Major mechanistic differences explain the higher clot lysis potency of reteplase over alteplase: lack of fibrin binding is an advantage for bolus application of fibrin-specific thrombolytics

Reteplase is a plasminogen activator composed of the kringle 2 and the protease domain of tissue-type plasminogen activator (alteplase). Two bolus injections of 10 U, 30 min apart, allow a more rapid and complete reperfusion of the occluded vessel than the accelerated infusion-regimen of alteplase. The longer half-life of reteplase is supposed to be a reason for the high in-vivo potency of the molecule. The standard in-vitro analysis revealed a lower plasmin-forming activity and a lack of high-affinity fibrin binding of reteplase compared to alteplase. In this paper, we determine the activity of reteplase and alteplase in a dynamic plasma clot lysis model. in contrast to other clot lysis activity assays, this model allows the pressure-driven plasma to flow through the clot, mimicking the in-vivo situation. At low concentrations, both reteplase and alteplase have a similar activity. At high concentrations, as achieved during the treatment of acute myocardial infarction, reteplase is more potent than alteplase. Immunostaining of plasma clots demonstrates that alteplase tightly binds to the fibrin matrix and accumulates at the surface of the clot. As a consequence, activation of plasminogen and the subsequent degradation of the fibrin matrix is supposed to occur from the surface to the interior of the clot, depending on the permanent supply of plasminogen from the plasma. In contrast, reteplase penetrates the clot due to its lack of fibrin binding which in turn allows the activation of plasminogen inside the clot. We discuss the ability of plasma clot penetration in relation to the achievement of a high in-vivo efficacy of fibrin-specific thrombolytic agents, especially when applied by bolus injection.