Product-dependent anti-factor VIII antibodies

The development of anti-factor (F)VIII antibodies in haemophilia A (HA) subjects undergoing replacement therapy has been well documented. The correlation between antibody development and the FVIII product used for replacement therapy remains a subject of discussion. The aim of this study was to evaluate the presence of anti-FVIII antibodies towards three commercial rFVIII products in 34 HA subjects' plasmas. Antibodies were quantitated by a Multiplex Fluorescence Immunoassay. All plasmas contained anti-FVIII antibodies at variable concentrations ranging from 50 nm to 570 μm. Eleven of the 20 HA subjects treated with one (r)FVIII product contained inhibitory anti-FVIII antibodies (0.8-3584 BU). The inhibitory antibody titre and the molar concentrations of total antibody were mildly correlated (r(2) = 0.6). Pronounced differences in antibody recognition with the three rFVIII products were observed. For the group treated with Product 'A', the titre towards this product was 2.4-fold higher than that observed with another full-length rFVIII-containing product (Product 'B') and almost four-fold higher than that measured with a B domain-less rFVIII product (Product 'C'). For the group of 14 HA subjects treated with FVIII other than Product 'A', only one showed higher antibody titre when measured with this product. Our data suggest that the development of anti-FVIII antibodies is biased towards the product used for treatment and that a significant fraction of antibodies bind to the B domain of FVIII.

Disulfide reduction abolishes tissue factor cofactor function

BACKGROUND

Tissue factor (TF), an in vivo initiator of blood coagulation, is a transmembrane protein and has two disulfides in the extracellular domain. The integrity of one cysteine pair, Cys186-Cys209, has been hypothesized to be essential for an allosteric "decryption" phenomenon, presumably regulating TF procoagulant function, which has been the subject of a lengthy debate. The conclusions of published studies on this subject are based on indirect evidences obtained by the use of reagents with potentially oxidizing/reducing properties.

METHODS

The status of disulfides in recombinant TF1-263 and natural placental TF in their non-reduced native and reduced forms was determined by mass-spectrometry. Functional assays were performed to assess TF cofactor function.

RESULTS

In native proteins, all four cysteines of the extracellular domain of TF are oxidized. Reduced TF retains factor VIIa binding capacity but completely loses the cofactor function.

CONCLUSION

The reduction of TF disulfides (with or without alkylation) eliminates TF regulation of factor VIIa catalytic function in both membrane dependent FX activation and membrane independent synthetic substrate hydrolysis.

GENERAL SIGNIFICANCE

Results of this study advance our knowledge on TF structure/function relationships.

From principle to practice: bridging the gap in patient profiling

The standard clinical coagulation assays, activated partial thromboplastin time (aPTT) and prothrombin time (PT) cannot predict thrombotic or bleeding risk. Since thrombin generation is central to haemorrhage control and when unregulated, is the likely cause of thrombosis, thrombin generation assays (TGA) have gained acceptance as "global assays" of haemostasis. These assays generate an enormous amount of data including four key thrombin parameters (lag time, maximum rate, peak and total thrombin) that may change to varying degrees over time in longitudinal studies. Currently, each thrombin parameter is averaged and presented individually in a table, bar graph or box plot; no method exists to visualize comprehensive thrombin generation data over time. To address this need, we have created a method that visualizes all four thrombin parameters simultaneously and can be animated to evaluate how thrombin generation changes over time. This method uses all thrombin parameters to intrinsically rank individuals based on their haemostatic status. The thrombin generation parameters can be derived empirically using TGA or simulated using computational models (CM). To establish the utility and diverse applicability of our method we demonstrate how warfarin therapy (CM), factor VIII prophylaxis for haemophilia A (CM), and pregnancy (TGA) affects thrombin generation over time. The method is especially suited to evaluate an individual's thrombotic and bleeding risk during "normal" processes (e.g pregnancy or aging) or during therapeutic challenges to the haemostatic system. Ultimately, our method is designed to visualize individualized patient profiles which are becoming evermore important as personalized medicine strategies become routine clinical practice.

Rivaroxaban delivery and reversal at a venous flow rate

OBJECTIVE

Rivaroxaban is an oral anticoagulant that directly targets both free factor Xa and factor Xa in complex with its protein cofactor, factor Va, in the prothrombinase complex. It is approved in the United States for the prophylaxis of deep vein thrombosis and stroke in patients with atrial fibrillation; however, it also carries a black box warning regarding the risk of thrombosis after discontinuation of treatment. The purpose of this study was to determine the degree to which rivaroxaban, over a range of physiologically relevant free plasma concentrations, inhibits preassembled prothrombinase at a typical venous shear rate (100 s(-1)) and to determine the dynamics of rivaroxaban washout.

METHODS AND RESULTS

Prothrombinase was assembled on phospholipid-coated glass capillaries. Its activity was characterized with respect to the activation of prothrombin (mean plasma concentration, 1.4 μmol/L) in the absence and presence of rivaroxaban (2, 5, and 10 nmol/L). The degree of inactivation of preassembled prothrombinase is sensitive to the solution-phase rivaroxaban concentration; however, prothrombinase unmasking upon removal of rivaroxaban is concentration independent.

CONCLUSIONS

The model system presented suggests that when rivaroxaban plasma concentrations decrease after cessation of therapy, there will be an unmasking of thrombus-associated prothrombinase that may be related to the reported rebound phenomena.

Prothrombin activation by platelet-associated prothrombinase proceeds through the prethrombin-2 pathway via a concerted mechanism

The protease α-thrombin is a key enzyme of the coagulation process as it is at the cross-roads of both the pro- and anti-coagulant pathways. The main source of α-thrombin in vivo is the activation of prothrombin by the prothrombinase complex assembled on either an activated cell membrane or cell fragment, the most relevant of which is the activated platelet surface. When prothrombinase is assembled on synthetic phospholipid vesicles, prothrombin activation proceeds with an initial cleavage at Arg-320 yielding the catalytically active, yet effectively anticoagulant intermediate meizothrombin, which is released from the enzyme complex ∼30-40% of the time. Prothrombinase assembled on the surface of activated platelets has been shown to proceed through the inactive intermediate prethrombin-2 via an initial cleavage at Arg-271 followed by cleavage at Arg-320. The current work tests whether or not platelet-associated prothrombinase proceeds via a concerted mechanism through a study of prothrombinase assembly and function on collagen-adhered, thrombin-activated, washed human platelets in a flow chamber. Prothrombinase assembly was demonstrated through visualization of bound factor Xa by confocal microscopy using a fluorophore-labeled anti-factor Xa antibody, which demonstrated the presence of distinct platelet subpopulations capable of binding factor Xa. When prothrombin activation was monitored at a typical venous shear rate over preassembled platelet-associated prothrombinase neither potential intermediate, meizothrombin or prethrombin-2, was observed in the effluent. Collectively, these findings suggest that platelet-associated prothrombinase activates prothrombin via an efficient concerted mechanism in which neither intermediate is released.

Modeling of human factor Va inactivation by activated protein C

BACKGROUND

Because understanding of the inventory, connectivity and dynamics of the components characterizing the process of coagulation is relatively mature, it has become an attractive target for physiochemical modeling. Such models can potentially improve the design of therapeutics. The prothrombinase complex (composed of the protease factor (F)Xa and its cofactor FVa) plays a central role in this network as the main producer of thrombin, which catalyses both the activation of platelets and the conversion of fibrinogen to fibrin, the main substances of a clot. A key negative feedback loop that prevents clot propagation beyond the site of injury is the thrombin-dependent generation of activated protein C (APC), an enzyme that inactivates FVa, thus neutralizing the prothrombinase complex. APC inactivation of FVa is complex, involving the production of partially active intermediates and "protection" of FVa from APC by both FXa and prothrombin. An empirically validated mathematical model of this process would be useful in advancing the predictive capacity of comprehensive models of coagulation.

RESULTS

A model of human APC inactivation of prothrombinase was constructed in a stepwise fashion by analyzing time courses of FVa inactivation in empirical reaction systems with increasing number of interacting components and generating corresponding model constructs of each reaction system. Reaction mechanisms, rate constants and equilibrium constants informing these model constructs were initially derived from various research groups reporting on APC inactivation of FVa in isolation, or in the presence of FXa or prothrombin. Model predictions were assessed against empirical data measuring the appearance and disappearance of multiple FVa degradation intermediates as well as prothrombinase activity changes, with plasma proteins derived from multiple preparations. Our work integrates previously published findings and through the cooperative analysis of in vitro experiments and mathematical constructs we are able to produce a final validated model that includes 24 chemical reactions and interactions with 14 unique rate constants which describe the flux in concentrations of 24 species.

CONCLUSION

This study highlights the complexity of the inactivation process and provides a module of equations describing the Protein C pathway that can be integrated into existing comprehensive mathematical models describing tissue factor initiated coagulation.

Tissue factor controversies

Tissue factor plays a primary role in both hemorrhage control and thrombosis depending upon whether its presentation is extravascular or intravascular. The molecular architecture and function of the tissue factor molecule and its role in the activations of factor IX and factor X have been elegantly elucidated but controversies prevail with respect to distinctions between tissue factor sources and tissue factor "activity." This presentation will review data on the architecture and functions of the tissue factor-factor VIIa complex and discuss the elements of the controversies associated with tissue factor presentation in both normal and pathologic milieu.

Defining the boundaries of normal thrombin generation: investigations into hemostasis

In terms of its soluble precursors, the coagulation proteome varies quantitatively among apparently healthy individuals. The significance of this variability remains obscure, in part because it is the backdrop against which the hemostatic consequences of more dramatic composition differences are studied. In this study we have defined the consequences of normal range variation of components of the coagulation proteome by using a mechanism-based computational approach that translates coagulation factor concentration data into a representation of an individual's thrombin generation potential. A novel graphical method is used to integrate standard measures that characterize thrombin generation in both empirical and computational models (e.g max rate, max level, total thrombin, time to 2 nM thrombin ("clot time")) to visualize how normal range variation in coagulation factors results in unique thrombin generation phenotypes. Unique ensembles of the 8 coagulation factors encompassing the limits of normal range variation were used as initial conditions for the computational modeling, each ensemble representing "an individual" in a theoretical healthy population. These "individuals" with unremarkable proteome composition was then compared to actual normal and "abnormal" individuals, i.e. factor ensembles measured in apparently healthy individuals, actual coagulopathic individuals or artificially constructed factor ensembles representing individuals with specific factor deficiencies. A sensitivity analysis was performed to rank either individual factors or all possible pairs of factors in terms of their contribution to the overall distribution of thrombin generation phenotypes. Key findings of these analyses include: normal range variation of coagulation factors yields thrombin generation phenotypes indistinguishable from individuals with some, but not all, coagulopathies examined; coordinate variation of certain pairs of factors within their normal ranges disproportionately results in extreme thrombin generation phenotypes, implying that measurement of a smaller set of factors may be sufficient to identify individuals with aberrant thrombin generation potential despite normal coagulation proteome composition.

Active tissue factor and activated factor XI in patients with acute ischemic cerebrovascular events

BACKGROUND

  Elevated factor (F)XI and tissue factor (TF) have been reported to occur in patients with acute ischemic stroke (AIS). We sought to investigate whether circulating activated FXI (FXIa) and TF on admission can predict clinical outcomes in patients with acute cerebrovascular events.

MATERIALS AND METHODS

  In the observational study, we evaluated 205 consecutive patients aged 70 years or less within the first 72 h of acute event, including 140 with AIS and 65 with transient ischemic attack (TIA). Plasma TF and FXIa activity were determined on admission in clotting assays by measuring the response to inhibitory monoclonal antibodies.

RESULTS

Active TF and FXIa activity were detected in 58 (28·9%) and 132 (64·4%) patients on admission, respectively. Active TF was detected in 45 of the 136 AIS patients with available TF levels (33·1%) and 13 of the 65 patients with acute TIA (20%; 0·05). Corresponding values for FXIa were 99 of the 140 (70·7%) and 33 of the 65 (50·8%; P= 0·006), respectively. Patients with detectable TF were more frequently women and hypertensive, while subjects with detectable FXIa had more often diabetes and higher levels of fibrinogen, C-reactive protein and interleukin-6 (all P < 0·05). Patients with detectable FXIa but not TF had higher National Institutes of Health Stroke Scale score, higher modified Rankin scale score and lower Barthel Index at discharge (all P < 0·05).

CONCLUSIONS

Circulating active TF and FXIa occur frequently in acute cerebrovascular ischemic events. Active FXIa in plasma might be useful as a novel risk marker of worse functional outcomes in patients with acute cerebrovascular events.

Measuring the mechanical properties of blood clots formed via the tissue factor pathway of coagulation

Thrombelastography (TEG) is a method that is used to conduct global assays that monitor fibrin formation and fibrinolysis and platelet aggregation in whole blood. The purpose of this study was to use a well-characterized tissue factor (Tf) reagent and contact pathway inhibitor (corn trypsin inhibitor, CTI) to develop a reproducible thrombelastography assay. In this study, blood was collected from 5 male subjects (three times). Clot formation was initiated in whole blood with 5 pM Tf in the presence of CTI, and fibrinolysis was induced by adding tissue plasminogen activator (tPA). Changes in viscoelasticity were then monitored by TEG. In quality control assays, our Tf reagent, when used at 5 pM, induced coagulation in whole blood in 3.93 ± 0.23 min and in plasma in 5.12 ± 0.23 min (n=3). In TEG assays, tPA significantly decreased clot strength (maximum amplitude, MA) in all individuals but had no effect on clot time (R time). The intraassay variability (CVa<10%) for R time, angle, and MA suggests that these parameters reliably describe the dynamics of fibrin formation and degradation in whole blood. Our Tf reagent reproducibly induces coagulation, making it an ideal tool to quantify the processes that contribute to mechanical clot strength in whole blood.

Posttranslational modifications of tissue factor

Tissue factor (TF), a membrane protein, is an initiator of blood coagulation in vivo. In this review we discuss how posttranslational modifications affect activity and other properties of TF. Glycosylation of the extracellular domain and the composition of carbohydrates at three glycosylation sites have an influence on TF activity in the extrinsic FXase by increasing the rate of FX proteolysis. No influence of TF glycosylation on the activity of the FVIIa/TF complex towards small synthetic substrates was observed, suggesting that glycosylation has no effect on TF interaction with FVIIa. There are no published data suggesting a direct influence of phosphorylation or palmitoylation in the cytoplasmic domain on TF procoagulant activity. There has been a debate in the recent literature related to the role and formation of the Cys¹⁸⁶-Cys²⁰⁹ disulfide bond. Published opinions from various laboratories range from this bond being essential for the expression of cell TF activity to having no role in it. Overall, it is clear that some modifications of TF have an effect on TF procoagulant activity, signaling functions and trafficking. The influences of other modifications are debatable.

Membrane binding events in the initiation and propagation phases of tissue factor-initiated zymogen activation under flow

This study investigates the dynamics of zymogen activation when both extrinsic tenase and prothrombinase are assembled on an appropriate membrane. Although the activation of prothrombin by surface-localized prothrombinase is clearly mediated by flow-induced dilutional effects, we find that when factor X is activated in isolation by surface-localized extrinsic tenase, it exhibits characteristics of diffusion-mediated activation in which diffusion of substrate to the catalytically active region is rate-limiting. When prothrombin and factor X are activated coincident with each other, competition for available membrane binding sites masks the diffusion-limiting effects of factor X activation. To verify the role of membrane binding in the activation of factor X by extrinsic tenase under flow conditions, we demonstrate that bovine lactadherin competes for both factor X and Xa binding sites, limiting factor X activation and forcing the release of bound factor Xa from the membrane at a venous shear rate (100 s(-1)). Finally, we present steady-state models of prothrombin and factor X activation under flow showing that zymogen and enzyme membrane binding events further regulate the coagulation process in an open system representative of the vasculature geometry.

Anticoagulants and the propagation phase of thrombin generation

The view that clot time-based assays do not provide a sufficient assessment of an individual's hemostatic competence, especially in the context of anticoagulant therapy, has provoked a search for new metrics, with significant focus directed at techniques that define the propagation phase of thrombin generation. Here we use our deterministic mathematical model of tissue-factor initiated thrombin generation in combination with reconstructions using purified protein components to characterize how the interplay between anticoagulant mechanisms and variable composition of the coagulation proteome result in differential regulation of the propagation phase of thrombin generation. Thrombin parameters were extracted from computationally derived thrombin generation profiles generated using coagulation proteome factor data from warfarin-treated individuals (N = 54) and matching groups of control individuals (N = 37). A computational clot time prolongation value (cINR) was devised that correlated with their actual International Normalized Ratio (INR) values, with differences between individual INR and cINR values shown to derive from the insensitivity of the INR to tissue factor pathway inhibitor (TFPI). The analysis suggests that normal range variation in TFPI levels could be an important contributor to the failure of the INR to adequately reflect the anticoagulated state in some individuals. Warfarin-induced changes in thrombin propagation phase parameters were then compared to those induced by unfractionated heparin, fondaparinux, rivaroxaban, and a reversible thrombin inhibitor. Anticoagulants were assessed at concentrations yielding equivalent cINR values, with each anticoagulant evaluated using 32 unique coagulation proteome compositions. The analyses showed that no anticoagulant recapitulated all features of warfarin propagation phase dynamics; differences in propagation phase effects suggest that anticoagulants that selectively target fXa or thrombin may provoke fewer bleeding episodes. More generally, the study shows that computational modeling of the response of core elements of the coagulation proteome to a physiologically relevant tissue factor stimulus may improve the monitoring of a broad range of anticoagulants.

The influence of prophylactic factor VIII in severe haemophilia A

Haemophilia A individuals displaying a similar genetic defect have heterogeneous clinical phenotypes. Our objective was to evaluate the underlying effect of exogenous factor (f)VIII on tissue factor (Tf)-initiated blood coagulation in severe haemophilia utilizing both empirical and computational models. We investigated twenty-five clinically severe haemophilia A patients. All individuals were on fVIII prophylaxis and had not received fVIII from 0.25 to 4 days prior to phlebotomy. Coagulation was initiated by the addition of Tf to contact-pathway inhibited whole blood ± an anti-fVIII antibody. Aliquots were quenched over 20 min and analyzed for thrombin generation and fibrin formation. Coagulation factor levels were obtained and used to computationally predict thrombin generation with fVIII set to either zero or its value at the time of the draw. As a result of prophylactic fVIII, at the time of the blood draw, the individuals had fVIII levels that ranged from <1% to 22%. Thrombin generation (maximum level and rate) in both empirical and computational systems increased as the level of fVIII increased. FXIII activation rates also increased as the fVIII level increased. Upon suppression of fVIII, thrombin generation became comparable in both systems. Plasma composition analysis showed a negative correlation between bleeding history and computational thrombin generation in the absence of fVIII. Residual prophylactic fVIII directly causes an increase in thrombin generation and fibrin cross-linking in individuals with clinically severe haemophilia A. The combination of each individual's coagulation factors (outside of fVIII) determine each individual's baseline thrombin potential and may affect bleeding risk.

Dilutional control of prothrombin activation at physiologically relevant shear rates

The generation of proteolyzed prothrombin species by preassembled prothrombinase in phospholipid-coated glass capillaries was studied at physiologic shear rates (100-1000 s(-1)). The concentration of active thrombin species (α-thrombin and meizothrombin) reaches a steady state, which varies inversely with shear rate. When corrected for shear rate, steady-state levels of active thrombin species exhibit no variation and a Michaelis-Menten analysis reveals that chemistry of this reaction is invariant between open and closed systems; collectively, these data imply that variations with shear rate arise from dilutional effects. Significantly, the major products observed include nonreactive species arising from the loss of prothrombin's phospholipid binding domain (des F1 species). A numerical model developed to investigate the spatial and temporal distribution of active thrombin species within the capillary reasonably approximates the observed output of total thrombin species at different shears; it also predicts concentrations of active thrombin species in the wall region sufficient to account for observed levels of des FI species. The predominant feedback formation of nonreactive species and high levels of the primarily anticoagulant intermediate meizothrombin (∼40% of total active thrombin species) may provide a mechanism to prevent thrombus propagation downstream of a site of thrombosis or hemorrhage.

Anticoagulation by factor Xa inhibitors

BACKGROUND

Therapeutic agents that regulate blood coagulation are critical to the management of thrombotic disorders, with the selective targeting of factor (F) Xa emerging as a promising approach.

OBJECTIVE

To assess anticoagulant strategies targeting FXa.

METHODS

A deterministic computational model of tissue factor (Tf)-initiated thrombin generation and two empirical experimental systems (a synthetic coagulation proteome reconstruction using purified proteins and a whole blood model) were used to evaluate clinically relevant examples of the two available types of FXa-directed anticoagulants [an antithrombin (AT)-dependent agent, fondaparinux, and an AT-independent inhibitor, Rivaroxaban] in experimental regimens relevant to long-term (suppression of new Tf-initiated events) and acute (suppression of ongoing coagulation processes) clinical applications.

RESULTS

Computational representations of each anticoagulant's efficacy in suppressing thrombin generation over a range of anticoagulant concentrations in both anticoagulation regimens were validated by results from corresponding empirical reconstructions and were consistent with those recommended for long-term and acute clinical applications, respectively. All three model systems suggested that Rivaroxaban would prove more effective in the suppression of an ongoing coagulation process than fondaparinux, reflecting its much higher reactivity toward the prothrombinase complex.

CONCLUSION

The success of fondaparinux in acute settings in vivo is not explained solely by its properties as an FXa inhibitor. We have reported that FIXa contributes to the long-term capacity of clot-associated catalysts to restart a coagulation process, suggesting that the enhanced anti-FIXa activity of fondaparinux-AT may be critical to its success in acute settings in vivo.

Taking the Thrombin “Fork”

The proverb that probably best exemplifies my career in research is attributable to Yogi Berra (http://www.yogiberra.com/), ie, “when you come to a fork in the road … take it.” My career is a consequence of chance interactions with great mentors and talented students and the opportunities provided by a succession of ground-breaking improvements in technology.