ABSTRACTProspective characterization of pharmacodynamics of tissue-type plasminogen activator (t-PA) is needed for diverse clinical applications. Accordingly, we used physiologically based, computer simulation of participating biochemical reactions in response to concentrations of circulating t-PA seen with infusions of 1 to 7 hr duration in 45 patients. Predicted values were compared with those from a "training set" obtained in six patients given t-PA for coronary thrombosis and six receiving therapy for peripheral arterial occlusion. Subsequently, results of simulation were compared prospectively with observations from a "test set" of 33 consecutive patients given low doses of t-PA for as long as 7 hr or higher doses for 1 to 2 hr and with data from 101 patients given t-PA in the European Cooperative Trial. Fits between observed and predicted values were close. Based on observations in the training set, the at2-macroglobulin reaction with circulating plasmin and ongoing synthesis of plasminogen were incorporated in the simulations. Fibrinogenolysis in vitro was documented despite supplementation of samples with aprotinin, particularly when concentrations of t-PA were high. This phenomenon can lead to overestimation of fibrinogen depletion and was found to be obviated by the use of PPACK, a novel serine protease inhibitor. Results indicate that the simulation approach developed permits economic, prospective evaluation of regimens of t-PA suitable for diverse conditions and delineation of the impact of individual constituents and reactions on pharmacodynamics of t-PA and on the risk of induction of a systemic lytic state.