Abstract 1033: Simultaneous Clot-targeted Factor Xa Inhibition And Selective Blockade Of Activated GPIIb/IIIa On Platelets Results In Delayed But Potent Antithrombotic Effects Without Bleeding Time Prolongation.
Background: We generated phage-display-derived anti-GPIIb/IIIa single-chain antibodies (e.g. scFv SCE5) that specifically bind to the activated GPIIb/IIIa only and thus specifically block activated platelets only. ScFv SCE5 demonstrates strong antithrombotic potency, comparable to the conformation-unspecific blockers tirofiban and eptifibatide. In contrast bleeding times were not prolonged with scFv SCE5. Here we now use the possibility to add effector molecules using molecular biology methods. The highly potent anticoagulant TAP (tick anticoagulant peptide), which is a direct factor Xa (fXa) inhibitor, was used as an effector molecule.
Methods and Results: We genetically fused the activation-specific scFv with TAP, expressed the constructs in E.coli and purified the 39 kDa protein via its Histag binding to Nickel beads. Specific binding of the fusion molecules MA2/SCE5-TAP and strong inhibition of fibrinogen binding was proven in flow cytometry; anti-fXa activity was demonstrated in chromogenic assays. In vivo anticoagulative efficiency was determined by Doppler-flow in a ferric chloride-induced carotid artery thrombosis model in mice. Prolongation in occlusion time with SCE5-TAP was significantly stronger compared to SCE5 alone, recombinant TAP, non-binding mut-scFv-TAP as well as the clinical used drugs enoxaparine and eptifibatide. In contrast to the other anticoagulants tested, bleeding time was not prolonged by SCE5-TAP. Flow experiments studying platelet adhesion on collagen revealed a possible mechanism for the unique finding of a fully normal bleeding time: LIBS exposure on adhering platelets and as such the anticoagulative targeting potency of SCE5-TAP was delayed until considerable layers of platelets were deposited.
Conclusions: The combination of activation-specific GPIIb/IIIa blockade and fXa inhibition in one clot-targeted molecule further improves in-vivo antithrombotic efficiency without causing any bleeding time prolongation. The delay of the observed targeting effect may allow a sealing of injuries with platelet layers but may be in time for the prevention of occlusive platelet aggregates. The described blockers represent a new type of highly selective drugs that warrant further clinical development.