How does alteplase compare to streptokinase in terms of selectivity at low doses?

Alteplase, a thrombolytic agent, exhibits a higher degree of selectivity for fibrin compared to streptokinase, especially at low doses. This selectivity is significant because alteplase primarily activates plasminogen that is bound to fibrin in a clot. This means that it works more effectively in breaking down existing clots, as the binding of plasminogen to fibrin facilitates alteplase's action specifically where it is needed.

In contrast, streptokinase has a broader activating effect on circulating plasminogen, leading to a less targeted approach and an increased risk of systemic fibrinolysis. This can result in more widespread clot degradation and potentially increase bleeding risks.

The ability of alteplase to selectively lyse fibrin in clots makes it a preferred agent in certain acute thrombotic conditions as it can specifically target and dissolve clots, minimizing effects on the fibrinogen and other components of the coagulation system that are not involved in the thrombus itself.