Summary
Fibrinolytic properties of four hybrids of u-PA and t-PA, all containing the u-PA
growth factor domain and binding to recombinant human u-PA receptor expressed in CHO
cells, were compared. Highest fibrin stimulation was observed with uK2tPA which when
compared to t-PA in the rabbit system, had a considerably prolonged circulatory half-life
in vivo. Compared to an equimolar dose of t-PA, 0.4 mg/kg uK2tPA caused a similar
consumption of α2-antiplasmin and fibrinogen and a considerably greater prolongation of the ex-vivo
blood clotting time. Nevertheless, this dose of uK2tPA was inactive in the jugular
vein thrombosis assay. This lack of thrombolytic activity is presumably due to the
presence of a functional u-PA growth factor domain, which in binding uK2tPA to cellular
blood elements possibly retards its penetration into the blood clot and in this manner
could neutralize the potential thrombolytic activity of the t-PA kringle 2 and protease
domains in uK2tPA.