Construction and functional evaluation of a single-chain antibody fusion protein with fibrin targeting and thrombin inhibition after activation by factor Xa

Background-Recombinant technology was used to produce a new anticoagulant t hat is preferentially localized and active at the site of the clot. Methods and Results-The variable regions of the heavy and light chains of a fibrin-specific antibody were amplified by polymerase chain reaction (PCR) with hybridoma cDNA. To obtain a functional single-chain antibody (scFv), a linker region consisting of (Gly(4)Ser)(3) was introduced by overlap PCR, After the scFv clones were ligated with DNA encoding the pill protein of t he M13 phage, high-affinity clones were selected by 10 rounds of panning on the B beta 15-22 peptide of fibrin (beta-peptide). Hirudin was genetically fused to the C-terminus of the variable region of the light chain. To rele ase the functionally essential N-terminus of hirudin at the site of a blood clot, a factor Xa recognition site was introduced between scFv(59D8) and h irudin. The fusion protein was characterized by its size on SDS-PAGE (36 kD a), by Western blotting, by its cleavage into a 29-kDa (single chain alone) and 7-kDa. (hirudin) fragment, by its binding to beta-peptide, and by thro mbin inhibition after Xa cleavage. Finally, the fusion protein inhibited ap positional growth of whole blood clots in vitro more efficiently than nativ e hirudin. Conclusions-A fusion protein was constructed that binds to a fibrin-specifi c epitope and inhibits thrombin after its activation by factor Xa. This rec ombinant anticoagulant effectively inhibits appositional clot growth in vit ro. Its efficient and fast production at low cost should facilitate a large -scale evaluation to determine whether an effective localized antithrombin activity can be achieved without systemic bleeding complications.