ATTEMPTS: a heparin/protamine-based delivery system for enzyme drugs.

A prodrug delivery system termed "Antibody Targeted, Triggered, Electrically Modified Prodrug-Type Strategy (ATTEMPTS)" has been developed to permit the antibody-directed administration of inactive enzyme drug including tissue-type plasminogen activator (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, and the active tPA can subsequently be 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 by either chemical conjugation by linking (Arg)7Cys to tPA with N-succinimidy-3-(2-pyridyldithio) propionate or by recombinant DNA methods. Results show that the modification process did not significantly alter the specific activity of tPA with regard to plasminogen activation, fibrin-binding ability, and response toward fibrinogen. The complexes of both modified tPA-heparin did not yield any intrinsic catalytic activity owing to the blockage of the active site of tPA by the attached heparin. On the other hand, heparin-induced inhibition of modified tPA activity was reversed by adding protamine, which is similar to that of a prodrug delivery system. These results suggest that heparin/protamine-based enzyme delivery systems may be a useful tool to improve current enzyme therapeutic status, as well as thrombolytic therapy, by both regulating the release of active enzyme and aborting the associated systemic toxic effect. Currently, modification of enzyme drugs has been optimized by recombinant DNA technology assisted by computer simulation. In addition, the original strategy has been revised to obtain enhanced therapeutic efficacy.