In vitro effect of plasmin on human platelet function in plasma. Inhibition of aggregation caused by fibrinogenolysis

Differentiating Pathologic from Physiologic Fibrinolysis: Not as Simple as Conventional Thrombelastography

BACKGROUND

Conventional rapid thrombelastography (rTEG) cannot differentiate fibrinolysis shutdown from hypofibrinolysis, as both of these patient populations have low fibrinolytic activity. Tissue plasminogen activator (tPA) TEG can identify depletion of fibrinolytic inhibitors, and its use in combination with rTEG has the potential to differentiate all 3 pathologic fibrinolytic phenotypes after trauma. We hypothesize tPA-TEG and rTEG in combination can further stratify fibrinolysis phenotypes postinjury to better stratify risk for mortality.

STUDY DESIGN

Adult trauma patients (981) with both rTEG and tPA-TEG performed less than 2 hours postinjury were included. rTEG lysis at 30 minutes after maximum amplitude (LY30) was used to initially define fibrinolysis phenotypes (hyperfibrinolysis >3%, physiologic 0.9% to 3%, and shutdown <0.9%), with Youden Index then used to define pathologic extremes of tPA-TEG LY30 (tPA sensitive [depletion of fibrinolytic inhibitors] vs resistant) resulting in 9 groups that were assessed for risk of death.

RESULTS

The median New Injury Severity Score was 22, 21% were female, 45% had penetrating injury, and overall mortality was 13%. The tPA-TEG LY30 inflection point for increased mortality was >35.5% (tPA sensitive, odds ratio mortality 9.2, p < 0.001) and <0.3% (tPA resistance, odds ratio mortality 6.3, p = 0.04). Of the 9 potential fibrinolytic phenotypes, 5 were associated with increased mortality. Overall, the 9 phenotypes provided a significantly better prediction of mortality than rTEG or tPA-TEG alone (areas under the operating characteristics curves = 0.80 vs 0.63 and 0.75, respectively, p < 0.0001). These could be condensed to 3 pathologic phenotypes (true hyperfibrinolysis, early fibrinolysis shutdown, and hypofibrinolysis).

CONCLUSIONS

The combination of rTEG and tPA-TEG increases the ability to predict mortality and suggests patient-specific strategies for improved outcomes.

Detection of cellular traction forces via the force-triggered Cas12a-mediated catalytic cleavage of a fluorogenic reporter strand

Molecular forces generated by cell receptors are infrequent and transient, and hence difficult to detect. Here we report an assay that leverages the CRISPR-associated protein 12a (Cas12a) to amplify the detection of cellular traction forces generated by as few as 50 adherent cells. The assay involves the immobilization of a DNA duplex modified with a ligand specific for a cell receptor. Traction forces of tens of piconewtons trigger the dehybridization of the duplex, exposing a cryptic Cas12-activating strand that sets off the indiscriminate Cas12-mediated cleavage of a fluorogenic reporter strand. We used the assay to perform hundreds of force measurements using human platelets from a single blood draw to extract individualized dose-response curves and half-maximal inhibitory concentrations for a panel of antiplatelet drugs. For seven patients who had undergone cardiopulmonary bypass, platelet dysfunction strongly correlated with the need for platelet transfusion to limit bleeding. The Cas12a-mediated detection of cellular traction forces may be used to assess cell state, and to screen for genes, cell-adhesion ligands, drugs or metabolites that modulate cell mechanics.

Role of Plasminogen Activation System in Platelet Pathophysiology: Emerging Concepts for Translational Applications

Traditionally, platelets have been exclusively considered for their procoagulant and antifibrinolytic effects during normal activation of hemostasis. Effectively, activated platelets secrete coagulation factors, expose phosphatidylserine, and promote thrombin and fibrin production. In addition to procoagulant activities, platelets confer resistance of thrombi to fibrinolysis by inducing clot retraction of the fibrin network and release of huge amounts of plasminogen activator inhibitor-1, which is the major physiologic inhibitor of the fibrinolytic cascade. However, the discovery of multiple relations with the fibrinolytic system, also termed Plasminogen Activation System (PAS), has introduced new perspectives on the platelet role in fibrinolysis. Indeed, the activated membrane surface of platelets provides binding sites on which fibrinolytic enzymes can be activated. This review discusses the evidence of the profibrinolytic properties of platelets through the description of PAS components and related proteins that are contained in or bind to platelets. Our analyses of literature data lead to the conclusion that in the initial phase of the hemostatic process, antifibrinolytic effects prevail over profibrinolytic activity, but at later stages, platelets might enhance fibrinolysis through the engagement of PAS components. A better understanding of spatial and temporal characteristics of platelet-mediated fibrinolysis during normal hemostasis could improve therapeutic options for bleeding and thrombotic disorders.

Abnormal platelet activity in dogs and cats - impact and measurement

Abnormal platelet activity can either lead to bleeding tendencies or inappropriate thrombus formation and can occur secondarily to a wide variety of disease processes, with a range of clinical consequences and severity. This article will discuss the pathophysiology of platelet function abnormalities and consider a logical diagnostic approach applicable to veterinary practice. Recent advances in platelet function testing will then be discussed, with regards to detection of platelet dysfunction and tailoring of pharmacological manipulation. Although many of these tests are still confined to research or academic institutions, techniques for indirectly assessing platelet function are starting to become more widely available. Although we still require further research to develop guidelines for the use of these tests in clinical decision-making, the recent advances in this field are an exciting step forward in being able to detect and manage platelet dysfunction in both primary care and referral practice.

Predictive factors of fatal bleeding in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is associated with a profound coagulopathy. Based on retrospective assessments, several potential risk factors for hemorrhagic morbidity and mortality have emerged. Several studies have shown elevated white blood cell (WBC) count at presentation to be a robust predictor of bleeding events. Other clinical and laboratory parameters have been evaluated with variable association with hemorrhagic morbidity or mortality. These include ECOG performance status, age, morphological subtype, platelet count, peripheral blood blast count, ethnicity, body mass index, prothrombin time, activated partial thromboplastin time, lactate dehydrogenase, d-dimers, creatinine and fibrinogen levels. Unfortunately, most of those assessments were based on a small patient sample and the results have been at times contradictory in terms of which parameters are independent predictors. More recently, two large retrospective studies have reported on the issue. They included data from several international trials of chemotherapy for APL, one on adults and the other focused on the pediatric population. Importantly, both analyses found that WBC count at presentation is the main predictor of early hemorrhagic death and early thrombo-hemorrhagic death, respectively. Much remains to be done if the rate of induction mortality in APL is going to be reduced significantly. One approach would be to incorporate the known risk factors for early hemorrhagic death into a risk stratification system and devise personalized transfusion interventions to meet an individual patient's risk, which could be evaluated in future randomized trials.

Effects of Plasmin on von Willebrand Factor and Platelets: A Narrative Review

Plasmin is the major fibrinolytic protease responsible for dissolving thrombi by cleavage of its primary substrate fibrin. In addition, emerging evidence points to other roles of plasmin: (1) as a back-up for ADAMTS13 in proteolysis of ultra-large von Willebrand factor (VWF) multimers and (2) as an activator of platelets. Although the molecular mechanisms of fibrinolysis are well defined, insights on the effects of plasmin on VWF and platelets are relatively scarce and sometimes conflicting. Hence, this review provides an overview of the literature on the effects of plasmin on VWF multimeric structures, on VWF binding to platelets, and on platelet activation. This information is placed in the context of possible applications of thrombolytic therapy for the condition thrombotic thrombocytopenic purpura.

Thrombolytic therapy

The therapeutic use of thrombolytic agents is the natural result of the increasing understanding of the pathophysiologic mechanisms underlying normal and deranged thrombosis and fibrinolysis. Plasminogen activators capable of increasing the production of plasmin exhibit considerable efficacy in the treatment of a variety of arterial and venous thrombotic disorders. The ideal thrombolytic agent has yet to be developed but the desired clinical result of rapid opening of the thrombosed vessel without reocclusion, without activation of systemic fibrinogenolysis, and without a risk of hemorrhage is well defined. Clinical studies clearly demonstrate that the addition of a variety of adjunctive agents to the available thrombolytics enhances benefit without inordinate risk. The addition of intravascular angioplasty and stenting to thrombolysis increases the potential long-term benefit. Newer thrombolytic agents and new protocols for the use of existing therapies offer the promise of saving many who would otherwise succumb to coronary or cerebral arterial thrombosis or to venous thromboembolism.

The platelet in cardiopulmonary bypass

Platelets are the smallest of the blood cells and are known to be activated during cardiopulmonary bypass. They play a role in many associated complications. Both quantitative and qualitative platelet defects have been demonstrated, resulting in microvascular hemorrhage and thromboembolism. As their interactions with endothelium and other blood cells are unraveled, the important contribution they make toward the systemic inflammatory response to operation seen in cardiopulmonary bypass is increasingly evident. In this review, we consider platelet activation during cardiopulmonary bypass, the resultant clinical effects, and potential approaches to therapy and prevention.

Endothelial Cell-Blood Interface Actions and the Procoagulant Response

Anesthesiologists and surgeons have focused on the problems of hypocoagulability and resulting hemorrhage after cardiopulmonary bypass. Recent work in endothelial cell biology has demonstrated that the interaction of inflammatory processes and coagulation dysfunction with the endothelium may contribute to either hypocoagulability (bleeding) or hypercoagulability (thrombosis). New work with endothelial cell function and intracellular signaling of procoagulant responses may allow for unique therapeutic interventions in the future.

Endothelial cell injury in cardiovascular surgery: the procoagulant response

The vascular endothelium plays a critical role in the regulation of coagulation through the constitutive expression and release of anticoagulants and the inducible expression of procoagulant substances. Cardiopulmonary bypass dysregulates this process by activating endothelial cells, initially promoting bleeding and then thrombosis. Endothelial cell activation in response to circulating inflammatory mediators leads to the initiation of coagulation when tissue factor is expressed throughout the intravascular space. This results in the widespread consumption of coagulation factors. Additionally, there is a cardiopulmonary bypass-related qualitative platelet defect that is exacerbated by thrombocytopenia as platelets are consumed from the circulation by clot and adherence to the cardiopulmonary bypass circuit. Finally, cardiopulmonary bypass results in the endothelial release of plasminogen activators, which lead to an increase in systemic fibrinolysis. The diffuse generation of thrombin, driven by the inducible intravascular expression of tissue factor, plays a major role in all of these processes. Efforts to understand the critical role of the endothelium in coagulation may lead to novel therapies to prevent bleeding or thrombosis in cardiovascular surgery patients.

Ischemia--a coagulation problem?

The causes of perioperative ischemia and myocardial infarction (MI) in coronary artery bypass graft (CABG) patients are almost certainly multifactorial, although not well understood. Ultimately, outcome after CABG is dependent on myocardial preservation and prevention of further myocardial ischemia. The largest number of ST-T-wave events come immediately after protamine is given, suggesting that re-establishment of coagulation function after cardiopulmonary bypass (CPB) may be an important event. CPB induces an inflammatory state that involves platelet-endothelial-cell interactions and vasospastic responses that result in low flow states in the coronary vasculature. The fibrinolytic system is activated during CPB, with raised tissue plasminogen activator (tPA) levels and related falls in plasminogen activator inhibitor (PAI-1). PAI-1 levels rise during the postoperative period. There is a huge variability in human response. However, the patients with the highest tPA surge are not the same patients who have the highest PAI surge. It could be postulated that patients with high PAI-1 levels are at highest risk for early ischemia. New data just being evaluated from the Multicenter Study of Perioperative Ischemia (McSPI) Research Groups' database in San Francisco may support the hypothesis that coagulation influences perioperative ischemia. The study of approximately 2,400 patients undergoing CABG surgery at 24 major institutions in the United States revealed that intensive care unit (ICU) entry hematocrit was significantly related to the risk for postoperative MI. Patients entering the ICU with hematocrits below 24% had the lowest MI rate (3.7%), whereas those with hematocrits greater than 34% had the highest rate (8.1%). Patients with ICU entry hematocrits below 18% had a zero incidence of perioperative MI. One possible explanation for these findings is that platelets are involved. As red cells stream down vessels, they marginate the smaller formed elements of the blood. As hematocrit is increased, the number of platelets moved to the outer sides of the vessels increases. Therefore, the number of endothelial-platelet interactions would increase over time with higher hematocrits.

Beyond thrombolysis: other effects of thrombolytic drugs

OBJECTIVE

To review the effects of plasminogen activators (tissue plasminogen activator, streptokinase, and anistreplase) on fibrinogen and thrombin, platelets, complement, blood rheology, and neutrophils.

DATA SOURCES

A MEDLINE search, as well as a review of recent scientific abstracts, was conducted to identify pertinent literature.

STUDY SELECTION

Focus was placed on studies conducted in humans. However, many in vitro studies have been performed to fully elucidate the effect of plasminogen activators on different aspects of hemostasis and on the fibrinolytic and immune systems.

DATA EXTRACTION

Data from in vitro, human, and animal studies were evaluated.

DATA SYNTHESIS

There is a discrepancy between 90-minute patency and mortality in acute myocardial infarction patients treated with thrombolytic drugs. This could be caused, in part, by other hematologic and immunologic effects of thrombolytic drugs. Though the emphasis of clinical trials has been infarct-related artery patency, left ventricular function, and mortality, some studies have evaluated the effect of thrombolytic agents on fibrinogen and thrombin, platelets, blood rheology, complement, and neutrophils. This review discusses the alteration of systemic hematologic and immunologic parameters by thrombolytic drugs and the possible clinical implications of these effects.

CONCLUSIONS

Although the interactions between thrombolytic drugs, hemostasis, and the fibrinolytic and immune systems are complex and still not fully understood, it appears that these drugs differ in their effects on these systems. A greater understanding of these properties and their clinical implications may ultimately enhance the care and outcome of acute myocardial infarction patients treated with thrombolytic therapy.

SIN-1 partially and RGDS totally counteracts platelet aggregation as assessed in vitro by two independent whole blood methods

The cyclic GMP stimulant SIN-1 and the GP IIB/IIIA receptor antagonist RGDS were compared with regard to platelet antiaggregatory effects as measured in vitro by filtragometry and by whole blood aggregometry. In filtragometry platelet aggregation is measured as the time to partial occlusion of a filter in the test unit. Beta-thromboglobulin concentrations increased over the filter (p < 0.002) indicating that in filtragometry part of the mechanism of aggregation could be platelet activation across the filter. In whole blood aggregometry platelet aggregation is induced by a chemical stimulant. As tested in blood from healthy volunteers, linear dose-effect relations were found with both methods, for SIN-1 in the 10(-7)-10(-6) M range (p < 0.02, filtragometry and p < 0.05, whole blood aggregometry) and for RGDS in the 10(-5)-10(-4) M range (p < 0.0001, filtragometry and p < 0.02, whole blood aggregometry). At the highest dose RGDS totally counteracted platelet aggregation in both test systems. Maximal SIN-1 platelet antiaggregatory effects were less (p < 0.04, filtragometry and p < 0.01, whole blood aggregometry) than for RGDS. SIN-1 concentrations in the 10(-4) M range had no further antiaggregatory effects. In conclusion, with two principally different methods for the assessment of whole blood platelet aggregation, SIN-1 was found to be a partial antagonist while RGDS a total antagonist.

Streptokinase, but not tissue plasminogen activator, attenuates platelet aggregation in patients with acute myocardial infarction

OBJECTIVES

To investigate if tissue plasminogen activator (tPA) and streptokinase given during acute myocardial infarction (AMI) have different effects on platelet aggregation which could contribute to the higher reocclusion rate observed after tPA.

DESIGN

Open labelled on consecutive patients.

SETTING

Coronary care unit.

SUBJECTS

Twenty patients with chest pain and ST elevations on an electrocardiogram suggestive of AMI.

INTERVENTIONS

Ten patients were treated with tPA (100 mg 3 h-1), 10 patients with streptokinase (1.5 x 10(6) IU 1 h-1).

MAIN OUTCOME MEASURES

Before, immediately after and 24 h after fibrinolytic therapy, platelet aggregation was estimated with filtragometry and whole blood aggregometry. Fibrinogen, beta-thromboglobulin, elastase and the fibrinogen-derived peptide B beta 30-43 were also measured.

RESULTS

The groups were comparable at baseline. Directly after treatment, streptokinase prolonged aggregation time in filtragometry with 112 +/- 140 s (P < 0.03) and reduced conductance in whole blood aggregometry by 6.2 +/- 6.1 omega (P < 0.03), both tests indicating inhibited platelet function. Fibrinogen decreased 2.5 +/- 1.0 g l-1 (P < 0.02). In the tPA-treated group corresponding changes were 68 +/- 225 s (NS) and 2.5 +/- 7 omega (NS) with no significant reduction in fibrinogen. After 24 h, at which time every patient was on acetylsalicylic acid, aggregation was inhibited in both groups as measured by aggregometry. Directly after fibrinolytic treatment, neutrophils were similarly activated in both groups with increments of elastase and B beta 30-43 by 26 +/- 46 micrograms l-1 (P < 0.03) and 280 +/- 381 pmol l-1 (P < 0.03) respectively (streptokinase) and by 12 +/- 6 micrograms l-1 (P < 0.02) and 919 +/- 856 pmol l-1 (P < 0.02) respectively (tPA).

CONCLUSIONS

Despite similar degrees of platelet and leucocyte activation, streptokinase but not tPA treatment appears to inhibit platelet aggregation. One possible reason could be a streptokinase-induced pronounced decrease of fibrinogen and increase of fibrinogen split products. Therefore, further development of adjuvant antiplatelet therapy could be of clinical importance.

Reciprocating autocatalytic interactions between platelets and the activation system

Under normal circumstances, the platelet surface serves as a site of assembly for plasminogen (PGN) and tissue-type plasminogen activator (t-PA) and facilitates PGN activation. Since the plasmin (Pn) produced on the platelet surface can modulate a variety of platelet properties, we examined the effects of Pn on platelet-surface PGN activation. We incubated platelets with Pn (one caseinolytic unit/ml for one hr at 37 degrees C) and measured the effects of this treatment on the binding of PGN, Pn, and t-PA to unactivated platelets; and on the kinetics of PGN activation on the platelet surface. Pn treatment increased the number of PGN binding sites by 78% (from 46,000 to 88,000 sites/platelet) without affecting affinity (KD = 2.2 microM). Pn treatment had a modest effect on (DFP-inactivated) Pn binding but did not modify t-PA binding; however, treatment increased the catalytic efficiency of t-PA approximately two-fold. Importantly, all of these effects occurred without evidence for platelet activation by Pn. These observations imply that PGN activation may be an autocatalytic process on the platelet surface and provide evidence for a unique reciprocating mechanism governing the interaction between platelets and the plasminogen activation system.

Randomized trial of thrombolysis versus heparin in unstable angina

BACKGROUND

The clinical usefulness of intravenous thrombolytic therapy in unstable angina is currently unknown, despite the pathogenetic similarity of this entity to acute myocardial infarction, for which thrombolysis has enjoyed great success. To compare the clinical benefit of intravenous urokinase with that of conventional antithrombotic therapy in preventing the progression of unstable angina to new myocardial infarction, intractable angina, or death within the first 96 hours after hospitalization, 149 patients with unstable angina were randomized to one of two intravenous thrombolytic strategies.

METHODS AND RESULTS

Forty-nine patients received 3 million units urokinase i.v. over 90 minutes plus intravenous heparin (group A); 47 patients received unblinded 3 million units urokinase i.v. plus 325 mg aspirin p.o. daily (group B); and 53 patients received placebo thrombolytic infusion plus full-dose heparin (group C). The primary end point of this trial was 96-hour clinical status. There were no significant differences in the baseline characteristics (age, sex, previous myocardial infarction, hypertension prevalence, diabetes, tobacco use, or previous revascularization) among the three groups. Despite an excess of minor untoward reactions for the urokinase groups (chills, 26.5% and 23.4% for groups A and B versus 0% for group C; p < 0.01), there was no significant difference with respect to major bleeds (two, none, and two for groups A, B, and C, respectively; p = NS). At 96 hours after presentation, no significant difference emerged in the incidence of new cardiac events: new myocardial infarctions developed in 10.2% of group A, 6.4% of group B, and 3.8% of group C (p = NS); intractable angina occurred in 6.1% of group A, 10.6% of group B, and 9.4% of group C (p = NS). There were no deaths. All three groups encountered a similar incidence of overall cardiac events: 16.3%, 17.0%, and 13.2% for groups A, B, and C, respectively (p = NS). Although trial enrollment was to extend to 600 patients, interim analysis led to early cessation of enrollment due to a negative trend in respect to outcome after thrombolysis.

CONCLUSIONS

High-dose intravenous urokinase followed by either heparin or aspirin can be safely administered to a broad, unselected group of patients with unstable angina. However, this study suggests that no clinical advantage is conferred by urokinase, with either adjunctive antithrombotic therapy over standard heparin therapy alone, when given relatively late (mean, 8.7 hours) after admission for unstable angina. A possible detrimental effect cannot be excluded.