ABSTRACT
The u-PA paradigm of fibrinolysis is predicated on pro-uPA (pro-UK) remaining a zymogen in
blood and being activated to two-chain u-PA (UK) selectively on the fibrin surface. Under these
conditions, pro-UK-induced thrombolysis is fibrin-specific, and lysis is augmented by a
hypercatalytic transitional state during the conversion of pro-UK to UK. Unfortunately, at the
pharmacological doses used for therapeutic thrombolysis, pro-UK is unstable in blood and is
activated to UK systemically. When this happens, the advantages of pro-UK over UK are lost.
This instability of pro-UK is related to its relatively high intrinsic activity, which activates free
plasminogen at pharmacological concentrations. Therefore, mutations were designed, produced,
and characterized to reduce this intrinsic activity. A histidine300 mutation (H-pro-UK) is
described that is more than four times more stable in blood, and that induces effective, fibrin-
specific clot lysis in vitro and in vivo, partly related to the fact that whereas its intrinsic activity is
reduced, H-UK is almost twice as active as UK. An additional property of H-pro-UK is that in a
study in dogs, it caused no increase in bleeding time at thrombolytic doses, in contrast to t-PA
and pro-UK. A mechanism for the selective sparing of hemostatic fibrin is proposed.
