The antithrombotic effects of CI-1028, an orally bioavailable direct thrombin inhibitor, in a canine model of venous and arterial thrombosis

Antithrombotic therapy in atrial fibrillation: ximelagatran, an oral direct thrombin inhibitor

The oral direct thrombin inhibitor ximelagatran (Exanta, AstraZeneca) is rapidly absorbed, is efficiently bioconverted to the active form, melagatran (AstraZeneca) and has shown efficacy and relative safety as an anticoagulant for prophylaxis and therapy of thromboembolism. Two Phase III trials, Stroke Prevention using an ORal Thrombin Inhibitor in atrial Fibrillation (SPORTIF V), have tested the hypothesis that oral ximelagatran, administered 36 mg twice daily without coagulation monitoring or dose adjustment, prevents stroke and systemic embolism at least as effectively as adjusted-dose warfarin (international normalized ratio, 2.0-3.0) in patients with nonvalvular atrial fibrillation. Both were randomized, multicenter trials (n > 3000 per trial) with blinded end-point assessment. The open-label SPORTIF III trial confirmed the noninferiority of ximelagatran versus warfarin. Publication of the full results from SPORTIF V is pending.

Adaptation of the Folts and electrolytic methods of arterial thrombosis for the study of anti-thrombotic molecules in small animals

INTRODUCTION

Animal preparations of arterial thrombosis play a crucial role in the discovery and validation of novel drug targets in vivo, aiding in the selection of new drugs for clinical evaluation. The Folts and electrolytic methods of arterial thrombosis are two of the most commonly used techniques to investigate drugs with novel anti-thrombotic potential. However, these techniques often involved the use of large animals such as dogs, and their application to small animals was limited. The aim of the present study was to adapt the Folts and electrolytic methods previously described in large animals to create highly reproducible, quantitative models of arterial thrombosis in mice, rats and rabbits.

METHODS

Carotid artery blood flow was measured in anaesthetised mice, rats and rabbits. In the Folts-like method, a silk suture was tied around one carotid artery distal to a flow probe and tightened to cause a concentric stenosis sufficient to decrease blood flow by 50%. Intimal damage was induced by pinching the artery at the site of stenosis using forceps. The sequential formation and mechanical dislodgement of the resultant platelet-rich occlusive thrombus caused cyclic carotid artery flow reductions. In the electrolytic method in mice, a platinum hook electrode was placed distal to a flow probe on one carotid artery. The artery was clamped distally to the electrode to cause stasis and an electrical current (4 mA for 1.25 min) was applied before clamp release. This induced vascular injury resulting in occlusive thrombus (platelet- and fibrin-rich) formation.

CONCLUSION

The Folts-like method of arterial thrombosis was successfully adapted for use in mice, rats and rabbits, and the electrolytic technique for use in mice. Compared with larger animals, these methods are highly reproducible and ideal for pre-clinical, cost-effective, low-cost routine screening of novel anti-thrombotic drugs.

Efficacious and orally bioavailable thrombin inhibitors based on a 2,5-thienylamidine at the P1 position: discovery of N-carboxymethyl-d-diphenylalanyl-l-prolyl[(5-amidino-2-thienyl)methyl]amide

Thrombin, a crucial enzyme in the blood coagulation, has been a target for antithrombotic therapy. Orally active thrombin inhibitors would provide effective and safe prophylaxis for venous and arterial thrombosis. We conducted optimization of a highly efficacious benzamidine-based thrombin inhibitor LB30812 (3, K(i) = 3 pM) to improve oral bioavailability. Of a variety of arylamidines investigated at the P1 position, 2,5-thienylamidine effectively replaced the benzamidine without compromising the thrombin inhibitory potency and oral absorption. The sulfamide and sulfonamide derivatization at the N-terminal position in general afforded highly potent thrombin inhibitors but with moderate oral absorption, while the well-absorbable N-carbamate derivatives exhibited limited metabolic stability in S9 fractions. The present work culminated in the discovery of the N-carboxymethyl- and 2,5-thienylamidine-containing compound 22 that exhibits the most favorable profiles of anticoagulant and antithrombotic activities as well as oral bioavilability (K(i) = 15 pM; F = 43%, 42%, and 15% in rats, dogs, and monkeys, respectively). This compound on a gravimetric basis was shown to be more effective than a low molecular weight heparin, enoxaparin, in the venous thrombosis models of rat and rabbit. Compound 22 (LB30870) was therefore selected for further preclinical and clinical development.

New antithrombotic drugs on the horizon

Venous and arterial thromboembolism are a major cause for morbidity and mortality. The list of established drugs for the prevention of thrombus formation and embolisation includes heparins, hirudin and derivatives, aspirin, ADP and glycoprotein IIb/IIIa receptor antagonists, as well as vitamin K antagonists. Several limitations exist for these drugs that have stimulated the search for new and better anticoagulants. A series of selective clotting factor Xa inhibitors and direct factor IIa (thrombin) inhibitors are on the horizon, two of which are getting close to broad clinical application. Additional therapeutics that are still under preclinical and clinical investigation include inhibitors of the tissue factor pathway/factor VII complex, clotting factor VIII and XIII inhibitors and modulators of the protein C pathway or of endogenous fibrinolysis, as well as novel antiplatelet drugs. This review is focused on the current status of development of novel antithrombotics and their clinical potential. Even though only a few of a broad array of antithrombotic agents have reached clinical testing, some hold the potential for significant improvement in efficacy and safety of anticoagulant therapy.

Are coagulation times biomarkers? Data from a phase I study of the oral thrombin inhibitor LB-30057 (CI-1028)

Ecarin clotting time and activated partial thromboplastin time are coagulation tests that meet the definition of a biomarker. Prolongation of these coagulation times closely correlated with blood concentrations of the oral thrombin inhibitor LB-30057 (CI-1028) during a phase 1 study. But this simply reflects their functioning as enzyme inhibition assays of drug concentration. Directly adding the drug to blood results in the same concentration-response relationship. Changes in coagulation tests only demonstrate that ex vivo clot formation has been altered, not that an in vivo process has been affected. To be most informative in drug development, biomarker assays should measure in vivo drug effects, not drug concentrations.

Effects of ximelagatran, the oral form of melagatran, in the treatment of caval vein thrombosis in conscious rats

The antithrombotic effects of direct (ximelagatran and hirudin) and indirect (dalteparin) anticoagulants were compared using a deep venous thrombosis (DVT) treatment model in conscious rats. Thrombus formation was induced in the inferior caval vein by total stasis plus topically applied ferric chloride. After 1-h thrombus maturation, one group of 10 rats were sacrificed and the mean thrombus weight in this group was 27.3 +/- 2.7 mg. This thrombus weight was handled as a reference to which all other results were compared. In all other groups, the total occlusion was removed after 1 h but a partial stasis was retained, permitting some blood flow around the thrombus. Groups of animals received subcutaneous (s.c.) dalteparin (200 IU/kg), s.c. hirudin (0.75 micromol/kg), one of four oral doses of ximelagatran (2.5, 5, 10 or 20 micromol/kg) or s.c. saline (control). After the 3-h treatment, mean thrombus weight in the saline group (26.5 +/- 3.3 mg) did not differ significantly from that of the reference group (27.3 +/- 2.7 mg, see above). Ximelagatran decreased thrombus weight in a dose-dependent manner, with an estimated ID(50) of 15 micromol/kg. Mean thrombus weight with the highest ximelagatran dose (11.1 +/- 1.3 mg) was similar to that with hirudin (13.0 +/- 1.5 mg). The effect of dalteparin on thrombus regression was much less pronounced (20.2 +/- 1.2 mg), compared with ximelagatran and hirudin, even though it was administered at a dose that yielded a similar activated partial thromboplastin time (APTT) prolongation. In conclusion, the results from this DVT treatment model showed that direct thrombin inhibitors ximelagatran and hirudin exhibited superior antithrombotic properties to low molecular weight heparin (LMWH).

Advances in the development of thrombin inhibitors

Thromboembolic diseases are a major cause of morbidity and mortality, particularly in the Western world, which has stimulated enormous research efforts by the pharmaceutical industry to introduce new antithrombotic therapies. One strategy is the development of direct inhibitors of the serine protease thrombin, which holds a central position in the final steps of the blood coagulation cascade and in platelet activation. At present there is only limited clinical use of some parenteral preparations of thrombin inhibitors in acute situations, especially when the common antithrombotic drugs heparin, warfarin and aspirin are ineffective or associated with side effects. However, for use in prophylaxis of thrombotic diseases such inhibitors should be orally available, must be safe to avoid bleeding complications and should have an appropriate half-life, allowing once or twice daily dosing to maintain adequate antithrombotically effective blood levels. Details of several new and potent thrombin inhibitors have been published during the last years. For some of them oral bioavailability is claimed and they are effective in in vitro coagulation assays. However, most of them showed only limited efficacy in animal studies with respect to the doses administered. For that reason, effort is concentrated on the evaluation and optimisation of the overall physicochemical characteristics of the inhibitors in order to improve the pharmacokinetics and, thus, the development of promising drug candidates. Nevertheless, only careful clinical studies can give clear answers about the true therapeutical benefit of new developments in this field. This review summarises the current status of direct thrombin inhibitors which are already in clinical use and clinical development and gives an overview on recently published and promising new compounds.