Novel and emerging therapies: thrombus-targeted fibrinolysis

Thrombolytic therapy by infusion of analogs of tissue plasminogen activator (tPA), other recombinant-based plasminogen activators (e.g., alteplase, reteplase, and tenecteplase), streptokinase, and urokinase (uPA) aims to clear blood clots and restore blood flow in occluded blood vessels. Thrombolytic therapy is thereby frequently used in patients with myocardial infarction, stroke, peripheral arterial disease, and massive pulmonary embolism. The leading drawbacks of thrombolysis and associated therapy are represented by a significant burden of inefficacy combined with a high risk of bleeding complications. Recent advances in understanding the complex pathophysiology of vascular occlusions, combined with important technological innovations, are notably improving the therapeutic armamentarium against thrombotic and occlusive disorders. Most of the past and ongoing research in this area have entailed thrombus-targeted fibrinolytic therapy with either tissue- and fibrin-specific immunoconjugates, fibrinolytic-bearing erythrocytes, or fibrinolytic-bearing nanoparticles. The greatest advantages of thrombus-targeted fibrinolysis, especially with biocompatible nanoparticles, are represented by their preferential localization within developing clots, effectual thrombolysis and enhanced safety due to substantial reduction of the dosage of fibrinolytic agents, and reduced onstream adverse effects. These positive biological features, coupled with minimal extravasation and favorable clearance from the circulation, appear advantageous for obtaining more efficacious and durable thrombolytic effects while concomitantly lowering or even eliminating the risk of systemic bleeding complications that typically accompany the injection of free or soluble plasminogen activators. Although an ideal technique has not been definitely established so far, tPA-bearing nanoparticles exhibiting affinity for clot-specific cells and biomolecules coupled with low-frequency ultrasound seem to bear the greatest advantages for prevention and therapy of acute thrombosis, with the possibility to specifically guide and concentrate the thrombolytic agent at the site of pathologic thrombi and clear preexisting clots by a series of mechanisms combining mechanical stress and increased penetration and effectiveness of the drugs employed.