Thrombolytic properties of tissue plasminogen activator (t-PA) and single chain urokinase plasminogen activator (scu-PA) in vitro: Absence of synergy as demonstrated by two quantitative analytical methods

Abstract

The thrombolytic effects of tissue-type plasminogen activator (t-PA), of single chain urokinase-type plasminogen activator (scu-PA) and of combinations of t-PA and scu-PA were measured in an in vitro system composed of a 125I-fibrin labelled human plasma clot immersed in citrated human plasma. Lysis of the plasma clot was concentration and time dependent; 50% lysis in 4 h was obtained with 1.2 nM t-PA, in 8 h with 0.5 nM t-PA and in 12 h with 0.4 nM t-PA. For scu-PA corresponding values were 7, 4 and 2 nM. Combinations of t-PA and scu-PA in molar ratios of 1:2 to 1:9, were equieffective at algebraic fractions which were not significantly smaller than 1 and using the geometric isobole method coincided with the straight line connecting the equieffective concentrations of the individual plasminogen activators. Thus, both analytical methods indicated that the effects of t-PA and scu-PA on in vitro clot lysis were purely additive and not synergistic, notwithstanding the fact that the thrombolytic potency of combinations was consistently higher than the sum of the effects of the individual agents at the same concentrations as used in the combination. Our results do not support the existence of in vitro synergism between t-PA and scu-PA.