Summary
A modified variant of human tissue-type plasminogen activator (t-PA) lacking the finger
domain (F), the growth factor domain (G) and the first kringle domain (K1), has an
extended plasma half-life in vivo, compared to that of t-PA. When the variant (denoted
K2P) was tested in vitro for its ability to lyse human plasma clots we found that
the activity was characterized by a time lag phase and a sigmoidal dose-response curve.
However, an attenuation of the lag phase in vitro was observed both when K2P was mixed
with t-PA in a w/w ratio of 4 : 1 and when K2P was allowed to lyse a clot that had
been pre-exposed to t-PA i.e. submitted to a limited plasmic digestion. Dosis that
in vitro caused 50% lysis within 6 h were calculated from individual dose-response
curves and were for K2P, t-PA and K2P/t-PA (4 : 1 w/w) 540 ng/ml, 360 ng/ml and 310
ng/ml, respectively. These results indicated a synergistic effect between K2P and
t-PA. However, the data from individual dose-response curves showed that the effect
of the K2P/t-PA mixture never was better than that of t-PA alone, and the synergistic
effect in vitro is therefore considered to be of limited use. The thrombolytic activity
in vivo was evaluated in a rabbit jugular vein thrombus model. Despite the lag phase
observed in vitro, K2P was approximately 3 times as effective as t-PA in vivo (bolus
injection). The thrombolytic effect of K2P was further potentiated when it was administred
together with a small amount of t-PA (4 : 1 w/w). This potentiation in vivo was, in
contrast to the effect in vitro, a useful synergistic effect as the dose-response
curve for the K2P/t-PA mixture was steeper than that of t-PA and K2P alone. Doses
that caused 50% lysis within 3 h were for t-PA, K2P and K2P/t-PA 1.28 mg/kg, 0.56
mg/kg and 0.35 mg/kg, respectively.
