Platelet function and fibrinolytic agents: two sides of a coin?

Comparative Analysis of Therapeutic Efficacy and Adverse Reactions among Various Thrombolytic Agents

Thrombosis is a major health concern that contributes to the development of several cardiovascular diseases and a significant number of fatalities worldwide. While stent surgery is the current recommended treatment according to the guidelines, percutaneous coronary intervention (PCI) is the optimal approach for acute myocardial infarction (AMI). However, in remote areas with limited resources, PCI procedures may not be feasible, leading to a delay in treatment and irreversible outcomes. In such cases, preoperative thrombolysis becomes the primary choice for managing AMI in remote settings. The market for thrombolytic drugs is continuously evolving, and identifying a safe and effective thrombolytic agent for treating AMI is crucial. This study evaluated Urokinase, Alteplase, and Recombinant Human TNK Tissue-type Plasminogen Activator for Injection (rhTNK) as representatives of first-, second-, and third-generation thrombolytic drugs, respectively. The research included in vitro thrombolysis experiments, exposure of human cardiomyocytes, zebrafish tail vein injections, and vascular endothelial transgenic zebrafish models. The findings revealed that rhTNK is the most effective thrombolytic drug with the least adverse effects and lowest bleeding rate, highlighting its potential as the preferred treatment option for AMI. The order of thrombolytic effectiveness was Urokinase < Alteplase < rhTNK, with adverse effects on cardiomyocytes post-thrombolytic therapy ranking similarly as Urokinase < Alteplase < rhTNK, while the bleeding rate after thrombolysis followed the order of Urokinase > Alteplase > rhTNK.

Lysis Onset Time as Diagnostic Rotational Thromboelastometry Parameter for Fast Detection of Hyperfibrinolysis

Background:

Rotational thromboelastometry is increasingly used to detect hyperfibrinolysis, which is a predictor of unfavorable outcome in patients with coagulation disturbances. In an in vitro study, the authors investigated which thromboelastometric hemostatic parameters could be valuable for fast diagnosis of the severity of hyperfibrinolysis and confirmed their findings in a patient population with hyperfibrinolysis.

Methods:

Thromboelastometry was performed after adding increasing concentrations of tissue plasminogen activator (0 to 400 ng/ml) to citrated blood samples of 15 healthy volunteers. Lysis parameters included the clotting time, maximum clot firmness, maximum lysis, and lysis onset time (LOT). The relation of tissue plasminogen activator with the LOT was further investigated in a patient population with out-of-hospital cardiac arrest and hyperfibrinolysis.

Results:

The LOT showed a dose-dependent association with increasing tissue plasminogen activator concentrations. Late, intermediate, or fulminant hyperfibrinolysis was associated with an average LOT (mean ± SD) of 42.7 ± 13.8, 23.2 ± 8.2, and 17.5 ± 4.6 min in the in vitro study and estimated 42.2 ± 8.3, 29.1 ± 1.2, and 14.6 ± 7.7 min in patients, respectively. The authors found a moderately negative correlation between patient plasma tissue plasminogen activator levels and the LOT (r = −0.67; P = 0.01).

Conclusion:

This study shows that the LOT may be used for fast detection of severe hyperfibrinolysis, with a better resolution than the maximum lysis, and should be further evaluated for optimization of therapeutic strategies in patients with severe clot breakdown.

Synthesis of novel peptide inhibitors of thrombin-induced platelet activation

Inhibitors of the activation of platelet aggregation have promise as important therapeutic agents for the management of acute coronary syndrome (ACS). Platelet activation by thrombin, a serine protease, occurs by binding to and cleavage of the extracellular N-terminal domains of protease-activated receptors 1 and 4 (PAR1 and PAR4). The proteolysis of the PARs exposes new tethered ligands that then signal through transmembrane domains to initiate platelet activation as a downstream effect. A pentapeptide cleavage product of bradykinin with the sequence Arg-Pro-Pro-Gly-Phe serves as a thrombin inhibitor by blocking alpha- and gamma-thrombin-induced platelet aggregation. Analogs of RPPGF have been prepared that result in improved inhibition of thrombin activation of platelets. Specific amino acid residues required for activity against platelet aggregation have been identified, and a lead compound, rOicPaPhe(p-Me)-NH(2) (FM19), has been developed. FM19, which completely inhibits threshold gamma-thrombin-induced platelet aggregation at a concentration of 16 +/- 4 microm, represents an important lead compound in the development of inhibitors of thrombin-mediated platelet aggregation for treatment of ACS.

The platelet-related effects of tenecteplase versus alteplase versus reteplase

Clinical studies have investigated the combination of glycoprotein (GP) IIb/IIIa inhibitors and thrombolytic agents for acute myocardial infarction. However, thrombolytic agents alone may possess direct antiplatelet properties that could affect reperfusion. Blood from 11 patients with coronary disease and five healthy subjects was incubated for 30 min with tenecteplase (4, 12, and 24 microg/ml), alteplase (1, 4, and 10 microg/ml), reteplase (1, 5, and 10 microg/ml) or control buffer. Platelet aggregation induced by 1, 20 and 50 micromol/l adenosine diphosphate (ADP), the stimulated expression of GP IIb/IIIa and P-selectin, and plasma fibrinogen levels were determined. Platelet aggregation in patients was inhibited by medium and high concentrations of alteplase when induced by 1 micromol/l ADP [1.6 +/- 0.5%, P = 0.001 and 0.9 +/- 0.2%, P = 0.002 versus 8.3 +/- 1.6% (control)] and 20 micromol/l ADP [46.9 +/- 3.9%, P = 0.001 and 46.2 +/- 4.8%, P = 0.001 versus 65.7 +/- 2.7% (control)]. High concentration tenecteplase was associated with lower aggregation by 20 micromol/l ADP (58 +/- 2.1% versus control, P = 0.033). There were no changes in GP IIb/IIIa activation or P-selectin expression in patients or healthy subjects. Platelet aggregation (1 micromol/l ADP) in healthy subjects was inhibited only by high doses of alteplase (P = 0.001). Plasma fibrinogen levels were significantly decreased after treatment with reteplase at 1 microg/ml(1.53 +/- 0.21 versus 2.65 +/- 0.31, P = .009) and 5 microg/ml(1.55 +/- 0.16 versus 2.65 +/- 0.31, P = .005). Alteplase inhibits platelet aggregation more than tenecteplase and reteplase. The attenuation of platelet aggregation by alteplase is dissociated from the expression of activated GP IIb/IIIa and P-selectin, and by fibrinogen degradation. These results suggest that alteplase exerts its antiplatelet effect independent of GP IIb/IIIa and P-selectin expressions and fibrinogen degradation. These findings may be directly relevant to the effect of alteplase on reperfusion and to future studies using combined platelet inhibitors and thrombolytic therapy.

The effect of anticoagulation with subcutaneously delivered polyethylene glycol conjugated hirudin and recombinant tissue plasminogen activator on recurrent stenosis in the rabbit double-balloon injury model

Myointimal hyperplasia is the condition usually responsible for recurrent stenosis (restenosis) after endarterectomy, bypass grafting and angioplasty. Its cause is still not known. The present study examined whether inhibition of thrombin by tissue plasminogen activator (r-TPA) or polyethylene glycol recombinant hirudin (PEG-hirudin) could reduce restenosis in an animal model. Restenosis was induced in 20 cholesterol-fed rabbits. The right carotid artery underwent a double-balloon injury while left carotid artery acted as a control. Recombinant tissue plasminogen activator (1 mg kg(-1) s.c.) and PEG-hirudin (0.7 mg kg(-1) s.c.) were given subcutaneously with normal saline acting as a control. Blood levels of PEG-hirudin were measured by both ELISA and an Ecarin (activity) assay. Vessel dimensions were measured in histological sections, obtained from perfusion-fixed tissue, using computerised planimetry. The model reproduced many of the histological changes found in human restenosis, such as intramural thrombus, rupture of the elastic lamina, macrophage infiltration and smooth muscle migration. Reinjury caused an almost three-fold reduction in the area of the lumen (median 0.25 mm(2)) compared with uninjured vessels (median 0.72 mm(2)). The mean plasma levels of PEG-hirudin and r-tPA achieved were 291 ng/ml (S.E.M. 28 ng/ml) and 34 IU/ml (S.E.M. 12 IU/ml), respectively. PEG-hirudin significantly inhibited the effect of balloon injury on luminal area compared with saline-treated controls (0.21 versus 0.44 mm(2), respectively, P<0.05). Recombinant tPA also had a similar inhibitory affect, but this did not reach statistical significance (0.16 versus 0.44 mm(2), respectively, P>0.05). The magnitude of luminal narrowing was significantly reduced by subcutaneous injection of PEG-hirudin. Further studies are required to determine whether this effect can be enhanced by other antithrombins or improved methods of delivery.

Plasminogen activator inhibitor-1 in patients with atrial arrhythmias during acute myocardial infarction, treated with streptokinase

Atrial arrhythmias (AA), especially atrial fibrillation (AF), during acute myocardial infarction (AMI) are often associated with increased mortality and heart failure. Impaired fibrinolysis with elevated plasminogen activator inhibitor-1 (PAI-1) activity is associated with resistance to fibrinolytic therapy in AMI patients, but it is also found in patients with AF. Our aim was a prospective study of the role of pre-treatment PAI-1 levels for the presence of AA in AMI patients and the influence of AA on in-hospital mortality. In 116 AMI patients, treated with streptokinase, pre-treatment PAI-1 levels were estimated by the chromogenic method (normal levels, 0.3-3.5 U/ml) and in-hospital AA were assessed as atrial fibrillation, flutter and/or tachycardias. Between patients with and without AA, a significant difference was observed in mean pre-treatment PAI-1 levels, in several in-hospital complications and mortality (24 versus 4.4%; P < 0.01; odds ratio, 6.45; 95% confidence interval, 1.66-25.0). The PAI-1 level > 7 U/ml was the most significant independent pre-treatment risk factor for AA (P < 0.05; odds ratio, 3.5; 95% confidence interval, 1.15-10.6). We conclude that AA were a significant risk for in-hospital mortality of AMI patients, treated with streptokinase. A pre-treatment PAI-1 level > 7 U/ml was the most significant pre-treatment risk for AA in these patients.

Effects of different thrombolytic treatment regimen with abciximab and tirofiban on platelet aggregation and platelet-leukocyte interactions: a subgroup analysis from the GUSTO V and FASTER trials

BACKGROUND

Due to considerably high rates of reocclusion under standard thrombolytic therapy GP IIb/IIIa inhibitors have been combined with thrombolytics to improve therapeutic outcomes. Potential reasons for arterial reocclusion may be increased platelet activation, interaction of platelets with other cell types such as leukocytes and inadequate drug dosing due to lack of ideal platelet monitoring. We compared combination therapy regimens consisting of GP IIb/IIIa inhibitors and thrombolytics with respect to platelet inhibition and platelet-leukocyte interactions.

METHODS AND RESULTS

From the GUSTO V trial (standard rPA vs. reduced dose rPA and abciximab) and the FASTER trial (standard TNK-tPA vs. reduced dose TNK-tPA and tirofiban) 15 patients were monitored by platelet aggregometry, rapid platelet function assay (RPFA) and flow cytometry (FC). rPA alone (n = 5) caused initial increases in platelet aggregation. However, platelet aggregation was significantly (p < 0.05) and sufficiently (>80%) inhibited by abciximab/rPA (n = 5) and tirofiban/TNK-tPA (n = 5). The platelet inhibitory effect of tirofiban/TNK-tPA was more pronounced compared to abciximab/rPA with a significant difference after 2 h (p < 0.05). Tirofiban/TNK-tPA and abciximab/rPA caused decreases in platelet-leukocyte aggregates as well as in binding of specific antibodies to the platelet vitronectin receptor and P-selectin (p < 0.05, respect.). No differences among the treatment groups were seen with respect to antibody binding to MAC-1 and CD154/CD40 ligand.

CONCLUSIONS

Taken together, GP IIb/IIIa inhibitors overcome the platelet activating effect of thrombolytics resulting in sufficient platelet inhibition. RPFA is a suitable monitoring tool to accurately assess platelet inhibition. Within the given combination treatment regimen tirofiban appears to be more effective compared to abciximab and to exert effects beyond the inhibition of GP IIb/IIIa.