Cardiovascular diseases remain the leading cause of deaths in industrialized countries. The underlying cause in these diseases is thrombosis of critically situated blood vessels. One approach to the treatment is dissolution of the thrombus with thrombolytic agents, namely plasminogen activators. Activation of plasminogen results in the formation of plasmin which degrades fibrin. Unfortunately, plasmin is a non-specific protease that also degrades other circulating clotting factors. Thus thrombolytic therapy, which introduces systemic generation of excess plasmin, carries the risk of hemorrhage. The quest for highly improved thrombolytic agents therefore remains. The """"""""ideal"""""""" agent, quoted directly from a highly publicized review in Science (243:51,1989), should: """"""""be capable of selectively attacking the components while sparing the circulating clotting proteins and platelet"""""""". We propose a novel approach which could fulfill these criteria. The approach comprises a large protein complex of two components: a fibrin-targeting antibody conjugated with heparin, and a plasminogen activator (PA) derivatized with quaternary ammonium species (QAS). The two components are linked via an electrostatic interaction between the anionic heparin and the cationic QAS species. Since QAS are small ions, the QAS-modified PA will fully retain its thrombolytic activity. This activity, however, will be inhibited after binding with the antibody component, primarily due to the blockage of the enzyme's catalytic site by the appended heparin macromolecule. Since protamine is a clinical heparin antagonist with a much stronger affinity to heparin, it can be used safely to trigger the release of the QAS-modified-PA from the large complex. Thus, the approach would permit the administration of a strong fibrin-targeting but inactive thrombolytic complex, and subsequently a triggered release of the active PA drug in close proximity of the fibrin deposit. These features would not only enhance the potency and specificity of a thrombolytic agent, but also alleviate the bleeding risk by allowing the agent to selectively attack the components of a thrombus while sparing the circulating proteins. In this proposal, we plan to use streptokinase and t-PA as two model compounds to examine the feasibility of the approach.
Our specific aims are: (1) to prepare the antibody-PA complexes; (2) to test their targeting and fibrinolytic functions in vitro; (3) to examine their pharmacokinetic/biodistribution properties in rats; (4) to test their fibrinolytic function in vivo, using a rabbit jugular vein model; and (5) to test their efficacy in recanalizing occluded coronary arteries, using a clinically-stimulated canine intracoronary thrombosis model.
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