Pharmacokinetics and fibrin specificity of alteplase during accelerated infusions in acute myocardial infarction.

Pharmacokinetics and fibrin specificity of alteplase (recombinant tissue-type plasminogen activator) were determined in 10 patients with acute myocardial infarction undergoing an accelerated infusion regimen during the alteplase/anistreplase patency study (TAPS). Fifteen milligrams of alteplase was administered as an intravenous bolus injection, followed by infusions of 50 mg over 30 min and 35 mg over a further 60 min. Mean steady state plasma concentrations of alteplase during the initial 30 min were 3.2 +/- 0.84 micrograms/ml, measured immunochemically, and 2.1 +/- 0.23 micrograms/ml, measured using a functional activity assay. These values were 45% and 51% higher, respectively, than those during the standard infusion schedule (p less than 0.01). However, the predominant plasma half-life determined by model fitting based on either assay (3.3 to 3.5 min) was unaltered compared with the standard regimen. Maximal concentrations of fibrin and fibrinogen degradation products were 5.1 +/- 2.2 and 1.9 +/- 1.1 micrograms/ml, respectively. Plasminogen decreased to 70% and alpha 2-antiplasmin to 35% of values before infusion. The results indicate that 1) improved coronary patency rates during "front-loaded" infusions can be rationalized in terms of higher plasma concentrations of both free and immunoreactive alteplase, 2) kinetic variables are comparable with those of other dosing strategies, and 3) fibrin specificity is not diminished relative to that of the standard infusion regimen.

Sixty-minute alteplase protocol: a new accelerated recombinant tissue-type plasminogen activator regimen for thrombolysis in acute myocardial infarction.

OBJECTIVES: Our aim was to design and evaluate a new and easily administered recombinant tissue-type plasminogen activator (rt-PA) regimen for thrombolysis in acute myocardial infarction (AMI) based on established pharmacokinetic data that improve the reperfusion success rate. BACKGROUND: Rapid restoration of Thrombolysis in Myocardial Infarction (TIMI) grade 3 flow is a primary predictor of mortality after thrombolysis in AMI. However, TIMI grade 3 patency rates 90 min into thrombolysis of only 50% to 60% indicate an obvious need for improved thrombolytic regimens. METHODS: Pharmacokinetic simulations were performed to design a new rt-PA regimen. We aimed for a plateau tissue-type plasminogen activator (t-PA) plasma level similar to that of the first plateau of the Neuhaus regimen. These aims were achieved with a 20-mg rt-PA intravenous (i.v.) bolus followed by an 80-mg i.v. infusion over 60 min (regimen A). This regimen was tested in a consecutive comparative trial in 80 patients versus 2.25 10(6) IU of streptokinase/60 min (B), and 70 mg (C) or 100 mg (D) of rt-PA over 90 min. Subsequently, a confirmation trial of regimen A in 254 consecutive patients was performed with angiographic assessment by independent investigators of patency at 90 min. RESULTS: The comparative phase of the trial yielded, respectively, TIMI grade 3 and total patency (TIMI grades 2 and 3) of 80% and 85% (regimen A), 35% and 50% (B), 50% and 55% (C) and 60% and 70% (D). In the confirmation phase of the trial, regimen A yielded 81.1% TIMI grade 3 and 87.0% total patency. At follow-up angiography 7 (4.1%) of 169 vessels had reoccluded. In-hospital mortality rate was 1.2%. Nadir levels of fibrinogen, plasminogen and alpha2-antiplasmin were 3.6 +/- 0.8 mg/ml, 60 +/- 21% and 42 +/- 16%, respectively (mean +/- SD). Fifty-seven patients (22.4%) suffered from bleeding; 3.5% needed blood transfusions. CONCLUSIONS: The 60-min alteplase thrombolysis in AMI protocol achieved a TIMI grade 3 patency rate of 81.1% at 90 min with no indication of an increased bleeding hazard; it was associated with a 1.2% overall mortality rate. These results are substantially better than those reported from all currently utilized regimens. Head to head comparison with established thrombolytic regimens in a large-scale randomized trial is warranted.

Pharmacokinetics and systemic effects of tissue-type plasminogen activator in normal subjects.

Pharmacokinetics and systemic effects of recombinant tissue-type plasminogen activator (rt-PA) were studied in 18 healthy male volunteers after 30-minute intravenous infusions of placebo, 0.25 mg/kg rt-PA, and 0.5 mg/kg rt-PA. Highly comparable pharmacokinetic parameters were obtained after analysis of rt-PA as both an antigen and an activity. Mean clearance (antigen) was 620 +/- 70 (SD) ml/min, volume of distribution at steady state was 8.1 +/- 0.8 L, initial volume of distribution was 4.4 +/- 0.6 L, and dominant half-life was 4.4 +/- 0.3 minutes. The pharmacokinetics of rt-PA were linear, showed low interindividual variation, and are compatible with rapid hepatic elimination of the protein. Systemic plasminogen activation was minimal as assessed by hemostatic assays of plasma samples treated with anti-rt-PA Immunoglobulin G (IgG) to inhibit in vitro fibrinogenolysis. Circulating fibrinogen levels, clotting times, and coagulation factors were unchanged; plasminogen and alpha 2-antiplasmin decreased maximally to 85% and 65% of baseline values, respectively. The data are consistent with the fibrin specificity of t-PA, which is derived from its role in physiologic fibrinolysis.

Automated monoclonal radioimmunoassays for pirenzepine, a selective muscarinic receptor antagonist, in plasma and urine.

Sensitive and specific monoclonal radioimmunoassays (RIA) for pirenzepine, a muscarinic receptor (M1) antagonist, were developed and validated for rapid automated routine analysis of plasma and urine samples from clinical studies. Three discrete stable hybridoma clones secreting monoclonal antibodies (MAb) to pirenzepine were produced by fusing the myeloma line X63-Ag8.653 to spleen cells of BALB/c mice immunized with pirenzepine-5-N-propionate-protein conjugates. Of three carrier proteins investigated (HSA, BSA and edestin), optimal humoral immune responses and affinity of hybridoma antibody were attained using HSA. All three MAb displayed high affinity to pirenzepine (Ka = 0.6-1.2 X 10(9) l/mol) but showed differing cross-reactivities with its 4'-N-desmethyl metabolite (less than 1%, 6% and 40% respectively). The MAb with essentially zero metabolite cross-reactivity, 58-7/7, was selected for RIA development. In comparison, eight rabbit polyclonal antisera raised against pirenzepine-5-N-propionate-HSA or pirenzepine-5-N-butyrate-HSA possessed a similar range of affinities to the MAbs, but none approached MAb 58-7/7 in specificity. The bridge length had no significant effect on antisera characteristics. Competitive solid-phase RIAs for pirenzepine in human plasma and urine were established using MAb 58-7/7 and [3H]pirenzepine as tracer. All fluid transfers were automated using a programmable sample processing system (Microlab 2,000). Detection limits were 0.25 ng/ml (plasma) and 4 ng/ml (urine) in 0.1 ml sample. The coefficient of within-assay variation was 4% or better in the range 2-100 ng/ml (plasma) or 30-1,000 ng/ml (urine), the between-assay CV was 5.3% or better in the range 5-90 ng/ml (plasma) or 40-660 ng/ml (urine). Excellent correlation was observed between the plasma monoclonal RIA and the hitherto used polyclonal RIA (n = 106, r = 0.994), and between the urine monoclonal RIA and HPLC (n = 82, r = 0.998). We expect that these assays will prove valuable in pharmacokinetic and pharmacological investigations of pirenzepine.

Comparison of specific antibody, D-Phe-Pro-Arg-CH2Cl and aprotinin for prevention of in vitro effects of recombinant tissue-type plasminogen activator on haemostasis parameters.

In vitro, concentration-dependent effects of rt-PA on a range of coagulation and fibrinolytic assays in thawed plasma samples were investigated. In absence of a fibrinolytic inhibitor, 2 micrograms rt-PA/ml blood (3.4 micrograms/ml plasma) caused prolongation of clotting time assays and decreases of plasminogen (to 44% of the control value), fibrinogen (to 27%), alpha 2-antiplasmin (to less than 5%), FV (to 67%), FVIII (to 41%) and FXIII (to 16%). Of three inhibitors tested, a specific polyclonal anti-rt-PA antibody prevented interferences in all fibrinolytic and most clotting assays. D-Phe-Pro-Arg-CH2Cl (PPACK) enabled correct assays of fibrinogen and fibrinolytic parameters but interfered with coagulometric assays dependent on endogenous thrombin generation. Aprotinin was suitable only for a restricted range of both assay types. Most in vitro effects were observed only with rt-PA plasma concentrations in excess of therapeutic values. Nevertheless it is concluded that for clinical application, collection of blood samples on either specific antibody or PPACK is essential for a correct assessment of in vivo effects of rt-PA on the haemostatic system in patients undergoing fibrinolytic therapy.

Pharmacokinetics and effects on fibrinolytic and coagulation parameters of two doses of recombinant tissue-type plasminogen activator in healthy volunteers.

Pharmacokinetics and pharmacological effects of two intravenous doses of recombinant tissue-type plasminogen activator (rt-PA) (40 and 60 mg over 90 min) were determined in healthy volunteers. Mean maximum plasma concentrations were 1080 and 1560 ng/ml respectively. The steady state level during subsequent maintenance infusion of 30 mg over 6 h was 250 ng/ml. The pharmacokinetics of rt-PA showed a bi-exponential disappearance from plasma consistent with a 2-compartment model of t1/2 alpha = 5.7 min, a t1/2 beta = 1.3 h and a total clearance of 380 ml/min. Mean fibrinogen levels at the end of the infusions of 40 mg or 60 mg rt-PA over 90 min, measured in thawed plasma samples collected on citrate/aprotinin, decreased to 74% and 57% of the preinfusion values respectively. Plasminogen fell to 55% and 48%, and alpha 2-antiplasmin to 28% and 18% of initial values. No further decrease of these parameters was observed during the infusion of 30 mg rt-PA over 6 h. Only 2% of the preinfusion fibrinogen levels could be recovered as fibrinogen-fibrin degradation products. This moderate extent of systemic fibrinogenolysis is much less than that reported for therapeutic i.v. infusions of streptokinase.

Pharmacokinetics and haemostatic status during consecutive infusions of recombinant tissue-type plasminogen activator in patients with acute myocardial infarction.

Pharmacokinetics and systemic effects of recombinant tissue-type plasminogen activator (rt-PA) were determined during coronary thrombolysis in 12 acute myocardial infarction patients using a consecutive intravenous infusion regimen. Ten mg rt-PA were infused in 2 minutes resulting in a peak plasma concentration (mean +/- SD) of 3310 +/- 950 ng/ml, followed by 50 mg in 1 h and 30 mg in 1.5 h yielding steady state plasma levels of 2210 +/- 470 ng/ml and 930 +/- 200 ng/ml, respectively. All patients received intravenous heparin. Total clearance of rt-PA was 380 +/- 74 ml/min, t1/2 alpha was 3.6 +/- 0.9 min and t1/2 beta was 16 +/- 5.4 min. After 90 min, in plasma samples containing anti-rt-PA-IgG to inhibit in vitro effects, fibrinogen was decreased to 54%, plasminogen to 52%, alpha 2-antiplasmin to 25%, alpha 2-macroglobulin to 90% and antithrombin III to 85% of initial values. Coagulation times were prolonged and fibrin D-dimer concentrations increased from 0.40 to 2.7 micrograms/ml. It is concluded that pharmacokinetics of rt-PA show low interpatient variability and that its short mean residence time in plasma allows precise control of therapy. Apart from its moderate effect on the haemostatic system, rt-PA appears to lyse a fibrin pool in addition to the coronary thrombus.

Influence of acute myocardial infarction and rt-PA therapy on circulating fibrinogen.

In 12 patients treated with 100 mg rt-PA/3 h for acute myocardial infarction (AMI), serial fibrinogen levels were measured with the Clauss clotting rate assay ("functional fibrinogen") and with a new enzyme immunoassay for immunologically intact fibrinogen ("intact fibrinogen"). Levels of functional and "intact fibrinogen" were strikingly different: functional levels were higher at baseline; showed a more pronounced breakdown during rt-PA therapy; and a rebound phenomenon which was not seen for "intact fibrinogen". The ratio of functional to "intact fibrinogen" was calculated for each individual patient and each time point. The mean ratio (n = 12) was 1.6 at baseline, 1.0 at 90 min, and increased markedly between 8 and 24 h to a maximum of 2.1 (p < 0.01), indicating that functionality of circulating fibrinogen changes during AMI and subsequent thrombolytic therapy. The increased ratio of functional to "intact fibrinogen" seems to reflect a more functional fibrinogen at baseline and following rt-PA infusion. This is in keeping with data that the relative amount of fast clotting "intact HMW fibrinogen" of total fibrinogen is increased in initial phase of AMI. The data suggest that about 20% of HMW fibrinogen are converted to partly degraded fibrinogen during rt-PA infusion. The rebound phenomenon exhibited by functional fibrinogen may result from newly synthesized fibrinogen with a high proportion of HMW fibrinogen with its known higher degree of phosphorylation. Fibrinogen- and fibrin degradation products were within normal range at baseline. Upon infusion of the thrombolytic agent, maximum median levels of 5.88 micrograms/ml and 5.28 micrograms/ml, respectively, were measured at 90 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Low incidence of antibodies to recombinant human tissue-type plasminogen activator in treated patients.

Sera from over 1,600 patients who received recombinant human tissue plasminogen activator (rt-PA) during clinical trials were assessed for the presence of antibodies to this therapeutic agent. The rt-PA was administered by a variety of dosage regimens for several different indications. Two different forms of rt-PA were used; one was predominantly two chain form, and the other was a predominantly one chain form. A sensitive radioimmunoprecipitation assay was used to measure antibodies to rt-PA in patients' serum before and after treatment. Of 932 patients tested with this assay, 929 were negative for antibodies to t-PA. Three patients developed low titers after treatment. Additional serum samples were obtained from these three patients within 2 years after rt-PA therapy and were negative for antibodies to t-PA. With the limited number of positive samples, no correlation could be found with dose or type of rt-PA, dosing regimen or clinical diagnosis. The virtual absence of antibody formation was confirmed in an additional 754 patients using a novel competitive two-site ELISA. It can be concluded that a single infusion of rt-PA was virtually unassociated with antibody formation, suggesting that repeat treatments could be given when necessary without the risk of immunologic complications as are seen with streptokinase or its derivatives.

Single-bolus injection of recombinant tissue-type plasminogen activator in acute myocardial infarction.

Recombinant tissue-type plasminogen activator (rt-PA) has hitherto been administered in acute myocardial infarction as an intravenous infusion with an initial bolus of about 10% of the total dose, both due to its short half-life and to avoid possible early reocclusions. A single-bolus dose would simplify the therapeutic regimen. Therefore, 20 patients with symptom duration of 125 +/- 58 minutes were given a single bolus of 50 mg of rt-PA over 2 minutes. Coronary angiography 60 minutes after the rt-PA bolus revealed a patent infarct-related artery in 15 of 20 patients (patency rate 75%, 95% confidence limits 51 to 91%). In the remaining patients, reperfusion was achieved by coronary angioplasty and intracoronary fibrinolysis; in 2 patients coronary artery bypass grafting was necessary. Control angiograms at 24 hours showed reocclusions in 4 of 18 patients. One woman died due to an intracranial hemorrhage 48 hours after the rt-PA bolus injection. Circulating fibrinogen decreased from 2.7 +/- 0.5 to 1.5 +/- 0.9 g/liter after 2 to 4 hours and reached the initial value within 24 hours. Pharmacokinetic parameters were obtained in 7 patients by measuring rt-PA antigen levels in multiple plasma samples. Mean peak rt-PA concentration was 9.8 +/- 3.6 micrograms/ml, total plasma clearance 476 +/- 148 ml/min and dominant half-life 4.8 +/- 1.0 minutes. Thus, rt-PA administered as a 50-mg single bolus appears to provide similar patency rates and shows similar kinetics in comparison with the conventional infusion regimen. Assessment of the incidence of bleeding complications requires further studies.

Pharmacokinetics and pharmacodynamics of tenecteplase: results from a phase II study in patients with acute myocardial infarction.

Tenecteplase is a site-specific engineered tissue plasminogen activator (t-PA) variant that can be administered as a single intravenous bolus injection because of its slower plasma clearance. The objective of this study was to investigate the dose-ranging pharmacokinetics and pharmacodynamics of intravenous bolus tenecteplase compared with intravenous alteplase recombinant t-PA in patients with acute myocardial infarction. A total of 103 patients received intravenous bolus doses of 30, 40, or 50 mg tenecteplase, and 56 patients received 100 mg rt-PA as the accelerated 90-minute infusion regimen in this randomized, open-label study. Tenecteplase and r-tPA plasma concentrations were measured for 6 hours. Tenecteplase exhibited biphasic elimination from the plasma with a mean initial half-life of 22 minutes and a mean terminal half-life of 115 minutes. The mean plasma clearance was 105 mL/min and did not depend on tenecteplase dose over the dose range studied. In comparison, rt-PA has a fourfold faster plasma clearance. Pharmacokinetic-pharmacodynamic evaluation showed that a dose of approximately 0.5 mg/kg results in a plasma AUC value that provides a TIMI 3 flow at 90 minutes that is comparable to that reported with accelerated r-tPA. In conclusion, tenecteplase has a fourfold slower plasma clearance compared with rt-PA, allowing dosing as an i.v. bolus injection. Weight-adjusted dosing of tenecteplase may optimize the therapeutic regimen of tenecteplase.

Thrombolysis using consecutive high dose bolus and infusion of alteplase in a patient with acute massive pulmonary embolism.

We describe the successful use of a novel dosage regimen of alteplase in a 77-year-old man with massive acute pulmonary embolism in the presence of a relative contraindication (recent cranial surgery). An intravenous alteplase bolus of 50 mg was administered, followed 40 min later by a 50 mg infusion over 2 h. Pulmonary arteriography showed a considerable increase in intraluminal filling of thrombotically occluded vessels as a result of alteplase administration, and was accompanied by marked clinical improvement. Peak alteplase plasma concentration after the bolus was 5.5 micrograms/ml, and the steady state concentration during infusion was 0.52 micrograms/ml. Detailed monitoring of haemostasis parameters showed elevated endogenous fibrinolytic activity at baseline, very high levels of fibrin degradation products during treatment resulting from lysis of an extensive thrombus mass, moderate fibrinogenolysis compared to fibrinolysis, long plasma half-lives of fibrin(ogen) degradation products (9.2-11.0 h), and increased thrombin generation. There were no bleeding side-effects. The regimen combines the advantages of rapid onset (bolus) and maintained potency (infusion) of thrombolytic effect in pulmonary embolism.

Studies on the functionality of fibrinogen during rt-PA therapy: results of three different methods of fibrinogen determination.

The conversion of soluble fibrinogen to insoluble fibrin is a crucial event in the process of haemostasis. In the present study three different methods were used for the determination of plasma fibrinogen levels in 12 patients with acute myocardial infarction during rt-PA therapy. Two methods were based on fibrin formation and thus measured 'functional' fibrinogen: a clotting rate assay according to the Clauss method and a photometric polymerization rate assay. Thirdly, fibrinogen antigen was measured with a new enzyme immunoassay (EIA) for immunologically intact fibrinogen. The levels of functional and intact fibrinogen were strikingly different: 4.1 g/l (Clauss) and 3.6 g/l (photometric) vs 2.6 g/l (EIA) at baseline, 2.2 g/l (54% of preinfusion value) and 2.1 g/l (59%) vs 2.1 g/l (81%) at 90 min, 5.7 g/l (146%) and 4.5 g/l (130%) vs 2.7 g/l (106%) at 72 h. The rebound phenomenon exhibited by functional fibrinogen (130-150%) was not observed for levels of intact fibrinogen. A ratio was calculated of functional (Clauss method) to intact fibrinogen (EIA) for each individual patient and each time point. The mean ratio (n = 12 patients) was 1.6 at baseline, 1.0 at 90 min, and increased markedly between 8 and 24 h to a maximum of 2.1 (P less than 0.01). Our data indicate that the 'functionality', i.e. the clotting rate per unit concentration of circulating fibrinogen, changed during acute myocardial infarction and subsequent thrombolytic therapy. This is in keeping with data from literature that the relative amount of fast clotting high molecular weight (HMW) fibrinogen of total fibrinogen was increased in the initial phase of acute myocardial infarction.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic-pharmacodynamic and metabolite modeling with TopFit.

Two examples illustrating the ease of use and powerful data fitting and simulation techniques provided by the validated program TopFit 2.0 for the PC are presented. In the first, kinetics of parent compound and metabolite for (+) and (-) enantiomers of a racemic compound X were determined during a Phase III clinical study. Four data sets were fitted simultaneously for each patient. The model could be defined by the user without programming differential equations. The fit results indicated enantiomer specific kinetics for the metabolite but not for parent compound. In the second example, a model with nonlinear elimination and an Emax-effect function was used to simultaneously fit data from six doses of compound Y in a Phase I study. The fitted parameters predicted the feasibility of a twice-daily dose regimen despite a very short plasma half-life of the compound. In conclusion, TopFit provides rapid and cost-effective support in analysis and design of clinical trials.

TopFit: a PC-based pharmacokinetic/pharmacodynamic data analysis program.

The program TopFit was developed and validated within the European pharmaceutical industry. It provides both pharmacokinetic data analysis support for international regulatory submissions of new drugs, and sophisticated techniques for model-based kinetic/dynamic evaluation during drug development. TopFit features are: (1) non-compartmental methods; (2) standard compartment models assembled from input and disposition modules; (3) a potentially unlimited number of linear user-defined models that accommodate metabolites, effects, and absorption profiles; (4) a library of 24 non-linear models. No user programming is required. A well-defined file structure allows ready exchange of data with other programs such as SAS. TopFit version 2.0 is now commercially available, with comprehensive documentation, in the form of an MS-DOS application for the PC.

Current developments in thrombolytic therapy using novel plasminogen activators.

As a result of intensive recent research, the molecular mechanisms governing the human fibrinolytic system could be elucidated. Clinical trials using streptokinase and urokinase demonstrated that therapeutic thrombolysis is a valuable regimen in the treatment of patients with thromboembolic diseases. After unsuccessful attempts to increase the clot specificity of these agents, efforts were increased towards developing fibrin-specific substances with high thrombolytic potency and negligible effects on systemic hemostasis. Tissue-type plasminogen activator manufactured by recombinant DNA technology (rt-PA) is currently the most comprehensively investigated fibrin-specific thrombolytic agent with the most clearly understood mechanism of action in vivo. Pro-urokinase has also become available through biotechnology. A large number of clinical trials studying thrombolytic therapy of myocardial infarction have proved that intravenous rt-PA posesses superior efficacy compared to conventional fibrinolytic agents. Comparatively few clinical studies have been performed using pro-urokinase. At therapeutic doses, the fibrin specificities of these agents differ. So far it has not been possible to correlate the incidence of bleeding complications during fibrinolytic therapy with alterations in hemostasis parameters. For routine laboratory monitoring of thrombolysis the determination of fibrinogen and thrombin clotting time can be recommended. New developments with potential for yielding the next generation of thrombolytic agents are mutants and chimeras of t-PA and pro-urokinase, and conjugates of these substances with fibrin specific antibodies.

Properties of molecular variants of tissue-type plasminogen activator.

Tissue-type plasminogen activator (t-PA), an important component of the fibrinolytic system, is now available as a biotechnologically manufactured recombinant protein for therapeutic use. It has proved highly effective in the clinical therapy of acute thromboembolic diseases such as myocardial infarction and pulmonary embolism. t-PA activates plasminogen to plasmin, which subsequently dissolves the fibrin network of a blood clot. This activation by t-PA occurs selectively on the clot surface, with negligible systemic side effects. The half-life of t-PA in vivo is in the order of minutes due to rapid hepatic elimination. t-PA is a glycoprotein with serine protease activity and consists of a polypeptide chain with 527 amino acids. Recently, intensive research efforts have been devoted to modification of the molecular structure of t-PA, with the objective of further increasing fibrin specificity and catalytic activity, or reducing the rate of elimination. As a result, considerable insights into structure-function relationships within the t-PA molecule have been gained, but as yet no clinically utilizable variant has been constructed which is in all respects superior to naturally occurring t-PA.

Pharmacokinetics and hepatic catabolism of tissue-type plasminogen activator.

Recombinant tissue-type plasminogen activator (rt-PA, alteplase, Actilyse, Activase; CAS 105857-23-6) is the most effective agent currently available for thrombolytic therapy of life-threatening diseases such as acute myocardial infarction. It acts by rapid, clot-specific lysis of pathological thrombi, with only limited effects on systemic hemostasis. Pharmacokinetics of rt-PA have been extensively characterized in animal species and man, and can be generally described by a 3-compartment model. Preferred analytical methods for rt-PA in plasma are ELISA and chromogenic activity assays. The dominant plasma half-life of rt-PA in myocardial infarction patients is short (3.6 min), which allows excellent control of plasma levels during therapy. Steady-state plasma concentrations effecting coronary thrombolysis using the current dosage regimen are 2.2 micrograms/ml. A deep compartment results in elevated rt-PA concentrations several hours after termination of infusions, which may contribute to short-term maintenance of patency of reperfused blood vessels. Clearance of rt-PA can be saturated in animals at very high plasma concentrations (Km = 12-15 micrograms/ml), however, pharmacokinetics in clinical settings are linear. Clearance occurs via hepatic receptor mediated endocytosis and intracellular degradation in liver parenchymal, endothelial and Kupffer cells. The catabolism involves coated pits, coated vesicles, endosomes, and finally degradation in lysosomes. Current evidence supports the existence of hepatic receptors recognizing carbohydrate as well as polypeptide determinants in rt-PA. In conclusion, increasing knowledge of rt-PA pharmacokinetics will contribute to the optimization of new clinical dosage regimens, such as front-loaded infusions and boluses, and to the identification of novel molecular targets for pharmacologic control of rt-PA catabolism and of circulating fibrinolytic activity.