A fusion protein with improved thrombolytic effect and low bleeding risk.

To resolve the therapeutic dilemma between efficacy of thrombolysis and bleeding risk associated with the use of a combination of thrombolytic and anticoagulant treatments, we created a fusion protein. Staphylokinase was fused to the N-terminus of hirudin using thrombin recognition sequence as linker peptide, resulting in a fusion protein STH. We hypothesised that STH would be cleaved by thrombin at the thrombus site, releasing staphylokinase and hirudin to perform bifunctionally, and attenuating bleeding risk. SDS-PAGE and Western blot analyses indicated that the linker peptide could be specially recognised and cleaved by thrombin. Amidolytic and thromboelastogram assays showed that the N-terminus of hirudin in STH was blocked by staphylokinase and linker peptide, impeding hirudin's anticoagulant activity. Once cleaved, STH displayed 35.7% of the anticoagulant activity of equimolar hirudin and exhibited anticoagulant effects in the fibrin clot lysis assay. Thrombin-binding and fibrin clot lysis assays showed that the C-terminus of hirudin retained its high affinity for thrombin. Moreover, STH showed improved thrombolytic effects and a lower bleeding risk in animals. Thus, STH may have the capacity to perform bifunctionally and release anticoagulant activity in a thrombus-targeted manner in vivo, which may reduce the bleeding risk that often accompanies high thrombolytic efficacy in the treatment of thromboembolic diseases.