Enhanced thrombolytic therapy for stroke: Targeted delivery of urokinase with magnetic nanoparticles

Thrombolytic therapy is the only approved treatment for ischemic stroke in acute setting. These thrombolytic drugs, such as urokinase and recombinant tissue plasminogen activator (rtPA), are enzymes that generate plasmin and degrade fibrin in the clot. However, these drugs tend to dissolve both pathological thrombi and fibrin deposit at sites of vascular injury, resulting in hemorrhagic toxicity at therapeutic doses. Targeting delivery of the thrombolytic drug to the location of thrombi in the artery may reduce systemic distribution of the drug and greatly reduce related adverse effects. We plan to resolve this problem by magnetic drug targeting using magnetic nanoparticles (MNP) as drug carriers, which is generally well tolerated in the biology system. Our recent studies have demonstrated the feasibility and efficacy of this approach using MNP-bound rtPA in a rat iliac embolic model. In the study, the major bioactivity of rtPA covalently bound to polyacrylic acid-coated magnetite (PAA-MNP) preserved and the composite retained to the target site in vivo by an external magnet. With hemodynamic measurement conducted with ultrasound flowmetry and laser Doppler flowmetry, hind limb blood flow was restored in response to magnetically guided MNP-rtPA. As the first demonstration of a successful magnetic target thrombolysis, our approach achieved reproducible and effective target thrombolysis with ＜20% of a regular dose of rtPA. Nevertheless, problems remain and further optimization is required. One of the most critical potential improvements is to further increase enzyme activity per MNP, in order to deliver more “active” drug with less MNP, which appears to be essential for application in smaller arteries, such as cerebral artery, without plugging the arteries. Since enzyme activity of MNP-rtPA has been optimized, using a more potent thrombolytic agent, such as urokinase, may be the solution. As a fibrinolytic agent, urokinase is as potent as rtPA in degrading fibrin in the clot; however, urokinase is a less specific drug that may degrade fibrin in circulation, as well as other coagulation-related factors, including fibrinogen. Therefore, it is conceivable that MNP-urokinase, once retained in the nearby of the clot by magnetic force, may exert enhanced thrombolytic activity with limited increase in systemic toxicity. In addition, magnetic guiding strategy appears to be critical for a prompt clot lysis. It appears that moving the magnet back and forth along the artery induced clot lysis; whereas stationary placement of the magnet by the clot caused no effect, presumably due to formation of pellet that are unable to interact with the substrate of the enzyme. Our preliminary data demonstrated that ultrasound energy may facilitate suspension of the composite under stationary magnet. Although ultrasound has been implicated useful in facilitating thrombolysis in treatment of thromboembolic diseases, a combined application of ultrasound and magnetic force has not been studied. We plan to conduct a positive test of the hypothesis that with magnetic guidance and ultrasound, enhanced thrombolytic activity per MNP in the composite may be achieved by using a less specific plasminogen activator, urokinase, which permits limited MNP required for drug delivery and achieves optimal efficacy in a rat embolic stroke model. These strategies will be tested with mathematical modeling, computer simulation and in vitro simulation prior to be realized in a rat iliac thromboembolic model and a rat stroke model. In addition to ischemic stroke, information we plan to acquire in the proposed study may also facilitate application of magnetic targeting in treatment of coronary embolism, restenosis after balloon injury, and angiogenesis/ tumor growth etc.

Project ID:PG10101-0180
External Project ID:NHRI-EX101-9937EI