ATTEMPTS: a heparin/protamine-based prodrug approach for delivery of thrombolytic drugs

The aim of this study is to develop a heparin/protamine-based prodrug system for the controlled delivery of enzyme such as tissue-type plasminogen activator (tPA). This approach, termed antibody targeted, triggered, electrically modified prodrug-type strategy (ATTEMPTS), would permit antibody-directed administration of inactive tPA, and allow a subsequent triggered release of the active tPA at the target site. Cation-modified tPA (mtPA) was attached to a heparin--antifibrin complex via ionic interaction. The active tPA can be subsequently released by the addition of protamine, a competitive heparin inhibitor. Anti-fibrin IgG was conjugated to heparin via an end-point attachment to form the heparin--antifibrin--complex which provides the targeting efficiency of the final heparin--mtPA complex. Cation-modification was performed either by chemical conjugation by linking (Arg)(7)Cys to tPA with N-succinimidy-3-(2-pyridyldithio) propionate or by recombinant DNA method. Results show that the chemical modification process did not significantly alter specific activity of tPA with regard to plasminogen activation, fibrin-binding ability, and response toward fibrinogen. Expressed modified tPA (EmtPA) produced by recombinant DNA methods retained the same catalytic activity of the parent tPA, as well as a dynamic catalytic behavior depending upon the presence of heparin and protamine. Both types of modified tPA, especially the mtPA demonstrated a significantly higher affinity toward heparin or heparin--antifibrin complex than native tPA. In addition, the complexes of mtPA--heparin did not yield any intrinsic clot lysis activity owing to the blockage of the active site of tPA by attached heparin. On the other hand, heparin-induced inhibition of both mtPA and EmtPA activity was reversed by adding protamine, as confirmed by chromogenic and in vitro clot lysis assays. These results suggested that a heparin/protamine-based tPA delivery system may be a useful tool to improve current thrombolytic therapeutic status, by both precisely regulating the release of active tPA and aborting the associated bleeding risk. Alternatively, this ATTEMPTS approach could also be used to deliver enzyme drugs while diminishing their associated toxic effects.