Urokinase is a serine protease that functions as a recombinant tissue plasminogen activator. It is administered intravenously and used to treat conditions caused by arterial blood clots such as acute ischemic stroke, acute myocardial infarction, acute massive pulmonary embolism and blocked central venous access devices. It targets plasminogen in blood vessels where these clots occur. The clotting process consists of two pathways, intrinsic and extrinsic, which converge to create stable fibrin which traps platelets and forms a hemostatic plug. The intrinsic pathway is activated by trauma inside the vasculature system, when there is exposed endothelial collagen. Endothelial collagen only becomes exposed when there is damage. The pathway starts with plasma kallikrein activating factor XII. The activated factor XIIa activates factor XI. Factor IX is then activated by factor XIa. Thrombin activates factor VIII and a Calicum-phospholipid-XIIa-VIIIa complex forms. This complex then activates factor X, the merging point of the two pathways. The extrinsic pathway is activated when external trauma causes blood to escape the vasculature system. Activation occurs through tissue factor released by endothelial cells after external damage. The tissue factor is a cellular receptor for factor VII. In the presence of calcium, the active site transitions and a TF-VIIa complex is formed. This complex aids in activation of factors IX and X. Factor V is activated by thrombin in the presence of calcium, then the activated factor Xa, in the presence of phospholipid, calcium and factor Va can convert prothrombin to thrombin. The extrinsic pathway occurs first, producing a small amount of thrombin, which then acts as a positive feedback on several components to increase the thrombin production. Thrombin converts fibrinogen to a loose, unstable fibrin and also activates factor XIII. Factors XIIIa strengthens the fibrin-fibrin and forms a stable, mesh fibrin which is essential for clot formation. The blood clot can be broken down by the enzyme plasmin. Plasmin is formed from plasminogen by tissue plasminogen activator. Urokinase acts as a tissue plasminogen activator. It binds to clots with fibrin where it causes hydrolysis of the arginine-valine bond in plasminogen, aiding its conversion to plasmin. The plasmin degrades the stable fibrin and causes lysis of the clot. The activity of Urokinase depends on the presence of fibrin. Only small amounts of plasmin is formed from plasminogen when there is no fibrin. Urokinase in the presence of fibrin obtains a higher affinity for plasminogen, thus leading to its increased activity. Urokinase undergoes metabolism by proteases and is excreted in bile and urine.

Urokinase References