Binding of alpha-thrombin to fibrin depends on the quality of the fibrin network

Investigating thrombin-loaded fibrin in whole blood clot microfluidic assay via fluorogenic peptide

During clotting under flow, thrombin rapidly generates fibrin, whereas fibrin potently sequesters thrombin. This co-regulation was studied using microfluidic whole blood clotting on collagen/tissue factor, followed by buffer wash, and a start/stop cycling flow assay using the thrombin fluorogenic substrate, Boc-Val-Pro-Arg-AMC. After 3 min of clotting (100 s-1) and 5 min of buffer wash, non-elutable thrombin activity was easily detected during cycles of flow cessation. Non-elutable thrombin was similarly detected in plasma clots or arterial whole blood clots (1000 s-1). This thrombin activity was ablated by Phe-Pro-Arg-chloromethylketone (PPACK), apixaban, or Gly-Pro-Arg-Pro to inhibit fibrin. Reaction-diffusion simulations predicted 108 nM thrombin within the clot. Heparin addition to the start/stop assay had little effect on fibrin-bound thrombin, whereas addition of heparin-antithrombin (AT) required over 6 min to inhibit the thrombin, indicating a substantial diffusion limitation. In contrast, heparin-AT rapidly inhibited thrombin within microfluidic plasma clots, indicating marked differences in fibrin structure and functionality between plasma clots and whole blood clots. Addition of GPVI-Fab to blood before venous or arterial clotting (200 or 1000 s-1) markedly reduced fibrin-bound thrombin, whereas GPVI-Fab addition after 90 s of clotting had no effect. Perfusion of AF647-fibrinogen over washed fluorescein isothiocyanate (FITC)-fibrin clots resulted in an intense red layer around, but not within, the original FITC-fibrin. Similarly, introduction of plasma/AF647-fibrinogen generated substantial red fibrin masses that did not penetrate the original green clots, demonstrating that fibrin cannot be re-clotted with fibrinogen. Overall, thrombin within fibrin is non-elutable, easily accessed by peptides, slowly accessed by average-sized proteins (heparin/AT), and not accessible to fresh fibrinogen.

Mathematical modeling to understand the role of bivalent thrombin-fibrin binding during polymerization

Thrombin is an enzyme produced during blood coagulation that is crucial to the formation of a stable clot. Thrombin cleaves soluble fibrinogen into fibrin, which polymerizes and forms an insoluble, stabilizing gel around the growing clot. A small fraction of circulating fibrinogen is the variant γA/γ′, which has been associated with high-affinity thrombin binding and implicated as a risk factor for myocardial infarctions, deep vein thrombosis, and coronary artery disease. Thrombin is also known to be strongly sequestered by polymerized fibrin for extended periods of time in a way that is partially regulated by γA/γ′. However, the role of γA/γ′-thrombin interactions during fibrin polymerization is not fully understood. Here, we present a mathematical model of fibrin polymerization that considered the interactions between thrombin, fibrinogen, and fibrin, including those with γA/γ′. In our model, bivalent thrombin-fibrin binding greatly increased thrombin residency times and allowed for thrombin-trapping during fibrin polymerization. Results from the model showed that early in fibrin polymerization, γ′ binding to thrombin served to localize the thrombin to the fibrin(ogen), which effectively enhanced the enzymatic conversion of fibrinogen to fibrin. When all the fibrin was fully generated, however, the fibrin-thrombin binding persisted but the effect of fibrin on thrombin switched quickly to serve as a sink, essentially removing all free thrombin from the system. This dual role for γ′-thrombin binding during polymerization led to a paradoxical decrease in trapped thrombin as the amount of γ′ was increased. The model highlighted biochemical and biophysical roles for fibrin-thrombin interactions during polymerization and agreed well with experimental observations.

Nonredundant Roles of Platelet Glycoprotein VI and Integrin αIIbβ3 in Fibrin-Mediated Microthrombus Formation

OBJECTIVE

Fibrin is considered to strengthen thrombus formation via integrin αIIbβ3, but recent findings indicate that fibrin can also act as ligand for platelet glycoprotein VI. Approach and Results: To investigate the thrombus-forming potential of fibrin and the roles of platelet receptors herein, we generated a range of immobilized fibrin surfaces, some of which were cross-linked with factor XIIIa and contained VWF-BP (von Willebrand factor-binding peptide). Multicolor microfluidics assays with whole-blood flowed at high shear rate (1000 s^-1) indicated that the fibrin surfaces, regardless of the presence of factor XIIIa or VWF-BP, supported platelet adhesion and activation (P-selectin expression), but only microthrombi were formed consisting of bilayers of platelets. Fibrinogen surfaces produced similar microthrombi. Markedly, tiggering of coagulation with tissue factor or blocking of thrombin no more than moderately affected the fibrin-induced microthrombus formation. Absence of αIIbβ3 in Glanzmann thrombasthenia annulled platelet adhesion. Blocking of glycoprotein VI with Fab 9O12 substantially, but incompletely reduced platelet secretion, Ca²⁺ signaling and aggregation, while inhibition of Syk further reduced these responses. In platelet suspension, glycoprotein VI blockage or Syk inhibition prevented fibrin-induced platelet aggregation. Microthrombi on fibrin surfaces triggered only minimal thrombin generation, in spite of thrombin binding to the fibrin fibers.

CONCLUSIONS

Together, these results indicate that fibrin fibers, regardless of their way of formation, act as a consolidating surface in microthrombus formation via nonredundant roles of platelet glycoprotein VI and integrin αIIbβ3 through signaling via Syk and low-level Ca²⁺ rises.

A Mathematical Model of Bivalent Binding Suggests Physical Trapping of Thrombin within Fibrin Fibers

Thrombin is an enzyme that plays many important roles in the blood clotting process; it activates platelets, cleaves coagulation proteins within feedback loops, and cleaves fibrinogen into fibrin, which polymerizes into fibers to form a stabilizing gel matrix in and around growing clots. Thrombin also binds to the formed fibrin matrix, but this interaction is not well understood. Thrombin-fibrin binding is often described as two independent, single-step binding events, one high-affinity and one low-affinity. However, kinetic schemes describing these single-step binding events do not explain experimentally-observed residency times of fibrin-bound thrombin. In this work, we study a bivalent, sequential-step binding scheme as an alternative to the high-affinity event and, in addition to the low-affinity one. We developed mathematical models for the single- and sequential-step schemes consisting of reaction-diffusion equations to compare to each other and to experimental data. We then used Bayesian inference, in the form of Markov chain Monte Carlo, to learn model parameter distributions from previously published experimental data. For the model to best fit the data, we made an additional assumption that thrombin was irreversibly sequestered; we hypothesized that this could be due to thrombin becoming physically trapped within fibrin fibers as they formed. We further estimated that ∼30% of thrombin in the experiments to which we compare our model output became physically trapped. The notion of physically trapped thrombin may provide new insights into conflicting observations regarding the speed of fibrinolysis. Finally, we show that our new model can be used to further probe scenarios dealing with thrombin allostery.

Establishing the Transient Mass Balance of Thrombosis: From Tissue Factor to Thrombin to Fibrin Under Venous Flow

Supplemental Digital Content is available in the text.

Objective—

We investigated the coregulation of thrombin and fibrin as blood flows over a procoagulant surface.

Approach and Results—

Using microfluidic perfusion of factor XIIa-inhibited human whole blood (200 s⁻¹ wall shear rate) over a 250-μm long patch of collagen/TF (tissue factor; ≈1 molecule per μm²) and immunoassays of the effluent for F1.2 (prothrombin fragment 1.2), TAT (thrombin–antithrombin complex), and D-dimer (post–end point plasmin digest), we sought to establish the transient mass balance for clotting under venous flow. F1.2 (but almost no free thrombin detected via TAT assay) continually eluted from clots when fibrin was allowed to form. Low-dose fluorescein-Phe-Pro-Arg-chloromethylketone stained fibrin-bound thrombin—a staining ablated by anti–γ′-fibrinogen or the fibrin inhibitor glypro-arg-pro but highly resistant to 7-minute buffer rinse, demonstrating tight binding of thrombin to γ′-fibrin. With fibrin polymerizing for 500 seconds, 92 000 thrombin molecules and 203 000 clot-associated fibrin monomer equivalents were generated per TF molecule (or per μm²). Fibrin reached 15 mg/mL in the pore space (porosity ≈0.5) of a 15-μm-thick thrombus core by 500 seconds and 30 mg/mL by 800 seconds. For a known rate of ≈60 FPA (fibrinopeptide-A) per thrombin per second, each thrombin molecule generated only 3 fibrin monomer equivalents during 500 seconds, indicating an intraclot thrombin half-life of ≈70 ms, much shorter than its diffusional escape time (≈10 seconds). By 800 seconds, gly-pro-arg-pro allowed 4-fold more F1.2 generation, consistent with gly-pro-arg-pro ablating fibrin’s antithrombin-I activity and facilitating thrombin-mediated FXIa activation.

Conclusions—

Under flow, fibrinogen continually penetrates the clot, and γ′-fibrin regulates thrombin.

Probing the Dynamics of Clot-Bound Thrombin at Venous Shear Rates

In closed system models of fibrin formation, exosite-mediated thrombin binding to fibrin contributes to clot stability and is resistant to inhibition by antithrombin/heparin while still susceptible to small, active-site inhibitors. Each molecule of fibrin can bind ∼1.6 thrombin molecules at low-affinity binding sites (K_d = 2.8 μM) and ∼0.3 molecules of thrombin at high-affinity binding sites (K_d = 0.15 μM). The goal of this study is to assess the stability of fibrin-bound thrombin under venous flow conditions and to determine both its accessibility and susceptibility to inhibition. A parallel-plate flow chamber (7 × 50 × 0.25 mm) for studying the stability of thrombin (0-1400 nM) adhered to a fibrin matrix (0.1-0.4 mg/mL fibrinogen, 10 nM thrombin) under a variety of venous flow conditions was developed using the thrombin-specific, fluorogenic substrate SN-59 (100 μM). The flow within this system is laminar (R_e < 1) and reaction rates are driven by enzyme kinetics (P_e = 100, D_a = 7000). A subpopulation of active thrombin remains stably adhered to a fibrin matrix over a range of venous shear rates (46-184 s^-1) for upwards of 30 min, and this population is saturable at loads >500 nM and sensitive to the initial fibrinogen concentration. These observations were also supported by a mathematical model of thrombin adhesion to fibrin, which demonstrates that thrombin initially binds to the low-affinity thrombin binding sites before preferentially equilibrating to higher affinity sites. Antithrombin (2.6 μM) plus heparin (4 U/mL) inhibits 72% of the active clot-bound thrombin after ∼10 min at 92 s^-1, while no inhibition is observed in the absence of heparin. Dabigatran (20 and 200 nM) inhibits (50 and 93%) clot-bound thrombin reversibly (87 and 66% recovery). This model illustrates that clot-bound thrombin stability is the result of a constant rearrangement of thrombin molecules within a dense matrix of binding sites.

Fibrin networks regulate protein transport during thrombus development

Thromboembolic disease is a leading cause of morbidity and mortality worldwide. In the last several years there have been a number of studies attempting to identify mechanisms that stop thrombus growth. This paper identifies a novel mechanism related to formation of a fibrin cap. In particular, protein transport through a fibrin network, an important component of a thrombus, was studied by integrating experiments with model simulations. The network permeability and the protein diffusivity were shown to be important factors determining the transport of proteins through the fibrin network. Our previous in vivo studies in mice have shown that stabilized non-occluding thrombi are covered by a fibrin network (‘fibrin cap’). Model simulations, calibrated using experiments in microfluidic devices and accounting for the permeable structure of the fibrin cap, demonstrated that thrombin generated inside the thrombus was washed downstream through the fibrin network, thus limiting exposure of platelets on the thrombus surface to thrombin. Moreover, by restricting the approach of resting platelets in the flowing blood to the thrombus core, the fibrin cap impaired platelets from reaching regions of high thrombin concentration necessary for platelet activation and limited thrombus growth. The formation of a fibrin cap prevents small thrombi that frequently develop in the absence of major injury in the 60000 km of vessels in the body from developing into life threatening events.

To restrict the loss of blood following rupture of blood vessels, the human body rapidly forms a clot consisting mainly of platelets and fibrin. However, to prevent formation of a pathological clot within vessels (thrombus) as a result of vessel damage or dysfunction, the response must be regulated, and clot formation must be limited. Our previous studies demonstrated that as a laser-induced thrombus stabilized in mice, the ratio of fibrin to platelets at the thrombus surface increased significantly. Stabilized non-occluding thrombi were observed to be covered by a fibrin network (‘fibrin cap’). In the present work the role of the fibrin network in protein transport is examined by integrating experiments in microfluidic devices with the hemodynamic thrombus model. The study reveals permeability of the fibrin network and protein diffusivity to be important factors determining the transport of blood proteins inside the thrombus. It is shown that the fibrin network does not dramatically limit the diffusion of thrombin but impairs flowing platelets in blood from reaching regions of high thrombin concentration thus, reducing the probability they are activated and stably integrated into the thrombus. This novel, counter-intuitive mechanism suggests that a fibrin network formed at early stages of thrombus initiation can prevent normally asymptomatic thrombi from developing into pathological clots.

Successful management by recombinant activated factor VII in a case of disseminated intravascular coagulopathy caused by obstetric hemorrhage

Postpartum hemorrhage (PPH) is a life-threatening emergency in obstetrics. Although recombinant activated factor VII (rFVIIa) has become used for the treatment of some cases of massive hemorrhage, its applications in the field of obstetrics are still limited. We describe a case of successful treatment with rFVIIa for PPH due to placenta accreta. The patient was a 33-year-old woman with placental previa. Cesarean section (CS) was performed at gestational week 35. During CS, there was massive hemorrhage due to placenta accreta. After CS, disseminated intravascular coagulopathy and hypovolemic shock were diagnosed. The PPH was not controlled by transfusion therapy. On the fourth day after CS, rFVIIa (90 microg/kg x 2) was given because of the persistent PPH. Bleeding decreased and no further transfusion was required from 2 days after administration. rFVIIa was useful in the treatment of this case of obstetric hemorrhage.

Polyphosphate enhances fibrin clot structure

Polyphosphate, a linear polymer of inorganic phosphate, is present in platelet dense granules and is secreted on platelet activation. We recently reported that polyphosphate is a potent hemostatic regulator, serving to activate the contact pathway of blood clotting and accelerate factor V activation. Because polyphosphate did not alter thrombin clotting times, it appeared to exert all its procoagulant actions upstream of thrombin. We now report that polyphosphate enhances fibrin clot structure in a calcium-dependent manner. Fibrin clots formed in the presence of polyphosphate had up to 3-fold higher turbidity, had higher mass-length ratios, and exhibited thicker fibers in scanning electron micrographs. The ability of polyphosphate to enhance fibrin clot turbidity was independent of factor XIIIa activity. When plasmin or a combination of plasminogen and tissue plasminogen activators were included in clotting reactions, fibrin clots formed in the presence of polyphosphate exhibited prolonged clot lysis times. Release of polyphosphate from activated platelets or infectious microorganisms may play an important role in modulating fibrin clot structure and increasing its resistance to fibrinolysis. Polyphosphate may also be useful in enhancing the structure of surgical fibrin sealants.

Hemostatic and neuroprotective effects of human recombinant activated factor VII therapy after traumatic brain injury in pigs

Human recombinant activated factor-VII (rFVIIa) has been used successfully in the treatment of spontaneous intracerebral hemorrhage. In addition, there is increasing interest in its use to treat uncontrolled bleeding of other origins, including trauma. The aim of this study was to evaluate the safety and potential effectiveness of rFVIIa to mitigate bleeding using a clinically relevant model of traumatic brain injury (TBI) in the pig. A double injury model was chosen consisting of (1) an expanding cerebral contusion induced by the application of negative pressure to the exposed cortical surface and (2) a rapid rotational acceleration of the head to induce diffuse axonal injury (DAI). Injuries were performed on 10 anesthetized pigs. Five minutes after injury, 720 microg/kg rFVIIa (n=5) or vehicle control (n=5) was administered intravenously. Magnetic resonance imaging (MRI) studies were performed within 30 min and at 3 days post-TBI to determine the temporal expansion of the cerebral contusion. Euthanasia and histopathologic analysis were performed at day 3. This included observations for hippocampal neuronal degeneration, axonal pathology and microclot formation. The expansion of contusion volume over the 3 days post-injury period was reduced significantly in animals treated with rFVIIa compared to vehicle controls. Surprisingly, immunohistochemical analysis demonstrated that the number of dead/dying hippocampal neurons and axonal pathology was reduced substantially by rFVIIa treatment compared to vehicle. In addition, there was no difference in the extent of microthrombi between groups. rFVIIa treatment after TBI in the pig reduced expansion of hemorrhagic cerebral contusion volume without exacerbating the severity of microclot formation. Finally, rFVIIa treatment provided a surprising neuroprotective effect by reducing hippocampal neuron degeneration as well as the extent of DAI.

Affinity purification of serine proteinase from Deinagkistrodon acutus venom

An affinity protocol was developed for the preparation of the main serine proteinase from Deinagkistrodon acutus venom on industrial scales. As affinity ligand, l-arginine was composed to medium and its structure was confirmed by ESI-MS analysis. The purification process consisted of one major affinity chromatography step to remove more than 95% of other proteins, and a polishing step of DEAE ion-exchange chromatography for removal of minor contaminants. The serine proteinase was 100% pure analyzed on HPLC Vydac C4 column, 99.4% on TSK G3000SW column, and 97.7% with SDS-PAGE analysis. The yield of the main serine proteinase was 3.6% of crude venom protein, the recoveries of typical fibrinogen (Fg) clotting activity and arginine esterase activity of serine proteinase were 82.2% and 84%, higher than those of other reported traditional protocols, the proteinase also showed arginine amidase activity. Reducing SDS-PAGE analysis showed that the arginine esterase was a single polypeptide with the mass of approximately 40 kDa, while MALDI-TOF-TOF-MS analysis showed that the purified proteinase should be a approximately 34 kDa glycoprotein. The desorption constant Kd and the theoretical maximum absorption Qmax on the affinity medium were 9.93 x 10(-5) and 38.1mg/g medium in absorption analysis.

Thrombin generation and fibrin clot structure

Generation of a hemostatic clot requires thrombin-mediated conversion of fibrinogen to fibrin. Previous in vitro studies have demonstrated that the thrombin concentration present at the time of gelation profoundly influences fibrin clot structure. Clots formed in the presence of low thrombin concentrations are composed of thick fibrin fibers and are highly susceptible to fibrinolysis; while, clots formed in the presence of high thrombin concentrations are composed of thin fibers and are relatively resistant to fibrinolysis. While most studies of clot formation have been performed by adding a fixed amount of purified thrombin to fibrinogen, clot formation in vivo occurs in a context of continuous, dynamic changes in thrombin concentration. These changes depend on the local concentrations of pro- and anti-coagulants and cellular activities. Recent studies suggest that patterns of abnormal thrombin generation produce clots with altered fibrin structure and that these changes are associated with an increased risk of bleeding or thrombosis. Furthermore, it is likely that clot structure also contributes to cellular events during wound healing. These findings suggest that studies explicitly evaluating fibrin formation during in situ thrombin generation are warranted to explain and fully appreciate mechanisms of normal and abnormal fibrin clot formation in vivo.

Recombinant factor VIIa in massive postpartum haemorrhage

Massive postpartum haemorrhage is a major cause of maternal and fetal morbidity and mortality. Management mainstays include transfusion therapy, uterotonic agents and surgery. The "off label" use of recombinant activated factor VII appears to have an evolving role in the management of massive postpartum haemorrhage refractory to conventional treatments. The current literature is reviewed.

Recombinant factor VIIa for life-threatening post-partum haemorrhage † †This article is accompanied by the Editorial

The treatment of life-threatening post-partum haemorrhage (PPH) still remains challenging, and hysterectomy may be required to control the bleeding. We present 12 cases of severe PPH treated with recombinant factor VIIa (rFVIIa). We briefly describe the causes of the haemorrhage and the medical and surgical interventions before rVIIa administration. In 11 women there was a partial or good response to rFVIIa administration, while in one there was no response. In the four women undergoing a subsequent selective arterial embolization, the bleeding was significantly reduced although not completely stopped. From our experience with these 12 cases, and from previously reported cases, the use of rFVIIa may be of benefit in life-threatening PPH. However, treatment with rFVIIa, in addition to standard surgical and medical interventions, may not be definitive in every patient and a selective arterial embolization may be needed.

Guidelines for the use of recombinant activated factor VII (rFVIIa) in uncontrolled bleeding: a report by the Israeli Multidisciplinary rFVIIa Task Force

BACKGROUND

Recombinant activated factor VII (rFVIIa) has been approved by the U.S. Food and Drug Administration (FDA) for almost a decade for hemophilic patients with inhibitors. Its off-label use as a hemostatic agent in massive bleeding caused by a wide array of clinical scenarios is rapidly expanding. While evidence-based guidelines exist for rFVIIa treatment in hemophilia, none are available for its off-label use.

OBJECTIVES

The aim of this study is to develop expert recommendations for the use of rFVIIa in patients suffering from uncontrolled bleeding (with special emphasis on trauma) until randomized, controlled trials allow for the introduction of more established evidence-based guidelines.

METHODS

A multidisciplinary task force comprising representatives of the relevant National Medical Associations, experts from the Medical Corps of the Army, Ministry of Health and the Israel National Trauma Advisory Board was established in Israel. Recommendations were construed based on the analysis of the first 36 multi-trauma patients accumulated in the prospective national registry of the use of rFVIIa in trauma, and an extensive literature search consisting of published and prepublished controlled animal trials, case reports and series. The final consensus guidelines, together with the data of the first 36 trauma patients treated in Israel, are presented in this article.

RESULTS

Results of the first 36 trauma patients: The prolonged clotting assays [prothrombin time (PT) and partial thromboplastin time (PTT)] shortened significantly within minutes following administration of rFVIIa. Cessation of bleeding was achieved in 26 of 36 (72%) patients. Acidosis diminished the hemostatic effect of the drug, while hypothermia did not affect it. The survival rate of 61% (22/36) seems to be favorable compared with published series of similar, or less severe, trauma patients (range 30%-57%).

CONCLUSIONS

As a result of the lack of controlled trials, our guidelines should be considered as suggestive rather than conclusive. However, they provide a valuable tool for physicians using rFVIIa for the expanding off-label clinical uses.

Recombinant factor VIIa

Human coagulation factor (F) VII is a single chain protease that circulates in the blood as a weakly active zymogen at concentrations of approximately 10 nmol/L. When converted to the active 2 chain form (FVIIa), it is a powerful initiator of haemostasis. Recombinant factor VIIa (rFVIIa, eptacog alfa, NovoSeven) is a genetically engineered product that was first introduced in 1988 for the treatment of patients with haemophilia A and B with high inhibitory antibody titres to factors VIII and IX. Recent reports in the form of case studies and series, and early trial data, have suggested a role for rFVIIa across a diverse range of indications including bleeding associated with trauma, surgery, thrombocytopaenia, liver disease and oral anticoagulant toxicity. This review describes the physiology of the coagulation pathway and in particular the role of recombinant factor VIIa. It will also focus on the emerging role of rFVIIa in both trauma and non-trauma bleeding and its potential use in the ED.

Elevated prothrombin results in clots with an altered fiber structure: a possible mechanism of the increased thrombotic risk

Individuals with elevated prothrombin levels are at increased risk of venous thrombosis. To understand the mechanism behind this observation, we studied the effect of prothrombin concentration on thrombin generation and fibrin clot structure. The pattern of thrombin generation was directly related to the prothrombin level at all concentrations tested. From 0% to 300% of normal plasma levels of prothrombin, increasing the prothrombin concentration increased the initial rate, peak, and total amount of thrombin generated. Importantly, fibrin clot structure was also affected by the prothrombin concentration. Fibrin clots made from prothrombin concentrations less than 10% of plasma levels were weak and poorly formed. Fibrin clots made at 10% to 100% of plasma levels of prothrombin had similar fiber structures (mass-to-length ratio; mu). However, the fiber mass-to-length ratio decreased with increasing prothrombin levels more than 100% of plasma levels, in a dose-dependent manner. These results suggest that increased levels of prothrombin alter thrombin generation and clot structure. Specifically, elevated prothrombin levels produce clots with reduced fibrin mass-to-length ratios compared with normal clots. We hypothesize that this alteration in fibrin clot structure is an important determinant of the risk of thrombosis.

Evaluation of DNA aptamers directed to thrombin as potential thrombus imaging agents

Two DNA aptamers directed against two separate exosites on human alpha-thrombin were evaluated for thrombus-imaging potential. Aptamer ODN 1 is directed to the thrombin substrate binding site (exosite 1). Our finding that ODN 1 competes with fibrin for binding to exosite 1 on thrombin suggests that ODN 1 will not be useful for thrombus imaging. Aptamer ODN 2 is directed against the thrombin heparin binding site (exosite 2). ODN 2 bound to model thrombi that were formed either by clotting purified fibrinogen with thrombin, or by recalcifying citrated plasma. As the thrombin content of thrombi was increased the rate of ODN 2 uptake into preformed thrombi increased, whereas the rate of release of ODN 2 out of preformed thrombi decreased. This in vitro data suggested that ODN 2 might be useful for thrombus imaging because it can bind to exosite 2 on fibrin-bound thrombin. However, in a rabbit jugular vein model using thrombus supplemented with human thrombin, ODN 2 uptake was equal to the ovalbumin control, and did not reflect thrombin content. While the in vitro results with ODN 2 were consistent with thrombus imaging, the rapid clearance of ODN 2 from circulation, combined with slow mass transfer in the clot, seem to work against in vivo thrombin-dependent imaging or washout analysis.

Recombinant activated factor VII for adjunctive hemorrhage control in trauma

BACKGROUND

Recombinant activated factor VII (rFVIIa) was approved for treatment of hemorrhages in patients with hemophilia who develop inhibitors to factors VIII or IX. Conditions with increased thromboembolic risk, including trauma with or without disseminated intravascular coagulation, were considered a contraindication for the drug. The mechanism of action of rFVIIa suggests enhancement of hemostasis limited to the site of injury without systemic activation of the coagulation cascade. Therefore, use of the drug in trauma patients suffering uncontrolled hemorrhage appears to be rational.

METHODS

Seven massively bleeding, multitransfused (median, 40 units [range, 25-49 units] of packed cells), coagulopathic trauma patients were treated with rFVIIa (median, 120 microg/kg [range, 120-212 microg/kg]) after failure of conventional measures to achieve hemostasis.

RESULTS

Administration of rFVIIa resulted in cessation of the diffuse bleed, with significant decrease of blood requirements to 2 units (range, 1-2 units) of packed cells (p < 0.05); shortening of prothrombin time and activated partial thromboplastin time from 24 seconds (range, 20-31.8 seconds) to 10.1 seconds (range, 8-12 seconds) (p < 0.005) and 79 seconds (range, 46-110 seconds) to 41 seconds (range, 28-46 seconds) (p < 0.05), respectively; and an increase of FVII level from 0.7 IU/mL (range, 0.7-0.92 IU/mL) to 23.7 IU/mL (range, 18-44 IU/mL) (p < 0.05). Three of the seven patients died of reasons other than bleeding or thromboembolism.

CONCLUSION

The results of this report suggest that in trauma patients rFVIIa may play a role as an adjunctive hemostatic measure, in addition to surgical hemostatic techniques, and provides the motivation for controlled animal and clinical trials.

Retransfusion of suctioned blood during cardiopulmonary bypass impairs hemostasis

In a previous study we observed extensive clotting and fibrinolysis in blood from the thoracic cavities during cardiopulmonary bypass. We hypothesized that retransfusion of this suctioned blood could impair hemostasis. In this prospective clinical study we investigated the effect of suctioned blood retransfusion on systemic blood activation and on postoperative hemostasis. During coronary artery bypass grafting in 40 patients, suctioned blood was collected separately. It then was retransfused to the patient at the end of the operation (n = 19), or it was retained (n = 21). During the study, 12 consecutive patients, randomized in two groups of 6, were analyzed for biochemical parameters indicating blood activation and clotting. The immediate and significant increase in circulating concentrations of thrombin-antithrombin III complex, tissue-type plasminogen activator, fibrin degradation products, and free plasma hemoglobin demonstrated the effect of suctioned blood retransfusion. Moreover, the increased concentrations of thrombin-antithrombin III complex and fibrin degradation products indicated renewed systemic clotting and fibrinolysis as a direct result of the retransfusion of suctioned blood. Concentrations of all indicators mentioned remained significantly lower in the retainment group. The clinical data showed that retainment of suctioned blood resulted in significantly decreased postoperative blood loss (822 mL in the retransfusion group versus 611 mL in the retainment group; p < 0.05) and similar or even reduced consumption of blood products (513 versus 414 mL red blood cell concentrate and 384 versus 150 mL single-donor plasma; both not significant). We conclude that retransfusion of highly activated suctioned blood during cardiopulmonary bypass exacerbates wound bleeding.