Regulation of tissue-type plasminogen activator concentrations by clearance via the mannose receptor and other receptors

Pharmacokinetics and pharmacodynamics of tenecteplase in fibrinolytic therapy of acute myocardial infarction

Tenecteplase is a novel fibrinolytic protein bioengineered from human tissue plasminogen activator (alteplase) for the therapy of acute ST-segment elevation myocardial infarction. Specific mutations at three sites in the alteplase molecule result in 15-fold higher fibrin specificity, 80-fold reduced binding affinity to the physiological plasminogen activator inhibitor PAI-1 and 6-fold prolonged plasma half-life (22 vs 3.5 minutes). Consequently, tenecteplase can be administered as a single intravenous bolus of 30-50mg (0.53 mg/kg bodyweight) over 5-10 seconds, in contrast to the 90-minute accelerated infusion regimen of alteplase. Tenecteplase plasma concentration-time profiles have been obtained from a total of 179 patients with acute myocardial infarction. Tenecteplase exhibited biphasic disposition; the initial disposition phase was predominant with a mean half-life of 17-24 minutes, and the mean terminal half-life was 65-132 min. Over the clinically relevant dose range of 30-50mg, mean clearance (CL) was 105 ml/min. The mean initial volume of distribution V(1) was 4.2-6.3L, approximating plasma volume, and volume of distribution at steady state was 6.1-9.9L, suggesting limited extravascular distribution or binding. Bodyweight and age were found to influence significantly both CL and V(1). Total bodyweight explained 19% of the variability in CL and 11% of the variability in V(1), and a 10kg increase in total bodyweight resulted in a 9.6 ml/min increase in CL. This relationship aided the development of a rationale for the weight-adjusted dose regimen for tenecteplase. Age explained only a further 11% of the variability in CL. The percentage of patients who achieved normal coronary blood flow was clearly related to AUC. More than 75% of patients achieved normal flow at 90 minutes after administration when their partial AUC(2-90) exceeded 320 microg.min/ml, corresponding to an average plasma concentration of 3.6 microg/ml. Systemic exposure to tenecteplase at all times after bolus administration of 30-50mg was higher than for alteplase 100mg. Tenecteplase has demonstrated equivalent efficacy and improved safety compared with the current gold standard alteplase in a large mortality trial (ASSENT-2). This suggests that the reduced clearance, greater fibrin specificity and higher PAI-1 resistance of tenecteplase allow higher plasma concentrations and thus a more rapid restoration of coronary patency to be attained, while providing a reduction in major non-cerebral bleeding events.

The fibrinolytic enzyme system. Basic principles and links to venous and arterial thrombosis

This article briefly describes some important aspects of the fibrinolytic system, its regulation, and possible disturbances of this system in connection with deep vein thrombosis and myocardial infarction.

Increased Clearance Explains Lower Plasma Levels of Tissue-Type Plasminogen Activator by Estradiol: Evidence for Potently Enhanced Mannose Receptor Expression in Mice

Several clinical studies have demonstrated an inverse relationship between circulating levels of estrogen and tissue-type plasminogen activator (t-PA). The present study was designed to test the hypothesis that estrogens lower plasma levels of t-PA by increasing its clearance from the bloodstream. 17-Ethinyl estradiol (EE) treatment resulted in a significant increase in the clearance rate of recombinant human t-PA in mice (0.46 mL/min in treated mice v 0.32 mL/min in controls; P < .01). The clearance of endogenous, bradykinin-released t-PA in rats was also significantly increased after EE treatment (area under the curve [AUC], 24.9 ng/mL · min in treated animals v 31.9 ng/mL · min in controls; P < .05). Two distinct t-PA clearance systems exist in vivo: the low-density lipoprotein receptor-related protein (LRP) on liver parenchymal cells and the mannose receptor on mainly liver endothelial cells. Inhibition of LRP by intravenous injection of receptor-associated protein (RAP) as a recombinant fusion protein with Salmonella japonicum glutathione S-transferase (GST) significantly retarded t-PA clearance in control mice (from 0.41 to 0.25 mL/min; n = 5, P < .001) and EE-treated mice (from 0.66 to 0.35 mL/min; n = 5, P < .005), but did not eliminate the difference in clearance capacity between the 2 experimental groups. Similar results were obtained in mice in which LRP was inhibited via overexpression of the RAP gene in liver by adenoviral gene transduction. In contrast, administration of mannan, a mannose receptor antagonist, resulted in identical clearances (0.22 mL/min in controls and 0.24 mL/min in EE-treated mice). Northern blot analysis showed a 6-fold increase in mannose receptor mRNA expression in the nonparenchymal liver cells of EE-treated mice, whereas the parenchymal LRP mRNA levels remained unchanged. These findings were confirmed at the protein level by ligand blotting and Western blotting analysis. Our results demonstrate that EE treatment results in increased plasma clearance rate of t-PA via induction of the mannose receptor and could explain for the inverse relationship between estrogen status and plasma t-PA concentrations as observed in humans.

Increased Clearance Explains Lower Plasma Levels of Tissue-Type Plasminogen Activator by Estradiol: Evidence for Potently Enhanced Mannose Receptor Expression in Mice

Several clinical studies have demonstrated an inverse relationship between circulating levels of estrogen and tissue-type plasminogen activator (t-PA). The present study was designed to test the hypothesis that estrogens lower plasma levels of t-PA by increasing its clearance from the bloodstream. 17-Ethinyl estradiol (EE) treatment resulted in a significant increase in the clearance rate of recombinant human t-PA in mice (0.46 mL/min in treated mice v 0.32 mL/min in controls; P &lt; .01). The clearance of endogenous, bradykinin-released t-PA in rats was also significantly increased after EE treatment (area under the curve [AUC], 24.9 ng/mL · min in treated animals v 31.9 ng/mL · min in controls; P &lt; .05). Two distinct t-PA clearance systems exist in vivo: the low-density lipoprotein receptor-related protein (LRP) on liver parenchymal cells and the mannose receptor on mainly liver endothelial cells. Inhibition of LRP by intravenous injection of receptor-associated protein (RAP) as a recombinant fusion protein with Salmonella japonicum glutathione S-transferase (GST) significantly retarded t-PA clearance in control mice (from 0.41 to 0.25 mL/min; n = 5, P &lt; .001) and EE-treated mice (from 0.66 to 0.35 mL/min; n = 5, P &lt; .005), but did not eliminate the difference in clearance capacity between the 2 experimental groups. Similar results were obtained in mice in which LRP was inhibited via overexpression of the RAP gene in liver by adenoviral gene transduction. In contrast, administration of mannan, a mannose receptor antagonist, resulted in identical clearances (0.22 mL/min in controls and 0.24 mL/min in EE-treated mice). Northern blot analysis showed a 6-fold increase in mannose receptor mRNA expression in the nonparenchymal liver cells of EE-treated mice, whereas the parenchymal LRP mRNA levels remained unchanged. These findings were confirmed at the protein level by ligand blotting and Western blotting analysis. Our results demonstrate that EE treatment results in increased plasma clearance rate of t-PA via induction of the mannose receptor and could explain for the inverse relationship between estrogen status and plasma t-PA concentrations as observed in humans.