Plenary Paper THROMBOSIS AND HEMOSTASIS A specific antidote for dabigatran: functional and structural characterization

Thromboembolic disorders such as myocardial infarction, stroke, and venous thromboembolism are the most common causes of mortality and morbidity in Western societies. These thromboembolic events can be triggered by excessive activation of coagulation, and thrombin plays a major role in these processes. The most widely used agents for antithrombotic therapy are indirect thrombin inhibitors (such as unfractionated and lowmolecular-weight heparin) and vitamin K antagonists (VKAs) such as warfarin. Recently with the introduction of new oral anticoagulants on the market, new options for patients who require antithrombotic therapy are available. The first of these, dabigatran etexilate, is a novel, potent, nonpeptidic direct thrombin inhibitor. The orally administered double prodrug is hydrolized in vivo by esterases into the active form, dabigatran. It binds reversibly to the active site of both free and clot-bound thrombin, thus effectively preventing the formation of fibrin clots. Currently, dabigatran etexilate has been approved in more than 70 countries worldwide for the prophylaxis of thromboembolism in patients undergoing orthopedic surgery and for the prevention of stroke in patients with atrialfibrillation. In one study, treatmentwith dabigatran in patientswith atrial fibrillation reduced the risk of stroke with a better safety profile compared with warfarin. In all indications, a fixed-dose regimen of dabigatran has provided effective anticoagulation with a favorable bleeding profile in patients without regular monitoring required. Despite these improvements in treatment with dabigatran, anticoagulation therapy is associated with an increased risk of bleeding. When VKAs are used, the anticoagulant effect in emergency situations is usually reversed with a combination of vitamin K, fresh-frozen plasma, prothrombin complex concentrate, or recombinant factor VIIa. Compared with VKAs, the new oral anticoagulants such as dabigatran and the factor Xa inhibitors rivaroxaban and apixaban have a shorter half life and shorter duration of anticoagulant effect. Although hemodialysis is recommended for the elimination of dabigatran and preclinical evidence suggests coagulation factor concentrates may also reverse bleeding, there is currently no specific antidote for dabigatran or for any of the other novel oral anticoagulants. Antibodies and antibody fragments such as fragment antigen binding (Fab) can be used as antidotes for toxins and drugs. With Fabs, intoxications with digoxin and colchicine can be treated and the effect of desipramine can be reversed. The preclinical development of a single-chain fragment variable for the neutralization of methamphetamine and a monoclonal antibody to treat cocaine overdoses have been reported recently. Here we present data on the identification, humanization, and Xray structure of the antibody fragment aDabi-Fab, which binds dabigatran and reverses its anticoagulant effects in vitro and in vivo.