safety for the treatment of deep venous thrombosis and pulmonary embolism, primary and secondary prevention of VTE, prevention of systemic embolism in patients with non-valvular atrial fibrillation, and prevention of cardiac events in patients with acute coronary syndromes. AZD0837 is the prodrug of ARH06737, a potent, competitive, reversible inhibitor of free and fibrin-bound thrombin. At present, only limited, preclinical, phase I and phase II clinical data have been presented. The drug has now entered a phase III clinical program in the population of patients with atrial fibrillation. Their properties and the oral administration render these compounds, theoretically, more convenient than both vitamin K antagonist and low molecular weight heparins. However, only reports from clinical practice patterns over the next months and years will tell us how and when to use the new DTIs.

How much does international normalized ratio monitoring cost during oral anticoagulation with a vitamin K antagonist? A systematic review

Next-generation oral anticoagulants offer the potential for effective prevention and treatment of thrombosis without the need for repeated monitoring of the international normalized ratio (INR). This systematic review evaluated the costs associated with INR monitoring tests performed as part of the standard management of oral anticoagulation with vitamin K antagonists. Studies published in or after 1990 reporting the costs of INR monitoring were identified from bibliographic databases and manual searches of reference lists. Cost data were extracted and inflated to the year 2006 before purchasing power parity conversion to US dollars. A total of 29 studies reported the cost of one INR test, which was shown to range from $6.19 to $145.70. Cost estimates were based on various combinations of direct medical costs, such as healthcare contacts, equipment, laboratory tests, clerical costs (postage and stationery), telephone calls, quality control, training/education and patient transportation, and indirect costs, such as time lost from work. In conclusion, the cost of INR monitoring varied substantially between studies depending on the monitoring modality and setting, and the cost categories included. When selecting a published estimate, healthcare decision makers should ensure that the chosen estimate reflects local service provision as closely as possible.

Structure and property based design of factor Xa inhibitors: pyrrolidin-2-ones with monoaryl P4 motifs

Structure and property based drug design was exploited in the synthesis of sulfonamidopyrrolidin-2-one-based factor Xa inhibitors, incorporating neutral and basic monoaryl P4 groups, ultimately producing potent inhibitors with effective levels of anticoagulant activity and extended oral pharmacokinetic profiles. However, time dependant inhibition of Cytochrome P450 3A4 was a particular issue with this series.

The new oral anticoagulants

Although their first application in clinical practice occurred in the 1940s, vitamin K antagonists remain the only form of oral anticoagulant medication approved for long-term use. Although the available vitamin K antagonists are highly effective for the prevention and/or treatment of most thrombotic disease, the significant interpatient and intrapatient variability in dose-response, the narrow therapeutic index, and the numerous drug and dietary interactions associated with these agents have led clinicians, patients, and investigators to search for alternative agents. Three new orally administered anticoagulants (apixaban, dabigatran, and rivaroxaban) are in the late stages of development and several others are just entering (or moving through) earlier phases of investigation. These novel anticoagulant medications are being studied for the prevention and treatment of venous thromboembolism, the treatment of acute coronary syndromes and the prevention of stroke in patients with atrial fibrillation. This review summarizes published clinical trial data pertinent to apixaban, dabigatran, and rivaroxaban.

Favorable therapeutic index of the direct factor Xa inhibitors, apixaban and rivaroxaban, compared with the thrombin inhibitor dabigatran in rabbits

BACKGROUND

Apixaban is an oral, direct factor Xa (FXa) inhibitor in late-stage clinical development. This study assessed effects of the direct FXa inhibitors, apixaban and rivaroxaban, vs. the direct thrombin inhibitor, dabigatran, on venous thrombosis (VT), bleeding time (BT) and clotting times in rabbits.

METHODS

We induced the formation of non-occlusive thrombus in VT models by placing threads in the vena cava, and induced bleeding by the incision of cuticles in anesthetized rabbits. Apixaban, rivaroxaban and dabigatran were infused IV to achieve a stable plasma level. Clotting times, including the activated partial thromboplastin time (aPTT), prothrombin time (PT), modified PT (mPT) and thrombin time (TT), were measured.

RESULTS

Apixaban, rivaroxaban and dabigatran exhibited dose-related efficacy in preventing VT with EC(50) of 65, 33 and 194 nm, respectively. At doses for 80% reduction of control thrombus, apixaban, rivaroxaban and dabigatran prolonged BT by 1.13 +/- 0.02-, 1.9 +/- 0.1-* and 4.4 +/- 0.4-fold*, respectively (*P < 0.05, vs. apixaban). In the treatment model, these inhibitors equally prevented growth of a preformed thrombus. Antithrombotic doses of apixaban and rivaroxaban prolonged aPTT and PT by <3-fold with no effect on TT. Dabigatran was > or = 50-fold more potent in prolonging TT than aPTT and PT. Of the clotting assays studied, apixaban, rivaroxaban and dabigatran responded the best to mPT.

CONCLUSION

Comparable antithrombotic efficacy was observed between apixaban, rivaroxaban and dabigatran in the prevention and treatment of VT in rabbits. Apixaban and rivaroxaban exhibited lower BT compared with dabigatran at equivalent antithrombotic doses. The clinical significance of these findings remains to be determined.

New antithrombotic drugs

Thrombosis, both venous and arterial, is a major cause of morbidity and mortality worldwide. Consequently, there is an ongoing search for new antithrombotic drugs, particularly novel antiplatelet agents and anticoagulants. A better understanding of the biochemical pathways involved in platelet activation and coagulation and of the links between these systems and the impact of thrombosis on inflammation has led to the identification of new targets for antithrombotic drugs. This paper focuses on these new targets and new antiplatelet drugs and anticoagulants and describes the major advances in the continuing search for more potent antithrombotic drugs that have limited effects on hemostasis.

Prevention of venous thromboembolism: consensus, controversies, and challenges

The last 50 years have witnessed a multitude of publications evaluating the efficacy, safety and cost effectiveness of many different thromboprophylaxis interventions. There is widespread consensus that thromboprophylaxis safely reduces morbidity and mortality. More than 25 evidence-based guidelines, published since 1986, also recommend routine thromboprophylaxis in the majority of hospitalized patients. As a result, thromboprophylaxis is recognized as a key safety priority for hospitals. Some of the remaining areas of controversy that will be discussed in this paper include the role of individual risk assessments to determine thrombosis risk and prophylaxis, replacement of low-dose heparin by low-molecular-weight heparin (LMWH), the optimal duration of prophylaxis, the role of combined thromboprophylaxis modalities, the safety of anticoagulant prophylaxis with regional analgesia, the use of LMWHs in chronic renal insufficiency, and the emerging role of new oral anticoagulants as thromboprophylactic agents. Despite the overwhelming evidence supporting thromboprophylaxis, rates of thromboprophylaxis use remain far from optimal. Successful implementation strategies to bridge this knowledge:care gap are the most important current challenges in this area. These strategies must be multifaceted, utilizing local, systems-based approaches as well as legislation and incentives that reinforce best practices.